Industrial wastes as low-cost potential adsorbents for the treatment of wastewater laden with heavy...

Advances in Colloid and Interface Science 166 (2011) 36–59

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Advances in Colloid and Interface Science

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Industrial wastes as low-cost potential adsorbents for the treatment of wastewaterladen with heavy metals

M. AhmaruzzamanDepartment of Chemistry, National Institute of Technology Silchar, Assam, 788010, India

E-mail address: [email protected].

0001-8686/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.cis.2011.04.005

a b s t r a c t
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Available online 30 April 2011

Keywords:Industrial wastesFly ashSludgeRed mudMetalsAdsorptionAdsorbentWastewater treatment

Industrial wastes, such as, fly ash, blast furnace slag and sludge, black liquor lignin, red mud, and waste slurry,etc. are currently being investigated as potential adsorbents for the removal of the heavy metals fromwastewater. It was found that modified industrial wastes showed higher adsorption capacity. The applicationof low-cost adsorbents obtained from the industrial wastes as a replacement for costly conventional methodsof removing heavy metal ions from wastewater has been reviewed. The adsorption mechanism, influencingfactors, favorable conditions, and competitive ions etc. on the adsorption of heavy metals have also beendiscussed in this article. From the review, it is evident that certain industrial wastematerials have demonstratedhigh removal capacities for the heavy metals laden with wastewater. However, it is to be mentioned thatadsorption capacities of the adsorbents vary depending on the characteristics of the adsorbents, the extent ofchemical modification and the concentration of adsorbates. There are also few issues and drawbacks on theutilization of industrial wastes as low-cost adsorbents that have been addressed. In order to find out the practicalutilization of industrial waste as low-cost adsorbents on the commercial scale, more research should beconducted in this direction.

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372. Current technologies available for treatment methods of heavy metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373. Industrial wastes as an adsorbent for removal of heavy metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.1. Adsorption of heavy metals on fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.2. Adsorption of heavy metals on blast furnace slag, sludge and dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.3. Adsorption of heavy metals on red mud . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.4. Adsorption of heavy metals on lignin, a black liquor waste of paper industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.5. Adsorption of heavy metals on waste hydroxide, a fertilizer industry waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.6. Adsorption of heavy metals on miscellaneous industrial adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

4. Competitive adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475. Leaching of fly ash, red mud, blast furnace slag, and other wastes in water system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486. Efficiency and cost comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 507. Combination of methodologies/techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508. Adsorption isotherms and mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519. Kinetic modeling of adsorption in a batch system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

10. Factors affecting adsorption of heavy metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5310.1. Effect of contact time and initial concentration of heavy metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5310.2. Effect of pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5310.3. Effect of particle size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5410.4. Effect of ionic strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

11. Column studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5512. Comparison of adsorption performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

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37M. Ahmaruzzaman / Advances in Colloid and Interface Science 166 (2011) 36–59

13. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5614. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

1. Introduction

Contamination of water by toxic heavy metals through the dis-charge of industrialwastewater is aworldwide environmental problem.Rapid industrialization has seriously contributed to the release of toxicheavymetals towater streams.Mining, electroplating,metal processing,textile, battery manufacturing, tanneries, petroleum refining, paintmanufacture, pesticides, pigment manufacture, printing and photo-graphic industries [1,2] are the main sources of heavy metal ion con-tamination. Metals such as lead, cadmium, copper, arsenic, nickel,chromium, zinc andmercury have been recognized as hazardous heavymetals. Unlike organicwastes, heavymetals are non-biodegradable andthey can be accumulated in living tissues, causing various diseases anddisorders; therefore they must be removed before discharge.

Heavy metal toxicity can result in damage or reduced mental andcentral nervous function, lower energy levels and damage to bloodcomposition, lungs, kidneys, liver and other vital organs. Presence ofmetals in water streams and marine water causes a significant healththreat to the aquatic community—most common being the damage ofthe gill of the fish [3,4]. Consequently, in many countries, more strictlegislation has been introduced to control water pollution. Variousregulatory bodies have set the maximum prescribed limits for thedischarge of toxic heavy metals in the aquatic systems. However, themetal ions are being added to the water stream at a much higherconcentration than the prescribed limits by industrial activities, thusleading to the health hazards and environmental degradation. Table 1shows the permissible limits and health effects of various toxic heavymetals [5–8]. Removal of metal ions from wastewater in an effectivemanner has become an important issue today. Precipitation followedby coagulation has been extensively employed for the removal of heavymetals from water. However, this process usually produces largevolumesof sludge consisting small amounts of heavymetals.Membranefiltration is a proven way to remove metal ions but its high cost limitsthe use in practice [3]. Adsorption is one of the most effective pro-cesses of advanced wastewater treatment, which industries employ to

Table 1Permissible limits and health effects of various toxic heavy metals [5–8].

Metals Permissible limits for industrialeffluent discharge (mg/L)

Permissible limits forpotable water (mg/L)

Indian standard WHO

Inlandsurfacewater

Publicsewers

Marinecoastalareas

Inlandsurfacewater

IndianstandardIS 10,500

WHO USEPA ES

Nickel 3.0 3.0 5.0 – 0.02 0.02 0.1 0Zinc 5.0 15.0 15.0 5.0–15.0 5.0 3.0 5.0 –

Copper 3.0 3.0 3.0 0.05–1.5 1.5 2.0 1.3 2

Cadmium 2.0 1.0 2.0 0.1 0.01 0.003 0.005 0Lead 0.10 1.0 2.0 0.1 0.05 0.01 0.015 0

Totalchromium

2.0 2.0 2.0 – 0.05 0.05 0.1 0

Arsenic 0.2 0.2 0.2 – 0.01 0.01 0.01 0Mercury 0.01 0.01 0.01 – 0.001 0.001 0.002 0

Iron 3.0 3.0 3.0 0.1–1.0 0.3 0.2 0.3 0

Manganese 2.0 2.0 2.0 0.05–0.5 0.1 0.5 0.05 0

Vanadium 0.2 0.2 0.2 – – 1.4 – –

reduce hazardous metals present in the effluents. Activated carbon –

produced by carbonizing organic materials – is the most widely usedadsorbent. Activated carbon has shown good metal ion adsorptioncapacities [9–13]. However, the high cost of the activation process limitsthe utilization in wastewater treatment. Over the last few years, a largenumber of investigations have been conducted to test the low costadsorbents for the removal of heavy metals. Waste biomass, industrialwaste, andmineral waste have been investigated bymanyworkers andbiomass has shown better adsorption properties [14].

An overview of some industrial wastes as low-cost adsorbents ispresented in this paper and their removal performance is compared.The main goal of this review is to provide a summary of informa-tion concerning the use of industrial waste materials as adsorbents forthe removal of heavy metals from wastewater. The review alsopresents some critical issues and drawbacks on the utilization ofindustrial wastes as low-cost adsorbents. This review also (i) presentsa critical analysis of these industrial waste materials; (ii) describestheir characteristics and (iii) discusses various factors and mecha-nisms involved. A comparison of the adsorption capacity of variousindustrial wastes on the removal of heavy metals is also presented inthis article.

2. Current technologies available for treatment methods ofheavy metals

Heavy metals are not biodegradable and tend to accumulate inliving organisms, causing various diseases and disorders. There arevarious treatment processes available for metal-contaminated wastestreams, such as, chemical precipitation, coagulation, solvent extrac-tion, ultra filtration, biological systems, electrolytic processes, reverseosmosis, oxidation with ozone/hydrogen peroxide, membrane filtra-tion, ion exchange, photo catalytic degradation, and adsorption. Thetechnologies can be divided into three categories: biological, chemicaland physical. Currently, the major methods of industrial wastewatertreatment involve physical and/or chemical processes. Because of the

Health hazards

Utandard

.02 Causes chronic bronchitis, reduced lung function, cancer of lungs.Causes short-term illness called “metal fume fever” and restlessness

.0 Long term exposure causes stomachache, irritation of nose,mouth, eyes, headache

.005 Carcinogenic, cause lung fibrosis, dyspnoea

.01 Suspected carcinogen, anemia, muscle and joint pains,kidney problem and high blood pressure

.05 Suspected human carcinogen, producing lung tumors

.01 Carcinogenic, producing liver tumors, and gastrointestinal effects

.001 Excess dose may cause headache, abdominal pain, and diarrhea,paralysis, and gum inflammation, loosening of teeth, loss of appetite, etc.

.2 Excess amounts cause rapid pulse rates, congestion ofblood vessels, hypertension

.05 Excess amounts toxic, and causes growth retardation, fever,sexual impotence, muscles fatigue, eye blindness.Very toxic, and may cause paralysis

Table 2Current treatment technologies for heavy metals removal involving physical and/orchemical processes [19].

Physical and/orchemical methods

Advantages Disadvantages

Oxidation Rapid process for toxicpollutants removal

High energy costsand formation ofby-products

Ion exchange Good removal of awide range of heavy metals

Absorbent requiresregeneration ordisposal

Membrane filtrationtechnologies

Good removal ofheavy metals

Concentrated sludgeproduction,expensive

Adsorption flexibility and simplicity ofdesign, ease of operation andinsensitivity to toxic pollutants

Adsorbents requiresregeneration

Coagulation/flocculation Economically feasible High sludgeproduction andformation of largeparticles

Electrochemicaltreatment

Rapid process and effectivefor certain metal ions

High energy costsand formation ofby-products

Ozonation Applied in gaseous state:alteration of volume

Short half life

Photochemical No sludge production Formation ofby-products

Irradiation Effective at lab scale Required a lotof dissolved 02

Electrokineticcoagulation

Economically feasible High sludgeproduction

Fentons reagents Effective and capable oftreating variety of wastesand no energy input necessaryto activate hydrogen peroxide

Sludge generation

Biological treatment Feasible in removing somemetals

Technology yet tobe established andcommercialized

38 M. Ahmaruzzaman / Advances in Colloid and Interface Science 166 (2011) 36–59

high cost and disposal problems, many of these conventional methodsfor treating wastewater have not been widely applied at large scale.

At the present time, there is no single process capable of ade-quate treatment, mainly due to the complex nature of the effluents. Inpractice, a combination of different processes is often used to achievethe desired water quality in the most economical way. Biologicaltreatment is often the most economical alternative when comparedwith other physical and chemical processes. Biodegradation methodsare commonly applied to the treatment of industrial effluents, becausemanymicroorganisms such as bacteria, yeasts, algae and fungi are ableto accumulate and degrade different pollutants [15–17]. However,their application is often restricted because of technical constraints.Biological treatment requires a large land area and is constrained bysensitivity toward diurnal variation as well as toxicity of some che-micals, and less flexibility in design and operation [18]. Chemicalmethods include coagulation or flocculation combined with flotationand filtration, precipitation–flocculation, electroflotation, electroki-netic coagulation, conventional oxidationmethods by oxidizing agents(ozone), irradiation or electrochemical processes. These chemicaltechniques are often expensive, and although the toxic pollutantsare removed, accumulation of concentrated sludge creates a disposalproblem. Different physical methods are also widely used, such asmembrane-filtration processes (nanofiltration, reverse osmosis, elec-trodialysis, etc.) and adsorption techniques. Themajor disadvantage ofthe membrane process is that of a limited lifetime, before membranefouling occurs and the cost of periodic replacement must be includedin any analysis of their economic viability. In accordancewith the veryabundant literature data, liquid-phase adsorption is one of the mostpopular methods for the removal of toxic pollutants fromwastewater,since proper design of the adsorption process will produce a high-quality treated effluent. This process provides an attractive alternativefor the treatment of contaminated water, especially if the adsorbent isinexpensive and does not require an additional pre-treatment stepbefore its application. Adsorption has been found to be superiorcompared to the other techniques for water re-use in terms of initialcost, flexibility and simplicity of design, ease of operation and insen-sitivity to toxic pollutants. Adsorption also does not result in theformation of harmful substances. Table 2 showed the advantages anddisadvantages of various processes used for the removal of heavymetals from wastewaters [19].

3. Industrial wastes as an adsorbent for removal of heavy metals

Heavy metals are nowadays the most important pollutants andbecoming a severe public health problem. Heavy metal and metalloidremoval from aqueous solutions has been commonly carried out byseveral processes such as, chemical precipitation, solvent extraction,ion-exchange, reverse osmosis or adsorption etc. Among these pro-cesses, the adsorption processmay be a simple and effective techniquefor the removal of heavy metals from wastewater.

Industrial waste is also one of the potentially low-cost adsorbentfor the removal of heavy metals from wastewaters. It requires littleprocessing to increase its adsorptive capacity. Generally industrialwastes are generated as by-products. Since these materials are locallyavailable in large quantities, they are inexpensive. Various types ofindustrial wastes such as fly ash, blast furnace sludge, waste slurry,lignin, iron (III) hydroxide, and red mud, have been explored for theirtechnical feasibility to remove toxic heavy metals from contaminatedwater. Other industrial wastes, coffee husks, Areca waste, tea factorywaste, sugar beet pulp, waste pomace of olive oil factory waste,battery industry waste, waste biogas residual slurry, sea noduleresidue, and grape stalk wastes have been utilized as low-costadsorbents for the removal of toxic heavy metals from wastewater.Although, many research works have been carried out recently to findthe potential of using various alternative adsorbents, so far no effortshave been made to obtain a comparative overview of all adsorbents

mentioned previously in terms of their removal performance,adsorption capacity, and cost effectiveness.

3.1. Adsorption of heavy metals on fly ash

Fly ash as coal combustion residue has a great potential in en-vironmental applications and an interesting alternative to replaceactivated carbon or zeolites as an adsorbent for the treatment ofwastewater. However, adsorption performance of fly ash stronglydepends on fly ash origin and chemical treatment. Economic barriershave to be overcome in terms of high value and high volume uti-lization. The chemical compositions of fly ash are high percentageof silica (60–65%), alumina (25–30%), magnetite, Fe2O3 (6–15%) andother important physicochemical characteristics of the fly ash, like bulkdensity, particle size, porosity, water holding capacity, and surfacearea — makes it suitable for use as adsorbent. Moreover, the alkalinenature of fly ash makes it a good neutralizing agent. Generally, in orderto maximize metal adsorption by hydrous oxides, it is necessary toadjust the pHofwastewater using lime and sodiumhydroxide [20]. Rawfly ash generally has low adsorption capacity. Modification by physicaland chemical treatment would enhance the adsorption capacity [21],thereby increasing the value for application.

Fly ash has been widely used as a low-cost adsorbent for the re-moval of heavy metals. Table 3 summarizes the results of the impor-tant metals investigated on fly ash. Among these metals, Ni, Cr, Pb, As,Cu, Cd and Hg are the most investigated. The use of fly ash for theremoval of heavy metals was reported as early as in 1975. Gangoliet al. [22] reported the utilization of fly ash for the removal of heavymetals from industrial wastewaters.

Removal of chromium ions including Cr(VI) and Cr(III) using flyash has been investigated by several researchers. Grover and

Table 3Summary of adsorption of metals on fly ash.

Metals Adsorbent Adsorption Capacity(mg/g)

Temperature(°C)

References

Zn2+ Fly ash 6.5–13.3 30–60 [44]Fe impregnated flyash

7.5–15.5 30–60 [44]

Al impregnated flyash

7.0–15.4 30–60 [44]

Fly ash 0.25–2.8 20 [46]Fly ash(I) 0.25–1.19 20 [47]Fly ash(II) 0.07–1.30 20 [47]Bagasse fly ash 2.34–2.54 30–50 [56]Bagasse fly ash 13.21 30 [57]Fly ash 4.64 23 [69]Fly ash 0.27 25 [70]Fly ash 0.068–0.75 0–55 [71]Fly ash 3.4 – [49]Fly ash zeolite 4A 30.80 [60]Rice husk ash 5.88 [167]Bagasse fly ash 7.03 [214]Fly ash 11.11 [156]Rice husk ash 14.30 [96]Fly ash 7.84 [156]

Cd2+ Fly ash 1.6–8.0 – [42]Fly ash zeolite 95.6 20 [42]Fly ash 0.67–0.83 20 [46]Fly ash(I) 0.08–0.29 20 [48]Fly ash(II) 0.0077–0.22 20 [48]Fly ash 198.2 25 [41]Fly ash-washed 195.2 25 [41]Fly ash-acid 180.4 25 [41]Bagasse fly ash 1.24–2.0 30–50 [58]Fly ash 0.05 25 [70]Coal Fly ash 18.98 25 [32]Rice husk ash 3.04 [167]Coal Fly ash pellets 18.92 [32]Afsin–Elbistan flyash

0.29 [28]

Seyitomer fly ash 0.21 [28]Bagasse fly ash 6.19 [214]Fly ash zeolite X 97.78 [67]

Pb2+ Fly ash zeolite 70.6 20 [101]Fly ash 444.7 25 [63]Fly ash-washed 483.4 25 [63]Fly ash-acid 437.0 25 [63]Fly ash 753 32 [63]Bagasse fly ash 285–566 30–50 [77]Fly ash 18.8 [72]Fly ash zeolite X 420.61 [67]Treated rice huskash

12.61 30 [39]

Cu2+ Fly ash 1.39 30 [30]Fly ash+wollastonite

1.18 30 [30]

Fly ash 1.7–8.1 – [42]Fly ash(I) 0.34–1.35 20 [47]Fly ash(II) 0.09–1.25 20 [47]Fly ash 207.3 25 [49]Fly ash-washed 205.8 25 [49]Fly ash-acid 198.5 25 [49]Fly ash 0.63–0.81 25 [31]Bagasse fly ash 2.26–2.36 30–50 [56]Fly ash 0.76 32 [62]Fly ash 7.5 – [49]Coal Fly ash pellets 20.92 25 [32]Fly ash zeolite 4A 50.45 [60]Fly ash 7.0 [165]CFA 178.5–249.1 30–60 [34]CFA-600 126.4–214.1 30–60 [34]CFA–NAOH 76.7–137.1 30–60 [34]Fly ash zeolite X 90.86 [67]Fly ash 7.0 [156]

Ni2+ Fly ash 9.0–14.0 30–60 [44]Fe impregnated flyash

9.8–14.93 30–60 [44]


10–15.75 30–60 [44]

Fly ash(I) 0.40–0.98 20 [47]

(continued on next page)

Table 3 (continued)


Temperature(°C)

References

Fly ash(II) 0.06–1.16 20 [47]Bagasse fly ash 1.12–1.70 30–50 [58]Fly ash 3.9 [49]Fly ash zeolite 4A 8.96 [60]Afsin–Elbistan flyash

0.98 [28]

Seyitomer fly ash 1.16 [28]Bagasse fly ash 6.48 [214]Fly ash 0.03 [33]

Cr3+ Fly ash 52.6–106.4 20–40 [53]Bagasse fly ash 4.35 [59]Fly ash zeolite 4A 41.61 [60]

Cr6+ Fly ash+wollastonite

2.92 – [25]

Fly ash+Chinaclay

0.31 – [25]

Fly ash 1.38 30–60 [45]Fe impregnated flyash

1.82 30–60 [45]


1.67 30–60 [45]

Fly ash(I) 0.55 20 [48]Fly ash(II) 0.82 20 [48]Bagasse fly ash 4.25–4.35 30–50 [59]Fly ash 23.86 – [26]Rice Husk Ash 25.64 – [26]

Co2+ Fly ash zeolite 4A 13.72 [60]Hg2+ Fly ash 2.82 30 [36]

Fly ash 11.0 30–60 [45]Fe impregnated flyash

12.5 30–60 [45]


13.4 30–60 [45]

Sulfo-calcic 5.0 30 [49]Silico-aluminousashes

3.2 30 [49]

Fly ash-C 0.63–0.73 5–21 [37]Treated rice huskash

9.32 15 [39]

As3+ Fly ash coal-char 3.7–89.2 25 [74]As5+ Fly ash 7.7–27.8 20 [72]

Fly ash coal-char 0.02–34.5 25 [74]Cs+ Fly ash zeolite 443.9 25 [80]


Narayanaswamy [23] studied the effects of hexavalent chro-mium concentrations, fly ash dosage, contact time, and pH on theremoval of Cr(VI) and found that removal was effective at lower pH.Dasmahapatra et al. [24] investigated the adsorption of Cr(VI) on flyash and reported that the percent removal of Cr(VI) by fly ash isaffected by its concentration in aqueous solution, temperature, par-ticle size, and pH. A homogeneous mixture of fly ash and wollastonite(1:1) was also utilized for the removal of Cr(VI) from aqueous solu-tions by adsorption [25]. Bhattacharya et al. [26] studied the removalof Cr(VI) from aqueous solution using fly ash. The influence of pH,adsorbent type, initial Cr(VI) concentration and contact time on theselectivity and sensitivity of the removal process was investigated.They also compared the adsorption capacity of fly ash with other low-cost adsorbents, such as, clarified sludge, saw dust, neem bark andrice husk ash, and found that clarified sludge was the most effectiveamong the studied adsorbents for the removal of Cr(VI) from aqueoussolution. The results suggest that adsorption of Cr(VI) on the selectedadsorbents involves a complex mechanism and that in the adsorptionprocess there are two distinct stages — the initial stages of boundarylayer diffusion was due to external mass transfer effects and in thelater stages it was due to intraparticle diffusion which contributes tothe rate determining step [26].

In another study, Sharma et al. [27] investigated the potential of flyash for the removal of chromium from aqueous solutions andwastewaters. Intraparticle diffusion was found to control the removalof Cr(VI) and the value of coefficient of intraparticle diffusion was


found to be 2.25×10−11 cm2/s at 298 K. The value of the co-efficientof mass transfer, βl, 2.15×10−2 cm/s at 298 K, suggested the transferof Cr(VI) onto the adsorbent surface to be rapid enough. The processof removal is highly dependent on pH of the solutions with maxi-mum removal (89.12%) at pH 2.5. Comparison of the adsorbent usedwith other non-conventional adsorbents shows that fly ash is a goodadsorbent and can be recommended for the treatment of metal richwastewater in general and that of Cr(VI) in particular.

The removal of Cr(VI) and Cd(II) from an aqueous solution onTurkish fly ash was compared by Bayat [28]. The effect of contact time,solution pH, and ash quality on the removal was investigated. Flyash was found to have a higher adsorption capacity for Cd(II) as com-pared to that of Cr(VI). The lime (crystalline CaO) content in the flyash seemed to be a significant factor in influencing the adsorption of Cr(VI) and Cd(II). Recently, fly ash obtained from the combustion ofpoultry litter was utilized as an adsorbent for the removal of Cr(III) fromaqueous solution [29].

Fly ash was also utilized for the removal of copper from aqueoussolution. The removal efficiency was found to be dependent on con-centration, pH and temperature [30]. The kinetics of adsorption indi-cated the process to be diffusion controlled. Fly ashes with differentquantities of carbon and minerals were also used for the removal ofCu(II) from aqueous solution [31]. The carbon fraction in fly ash wasimportant in the removal of Cu(II). The specific adsorption capacitiesof carbon ranged from 2.2 to 2.8 mg Cu/g carbon, while those formineral were only about 0.63–0.81 mg Cu/g mineral. Fly ash can alsobe shaped into pellets and used for the removal of copper andcadmium ions from aqueous solutions [32]. The adsorption of copperand cadmium fitted well with the Langmuir isotherm. The calculatedadsorption capacities for copper and cadmiumwere found to be 20.92and 18.98 mg/g, respectively. They reported that fly ash shaped intopellets could be considered as a potential adsorbent for the removal ofcopper and cadmium from wastewaters.

Raw bagasse and fly ash have also been used as low-cost adsorbentsfor the removal of chromium and nickel from an aqueous solution [33].The extent of adsorption at equilibrium was found to be dependent onthe physical and chemical characteristics of the adsorbent, adsorbateand experimental system. Raw and modified coal fly ash effectivelyadsorb Cu(II) fromwastewater [34]. These adsorptions were endother-mic in nature; the values of activation energy (between 1.3 and9.6 kJ mol−1) were consistent with an ion-exchange adsorptionmechanism. The adsorptions of Cu(II) onto coal fly ash (CFA), CFA-600, and CFA–NaOH followed pseudo-second-order kinetics. CFA wasfound to be a cheap and effective adsorbent for the removal of Cu(II)ions fromwater. The nature of CFA did not improve its ability to adsorbCu(II). The removal characteristics of Pb(II) and Cu(II) ions fromaqueous solution byfly ashwere investigated [35]. The level of uptake ofPb(II) and Cu(II) ions by the fly ash generally increased, but not in aprogressive manner, at higher pH values. The main mechanismsinvolved in the removal of heavy metals from solution were adsorptionat the surface of the fly ash and precipitation.

Fly ash was also found to be effective for the removal of mercuryfrom aqueous solution. Sen and De [36] showed that the adsorptioncapacity of coal fly ash formercurywas comparable to that of activatedpowdered charcoal. The pH of the solution was found to be the mostimportant parameter affecting the adsorption. The effectiveness of flyash in adsorbing mercury from wastewater has been studied [37].Batch kinetic and isotherm studies have been carried out to determinethe effect of contact time, pH and temperature on the adsorption.

Rice husk ash was found to be a good adsorbent for the removal ofHg (II) from aqueous solution [38]. Batch studies indicated thatpercent adsorption decreased with increased initial concentration ofHg(II) and particle size of the adsorbent. Maximum Hg(II) removalwas observed near a pH of 6.0. Column experiments indicated that theadsorbed amount of Hg(II) decreased with increased flow rate anddecreased bed height. An attempt was made for the removal of lead

and mercury from aqueous water by using rice husk ash as anadsorbent and found to be a suitable adsorbent for the adsorption oflead and mercury ions [39]. The Bangham equation was used toexpress the mechanism for adsorption of lead and mercury ions byrice husk ash. Its adsorption capability and adsorption rate are con-siderably higher and faster for lead ions than for mercury ions. Thefiner the rice husk ash particles used, the higher the pH of the solutionand the lower the concentration of the supporting electrolyte, potas-sium nitrate solution, the more lead and mercury ions adsorbed onrice husk ash.

Papachristou et al. [40] determined the selective adsorption ofvarious metal ions (Na, K, Mg, Ca, Cu, Cd, Mn, Hg, Cr, Pb, and Fe) bytwo different fly ashes. Lead ions were found to be selectivelyadsorbed at a mean value of 19 meq of Pb(II) per 100 g of fly ash. Thisselective adsorption could be due to the formation of crystallineettringite mineral after the hydration of the fly ash. Coal fly ashes havealso been used for the removal of toxic heavy metals, i.e. Cu(II), Pb(II)and Cd(II) from water [41]. The breakthrough volumes of the heavymetal solutions have beenmeasured by dynamic column experimentsso as to determine the saturation capacities of the adsorbents. Theadsorption sequence is CuNPbNCd in accordance with the order ofinsolubility of the corresponding metal hydroxides. Similar resultson the adsorption of Cd and Cu by fly ash were also reported by Ayalaet al. [42]. Adsorption capacity increased with decreased initial con-centration of the metals. The presence of high ionic strength or ap-preciable quantities of calcium and chloride ions does not have asignificant effect on the adsorption of these metals by fly ash.

The utilization of zeolites synthesized from fly ash (FA) and relatedco-disposal filtrates as low-cost adsorbent material was investigated[43]. When raw FA and co-disposal filtrates were subjected to alkalinehydrothermal zeolite synthesis, the zeolites, faujasite, sodalite andzeolite A were formed. The synthesized zeolites were explored toestablish its ability to remove lead and mercury ions from aqueoussolution in batch experiments, to which various dosages of the syn-thesized zeolites were added. The lead concentrations were reducedfrom 3.23 to 0.38 and 0.17 μg/kg, respectively, at an average pH of 4.5,after the addition of raw FA zeolite and co-disposal filtrate zeolite tothe AMD wastewater. On the other hand, Somerset et al. [43] showedthat the mercury concentration was reduced from 0.47 to 0.17 μg/kgat pH=4.5 when increasing amounts of co-disposal filtrate zeolitewere added to the wastewater. The experimental results had shownthat the zeolites synthesized from the co-disposal filtrates were ef-fective in reducing the lead and mercury concentrations in the AMDwastewater by 95% and 30%, respectively [43].

Banerjee et al. [44,45] studied the adsorption of various toxicmetalions, Ni(II) and Zn(II), Cr(II) and Hg(II), on fly ash and Al- and Fe-impregnated fly ash. The impregnated fly ash showed much higheradsorption capacity for all the ions as compared to that of untreatedfly ash. The adsorption was found to be exothermic for Ni(II) andendothermic in case of Zn(II). The adsorption capacity of FA, Al-FA,and Fe-FA for Cr(VI) was found to be 1.379, 1.820, and 1.667 mg/g andthat of Hg(II) was 11.00, 12.50, and 13.40 mg/g. Bayat investigatedthe removal of Zn(II) and Cd(II) [46], Ni(II) and Cu(II) [47], and Cr(VI)[48] using lignite-based fly ash and activated carbon and found thatthe fly ash was effective as activated carbon. The parameters studiedinclude contact time, pH, temperature, initial concentration of theadsorbate, and fly ash dosage. The percent adsorption of Zn(II) and Cd(II) increased with an increase in concentration of Zn(II) and Cd(II),dosage of fly ash and temperature and the maximum adsorptionoccurred in the pH range of 7.0–7.5. Thermodynamic parameters sug-gested the endothermic nature of the adsorption process. The effec-tiveness of the fly ash as an adsorbent improvedwith increased amountof calcium (CaO). Fly ashes were found to have a higher adsorptioncapacity for Cd(II) as compared to that of Cr(VI).

Ricou et al. [49,50] studied the removal of Cu, Ni, Zn, Cd and Pb byfly ash and fly ash/lime mixture. The removal extent was achieved in


the order of Pb(II)NCu(II)NNi(II)NZn(II)NCd(II). Formation of calci-um silicate hydrates (CSH) was supposed to be responsible for in-creasing removal as also for decreasing desorption. Two fluidized-bedsourced fly asheswith different chemical composition, silico-aluminousfly ashes and sulfo-calcic fly ashes, were tested to remove Pb(II), Cu(II),Cr(III), Ni(II), Zn(II), Cr(VI) [51] and Hg(II) [52] from aqueous solutions.The percentage of adsorbed ions was found to be greater when theywere in contact with silico-aluminous fly ashes than sulfo-calcic flyashes, except in the case of the ion Ni(II). However, sulfo-calcic fly ashremoved mercury more efficiently and more steadily. The resultsshowed that mercury is bound to ash surface due to several chemicalreactions between mercury and various oxides (silicon, aluminum andcalcium silicate) of the surface of fly ash.

Cetin and Pehlivan [53] compared fly ash and activated carbonfor their ability to remove nickel [Ni(II)] and zinc [Zn(II)] from anaqueous solution. Batch kinetic studies showed that an equilibriumtime of 1 hwas required for the adsorption of Ni(II) and Zn(II) on bothadsorbents. The maximummetal removal was found to be dependenton solution pH.With an increase in the concentrations of thesemetals,the adsorption of Ni(II) and Zn(II) decreased on both of the ad-sorbents. The effectiveness of fly ash as an adsorbent improved withincreasing calcium (CaO) content. Adsorption data in the range of pHvalues (3.0–8.0) using Ni(II) concentration of 25±2 mg/L and Zn(II)concentration of 30±2 mg/L in solution were correlated using thelinear forms of the Langmuir and Freundlich equations.

The changes in toxicity and heavy metals in a municipal waste-water treatment plant effluent on treatment with fly ash weremeasured [54]. The effluent after the treatment with fly ash showeda significant reduction in the toxicity, Cu, Pb and PO43− and NO3

−

contents. Fly ash removed Cu and Pb from the effluent and the re-moval of these toxic heavymetals resulted in the reduction of toxicity.Gupta research group [55–59] conducted a series of investigations onthe adsorption of heavy metals by using fly ash as adsorbents. Theyhave used bagasse fly ash from sugar industries for the removal of lead[55], copper and zinc [56,57], cadmium and nickel [58] and chromium[59] from aqueous solutions. Copper and zinc are adsorbed by thedeveloped adsorbent up to 90–95% in batch and column experiments.The adsorption was found to be endothermic in nature and followsboth the Langmuir and Freundlich models. The batch test showed 90%removal for Cd and Ni, in about 60 and 80 min, respectively. Theremoval of Zn is 100% at low concentrations, whereas it is 60–65% athigher concentrations. The uptake decreases with increase in tem-perature indicating the process to be exothermic in nature. Lead andchromium are also adsorbed by the developed adsorbent up to 96–98%. The removal of these two metal ions up to 95–96% was achievedby column experiments at a flow rate of 0.5 mL/min.

Removal of mixed heavy metal ions in wastewater by zeolite 4Aand residual products from recycled coal fly ash was investigated byHui et al. [60]. The adsorption capacities of fly ash zeolite 4A werereported to be 30.80, 50.45, 8.96, 41.60, and 13.72 (mg/g) for Zn(II),Cu(II), Ni(II), Cr(III), and Co(II) ions respectively. The feasibility of flyash for the removal of Cu(II) and Pb(II) from wastewater was studiedby Lin et al. [61]. Results showed that the cation exchange capacityand specific surface area of fly ash increased with increasing thecarbon content. The adsorption of metal ions onto the surfaces of flyashes was found to be proportional to the carbon contents. This isbecause the amounts of adsorption or ion exchange sites on carbonsoot are higher than those on mineral surface. This is consistent withthe cation exchange capacity and specific surface area. Consequently,carbon residual in the fly ashes played an important role than mineralmatter in the removal of metals by the fly ashes. Rao et al. [62] alsoutilized fly ash along with other low-cost adsorbents for the removalof copper and lead.

Yadava et al. [63] investigated the removal of cadmium fromaqueous solution by fly ash. The removal of cadmium from aqueoussolutions by adsorption on fly ash increased with time and that

equilibrium is attained within 2 h. The adsorption of cadmium on flyash could be explained by the Langmuir equation. Prabhu et al. [64]showed that fly ash was a good adsorbent for the removal of zincfrom aqueous solutions; maximum removal was obtained in the pHrange of 3 to 4. They found that adsorption fitted the Freundlichisotherm. Studies by Gashi et al. [65] indicated that fly ash showedgood adsorptive properties of removal of lead, zinc, cadmium andcopper from effluents of battery industry and fertilizer industry.Removal efficiencies were greater than 70%. Adsorption studiescarried out to estimate heavy metal removal using fly ash on waste-water at Varnasi, India showed that removal was in the followingorder: PbNZnNCuNCrNCdNCoNNiNMn [66].

Coal fly ash (CFA) was modified to zeolite X and used as aneffective sorbent for removing of Cu(II), Cd(II), and Pb(II) from theiraqueous solution [67]. It required a longer time to reach equilibriumfor higher initial metal concentration and lower sorbent dose but allreached equilibrium within 120 min. External mass transfer stepseemed to take part as a rate limiting step for the sorption of Pb(II) atlow initial concentration and high adsorbent dose, on the contrary,the process was controlled more significantly by intraparticle dif-fusion step at high initial concentration and low sorbent dose. How-ever, Cu(II) was found to be generally controlled by intraparticlediffusion step within all concentration ranges studied. The adsorptionof Cd(II), on the other hand was controlled both by external masstransfer and intraparticle diffusion steps at all ranges of initial con-centration. The order of maximum adsorption capacity in a unit ofmol/kg was: Pb(II)NCu(II)NCd (II). The adsorption energy falls in therange of physico-sorption. Equilibrium sorption capacity and removalpercentage were governed by both initial concentration and sorbentdose. A general mathematical model was developed for describing theadsorption under the variations in initial metal concentration andzeolite doses. Solid state conversion of fly ash to an amorphous alu-minosilicate adsorbent (geopolymer) has been investigated for theremoval of Cu(II) from aqueous solution [68]. It has been found thathigher reaction temperature and sodium:fly ash ratio will make theadsorbent achieve higher removal efficiency. The adsorption capacityof the synthesized adsorbent (92 mg/g) shows a much higher valuethan fly ash (0.1 mg/g) and natural zeolite (3.5 mg/g).

A process for the treatment of industrial wastewater containingheavy metals, using fly ash adsorption and cement fixation of themetal-laden adsorbent, was investigated by Huang research group[69,70]. Results showed that the fly ash could be an effective metaladsorbent, at least for Zn(II) and Cd(II) in dilute industrialwastewaters. Fly ash adsorption capacities for Zn(II) and Cd(II)were 0.27 and 0.05 mg/g, respectively. A 10% metal laden fly ash wastested for leaching and it exhibited metal concentrations lower thanthe drinking water standards. They [71] further examined theadsorption characteristics of Zn(II) onto fly ash. In general, theamount of Zn adsorbed increased as the solid concentration and pHincreased, and sharply reached a 99% removal at a specific pH value,then it remained constant over a wide pH region. The fly ashadsorption capacities of Zn ranged from 1.04 × 10− 6 to1.15×10−5 mol/g in the pH range of 6.0–7.5. Experimental resultsindicated that the adsorption was favorable at lower ionic strength,higher pH, and higher temperature and that the adsorption is aphysical process enhanced by the electrostatic effect. Fly ash was alsofound to be effective for the removal of arsenic from aqueous solution.Fly ash obtained from coal power stations was examined for As (V)removal from water [72]. Kinetic and equilibrium experiments wereperformed to evaluate the As(V) removal efficiency by lignite-basedfly ash. Removal at pH 4 was significantly higher than that at pH 7 or10. Maple wood ash without any chemical treatment was also utilizedto remediate As(III) and As(V) from contaminated aqueous streams inlow concentrations [73]. Static tests removed ≤80% arsenic while thearsenic concentration was reduced from 500 to b5 ppb in dynamiccolumn experiments.


The As(V) and As(III) removal efficiency of a char-carbon (CC),derived fromfly ash is comparedwith those of a commercially activatedcarbon [74]. The results indicate that CC and AC adsorbents removealmost equal amounts of As(V) at optimum conditions; however, ona percent basis CC removesmore As(III) than does AC. The investigationrevealed that the adsorption of As(V) onto CC is influenced by pH, initialmetal concentration and temperature. Zeta potential measurementswere obtained to explain themetal removal behavior of the adsorbentsused in this investigation.

A special iron-abundant fly ash was used to develop a novel adsor-bent for arsenic (V) removal fromwastewater through simple chemicalprocesses [75]. The inherent iron in the fly ash was rearranged andloaded on the surface of the fly ash by dissolution and precipitationprocesses. The results showed that porous amorphous FeOOH wasloaded on the surface of the adsorbent (HIOFAA) successfully. The ad-sorption capacity for arsenic removal was found to be 19.46 mg/g.

A cancrinite-type zeolite was synthesized from Class C fly ash bymolten-salt method and the product obtained (ZFA) was used as theadsorbent for the arsenate removal from water [76]. The adsorptionequilibriums of arsenate are investigated on various adsorbents. ZFAshowed a higher adsorption capacity (5.1 mg/g) than activated carbon(4.0 mg/g), silica gel (0.46 mg/g), zeolite NaY (1.4 mg/g), and zeolite5A (4.1 mg/g). The relatively higher adsorption capacity of ZFA thanzeolite NaY and 5A was attributed to the low Si/Al ratio and themesoporous secondary pore structure of ZFA. However, it was foundthat the adsorption capacity of zeolites was generally lower thanactivated alumina (16.6 mg/g), which is ascribed to the small pores inzeolite frameworks. The adsorption capacity of ZFA was significantlyimproved after alumina loading via a wet-impregnation method. Themodified ZFA (ZFAAl50) with the optimum alumina loading showedan adsorption capacity of 34.5 mg/g, which was 2.1 times higher thanactivated alumina. The Toxicity Characteristic Leaching Procedure(TCLP) leachability tests indicated that the spent ZFA and alumina-modified ZFA complied with the EPA regulations for safe disposal.

The removal of arsenic from drinking water by filtration throughmodified fly-ash bed is also reported [77]. The preparation and char-acteristics of the bed material and the effects of different parameterslike pH and the presence of other constituents are described. Variousarsenic compounds in synthetic mixtures as well as drinking watersamples containing arsenic have been investigated. The effectivenessof the modified fly-ash bed for the control of arsenic has been dem-onstrated by taking different quantities of arsenic(III) and arsenic(V)salts. The results of filtering through a fly-ash bed were found tobe satisfactory which makes it a low cost treatment method forthe removal of arsenic, improving the quality of drinking water. Theutilization of the bed material after use is also discussed.

Surface-modified zeolite materials have been developed fromcommercial zeolites and fly ash-based zeolites by treating them withsurface modifiers like hexadecyltrimethylammonium bromide andtetramethylammonium bromide [78]. The adsorbent has been eval-uated for the removal of arsenic and chromate anions. High selectivity,faster kinetics and high adsorption capacity ensure cost effectivenessof these materials compared to other conventional materials fordearsenification.

It has been reported that coal fly ash is a good adsorbent for bothradionuclides of 137Cs and 90Sr [79]. Radiocesium adsorption is max-imal around the neutral region whereas radiostrontium adsorptionincreases with pH, especially above pH 8. The retention of Cesiumsharply drops with ionic strength, whereas the adsorption of stron-tium increases sharply and steadily at low and moderate concentra-tions of the inert electrolyte, respectively. The suggested mechanismof radionuclide retention by fly ash is specific adsorption of Cs(I) andirreversible ion-exchange uptake of Sr(II).

In addition, fly ash has been converted to zeolites by hydrothermaltreatment and used as an adsorbent for the removal of metal ions,radiocesium [80], lead and cadmium [81]. The maximum uptake

capacity was found to be 3.34 mmol Cs/g, which was 2–3 timeshigher than those of synthetic zeolite P and natural mordenites.The adsorption capacity for lead and cadmium was found to be70.58 mg lead/g-zeolite and 95.6 mg cadmium/g-zeolite, respectively,when the initial concentration for both the ions was 100 mg/L. Recently,Wang and Wu [82] had written a nice review on the environmentalbenign utilization of fly ash as low-cost adsorbents for the removal ofvarious pollutants from wastewaters. It is recognized that fly ash is apromising adsorbent for removal of various pollutants. Chemical treat-ment of fly ash will make conversion of fly ash into a more efficientadsorbent for water cleaning. Investigations also revealed that theunburned carbon component in fly ash plays an important role in ad-sorption capacity. The technical feasibility of utilization of fly ash as alow-cost adsorbent for various adsorption processes for removal ofpollutants in water systems has been reviewed.

3.2. Adsorption of heavy metals on blast furnace slag, sludge and dust

The sludge is a dried waste from the electroplating industry, whichis produced by precipitation of metal ions in wastewater with calciumhydroxide. It contains insoluble metal hydroxides and other salts.Another low-cost adsorbent showing capability to adsorb heavymetals is blast furnace slag, an industrial by-product generated in thesteel plants. Steel plants generate a large volume of granular blastfurnace slag which is also being used as filler or in the production ofslag cement. Blast furnace flue dust is a waste material from steelindustries and may be used to remove heavy metal ions from aqueoussolutions. The adsorption of Cu(II), Zn(II), and Ni(III) using blastfurnace slag was studied by Dimitrova [83]. It was found that theadsorption of metal ions takes place in the form of hydro-oxo com-plexes and that the high adsorption capacity is related to the for-mation of soluble compounds on the internal surface of the adsorbent.Srivastava et al. [84] investigated the activated blast furnace slag forthe removal of lead and chromium. The maximum uptake of metalswas found to be 40 and 7.5 mg/g for lead and chromium, respectively.The adsorption on activated slag followed both Feundlich and Lang-miur isotherms. It was also reported that the performance of activatedslag is comparable to that of commercially available activated carbon.

The Lopez research group investigated the potential of blastfurnace sludge for the removal of heavymetals from aqueous solution.The effectiveness of blast furnace sludge to purify Cu(II) containingsolutions was studied by determining the adsorption of Cu(II) ionsfrom aqueous solution on flue dust [85] and corresponds to a typicalendothermic physical adsorption process. The results suggest thatblast furnace sludge is a good adsorbent for the removal of Cu(II) fromaqueous solutions. They [86,87] also studied the adsorption of Pb(II),Zn(II), Cd(II), Cu(II) and Cr(III) on the sludge. Blast furnace sludgewasfound to be an effective adsorbent for Pb, Zn, Cd, Cu and Cr-ionswithinthe range of ion concentrations employed. High temperature favorsthe adsorption of lead ions from aqueous solutions on samples withlower carbon content. However, the adsorption capacity of sampleswith higher carbon contents decreased with increased temperature.The increase of adsorption capacitywith increasing Fe2O3/C ratio in thesludge indicates that under these experimental conditions, the processof lead ion adsorption occurs more strongly on the hematite phasethan on the carbon phase. This research group [88] also investigatedthe potential of blast furnace sludge for the removal of Pb(II) fromindustrial effluents. The blast furnace sludge was found to have a highadsorption capability, up to 80 mg Pb per g dried sludge at saturation.The order ofmagnitude of the adsorption enthalpy, calculated from theapparent equilibrium constant, indicates that the adsorption of Pb(II)on the sludge is of a physical nature. XPS and EDAX analysis suggestthat the physical adsorption process partially occurs through ionicexchange between Pb(II) and other ions such as Ca(II).

Blast furnace flue dust may also be used for the removal of divalentCu(II) and Pb (II) ions from aqueous solutions [89]. The carbon,


metallic iron and aluminosilicate phase present in the flue dustsample are responsible for the removal of metal ions. The adsorptionkinetic was rapid over an initial time period followed by the slowerrate at final stage. The removal of Pb(II) by adsorption on granulatedblast-furnace slag was investigated as a function of pH, the metal ionconcentration, the particle size and the amount of sorbent [90]. Thepercentage of lead removal at equilibrium increases with increasingslag amount but the adsorption capacity decreases. The results ob-tained could be useful for the application of granulated slag for the Pb-ions removal from industrial waste water. The utilization of granularblast furnace slag packed columns to treat Pb(II)-containing solutionshas been investigated [91]. The results obtained indicated that the slagusage rate decreasedwith increasing flow velocity, particle size, initiallead concentration and decreasing bed height. It was found that Pb(II)was removed selectively in the presence of other heavy metal ions.High concentrations of sodium and especially calcium in the solutionsimpeded the uptake of Pb(II). Column pHwas an important parameterin Pb(II) removal under dynamic conditions and reflected the in-fluence of the investigated factors. During all runs Pb(II) breakthroughcoincided with an abrupt drop in effluent pH. The apparent mech-anisms of Pb(II) removal in the blast furnace slag packed columnare sorption (ion exchange and adsorption) on the slag surface andprecipitation.

A comparative study of the adsorbents prepared from severalindustrial wastes viz. carbon slurry and steel plant wastes viz. blastfurnace (B.F.) slag, dust, and sludge for the removal of Pb(II) has beencarried out [92]. The adsorption of Pb(II) on different adsorbents hasbeen found in the order: B.F. sludgeNB.F. dustNB.F. slagNcarbonaceousadsorbent. The least adsorption of Pb(II) on carbonaceous adsorbenteven having high porosity and consequently greater surface area ascompared to other three adsorbents, indicates that surface area andporosity are not important factors for Pb(II) removal from aqueoussolutions. Since all threewaste products from the steel industry showedhigher potential for the removal of lead from water, therefore, it issuggested that thesemetallurgical wastes can be fruitfully employed aslow-cost adsorbents for effluent treatment containing toxic metal ions.

The adsorbent derived from sewage sludge through chemicalpyrolysis has been used for the removal of Cd(II) and Ni(II) fromaqueous solution [93]. The capacity of adsorption calculated from theLangmuir isotherm was 16 and 9 mg/g for Cd(II) and Ni(II), respec-tively. The mechanism of adsorption seems to be ion exchange. As thesewage sludge is discarded as a waste from wastewater treatmentprocessing plant, the adsorbent derived from sewage sludge is ex-pected to be an economical product for metal ion remediation fromwater and wastewater.

Storm water and landfill leachate can both contain significantamounts of toxic metals such as Zn, Cu, Pb, Cr and Ni. Pine bark andblast furnace slag are both residual waste products that have shown alarge potential for metal removal from contaminated water [94]. Theresults showed that pine bark was generally more efficient than blastfurnace slag when the metal concentrations were relatively small,whereas blast furnace slag adsorbed most metals to a larger extent atincreasedmetal loads. In addition, adsorption to blast furnace slagwasfound to be faster than metal binding to pine bark.

In another study, the removal of chromium (VI) from aqueous andindustrial effluent was carried out using distillery sludge [95]. TheLangmuir adsorption capacity was found to be 5.7 mg/g. Desorptionstudies indicated the removal of 82% of hexavalent chromium. Theefficiency of adsorbent towards the removal of chromium was alsotested using chromium-plating wastewater.

Clarified sludge (a steel industry waste material) was also used forthe removal of Zn(II) from aqueous solution [96]. The adsorptioncapacity (qmax) calculated from Langmuir isotherm and the values ofGibbs free energy obtained showed that clarified sludge has thelargest capacity and affinity for the removal of Zn(II) compared toother adsorbents, such as, rice husk ash, activated alumina and neem

bark, respectively. Bhatnagar et al. [97] investigated the potential ofmetal sludge, a waste product of electroplating industry, for the re-moval of vanadium from water. The adsorption capacity of metalsludge for vanadium was found to be 24.8 mg/g at 25 °C. After ad-sorption studies, the metal-laden sludge adsorbent was immobilizedinto the cement for its ultimate disposal. Physical properties suchas initial and final setting time, and compressive strength of cementstabilized wastes were tested to see the effect of metal-laden sludge incement. The study clearly revealed that metal sludge can be fruitfullyemployed in treating industrial effluents containing vanadium andfurther safely disposed of by immobilizing it into cement. The pro-posed technology provides a two-fold advantage of wastewater treat-ment and solid waste management.

Waste sludge from laboratory units at four different steady-stateconditions (5–20 days sludge ages) was employed for studying heavymetal shock load by the jar test system [98]. Metal uptake (Hg, Cd, Ni)and oxygen depletion rates were simultaneously followed. The po-tential adsorption constants (kam) measured reveal a metal affinitysequence of HgNCdNNi, in partial contrast to the metal toxicity (MT)one, HgNNiNCd. This suggests different sludge adsorption sites. Hgis preferentially adsorbed on the cell, while Cd is adsorbed on extra-cellular polymer slimes and Ni on capsular polymers and the cellularwall. Therefore sludge age influences Cd and Ni toxicity, while noeffect is observed for Hg.

Blast furnace slag was converted into an effective and economicalscavenger and utilized for the remediation of aqueous chromium [99].Erdem et al. [99] applied ferrochromium slag for the removal of Cr(VI)from aqueous solution. The Cr(VI) concentration in water that con-tacted with the ferrochromium slag (W/FS=10) was 0.61 mg/L after50 batches. Ten grams per liter ferrochromium slag dosage and3.5 mL/L H2SO4 (5 M)were sufficient to reduce all Cr(VI) in the modelsolution containing 10 mg/L Cr(VI) during contact time of 60 min at25 °C.

For years, the microwave technique, offering the advantage ofuniform and rapid heating, has beenwidely applied for the treatment ofsome environmental materials. Moreover, the stabilization and immo-bilization of metal ions in soil and metal sludge through the microwavetreatment are also reported [100–102]. This demonstrates thatmicrowave treatment is quite efficient in inhibiting the leaching ofmetal ions from soil or sludge. Hence, the treated soil or sludge becomesa stable matrix for application. For reducing the problems of metalions in aqueous solution, sewage sludge has been used efficiently aslow-cost adsorbents [103]. Microwave stabilized heavy metal sludgewas used as an adsorbent to remove the copper ions from aqueoussolution [104]. Results show that the pHzpc of stabilized-sludge was at9.2–9.5. Moreover, the adsorption of copper ions onto the stabilized-sludge surface was mainly on account of the heterogeneous surface ofthe stabilized-sludge. In the dynamic study, the experimental data wasfitted to the intraparticle diffusionmodel, pseudo-first ordermodel andpseudo-second order model. From the Langmuir equation, the adsorp-tion capacity increased from18 to28 mg/g as the temperature increasedfrom 15 to 55 °C, since this adsorption process was an endothermicreaction.

3.3. Adsorption of heavy metals on red mud

Another abundant industrial by-product is a red mud. Waste redmud is a bauxite processing residue discarded in alumina production.Red mud is a waste material formed during the production of aluminawhen bauxite ore is subjected to caustic leaching. A typical Bayerprocess plant generates a 1–2 tonnes of red mud per ton of aluminaproduced [105]. Red mud is principally composed of fine particles ofsilica, aluminum, iron, calcium and titanium oxides and hydroxides,which are responsible for its high surface reactivity. Because of thesecharacteristics redmuds have been the subject ofmany investigations,including some on the removal of toxic heavymetals fromwastewater


and acid mine drainage. Red mud has been explored as an alternateadsorbent for arsenic [105,106]. An alkaline aqueousmedium (pH 9.5)favored As(III) removal, whereas the acidic pH range (1.1–3.2) waseffective for As(V) removal [106]. Heat and acid treatments on redmud increased its adsorptive capacity [107]. Arsenic adsorption onacid and heat treated redmud is also pH-dependent, with an optimumrange of 5.8–7.5 for As(III) and 1.8–3.5 for As(V) [95]. Adsorptionfollowed a first-order rate expression and fit the Langmuir isothermwell. The As(III) adsorption was exothermic, whereas As(V) adsorp-tion was endothermic [94,95]. As(V) removal by using liquid phase ofred mud (LPRM) was also reported [108]. Authors suggested that it isadvantageous to use a waste material of red mud liquid phase in thetreatment of arsenical wastewater, possibly in conjunction with redmud solids as an adsorbent.

Seawater-neutralized red muds (Bauxol) [109], Bauxsol activatedby acid treatment, and by combined acid and heat treatment, andBauxsol with added ferric sulfate or aluminum sulfate [110], activatedBauxsol (AB), and chemically modified and activated Bauxsol (AB)-coated sand [111] were also utilized for the removal of arsenic. Theacid, and heat treatment, increased the removal efficiency of arsenic[110,111]. Addition of ferric sulfate or aluminum sulfate suppressedthe removal of arsenic. The activated Bauxsol (AB) produced usingcombined acid and heat treatment removed approximately 100% ofthe arsenate (at pH 4.5) with or without the presence of competinganions (i.e., phosphate, bicarbonate, and sulfate).

Red mud was found to remove effectively nickel ions from diluteaqueous solutions [112]. Red mud, being an industrial solid by-product/waste from alumina production during bauxite processing,was found to act simultaneously as an alkalinity regulator, causingprecipitation of nickel as the insoluble hydroxide, as an adsorbent ofthe formed nickel hydroxide and as a flocculant of the resultant fineparticulate matter. Sedimentation was subsequently considered as apossible solid/liquid separation technique. Adsorption by activatedred mud (ARM) is investigated as a possible alternative to the con-ventional methods of Cr(VI) removal from aqueous synthetic solu-tions and industrial effluents [113]. Adsorption characteristics suggestthe heterogeneous nature of the adsorbent surface sites with respectto the energy of adsorption. The influence of the addition of anions onthe adsorption of Cr(VI) depends on the relative affinity of the anionsfor the surface and the relative concentrations of the anions.

Santona et al. [114] investigated the heavy metal adsorption ofnon-treated (RMnt) and acid-treated red muds (RMa), in order toevaluate how efficient they are in reducing metal solubility and bio-availability in polluted soils. Red mud samples were artificially pol-luted with solutions containing increasing concentrations of Pb, Cdand Zn. Cancrinite and hematite were the main phases of the redmuds, and also the components which adsorbed most heavy metals.The results showed that the RMnt adsorption capacity for the threeheavy metals was Zn≥PbNCd. Acid treatment with HCl decreased theadsorption capacity of red mud for heavy metals by 30%. To study theheavy metal–RM interaction mechanisms, all samples after artificialcontamination were treated with solutions with gradually increasingextraction capacity. H2O and Ca(NO3)2 treatments only extracted verylow concentrations of Pb, Cd and Zn, while EDTA treatment extractedthe most adsorbed heavy metals from the sorbent particles. In par-ticular the water-soluble and exchangeable metal fractions werehigher in the RMa than they were in the RMnt, while the concen-trations of Pb, Cd and Zn extracted with EDTA were lower.

The adsorption of Cu(II), Cd(II), and Pb(II), on red mud has beeninterpreted with respect to pH and metal concentration by means ofthe modified Langmuir single-site and double-site models [115].Although red mud is a heterogeneous adsorbent, the binding of thesole metal cation (M2+) onto one or two types of surface sites at pH6.0 and less than 50% surface coverage in the form of ~SOM+ mono-dentate surface complex, which results in the release of protons fromthe surface, effectively explains the observed metal adsorption. The

adsorption intensity and surface binding constant of Cu(II) werehighest among the studied metals, in accordance with the Irwing–Williams order of complex formation. They [116] have also used themixture of bauxite waste red mud and coal fly ash for the removal ofcopper(II), lead(II) and cadmium(II) from aqueous solution. Theseheavy-metal-loaded solid wastes may then be solidified by addingcement to a durable concrete mass assuring their safe disposal. Thedistribution ratios of metals between the solid adsorbent and aqueoussolution have been found as a function of adsorbent type, equilibriumaqueous concentration of metal and temperature. Guclu and Apak[117] investigated the adsorption of Cu(II), Cd(II), and Pb(II), frommetal–EDTA mixture solutions on a composite adsorbent having aheterogeneous surface, i.e., bauxite waste red mud, and modeled withthe aid of a modified surface complexation approach in respect to pHand complexant dependency of heavy metal adsorption. EDTA wasselected as the modeling ligand in view of its wide usage as ananthropogenic chelating agent and abundance in natural waters. Theadsorption experiments were conducted for metal salts (nitrates),metal–EDTA complexes alone, or in mixtures containing (metal–metal–EDTA). For all studied cases, the solid adsorbent phase con-centrations of the adsorbed metal and metal–EDTA complexes werefound by using the derived model equations with excellent compat-ibility of experimental and theoretically generated adsorption iso-therms. The model was useful for metal and metal–EDTA mixturesolutions either at their natural pH of equilibration with the sor-bent, or after pH elevation with NaOH titration up to a certain pH.Thus adsorption of every single species (M2+ or MY2−) or of possiblemixtures (M2+ or MY2−) at natural pH or after NaOH titration couldbe calculated by the use of simple quadratic model equations, oncethe initial concentrations of the corresponding species, i.e., [M2+]0 or[MY2−]0, were known. The findings of this study can be further de-veloped so as to serve environmental risk assessment concerning theexpansion of a heavy metal contaminant plume with groundwatermovement in soil consisting of hydrated-oxide type minerals. Lopezet al. [118] assessed the feasibility of using red mud (RM), a residuefrom bauxite refining, for waste-water treatment.Moistenedmixturesof RM and 8% (w/w) CaSO4 form aggregates which are stable inaqueous media. The adsorption capacities were reported to be 19.72,12.59, 10.95 and 10.57 mg/g for Cu(II), Zn(II), Ni(II) and Cd(II),respectively. Red mud has been converted into an inexpensive andefficient adsorbent and used for the removal of lead and chromiumfrom aqueous solutions [119]. The effect of presence of other metalions/surfactants on the removal of Pb(II) and Cr(VI) has also beenstudied. The maximum adsorption capacity for Pb(II) and Cr(VI) byred mud were reported as 64.79 mg/g (batch capacity), 88.20 mg/g(column capacity), and 35.66 mg/g (batch capacity), 75.00 mg/g(column capacity) respectively. Wang et al. [120] presents an excellentreview on the utilization of redmud inwater treatment for the removalof toxic heavy metal and metalloid ions from their aqueous solu-tion. They have also discussed the environmental compatibility of redmud. Red mud presents a promising application in water treatmentfor the removal of toxic heavy metal and metalloids from wastewaters.

3.4. Adsorption of heavy metals on lignin, a black liquor waste of paperindustry

Black liquor is a waste product originated from paper industry.Lignin, extracted from black liquor, could be used as adsorbents forthe removal of heavy metals from wastewater. Lignin is a naturalamorphous cross-linked resin that has an aromatic three-dimensionalpolymer structure containing a number of functional groups such asphenolic, hydroxyl, carboxyl, benzyl alcohol, methoxyl, and aldehydegroups [121], making it potentially useful as an adsorbent material forremoval of heavy metals from wastewater. The elemental analysisshows that the lignin had the following percentage composition (%)[122]: C, 60.8; H, 5.8; N, 1.3; S, 2.1; ash, 5.5; K, 0.24; Ca, 0.06; Na, 1.4;


Mg, 0.01; Fe, 0.007; and Al, 0.03. Suhas et al. [123] have reviewed theliterature on the utilization of lignin for the removal of harmfulsubstances from wastewater and focused on the utilization of ligninas an adsorbent, its conversion into chars and activated carbons.Srivastava et al. [124] obtained remarkably high uptake of Pb(II) andZn(II), up to 1587 and 73 mg/g for Pb(II) and Zn(II), respectively, byusing lignin extracted from black liquor. The uptake of lead is found tobe greater than the uptake of zinc, and the sorption capacity increasedwith increased pH. Several polyhydric phenols and other substitutedanalogs may be involved in the uptake of metal ions. Demirbas [125]reported a maximum adsorption capacity of 8.2–9.0 mg/g for Pb(II)and 6.7–7.5 mg/g for Cd(II) on lignin from beech and poplar woodmodified by alkaline glycerol delignification. Mohan et al. [126]extracted lignin from black liquor waste, and utilized for the removalof copper and cadmium from aqueous solutions. The maximum ligninadsorption capacities at 25 °C were 87.05 mg/g and 137.14 mg/g forCu(II) and Cd(II), respectively. They reported that adsorption occursthrough a particle diffusion mechanism at temperatures of 10 and25°C while at 40°C it occurs through a film diffusion mechanism.To assess the possibility of using lignin to remove Cr(III) from waters,the adsorption of Cr(III) on lignin isolated from black liquor, wasinvestigated [127]. The equilibrium data can be well fitted usingLangmuir two-surface model with a maximum adsorption capacity of17.97 mg/g. The authors suggested that Cr(III) adsorption on lignin wasmainly through the ion-exchangemechanism and formed inner-spherecomplexeswith lignin. Crist et al. [128] utilizedkraft lignin, a by-productof paper production, for the removal of toxic metals from industrialprocess water. They reported that uptake of divalent toxic metals isaccompanied by a release of protons or existing metals from the lignin.Equilibrium constants for displacement of protons decreased withpH, and equilibrium constants for metal-metal exchange showed thatbinding strengths followed the order: PbNCuNZnNCdNCaNSr.

In another study, Guo et al. [122] investigated the adsorption ofheavy metal ions Pb(II), Cu(II), Cd(II), Zn(II), and Ni(II) on ligninisolated from black liquor. Lignin has affinity with metal ions in thefollowing order: Pb(II)NCu(II)NCd(II)NZn(II)NNi(II). They reportedthat lignin surfaces contain two main types of acid sites attributed tocarboxylic- and phenolic-type surface groups and the phenolic siteshave a higher affinity for metal ions than the carboxylic sites. Metalion adsorption onto deprotonated carboxyl and phenolic sites was thedominant mechanism that could reasonably explain the observedadsorption behavior [122].

Lignin was also utilized for the removal of Cr(III) from waters[129]. The Cr(III) adsorption was strongly dependent on pH andadsorbent dosage, but independent of ionic strength and other metalions. The adsorption kinetic data can be described well with pseudo-second-order model and the equilibrium data can be well fitted usingLangmuir two-surface model with a maximum adsorption capacityof 17.97 mg/g. Cr(III) adsorption on lignin was mainly through theion-exchange mechanism and formed inner-sphere complexes withlignin. The independence of ionic strength suggests that Cr(III) pri-marily forms inner-sphere complexes with lignin. The low cost, thehigh adsorption capacity and the successful application in removingCr(III) from wastewater demonstrate that lignin has a great potentialto be an economical and efficient adsorbent for the removal of Cr(III)from industrial effluents. Carboxymethylated lignin isolated fromsugar cane bagasse has been investigated for the removal of Cd(II) andPb(II) fromwastewater [130]. A factorial design showed that themostimportant variables are temperature and ionic strength for the Pb(II)adsorption in single and binary systems respectively. For both metals,maximum binding capacity decreased with increased ionic strength.

From the above data and the review by Suhas et al. [123] there aresignificant differences in the metal adsorption capacities of differenttypes of lignin. Moreover, the relatedmechanisms of metal adsorptionby lignin are still subject to debate. Some studies have found that ion-exchange mechanisms may be responsible for the adsorption of metal

ions on lignin [131,132]. Several processes including ion exchange,surface adsorption, and complexation, have been suggested to ex-plain the mechanisms involved. Detailed studies are therefore re-quired to achieve a quantitative and mechanistic understanding ofthe adsorption of metal ions by lignin.

3.5. Adsorption of heavy metals on waste hydroxide, a fertilizer industrywaste

Iron (III)/Cr(III) hydroxide is one of the waste materials obtainedfrom fertilizer industries. It can be used for the removal of heavymetals from wastewater. Navasivayam and Ranganathan [133–135]investigated the potential of Fe(III)/Cr(III) hydroxide for the removalof Cr(VI), Cd(II), Pb(II), and Ni(II) ions from wastewater. The max-imum adsorption capacity of Cr(VI) was found to be 0.47 mg/g. Theintraparticle diffusion of Cd(II) through pores of adsorbent was shownto be main rate limiting step. They [135] also studied the effect ofligands, such as, ethylenediamine tetraacetic acid, citrate and acetate,on the adsorption of metal ions such as Pb(II), Ni(II) and Cd(II). Allthree ligands decreased the adsorption of Pb(II) in a pH range of 3.5–8.5. The presence of citrate and EDTA considerably decreased theadsorption of Ni(II), however, acetate showed a slight increase inadsorption capacity. The adsorption of cadmium decreased signifi-cantly in the presence of acetate and citrate.

Fe(III) hydroxide was also used for the removal of metal ionsincluding Cr(VI), Ni(II), Cu(II), Cd(II) and Zn(II) from electroplatingwaste water [136]. The adsorption of heavy metals on hydrous oxidesdepends upon the oxidation potential, pH and complex formingability. These factors also play an important role in the transport ofheavy metals in the aquatic environment [137]. Hydrous oxides of Fe(III) andMn(II) have been found to be very effective in controlling theconcentration of heavy metals in sea water [138]. The adsorption ofmetal ions on hydrous oxides of Fe(III) and Mn(II) has also beenstudied [139]. Huang et al. [140] investigated the potential of wasteiron oxide material for the removal of Cu(II) from aqueous solutions.The highest Cu(II) adsorption capacity of waste iron oxide adsorbentwas determined as 0.21 mmol/g for 0.8 mmol dm−3 initial Cu(II)concentration at pH 6.0 and 300 K. Chromium(VI) [Cr(VI)] is adsorbedas HCrO−

4 on iron(III) hydroxide at a pH below 8.5 [141]. The Cr(VI)adsorption is suppressed by the presence of other anions such as SO4

2−

and SCN−, and enhanced by the presence of metal ions such as Cd(II)and Pb(II). The suppression is due to the competitive adsorption ofother anions, depending upon the stability of their iron complexes.The enhancement is probably due to the increase in adsorption sitesas a result of co-precipitation of metal ion with iron(III) hydroxide.

The effects of chromate, cyanide, pyrophosphate and tripolypho-sphate on the adsorption of metals (Cd, Cu, Cr, Zn, Ni and Pb) onto ironoxyhydroxide have been investigated [142]. In absence of complexingagents, adsorption of the strongest adsorbing metals (Pb, Cu, Cr)slightly inhibited that of the remaining metals when all were presentat concentrations of 10−4 M and 10−3 M and total iron was availableas the adsorbent. The presence of up to 10−3 MCrO4 had no significanteffect on removal of the cationic metals from solution. The presence ofcyanide (CN) strongly inhibited the adsorption of Ni and Cu, but didnot affect other metals. These effects can be explained based onchanges in metal speciation when the CN was added. Cyanide wasfound to form strong soluble complexes with Ni. Also, reduction of Cu(II) to Cu(I) by CN, and subsequent formation of Cu+–CN complexes,was thermodynamically favorable. The remainingmetals in the systemdid not form strong complexes under the experimental conditions andwere therefore unaffected. At low concentration, pyro- and tripoly-phosphate enhanced adsorption of all the metals, but at high concen-tration they led to a qualitative change in themetal sorption behavior.Whereas in absence of the phosphate ligands, metal sorption isstrongly pH dependent, at high polyphosphate concentration, thisdependence almost disappears. Simultaneously, some of the Fe

Table 4Summary of adsorption of metals on other industrial by-products/wastes.


References

Zn2+ Blast furnace slag 103.3 [83]Lignin 73 [124]Acid washed-tea industry waste 12 [143]Tea industry waste 11 [144]Clarified sludge 15.53 [96]Sugar beet pulp 35.6 [184]Lignin 11.24 [122]Solid residue of olive mill products 5.40 [145]Red mud 12.59 [118]Blast furnace slag 17.65 [146]Powered waste sludge 168 [147]Coffee Husk 5.57 [173]

Ni2+ Red mud 160 [112]Tea industry waste 5 [144]Tea factory waste/waste tea 18.42 [259]Grape stalk wastes 1.06 [187]Lignin 5.98 [129]Exhausted coffee wastes 7.25 [198]Grape stalk wastes 38.31 [198]Sugar beet pulp 11.85 [179]Crop milling waste 20.15 [148]Olive stone waste 2.13 [149]Treated sewage sludge 9.09 [93]

Cu2+ Blast furnace slag 133.3 [83]Waste slurry 20.97 [172]Sour orange residue 21.7 [151]Olive mill residue 13.5 [150]Red mud 106.44 [153]Rud mud/blast-furnace sludge 10.57 [154]Slag 11.23 [155]Lignin 6.7–7.7 [125]Seafood processing waste sludge 15.73 [172]Tea industry waste 11.29 [192]Iron oxide 98.0 [157]Fe(III)/Cr(III) 39.0 [158]Acid washed-tea industry waste 27 [143]Tea industry waste 48 [175]Olive stone waste 2.03 [149]Sugar beet pulp 30.9 [184]Grape stalk wastes 1.01 [187]Cellulose pulp waste 4.98 [197]Lignin 22.87 [129]Areca waste (AW) 1.12 [177]Grape stalk wastes 42.92 [198]Exhausted coffee wastes 11.60 [198]Sugar beet pulp 21.16 [179]Crop milling waste 27.97 [148]Olive stone waste 2.02 [149]Coffeee Husk 7.50 [173]Dried sugar pulp 18.2 [194]

Cr6+ Waste slurry 640 [159]Iron(III) hydroxide 0.47 [160]Blast furnace slag 7.5 [161]Activated red mud 1.6 [162]Paper mill sludge 7.4 [163]Red mud 75 [119]Clarified sludge 26.31 [164]Tea industry waste 455 [165]Leather industry waste 133 [180]Distillery sludge 5.70 [167]Olive oil factory wastes 12.15 [181]Tea factory waste 54.65 [174]Sugar beet pulp 17.2 [166]Sugar cane Bagasse 13.4 [166]Coffee Husk 6.96 [173]

Cr3+ Lignin 17.97 [122]Hg2+ Waste slurry 560 [159]Cd2+ Waste slurry 15.73 [172]

Lignin 6.7–7.5 [125]Fly ash 207.3 [152]Red mud 66.67 [153]Rud mud/blast-furnace sludge 16.07 [154]Blast furnace slag 38 [156]Seafood processing waste sludge 20.97 [172]

Table 4 (continued)


References

Tea industry waste 8.64 [192]Fly ash 1.38 [168]Apple waste 10.8 [169]Phosphated apple waste 36.2 [169]Sugar beet pulp 38.0 [179]Orgabosolv lignin 1.10 [83]Acid-washed-tea industry waste 31 [143]Tea industry waste 11.29 [192]Olive stone waste 7.73 [149]Crop milling waste 39.99 [148]Cellulose pulp waste 5.82 [197]Lignin 25.40 [129]Carboxymethylated lignin 107.53 [130]Areca waste (AW) 1.12 [177]Sugar beet pulp 46.1 [183]Native sugar beet pulp 38.0 [179]Treated sewage sludge 16.7 [93]Coffeee Husk 6.85 [173]

Pb2+ Waste slurry 1030 [159]Lignin 1865 [124]Blast furnace slag 40.0 [84]Lignin 1587 [124]Lignin 8.2–9.0 [125]Acid washed-tea industry waste 79 [143]Tea industry waste 2 [144]Tea industry waste 65 [175]Olive stone waste 9.26 [149]Crop milling waste 49.97 [148]Lignin 89.51 [129]Carboxymethylated lignin 37.99 [130]Sugar beet pulp 43.5 [183]Sludge 39.3 [178]Waste biogas residual slurry 28.0 [186]

Fe3+ Tea industry waste 24 [144]As3+ Zr(IV)-loaded orange waste 130 [170]As5+ Leather industry waste 26 [180]

Zr(IV)-loaded orange waste 88.0 [170]V5+ Waste metal sludge 24.8 [97]


adsorbent dissolves as Fe–P complexes. The changes inmetal behaviorwith increasing ligand concentration reflect the totality of interactionsin the system, which includes complexation, adsorption of free metaland complexes, iron dissolution and changes in the surface chargeassociated with the various sorption reactions. The effects of metal,chromate and CN concentrations do not appear to be major impe-diments to the utilization of adsorption as a treatment process. Theeffects of P-containing ligands are more complicated: they may eitherincrease or decrease metal adsorption depending on the particularwater under study.

3.6. Adsorption of heavy metals on miscellaneous industrial adsorbents

The literature also showed that other industrial wastes, such as,waste slurry, coffee husks, Areca waste, tea factory waste, sugar beetpulp, waste pomace of olive oil factory waste, battery industry waste,waste biogas residual slurry, sea nodule residue, sour orange residue,and grape stalk wastes have been utilized as low-cost adsorbents forthe removal of toxic heavy metals from wastewater. Table 4 showedthe results of important metals reported on various industrial basedadsorbents [143–170].

The potential of sour orange residue for the removal of copperions from aquatic systems was investigated by Khormaei et al. [170].Effects of particle size on adsorption efficiencies were small, but rateof adsorption decreased with increasing the particles size. Additionalchemical treatment of the adsorbent by NaOH, increased the adsorp-tion capacity. The interactions between Cu(II) ions and functionalgroups on the surface of the orange residue are confirmed by FTIR

Table 5Adsorption capacities for various metal ions in multi-component systems on variousadsorbents.

Metal System Adsorbent Adsorptioncapacity (mg/g)

Reference

Cu Cu(Cu–Cd) Black liquor lignin 38.71 [126]Cu Cu(Cu–Zn) Black liquor lignin 43.81 [126]Cu Cu(Cd–Cu–Zn) Black liquor lignin 9.95 [126]Cd Cd(Cd–Cu) Black liquor lignin 25.71 [126]Cd Cd(Cd–Zn) Black liquor lignin 36.93 [126]Cd Cd(Cd–Cu–Zn) Black liquor lignin 16.12 [126]Cu Cu(Cu–Cd) Tea industry waste 6.65 [192]Cd Cd(Cu–Cd) Tea industry waste 2.59 [192]Cu Cu(Cu–HA) Fly ash 28.0 [193]Pb Pb(Pb–HA) Fly ash 37.0 [193]Cd Cd(Cu–Cd) Anaerobic sludge 8.94 [197]Cu Cd(Cu-Cd) Cellulose pulp waste 4.55 [197]Cd Cd(Cu–Cd) Cellulose pulp waste 1.82 [197]Pb Pb(Pb–Cd) Carboxymehtylated lignin 123.07 [130]Cd Cd(Pb–Cd) Carboxymehtylated lignin 9.33 [130]


analysis and the spectra showed that carboxyl and hydroxyl groupsare involved in Cu binding to the sour orange residue.

Waste slurry, generated by the combustion of liquid fuel in afertilizer production plant was pretreated with peroxide and thenair activated at 450 °C and utilized for the removal of Cu(II), Cr(VI),Hg(II), and Pb(II) from aqueous solution [171]. Lee and Davis [172]studied the adsorption of Cu(II) and Cd(II) using waste slurry fromseafood processing factories. The adsorption capacity of the wasteslurry was found to be 20.97 and 15.73 mg/g for Cu(II) and Cd(II),respectively. Coffee husks, a coffee processing residue, were used forthe removal of heavy metal ions from aqueous solutions [173]. Theuptake of hexavalent chromium Cr(VI) from aqueous solutions ontotea factory waste (TFW) has been assessed [174]. The maximumadsorption capacity was found to be 54.65 mg/g of Cr(VI) ions onTFW. Adsorption of copper and lead ions onto tea waste from aqueoussolutions was also studied [175] and highest metal uptake of 48 and65 mg/g were observed for Cu and Pb, respectively. Pb showed higheraffinity and adsorption rate compared to Cu under all the experi-mental conditions. Increase in the total adsorption capacity wasobserved when both Cu and Pb ions are present in the solution. Astudy is made on the use of a steel-making by-product (rolling millscale) as a material for removing Cu(II) ions from aqueous solutions[176]. The removal of Cu(II) ions from an aqueous solution involvestwo processes: (i) adsorptions of Cu(II) ions on the surface of millscale due to the presence of iron oxides and (ii) cementation of Cu(II)onto metallic iron present in the mill scale.

Areca waste (AW) has been investigated for the removal ofcadmium and copper from aqueous solution [177]. The highest valueof Langmuir maximum uptake was found for cadmium (1.12 mg/g)and copper (2.84 mg/g). Ion-exchange and surface adsorption mightbe involved in the adsorption process of cadmium and copper. De-sorption studies revealed that cadmium and copper can be easilyremoved from AW by altering the pH values of the solution usingHNO3. The adsorption of cadmium, Cu(II), lead and zinc from aqueoussolution by paper mill waste (PMW) and composted PMW wasinvestigated by Lister and Line [178]. Sugar beet pulp generated bysugar-refining factories has been shown to be an effective adsorbentfor the removal of heavy metals from aqueous solutions [179]. Thestructural components related to the metallic adsorption being deter-mined, batch adsorption studies were performed for several metalions, namely, Pb(II), Cu(II), Zn(II), Cd(II), and Ni(II). Ion exchangewith Cd(II) ions neutralizing the carboxyl groups of the polysaccha-ride was found to be the predominant mechanism, added with com-plexation for Pb(II), Cu(II), and Zn(II) metals.

The feasibility of using a solid waste from the leather industry asan adsorbent for removal of Cr(VI) and As(V) from aqueous mediawas studied [180]. The high amounts of Cr(VI) — 133 mg/g and As(V) — 26 mg/g adsorbed demonstrates the good potential for usingthis leather industry waste as a low-cost alternative to the tradi-tionally used adsorbent materials. The waste pomace of olive oilfactory (WPOOF) was tested for its ability to remove chromium(VI)from aqueous solution [181]. The adsorption potential of batteryindustry waste as an adsorbent has been investigated for the removalof cobalt from aqueous solutions [182] and showed that the preparedadsorbent adsorbs cobalt to a sufficient extent (35 mg/g). Further, themetal-laden adsorbent was immobilized into cement for ultimatedisposal and no significant leaching was observed from the stabilizedproducts. The adsorption of Cd(II) and Pb(II) on sugar beet pulp (SBP)has been reported [183] and overall uptake for the (SBP) is found tobe 46.1 mg/g for Cd(II) at a pH of 5.3 and 43.5 mg/g for Pb(II) at a pHof 5.0, which seems to be removed exclusively by ion exchange,physical adsorption and chelation. The abilities of native sugar beetpulp (SBP) to remove copper Cu(II) and zinc Zn(II) ions from aqueoussolutions were compared [184]. The maximum overall uptake for theSBP is 30.9 mg/g for copper at a pH of 5.5 and 35.6 mg/g for zinc at apH of 6.0. The presence of low ionic strength or low concentration of

Na and Cl ions does not have a significant effect on the adsorption ofthese metals by SBP.

Sea nodule residue (SNR), a solid waste generated during the pro-cessing of polymetallic sea nodules for copper, nickel, and cobalt re-covery, hasbeendemonstrated to be aneffective adsorbent for removingan inorganic pollutant such as zinc from wastewaters/industrial ef-fluents by adsorption [185]. Waste biogas residual slurry (BRS) wasinvestigated for the adsorption of Pb(II) from aqueous solution [186]and the adsorption capacity was found at 28 mg/g. The usefulness ofgrape stalk wastes generated in the wine production process has beeninvestigated for the removal of copper and nickel ions from aqueoussolutions [187]. Maximum uptake obtained was 1.59×10−4 mol ofcopper and 1.81×10−4 mol of nickel per gram of dry sorbent. Sorptionof copper and nickel on grape stalks released an equivalent amount ofalkaline and alkaline earth metals (K+, Mg2+, Ca2+) and protons, in-dicating that ionic exchange is predominantly responsible for metalion uptake. Fourier transform infrared (FTIR) spectrometry analysisindicated that lignin C―O bond might be involved in metal uptake. Inanother study, Vegliò et al. [188] utilized olive mill residues (OMR) ascopper adsorbing material. Regenerated residues by acid solutions gavea copper removal of about 40%, in the same experimental conditions ofthe first adsorption test: regeneration with EDTA at different concen-trations suggested that it presents a damage of adsorption active sites.On the other hand, the utilization of HCl and CaCl2 found to completelyregenerate the adsorbent material.

4. Competitive adsorption

Industrial effluents rarely contain a single component; hence,design of adsorption systems must be based on multi-componentsystems. Therefore, competitive adsorption is very important in waterand wastewater treatment because most metal ions to be adsorbedexist in solution with other adsorbable metal ions. Industrial effluentsalso contain apart from various pollutants, different salts at differentlevels of concentration. Table 5 showed the adsorption capacities formetal ions in multi-component systems on various adsorbents. More-over, the utilization offixed bed ismore preferable than a batch reactorin industrial applications, as it is able to treat wastewater with largequantity. Different reasons have been given regarding the adsorptionaffinity of industrial waste materials on heavy metals. The amount ofadsorbed ions depends on the equilibrium between adsorption com-petition from all the cations, ionic size, stability of bonds betweenmetal ions and adsorbent [189]. Wong et al. [190] investigated theeffect of competitive ions between Pb and Cu and shows that theaffinity of amodified rice husk (MRH) formetal ions is PbNCu inmixed


metal solutions. The stability and enthalpy of formation are greater forPb-MRH than for Cu-MRH.

Aksu and Gonen [191] reported the simultaneous adsorption ofphenol and chromium(VI) ions on the Mowital®B30H resin immo-bilized activated sludge from binary mixture and compared withsingle phenol or chromium(VI) situation in a packed bed column. Ithas been demonstrated that dried activated sludge offers interestingpossibilities as a metal ion and an organic adsorbent, showing rapidbinding. The data obtained in the single and dual systems indicatedthat the adsorption capacity of Mowital®B30H resin immobilizedactivated sludge for chromium(VI) is generally higher than that ofphenol. The uptake capacity of dried activated sludge for chromium(VI) and phenol increased with increased influent concentration ofeach component from bi-solute aqueous solutions. However, thepresence of a second component decreased both the capacities. Thismay be attributed due to the fact that both the components are com-peting for same adsorption sites on the activated sludge. The mono-and two-component sorption phenomena were expressed byYoon and Nelson model to predict the breakthrough curves for eachcomponent. The relation between Yoon and Nelson model rate con-stant of each component and the concentrations of each componentin the binary sorption system was determined using the methodof response surface analysis. Such relationships can be used to findrate constants in a mixture containing unstudied concentrations ofphenol and chromium (VI). The adsorption ability of Turkish teawaste (fibrous) obtained from various tea-processing factories wasinvestigated for the removal of Cu(II) and Cd(II) from single (non-competitive) and binary (competitive) aqueous systems [192]. Themaximum adsorption capacities of Cu(II) and Cd(II) per gram teawaste were calculated as 8.64±0.51 and 11.29±0.48 mg for singleand 6.65±0.31 and 2.59±0.28 mg for binary systems, respectively.

Heavymetals and humic acid (HA) are two important pollutants insurface andwaste water systems.Wang et al. [193] utilized fly ash as alow-cost adsorbent for the simultaneous removal of heavy metalsand humic acid. The investigations were conducted for individualpollutant adsorption along with co-adsorption of both pollutantsand found that, for a single pollutant system, fly ash can achieveadsorption of lead ion at 18 mg/g, copper ion at 7 mg/g and humic acidat 36 mg/g, respectively. For co-adsorption, complexation of heavymetals and humic acid plays an important role. The presence of humicacid in water will provide additional binding sites for heavy metals,thus promoting metal adsorption on fly ash. For Pb–HA and Cu–HAsystems, Pb(II) and Cu(II) adsorption can increase to 37 and 28 mg/g,respectively, at pH 5 and 30 °C. However, the heavymetal ions presentin the system will compete with the adsorption of humic acid on flyash, thus resulting in a decrease in humic acid adsorption. Simulta-neous adsorption of Gemazol Turquoise Blue-G reactive dye anionsand copper(II) cations to dried sugar beet pulp, from binary mixtureswas studied and comparedwith single dye andmetal ion situation in abatch system [194]. The presence of increasing concentrations ofcopper(II) ions increased the equilibrium uptake of dye anions whilethe increasing concentrations of dye diminished the copper(II) ionuptake for both pH values studied. This situation showed the syn-ergistic effect of copper(II) cations on dye adsorption and the antag-onistic effect of dye anions on copper(II) adsorption. Adsorptionisotherms were developed for single-dye, single copper(II) and dual-dye-copper(II) ion systems at these two pH values and expressed bythe mono-component Langmuir model and multi-component syner-gistic and antagonistic Langmuir models and model parameters wereestimated by the non-linear regression. The competitive adsorptiveremoval of Cd(II) and Zn(II) ions from binary systems using ricehusk ash (RHA), as an adsorbent was reported by Srivastava et al.[195]. The initial pH (pH0) significantly affects the capacity of RHA foradsorbing the metallic ions in the aqueous solution. The pH0≈6.0 isfound to be the optimum for the removal of Cd(II) and Zn(II) ions byRHA. The single ion equilibrium adsorption from the binary solution is

better represented by the non-competitive Redlich–Peterson (R–P)and the Freundlich models than by Langmuir model in the initialmetal concentration range of 10–100 mg/l. The adsorption of Zn(II)ion is more than that of Cd(II) ion, and this trend is in agreementwith the single-component adsorption data. The equilibrium metalremoval decreased with increased concentrations of other metal ionsand the combined effect of Cd(II) and Zn(II) ions on RHA is found tobe antagonistic. The extended-Freundlich model satisfactorily repre-sents the adsorption equilibrium data of Cd(II) and Zn(II) ions ontoRHA. Heavy metal adsorption onto solid wastes from olive oil pro-duction plants, olive pomace, has also been investigated [196]. Acid–base properties of the active sites of olive pomace were determinedby potentiometric titrations and represented by a continuous modelaccounting for two main kinds of active sites. Competition among pro-tons and heavy metals in solution was considered by performing ad-sorption tests at different equilibrium pH with single (Cu and Cd) andbinary metal systems (Cu–Cd).

The adsorption of copper and cadmium ions using waste materialssuch as compost, cellulose pulp waste and anaerobic sludge as adsor-bents is reported [197]. A competitive uptake was observed whenboth metals are present; copper being preferentially adsorbed by allmaterials with the exception of anaerobic sludge. Anaerobic sludgehas a greater preference for cadmium, even in the presence of copper,with a removal efficiency of 98%. Two industrial vegetable wastes,grape stalk, coming from a wine producer, and exhausted coffee,coming from a soluble coffee manufacturer, have been investigatedfor the removal of Cu(II) and Ni(II) from aqueous solutions in pre-sence and in absence of the strongly complexing agent EDTA [198].Effects of pH and metal–EDTA molar ratio, kinetics as a function ofsorbent concentration, and sorption equilibrium for both metals ontoboth sorbents were evaluated in batch experiments. EDTA was foundto dramatically reduce metal adsorption, reaching total uptakeinhibition for both metals onto both adsorbents at equimolar metal:ligand concentrations. Grape stalk showed the best performance forCu(II) and Ni(II) removal in presence and in absence of EDTA, despiteexhausted coffee appearing as less sensitive to the presence ofcomplexing agent. The performance of Cu(II) and Ni(II) sorption ontogrape stalk in a continuous flow process was evaluated. In solutionscontaining EDTA, an initial metal concentration in the outlet flowcorresponding to the complexed metal fraction was observed fromthe beginning of the process. A high metal recovery yield (N97%) wasachieved by feeding the metal-loaded column with 0.05 M HCl. Fromthe above studies, it has been found that very few studies have beenreported for the removal of heavy metals from real wastewaters.However, some investigations have been carried out to see the effect ofmetal adsorption in presence of other organic substances. Therefore,competitive metal adsorption either in presence of other metals ororganic impurities is very important and needs to be addressed indetail in the future.

5. Leachingofflyash, redmud,blast furnace slag, andotherwastes inwater system

Utilization of fly ash in water involves the potential leachingof some elements into water. This creates a problem of secondaryenvironmental pollution. It was found that the surface layer of fly ashparticles probably microns in thickness contains a significant amountof readily leachable material which is deposited during cooling aftercombustion. Therefore, the charge on the surface of fly ash particleand formation of the diffuse double layer plays a significant role inleaching. The elements present were divided into two groups on thebasis of their concentration dependence on particle size. Results ofthe analysis by particle size indicates that the elements Mn, Ba, V, Co,Cr, Ni, Ln, Ga, Nd, As, Sb, Sn, Br, Zn, Se, Pb, Hg and S are usually volatileto a significant extent in the combustion process. The volatility forthese elements is inversely proportional to the particle size. Elements,


such as, Mg, Na, K, Mo, Ce, Rb, Cs and Nb appear to have a smallerfraction volatilized during coal combustion [199]. Here the volatilityis directly proportional to particle size. The elements Si, As, Fe, Ca,Sr, La, Sm, Eu, Tb, Py, Yb, Y, Se, Zr, Ta, Na, Th, Ag and Zn are either notvolatilized or may show minor trends which are related to thegeochemistry of the mineral matter. The volatility of trace elementsincreased from a larger particle size to a smaller particle size whichestablishes an inverse relationship of volatility and particle size [200].

Studies have shown that only about 1–3% fly ashmaterial is solublein water with lignite fly ashes having a higher proportion of watersoluble constituents [201]. The leaching of major elements fromcoal fly ash has been extensively reviewed [202]. Analysis of waterextracts [203,204] showed that the principal cations in water extractsare calcium and sodium whereas anions are dominated by OH−, CO3

with aqueous extracts of the ash nearly saturated with Ca(OH)2. Thealkalinity and acidity controlled the extractability of elements like As,B, Be, Cd, Cr, Cu, F, Mo, Se, V and Zn. Aqueous extracts of an acidic flyash contained concentrations of Cd, Co, Cu, Mn, Ni, Zn, As, B, Be, Cd, F,Mo, Se and V [205,206]. Leachate waters can have markedly differentcompositions, depending on the surface of fly ash, flue gas processconditions design of combustion systems and whether lime or limestone injection processes were implemented for desulfurization. Totaldissolved solid concentrations may vary from hundreds to tens ofthousands of milligrams/liter. Even a small sample can show markeddifferences in leachate water chemistry, depending on reaction timeand water/solid ratio in batch equilibrations or with column lengthand flow rate in a dynamic leaching test. The mineral and glassphases that constitute fly ashmaterial are formed over a wide range oftemperatures in the furnace environment. All these phases are unstable.They dissolve and then precipitate as stable and less soluble secondaryphases. The primary phases even though highly soluble especially inwater are dissolved very slowly as they are trapped in the glass andcrystalline alumina silicates. Secondary hydrous alumino silicateproducts are shown to be very insoluble [207] and build up on rindson the surfaces of primary phases. The dissolution of primary phases isslowed down as the mass transport of ions and water between phasesbecomes diffusion controlled.

Many important aspects of the leachingbehavior offly ashhave beencovered by a number of researchers [199,208–210]. Experimentalresults indicate that leachability of heavy metals from the coal fly ash isrelatively low and leaching extent is dependent on the conditions ofwater system. Trace metal concentration in the leachate depends on flyash weight/solution, pH, concentration of the elements, temperature,pressure and time. In water, rapid leaching of most of the trace metals(except Cu) takes place from the surface of ash particles in the lower pHrange; all the trace elements lie within acceptable limits [210].

A complex mixture of organic compounds is also associated withfly ash particles. The organic compounds identified in fly ash extractsinclude known mutagens and carcinogens. There are leaching pro-cedures for the dissolution of the organic compounds of FA in 30%–40% hydrogen peroxide, concentrated nitric acid under microwaveconditions, or benzene. However, this chemical treatment also leachescomponents associated with both organic and inorganic compoundsin fly ash. For example, some unstable minerals from the chloride,sulfate, sulfide, oxide, and carbonate classes may be leached, destroyed,or altered during this procedure. Formations of artificial minerals andphases such as oxyhydroxides, calcium oxalate, nitrates, and others arealso possible. Hence, such minerals are not available or actual in the flyash residue for any future mineralogical study. On the other hand,portions of organics may remain in the residue, because of the less-soluble behavior or when the solvent cannot contact with organicsencapsulated in other inorganic matrixes (especially glass). There aretraces of polycyclic aromatic hydrocarbons present in the coal fly ash,typically up to 25 mg/kg. Due to the low solubility, the leachate fromcoal fly ash contains very low concentrations of polycyclic aromatichydrocarbons (PAHs).

Red mud may impose a risk for living organisms, due to its causticnature. Neutralization of red mud allowed its reuse to achieve betterperformance. Red mud can be used for the treatment of wastewater,after its neutralization. However, leaching of red mud will be a bigproblem and the environmental compatibility has to be evaluated forits ultimate disposal.

Some investigations have been carried out to understand theleaching behavior of redmud. Leachability of chromium and iron fromthe reacted wastes was determined by Singh and Singh [211] usingthe TCLP. This test showed a very low level of leachability of chro-mium as Cr(III) and iron from the reacted wastes. To minimize theirleachability further, Cr(VI)-reacted solid wastes were stabilized withPortland cement. Leachability tests of stabilized wastes by the TCLPindicated a considerable decrease in leachability of chromium andiron compared with that of reacted wastes alone. McConchie et al.[212] also conducted TCLP tests of Bauxsol TM and found that themetal leaching capacity was far below the standard limits. Brunoriet al. [213] reported a leaching test on unwashed and seawater-washed red muds (BauxsolTM). The BauxsolTM showed a generalhigh metal trapping capacity and the release of the trapped metalswas also generally low even at low pH values. The leaching tests atpH 5 showed that less than 6% of trapped metal was leached (As, Cdand Cu), while 10–30% was leached in the case of Mn and Zn. Ho et al.[214] investigated a long-term effect of leaching of red mud. Themajor salts released were sodium sulfate, a product of red mudalkalinity neutralization by gypsum, and excess gypsum released at itssolubility concentration. The leaching of Al, Fe, and Cd from the redmud and gypsum was negligible, while the retention of superphos-phate was over 99%. Fluoride from the waste gypsum was leachedrapidly and reduced to background concentration (less than 1 mg/l).The physical, chemical and genotoxic properties of ARM and its wastemud remaining after waste water treatment were also studied byOrešcanin et al. [215]. The sequential leaching of waste mud underdifferent conditions was investigated. Heavy metal content and otherparameters, as well as the genotoxicity of water extract of newcoagulant produced for industrial waste water treatment, were inves-tigated to confirm its non-toxicity before its commercial productionand usage. Kutle et al. [216] reported an assessment of environmentaleffects of leaching from red mud with waste base in a waste disposalsite. They reported that a possible threat to the environment from redmud under normal environmental conditions could be caused only byleaching of alkalis and V. Wang et al. [120] reported that leaching andeco-toxicological tests indicate that red mud does not present hightoxicity to the environment before or after reuse. Leaching tests alsoindicate that red mud presents a much lesser leachability and thus noserious secondary contamination will occur. All of the above inves-tigations suggest that red mud exhibits low leaching of its compo-nents. After reuse for some wastewater treatment, red mud can beemployed for brick production or disposed in nonhazardous landfillswithout causing significant environmental toxicity.

The BF slag is basically inorganic in nature. It contains mainlyinorganic constituents such as silica (30–35%), calcium oxide (28–35%), magnesium oxide (1–6%), and Al2O3/Fe2O3 18–258%. Two typesof blast furnace slag such as air-cooled slag and granulated slag arebeing generated from the steel plants. The crystalline phases of slagwere identified as gehlenite, diopside pyroxene and barium alumi-num silicate. It is necessary to assess the environmental impact of slagand sludge, especially for the hazardous elements (heavy metals)being present and the possible risk of their leacheability into soil andgroundwater, according to leaching test. The recovery of silica gelfrom blast furnace slag has been attempted by leaching with H2SO4,separation of gypsum, precipitation of silica gel at a pH of 3.2, followedby the washing of the raw precipitate [217].

Matei et al. [218] tested blast oxygen furnace slag (BOF) andelectric arc furnace (EAF) slags to see their leaching of heavy metals(Ni, Pb, Zn, Cd, Cr, Cu, Ca and Fe) from aqueous solution. The obtained

Table 6Cost of low-cost adsorbents and commercial activated carbons as reported in literature.

Adsorbent Price (US$kg−1) Reference

Blast furnace waste 0.038 [84]Carbonaceous adsorbent (Blast furnace waste) 0.052 [84]Carbonaceous adsorbent (waste metal sludge) 0.1–0.2 [97]Waste metal sludge 0.004–0.005 [97]Bagasse fly ash 0.009 [224]Carbonaceous adsorbent (fertilizer industry waste) 0.1 [225]Blast furnace slag 0.04 [226]Commercial activated carbon 20 [226]Commercial granular activated carbon 3.30 [227]


results showed the presence of calcium, silicium and iron oxides, inthe large amounts, as a result of used fluxes. Applying this laboratorytest on steel wastes (basic oxygen furnace slags and electric arcfurnace slags) it is observed that not one of these heavy metal ions ispresented in high quantities in the leachate over the permissiblevalues. There are high quantities of some elements from leachate,such as iron and calcium, due to their presence in initial compositionof the slags; the concentration values of these ions do not exceed themaximum admitted values for potable water. The leaching of copperions from the industrial sludge was investigated using sulfuric acidand reported that the concentration exceeds 15 mg/L, which is theleaching standard of the toxicity characteristic leaching procedure(TCLP) for hazardous waste in Taiwan [219]. A study on the metalleaching from blast furnace steel slag has been studied and showedthat although blast furnace slags do leach heavy metals, this occurs toa very low degree [220]. The leaching from the slag was found to be inthe same range as the corresponding leaching from natural gravel.Similarly, the Landcare Research Institute from New Zealand andnatural Steel Slag Association in the United States performed researchon metal leaching from eight steel slag materials and found thatleaching from steel slag is not detrimental to the environment orhuman health [221–223].

Lignin has an aromatic, three-dimensional polymer structure withapparent infinite molecular weight and contains several functionalgroups such as methoxyl, hydroxylaliphatic and phenolic, carboxyl,etc. The lignin obtained from many of these precursors containssignificant amounts, up to 15%, of ash. This appears to be mainly silica,with lower amounts of sodium and calcium, and trace quantities ofother metals but can be readily removed by sulfuric acid washing.Thus, there is a lesser chance of metal leaching from the aqueouslignin solution. However, leaching of lignin from their aqueoussolution should be performed and evaluated before it can be used asan adsorbent.

Similarly, other industrial wastes, such as, waste slurry, coffeehusks, Areca waste, tea factory waste, sugar beet pulp, waste pomaceof olive oil factory waste, battery industry waste, waste biogasresidual slurry, sea nodule residue, and grape stalk wastes should betested for their metal leaching behavior in aqueous solution beforebeing used as an adsorbent for the removal of heavymetals. Therefore,it is recommended that the following measures/steps should betaken before any industrial waste material is used as adsorbent forthe treatment of wastewater: (1) leaching test of the industrial wastesas adsorbent should be preformed for the investigated water system;(2) forced extraction of mobile/labile substances present in the in-dustrial waste; (3) destruction or ultimate disposal of persistentheavy metals from industrial waste (adsorbent)-loaded heavy metals.

6. Efficiency and cost comparison

The two most important factors for the adsorption process to beeconomically feasible are efficiency (adsorption capacity) and cost ofthe adsorbent used. However, adsorption capacity is relative ascompared with some standard adsorbent, as it depends on variousfactors, including initial concentration of the adsorbate under inves-tigation. The cost of the precursor or the final material (adsorbents)depends on various factors which include its availability, whether itis natural, industrial/agricultural/domestic wastes or by-products orsynthesized products, the processing required, the treatment condi-tions and both recycle and lifetime issues. For example, fly ash isavailable free of cost at the power plant and hence only transportationcost, laying and rolling cost are there in case of fly ash [20]. Wastebaggase fly ash is available for US $ 0.002 kg−1, and considering thecost of transport, chemicals, electrical energy, etc., used in the process,the finished product would cost approximately US $0.009 kg−1[224].However, the cost of adsorbent is rarely reported in the literature.Table 6 presents cost estimates of some industrial based adsorbents

along with commercial activated carbons [225–227]. According tothese coal fly ash, bagasse fly ash, blast furnace waste, waste metalsludge, carbonaceous adsorbent (fertilizer and waste metal sludgeindustry waste) are the materials costing ≤0.1 US$ per kg makingthem useful materials in terms of cost as compared to commercialactivated carbons which normally cost more than 3.0 US $per kg.Mainly a combination of these two factors, i.e., adsorption capacity andcost of the adsorbent can be helpful in predicting the actual applicabilityof these industrially available waste materials.

It is difficult to estimate a reliable treatment cost for metal-contaminated wastewater because of the involvement of many costcomponents, such as, pumping equipment and treatment facility. Inaddition, changes in the quality and quantity of the variouswastewatersdue to the fluctuating market demand also contribute to the variationsof its treatment cost. Therefore, full information on the treatment costof wastewater involving adsorption is rarely reported in the literature.Therefore, total cost–benefit analysis of adsorption process for theremoval of heavy metals should be studied in detail and recommendedfor future. Basically, the treatment cost of contaminated wastewatervaries, depending on its strength and quantity, the process employed,the amount and composition of impurities, as well as the extent ofpurification. The overall treatment cost includes the construction costsas well as the operational and maintenance costs (O&M). The con-struction costs normally depend on the effluent quality required andthe capacity of the installation, while the O&M costs cover manpower,energy, chemicals and maintenance. The manpower cost varies fromone country to another. To obtain an accurate assessment of theoperational cost for the treatment of wastewater, a pilot-scale studyneeds to be carried out. Most of the data presented in this review arederived from research conducted on a laboratory scale and therefore,further experiments on a pilot scale are needed to quantify the overalltreatment cost associated with the proposed treatment. A direct com-parison of the overall treatment cost of each technique presented abovemay be difficult due to their different operating conditions. In addition,the overall treatment cost for various wastewaters varies depending onthe process employed and the local conditions. This may be attributedto the fact that most of the industries are located either in the com-mercial area of a town or in industrial estates, where wastewateris discharged into sewers after neutralization with acids/alkalis. Widevariations in the flow and the characteristics of the effluent wastewateralsopresentdifficulties in estimating the treatment cost accurately. Suchinconsistency in data presentation makes a cost comparison amongthe available treatment technologies for wastewater laden with heavymetals difficult to materialize.

7. Combination of methodologies/techniques

Some researchers have used the adsorption methods in conjunctionwith other process. Adsorption/catalytic oxidation, adsorption coupledwith redox process and adsorption/catalytic reduction process can besuggested asprocesses for the removal of heavymetals fromwastewater.Adsorption/catalytic oxidationprocess canbe categorized into twoparts:in situ and ex situ adsorption with catalytic oxidation, according to


whether adsorption process and oxidation process occur in thesame reactor. The photocatalytic degradation of heavy metals by adsor-bents supported with TiO2 belongs to the former. Adsorption/catalyticreduction process has been observed to be efficiently utilized in thepurification of wastewater as well as water. Recently, various types ofadsorbents with the catalytic reduction function, for example, modifiedhydrotalcite-type (HT) adsorbent, were developed and utilized in theremoval of heavy metals from wastewater. Several redox compositeadsorbents have been developed and applied in water purification, bythe combination of oxidation and reduction effect in the adsorptionprocess. Arsenic is primarily present in inorganic forms and exists in twopredominant species, namely, arsenate (As(V)) and arsenite (As(III)) inwastewater. As(III) is found to be much more toxic [228], more soluble,and more difficult to dispose than that of As(V). To achieve higherremoval of arsenic, a pre-treatment for As(III) oxidation is, involvedfollowed by adsorption of the As(V) formed ontometal oxy-hydroxides.The pre-treatment method not only increased the running cost but alsothe complexity of the operation. Traditional oxidants such as chlorine,ozone, and hydrogen peroxide are not suitable for a specific oxidationof As(III), due to some side reactionswith natural organicmatter presentin wastewater. Therefore, the development of adsorbents from wastematerials which could simultaneously oxidize arsenite and adsorb theformed arsenate is becoming the focus of research in recent years[229,230]. More research should be conducted for the removal of heavymetals fromwastewater by using adsorption in combination with othermethods.

Table 7Single-component adsorption models reported in the literature.

Isotherm types Equations Nomenclature

Langmuirqe =

Q0bCe

1 + bCe

qe is equilibrium metalqmax and b are Langmuand bonding energy of a

Freundlich qe=KFCe1/n KF is a adsorption equil

constant indicative of aSips Isotherm

qe =KsC

βSe

1 + asCβSe

Where, βs is the Sips co

BET model(multilayer sorption)

qe =qsCBETCe

Cs−Ceð Þ 1 + CBET−1ð Þ Ce = Csð Þ½ �Where CBET, Cs, qs and qsaturation concentratioequilibrium adsorption

Redlich–Petersonqe =

kRCe

1 + aRCβe

KR, aR, and β are the ReFor β=1 the model co

Radke–Prausnitzqe =

aRPrRCβRe

aRP + rRCβR−1e

Its model exponent is re

(Frenkel–Halsey–Hill)FHH ln

Ce

Cs

� �= − α

RTqsqde

� �r Where d, α and r are thand inverse power of d

Tempkinqe =

RTbT

lnATCeWhere AT (L/g) and bTqe versus lnCe

TothQe = Qmax

bTCe

1 + bTCeð Þ1=ηTh iηT

Where, bT and nT are twLangmuir-type isotherm

Flory–Huggins θC0

= KFH 1−θð ÞnFH Where, θ is the degreeequilibrium constant an

Koble–Corrigan qe =ACn

e

1 + BCne

The isotherm constantserror optimization

(MacMillan–Teller) METqe = qs

kln Cs =Ceð Þ� �1 =3 Where k is an isotherm

Dubinin–Radushkevich qe=(qs)exp(−kdε2) where, kd is denoted as

correlated as: ε = RT ln

(8.314 J/mol K), absolurespectively

Khanqe =

qsbKCe

1 + bKCeð ÞαK

bK and aK are devoted t

8. Adsorption isotherms and mechanism

Assessment of a solid–liquid adsorption system is generally depen-dent on two types of investigations: (i) equilibrium batch adsorption,and (ii) dynamic continuous-flow adsorption studies. Equilibriumisotherm models are generally divided into empirical equations andmechanistic models. The mechanistic models are based on the mech-anism of metal ion adsorption and able to explain and predict theobserved experimental behavior. Several empirical models are re-ported for single solute adsorption systems [231–244] and shown inTable 7. The Langmuir and Freundlichmodels are found to be themostwidely used isotherm models reported in the literature. TheBET model assumes that the Langmuir isotherm applies to eachlayer and describes multi-layer adsorption at the adsorbent surface.These models can provide information on the adsorption capacity ofmetals from their aqueous solution. The models, such as, Langmuirand Freundlich also do not incorporate the effects of any externalvariable environmental factors. However, they are capable ofdescribing many adsorption isotherms in most of the cases. Themechanistic conclusions from the good fit of the observed experi-mental data with the empirical models should be avoided. However,adsorption isotherms may exhibit an irregular pattern due to thecomplex nature of both adsorbents and its variedmultiple active sites,as well as the complex solution chemistry of some metalliccompounds. Foo and Hameed [245] recently, presents a state of artreview of adsorption isotherms modeling, its fundamental

References

sorption capacity; Ce is equilibrium solute concentration in solution;ir constants related to maximum sorption capacity (monolayer capacity)dsorption (or “affinity”), respectively

[231]

ibrium constant, representative of the sorption capacity; and n is adsorption intensity

[232]

nstant [233]

e are the BET adsorption isotherm (L/mg), adsorbate monolayern (mg/L), theoretical isotherm saturation capacity (mg/g) andcapacity (mg/g), respectively

[234]

dlich–Peterson parameters. The exponent β lies between 0 and 1.nverts to the Langmuir form

[235]

presented by aRP, where aR and rR are referred to the model constants [236]

e sign of the interlayer spacing (m), isotherm constant (Jmr/mole)istance from the surface (about 3), respectively

[237]

are the Tempkin constants which can be determined from a plot of [238]

o constants. Obviously, if nT=1, Toth isotherm reduces to theequation

[239]

of surface coverage, where KFH and nFH are the indication of itsd model exponent

[240]

, A, B and n are evaluated from the linear plot using a trial and [241]

constant [242]

the isotherm constant. Meanwhile, the parameter ε can be

1 +1Ce

� �,where R, T and Ce represent the gas constant

te temperature (K) and adsorbate equilibrium concentration (mg/L),

[243]

o the model constant and model exponent. [244]


characteristics and mathematical derivations. The key advance of theerror functions, its utilization principles together with the comparisonsof linearized and non-linearized isotherm models has been highlightedand discussed. They also concluded that, the expanding of the nonlinearisotherms represents a potentially viable and powerful tool, leading tothe superior improvement in the area of adsorption science. They alsohighlighted that the next real challenge in the adsorption field is theidentification and clarification of both isotherm models in various ad-sorption systems. Further explorations on developing in this areaare recommended. Real wastewaters commonly contain a mixture ofmetal ions. Therefore, multi-component metal adsorption models haveto be developed to describe the binary or ternary, and multi-componentadsorption systems. To describe two- or multi-metal ion adsorptionsystems, variousmodels, suchas, extendedLangmuir,modifiedLangmuir,non-modified Redlich–Peterson, Sheindorf–Rebuhn–Sheintuch (SRS)and extended Freundlich models have been developed and reported inthe literature [246–250]. Table 8 showed some of multi-componentadsorption models reported in the literature. However, these empiricalmodels hardly reflect any adsorption mechanism. These empiricalmodels hardly reflect any mechanisms of metal uptake and do not havea meaningful physical interpretation for the adsorption process.

The mechanisms by which the metal ions are adsorbed ontovarious industrialwastes are very complicated and appear attributableto electrostatic attraction, ion-exchange, adsorption–precipitation,hydrogen bonding, and chemical interaction between the metal ionsand the surface functional groups of the various industrial adsor-bents. It is commonly believed that the chemical interaction betweenthe metal ions and the surface functional groups of various industrialwastes is the major adsorption mechanism. It is known that heavymetal adsorption is effective in an alkaline region. However, whenthe medium pH is controlled by the adsorbent (as in the cases ofvarious alkali and alkali earth silicates), the mechanism of heavymetals removal is difficult to determine. According to Dushina andAleskovaski [251], the ion exchange is a first step of metal ion de-tention on the surface of various calcium silicate materials, includingslag. Depending on the nature of the material and the conditions of

Table 8Multi-component adsorption models reported in the literature.

Isotherm types Equations Nomenclature

Non-modified Langmuirqe;i =

qm;iKL;iCe;i

1 + ∑N

j=1KL;jCe;j

Where, Ce,i and qe,i areequilibrium and the arespectively, KL,i and qisotherm equations.

Modified Langmuirqe;i =

qm;iKL;i Ce;i = ηL;i� �

1 + ∑N

j=1KL;j Ce;j = ηL;j� �

Where, ηL,i is the Lanfrom competitive ads

Extended Langmuirqe;i =

qmaxKEL;iCe;i

1 + ∑N

j=1KEL;jCe;j

qmax and K EL,i are thecorresponding adsorb

Sheindorf–Rebuhn–Sheintuch(SRS) qe;i = KF;iCe;i ∑

N

j=1aijCe; j

! 1=nið Þ−1 The competition coefof component i by coor more likely, from

Non-modifiedRedlich–Peterson qe;i =

KR;iCe;i

1 + ∑N

j=1aR;jC

β; je; j

Where, KR,i, Ce,i and βindividual Redlich–Pe

Modified Redlich–Petersonqe;i =

KR;i Ce;i = ηR;i� �

1 + ∑N

j=1aR; J Ce;i =ηR; j� �β; j

Where, ηR,i is the Lancompetitive adsorpti

Extended Freundlichqe;1 =

KF;1C1=n1ð Þ+ x1e;1

C x1e;1 + y1C

z1e;2

Where, KF,1, KF,2 and nisotherm equations anFreundlcih adsorptionfor binary system) are

qe;2 =KF;2C

1=n2ð Þ+ x2e;2

Cx2e;2 + y2C

z2e;1

the experiment, the process may continue with further interac-tions between the metal ions and the solid matrix. The formation ofsparingly soluble compounds of the type of metal silicates (surfaceprecipitation) is possible. Yamashita et al. [252] have reported thatthe mechanism of adsorbing and fixing heavy metals of the converterfurnace slag can be attributed to one or more of following effects:adsorption, co-precipitation, hydroxide precipitation as hydroxide,sulfide and ion exchange. Lopez et al. [87] evaluated the applicabilityof blast furnace sludge for the removal of Pb(II) from their aqueoussolution and showed that adsorption is of a physical nature andphysical adsorption process partially occurs through ionic exchangebetween Pb (II) and other ions such as Ca (II). The mechanismsinvolved in metal retention by slag are thought to be ion exchangewith calcium on particle surfaces and precipitation on Al(OH)3 andSiO2 [90].

Guo et al. [122] investigated the adsorption of the heavy metalions Pb(II), Cu(II), Cd(II), Zn(II), and Ni(II) on a lignin isolated fromblack liquor and reported that surface complexation mechanismwas involved in the adsorption process. They also found that ligninsurfaces contain two main types of acid sites attributed to carboxylic-and phenolic-type surface groups and the phenolic sites have a higheraffinity for metal ions than the carboxylic sites. Metal ion adsorptiononto deprotonated carboxyl and phenolic sites was the dominantmechanism that could reasonably explain the observed adsorptionbehavior. Crist et al. [128,131,132] observed that metal ion sorptionwas accompanied by stoichiometric release of protons and existingmetals, and concluded that ion exchange mechanisms were re-sponsible for the removal of metals by lignin. In order to see if asimilar process was occurring in the study, release of the cations Na(I),Mg(II), and Ca(II) was recorded while the metal ion was adsorbed, bytaking Cu as an example. Compared to the amount of Na(I) released,the release of Mg(II) and Ca(II) was negligible. When the pH valuewas increased from 2.5 to 4.5, the molar ratios of Na(I) released to Cu(II) adsorbed were found between 1.28 and 1.39, but when the pHvalue reached 4.8, the ratio decreased to 0.66. The amount of Cu(II)adsorbed was more than the equivalent cations released, indicating

References

the unadsorbed concentration of each component in the mixture atdsorbed quantity of each component per gm of adsorbent at equilibriumm,i are derived from the corresponding individual Langmuir

[246]

gmuir correction coefficient of the i component where estimatedorption data

[246]

model parameters (the subscripts 1 and 2 represent theates) obtained experiemtnally single-solute Langmuir isotherm.

[247]

ficients aij in the SRS model describe the inhibition to the adsorptionmponent j, and can be determined from the thermodynamic data,the experimental data of multi-component systems.

[248]

j are the Redlich–Peterson parameters derived from the correspondingterson isotherm equations

[249]

gmuir correction coefficient of the i component where estimated fromon data.

[249]

1 and n2 are derived from the corresponding individual Freundlichd the six other parameters (x1,y1,z1 and x2,y2,z2 are the multi-componentconstants of the first and the second components, respectively,the competition coefficients for two species.

[250]

Table 9Single-component kinetic models used in the literature.

Isotherm types Equations Nomenclature References

Lagergren's first orderrate equation

log qe−qtð Þ = log qe−k1

2:303t

Where qe and q are the amounts of phenol adsorbed at equilibrium and at time t inmilligrams per gram, respectively, k1 is the rate constant of Lagergren first-order adsorption

[255]

Pseudo-second-orderkinetic model

tqt

=1

k2q2e+

1qe

tWhere k2 is the rate constant of second-order adsorption [257]

Elovich Equationqt =

1b

� �ln abð Þ + 1

b

� �ln t

Where, a and b are the constants for this model obtained from the slope and interceptof the linear plot of qt versus ln t.

[258]


that there may some other mechanisms at work in addition to ionexchange.

Moreover, the related mechanisms of metal sorption by lignin arestill subject to debate. However, ion-exchange mechanisms may beresponsible for the adsorption of metal ions on lignin as mentionedpreviously [131,132]. Srivastava et al. [124] and Mohan et al. [126]suggested that no single mechanism could explain the process ofmetal removal by lignin. Ion exchange may also dominate whereheavier and higher valence metal ions replace H+, Na+ and other ionson lignin surface functions. Traces of metals present in the as-obtainedlignin can enter into reactions with metal ions lower in the elec-trochemical series, causing heavy metals to deposit on the surface[126]. Several processes including ion exchange, surface adsorption,and complexation, have been suggested to explain the mechanismsinvolved. Detailed studies are therefore required to achieve a quan-titative and mechanistic understanding of the sorption of metal ionsby lignin.

The Al, Fe oxides and hydroxides of the red muds, principally thehematite, were the other active components in heavy metal adsorp-tion. Since the pH of solutions at equilibrium in the experiments waslower of the point of zero charge of the red muds, as reported byseveral authors (pzc=8–8.5) [113,253,254]; the adsorption of heavymetals on Al, Fe oxides and hydroxides occurred on positively chargedsurfaces through the formation of specific inner-sphere bonds.

9. Kinetic modeling of adsorption in a batch system

Numerous kinetic models have been suggested to describe thereaction order of adsorption systems based on concentration of thesolution [255,257,258] and reported in Table 9. Kineticmodels based onthe capacity of the adsorbent have also been presented, such as theLagergren'sfirst-order equation [255] andHo's second-order expression[256]. The first-order equation of Lagergren [255] and the pseudosecond-order equation are the most widely used kinetic models todescribe the adsorption of a solute from a liquid solution. The pseudosecond-order equation fitted the adsorption data very well in a largequantity of literature reported [256–257]. Ho [255] reviewed the ap-plication of second-order kinetic models to adsorption systems,including an earlier adsorption rate equation based on the solid capac-ity for a system of liquids and solids, the Elovich equation for adsorp-tion of gases onto a solid, and applying a second-order rate equationfor gas/solid and solution/solid adsorption systems, a second-order rateexpression for ion exchange reactions, and a pseudo-second-orderexpression. The pseudo-second-order rate expression was used to de-scribe chemisorption involving valency forces through the sharing orexchange of electrons between the adsorbent and adsorbate as covalentforces, and ion exchange. In recent years, the pseudo-second-orderrate expression has been widely applied to the adsorption of pollutantsfrom aqueous solutions [255]. The advantage of using this model isthat there is no need to know the equilibrium capacity from the experi-ments, as it can be calculated from the model [255]. In addition, theinitial adsorption rate can also be obtained from the model. Kineticsstudies and dynamic continuous-flow investigations, offering informa-tion on the rate of the adsorption uptake of metals, together with the

hydrodynamic parameters, are very important for the design ofadsorption process. However, adsorption kinetics studies are insuffi-cient according to the literature published so far. The kinetics ofadsorption is very important from the point of view that it controls theprocess efficiency.

10. Factors affecting adsorption of heavy metals

10.1. Effect of contact time and initial concentration of heavy metals

The initial concentrations of metals have a strong effect on theadsorption capacity of various industrial waste materials. Generally,adsorption capacity increased with increased initial concentrations ofthe heavy metals. The initial concentration provides an importantdriving force to overcome all mass transfer resistance of heavy metalsbetween the aqueous and solid phases. The removal of heavy metalsto reach the equilibrium also varied with contact time. Bhattacharyaet al. [96] reported the effect of contact time and initial concentrationof heavy metals on the adsorption of Zn(II) on various adsorbents andshowed that increase in contact time from 0.5 to 2.0 h enhancedsignificantly the percent removal of Zn(II). The initial rapid adsorptiongives away a very slow approach to equilibrium. The nature of adsor-bent and its available adsorption sites affected the time needed toreach the equilibrium. They [96] reported that times needed to reachthe equilibrium are 1 h, and 3 h for clarified sludge and rice huskash, respectively. For the other two adsorbents, activated aluminaand neem bark, a contact time of 4 h is needed for equilibrium to beestablished. The results are similar to those reported by Zheng et al.[177] for the adsorption of heavymetals by areca— a foodwaste (AW)in which equilibrium was attained within 2 h. The Nickel(II) adsorp-tion capacities of waste tea were presented as a function of contacttime in Fig. 1 [259]. The initial concentration (C0, mg/L) was changedin the range of 50–300 mg/L. When the initial nickel(II) ion con-centration increased from 50 to 300 mg/L, the residual Ni(II) concen-tration (Cres, mg/L) of solution increased from 5.8 to 159 mg/L. Theremoval of Ni(II) ions increased rapidly with time up to 20 min andthereafter increased slowly. While initial metal concentration was100 mg/L, the uptake of Ni(II) ions was 8.12 mg/g at 30 min and8.24 mg/g at 120 min. The amount of nickel(II) ions adsorbed per unitmass of the adsorbent increased with the initial concentration ofmetal ions. When the initial nickel(II) ion concentration increasedfrom 50 to 300 mg/L, the initial adsorption capacity of waste teaincreased from 4.413 to 14.04 mg/g. The effect of initial chromium ionconcentrations on the adsorption of Cr(VI) ions from aqueous solutionon tea factory waste was also investigated [174].

10.2. Effect of pH

The solution pH has a pronounced influence on the adsorption ofmetal ions on various industrial wastes. In a certain pH range, mostmetal adsorption increases with increasing pH up to a certain valueand then decreases with further increase in pH. This is readily ex-plained by the adsorption mechanism.

Fig. 2. Effect of pH on Ni(II) uptake and residual concentration by waste tea (adsorbentdosage=10 g/L, agitating rate=360 rpm, C0=100 mg L−1) [259].

Fig. 1. The uptake and residual amount of Ni(II) by waste tea as a function of initial metalconcentration.(adsorbent dose=10 g L−1, agitating rate=360 rpm, pH 4.0) [237].


Therefore, there is a favorable pH range for the adsorption ofevery metal on a certain industrial waste material. The effect of pHmay also be accounted in terms of pHzpc of the adsorbent, at whichthe adsorbent is neutral. The surface charge of the adsorbent is posi-tive when the media pH is below the pHzpc value, while it is nega-tive at a pH over the pHzpc. On the other hand at pH below the pHzpc,the predominant metal species (M2+ and M(OH)+) are positivelycharged and therefore, the uptake of metals in the pH range belowpHzpc is H+―M2+ (or M(OH)+) exchange process. With an increasein pH above pHzpc, although the surface of the adsorbent is negativelycharged, the adsorption still increases as long as the metal species arestill positively charged or neutral.When both the surface charge of theadsorbent and metal species charge become negative, the adsorptionwill decrease significantly. According to the mechanism and thediscussion of pH effect, the adsorption will lead to a decrease in pH asequivalent H+ will be released along with the adsorption. This is thecase for most metal adsorption, but there is always exception. Somemetals existing as negative species in solution, such as hexavalentchromium, may release hydroxide (OH−) instead of proton (H+)when they are adsorbed by waste materials, and therefore result in anincrease in pH. Fig. 2 shows the effect of pH on Ni(II) uptake andresidual concentration by waste tea [259].

10.3. Effect of particle size

Intra-particle diffusion study shows that particle size of the wastematerials used greatly influences the adsorption rate. Decrease inparticle size would lead to increase in surface area and then increasein the adsorption opportunity at the outer surface of the waste mate-rials. Besides adsorption at the outer surface of the waste materialthere is also a possibility of intra-particle diffusion from the outersurface into the pores of the material. The diffusional resistance tomass transfer is higher for large particles. Due to various factors, suchas diffusional path length or mass transfer resistance, contact time,and blockage of some diffusional path, most of the internal surfaceof the particle may not be utilized for adsorption; consequently theadsorption efficiency may become low.

The adsorption capacity of waste materials very much depends onthe surface activities — in other words, specific surface area availablefor solute surface interaction, which is accessible to the solute. It isexpected that adsorption capacity will be increased with a larger sur-

face area. In other words, smaller particle size increases the adsorp-tion capacity. Amarasinghe and Williams [175] studied the effect ofparticle size of tea waste on the adsorption capacity of heavy metals.The results showed that for tea waste particles of mean size 1250, 925and 575 μm, the percentage removal of Cu ion were 41, 53 and 57,respectively. Batch adsorption experiments were also carried out toinvestigate the effect of particle size of bagasse fly ash on the removalof Pb and Cr (VI) from their aqueous solution [59]. Studieswere carriedout using three different particle sizes, 100–150, 200–250, and 300–350 μm at pH 6.0 for lead and 5.0 for chromium, with an adsorbentdose of 10 g/L, shaking time 80 min for lead and 60 min for chromiumat 30 °C. The adsorption of lead and chromium was found to be 99.9,95.0, and 88.0%, respectively using the above mentioned sizes.

10.4. Effect of ionic strength

Ionic strength is a general property of the solution affecting theaffinity between the solute and the aqueous phase. This is one of theimportant factors which influence the aqueous phase equilibrium.Generally, adsorption decreases with increasing ionic strength of theaqueous solution. This effect may be ascribed to changes in the metalactivity, or in the properties of the electrical double layer. According tosurface chemistry theory, when two phases, e.g. waste particles andmetal species in aqueous solution, are in contact, they are bound to besurrounded by an electrical double layer owing to electrostaticinteraction. If the adsorptionmechanism is significantly the electrostaticattraction, adsorption decreases with increase in ionic strength. Someinorganic anions, such as chloride, may form complexes with somemetal ions and therefore, affect the adsorption process. The effect ofdifferent competitive ions like nitrate (NO3

−), sulfate (SO42−), and

phosphate (PO43−) on the adsorption of Cr(VI) was studied at various

concentrations [101]. It was observed that the percentage of adsorp-tion of Cr(VI) decreased with increasing concentrations of externallyadded ions. The affinity sequence for adsorption of such anions onactivated red mud is PO4

3−NSO42−NNO3

−. Increasing dosages of theseions from 5 to 20 mg/L had little effect. Similar observations weremadefor fluoride removal using treated alum sludge [260]. Weng and Huang[71] also studied the effect of ionic strength on the removal of Zn(II)from aqueous solution by usingfly ash and reported that Zn(II) removalis higher at a lower ionic strength under the same pH value.

Fiol et al. [149] shows the effect of NaCl and NaClO4 on the removalof metal by olive stone waste. The presence of both salts indicated adramatic decrease on the uptake of metal (N70%) with the exception

image of Fig.�1

Fig. 3. Breakthrough curves for adsorption of Cu and Pb onto tea waste: 15 g of tea, flowrate — 20 mL/min [175].


of copper. The presence of NaCl was found to exert a slightly greaterinfluence than NaClO4 on the removal of metal. The decrease of metalremoval in the presence of NaCl was also observed when grape stalkwastes [187] and cork [261] were used to remove copper and nickelfrom aqueous solutions. Some authors explain that the percentagereduction of metal removal is due to the increase of NaCl and thusby the presence of competing Na+ ions for metal binding [262].Other authors reported that there was a decrease of adsorption withincreasing ionic strength, where the adsorption process involvedelectrostatic attraction between adsorbent and adsorbate [263]. Thus,in the case of metal adsorption by olive stones, both phenomena couldbe considered to provoke the decrease of metal uptake. In addition, allthe studied metals form complexes with chloride ions; therefore, theformation of chloro-complexes would explain the slight difference onmetal adsorptionwhen this mediumwas employed instead of NaClO4.

11. Column studies

Column-type continuous flow operations appear to have a distinctadvantage over batch type operations because the rate of adsorptiondepends on the concentration of solute in the solution being treated.For column operation the adsorbents are continuously in contactwith a fresh solution. Consequently the concentration in the solutionin contact with a given layer of adsorbent in a column is relativelyconstant. For batch treatment, the concentration of solute in contactwith a specific quantity of adsorbent steadily decreases as adsorptionproceeds, thereby decreasing the effectiveness of the adsorbent forremoving the solute. The breakthrough capacity, which is the amountadsorbed before the appearance of adsorbates in the effluent, and thetotal capacity, which is the amount adsorbed until the effluentconcentration of the adsorbate is equal to the influent solutionconcentration, are computed from the breakthrough curves. Guptaand Ali [59] reported that the removal of lead and chromium ions upto 95–96% was achieved using baggase fly ash by column experimentsat a flow rate of 0.5 mL/min. The flow rate was varied to achieve themaximum removal of the adsorbates and it was found that themaximumuptake of lead and chromiumwas achieved at a flow rate of1.0 mL/min. The column adsorption capacity at complete exhaustionwas found to be 35.0 and 25.0 mg/g for lead and chromium,respectively, which is greater than in the batch experiments (30.0and 20 mg/g of lead and chromium, respectively). The high columncapacity may be due to the fact that a continuously large concentra-tion gradient occurred at the interface zones as it passes through thecolumn, while the concentration gradient decreased with time in thebatch experiments.

Regeneration and recovery of the column is a very important aspectin wastewater treatment processes and, therefore, desorption of leadand chromiumwas triedwith a number of eluents (methanol, ethanol,sodiumhydroxide, sulfuric acid, hydrochloric acid, nitric acid, etc.) anditwas found that the desorption of these twometal ions occurred by 5%HNO3 [59]. The column was allowed to pass 5% HNO3 at a flow rate of2.5 mL/min for about 3 h and then again allowed to pass deionizedwater at a flow rate of 2.50 mL/min for about 3 h. It was observed thatthe column loses about 2% capacity after the first run and about 3–10%aftermore thanfive runs. Therefore, the column can be used for at least10 runs without any problem. It is very important to mention thatbagasse fly ash is an easily available, inexpensive material, and its costis very low in comparison to the cost of regeneration. Moreover, theused bagassefly ash can be used as rawmaterial for buildingmaterials.Therefore, there is no need to regenerate bagasse fly ash in columns.

The use of granular blast furnace slag (GBFS)-packed columns totreat lead-containing solutionshas been reportedbyDimitrova [91]. Theresults obtained indicated that the slag usage rate decreased withincreased flow velocity, particle size, and initial lead concentration anddecreased with bed height. Lead removed selectively in the presence ofother heavy metal ions. High concentrations of sodium and especially

calcium in the solutions impeded the uptake of lead. The apparentmechanismsof lead removal inGBFS columnare sorption (ion exchangeand adsorption) on the slag surface and precipitation. Adsorption ofcopper and lead ions onto teawaste fromaqueous solutionswas studiedin column mode to enable comparison with alternative commonlyavailable absorbents [175]. Fixed bed column experiments wereperformed to study practical applicability and breakthrough curveswere obtained. Breakthrough curves for Cu and Pb adsorption onto teawaste from single metal ion solutions of concentration 100 mg/L areshown in Fig. 3. The removal efficiency of waste tea from nickel con-taining aqueous solutions in fixed-bed column was investigated [259].The total adsorbed quantities, equilibrium uptakes and total percentremoval of Ni(II) related to the effluent volumes were determined byevaluating the breakthrough curves obtained at different flow rates,different inlet Ni(II) concentrations, different pH value, different bedheight and different particle size for waste tea.

12. Comparison of adsorption performance

Suitable industrial waste materials should possess several require-ments for the adsorption of metals, such as, (i) efficient for the removalof a wide variety of heavy metals (ii) maximum removal of specificmetals (iii) high adsorption capacity (iv) low-processing cost (v) easilyregenerated/disposed off after adsorption and (vi) tolerant of a widerange of concentration of heavy metals and other wastewaterparameters.

Certain industrial wastes, such as, fly ash, blast furnace slag and redmud have been tested by various workers and proposed for theremoval of heavy metals. Each industrial waste material has its ownspecific physical and chemical characteristics such as porosity, surfacearea and physical strength. The waste materials have also their in-herent advantages and disadvantages in the wastewater treatment.Adsorption capacities of the adsorbent were also found to vary, de-pending on their experimental conditions. It is evident from thepresent review that industrial waste material as adsorbents may havepotential as readily available, inexpensive and effective adsorbents.They also possess several other advantages that make them excellentmaterials for environmental purposes, such as high capacity and rateof adsorption (Tables 3, 4, and 5).

Table 3 presents a summary of the adsorption capacities of fly ash forthe removal of heavy metals. However, the adsorption capacities formetal ions in multi-component systems are reported in Table 4. Table 5shows the adsorption capacities for heavy metals on other industrialadsorbents. From the recent literature reviewed, adsorbents that standout for high adsorption capacities for the removal of Zn(II) metals are

image of Fig.�3


bagasse fly ash (13.21 mg/g), rice husk ash (14.30 mg/g), coal fly ash(11.11 mg/g), fly ash zeolite 4A (30.80 mg/g), blast furnace slag(103.3 mg/g), lignin (73.3 mg/g), powered waste sludge (168 mg/g),sugar beet pulp (35.6 mg/g), and clarified sludge (15.53 mg/g).Adsorbents showing high adsorption capacities for the removal of Cd(II) metals are bagasse fly ash (6.19 mg/g), fly ash (198.2 mg/g), fly ashzeolite (95.6 mg/g), fly ash zeolite X (97.78 mg/g), waste slurry(15.73 mg/g), red mud (66.67 mg/g), lignin (25.40 mg/g), carboxy-mehtylated lignin (107.53 mg/g), sugar beet pulp (38.6 mg/g), andphosphate apple waste (36.2 mg/g). The literature also indicatedsome of the adsorbents that showed high adsorption capacities forthe removal of Pb(II) metals which are bagasse fly ash (285.56 mg/g),fly ash (444.7 mg/g), fly ash zeolite (70.6 mg/g), fly ash zeolite X(420.61 mg/g), blast furnace slag (40 mg/g), waste slurry (1030 mg/g),lignin (1865 mg/g), sugar beet pulp (43.5 mg/g),tea industry waste(65 mg/g), crop milling waste (49.97 mg/g/m), carboxymehtylatedlignin (37.99 mg/g), sludge (39.3 mg/g), and waste biogas residualslurry (28.0 mg/g).Adsorbents, suchas, coalflyash (178.5–249.1 mg/g),fly ash zeolite (50.45 mg/g),fly ash zeolite X (90.86 mg/g), blast furnaceslag (133.3 mg/g), waste slurry (20.97 mg/g), red mud (106.44 mg/g),sugar beet pulp (30.9 mg/g), iron oxide waste (98.0 mg/g), and grapestalk wastes (42.92 mg/g) showed high adsorption capacities forthe removal of Cu(II) metals. Adsorbents that indicated high adsorp-tion capacities for the removal of Ni(II) metals are bagasse fly ash(6.48 mg/g), Al-impregnated fly ash (10–15.7 mg/g), Fe-impregnatedfly ash (9.8–14.93 mg/g), fly ash zeolite 4A (8.96 mg/g), red mud(160 mg/g), tea factory waste (18.42 mg/g), grape stalk wastes(38.31 mg/g), sugar beet pulp (11.85 mg/g), crop milling waste(20.15 mg/g/m), and treated sewage sludge (9.09 mg/g). Lastly,adsorbents that stand out for high adsorption capacities for the re-moval of Cr(VI) metals are bagasse fly ash (4.25–4.35 mg/g), fly ash(23.86 mg/g), rice husk ash (25.64 mg/g), waste slurry (640 mg/g),clarified sludge (26.31 mg/g), tea industry waste (455 mg/g), leatherindustrywaste (133 mg/g), sugar beet pulp (17.2 mg/g), and sugar canebagasse (13.4 mg/g).

13. Discussion

The adsorption of heavy metal ions from aqueous solutions usingindustrial wastes as an adsorbent has been extensively studied. How-ever, there are several issues and drawbacks concerned that should beaddressed. These can be summarized as shown below:

(i) In most of the article, no attempts were made to relate thecharacterization results with the performance of the adsor-bents for the removal of heavy metals.

(ii) Metal ion adsorption is well known to be pH dependent.However, in most of the cases, the pH effect was invariablyinvestigated in terms of the initial pH of the solution. Not in asingle case data has been reported for the changes of thesolution pH during the course of the adsorption.

(iii) The equilibrium data reported, without exception, were empiri-cally correlatedwith conventional isotherm(Freundlich, Langmuirand a few others). In a few cases, the authors have proposed themechanismof theadsorption.However, theproposedmechanismswere hardly used in interpreting the equilibrium data obtained.

(iv) The kinetic data were fitted with the Lagergren equation (or itsvariant). How this equation and the parameter values obtainedcan be used in the design calculations of fixed-bed operationwas ignored in most of the studies.

(v) The competitive metal adsorption on various adsorbentswas reported in a few cases. However, there is scarce dataavailable for the adsorption of heavy metals in presence oforganic and other contaminants. Therefore, the potential ofindustrialwastes as low-cost adsorbents undermulti-componentpollutants/contaminants need to be addressed. This would make

a significant impact on the commercial application of industrialwaste as low-cost adsorbents for the treatment of industrialwastewater.

(vi) No literature is available to apply the well-developed surfaceionization/complexation model or the double layer retentionmodel to investigate the effect of ionic strength or pH on theextent of adsorption.

(vii) Further, detailed systematic studies are needed to find outwhich component in the precursor is primarily responsible forthe development of surface area or micro/meso porosity or ishelpful in the development of functional groups.

(viii) Combination of methodologies/system should also be encour-aged since one perfect method/system is practically difficult.Besides these, strategies to minimize metal and related chemicaleffluent at all levels anddesigningmoreenvironmentally friendlychemicals are also needed. More comprehensive or specificmodels for equilibrium or kinetic studies that can simulate morecomplex adsorption systems, such as hybrid adsorption system,should be developed.

(ix) The cost of the various industrial wastes (along with the cost ofprocessing) is seldom reported in any of the publications.However, the expense of individual adsorbents varies, depend-ing on the processing conditions and local availability. In spite ofthe scarcity of consistent cost information, the widespread usesof low-cost adsorbents in industries for wastewater treatmentapplications today are strongly recommended due to theirlocal availability, technical feasibility, engineering applicability,and cost effectiveness. If low-cost adsorbents such asfly ash, redmud, waste slurry, and lignin perform well in removing heavymetals at a low cost, they can be adopted and widely used inindustries not only to minimize cost inefficiency, but alsoimprove profitability. Therefore, a cost–benefit analysis of usinglow-cost adsorbents for heavy metal removal needs to beconducted to judge the economic feasibility of its practical usein wastewater treatment applications today.

(x) It is also evident from the literature that most of the adsorptionstudies were carried out in a batch operation. Very few studieswere reported on the adsorption of heavy metals on columnoperation. Thus, it is required to carry the adsorption experi-ments in column mode in the laboratory and then the studiesshould be conducted on pilot plant scale utilizing industrialwaste as low-cost adsorbents to see their feasibility on theutilization in the commercial scale.

(xi) Selection and identification of an appropriate industrial waste aslow-cost adsorbent is one of the key issues to achieve the max-imum removal of specific metals depending on the adsorbent–adsorbate interaction.

(xii) Leaching of industrial wastes in water is very important inorder to see the dissolution of the various substances present inthe wastes. This interference will lead to the erroneous resultsin the adsorption experiments. However, very few authorsstudied the leaching of industrial wastes when it is as such usedas an adsorbent. Therefore, more research should be performedin this direction in order to see the practical utility of as suchindustrial waste.

(xiii) The conditions for the production of suitable adsorbents fromindustrial wastes by treating with various reagents for higheruptake of heavy metals need to be optimized.

(xiv) Adsorption mechanisms of heavy metals from wastewater needto be studied in detail. It is required to propose a correct bindingmechanism of heavy metals with industrial waste. This can bedone by blocking the functional group responsible for the ad-sorption of heavymetals. However, in very few cases the bindingmechanism is reported. Therefore, more emphasis should begiven on this direction to impart knowledge/highlight the newdimension regarding adsorption phenomenon.


(xv) Regeneration/recovery of heavy metals from metal-loadedadsorbent should be performed in detail. Disposal of themetal-laden industrial waste is again a big problem. Therefore,more research should be conducted on the various methodsutilized for the environmental friendly safe disposal of metal-laden adsorbents. It will enhance the economic feasibility ofthe process as well as reduce the environmental pollution.

(xvi) To promote the adsorption rate, some special type of compositeadsorbents with the redox, catalytic, and adsorptive functionswhich can carry out the processes of oxidation/reduction andadsorption of pollutants simultaneously, may be developed.

14. Conclusion

The role of industrial wastes in the removal of heavy metals fromwastewater has been investigated. Among the industrial wasteproducts, fly ash, waste slurry, red mud, lignin, sugar beet pulp, blastfurnace slag, tea industry waste, and sugar cane bagasse have beenproven to be a promising material for the removal of heavy metalsfrom waste waters. The adsorption phenomenon could be generallymodeled by Langmuir and Freundlich isotherms. However, othermodels, such as, Redlich–Peterson, Dubinin KR, Tempkin, Toth, BET,Radke–Prausnitz, Flory–Huggins, and Sips models are also used. Thevarious parameters such as adsorption capacity, adsorption intensityand energy of adsorption can be determined by linear regression.

The adsorption could be influenced by a number of factors, such as,adsorbent dose and size, contact time, agitation speed, temperature,pH and ionic strength of the aqueous solution. Generally, percentadsorption increased with increased adsorbent dose, contact time,and agitation speed. However, favorable conditions may be differentfor differentmaterials and adsorptions. For each type ofmaterial, thereis a neutral pH beyond which the material will be either positivelyor negatively charged. For every heavy metal, there is a favorablepH range in whichmaximum adsorption was observed. If it is possibleto develop such adsorbents from industrial wastes possessing all theabove-mentioned characteristics, then these adsorbents may offer sig-nificant advantages over currently available expensive commerciallyactivated carbons, and in addition contribute to an overall waste mini-mization strategy. Further studies are required to apply the design andsimulation model to larger scale pilot plant and not on small scalelaboratory applications.

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