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Journal of Hazardous Materials 181 (2010) 836–844

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

imultaneous removal of heavy-metal ions in wastewater samples usingano-alumina modified with 2,4-dinitrophenylhydrazine

bbas Afkhamia,∗, Mohammad Saber-Tehranib, Hasan Bagherib

Faculty of Chemistry, Bu-Ali Sina University, Hamadan, IranDepartment of Chemistry, Islamic Azad University, Science and Research Branch, P.O. Box 1415-775, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 9 March 2010eceived in revised form 19 May 2010ccepted 19 May 2010vailable online 25 May 2010

a b s t r a c t

2,4-Dinitrophenylhydrazine (DNPH) immobilized on sodium dodecyl sulfate coated nano-alumina wasdeveloped for the removal of metal cations Pb(II), Cd(II), Cr(III), Co(II), Ni(II) and Mn(II) from water sam-ples. The research results displayed that adsorbent has the highest adsorption capacity for Pb(II), Cr(III)and Cd(II) in ions mixture system. Optimal experimental conditions including pH, adsorbent dosage andcontact time have been established. Langmuir and Freundlich isotherm models were applied to analyze

eywords:eavy-metal ions removal

sothermslumina nanoparticlesodified nanoparticlesastewater

the experimental data. The best interpretation for the experimental data was given by the Freundlichadsorption isotherm equation for Mn(II), Pb(II), Cr(III) and Cd(II) ions and by Langmuir isotherm equa-tion for Ni(II) and Co(II) ions. Desorption experiments by elution of the adsorbent with a mixture of nitricacid and methanol show that the modified alumina nanoparticles could be reused without significantlosses of its initial properties even after three adsorption–desorption cycles. Thus, modified nano-aluminawith DNPH is favorable and useful for the removal of these metal ions, and the high adsorption capacitymakes it a good promising candidate material for Pb(II),Cr(III) and Cd(II) removal.

. Introduction

Heavy metals released into the environment have posed a signif-cant threat to the environment and public health because of theiroxicity and persistence in environment [1–3]. The contaminationf wastewaters and surface waters by toxic heavy metals is a world-ide environmental problem. These toxic metal ions commonly

xist in process waste streams from mining operations, metal-lating facilities, power generation facilities, electronic deviceanufacturing units, and tanneries. Because of economic and envi-

onmental factors, the removal and recovery of heavy-metal ionsrom industrial wastewater have been a significant concern in mostndustrial branches [3–5]. The most commonly applied physico-hemical treatment methods are: (i) precipitation as hydroxides,arbonates or sulfides and subsequent liquid-solids separation byravity settling, and flotation or filtration, (ii) sorption (adsorption,on exchange), (iii) membrane processes, (iv) electrolytic recovery

nd (v) liquid-liquid extraction [4,6–8]. However, each method hasts merits and limitations in application and they are often limitedy technical and economical issues [4,9].

∗ Corresponding author. Tel.: +98 811 8272404; fax: +98 811 8272404.E-mail address: afkhami@basu.ac.ir (A. Afkhami).

304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2010.05.089

© 2010 Elsevier B.V. All rights reserved.

The adsorption process is arguably one of the more popu-lar methods for the removal of heavy-metal ions because of itssimplicity, convenience, and high removal efficiency [10–13]. Theadsorption process with activated carbon is attractive to many sci-entists because of the effectiveness of the removal of heavy-metalions at trace quantities [14,15]. However, the process has not beenused extensively because of its high cost [14–17]. Therefore, the useof low-cost materials as sorbents for metal removal from wastewa-ter has been a focus. Using low-cost biosorbents such as agriculturalwastes, clay materials, biomass, and seafood processing wastesmay be an alternative wastewater technology because they areinexpensive and capable of removing trace levels of heavy-metalions [18–20]. However, to improve their absorption capacity andenhance the separation rate, the design and exploration of noveladsorbents are still necessary.

Recently, application of nanoparticles for the removal of pol-lutants has come up as an interesting area of research. Theunique properties of nanosorbents are providing unprecedentedopportunities for the removal of metals in highly efficient and cost-effective approaches, and various nanoparticles and dendrimershave been exploited for this purpose [9,21–28]. Nanoparti-

cles exhibit good adsorption efficiency especially due to highersurface area and greater active sites for interaction with metal-lic species. Furthermore, adsorbents with specific functionalgroups have been developed to improve the adsorption capacity[29].

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A. Afkhami et al. / Journal of Haz

Alumina is a classical adsorbent and the form of �-alumina isnticipated to be more adsorptive active than �-alumina [30,31],nd if indeed sorption were the key mechanism, then the sub-tantial increase in surface area of the nano-form would increaseapacities very significantly. �-Al2O3 nanoparticles is a promisingaterial as a solid-phase adsorbent because of its large specific

urface area, high adsorption capacity, mechanical strength andow temperature modification [29,32,33]. But, some heavy-metalations are poorly adsorbed on it. To overcome this problem, chem-cal or physical modification of the surface �-Al2O3 nanoparticles

ith certain functional groups containing some donor atoms suchs oxygen, nitrogen, sulfur and phosphorus is necessary [34–38].hen a modifier is immobilized at the surface of alumina, the

emoval mechanism has changed. The target metals are not onlyemoved by adsorption on the surface of the alumina but it could beemoved by a surface attraction/chemical-bonding phenomenonn the newly added chemicals. A very common procedure toeposit an organic coating on inorganic oxide such as alumina, ironxide and silica is to dissolve it in a proper medium and mix theolution with inorganic oxide particles for a period of time, followedy evaporation of the solvent and air drying of the adsorbent.

2,4-Dinitrophenlhydrazin (DNPH) is an important reagent butp to present, it has not been used to separation and removal ofetal ions [39,40]. In our work, DNPH was chemically immobilized

n the surface of nano-�-Al2O3 as a new solid-phase adsorbentor removing of trace Pb(II), Cr(III), Cd(II), Ni(II), Co(II) and Mn(II).he adsorption behavior of analytes on the DNPH-�-Al2O3 and thexperimental conditions for the removal process were investigatedn detail. This system has several advantages that the process doesot generate secondary waste and the materials involved can beecycled and facilely used on an industrial scale.

. Experimental

.1. Materials

A Shimadzu model AA-670/g V-7 atomic absorption spectrom-ter with deuterium lamp background correction and hallowathode lamps at respective wavelengths (resonance line) asadiation sources, was used throughout the measurements. An air-cetylene flame was used for the determination of the metals ions.ll instrumental settings were those recommended by the manu-

acturer.An infrared spectrum was obtained using a Fourier transform-

nfrared spectrometer (FT-IR, Perkin-Elmer, spectrum 100) todentify the functional groups and chemical bonding of the coated

aterials. Scanning electron microscopy (SEM) was performedo measure the particle size and shape (SEM-EDX, XL30 andhilips Netherland). Surface area and porosity were defined by N2dsorption–desorption porosimetry (77 K) using a porosimeter (Belapan, Inc.). pH adjustments were carried out by a Metrohm model13 (Herisau, Switzerland) pH-Meter.

All the reagents used were of analytical grade. Double dis-illed water was used throughout. Laboratory glassware was keptvernight in a 10% (v/v) HNO3 solution and then rinsed with deion-zed double distilled water.

The nitrate salts of Pb(II), Cd(II), Cr(III), Co(II), Ni(II), and Mn(II)f analytical grade (Merck) were used without further purifica-ion. More dilute solutions were prepared daily from 1000 mg L−1

tock solutions. 2,4-Dinitrophenylhydrazine (DNPH) and sodium

odecyl sulfate (SDS) were purchased from Merck (Darmstadt, Ger-any). Alumina nanoparticles (average size of 40 nm, �-type (X-ray

nalysis), spherical shape) with purity 99.9% was purchased fromlasmachem GmbH (Germany). pH adjustments were performedith 0.01–1.0 mol L−1 HCl and NaOH solutions.

s Materials 181 (2010) 836–844 837

2.2. Preparation of �-Al2O3 modified with2,4-dinitrophenylhydrazine

2.0 g of alumina nanoparticles was suspended in 50 mL of waterand was mixed with 100 mg of SDS. Then, 20 mL of solution of2,4-dinitrophenylhydrazine (0.90 g DNPH in HCl + acetonitrile) wasadded. The suspension was stirred at 60 ◦C for 3 h. The mixture fol-lowed by evaporation of the solvent and washed, air-dried, andstored in a closed bottle for subsequent use.

2.3. Batch removal experiments

The adsorption of cations by modified nano-alumina was stud-ied by a batch operation at room temperature. The batch modeadsorption was selected due to its simplicity. A 0.05 g adsorbentwas suspended in 50 mL solution of the mixture of 50 mg L−1 of eachPb(II), Cd(II), Ni(II), Co(II), Mn(II) and Cr(III) at pH 5.0. The solutionswere not buffered to mimic potential industrial practice. Thesesamples were stirred for 90 min at 150 rpm. After a time period ofagitation, the suspensions were centrifuged at 2500 rpm for 5 min.The supernatants were then collected and analyzed for cations con-centration by flame atomic adsorption spectrometry. The effects ofthe experimental parameters to the adsorption capacity of modifiedalumina in the experiments were investigated.

The percent of removed metal ions by the adsorbents was cal-culated by the following equation:

R (%) = (C0 − Ct)C0

× 100 (1)

where R is the removal efficiency of the metals ions, C0 is the initialconcentration of the metal ions in mg L−1, and Ct is the concentra-tion of the metal ions at any time in mg L−1. All the investigationswere carried out in triplicate to avoid any discrepancy in experi-mental results with the repeatability and the relative deviation ofthe order of ±1.5%.

3. Results and discussions

3.1. Characterization of adsorbents

The surface and textural morphology of alumina nanoparti-cles by SEM image is illustrated in Fig. 1. As shown in Fig. 1, thenaked alumina nanoparticles had a mean diameter of 53 nm. Aftermodification process, the modified nanoparticles prepared are inthe range of 68–87 nm in diameter. This shows that the aluminananoparticles have been completely coated by the DNPH. It revealsthat the coating process significantly result in the agglomerationand the change in size of the particles [41]. Also, this could beattributed to the reaction occurring only on the particle surface,and thus our attempt to prepare DNPH-�-Al2O3 nanoparticles inthis work has been achieved. Although some agglomerations of thenanoparticles, which causes irregularity of the shape and size of thenanoparticles, can also be observed from Fig. 1, it was found thatthese agglomerations had not obvious effect on adsorbing applica-tions of the nanoparticles. It was clear that modified nanoparticleshad a mean diameter of 75 nm since the agglomeration is not seri-ous.

The specific surface area was determined using the BET equa-tion applied to the adsorption data. The results of the BET methodshowed that the average specific surface area of alumina nanopar-ticles was 42.62 m2 g−1. It can be concluded from these values that

this type alumina is nanoparticles with relatively large specific sur-face area. The surface area of the modified alumina nanoparticles,measured using the BET was 30.38 m2 g−1. The decrease in BETsurface area of modified �-Al2O3 with DNPH is due to the bulksize of the organic ligand, which blocks the surface of the alu-

838 A. Afkhami et al. / Journal of Hazardous Materials 181 (2010) 836–844

ticles

modoaw

aths8ttpatSoflt

wSmomoc(r

�bcw((

Fig. 1. The SEM image of naked �-alumina nanopar

ina nanoparticles [42–45]. Also, decrease in surface area incasef DNPH-�-Al2O3 as compared to nano-alumina which is possiblyue to aggregation after surface modification. According to BET the-ry [42] this decrease indicates the decrease in interaction betweendsorbate N2 molecules and the particle surface after modificationith DNPH.

Generally, the alumina surface is hydrophilic and has lowdsorption affinity for organic compounds; however, when it isreated with sodium dodecyl sulfate (SDS), alumina will acquireigh adsorption capability. This is due to the fact that when theolution pH is below the point of zero charge (PZC) of alumina (pH.5), the alumina surface is positively charged and anionic surfac-ants such as SDS molecules will adsorb onto the surface throughhe negative moiety sulfate [30]. In 1994, Hiraide et al. [34] pro-osed that water-insoluble organic ligand could be trapped into theggregate of sodium dodecyl sulfate (SDS) on alumina particles. Inhe preliminary studies it was found that when DNPH is mixed withDS coated alumina particles, the ligand is trapped homogenouslyn the hemi-micelles or ad-micelles formed by SDS on alumina sur-ace in a manner similar to that demonstrated for the other organicigands [34–38] and the color of alumina was changed from whiteo orange.

In this work, the concentration of SDS was fixed at 5 × 10−3 M,hich is below the critical micellization concentration (c.m.c.) of

DS (8 × 10−3 M). Above the c.m.c., the excess of SDS would formicelles in the aqueous solution, which does not become adsorbed

n the alumina surface. The SDS can form hemi-micelles or ad-icelles on the alumina by strong adsorption. The hemi-micelles

r ad-micelles can trap molecules of DNPH homogeneously, whichauses the alumina to change color from white to orange (Fig. 2)For interpretation of the references to color in this sentence, theeader is referred to the web version of the article).

The FT-IR spectra using KBr of DNPH, modified �-alumina and-alumina nanoparticles are shown in Fig. 3. The broad absorptionand in the region 3358–3100 cm−1 and weak band at 1628 cm−1

an be attributed to the stretching of the framework Al–OH groupith the defective sites and physically adsorbed water molecules

Fig. 3a) [41]. The vibrations of Al–O–Al can be seen at 914 cm−1

asymmetric stretching), 803 cm−1 (symmetric stretching) and at

Fig. 2. Schematic of the functionalizing DNPH on the alumina nanoparticles.

(left) and modified �-alumina nanoparticles (right).

633 cm−1 (bending). After modification of the alumina nanopar-ticles with DNPH, DNPH-�-Al2O3, it shows a visible broad band inthe 3500–3200 cm−1 region which is due to stretching vibrations ofOH or N–H groups with varying degrees of H bonding. The absorp-tion spectrum showed that the absorption bands at 1490 cm−1 andat 1620 cm−1 are corresponding to the bending vibration of N–Hgroup, while N–H stretching (3200–3500 cm−1) and C–N stretch-ing (1030–1230 cm−1) overlap with the broad absorption band ofthe Al–OH group and the Al–O–Al vibrations (Fig. 3b) [41,46–47].A comparison of these characteristic spectral bands indicated thatthe surface DNPH-�-Al2O3 contained –NH–functional group as aresult of the immobilization procedure. Also, the small shift in the633 cm−1 peak in the unmodified sample to 629 cm−1 in modifiedsample also confirms the involvement of the Al–O bond linkage inthe modification.

After adsorption of metals onto DNPH-�-Al2O3, the bandbetween 3358 and 3100 cm−1 appeared to be sharper in compar-ison with DNPH-�-Al2O3, indicating the major contribution of theN–H bond to that band. After adsorption of metal ions, an increasein the intensity of the band at 3324 cm−1 (�NH2 ) in the IR spectra ofthe metal ions was observed. The peak at 1620 cm−1, assigned tothe vibration of N–H in secondary amine (–NH–), shifts from 1620to 1630 cm−1, indicating –NH also takes part in adsorption interac-tions [45–47]. Also, after uptake the Pb2+, Cd2+, Cr3+, Co2+ and Ni2+

ions, in their spectra, the intensity of the peaks at 1030–1100 cm−1

and 1630 cm−1 decreased, indicating a stronger metal–adsorbentinteractions has formed between metal ions and functional sites inDNPH-�-Al2O3. While the intensities of the above bands decreasedafter metal ions adsorption using DNPH-�-Al2O3, an increase inthe intensity of the band at 1408 cm−1 was observed. The mainchange that took place in the FT-IR spectra, after adsorption of ions,is the appearance of a new adsorption band at 954 cm−1, whichwas considered as being a characteristic peak of the association ofDNPH-�-Al2O3 and metal ion.

SDS coated alumina nanoparticles did not adsorb metal ionsfrom the solution. It was observed that for DNPH-�-Al2O3 theuptake of heavy metals took place with high efficiency. It is dueto the immobilization of DNPH on nanoparticles that can favor theuptake of metal ions by formation of complexes with them (Fig. 4).

3.2. Effect of solution pH

pH is an important factor affecting the removal of cations fromaqueous solutions. The dependence of metal sorption on pH isrelated to both the metal chemistry in the solution and the ion-

ization state of the functional groups of the sorbent which affectsthe availability of binding sites [3,48].

In order to evaluate the influence of this parameter on theadsorption of cations, the experiments were carried out with thepH range of 1.5–5.5. A 0.05 g sample of adsorbent was suspended

A. Afkhami et al. / Journal of Hazardous Materials 181 (2010) 836–844 839

Fig. 3. FT-IR spectrum of the DNPH (a), DNPH-�-Al2O3 (b) and �-Al2O3 (c).

of the metal ions adsorption using DNPH-�-Al2O3.

iuH1a

cmRtaNapbnp

Fig. 4. Illustration of suggested mechanism

n 50 mL solution of 50 mg L−1 each of metal ions at several pH val-es (1.5, 2.5,3.5, 4.5,5.0 and 5.5) using either 0.1 M NaOH or 0.1 MCl for pH adjustment. These samples were stirred for 120 min at50 rpm. Then the samples were centrifuged at 2500 rpm for 5 mint room temperature to separate the adsorbent.

The pH range was chosen as 1.5–5.5 in order to avoid pre-ipitation of metal cations in the form of chloride or hydroxidesetals. The effect of pH on adsorption efficiencies is shown in Fig. 5.

emoval of Pb(II), Cd(II) and Cr(III) increases with increasing solu-ion pH and a maximum value was reached at an equilibrium pH ofround 5.0. Also, Fig. 5 illustrated that removal efficiency of Co(II),i(II) and Mn(II) increased by increase pH from 1.5 to 4.5. A weak

dsorption occurs in acid medium but it can be seen that higherH leads to higher metal uptake. Acid conditions are not favorableecause most of the functional groups of the components are proto-ated leaving few available ionized groups. Competition betweenrotons and metal species could thus explain the weak adsorption

Fig. 5. Percentage of metal ions removal at different pHs. Conditions: 0.05 g adsor-bent, 50 mL of 50 mg L−1 of heavy-metal ions, agitation time of 120 min.

840 A. Afkhami et al. / Journal of Hazardous Materials 181 (2010) 836–844

Fbo

i4oa

si

M

M

o

M

FrtM5pb

sesias

3

a5wstTocrt

3

ct

ig. 6. Percentage removal of heavy-metal ions at different amounts of adsor-ent. Conditions: 50 mL of 50 mg L−1 of heavy-metal ions, pH 5.0, agitation timef 120 min.

n acid medium. The highest uptake value was recorded at the pH.5 and 5.0. This may be attributed to the presence of free lone pairf electrons on nitrogen and deprotonated oxygen atoms, whichre suitable for coordination with the metal ions.

Generally, the metal ions in the aqueous solution may undergoolvation and hydrolysis. The process involved for metal adsorptions as follows [49]:

2+ + nH2O ↔ M(H2O)n2+, (2)

(H2O)n2+ ↔ [M(H2O)n−1(OH)]+ + H+, (3)

r totally

2+ + nH2O ↔ [M(H2O)n−1(OH)]+ + H+. (4)

or example, the pKa value for Pb(II) and Cd(II) are 7.7 and 10.1,espectively. Perusal of the literature on metal speciation showshat the dominant species is M(OH)2 at pH > 6.0 and M2+ and

(OH)+ at pH < 6.0. Maximum removal of metal was observed at pH.0 for adsorption. On further increase of pH adsorption decreasesrobably due to the formation of hydroxide of cadmium and leadecause of chemical precipitation [50–52].

To achieve high efficiency and good selectivity a pH of 5.0 waselected for subsequent work. Several researchers investigated theffect of pH on adsorption of metals by using different kinds oforbents [51–53]. In all cases, they observed a maximum metalons adsorption between pH 5 and 6. The results indicate that thedsorption percent of Pb2+ is higher than other ions in all pH rangestudied, suggesting possible selectivity for this metal.

.3. Effect of the amount of adsorbent

We studied the dependence of the adsorption of cations on themount of modified nanoparticles at room temperature and at pH.0 by varying the adsorbent amount from 0.01 to 0.07 g in contactith 50 mL solution of the mixture of 50 mg L−1 each of cations. The

uspension was then stirred for 120 min by a magnet. After cen-rifuging, the supernatant was analyzed for the remaining cations.he results are shown in Fig. 6. Apparently, the percentage removalf cations increased by increasing amount of modified nanoparti-les due to the greater availability of the adsorbent. The adsorptioneached a maximum with 0.06 g of adsorbent for all cations, andhe maximum percentage removal was about 98% for lead ions.

.4. Effect of contact time

The importance of stirring time comes from the need for identifi-ation of the possible rapidness of binding and removal processes ofhe tested metal ions by the newly modified adsorbents and obtain-

Fig. 7. Percentage removal of heavy metals at different times. Conditions: 50 mL of50 mg L−1 of heavy-metal ions, pH 5.0 and 0.06 g adsorbent.

ing the optimum time for complete removal of the target metalions.

The analysis of batch adsorption of metal ions was carried out in30 min steps and the concentration of each sample was measuredby atomic absorption spectroscopy after 180 min contact time. Thedata for adsorption experiment were replicated three times andthe results were averaged. The standard deviation was less than1.5%. The results were recorded and the time profile of heavy-metalions adsorption was plotted (Fig. 7). After 90 min the adsorbentremoved the metal ions from the solution. Therefore, the optimumcontact time for adsorption of the heavy metals was considered tobe 90 min.

3.5. Equilibrium adsorption study

The relationship between the amount of a substance adsorbedper unit mass of adsorbent at constant temperature and its con-centration in the equilibrium solution is called the adsorptionisotherm. Adsorption isotherm is important to describe how solutesinteract with the sorbent. Developing an appropriate isothermmodel for adsorption is essential to the design and optimiza-tion of adsorption processes. Several isotherm models have beendeveloped for evaluating the equilibrium adsorption of compoundsfrom solutions, such as Langmuir, Freundlich, Redlich–Peterson,Dubinin–Radushkevich, Sips, and Temkin [54]. Since the morecommon models used to investigate the adsorption isotherm areLangmuir and Freundlich equations, the experimental results ofthis study were fitted with these two models. The equilibriumadsorption isotherms are important in determining the adsorptioncapacity of metal ions [Pb(II), Cd(II), Cr(III), Ni(II), Co(II) and Mn(II)]and diagnose the nature of adsorption onto the DNPH-�-Al2O3.

The equilibrium adsorption capacity of adsorbent was calcu-lated by the following equation:

qe = V(C0 − Ce)W

(5)

where qe is the equilibrium adsorption capacity of adsorbent inmg metal/g adsorbent, C0 is the initial concentration of the metalions in mg L−1, Ce is the equilibrium concentration of metal ions inmg L−1, V is the volume of metal ions solution in L, and W is theweight of the adsorbent in g. The equilibrium adsorption of metalion solutions by adsorbent was measured (50 mL of 1–150 mg L−1)after equilibrium time.

The most widely used Langmuir equation, which is valid for

monolayer sorption on a surface with a finite number of identicalsites, is given by [55]:

Ce

qe= Ce

qmax+ Kads

qmax(6)

A. Afkhami et al. / Journal of Hazardou

Fn

wta(bg

bf

R

wt[Ra

tiiawe

q

a

l

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TA

ig. 8. Langmuir adsorption isotherm of metal ion s for modified �-aluminaanoparticles.

here qmax is the maximum adsorption at monolayer (mg g−1), Ce ishe equilibrium concentration of metal ion, qe is the amount of iondsorbed per unit weight of adsorbent at equilibrium concentrationmg g−1) and Kads is the Langmuir constant related to the affinity ofinding sites (mg L−1) (Fig. 8). A linearized plot of Ce/qe against Ce

ives qmax and Kads.The essential characteristics of the Langmuir isotherm can also

e expressed in terms of a dimensionless constant of separationactor or equilibrium parameter, RL, which is defined as

L = 11 + bC0

(7)

here b is the Langmuir constant and C0 is the initial concentra-ion of metal ions. The RL value indicates the shape of isotherm56]. RL values between 0 and 1 indicate favorable adsorption, whileL > 1, RL = 1, and RL = 0 indicate unfavorable, linear, and irreversibledsorption isotherms.

The widely used empirical Freundlich equation is based on sorp-ion on a heterogeneous surface. The heat of adsorption decreasesn magnitude with increasing extent of adsorption. If the declinen heat of adsorption is logarithmic, it implies that adsorption sitesre distributed exponentially with respect to an adsorption energyhich differs between groups of adsorption sites. The Freundlich

quation is given as [57]:

e = KF C1/ne (8)

nd the linear form of the Freundlich isotherm is

og qe = log KF + 1/n log Ce (9)

here KF and n are Freundlich constants indicating sorption capac-ty (mg g−1) and intensity, respectively. KF and n can be determinedrom linear plot of log qe against log Ce. The calculated results of the

angmuir and Freundlich isotherm constants are given in Table 1.

The adsorption capacities for the adsorption of Pb(II),d(II), Cr(III), Ni(II), Co(II) and Mn(II) by the DNPH-�-l2O3 expressed by Langmuir coefficient qmax demonstrate

hat adsorption capacity increased in the sequence of

able 1dsorption isotherms parameters of metal ions onto DNPH-�-Al2O3.

Langmuir model

qmax (mg g−1) b KL r

Pb2+ 100.0 1.667 166.7 0.993Cd2+ 83.33 0.121 10.10 0.986Cr3+ 100.0 1.250 125.0 0.995Co2+ 41.66 0.089 3.703 0.990Ni2+ 18.18 0.078 1.426 0.985Mn2+ 6.289 0.118 0.743 0.994

s Materials 181 (2010) 836–844 841

Mn(II) < Ni(II) < Co(II) � Cd(II) < Pb(II) = Cr(III), where the differ-ent adsorption capacity is a consequence of disparity in cationsradia and interaction enthalpy values [58]. The dimensionlessparameter (RL) value, which is defined in Eq. (7) described abovecan be computed by substituting the values of b and C0 (50 mg L−1)to the equation. The RL values were 0.012 for Pb(II), 0.142 forCd(II), 0.0157 for Cr(III), 0.183 for Co(II), 0.204 for Ni(II) and0.145 for Mn(II). For the six metal ions the values of RL werebetween 0 and 1, pointing out the favorable adsorption ontothe DNPH-�-Al2O3. Freundlich coefficient KF, which representsthe adsorption capacity was found to increases in the sequenceNi(II) < Mn(II) < Co(II) < Cd(II) < Pb(II) < Cr(III), which is nearly thesame as that revealed by Langmuir coefficient qmax (Table 1).The other Freundlich coefficient “n” values fulfilled the conditionof 0 < n < 10 for favorable adsorption [14]. As can be seen fromthe isotherms table the surface of DNPH-�-Al2O3 is favorable foradsorption phenomenon.

Also, the larger the constant b for Pb(II), Cr(III) and Cd(II), thehigher is the adsorption energy, reflected by a fast increase inadsorption at low concentrations of adsorbate [59]. The data fitwell to the model with correlation coefficients (r) in the range of0.985–0.995.

In order to determine the best isotherm that could describeadsorption process of the metal ions, data analysis was performedusing linear regression that fitness of the models is often evaluatedbased on the value of the correlation coefficients (r). But, both Lang-muir and Freundlich isotherms give very high and close r values thatconclusion is not easy. Due to the resulting correlation coefficients(r) of the isotherms that were very close and high, we used Chi-square test to determine best isotherm models. Chi-square analysishas the advantage that all isotherms were compared on the sameabscissa and ordinate, could be used to determine the best fit-ted model for isotherm. The Chi-square statistic test (Eq. (10)) isbasically the sum of the squares of the differences between theexperimental data and theoretically predicted data from models.The Chi-square value is given as [60,61]:

�2 =i=N∑

i=1

(qe − qem)2

qem(10)

where qem equilibrium capacity obtained by calculated from model(mg g−1) and qe was the equilibrium capacity (mg g−1) from theexperimental data. If data from the model were similar to the exper-imental data, �2 would be a small value and vice versa. Therefore,it is necessary to also analyze the data set using the Chi-squaretest to confirm the best fit isotherm for the sorption system. Thevalues of �2 of each model were shown in Table 1. The Freundlichisotherm has the lowest �2 values for Pb2+, Cd2+, Cr3+ and Mn2+,suggesting that the Freundlich isotherm provides the best fit to the

experimental data. Therefore, adsorption of these ions preferablyfollows the heterolayer adsorption process. In the case of Co2+ andNi2+, the �2 values of Langmuir isotherm have partially lower thanthe �2 values of Freundlich isotherm. In this study, we would liketo point out that it is not appropriate to use the correlation coeffi-

Freundlich model

�2 KF n r �2

26.86 37.79 3.649 0.992 0.4895.473 11.73 2.160 0.986 2.73

16.75 40.17 3.861 0.985 0.7781.723 5.03 2.128 0.981 2.4721.677 1.95 2.037 0.926 3.5020.382 2.21 4.878 0.987 0.034

842 A. Afkhami et al. / Journal of Hazardous Materials 181 (2010) 836–844

Table 2Effect of different eluants on desorption recovery (%) for metal ions adsorbed on DNPH-�-Al2O3 (N = 3).

Eluant Recovery (%)

Pb2+ Cd2+ Cr3+ Co2+ Ni2+ Mn2+

1 mol L−1 HCl 84 ± 1 81 ± 1 79 ± 2 67 ± 3 84 ± 1 75 ± 11 mol L−1 HNO3 92 ± 2 93 ± 1 94 ± 1 89 ± 1 92 ± 1 90 ± 10.1 M EDTA 12 ± 3 8 ± 2 11 ± 2 18 ± 1 21 ± 2 24 ± 24.5 mL of 1 mol L−1 HNO3 + 1.5 mL of methanol 99 ± 1 98 ± 1 99 ± 1 97 ± 1 98 ± 2 99 ± 16 mL of 1 mol L−1 HNO3 + 1.5 mL of methanol 99 ± 1 96 ± 1 97 ± 1 96 ± 1 95 ± 2 97 ± 2

Table 3Removal (%) after treating 50 mL of water with 0.06 mg DNPH-�-Al2O3.

Sample Added metal ions (mg L−1)

Pb Cr Cd Co Ni Mn

Added Removal, % Added Removal, % Added Removal, % Added Removal, % Added Removal, % Added Removal, %

Tap water (1) 1.00 100 1.00 99.3 1.00 98.5 1.00 98.0 1.00 97.7 1.00 97.5Tap water (2) 20.0 99.2 20.0 98.7 20.0 89.5 20.0 69.0 20.0 38.5 20.0 23.4

.4

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Tap water (3) 50.0 97.0 50.0 97.4 50.0 80

ercentage removal values are based on triplicate analysis.

ient (r) of linear regression analysis for comparing the best-fittingf Freundlich and Langmuir isotherms. Chi-square analysis woulde a better criterion for avoiding such errors.

.6. Desorption experiments

Desorption of metal ions from adsorbent and re-generation ofhe adsorbent is an important issue in view of re-usability of thedsorbent. For desorption studies, metal-adsorbed modified alu-ina nanoparticles were first washed by ultrapure water to remove

he unadsorbed metals loosely attached to the vial and adsorbent. Inrder to estimate the recovery of Mn(II), Co(II), Ni(II), Cd(II), Cr(III)nd Pb(II) from modified alumina, desorption experiments withifferent reagents (1 M HCl, 1 M HNO3 and 0.1 M EDTA solutions,ixture of methanol:1 M HNO3 (1:3) and 1 M HNO3:EDTA (4:1))ere performed. After adsorption of metal ions the adsorbent was

eparated by centrifugation (at 2500 rpm for 5 min) and the residualolid was dried at 40 ◦C. Then the obtained solid-phase mass wasdded to 6 mL of the effluents. Samples were collected after 5, 10,0, 60 and 90 min contact times with the effluent to evaluate metalecovery by means of an atomic absorption spectrophotometer. Theetal recovery was calculated by the following equation:

ecovery (%) = amount of desorbed metalsamount of adsorbed metals

× 100 (11)

The results showed that mixture of methanol with 1 M nitriccid (1.5 + 4.5 mL) is effective as a back-extractant and can besed for the quantitative recovery of the ion metals (Table 2). Incidic solution, the N donor, which serves as an electron donoran be protonated, resulting in a positive charge that repels metalons. Nevertheless, this can be beneficial to the back-extractiontep, because lowering solution pH facilitates decomplexation ofhe metal ion from the sorbent. Desorption rate was found to beery rapid as almost 100% desorption completed within 5–10 minor all metal ions. To test the re-usability of the adsorbent, thisdsorption–desorption cycle was repeated three times. Regener-ted adsorbent was reused for three consecutive adsorption cyclesith 50 mg L−1 metal ions solution under similar condition as

escribed above. The results show that the efficiencies of the recy-led adsorbent for removing cations are nearly the same as thoseor the fresh ones even after three times recycling.

Decrease in adsorption capacity in subsequent cycles may beue to the displacement of some adsorbed modifier molecules from

50.0 55.7 50.0 34.0 50.0 11.4

nano-alumina surface while treated with acid and alcohol solu-tions. Still, complete displacement of DNPH was not occurred asconsiderable adsorption efficiency observed in next cycles. Reason-able adsorption capacity of this adsorbent in subsequent adsorptioncycles made it a good choice for multi-cyclic use.

3.7. Application for real samples

Water samples were collected from the laboratory’s tap waterin Hamadan, I.R. Iran. The samples were filtered before analysisthrough a 0.45 �m membrane filter and stored in polyethylene con-tainer for subsequent usage after acidified. In first part, real watersspiked with 1.00 mg L−1 each of the target metals were used to fur-ther evaluate the effects of real matrices on the removal efficiencyof heavy metals under optimized conditions. The second and thirdsamples were spiked with 20.0 and 50 mg L−1 each heavy-metalions and removed by adsorbent, respectively (Table 3). The con-centrations of these spiked heavy-metal ions in the three sampleswere determined by flame atomic absorption analysis as an initialstep. The results show that the removals of the target heavy metalswere not affected by the presence of excess amounts of Ca and Mgin real waters.

4. Conclusions

The present study proves the capability and effectivenessof newly modified alumina nanoparticles – immobilized-DNPHadsorbent for heavy metals removal from various water samples.Sorption process was affected by experimental parameters suchas pH, contact time, and adsorbent dosage. Using Chi-square test,it was found that sorption isotherms were better described byFreundlich model for Mn(II), Pb(II), Cr(III) and Cd(II) ions and byLangmuir model for Ni(II) and Co(II) ions. The maximum adsorp-tion capacity values of Cr(III), Cd(II) and Pb(II) ions in a mixture ofsix ion metals with modified alumina nanoparticles (from Langmuirequation) were 100.0, 83.33 and 100.0 mg g−1, respectively.

The adsorption–desorption cycle results open an important areafor future applications of the modified alumina-based materials for

ultratrace preconcentration of some heavy-metal ions. Studies onbatch adsorption using real samples in order to remove the Pb(II),Cd(II), Cr(III), Co(II), Ni(II) and Mn(II) ions indicated that the adsor-bent has a good potential to remove the heavy-metal ions fromwastewater samples in practical applications.

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cknowledgements

The authors gratefully acknowledge the financial and technicalupport provided by the Bu-Ali Sina University Research Councilnd Center of Excellence in Development of Chemical MethodsCEDCM).

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