Removal of chromium (VI) from aqueous solution using walnut hull

Journal of Environmental Management 90 (2009) 721e729www.elsevier.com/locate/jenvman

Removal of chromium (VI) from aqueous solution using walnut hull

Xue Song Wang*, Zhi Zhong Li, Sheng Rong Tao

Department of Chemical Engineering, Huaihai Institute of Technology, Lianyungang, Jiangsu 222005, China

Received 5 June 2007; received in revised form 30 October 2007; accepted 12 January 2008

Available online 17 March 2008

Abstract

In this study, removal of chromium (VI) from aqueous solution by walnut hull (a local low-cost adsorbent) was studied. The extent of ad-sorption was investigated as a function of solution pH, contact time, adsorbent and adsorbate concentration, reaction temperature and supportingelectrolyte (sodium chloride). The Cr (VI) removal was pH-dependent, reaching a maximum (97.3%) at pH 1.0. The kinetic experimental datawere fitted to the first-order, modified Freundlich, intraparticle diffusion and Elovich models and the corresponding parameters were obtained. A102.78 kJ/mol Ea (activation energy) for the reaction of chromium (VI) adsorption onto walnut indicated that the rate-limiting step in this casemight be a chemically controlled process. Both the Langmuir and Freundlich isotherms were suitable for describing the biosorption of chromium(VI) onto walnut hull. The uptake of chromium (VI) per weight of adsorbent increased with increasing initial chromium (VI) concentration up to240e480 mg/L, and decreased sharply with increasing adsorbent concentration ranging from 1.0 to 5.0 g/L. An increase in sodium chloride (assupporting electrolyte) concentration was found to induce a negative effect while an increase in temperature was found to give rise to a positiveeffect on the chromium (VI) adsorption process. Compared to the various other adsorbents reported in the literature, the walnut hull in this studyshows very good promise for practical applicability.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Biosorption; Walnut hull; Chromium (VI); Kinetics; Isotherm; Thermodynamics

1. Introduction

Water pollution by heavy metals is an important economicand environmental issue in numerous parts of the world(Kohler et al., 2007). Among these heavy metals, chromium(Cr) is a common contaminant in surface water and groundwa-ter resulting from numerous industrial activities such as thepreservation of wood, textile dyeing, leather tanning, electro-plating and metal finishing (Kim et al., 2002; Donmez andAksu, 2002). The element exists mainly in the Cr (III) andCr (VI) valence states, although Cr (0), Cr (II), and Cr (V)have also been observed. Both of the common Cr (VI) anions,chromate (CrO4

2�) and dichromate (Cr2O72�), are strong

oxidants, and chromate is a known carcinogen and a suspectedmutagen and teratogen. By contrast, Cr (III) toxicity is negli-gible because it often forms insoluble hydroxides at circum-

* Corresponding author. Tel.: þ86 518 85895408; fax: þ86 518 85895409.

E-mail address: [email protected] (X.S. Wang).

0301-4797/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jenvman.2008.01.011

neutral pH (Cummings et al., 2007). The US EPA requires0.05 and 0.1 mg/L of Cr (VI) in drinking water and inland sur-face waters, respectively. Accordingly, chromium containingwastewaters must be treated to lower the Cr (VI) to allowablelimits before discharging into the environment. Conventionalmethods utilized to remove the Cr (VI) from industrial waste-waters include reduction followed by chemical precipitation(Ozer et al., 1997), activated carbon adsorption (Lotfi andAdhoum, 2002), electrochemical precipitation (Namasivayamand Yamuna, 1995), ion exchange (Rengaraj et al., 2003), sol-vent extraction (Mauri et al., 2001), reverse osmosis (Padillaand Tavani, 1999), etc. These processes apart from beingeconomically expensive have disadvantages such as highreagent and energy requirements, incomplete metal removal,and generation of a large quantity of toxic waste sludge, whichnecessitates careful disposal in further steps (Bai andAbraham, 2003). Recently, a search for a low-cost and easilyavailable adsorbent has led to the investigation of materials ofagricultural and biological origin, along with industrial

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0 1 2 3 4 5 6 7 80

4

8

12

16

20

24

pH

q (m

g/g

)

0

20

40

60

80

100

Rem

oval (%

)

Fig. 1. The effect of solution initial pH on the adsorption of chromium (VI).

722 X.S. Wang et al. / Journal of Environmental Management 90 (2009) 721e729

byproducts, as potential metal adsorbents. The variety ofmaterials tested as Cr (VI) adsorbents includes algae (Guptaet al., 2001; Donmez and Aksu, 2002), charcoal, wool, olivecake, sawdust, pine needles, almond shells, cactus leaves(Dakiky et al., 2002), rice husk (Guo et al., 2002), crushedcoconut shell, peat moss, exhausted coffee, waste tea, moulds,yeast, bacteria, crab shells, soybean hulls and cotton seedhulls, hazenut shell, wheat brans, sawdust, mustard seed cakes,bark and straw (Ahalya et al., 2005 and references therein).

Johns et al. (1998) utilized granular activated carbons(GACs) made from walnut hull to successfully remove higherlevels of benzene, toluene, methanol, acetonitrile, acetone, and1,4-dioxane from an aqueous mixture than commercial GACs.However, very little is known about the performance of Cr(VI) reduction by this biomass. In this study, the locallyavailable walnut hull was applied to study the adsorption per-formance of chromium (VI) from single aqueous solution. Theeffects of solution pH, contact time, adsorbent concentration,ionic strength, and reaction temperature on chromium (VI)adsorption were investigated in detail.

2. Materials and methods

The walnut hull was taken from local natural resources. Thematerial was cleaned and dried to constant weight at roomtemperature and then ground to pass through a 100-mesh sievefor further experiments.

The stock solution was prepared by dissolving a knownquantity of potassium dichromate (K2Cr2O7) (AR grade) inde-ionized water. The stock solution was finally diluted toobtain standard solutions.

Batch kinetic experiments were carried out at constant pH1.0 with initial concentration (100 mg/L) and adsorbent doseof 5 g/L at a temperature of 30 �C. After shaking, the solutionsamples were withdrawn at suitable time intervals. Batchequilibrium experiments were conducted using 125-mlpolypropylene bottles at a total sample volume of 50 mL foreach adsorption run. The samples were agitated in a reciprocat-ing shaker to reach equilibrium. At the end of the reactiontime, a known volume of the solution was removed and centri-fuged for Cr (VI) analysis. The residual Cr (VI) concentrationswere determined using diphenylcarbazide method (Eatonet al., 1995). Diphenylcarbazide forms a redeviolet complexselectively with Cr (VI), and the intensity of this complexwas read at 542 nm using a UVevisible spectrophotometer.Experiments were triplicated and the results averaged. Thepercent standard deviation of the adsorption parameters wasunder 5.0%.

The effect of pH on adsorption of Cr (VI) onto the biosorb-ent was investigated by varying the solution pH from 1.0 to7.0. The effects of temperature and adsorbent concentrationson uptake of Cr (VI) were then examined. Finally, sodiumchloride was employed as background electrolyte to investi-gate the effect of ion strength on uptake of Cr (VI). Tempera-ture control was provided by the water bath shaker units. Thesolution pH was adjusted with strong acid (HCl) and/or strongbase (NaOH) and recorded with a pH meter (PHS-3C).

The amount of Cr (VI) sorbed by biosorbents (q) in thesorption system was calculated using the mass balance:

q¼ VðCi�CeÞm1

ð1Þ

where V is the solution volume (L), m1 is the amount of sor-bent (g), and Ci and Ce (mg/L) are the initial and equilibriummetal concentrations, respectively.

The Cr (VI) percent removal (%) was calculated using thefollowing equation:

Removalð%Þ ¼ Ci�Ce

Ci

100 ð2Þ

3. Results and discussion

3.1. Effect of solution initial pH on thechromium (VI) adsorption

Earlier studies have shown that solution pH is an importantparameter influencing the biosorption of metal ions (Guptaet al., 2001; Donmez and Aksu, 2002). Chromium (VI)removal was investigated as a function of solution pH andthe result is indicated in Fig. 1. As seen from this figure, thebiosorption of chromium (VI) onto walnut hull is stronglypH-dependent. The optimum uptake occurred when the initialpH was 1.0 when 97.3% of the chromium (VI) was removedfrom the solution. There is a sharp decrease in the removalwhen the solution pH is raised from 2.0 to 4.0 and thereafterthe effect becomes negligible. Similar results were reportedby other several workers (Guo et al., 2002; Kobya, 2004).The decrease in the adsorption with the increase in solutionpH may be ascribed to the decrease in electrostatic force ofattraction between the sorbent and the sorbate ions. The rawwalnut hull sample was analyzed using a Fourier transforminfrared spectrometer (FTIR) (data not shown) and the resultshows that presence of several functional groups (amine,carboxyl and hydroxyl groups) responsible for the binding of

0 100 200 300 400 500

0

20

40

60

80

100

0 10 20 30 40 50 60

0

20

40

60

80

100

ct (m

g/L

)

Reaction time (min)

30°C40°C50°C

Fig. 2. The evolution of chromium (VI) concentration as a function of time at

various temperatures. The inset corresponds to the evolution of chromium (VI)

concentration at a period of 60 min.

723X.S. Wang et al. / Journal of Environmental Management 90 (2009) 721e729

chromium (VI) onto the surface of walnut hull. These func-tional groups were positively charged when protonated andmay electrostatically interact with negatively charged metalcomplex.

The distribution of the Cr (VI) species in solution dependson pH and Cr (VI) concentration in the following form(Cimino et al., 2000 and references therein):

log K = 0.382 log K = -6.14

H2CrO4 HCrO4- CrO4

2-

log K = 1.706

Cr2O72- (3)

Therefore at low pH values, the dichromate and acidchromate ion species were predominant in solution. In thepresence of a reducing substrate, these species are quicklyconverted according to the following equations (Weng et al.,1997; Cimino et al., 2000):

3CxOH þ Cr2O2�7 þ 4Hþ ¼ 3CxO þ HCrO�4

þ Cr3þ þ 3H2O ð4Þ

3CxOH þ HCrO�4 þ 4Hþ ¼ 3CxO þ Cr3þ þ 4H2O ð5Þ

where CxO represents the oxo groups of the sorption sites.Chromium (III) ions remained in solution at pH 1.0 and for

a chromium (VI) initial concentration of 100 mg/L the concen-tration of the Cr2O7

2� could be assumed negligible in theadsorption process (Weng et al., 1997; Cimino et al., 2000).Hence only the acid chromate ion species (HCrO4

�) could beadsorbed on the protonated active sites of the biosorbent sub-strate. A sharp decrease in adsorption above pH 2.0 may alsobe related to the occupation of the adsorption sites by anionicspecies like CrO4

2�, Cr2O72�, etc., which retard the approach of

such ions further towards the sorbent surface (Das et al., 2000;Agarwal et al., 2006). The decrease in adsorption at higher pHvalues (pH> 4.0) may be ascribed to the competitiveness ofthe oxyanions of chromium and OH�. Hence, in the followingexperiments, the initial solution pH 1.0 was used.

3.2. Adsorption kinetics

Equilibrium and kinetics are the two important aspects inthe adsorption process as a unit operation. The adsorbentwith both higher adsorption capacity and kinetic rates ismost desirable. Fig. 2 shows the change of chromium (VI)concentration as a function of time at various temperatures.The main plot in Fig. 2 shows that at temperature of 30 �C,the chromium (VI) removal rate could generally divided intotwo steps. A rapid initial uptake of chromium (VI) ions isobserved at the period of less than 60 min. This may primarilybe ascribed to a rapid attachment of chromium (VI) ions to thesurfaces of the walnut hull. After this period of time, theadsorption rate becomes slower, which is probably due tothe diffusion of the chromium (VI) ions into the pores of thewalnut hull (Jang et al., 2005). To ensure complete adsorption

equilibrium, in all subsequent experiments samples withadsorbents were left shaking for 420 min at a temperature of30 �C.

The inset in Fig. 2 indicates that at the same initial chromium(VI) concentration of 100 mg/L, the higher the reaction temper-ature, the faster the adsorption equilibrium is reached. Theincrease in uptake rate with rise in temperature may be due tocreation of some new sorption sites on the surface of walnuthull or the increased rate of intraparticle diffusion of chromium(VI) ions into the pores of adsorbent at higher temperature asdiffusion is an endothermic process (Das et al., 2000).

3.3. Kinetic modeling

Different kinetic models described below were used todescribe the chromium (VI) adsorption onto the walnut hull.The goodness of conformity between experimental data andthe model-predicted values was expressed by the coefficientof determination (r), level of significance ( p), and the standarddeviation (SD). A relatively high r value and relatively low pand SD values for the relationship between measured andpredicted chromium (VI) adsorption indicated that the modelsuccessfully described the kinetics of chromium (VI) adsorp-tion onto walnut hull. It should be noted that a relativelyhigh r value for a particular kinetic model does not necessarilymean that this model is the best. A model also cannot be usedto definitely determine the mechanism of chromium (VI)adsorption (Kithome, 1998).

3.3.1. First-order modelThe first-order kinetic model can be generally expressed as:

ln Ct ¼ ln Ci� k1t ð6Þ

where Ct (mg/L) is the chromium (VI) concentration at time t(min), Ci (mg/L) is the initial chromium (VI) concentration, k1

(min�1) is the rate constant of adsorption at 30 �C. The plot of

0 10 20 30 40 50 60-1

0

1

2

3

4

5a b

dc

ln

Ct

Reaction time (min)

1 2 3 40.5

1.0

1.5

2.0

2.5

3.0

ln

(q

t)

ln (Reaction time)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.50

4

8

12

16

20

24

qt (m

g/g

)

ln (Reaction time)

0 1 2 3 4 5 6 7 80

4

8

12

16

20

24

qt (m

g/g

)

(Reaction time)0.5

30°C40°C50°C

30°C40°C50°C

30°C40°C50°C

30°C40°C50°C

Fig. 3. Different kinetic models ((a) first-order; (b) modified Freundlich; (c) Elovich; and (d) intraparticle diffusion) fitted to the Cr (VI) adsorption kinetics on

walnut hull at various temperatures.

Table 1

The values of the parameters for different kinetic models fitted to the Cr (VI) adsorption kinetics on walnut hull at various temperatures

First-orderTemperature (K) k1 (min�1) r p SD

303 0.01796 0.9698 2.98� 10�4 0.1041

313 0.11236 0.9908 <0.0001 0.1741

323 0.22205 0.9802 0.0033 0.3777

Modified FreundlichTemperature (K) k2 (L/g min) m r p SD

303 0.05412 1.8097 0.9632 4.86� 10�4 0.2009

313 0.35624 6.0190 0.9963 <0.0001 0.0150

323 0.35006 4.9902 0.9914 9.51� 10�4 0.0282

Intraparticle diffusion

Temperature (K) kid (mg/g (min)0.5) r p SD

303 2.2972 0.9771 1.51� 10�4 1.2357

313 2.1408 0.9781 1.35� 10�4 0.7384

323 2.9493 0.9726 5.42� 10�3 1.0011

Elovich

Temperature (K) a b r p SD

303 3.6578 0.2254 0.9966 <0.0001 0.4755

313 191.1446 0.3005 0.9991 <0.0001 0.1482

323 106.5611 0.2489 0.9968 2.12� 10�4 0.3418


0.00310 0.00315 0.00320 0.00325 0.00330

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

R = 0.9713

ln

k

1

1/T

Fig. 4. Arrhenius plot of Cr (VI) adsorption kinetics, where k1 (min�1) is the

first-order rate constant and T is absolute temperature (K).

200 250 300 350 400 450 50056

60

64

68

72

76

80

84

88

92

Initial adsorbate concentration

qe (m

g/g

)

76

80

84

88

92

96

100

Rem

oval (%

)

Fig. 5. Effect of initial chromium (VI) concentration on adsorption of chro-

mium (VI).


ln(Ct) versus time (t) shows a straight line indicating theapplicability of assuming first-order kinetics.

3.3.2. Modified Freundlich modelThe modified Freundlich model proposed by Kuo and Lotse

(1974) is:

ln qt ¼ ln k2 þ ln Ciþ1

mln t ð7Þ

where qt (mg/g) is the amount of chromium (VI) sorbed attime t per unit of weight of walnut hull, k2 (L/g min) is theadsorption rate coefficient, Ci is the initial chromium (VI)concentration, t is the reaction time (min), m is a constant.A plot of ln qt versus ln t should be linear if the chromium(VI) adsorption process conforms to the modified Freundlichmodel.

3.3.3. Intraparticle diffusion modelThe rate constant for intraparticle diffusion (kid (mg/g

(min)0.5)) is given by Weber and Morris (1963):

qt ¼ kidt0:5 ð8Þ

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

ce/q

e (g

/L

)

ce (mg/L)

30°C40°C50°C

Fig. 6. Linearized Langmuir and Freundlich isotherms of chromium (V

where qt (mg/g) is the amount of chromium (VI) sorbed attime t (min) per unit of weight of walnut hull. A plot of qt

against t0.5 is often used to test a diffusion-controlled reaction.

3.3.4. Elovich modelThe Elovich model as modified by Chien and Clayton

(1980) was also used to study the kinetics of chromium (VI)adsorption onto walnut hull. The equation is expressed as:

qt ¼�

1

b

�lnðabÞ þ

�1

b

�ln t ð9Þ

where qt (mg/g) is the amount of chromium (VI) sorbed attime t (min) per unit of weight of walnut hull and a andb are constants. If chromium (VI) adsorption conforms tothe Elovich model, a plot of qt versus ln t should yield a linearrelationship with a slope of 1/b and intercept of (1/b)ln(ab).The chemical significance of these constants has not beenclearly resolved (Kithome, 1998).

Fig. 3 shows the various kinetic plots for chromium (VI)adsorption onto the walnut hull at different temperatures.The kinetic data followed very well all four kinetic models.

0 1 2 3 40

1

2

3

4

ln

qe

lnce

30°C40°C50°C

I) biosorption by walnut hull at various temperatures (pH¼ 1.0).

Table 2

Parameters fitted by Langmuir and Freundlich equations along with thermodynamic parameters at various temperatures

Temperature

(K)

Langmuir isotherm Freundlich isotherm Thermodynamic parameters

qmax

(mg/g)

b

(L/mg)

r

(mg/g)/(mg/L)nkf

(mg/g)/(mg/L)nn r DG0

(kJ/mol)

DH0

(kJ/mol)

DS0

(J/mol K)

303 98.13 0.1795 0.9979 45.60 6.8714 0.9803 �23.03 64.14 287.4

313 109.17 0.3641 0.9957 78.07 15.299 0.9510 �25.63

323 132.80 0.8665 0.9978 77.35 6.5457 0.9927 �28.77


These plots were utilized to obtain the parameters given inTable 1.

The effect of temperature on chromium (VI) adsorption rateby walnut hull was examined further by applying theArrhenius equation on the first-order rate constant, k1. TheArrhenius equation is:

k1 ¼ A exp

��Ea

RT

�ð10Þ

where k1 is the first-order rate constant (min�1), A the pre-exponent factor (frequency factor) (min�1), which is a measureof the accessibility of the reactive sites to the reactant, Ea theArrhenius activation energy (kJ/mol), which must beovercome before adsorption can take place, R the gas constant(8.314 J/mol K), and T the absolute temperature (K). A plot ofln k versus (1/T ) yields a straight line, from which the Ea andA can be obtained on the slope and intercept, respectively.

Fig. 4 shows the Arrhenius plot of the chromium (VI)adsorption onto the walnut hull. The estimated values of Aand Ea are 1.14� 1016 min�1 and 102.78 kJ/mol, respectively.In general, in heterogeneous system, low Ea values (<42 kJ/mol) indicate diffusion-controlled processes whereas higherEa values (>42 kJ/mol) suggest chemically controlled pro-cesses (Saha et al., 2004). A 102.78 kJ/mol Ea for the reactionof chromium (VI) adsorption onto walnut indicates that therate-limiting step in this case might be a chemically controlledprocess.

300 305 310 315 320 325

-29

-28

-27

-26

-25

-24

-23

R=-0.9985Gib

bs free en

erg

y ch

an

ge

(kJ/m

ol)

T(K)

Fig. 7. The plot of DG0 versus T (K).

3.4. Effect of initial chromium (VI) concentrationon adsorption

The initial adsorbate concentration provides an importantdriving force to overcome all mass transfer resistance of metalions between the aqueous and solid phases (Donmez andAksu, 2002). Fig. 5 clearly shows that the chromium (VI)removal efficiency decreases with increasing adsorbate con-centration. The reduction in chromium (VI) removal efficiencymay be ascribed to the fact that the number of active adsorp-tion sites to accommodate adsorbate ions remains constant butwith increasing adsorbate concentration, the adsorbate ions tobe accommodated increases and hence the removal efficiencyof adsorption reduces. On the other hand, Fig. 5 also indicatesthat the equilibrium adsorption capacity increases with in-creasing initial chromium (VI) concentration. When the initialchromium (VI) concentration increased from 240 to 480 mg/L,the equilibrium adsorption capacity increased from 59.2 to90.8 mg/g. This may be due to the increase in the number ofchromium (VI) ions competing for the available binding sitesin the surface of the adsorbent.

3.5. Adsorption isotherm

The linearized Langmuir and Freundlich equations shownbelow were used to fit the isothermal adsorption data:

ce

qe

¼ 1

bqmax

þ ce

qmax

ð11Þ

0 1 2 3 4 5 6

60

70

80

90

100

110

120

Adsorbent concentration (g/L)

qe (m

g/g

)

30

40

50

60

70

80

90

100

Rem

oval (%

)

Fig. 8. The effect of adsorbent concentration on chromium (VI).

0 20 40 60 80 100 120 140 160 18035

40

45

50

55

60

65

70

75

80

85

90

95

qe (m

g/g

)

ce (mg/L)

0 g/L NaCl8 g/L NaCl16 g/L NaCl24 g/L NaCl

Fig. 9. The effect of sodium chloride concentration on the equilibrium chro-

mium (VI) adsorption.


ln qe ¼ ln kf þ1

nln ce ð12Þ

where qe is the concentration of adsorbate in solid phase atequilibrium (mg/g), Ce is the concentration of adsorbate inliquid phase at equilibrium (mg/L), qmax (mg/g) and b (L/mg) are the Langmuir consants, and kf ((mg/g)/(mg/L)n) andn (dimensionless) are the Freundlich constants.

6 8 10

R = - 0.9745y = 98.28 - 0.07688x

12 14 16 18 20 22 24 2696.4

96.6

96.8

97.0

97.2

97.4

97.6

97.8

qm

ax (m

g/g

)

NaCl concentration (g/L)

6 8 10 12 14 16 18 20 22 24 26

34.2

34.4

34.6

34.8

35.0

35.2

35.4

35.6

kf


Fig. 10. The constants of Langmuir and Freundlich

Fig. 6 presents the linear Langmuir and Freundlichisotherm plots for chromium (VI) adsorption onto walnuthull at various temperatures. The equilibrium data were fittedvery well to the two adsorption isotherms. These plots wereapplied to calculate the isotherm constants given in Table 2for chromium (VI) at various temperatures.

Table 2 shows that the value of qmax increases with the risein temperature from 30 to 50 �C, indicating that the process tobe chemisorption (Das et al., 2000). The increase in adsorptioncapacity with rise in temperature may be due to the increase inchemical interaction between adsorbate and sorbent surface orcreation of some new sorption on the sorbent surface (Daset al., 2000). The constant, b, is related to the affinity of thebinding sites. The value of b also increases with the rise intemperatures from 30 to 50 �C.

kf and n are the Freundlich isothermal constants related to theadsorption capacity and adsorption intensity of the sorbent,respectively. The values of n at various temperatures are greaterthan unity, indicating favorable adsorption (McKay et al., 1982).

Thermodynamic parameters (changes in standard Gibbsfree energy, DG0, enthalpy change, DH0, and entropy change,DS0) were calculated using the following equations:

DG0 ¼�RTln b0 ð13Þ

DG0 ¼ DH0� TDS0 ð14Þ

where b0 is Langmuir constant when concentration terms areexpressed in L/mol. R is the gas constant (8.314 J/mol K)and T is the absolute temperate (K). The plot of DG0 versus

R = - 0.9986y = 0.06636 - 7.506E - 04x

6 8 10 12 14 16 18 20 22 24 26

0.048

0.050

0.052

0.054

0.056

0.058

0.060

0.062

b (L

/m

g)


6 8 10 12 14 16 18 20 22 24 265.1

5.2

5.3

5.4

5.5

5.6

n


isotherms versus sodium chloride concentration.

Table 3

Comparison of maximum adsorption capacity for Cr (VI) with various

adsorbents

Adsorbent pH qmax

(mg/g)

References

Sugar cane bagasse 2.0 13.4 Kobya (2004)

Maize cob 1.5 13.8

Palm pressed fibres 2.0 14.0

Sugar beet pulp 2.0 17.2

Hazelnut shell

activated carbon

2.0 17.7

Pine needle 2.0 21.5 Dakiky et al. (2002)

Olive cake 2.0 33.4

Sawdust 2.0 39.7


T was found to be linear (Fig. 7) and DH0 and DS0 values werecalculated from the intercept and slope of the plot by linearregression method and are listed in Table 2. The positivevalues of DH0 and DS0 show that adsorption process isendothermic in nature and increases the randomness in thesystem. The negative DG0 value indicates the feasibility ofthe adsorption process. That the DG0 value becomes morenegative with increasing temperature (Table 2) suggests thatchromium (VI) adsorption onto walnut hull is favored withthe increase in temperature. Similar result was reported byDas et al. (2000) using activated cow dung carbon as adsorbentto remove the Cr (VI) from aqueous solution.

Wool 2.0 41.2

Rice husk carbon 2.5 45.6 Kobya, 2004

Almond 2.0 10.6 Dakiky et al. (2002)

3.6. Effect of adsorbent concentration on thechromium (VI) adsorption

Coal 2.0 6.78

Cactus 2.0 7.08

Resin

1200H 3.8 84.04 Rengaraj et al. (2003)

1500H 3.8 188.67

IRN97H 3.8 58.14

Aeromonas caviae 2.5 124.46 Loukidou et al. (2004)

Chlorella vulgaris 2 24

Zooglera ramigera 2 3

Halimeda opuntia 4.1 40

Rhizopus arrhizus 2 62

Rhizopus nigricans 2 123.45

Sargassum 2 40

Spirogyra 2 14.7

Pinus sylvestris 1 201.81

The effect of adsorbent concentration ranged from 1 to 5 g/Lon the chromium (VI) adsorption at solution pH 1.0 at 30 �C ispresented in Fig. 8. It shows by increasing the adsorbent con-centration, the removal efficiency increases but the uptake ofper gram of adsorbent (adsorption density) decreases. The de-crease in adsorption density can be attributed to the fact thatsome of the adsorption sites remain unsaturated during the ad-sorption process; whereas the number of available adsorptionsites increases by an increase in adsorbent and this leads to anincrease in removal efficiency. Similar results were reportedby several workers (Das et al., 2000; Rengaraj et al., 2003).

Bengal gram

(Cicer arientinum)

2 91.64 Ahalya et al. (2005)

Tamarindus indica seed 4 98.04 Agarwal et al. (2006)

3.7. Effect of supporting electrolyte (sodium chloride)concentration on chromium (VI) adsorption

Ground nut shell 4 5.88

Almond shell 2 22.04

Aspergillus sp. 2 34.8 Sen and Ghosh

Dastidar (2007)

Walnut hull 1 98.13 This study

Wastewaters generated by such industries as the textiledyeing, leather tanning, electroplating often contain significantquantities of salts. One of the major pollutants from tannerywastewaters has been identified as sodium chloride (Donmezand Aksu, 2002). Fig. 9 suggests the effect of sodium chlorideconcentration on the biosorption of chromium (VI) ions ontothe walnut hull. The results clearly indicate that increasingthe sodium chloride concentration leads to the decreaseof the equilibrium adsorption capacity. This phenomenoncan be attributed to (1) increasing Cl� concentration and there-fore increasing competition for the adsorption sites on the wal-nut hull, and (2) decreasing activity of chromium (VI) ions insolution due to increasing non-ideality of solution. This non-ideality is due to increasing electrostatic interaction betweenNaþ and chromium (VI) oxyanions. The results of the param-eters of Langmuir and Freundlich isotherms (qmax, b, kf and n)as a function of sodium chloride concentration are shown inFig. 10. The increase in sodium chloride concentration leadsto the linear decrease in numerical values of qmax and b.However, for the kf and n, linear decrease with the increasein sodium chloride concentration is not found.

3.8. Comparison with other adsorbents

The value of qmax is of importance to identify whichsorbent shows the highest adsorption capacity and is useful

in scale-up considerations (Loukidou et al., 2004). The directcomparison of adsorbent capacity of walnut hull with othersorbents reported in the literature is difficult due to the varyingexperimental conditions employed in those studies; however,the walnut hull in this study possesses reasonable adsorptioncapacity in comparison with other sorbents (Table 3).

4. Conclusions

The walnut hull acts as a reasonably good adsorbent for theremoval of chromium (VI) from aqueous solution. The opti-mum pH for removal was found to be 1.0, at which chromium(VI) removal was 97.3%. The removal increased with theincrease in the adsorbent concentration and the decrease inadsorbate concentration. An increasing equilibrium adsorptioncapacity with the rise in temperature indicated that the natureof adsorption process is endothermic, which is furthersupported by the thermodynamic parameters calculated fromthe Langmuir isotherm at various temperatures. The adsorp-tion process was found to follow the first-order, modified


Freundlich, intraparticle diffusion and Elovich models. Com-pared to the various other sorbents reported in the literature,the walnut hull in this study shows very good promise forpractical applicability. However, more studies are needed tooptimize the system from the regeneration point of view andto investigate the economic aspects.

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Removal of chromium (VI) from aqueous solution using walnut hull

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