ORIGINAL PAPER

Retention studies of chromium (VI) from aqueous solutionon the surface of a novel carbonaceous material

Sajjad Hussain & Saima Gul & Sabir Khan &

Habib ur Rehman

Received: 5 January 2012 /Accepted: 31 October 2012# Saudi Society for Geosciences 2012

Abstract In the present study, the retention capacity of car-bonaceous material obtained from the diesel engine exhaustmufflers for Cr(VI) removal has been investigated. The phys-icochemical properties such as density, pH of aqueous slurry,pH at point of zero charge, ash content, moisture content,volatile matter, surface area, scanning electron microscopyand electron dispersive spectroscopy of the carbonaceous ma-terial were determined. The capacity of adsorbent for removalof Cr(VI) from aqueous solution was observed under differentexperimental condition like contact time, initial concentrationof metal ions, pH and temperatures on the adsorption capacityof the adsorbent. Maximum adsorption of Cr(VI) ions wasfound at low pH. The adsorption process was found to followsecond-order kinetics. The rate constant was evaluated at dif-ferent temperatures along with other thermodynamic parame-ters like activation energy, Gibbs free energy change, enthalpychange and entropy change. Both Langmuir and Freundlichisotherms were used to describe the adsorption equilibrium ofcarbonaceous material at different temperatures. Langmuirisotherm shows better fit than Freundlich isotherm at givenconditions. The result shows that low-cost carbonaceous ma-terial from diesel engine exhaust mufflers can be efficientlyused for wastewater treatment containing Cr(VI) ions.

Keywords Carbonaceous material . Chromium .

Adsorption . Kinetics . Thermodynamic

Introduction

Heavy metals removal from industrial and drinking water isalways in focus because of their diverse health effects(Hayes 1982; Dupont and Guillon 2003; Cummings et al.2007; Verma et al. 2006; Patterson 1985; Li et al. 2007).Chromium, among the top 16th toxic pollutants (Gardea-Torresdey et al. 2000; ATSDR 2000), is of greater concernbecause of its wide utility in different industrial processessuch as tanning, textile manufacturing, welding, electroplat-ing, manufacturing of safety matches, metal finishing, pro-duction of pigments as well as wood preservation andcorrosion inhibition (Kimbrough et al. 1999). The releaseof effluents into natural waters from these anthropogenicsources generally contains more than permissible amountsof Cr(III) and Cr(IV), which is posing greater health prob-lems. Cr(IV) is a strong oxidizing agent, water soluble,easily enters in to the living cells to form complexes and ismuch more toxic than Cr(III) (Park and Jung 2001). Itcauses several disorders such as pulmonary, pancreatic andstomach cancer, gastrointestinal effects, skin and mucousmembrane irritation, dermatitis, chromosomal aberrations,sister chromatids exchanges and DNA damage in animals(Costa 1997), while in plants, it can affect seed germinationand growth Shanker et al. 2005).

According to WHO guidelines for drinking water quality(World Health Organization 2006), the acceptable limit of Cr(IV) in drinking water is 0.05 mgL−1. The presence of Cr(IV)beyond this limit can cause health problems mentioned above.Consequently, a large number of methods have been developedfor the removal of chromium from industrial wastewater whichinclude chemical precipitation, electrodialysis (Changshenget al. 2004), membrane process (Larisa and Maria 2008), ionexchange (Cavaco et al. 2007), liquid extraction (Sahu et al.2008), reverse osmosis, nanofiltratin, ultrafiltraion (Jaekyung etal. 2009) and emulsion pertraction technology (Ortiz et al.2003). However, most of these methods require high capital

S. Hussain (*) : S. GulInstituto de Química de São Carlos, Universidade de São Paulo,CP 780, CEP 13560-970 São Carlos, SP, Brazile-mail: sajjad@iqsc.usp.br

S. KhanInstituto de Química, Universidade Estadual de Campinas,CP 6154, CEP 13083-970 Campinas, SP, Brazil

H. ur RehmanInstitute of Chemical Sciences, University of Peshawar,Peshawar 25120, Pakistan

Arab J GeosciDOI 10.1007/s12517-012-0745-9

and operational cost, higher reagents consumption besides caus-ing problems such as incomplete removal of metal ions anddisposal of the resulting sludge.

However, adsorption is one of the most promising, con-venient, efficient and economical techniques. Its cost canfurther be reduced using low-cost, readily available non-conventional materials. A few examples of these low-costmaterials include agriculture wastes and biomaterials suchas maize bran and maize tassel (Zvinowanda et al. 2009;Singh et al. 2006), activated carbon from cornelian cherry,apricot stone and almond shell, hazelnut shell (Demirbas etal. 2004; Erol and Tuerkan 2008), sunflower stalks (Sun andShi 1998), coconut shell, waste tea, rice straw, tree leaves,peanut and walnut husks (Karthikeyan et al. 2005), rawbagasse and fly ash (Rao et al. 2002), biomaterials(Donghee et al. 2011), dried powder marine algae (Leeet al. 2000), activated tamarind seed (Babu and Gupta2008), sawdust and used tyre carbon (Hamadi et al.2001), biogas residual slurry(Namasivayam and Yamuna1995), activated lignin (Nassima and Moussa 2010),surface-modified coconut shell charcoal and commercialactivated carbon (Babel and Kurniawan 2004).

The present study introduces carbonaceous material fromdiesel engine exhaust mufflers as an adsorbent for Cr(IV)removal, which has the advantage of high separation efficien-cy, cost-effectiveness, less reagent consumption, simple prac-tical steps and easy availability; moreover, it may reduce theeconomic burden and will be useful in the provision of puredrinking water for public as well as for agricultural andindustrial use.

Therefore, an attempt was made to study the adsorptionof Cr(VI) from aqueous solution onto the carbonaceousmaterial obtained from diesel engine exhaust mufflers. Var-ious influential parameters such as adsorbent concentration;pH, contact time and initial metal ion concentration wereinvestigated. The adsorption behaviour of carbonaceousmaterial was examined by using adsorption isotherm modelsand thermodynamic parameters. The kinetics and equilibri-um parameters were also calculated to determine rate con-stants and activation energy of the adsorption process.

Experimental

Material development and physical characterization

The carbonaceous material used in this study was obtainedfrom diesel engines’ exhaust muffler of buses and trucksfrom Peshawar region (Pakistan). The carbonaceous mate-rial was washed thoroughly with doubly distilled water toremove dust and other foreign matter and then dried at 60 °Cfor a period of 5–6 h. Dried carbonaceous material was then

soaked in 0.1 M H2SO4 for 24 h, and after that, it waswashed again with doubly distilled water. The adsorbentwas soaked in 0.1 M NaOH for a period of 4–5 h to removetraces of acid and then dried in an oven at 80 °C for 1 h. Thesample was ground into fine particles by means of agatemortar and passed through 200- and 150-μm sieves. Thesample was transferred into a large-sized China dish andwas activated by placing in a vacuum oven at 200 °C for 2 h.After activation, the sample was stored in a vacuum desic-cator for further study.

Equipment

The pH was measured using pH meter model Hanna Instru-ments® HAN-HI8314. The pH meter was calibrated usingstandard buffer solutions. A spectrophotometer model SP-300 Microprocessor Controlled Spectrophotometer OPTI-MA INC, Japan was used for the absorbance measurements.Scanning electron microscope model JSM 5910, JEOL-JAPAN was used for the analysis of surface micrograph.EDX was done using energy-dispersive X-ray analyzerINCA-200, Oxford Instruments, UK. The surface area andpore size of the adsorbent were determined by BET-N2

adsorption method using Surface Area Analyzer NOVA2200e Quanta Chrome, USA. The samples were degassedat 100 °C for 1 h for the surface area analysis.

Preparation of Cr(VI) stock and working solutions

All the reagents used were of analytical grade. Potassiumchromate, hydrochloric acid, sodium hydroxide, 1, 5-diphenylcarbazide, etc. were purchased from Merck (Pvt.)Ltd. Pakistan.

A stock solution of Cr(VI) (1,000 mgL−1) was preparedby dissolving 2.8286 g of potassium dichromate (K2Cr2O7

molar mass, 294.185 gmol−1) in deionized water. The de-sired concentration of working solutions (5–400 mgL−1)was prepared by proper dilution from the stock solution.The pH was adjusted using NaOH and HCl solutions.

Batch adsorption experiments

A series of 100-mL stoppered conical flasks were taken,and each one was filled with 30 mL of Cr(IV) solutionof varying concentrations. A known amount of preparedadsorbent was added to each conical flask and wasshaken in a mechanical agitator with a speed of 120 rpm atthe temperature range of 10–40 °C. After different timeintervals, the samples were taken out and filtered throughthe Whatman no. 41 filter paper. The equilibrium concentra-tion of Cr(VI) in the sample was determined by reacting with1,5-diphenyl carbazide, and the absorption was measured by

Arab J Geosci

SP-300 Microprocessor Controlled spectrophotometer at540 nm.

The amount adsorbed “q” (milligrams per gram) wascalculated using the following relation.

q ¼ Ci � Ce � V

wð1Þ

Where Ci is the initial concentration (milligrams perliter), Ce is the equilibrium concentration (milligrams perlitre), V is the volume of solution in milliliters and w is theamount of adsorbent in grams. The experimental parametersvaried were the Cr(VI) initial concentration (5–400 mgL−1)and the temperature (10, 20, 30 and 40 °C). All the experi-ments were performed in triplicate, and the mean values arereported in this paper.

Effect of solution initial pH

To study the effect of solution pH, 50 mL of Cr(VI) solutionof initial concentration 100 mgL−1 at different pH values(2.0–12.0) was agitated with 0.5 g of carbonaceous materialat 25 °C. The pH was adjusted using 0.1 M NaOH or 0.1 MHCl and measured using a digital pH meter (describedabove).

Point of zero charge determination

The pH at the point of zero charge (pHPZC) of carbonaceousmaterials was determined by the solid addition method(Lataye et al. 2006), 50 mL NaCl solution of 0.1 M concen-tration was transferred to a series of 100-mL Erlenmeyerflasks. The pHi (initial pH) values of the solutions wereadjusted from pH 2 to 12 by adding 0.1 M HCl or 0.1 MNaOH. And 0.5 g of carbonaceous material was added to eachflask, and was agitated in a thermostatic water bath shaker at25 °C for 24 h. The final pH of the (pHf) was determined, andthe difference between pHi and pHf values (ΔpH0pHf−pHi)was plotted versus the pHi. The pHPZC was determined fromthe point of intersection of the curve at which ΔpH00.

Result and discussion

Physicochemical characteristics of the adsorbent material

Physicochemical characteristics and proximate analysissuch as ash content, moisture content and volatile matterwere determined by ASTM D3173 method (Berkowitz1979), and the results are tabulated in Table 1. It is notedfrom Table 1. that the surface area of the adsorbent is quitegreater than the reported values of neem bark, saw dust andfly ash (Bhattacharya et al. 2008). So it is expected that it

will be an effective adsorbent for the removal of Cr(VI)from aqueous solution. The low moisture content, ash con-tent, volatile matter contents and high percentage of carbonare the characteristics of an effective adsorbent. The pHPZC

(point of zero charge) of carbonaceous material is around3.5, the surface of adsorbent should be positive at pH<3.7as shown in Fig. 7. Therefore, a positively charged surfacecan strongly bind negatively charged Cr(VI) species at lowpH. The EDX and SEM studies were carried out for theadsorbent material, and the spectra are given in Figs. 1, 2, 3,4 and 5. Figure 1 indicates that this carbonaceous material iscomposed of 79.97 % carbon, 19.09 % oxygen, 0.53 %sulphur and 0.41 % silicon. It is obvious that the adsorbentcontains a large amount of carbon which is mainly respon-sible for adsorption. SEM images at ×2,000 and ×4,000magnification obtained before and after adsorption to deter-mine the surface morphology, as shown in Figs. 2, 3, 4 and5. Figures 2 and 3 show an irregular structure with a largenumber of small pores available for adsorption. But theSEM micrographs in Figs. 3 and 4 are indicating that thesepores become filled with the adsorbate after adsorption.

Point of zero charge

The pHPZC of an adsorbent is one of the important character-istics that determine the pH at which the adsorbent surface is

Table 1 Physicochemical parameters of carbonaceous material

S. no. Parameters Values

1 pH (slurry) 4.87

2 Bulk density (g/cm3) 0.76

3 Moisture (%) 6.94

4 pHPZC 3.5

5 Ash content (%) 6.02

6 Volatile matter (%) 8.79

8 Surface area (BET), m2/g 36.97

Fig. 1 EDX spectra of the adsorbent

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electrically neutral, and thus, the acidic or basic functionalgroups no longer contribute to the pH of the solution (Wan etal. 2008). Figures 6 and 7 show a plot of the ΔpH of carbo-naceousmaterial versus pHi. The values of the ΔpHi decreasedas the pH was increased. At pH >3.7, it was observed that thecarbonaceous material exhibited negative ΔpH values, and thevalue of pHPZC was around 3.7. The surface of adsorbent ispositive at pH <3.7. Therefore, the positively charged surfacecan strongly bind negatively charged chromate ions at low pH,and the negative surface charge facilitated electrostatic repul-sion of the chromate ions.

Effect of pH

The existence and stability of Cr(VI) in aqueous solutiondepend upon the pH of the system. The dominant forms of

Cr(VI) at low pH are HCrO4−, Cr2O7

2−, Cr3O102−

and Cr4O132−.

When the pH is increased, the concentration of HCrO4−

will shift to Cr2O72− and other forms as CrO4

−. The uptake ofCr(VI) as a function of hydrogen ion concentration was ex-amined over a pH range of 2–12 as shown in Fig. 6. From thefigure, it is observed that the adsorption of Cr(VI) increasesfrom 15.24 to 80.36 % as pH decreases from 12 to 2. Maxi-mum adsorption at pH 2 indicates that it is HCrO4

−, which is apredominant species at this pH range and adsorbed preferen-tially on the adsorbents. Higher adsorption at low solution pHis due to the existence of a large quantity of hydronium ions(H+), which make the surface of carbonaceous material morepositive, thereby facilitating the diffusion of hydrogenchra-mate ions (HCrO4

−) and their subsequent adsorption. Thesecond reason for the high adsorption of Cr(VI) onto carbo-

Fig. 2 SEM spectra of the adsorbent before adsorption of Cr(VI) at×2,000 magnification

Fig. 3 SEM spectra of the adsorbent before adsorption of Cr(VI) at×4,000 magnification

Fig. 4 SEM spectra of adsorbent after adsorption of Cr(VI) at ×2,000magnification

Fig. 5 SEM spectra of adsorbent after adsorption of Cr(VI) at ×4,000magnification

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naceous material could be the reduction of Cr(VI) to Cr(III)ions in the acidic medium. Reduction of Cr(VI) to Cr (III) ionsoccurs at low pH, but the amounts of total Cr and Cr(VI) atlow pH are approximately equivalent which implies that thepresence of Cr(III) in the final solution is insignificant (Guptaand Babu 2009) and thus can be easily adsorbed by thepositively charged ions present on the highly porous adsorbentsurface. So the amount of Cr(III) was not determined in thefinal solution in this study. The higher percentage removal ofCr(VI) under acidic conditions is in agreement with the pointof zero charge determination, although the optimum pH wasfound to be 2.5 for high removal of Cr(VI).

Effect of contact time

The effect of contact time on the rate of uptake of Cr(VI)(100 mgL−1) at different temperatures is shown in Fig. 8. It

is clear from the figure that increase in contact time en-hanced significantly the percentage removal of Cr(VI) fromthe solution and attain equilibrium within 140 min. It is alsoevident that the amount of Cr(VI) adsorbed by adsorbentincreases rapidly in the initial 60 min and subsequentlyattains a constant value when adsorption equilibrium wasestablished. The adsorption process takes place in twostages, a relatively fast one followed by a slower one.

Kinetic study of adsorption

Kinetics of adsorption describes the rate of uptake of Cr(VI)onto the carbonaceous material, and this controls the equi-librium time. The pseudo first-order (Lagergren 1898) andpseudo second-order equation (Ho et al. 2000) were appliedto the experimental adsorption data. The kinetic data werefitted to these equations. The most popular forms of theseequations can be expressed as

1 2 3 4 5 6 7 8 9 10 11 12 130

10

20

30

40

50

60

70

80

90

100%

Ads

orpt

ion

pH

Fig. 6 pH study of Cr(VI) adsorption on carbonaceous material 293 K

1 2 3 4 5 6 7 8 9 10 11 12

-5

-4

-3

-2

-1

0

1

ΔpH

pHi

Fig. 7 Determination of point of zero charge (PZC)

0

1

2

3

4

5

6

0 50 100 150 200 250 300

q (

mg/

g)

t (mint)

283 K

293 K

303 K

313 K

Fig. 8 Effect of contact time on adsorption of Cr(VI) at differenttemperatures

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.60 50 100 150 200 250

log(

qe-q

t)

t, (min)

283 K

293 K

303 K

313 K

Fig. 9 Lagergren plot for the adsorption Cr(IV) at differenttemperatures

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log qe � qtð Þ ¼ �k1t

2:303þ log qe ð2Þ

Where qt is the adsorption time t (minutes; milligrams pergram), k1 the rate constant of the equation (l/min); qe is theamount of adsorption equilibrium (milligrams per gram).The adsorption rate constant, k1, can be determined experi-mentally by plotting of log(qe−qt) against t.

t

q1¼ 1

k2qe2þ 1

qeð3Þ

Where k2 is the rate constant of the second-order equation(grams per milligram per minute), qt the adsorption time t(minutes; milligrams per gram) and qe is the amount ofadsorption equilibrium (milligrams per gram).

Figure 9 shows the plot of log(qe−qt) versus t (minutes),and Fig. 10 shows the second order for the adsorptionprocess. The amount of adsorption equilibrium ‘qe’, rateconstant ‘k’ and correlation coefficient “R2” for Lagergrenfirst order as well as for second order are given in Table 2.The validity of the kinetic models is tested by the magnitudeof the correlation coefficient R2 which is given in Table 2. Itis important to note that the correlation coefficient for firstorder is less than the second-order correlation coefficient atevery experimental temperature. Therefore, the first-order

model is not suitable for modeling the adsorption of Cr(VI)on this carbonaceous material. Table 2 shows the pseudo-second-order rate constants and correlation coefficients forremoval of Cr(VI) from aqueous solutions. These valuesindicate a better fit of pseudo-second-order model with theexperimental data as compared to the Lagergren first-ordermodel. The results of pseudo-second-order kinetics found inthis study are also supported by many investigators (Rao etal. 2002; Bhattacharya et al. 2008)

Activation energy

The activation energy, Ea, for adsorption of Cr(VI) wascalculated by using the following Arrhenius equation

In k ¼ � Ea

RTþ constant ð4Þ

Where Ea is the activation energy, R is gas constant and Tis the absolute temperature.

The value of activation energy was determined from theslope of the plot of ln k versus 1/T as shown in Fig. 11. Thevalue of Ea was found to be 7.291 kJmol−1. From thecalculated value of Ea, it can be suggested that the processis physical adsorption.

Effect of initial Cr(VI) concentration on adsorption process

Adsorption process is significantly influenced by the initialconcentration of Cr(VI) in aqueous solution. The investiga-tion was done by varying the concentration from 25 to200 mgL−1 at constant pH 2.5 while maintaining dose of10 gL−1 and at temperatures range 283–313 K. The resultsare demonstrated in Fig. 12. It is clear that the amount of Cr(VI) adsorbed increases as the concentration increases.

At low initial concentration, most of the Cr(VI) adsorbedbecause of the availability of a large number of adsorptionsites on the surface of the carbonaceous material, but whenthe concentration was increased, the adsorption processdecreases slowly due to the increase in the number of ions.Also, the adsorption efficiency is decreased steadily be-cause the higher energy sites are saturated, and adsorp-tion begins at lower energy sites, and as a result, there is

0

10

20

30

40

50

60

0 50 100 150 200 250 300

t/qt

t, min

283 K

293 K

303 K

313 K

Fig. 10 Second-order plot for the adsorption of Cr(VI) at differenttemperatures

Table 2 Rate adsorption con-stants for two kinetic models atdifferent temperatures

T (K) First order Second order

qe (mg/g) k1 (×10−2min−1) R2 qe (mg/g) k1 (×10

−2min−1) R2

283 4.761 1.19 0.9700 4.761 1.19 0.9971

293 5.0505 1.24 0.9781 5.0505 1.76 0.9970

303 5.347 1.38 0.9608 5.347 1.92 0.9972

313 5.434 1.47 0.9906 5.434 2.05 0.9971

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a decrease in the adsorption efficiency. Maximum removal of80 % was achieved at a concentration of 100 mgL−1 at thetemperature 40 K.

Effect of temperature

Temperature has an important effect on the process of ad-sorption. The Cr(VI) adsorption on the carbonaceous mate-rial was studied as a function of temperature range 283–313 K. The equilibrium adsorption capacity of the carbona-ceous material was found to be about 4.6 mgg−1 at 283 Kwhich was increased up to 5.4 mgg−1 at 313 K. It is clearfrom Figs. 8 and 12 that the amount of Cr(VI) ions adsorbedincreases with the rise in temperature, which means that theadsorption of Cr(VI) ions on the surface of carbonaceousmaterial from aqueous solution is endothermic in nature.

The adsorption isotherms

Adsorption isotherm is very important in designing thenature of an adsorption system. Adsorption isotherm is

characterized by certain constants that express the sur-face properties and affinity of the adsorbent towards Cr(VI). The present data were evaluated by applying theLangmuir (Langmuir 1918) and Freundlich isotherms(Freundlich 1906).

Langmuir isotherm

The linear form of Langmuir adsorption isotherm is shownby the following equation

Ce

qt¼ 1

K1Xmþ Ce

Xmð5Þ

Where Ce is the equilibrium concentration (milligramsper liter), qt is the amount (milligrams per gram) of Cr(VI)adsorbed, Xm (milligrams per gram) is adsorption capacity(amount of adsorbate adsorbed per unit mass of the adsor-bent to complete the monolayer coverage) and K1 (liters permilligram) are Langmuir constants (which is the bindingenergy constant of adsorption or energy of adsorption),representing the adsorption capacity (amount of adsorbateadsorbed per unit mass of the adsorbent to complete mono-layer coverage) and energy of adsorption, respectively.Langmuir constants were calculated from the slopes andintercepts of the linear plots of the Ce/qt against Ce whichis shown in Fig. 13 and listed in Table 3, which indicate theapplicability of Langmuir adsorption isotherm, consequent-ly the formation of monolayer surface of the adsorbate onthe surface of the adsorbent. For each experimental temper-ature, reasonable high values of correlation coefficient R2

were obtained, which indicates a good agreement betweenexperimental value and isotherm parameters and also con-firms monolayer coverage.

RL is one of the essential characteristics of Langmuirisotherm which is a dimensionless separation factor definedby Weber and Chakkravorti (1974) and described as

y = -0.6548x - 2.1306R² = 0.9697

-4.5

-4.45

-4.4

-4.35

-4.3

-4.25

-4.23.15 3.25 3.35 3.45 3.55 3.65

lnk

1/T x10-3

Fig. 11 Arrhenius plot for the adsorption of Cr(VI) carbonaceousmaterial

0

1

2

3

4

5

6

7

8

9

20 40 60 80 100 120 140 160 180 200 220

q (m

g/g)

Ci (mg/L)

283 K

293 K

303 K

313 K

Fig. 12 Effect of initial concentration of Cr(VI) on the adsorption atdifferent temperatures

0

2

4

6

8

10

12

14

0 20 40 60 80 100

Ce/

qt

Ce (mg/L)

283 K

293 K

303 K

313 K

Fig. 13 Langmuir isotherm for the adsorption of Cr(VI) at differenttemperatures

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RL ¼ 1

1þ K Coð6Þ

The magnitude of RL shows the feasibility of adsorptionprocess; values lie between 0 and 1 which indicates favour-able adsorption. The RL value for the adsorption of Cr(VI)at all temperatures at 25 mg/L initial concentration has beengiven in Table 3. The RL values lying between 0 and 1signify favourable adsorption.

Freundlich isotherm

The Freundlich isotherm is an empirical equation employedto describe a heterogeneous system. The data were alsofitted to the Freundlich adsorption isotherm which describesequilibrium on heterogeneous surface. The Freundlich iso-therm is expressed by the following equation

logX

m¼ log k þ 1

nlog Ce ð7Þ

Where k (micromoles per gram) and n (grams per liter)are Freundlich constants, indicating the adsorption capacityand adsorption intensity, respectively. The constants K and nwere calculated from slope and intercept of log X/m versuslog Ce at different temperatures as shown in Fig. 14 andtabulated in Table 3. The values of n lie between 1 and 10,which represent favourable adsorption (Calace et al. 2002).

It also indicates the formation of relatively stronger bondbetween adsorbate and adsorbent as temperature rises andthus a favourable adsorption. The value of correlation coef-ficient R2 for Langmuir isotherm is greater than that of theR2 of Freundlich isotherm; it means that Langmuir isothermis more suitable for the adsorption of Cr(VI) on carbona-ceous material.

Thermodynamics of adsorption

Various thermodynamics parameters such as Gibb’s freeenergy change ΔGo, entropy change ΔSo and enthalpychange ΔHo of Cr(VI) adsorption were calculated usingthe following relations

InK1 ¼ �ΔH

RTþ S�

Rð8Þ

ΔG� ¼ ΔH� � TΔS� ð9ÞThe value of ΔHo was calculated from the slope, and the

value of ΔSo was calculated from intercept of the linearvariation of ln K1 versus 1/T shown in Fig. 15, and thevalues of ΔGo were calculated by using Eq. 8.

The values ofΔSo,ΔGo andΔHo are recorded in Table 4.The negative value ofΔGo obtained at various temperaturesshow the spontaneity and feasibility of adsorption process.More negative values of ΔGo predict that an increase in

Table 3 Langmuir and Freund-lich constants for the adsorptionof Cr(VI) at differenttemperatures

T (K) Langmuir constant Freundlich constant

Xm1 (mgg−1) K1 (Lmg−1) RL R2 n (gL−1) K R2

283 11.494 0.022 0.645 0.9637 2.808 1.629 0.9165

293 10.526 0.041 0.493 0.9696 2.865 1.749 0.9068

303 10.683 0.077 0.341 0.9774 2.906 1.874 0.9161

131 10.288 0.411 0.088 0.9806 3,039 2.046 0.9189

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1

log

x/m

log Ce

283 K

293 K

303 K

313 K

Fig. 14 Freundlich isotherm for the adsorption of Cr(VI) at differenttemperatures

y = -8.2811x + 25.211R² = 0.9159

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

03.15 3.2 3.25 3.3 3.35 3.4 3.45 3.5 3.55

ln K

1/T x 10-3

Fig. 15 Plot of ln K vs 1/T×10−3

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temperature favoured adsorption process. The positive valueof ΔHo shows that the process of adsorption is endothermicin nature. It also shows that adsorption is physical in natureinvolving weak forces of attraction, which means that theprocess is energetically stable. The positive value of ΔSo

suggests the increased randomness at the solid–solutioninterface, and it may be due to some structural changes inboth adsorbate and adsorbent during the adsorption.

Conclusions

Batch adsorption studies of Cr(VI) ions on carbonaceousmaterial obtained from diesel engine exhaust mufflerindicated that adsorption equilibrium was attained within140 min. Initially, adsorption was very fast, but itbecomes constant after equilibrium. Lagergren first-orderand pseudo-second-order kinetics models were appliedfor adsorption of Cr(VI), and it was found that that theadsorption process follows second-order rate kinetics.Both Langmuir and Freundlich adsorption isotherm mod-els have been found to fit well with the experimentaldata. Temperature, pH and initial metal ion concentrationhave important influence on the adsorption process; therate of adsorption was found to increase with a rise intemperature. The negative values of ΔGo obtained atvarious temperatures show the spontaneous nature ofthe adsorption process. The values obtained for ΔGo

are nearly constant, showing that there is no effect oftemperature on the free energy of adsorption. The posi-tive value of ΔHo shows that the process of adsorptionis endothermic in nature. The positive value of entropysuggests an increase in randomness at the solid–solution in-terface which may be due to some structural changes in bothadsorbate and adsorbent during the adsorption. Thus, theresults show that the carbonaceous material obtained fromdiesel engine exhaust mufflers can be effectively applied toreduce Cr(VI) concentration from 100 to 20 mgL−1. Theadsorbent may be viewed as a useful material while consider-ing the economic aspects of wastewater treatment.

Acknowledgments The authors gratefully acknowledge the cooper-ation of Lab technician of Centralized Resource Laboratory (CRL)

Department of Physics, University of Peshawar, Pakistan for the anal-ysis of the carbonaceous material.

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Table 4 Thermodynamic parameters for the adsorption of Cr(VI) at various temperatures

T (K) 1/T×10−3 (Kelvin−1) ln K1 ΔHo (kJmol−1) ΔSo (kJmol−1K−1) ΔGo (kJmol−1) R2

283 3.533 −3.816 68.84 209.60 −59.24 0.9159293 3.413 −3.194 −61.34

303 3.300 −2.563 −63.44

313 3.195 −0.889 −6553

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