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Hexavalent chromium removal from aqueous medium by activated carbonprepared from peanut shell: Adsorption kinetics, equilibrium andthermodynamic studies
Z.A. AL-Othman, R. Ali ∗, Mu. Naushad∗
Department of Chemistry, College of Science, Building-5, KingSaud University, Riyadh, Saudi Arabia
a r t i c l e i n f o
Article history:
Received 21 November 2011Received in revised form 9 January 2012Accepted 9 January 2012
Activated carbon was prepared from peanut shell by chemical activation with KOH. Unoxidized activatedcarbon was prepared in nitrogen atmosphere which was then heated in air at a desired temperatureto get oxidized activated carbon. The prepared carbons were characterized for surface area and porevolume and utilized for the removal of Cr(VI) from aqueous solution. The effects of pH, contact time,initial concentration of adsorbate and temperature on adsorption of Cr(VI) were investigated. Adsorptionkinetics of Cr(VI) was analyzed by pseudo first order, pseudo second order and intraparticle diffusionkinetic models. Results showed that Cr(VI) adsorption on both oxidized and unoxidized samples followedthe first and second order kinetics models most appropriately. Isotherm data were treated according toLangmuir and Freundlich models. The results showed that both Langmuir and Freundlich models fittedthe data reasonably but the Langmuir adsorption isotherm model fitted better in the temperature rangestudied. The adsorption capacity was found to increase with temperature, showed endothermic natureof Cr(VI) adsorption. The thermodynamic parameters, such as Gibb’s free energy change (G◦), standardenthalpy change (H ◦), standard entropy change (S ◦) were evaluated. The value of G◦ was foundnegative for the adsorption of Cr(VI) which confirmed the feasibility and spontaneity of the adsorptionprocess.
Industrial progress has made life more comfortable and easy.But at the same time the natural environment had suffered fromthe unfavorable effects of pollution. Heavy metals are unpleasantlyaffecting our ecosystem due to theirtoxicological and physiologicaleffects in environment. Thesemetals, if present beyond certain con-centration can be a serious health hazard which can leads to manydisorders in normal functioning of human beings and animals [1].The main reason for heavy metal pollution is due to metal-platingfacilities, battery manufacturing processes, mining and metallurgi-cal engineering, dyeing operations, electroplating, nuclear powerplants, aerospace industries, the productionof paints and pigmentsand glass production industries [2]. The main heavy metals whichcause metal ion pollution are Th, Cd, Pb, Cr, As, Hg, Cu and Ni.Unlike most organic pollutants, heavy metals are generally refrac-tory and cannot be degraded or readily detoxified biologically [2].Chromium is one of the most toxic pollutants which cause severe
environmental and public health problems. When accumulated athigh levels, chromium can generate serious problems and whenconcentration reaches 0.1mg/g body weight, it can ultimatelybecome lethal [3]. The most common forms of chromium are Cr(0),Cr(III), and Cr(VI). Hexavalent form is more toxic than trivalent andrequires more concern. Strong exposure to Cr(VI) causes cancer inthedigestive tract andlungsand may causeepigastricpain, nausea,vomiting, severe diarrhea and hemorrhage [4]. In aqueous solu-tion, Cr(VI) exists in the form viz. – chromate CrO4
2−, dichromateCr2O7
2− and hydrogen chromate HCrO42−. CrO4
2− is predomi-nant in basic solutions, H2CrO4 is predominant at pH< 1 whileHCrO42− and Cr2O72− are predominant at pH 2–6. The removalof toxic metals from waste water has been achieved by severalprocesses such as ion exchange [5], sedimentation [6], electro-chemical processes [7,8], cementation [9], biological operations[10], coagulation/flocculation [11], filtration and membrane pro-cesses [12,13], chemical precipitation, adsorption [14] and solventextraction [15,16]. Most of these methods suffer from drawbackslike high capital and operational cost and there are problems indisposal of residual metal sludge [17]. In contrast, the adsorptiontechnique has become one of the most preferred methods for theremoval of heavy metals due to its high efficiency and low cost.Many agricultural wastes had directly been used as sorbents for
Z.A. AL-Othman et al. / Chemical Engineering Journal184 (2012) 238–247 239
heavy metal adsorption from wastewater which included soybeanhull [18], olivecake [19], wheatstraw [20], maizecob [21], ricehusk[22], barley straw [23], bagasse pith [24], coconut husk [25], cocoashells [26], tea leaves [27], orange peel and banana peel [28]. Acti-vatedcarbonsaremoreeffectiveintheremovalofheavymetalsdueto some specific characteristics that enhance the use of activatedcarbon for the removal of contaminants including heavy metalsfrom water supplies and wastewater [29]. Many studies have useddifferent type of activated carbon to remove Cr(VI) by adsorption.Coconut shell activated carbon [30], wood and dust coal activatedcarbons [31], hazelnut activated carbon [32], sawdust and usedtyre activated carbon [33] were used for Cr(VI) uptake. Commod-ity crops such as peanuts generate considerable quantities of shellseach year which have little or no value. Peanut shells are low indensity and high in volume and are used in animal feed or burnedforenergy.Chinaranksfirstinpeanutproductionintheworldandapotential of 4.5million tons of peanut shells are produced annually[34]. North Carolina currently ranks fourth in peanut production,producing 95.2 thousand metric tons or 6.3% of the United Statesproduction [35]. This represents a potential of 26 thousand met-ric tons of peanut shells produced each year that have little value.This leads to a need to convert these by-products to useful, valueaddedproducts, such as activatedcarbons. There have been several
reports that peanut shellsconvertedinto activatedcarbon and usedto absorb various metal ions and organic compounds [36–39].
The present paper is concerned with the synthesis of acti-vated carbons derived from peanut shells by chemical activationwith KOH and the removal of hexavalent chromium from aqueoussolution. The kinetics, isotherms and thermodynamics about thesorption of Cr(VI) on the prepared samples were studied. The influ-ence of several operating parameters, such as pH, contact time andinitial concentrationsof adsorbateon the adsorption capacity,werealso investigated.
2. Experimental
2.1. Materials and methods
All AR grade chemicals were used. K2Cr2O7, NaOH, HCl andH2SO4 were purchased from Sigma–Aldrich, Germany. A stocksolution of 1000mg/L of Cr(VI) was prepared from potassiumdichromate salt. The working solutions of desired concentrationswere prepared by appropriate dilution of the stock solutions. Theinitial pH of the test solutions was adjusted to the desired value byusing dilute solutions of HCl and NaOH.
2.2. Equipments
pH measurement were made with a pH meter (model 744,Metrohm) equipped witha combined glass-saturatedcalomel elec-trode calibrated with buffer solutions of pH 4.0, 7.0 and 9.2. The
nitrogen adsorption isotherms were determined with a quan-tachrome NOVA 2200e, surface area and pore size analyzer. Thecarbon samples weredried in program controllerNebertherm C-19,model N 7/4 W – Germany. Absorbance measurements were madewith a UV–visible spectrophotometer model UV-160A Shimadzu
Japan equipped with a 1 cm path length quartz cell. Agitation of thesystem wascarried outon a thermostat-cum-shaking assembly(model MSW 275).
2.3. Adsorbents development and characterization
The raw peanut shells for the production of activated carbonswere collected from local market in Riyadh (City in Saudi Ara-bia). Peanut shells were first washed with single distilled water
to remove dust, dried at 110◦
C for 24h and then crushed to the
desired particle size. The crushed peanut shells were mixed with20% KOH solution in a ratio of 1:1 by mass and allowed to sock for24h at room temperature. Carbonization of theimpregnated mate-rial was carried out in a horizontal tube furnace. Samples (25 g)were placed into the reactor and heated from room temperature to170 ◦C (±5 ◦C) for 1 h under nitrogen flow (flow rate 100 mL min−1)and then at 450 ◦C (±5 ◦C) for 1h at the same flow rate of nitrogen.At the end of activation period, the samples were cooled down toroom temperature in nitrogen flow. Half of the amount of samplesobtained under the flow of nitrogen only, were oxidized by usingbreathing grade air (flow rate 100 mL min−1) at 450 ◦C for 1 h andthen cooled. The products obtained, was rinsed with double dis-tilled water (DDW) in a soxhlet extractor at 100 ◦C until the pH of the rinse water was neutral and finally dried at 110 ◦C for 24h. Theprepared samples were cooled in desiccators and sieved to desiredparticle size (170–400 mesh). Theseactivatedcarbon samples wereclassed as oxidized and unoxidized.
BET surface area, pore volumes, micropore surface area andaverage pore diameter were determined by nitrogen adsorptionat 77 K , using Quanta Chrome NOVA 2200e, surface area andpore size analyzer. Before adsorption measurements, respectivesamples were degassed at 150 ◦C for 2 h at a final pressure of 133.32×10−4 Pa. The BET surface area was calculated by the
Brunauer, Emmett, and Teller (BET) method using the adsorptionisotherms [40]. The cross-sectional area of nitrogen molecules wastaken 16.2 A2/mol. The total pore volume was determined by BJHmethod [41] f romthe amountof nitrogen adsorbedat P /P o 0.95.Themicropore volume, micropore surface area and average pore widthwas determined by Dubinin–Radushkevich (DR) equation [42]. Themesopore volume was calculated by subtracting the microporevolume from the total pore volume [43]. The microstructure of the activated carbons prepared, was examined by SEM (JEOL-JSM-5910, Japan). The elemental analysis of the prepared samples wascarried out by Energy Dispersive Spectrometer (EDS), Inca Oxford.
2.4. Adsorption procedure
The adsorption capability of the prepared activated carbonstoward Cr(VI) was investigated using their aqueous solutions. Thestock solution (1000 mg/L) was diluted as required to obtain stan-dard solutionsof concentration ranging from 10 to100 mg/L. Alltheadsorptionexperiments were performed by the batch technique byusing 400 mesh average particle size of carbons for all the adsorp-tion studies. Thecarbon samples were dried for 24h at 110 ◦C priorto analysis.
The amount of Cr(VI) per unit weight of adsorbent, qe (mg/g)was calculated by the following equation:
qe =V (C i − C e)W × 1000
(1)
where V is the volume of Cr(VI) solution in litre, C i and C e are the
initial and final concentrations (mg/L) of Cr(VI) in solution, respec-tively, andW is the weight (g) of adsorbent.
2.5. Kinetic study
Pseudo-first order, pseudo-second order and intraparticle dif-fusion rate equations have been used for modeling the kineticsof Cr(VI) adsorption. The batch technique was employed to studythe effect of contact time and adsorbate concentration for Cr(VI)adsorption. For this purpose, a number of 100 mL air tight flaskscontaining 40 mL solution of desired concentrations of Cr(VI),wereagitated in a thermostat shakerat 200rpm. 0.1g of activatedcarbonwas added to each flask at the desired temperature. The solutionof the specified flask was separated from carbon at different timeinterval and analyzed for the uptake of Cr(VI).
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Batch adsorption studies were performed in a series of 100 mL conical, airtight Pyrex glass flasks. Each flask was filled with40mL solution of Cr(VI) of desired concentration and adjusted tothe desired pH and temperature. A known amount of activatedcarbon was added to each flask and kept in isothermal shaker(25 ◦C) at 200rpm until equilibrium was reached. Preliminary testsshowed that after 10h, Cr(VI) concentration remain unchanged.The allowed contact time was 24h to reach the equilibrium. Afterthis period, thesolutionwas filtered to remove thecarbon particlesand analyzed spectrophotometrically at the correspondingmax forthe concentration of Cr(VI) remained in the solution.
The effect of pH on the adsorption of Cr(VI) over a pH range of 2–10 was investigated. Cr(VI) adsorption was also studied in con-centration range of 10–100mg/L at different temperatures (20 ◦C,30 ◦C, 40◦C) to elucidate the effect of temperature and adsorptionthermodynamic parameters. The amount of Cr(VI) adsorbed wascalculated by the above method (Eq. (1)).
2.7. Analysis of Cr(VI)
The analysis of Cr(VI) was carried out calorimetrically.Absorbance values were obtained at the wavelength for maximumabsorbance (max =540nm) by making a purple-violet coloredcomplex of Cr(VI) with 1,5-diphenylcarbazide in the acidic condi-tion whichwas converted into concentration datausing calibration
relation pre-determined at the wavelength of interest [44].
3. Results and discussion
3.1. Characterization of prepared samples
Information about the physical properties of adsorbent such ascarbon content, pH and pore structure is essential procedure priorto adsorption process. The various characteristics of activated car-bon prepared from peanut shells by chemical activation with KOHand carbonization at 450 ◦C under inert and air atmosphere arelisted in Tables1and2. Asshownin Table1, the oxidizedcarbon hashigherpH (9.68)and carbon(weight percent, 80.16) than theunox-idizedcarbon (pH8.96and carbonweight percent, 77.32). Thus, the
oxidized carbon was more basic than unoxidized one. The specificsurface area (S BET) and micropore volume (V D–R ) of the preparedcarbon samples was evaluated by applying the Brunauer, Emmettand Teller (BET) and Dubinin–Radushkevich equations, respec-tively. It can be observed from Table 2 that the specific surface area(S BET), micropore surface area (S D–R ), micropore volume (V micro)
Fig. 1. (a) Scanning electron micrograph of KOHtreated oxidized carbon. (b) Scan-ning electron micrograph of KOH treated unoxidized carbon.
and mesopore volume (V meso) of oxidized sample was greater thanthe unoxidized sample which may be due to the air oxidation of the impregnated material during carbonization that has facilitatedthe evolution of volatile matter from the precursor material andthereby enhanced the porosity in the carbon texture. It is alsoclear from Table 2 that both oxidized and unoxidized carbons weremesoporous as the mesopore volume of oxidized carbon occupied74% of the total pore volume while the unoxidized carbon occu-pied 75% of the total pore volume. According to the InternationalUnion of Pure and Applied Chemistry (IUPAC), the pore structuresof activated carbons are classified into three groups as microp-
ore (≤2 nm), mesopore (2–50 nm) and macropore (≥50nm). Bothtypeof activated carbons contained microporesand mesopores butthe mesopore volume was larger than the micropore volume. Theoxidationduringcarbonization hada considerable effect on thetex-tural properties of carbonsamples.The prepared samples were alsocharacterized for their efficiency for Cr(VI) adsorption and showed
Table 2
Physical properties of oxidized and unoxidized activated carbon samples.
Z.A. AL-Othman et al. / Chemical Engineering Journal184 (2012) 238–247 241
0
5
1015
20
25
30
35
40
45
0 2 4 6 8 10 12
% R
e m o v a l
pHi
Oxidized
Un-oxidized
Fig. 2. Effect of pH on Cr(VI) adsorption for oxidized and unoxidized carbons at25 ◦C.
that the oxidized carbon had a higher adsorption capacity than theunoxidized carbon.
3.2. SEM images
Scanning electron micrographs (SEM) of the prepared carbonsamples, are shown in Fig. 1(a) and (b). It is clear from the SEMfigures that the external surfaces of both samples were rough andcontained pores of different size and shapes. The oxidized samplesurface had narrow elongated pores while the unoxidized sam-ple had wide pores which showed that air oxidation resulted inactivated carbon with well developed porosity. The micrographsshowed that the cavities on the surfaces of the carbon samplesresulted from the evaporation of the KOH during carbonization,leaving the space open that was previously occupied by KOH.During impregnation, the molecules of the chemical impregnat-ing agent diffused into the texture of the lignocellulosic material.On carbonization at thedesired temperature, thechemical impreg-
nating agent evaporated and made the remaining carbon textureporous. The major elements present in both samples were car-bon and oxygen with a small amount of potassium. Table 1 showsthat the percentage of carbon in oxidized sample was greater thanthe unoxidized sample while the unoxidized sample had a higherpercentage of oxygen.
3.3. Effect of pH
Activated carbons are species with amphoteric character, thusdepend on the pH of the solution. Their surface might be positivelycharged or negatively charged. The pore wall of activated carboncontained a large number of surface functional groups. The pHdependence of Cr(VI) adsorption can largely be related to the type
andionic state of these functional groupsand also on the adsorbatechemistry in the solution. The solution pH is one of the importantparameter for the removal of heavy metals from aqueous solutionbecause it affects the solubility of adsorbates, concentration of thecounter ions on the functional groups of the adsorbent and thedegree of ionization of the adsorbate during reaction [45]. Cr(VI)removal was studied as a function of pH over a pH range of 2–10on oxidized and unoxidized samples at the initial concentration of 50mg/L as shown in Fig. 2. It is clear from Fig. 2 that the preparedactivated carbons were more active in the acidic range and max-imum adsorption occurred at pH 2.0. There was a sharp decreasein the sorption capacity when pH was raised from 2.0 to 7.0 andthereafter the effect became negligible. Cr(VI) may exist in threedifferent ionic forms (HCrO4
−, Cr2O72−, CrO4
2−) in aqueous solu-
tions andthe stabilityof these ions in aqueous systems is mainlypH
0
1
23
4
5
6
7
8
9
10
0 5 10 15 20 25 30 35 40 45 50
q e ( m g / g )
t (hrs)
30 mg/l
40 mg/l
50 mg/l
0
1
2
3
4
5
6
7
0 4 8 12 16 20 24 28 32 36 40 44 48
q e
( m g / g )
t (hrs)
30 mg/l
40 mg/l
50 mg/l
a
b
Fig. 3. Effect of time on Cr(VI) adsorption for (a) oxidized carbon, (b) unoxidizedcarbon at25 ◦C.
dependent [46]. The percentage of Cr(VI) removal washigherin thelower pH ranges due to high electrostatic force of attraction. As the
number of H+ ions increased with lowering the solution pH, whichneutralized the negative charge on adsorbent surface and therebyincreased the diffusion of chromate ions into the bulk of the adsor-bent [50]. As reported by other workers that the dominant form of Cr(VI) at pH up to 4.0 is HCrO4
− [46]. So, Cr(VI) was adsorbed onthe surface of activated carbon mostly in the form of HCrO4
− ions.The decrease in the adsorption with increase in pH may be due tothe increased number of OH− ions in the bulk which retarded thediffusion of chromate ions. The decrease in adsorption at higher pHmay be due to the competitiveness of the oxyanions of chromium.Hence pH 4.0 was taken as the optimal values for further studies of Cr(VI) adsorption on oxidized and unoxidized carbons.
3.4. Effect of contact time and initial concentration
The amount of Cr(VI) adsorbed on oxidized and unoxidized car-bons was studied as a function of shaking time at three differentinitial concentrations (20, 30, 40mg/L) of Cr(VI) at 25◦C, 0.1 g of adsorbent and desired pH. The results are given in Fig. 3(a) and (b),respectively. It is evident from these figures that the adsorption of Cr(VI) increased with increase in contact time from 10min to 7 h,thenbecameslowupto20handthesaturationisalmostreachedin24h in case of both oxidized and unoxidized samples. The natureand compactness of the adsorbent affected the equilibrium time.The removal of Cr(VI) was found to be dependent on the initialconcentration. The amount of Cr(VI) adsorbed,qe (mg/g), increasedwith increase in initial concentration. Further, the adsorption wasrapid in the early stages and then gradually decreased and became
almost constant after the equilibrium point. At low concentrations,
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242 Z.A. AL-Othmanet al./ Chemical Engineering Journal 184 (2012) 238–247
the ratio of available surface to initial Cr(VI) concentration waslarger, so the removal became independent of initial concentra-tions. However, in the case of higher concentrations, this ratio waslow. The percentage removal then depended upon the initial con-centration. The curves also indicated that the adsorption led tosaturation, suggested the possible monolayer coverage of Cr(VI) onthe surface of adsorbent [51]. In the case of oxidized carbon, theCr(VI) removal was decreased from 46.63 to 41.56% as the Cr(VI)concentration was increased from 30 to 50mg/L and the amount of Cr(VI) adsorbed increased from 5.59 to 8.31mg/g. while in case of unoxidized carbon, the Cr(VI) removal was decreased from 35.96to 31.24% as the Cr(VI) concentration was increased from 30 to50mg/L and the amount of Cr(VI) adsorbed increased from 4.31to 6.25 mg/g.
3.5. Kinetics of adsorption
Adsorption kinetics provides valuable information about thereaction pathways and mechanism of the reactions. The kinet-ics of Cr(VI) adsorption on oxidized and unoxidized carbons wereanalyzed using pseudo first-order [46], pseudo second-order [47]and intraparticle diffusion[48,49] models. The conformity betweenexperimental data and the model predicted values was expressedby the correlation coefficients (R2). A relatively high R2 value indi-cated that the model successfully describes the kinetics of Cr(VI)adsorption.
3.5.1. The pseudo-first order equation
The pseudo-first order equation [46], is generally expressed as:
log(qe − qt ) = log qe −k1t
2.303 (2)
where qe and qt are the amounts of Cr(VI) adsorbed (mg/g) at equi-librium and at time t , respectively, and k1 is the rate constant of first order adsorption (h−1).
A straight lines were obtained by plotting log(qe−qt ) against t ,as shown in Fig. 4(a) and (b). The values of the rate constant k
1and qe were obtained from the slopes and intercepts of the plots,respectively (Table 3).
3.5.2. The pseudo second-order equation
The pseudo second-order adsorption kinetic rate equation isexpressed as [47]:
dqt dt = k2(qe − qt )
2 (3)
where qe and qt are the sorption capacity at equilibrium and timet (mg/g), respectively, k2 is the rate constant of the pseudo-secondorder sorption (g/mgh). For the boundary conditions t = 0 to t = t and qt = 0 t o qt = qt , the integrated form of Eq. (3) will be as:
1qe − qt
= 1qe
+ k2t (4)
which is theintegratedrate lawfor a pseudosecond-order reaction.Eq. (4) can be rearranged to obtain Eq. (5), which has a linear form:
t
qt =
1
k2q2e
+t
qe(5)
The initial adsorption rate, h (mg/g h) is given as:
h = k2q2e (6)
Furthermore, Eq. (5) can be written as:
t
qt
=1
h
+t
qe
(7)
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
0 2 4 6 8 10 12 14 16 18
l o g ( q e - q t )
t (hrs)
30 mg/l
40 mg/l
50 mg/l
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
0 2 4 6 8 10 12 14 16 18
l o g ( q e - q t )
t (hrs)
30 mg/l
40 mg/l
50 mg/l
-2
-1.5
-1
-0.5
0
0.5
1
0 2 4 6 8 10 12 14 16 18
l o g ( q e - q t )
t (hrs)
30 mg/l
40 mg/l
50 mg/l
a
b
Fig. 4. Lagergren first order plot for Cr(VI) adsorption on (a) oxidized carbon, (b)unoxidized.
The plots of t /qt versus t of Eq. (5) gave linear plots (Fig. 5(a) and(b)). The values of qe and k2 were determined from the slopes andintercepts of the plots, respectively (Table 3).
3.5.3. The intraparticle diffusion model
The intraparticle diffusion model [49] is expressed as:
qt = kidt 1/2
+ C (8)
where kid is the intraparticle diffusion rate constant (mg/g h1/2), C is the intercept (mg/g).
The plot of qt versus t 1/2
gave straight line and the values of kid were calculated from the slopes of the plots. Values of C gavean idea about the thickness of boundary layer, i.e., the larger theintercept, greater the contribution of the surface sorption in therate controlling step. The data for the adsorption of Cr(VI) on tooxidized and unoxidized activated carbons applied to intraparticlediffusion model is shown in Fig. 6(a) and (b) and the results aregiven in Table 3.
0
1
2
3
4
5
6
7
8
9
10
4.543.532.521.510.50
q t ( m g / g )
t½ (h½)
30 mg/l
40 mg/l
50 mg/l
0
1
2
3
4
5
6
7
4.543.532.521.510.50
q t ( m g / g )
t½ (h½)
30 mg/l
40 mg/l
50 mg/l
a
b
Fig. 6. Intraparticle diffusion kinetic plot for Cr(VI) adsorption on (a) oxidized car-
bon, (b) unoxidized carbon.
Table 3 shows the values of the correlation coefficient (R2) of pseudo-first order, pseudo-second orderand intraparticle diffusionkinetic models. The results demonstrated that among these threemodels, pseudo-first order and pseudo-second order kinetic equa-tions had high R2 values. So, both theses kinetic models were takenas the best fit equations for the description of the mechanism of sorption of Cr(VI) ions. Therefore, the sorption of Cr(VI) ions fromaqueous solution onto oxidized and unoxidized activated carbonswere found to follow both pseudo-first order and pseudo-secondorder kinetic equations.
3.6. Adsorption isotherms
The equilibrium isotherms for the adsorption of Cr(VI) onto oxi-dized and unoxidized samples over a wide range of concentration(10–100 mg/L), optimum pH of adsorption and different tempera-tures are shown in Fig. 7(a) and (b), respectively. These isothermsshowed the relationship between the amounts of Cr(VI) adsorbed
0
2
4
6
8
10
12
14
0 10 20 30 40 50 60 70 80
q e ( m g
/ g )
Ce (mg/l)
20 ⁰C
30 ⁰C
40 ⁰C
0
2
4
6
8
10
12
0 10 20 30 40 50 60 70 80 90
q e ( m g / g )
Ce (mg/l)
20 ⁰C
30 ⁰C
40 ⁰C
a
b
Fig. 7. Isotherm study of Cr(VI) adsorption on (a) oxidized carbon, (b) unoxidized
carbon at different temperatures.
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244 Z.A. AL-Othmanet al./ Chemical Engineering Journal 184 (2012) 238–247
(qe) and its equilibrium concentration (C e) in solution. It is alsoclear from these figures that the adsorptivity of Cr(VI) increasedwith increase in temperature. This suggested that Cr(VI) adsorp-tion from aqueous solutions on oxidized and unoxidized carbonwas endothermic process. The increase in the adsorption capac-ity may be due to the chemical interaction between adsorbate andadsorbent, creation of some new adsorption sites or the increasedrate of intra particle diffusion of Cr(VI) ions into the pores of theadsorbent at higher temperature [52,53].
To examine the relationship between sorbed (qe) and aque-ous concentration (C e) at equilibrium, sorption isotherm modelsare widely employed for fitting the data, of which Langmuir andFreundlich equations are most widely used. The Langmuir modelassumes that the uptake of adsorbate molecules occurs on ahomogenoussurfaceby monolayer adsorption without anyinterac-tionbetweenadsorbedmolecules[54]. Freundlichmodelissuitablefor non-ideal adsorption on heterogeneous surfaces. The hetero-geneity is caused by the presence of different functional groups onthe surface, and various adsorbent–adsorbate interactions [54]. Toget the equilibrium data, initial Cr(VI) concentrations were variedwhile the adsorbent mass for both samples were kept constant andequilibrium time 24h, were used for sorption experiments on bothoxidized and unoxidized samples.
To ensure equilibrium conditions, the linear form of the Lang-muir equation was applied to the experimental data.
C eqe
=1
Q ob +
C eQ o
(9)
where qe was the quantity of Cr(VI) adsorbed per unit weightof adsorbent (mg/g) at equilibrium, C e was the equilibrium con-centration (mg/L) of Cr(VI) in solution. The constant Q o gives thetheoretical monolayer adsorption capacity (mg/g) and b is relatedto the energy of adsorption (L/mg). Straight lines were obtainedby plotting C e/qe against C e as shown in Fig. 8(a) and (b) for oxi-dized and unoxidized samples, respectively. The linear plot of C e/qe against C e indicated the applicability of Langmuir adsorp-tionisotherm.Consequently,suggestedtheformationofmonolayer
coverage of theadsorbate on thesurface of theadsorbent.Langmuirconstants, Q o and b were calculated from the slopes and interceptsof plots of C e/qe versus C e, respectively, and are given in Table 4along with correlation coefficients (R2). It is clear from Table 4 thatthebvalues werehigherat highertemperatures, showedendother-mic nature of Cr(VI) adsorption.
The essential characteristics of the Langmuir isotherm canbe expressed by a dimensionless equilibrium parameter, RL, alsoknown as theseparation factor, definedby Weber andChackravorti[55].
RL =1
1+ bC i(10)
where b is the Langmuir constant and C i is the lowest initial Cr(VI)
concentration (mg/L), RL values indicate the type of isotherm. Theaverage values of RL for each of the different initial concentrationand temperatures used, was between 0 and 1, which indicated thefavorable adsorption of Cr(VI) on both oxidized and unoxidizedsamples.
The linear form of Freundlich isotherm as expressed by Eq. (11),was also applied to the adsorption data of Cr(VI)
lnqe = lnK 1 +1n
ln C e (11)
where K 1 (mg/g) and 1/n (g/l) are Freundlich adsorption constants,indicatingtheadsorptioncapacityandadsorptionintensity,respec-tively. Straight lines were obtained by plotting lnqe against lnC e asshown by Fig. 9(a) and (b) for the adsorptionon oxidizedand unox-
idized carbon samples, respectively, which showed that adsorption
0
1
2
3
4
5
6
7
8
9
0 10 20 30 40 50 60 70 80
C e / q e ( g / l )
Ce (mg/l)
20 ⁰C
30 ⁰C
40 ⁰C
0
1
2
3
4
5
6
7
8
9
10
0 10 20 30 40 50 60 70 80 90
C e / q e ( g / l )
Ce (mg/l)
20 ⁰C
30 ⁰C
40 ⁰C
a
b
Fig. 8. Langmuir adsorptionisothermsfor Cr(VI) adsorptionon (a) oxidized carbon,(b) unoxidized at different temperatures.
0
0.5
1
1.5
2
2.5
3
1 1.5 2 2.5 3 3.5 4 4.5
l n q e
ln Ce
20 ⁰C
30 ⁰C
40 ⁰C
0
0.5
1
1.5
2
2.5
3
1 1.5
2 2.5
3 3.5 4 4.5
l n q e
ln Ce
20 ⁰C
30 ⁰C
40 ⁰C
a
b
Fig.9. Freundlichadsorptionisotherms forCr(VI)adsorption on(a) oxidizedcarbon,(b) unoxidized at different temperatures.
8/11/2019 Hexavalent Chromium Removal From Aqueous Medium by Activated Carbon
of Cr(VI) obeyed Freundlich isotherm not very well. Values of Freundlich constants and correlation coefficient (R2) are given inTable 4. It is also evident from the correlation coefficient (R2) val-ues that the Freundlich isotherm did not fitted the experimentaldata very well. The values of K 1 and n changed with the rise intemperature. The value of n showed an indication of the favorabil-ity of adsorption. Values of n larger than 1, showed the favorablenatureofadsorption[56,57]. Thevalueof n suggestedthatCr(VI)arefavorably adsorbed by the activated carbon prepared from peanutshells. The values of Q o (monolayer adsorption capacity), as calcu-lated from Langmuir adsorption isotherms for Cr(VI) was found tobe higher for oxidized carbon than unoxidized carbon which wasin agreement with the high surface area, high carbon content andmicropore volume of oxidized carbons.
3.7. Adsorption thermodynamics
The thermodynamic parameters such as H (enthalpy change)andS (entropy change) werecalculatedfrom theslopes and inter-cepts of the plots of lnK c versus 1/T as shown in Fig. 10(a) and (b),
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
3.453.43.353.33.253.23.15
l n K c
1/T (Kx10-3 )
1/T (Kx10-3 )
-3.9
-3.8
-3.7
-3.6
-3.5
-3.4
-3.3
-3.2
-3.1
3.453.43.353.33.253.23.15
l n K c
a
b
Fig. 10. Plot of lnK c versus 1/T for Cr(VI) adsorption on (a) oxidized carbon, (b)
unoxidized carbon.
Table 5
Thermodynamic parameters for Cr(VI) adsorption on oxidized and unoxidizedcarbons.
Adsorbent Thermodynamic parameters
T (K) G◦ H ◦ S ◦
Oxidized
293 −28.38
0.04 0.10303 −29.39313 −30.32
Unoxidized
293 −11.73
0.02 0.04303 −11.13313 −12.53
for adsorption on oxidized and unoxidized carbons, respectively,by using the following relation.
ln K c = −H ◦
RT +
S◦
R (12)
The G◦ (free energy change) was calculated from the followingrelation:
G◦ =H ◦ − TS◦ (13)
whereR (8.314J/mol K) is thegas constant,T (K), absolute tempera-ture andK c (L/mg), standard thermodynamic equilibrium constantdefined by qe/C e.
The values of H ◦, S ◦, and G◦ for Cr(VI) adsorption on oxi-dized and unoxidized carbons are given in Table 5. It may be
observed from Table 5 that the values of H ◦
was positive indi-cated the endothermic process of adsorption and the values of S ◦
were positive which showed that Cr(VI) adsorption caused disor-derness in the system. The value of G◦ indicated the degree of spontaneity of the adsorption process and a more negative valueshowed an adsorption process which was favorable energetically.The increase in G◦ with increasing temperature showed that theadsorptionwas more favorableat high temperature. Other workers[58,59] have also been reported similar results for the adsorptionof Cr(VI).The value of G◦ was found negative in the adsorption of Cr(VI) at all temperatures but more negative values were obtainedin case of oxidized carbon, which confirmed the feasibility of thisadsorbentand spontaneity of theadsorption process. So,the capac-ity of the oxidized carbon for the removal of Cr(VI) was higher than
unoxidized carbon.
4. Conclusion
Activated carbon was prepared from peanut shell by chemicalactivation with KOH, characterized and utilized for the removalof Cr(VI) from aqueous solutions in the concentration range of 10–100mg/L. The oxidized carbon had high surface area and porevolume and higher capacity for Cr(VI) adsorption than the unox-idized carbon. Cr(VI) adsorption was found to be pH dependent.Effective adsorption was occurred in the pH range of 2–4. Thekinetics of Cr(VI) followed both pseudo first order and pseudo-second order rate expressions. The removal of Cr(VI) was found tobe dependent on the initial concentration. The percentage removal
was decreased with increase in initial concentration. Isotherm
8/11/2019 Hexavalent Chromium Removal From Aqueous Medium by Activated Carbon
Z.A. AL-Othman et al. / Chemical Engineering Journal184 (2012) 238–247 247
[56] N. Daneshvar, D. Salari, S. Aber, Chromium adsorption and Cr(VI) reductionto trivalent chromium in aqueous solutions by soya cake, J. Hazard. Mater. 94(2002) 49–61.
[57] P.K. Malik, Dye removal from wastewater using activated carbon developedfrom sawdust: adsorption equilibrium and kinetics, J. Hazard. Mater. 113(2004) 81–88.
[58] J. Romero-Gonzalez, J.R. Peralta-Videa, E. Rodriguez, S.L. Ramirez, J.L. Gardea-orresdey, Determination of thermodynamic parameters of Cr(VI) adsorptionfrom aqueous solution onto Agave lechuguilla biomass, J. Chem. Thermodyn.37 (2005) 343–347.
[59] E. Oguz,Adsorption characteristics and the kinetics of the Cr(VI) on the Thujaoriantalis, Colloids Surf. 252 (2005) 121–128.
