Hexavalent chromium removal from aqueous medium

Chemical Engineering Journal 184 (2012) 238–247

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Hexavalent chromium removal from aqueous medium by activated carbon prepared from peanut shell: Adsorption kinetics, equilibrium and thermodynamic studies Z.A. AL-Othman, R. Ali ∗ , Mu. Naushad ∗ Department of Chemistry, College of Science, Building-5, King Saud University, Riyadh, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 21 November 2011 Received in revised form 9 January 2012 Accepted 9 January 2012 Keywords: Activated carbon Cr(VI) adsorption Peanut shell Isotherms model Lagergren model Thermodynamics

a b s t r a c t Activated carbon was prepared from peanut shell by chemical activation with KOH. Unoxidized activated carbon was prepared in nitrogen atmosphere which was then heated in air at a desired temperature to get oxidized activated carbon. The prepared carbons were characterized for surface area and pore volume 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. Adsorption kinetics of Cr(VI) was analyzed by pseudo first order, pseudo second order and intraparticle diffusion kinetic models. Results showed that Cr(VI) adsorption on both oxidized and unoxidized samples followed the first and second order kinetics models most appropriately. Isotherm data were treated according to Langmuir and Freundlich models. The results showed that both Langmuir and Freundlich models fitted the data reasonably but the Langmuir adsorption isotherm model fitted better in the temperature range studied. The adsorption capacity was found to increase with temperature, showed endothermic nature of Cr(VI) adsorption. The thermodynamic parameters, such as Gibb’s free energy change (G◦ ), standard enthalpy change (H◦ ), standard entropy change (S◦ ) were evaluated. The value of G◦ was found negative for the adsorption of Cr(VI) which confirmed the feasibility and spontaneity of the adsorption process. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Industrial progress has made life more comfortable and easy. But at the same time the natural environment had suffered from the unfavorable effects of pollution. Heavy metals are unpleasantly affecting our ecosystem due to their toxicological and physiological effects in environment. These metals, if present beyond certain concentration can be a serious health hazard which can leads to many disorders in normal functioning of human beings and animals [1]. The main reason for heavy metal pollution is due to metal-plating facilities, battery manufacturing processes, mining and metallurgical engineering, dyeing operations, electroplating, nuclear power plants, aerospace industries, the production of paints and pigments and glass production industries [2]. The main heavy metals which cause metal ion pollution are Th, Cd, Pb, Cr, As, Hg, Cu and Ni. Unlike most organic pollutants, heavy metals are generally refractory and cannot be degraded or readily detoxified biologically [2]. Chromium is one of the most toxic pollutants which cause severe

∗ Corresponding authors. Tel.: +966 560467434. E-mail addresses: rahmat [email protected] (R. Ali), [email protected] (Mu. Naushad). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.048

environmental and public health problems. When accumulated at high levels, chromium can generate serious problems and when concentration reaches 0.1 mg/g body weight, it can ultimately become lethal [3]. The most common forms of chromium are Cr(0), Cr(III), and Cr(VI). Hexavalent form is more toxic than trivalent and requires more concern. Strong exposure to Cr(VI) causes cancer in the digestive tract and lungs and may cause epigastric pain, nausea, vomiting, severe diarrhea and hemorrhage [4]. In aqueous solution, Cr(VI) exists in the form viz. – chromate CrO4 2− , dichromate Cr2 O7 2− and hydrogen chromate HCrO4 2− . CrO4 2− is predominant in basic solutions, H2 CrO4 is predominant at pH < 1 while HCrO4 2− and Cr2 O7 2− are predominant at pH 2–6. The removal of toxic metals from waste water has been achieved by several processes such as ion exchange [5], sedimentation [6], electrochemical processes [7,8], cementation [9], biological operations [10], coagulation/flocculation [11], filtration and membrane processes [12,13], chemical precipitation, adsorption [14] and solvent extraction [15,16]. Most of these methods suffer from drawbacks like high capital and operational cost and there are problems in disposal of residual metal sludge [17]. In contrast, the adsorption technique has become one of the most preferred methods for the removal 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 Journal 184 (2012) 238–247

heavy metal adsorption from wastewater which included soybean hull [18], olive cake [19], wheat straw [20], maize cob [21], rice husk [22], barley straw [23], bagasse pith [24], coconut husk [25], cocoa shells [26], tea leaves [27], orange peel and banana peel [28]. Activated carbons are more effective in the removal of heavy metals due to some specific characteristics that enhance the use of activated carbon for the removal of contaminants including heavy metals from water supplies and wastewater [29]. Many studies have used different type of activated carbon to remove Cr(VI) by adsorption. Coconut shell activated carbon [30], wood and dust coal activated carbons [31], hazelnut activated carbon [32], sawdust and used tyre activated carbon [33] were used for Cr(VI) uptake. Commodity crops such as peanuts generate considerable quantities of shells each year which have little or no value. Peanut shells are low in density and high in volume and are used in animal feed or burned for energy. China ranks first in peanut production in the world and a potential of 4.5 million 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 States production [35]. This represents a potential of 26 thousand metric tons of peanut shells produced each year that have little value. This leads to a need to convert these by-products to useful, value added products, such as activated carbons. There have been several reports that peanut shells converted into activated carbon and used to absorb various metal ions and organic compounds [36–39]. The present paper is concerned with the synthesis of activated carbons derived from peanut shells by chemical activation with KOH and the removal of hexavalent chromium from aqueous solution. The kinetics, isotherms and thermodynamics about the sorption of Cr(VI) on the prepared samples were studied. The influence of several operating parameters, such as pH, contact time and initial concentrations of adsorbate on the adsorption capacity, were also investigated.

239

desired particle size. The crushed peanut shells were mixed with 20% KOH solution in a ratio of 1:1 by mass and allowed to sock for 24 h at room temperature. Carbonization of the impregnated material was carried out in a horizontal tube furnace. Samples (25 g) were placed into the reactor and heated from room temperature to 170 ◦ C (±5 ◦ C) for 1 h under nitrogen flow (flow rate 100 mL min−1 ) and then at 450 ◦ C (±5 ◦ C) for 1 h at the same flow rate of nitrogen. At the end of activation period, the samples were cooled down to room temperature in nitrogen flow. Half of the amount of samples obtained under the flow of nitrogen only, were oxidized by using breathing grade air (flow rate 100 mL min−1 ) at 450 ◦ C for 1 h and then cooled. The products obtained, was rinsed with double distilled 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 24 h. The prepared samples were cooled in desiccators and sieved to desired particle size (170–400 mesh). These activated carbon samples were classed as oxidized and unoxidized. BET surface area, pore volumes, micropore surface area and average pore diameter were determined by nitrogen adsorption at 77 K, using Quanta Chrome NOVA 2200e, surface area and pore size analyzer. Before adsorption measurements, respective samples 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 adsorption isotherms [40]. The cross-sectional area of nitrogen molecules was taken 16.2 A˚ 2 /mol. The total pore volume was determined by BJH method [41] from the amount of nitrogen adsorbed at P/Po 0.95. The micropore volume, micropore surface area and average pore width was determined by Dubinin–Radushkevich (DR) equation [42]. The mesopore volume was calculated by subtracting the micropore volume from the total pore volume [43]. The microstructure of the activated carbons prepared, was examined by SEM (JEOL-JSM5910, Japan). The elemental analysis of the prepared samples was carried out by Energy Dispersive Spectrometer (EDS), Inca Oxford.

2. Experimental 2.4. Adsorption procedure 2.1. Materials and methods All AR grade chemicals were used. K2 Cr2 O7 , NaOH, HCl and H2 SO4 were purchased from Sigma–Aldrich, Germany. A stock solution of 1000 mg/L of Cr(VI) was prepared from potassium dichromate salt. The working solutions of desired concentrations were prepared by appropriate dilution of the stock solutions. The initial pH of the test solutions was adjusted to the desired value by using dilute solutions of HCl and NaOH. 2.2. Equipments pH measurement were made with a pH meter (model 744, Metrohm) equipped with a combined glass-saturated calomel electrode calibrated with buffer solutions of pH 4.0, 7.0 and 9.2. The nitrogen adsorption isotherms were determined with a quantachrome NOVA 2200e, surface area and pore size analyzer. The carbon samples were dried in program controller Nebertherm C-19, model N 7/4 W – Germany. Absorbance measurements were made with a UV–visible spectrophotometer model UV-160A Shimadzu Japan equipped with a 1 cm path length quartz cell. Agitation of the system was carried out on a thermostat-cum-shaking assembly (model MSW 275). 2.3. Adsorbents development and characterization The raw peanut shells for the production of activated carbons were collected from local market in Riyadh (City in Saudi Arabia). Peanut shells were first washed with single distilled water to remove dust, dried at 110 ◦ C for 24 h and then crushed to the

The adsorption capability of the prepared activated carbons toward Cr(VI) was investigated using their aqueous solutions. The stock solution (1000 mg/L) was diluted as required to obtain standard solutions of concentration ranging from 10 to 100 mg/L. All the adsorption experiments were performed by the batch technique by using 400 mesh average particle size of carbons for all the adsorption studies. The carbon samples were dried for 24 h at 110 ◦ C prior to analysis. The amount of Cr(VI) per unit weight of adsorbent, qe (mg/g) was calculated by the following equation: qe =

V (Ci − Ce ) W × 1000

(1)

where V is the volume of Cr(VI) solution in litre, Ci and Ce are the initial and final concentrations (mg/L) of Cr(VI) in solution, respectively, and W is the weight (g) of adsorbent. 2.5. Kinetic study Pseudo-first order, pseudo-second order and intraparticle diffusion rate equations have been used for modeling the kinetics of Cr(VI) adsorption. The batch technique was employed to study the effect of contact time and adsorbate concentration for Cr(VI) adsorption. For this purpose, a number of 100 mL air tight flasks containing 40 mL solution of desired concentrations of Cr(VI), were agitated in a thermostat shaker at 200 rpm. 0.1 g of activated carbon was added to each flask at the desired temperature. The solution of the specified flask was separated from carbon at different time interval and analyzed for the uptake of Cr(VI).

240


Table 1 pH and elemental analysis of KOH treated oxidized and unoxidized carbons. Parameters

Oxidized

Unoxidized

pH PZC C (weight %) O (weight % K (weight %

9.68 9.12 80.16 22.11 1.76

8.96 8.37 77.32 24.12 1.98

2.6. Batch equilibrium studies Batch adsorption studies were performed in a series of 100 mL conical, airtight Pyrex glass flasks. Each flask was filled with 40 mL solution of Cr(VI) of desired concentration and adjusted to the desired pH and temperature. A known amount of activated carbon was added to each flask and kept in isothermal shaker (25 ◦ C) at 200 rpm until equilibrium was reached. Preliminary tests showed that after 10 h, Cr(VI) concentration remain unchanged. The allowed contact time was 24 h to reach the equilibrium. After this period, the solution was filtered to remove the carbon particles and analyzed spectrophotometrically at the corresponding max for the 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 concentration range of 10–100 mg/L at different temperatures (20 ◦ C, 30 ◦ C, 40 ◦ C) to elucidate the effect of temperature and adsorption thermodynamic parameters. The amount of Cr(VI) adsorbed was calculated 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 maximum absorbance (max = 540 nm) by making a purple-violet colored complex of Cr(VI) with 1,5-diphenylcarbazide in the acidic condition which was converted into concentration data using calibration relation pre-determined at the wavelength of interest [44].

Fig. 1. (a) Scanning electron micrograph of KOH treated oxidized carbon. (b) Scanning electron micrograph of KOH treated unoxidized carbon.

3. Results and discussion 3.1. Characterization of prepared samples Information about the physical properties of adsorbent such as carbon content, pH and pore structure is essential procedure prior to adsorption process. The various characteristics of activated carbon prepared from peanut shells by chemical activation with KOH and carbonization at 450 ◦ C under inert and air atmosphere are listed in Tables 1 and 2. As shown in Table 1, the oxidized carbon has higher pH (9.68) and carbon (weight percent, 80.16) than the unoxidized carbon (pH 8.96 and carbon weight percent, 77.32). Thus, the oxidized carbon was more basic than unoxidized one. The specific surface area (SBET ) and micropore volume (VD–R ) of the prepared carbon samples was evaluated by applying the Brunauer, Emmett and Teller (BET) and Dubinin–Radushkevich equations, respectively. It can be observed from Table 2 that the specific surface area (SBET ), micropore surface area (SD–R ), micropore volume (Vmicro )

and mesopore volume (Vmeso ) of oxidized sample was greater than the unoxidized sample which may be due to the air oxidation of the impregnated material during carbonization that has facilitated the evolution of volatile matter from the precursor material and thereby enhanced the porosity in the carbon texture. It is also clear from Table 2 that both oxidized and unoxidized carbons were mesoporous as the mesopore volume of oxidized carbon occupied 74% of the total pore volume while the unoxidized carbon occupied 75% of the total pore volume. According to the International Union of Pure and Applied Chemistry (IUPAC), the pore structures of activated carbons are classified into three groups as micropore (≤2 nm), mesopore (2–50 nm) and macropore (≥50 nm). Both type of activated carbons contained micropores and mesopores but the mesopore volume was larger than the micropore volume. The oxidation during carbonization had a considerable effect on the textural properties of carbon samples. The prepared samples were also characterized for their efficiency for Cr(VI) adsorption and showed

Table 2 Physical properties of oxidized and unoxidized activated carbon samples. Sample

Properties Oxidized Unoxidized

Surface area (SBET ), m2 /g

Average pore width (DR) (Å)

Micropore volume (DR) (cc/g)

Micropore surface area (DR) (m2 /g)

Pore diameter (BJH) (Å)

Surface area (BJH) (m2 /g)

Pore volume (BJH) (cc/g)

Mesopore volume (cc/g)

95.51 88.85

141.50 142.84

0.09 0.08

180.16 179.34

115.10 114.27

133.31 129.89

0.35 0.33

0.26 0.25


45

a

40 Oxidized

30

Un-oxidized

25

qe (mg/g)

% Removal

35

20 15 10 5 0 0

2

4

6 pHi

8

10

10 9 8 7 6 5 4 3 2 1 0

b

Scanning electron micrographs (SEM) of the prepared carbon samples, are shown in Fig. 1(a) and (b). It is clear from the SEM figures that the external surfaces of both samples were rough and contained pores of different size and shapes. The oxidized sample surface had narrow elongated pores while the unoxidized sample had wide pores which showed that air oxidation resulted in activated carbon with well developed porosity. The micrographs showed that the cavities on the surfaces of the carbon samples resulted 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 impregnating agent diffused into the texture of the lignocellulosic material. On carbonization at the desired temperature, the chemical impregnating agent evaporated and made the remaining carbon texture porous. The major elements present in both samples were carbon and oxygen with a small amount of potassium. Table 1 shows that the percentage of carbon in oxidized sample was greater than the unoxidized sample while the unoxidized sample had a higher percentage of oxygen. 3.3. Effect of pH Activated carbons are species with amphoteric character, thus depend on the pH of the solution. Their surface might be positively charged or negatively charged. The pore wall of activated carbon contained a large number of surface functional groups. The pH dependence of Cr(VI) adsorption can largely be related to the type and ionic state of these functional groups and also on the adsorbate chemistry in the solution. The solution pH is one of the important parameter for the removal of heavy metals from aqueous solution because it affects the solubility of adsorbates, concentration of the counter ions on the functional groups of the adsorbent and the degree of ionization of the adsorbate during reaction [45]. Cr(VI) removal was studied as a function of pH over a pH range of 2–10 on oxidized and unoxidized samples at the initial concentration of 50 mg/L as shown in Fig. 2. It is clear from Fig. 2 that the prepared activated carbons were more active in the acidic range and maximum adsorption occurred at pH 2.0. There was a sharp decrease in the sorption capacity when pH was raised from 2.0 to 7.0 and thereafter the effect became negligible. Cr(VI) may exist in three different ionic forms (HCrO4 − , Cr2 O7 2− , CrO4 2− ) in aqueous solutions and the stability of these ions in aqueous systems is mainly pH

40 mg/l 50 mg/l 5

10

15

20

25 30 t (hrs)

35

40

45

50

7 6

that the oxidized carbon had a higher adsorption capacity than the unoxidized carbon.

5 qe (mg/g)

3.2. SEM images

30 mg/l

0

12

Fig. 2. Effect of pH on Cr(VI) adsorption for oxidized and unoxidized carbons at 25 ◦ C.

241

4 30 mg/l 3 40 mg/l

2 1

50 mg/l

0 0

4

8

12 16 20 24 28 32 36 40 44 48 t (hrs)

Fig. 3. Effect of time on Cr(VI) adsorption for (a) oxidized carbon, (b) unoxidized carbon at 25 ◦ C.

dependent [46]. The percentage of Cr(VI) removal was higher in the lower pH ranges due to high electrostatic force of attraction. As the number of H+ ions increased with lowering the solution pH, which neutralized the negative charge on adsorbent surface and thereby increased the diffusion of chromate ions into the bulk of the adsorbent [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 on the surface of activated carbon mostly in the form of HCrO4 − ions. The decrease in the adsorption with increase in pH may be due to the increased number of OH− ions in the bulk which retarded the diffusion of chromate ions. The decrease in adsorption at higher pH may 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 carbons was studied as a function of shaking time at three different initial concentrations (20, 30, 40 mg/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 10 min to 7 h, then became slow up to 20 h and the saturation is almost reached in 24 h in case of both oxidized and unoxidized samples. The nature and compactness of the adsorbent affected the equilibrium time. The removal of Cr(VI) was found to be dependent on the initial concentration. The amount of Cr(VI) adsorbed, qe (mg/g), increased with increase in initial concentration. Further, the adsorption was rapid in the early stages and then gradually decreased and became almost constant after the equilibrium point. At low concentrations,

242


the ratio of available surface to initial Cr(VI) concentration was larger, so the removal became independent of initial concentrations. However, in the case of higher concentrations, this ratio was low. The percentage removal then depended upon the initial concentration. The curves also indicated that the adsorption led to saturation, suggested the possible monolayer coverage of Cr(VI) on the surface of adsorbent [51]. In the case of oxidized carbon, the Cr(VI) removal was decreased from 46.63 to 41.56% as the Cr(VI) concentration was increased from 30 to 50 mg/L and the amount of Cr(VI) adsorbed increased from 5.59 to 8.31 mg/g. while in case of unoxidized carbon, the Cr(VI) removal was decreased from 35.96 to 31.24% as the Cr(VI) concentration was increased from 30 to 50 mg/L and the amount of Cr(VI) adsorbed increased from 4.31 to 6.25 mg/g.

a

3.5. Kinetics of adsorption

b

(2)

where qe and qt are the amounts of Cr(VI) adsorbed (mg/g) at equilibrium 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 k1 and 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 is expressed as [47]: dqt = k2 (qe − qt )2 dt

log (qe -qt)

0.5

40 mg/l

0

-2 -2.5 0

10 8 t (hrs)

12

14

16

18

30 mg/l 40 mg/l 50 mg/l

-0.5 -1 -1.5 -2 0

2

4

6

8 10 t (hrs)

12

14

16

18

Fig. 4. Lagergren first order plot for Cr(VI) adsorption on (a) oxidized carbon, (b) unoxidized.

a

4 3.5 3 2.5 2 1.5

30 mg/l

1

40 mg/l

0

50 mg/l

0

2

4

6

8

10

12

14

16

18

20

22

t (hrs)

b

4 3.5

(4)

(5)

The initial adsorption rate, h (mg/g h) is given as: (6)

Furthermore, Eq. (5) can be written as: (7)

3 2.5 t/qt

which is the integrated rate law for a pseudo second-order reaction. Eq. (4) can be rearranged to obtain Eq. (5), which has a linear form:

t t 1 = + qt qe h

6

0

where qe and qt are the sorption capacity at equilibrium and time t (mg/g), respectively, k2 is the rate constant of the pseudo-second order sorption (g/mg h). For the boundary conditions t = 0 to t = t and qt = 0 to qt = qt , the integrated form of Eq. (3) will be as:

h = k2 q2e

4

0.5

0.5

t 1 t = + qt qe k2 q2e

2

1

(3)

1 1 = + k2 t qe − qt qe

50 mg/l

-1

t/qt

k1 t 2.303

30 mg/l

-0.5

3.5.1. The pseudo-first order equation The pseudo-first order equation [46], is generally expressed as: log(qe − qt ) = log qe −

1

-1.5

log (qe - qt)

Adsorption kinetics provides valuable information about the reaction pathways and mechanism of the reactions. The kinetics of Cr(VI) adsorption on oxidized and unoxidized carbons were analyzed using pseudo first-order [46], pseudo second-order [47] and intraparticle diffusion [48,49] models. The conformity between experimental data and the model predicted values was expressed by the correlation coefficients (R2 ). A relatively high R2 value indicated that the model successfully describes the kinetics of Cr(VI) adsorption.

1.5

2 30 mg/l

1.5 1

40 mg/l

0.5

50 mg/l

0 0

2

4

6

8 10 t (hrs)

12

14

16

18

Fig. 5. Pseudo second order kinetic plot for Cr(VI) adsorption on (a) oxidized carbon, (b) unoxidized carbon.


243

Table 3 Kinetic constant parameters obtained for Cr(VI) adsorption on oxidize and unoxidized carbons. Sample

Pseudo-first order

Oxidized

Unoxidized

Pseudo-second order

Intraparticle diffusion model

Ci (mg/l)

k1 (h−1 )

R2

k2 (g/mg h)

h (mg/g h)

R2

kid (mg/g h)

C (mg/g)

R2

30 40 50 30 40 50

0.326 0.320 0.307 0.215 0.229 0.219

0.98 0.99 0.98 0.96 0.98 0.98

0.417 0.094 0.023 0.875 0.331 0.127

13.624 5.199 2.492 16.313 9.88 5.279

0.999 0.998 0.958 0.999 0.998 0.993

0.666 1.455 2.181 0.372 0.756 1.263

3.325 1.787 0.268 2.985 2.673 1.567

0.835 0.942 0.949 0.859 0.931 0.957

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 and intercepts of the plots, respectively (Table 3). 3.5.3. The intraparticle diffusion model The intraparticle diffusion model [49] is expressed as: qt = kid t 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 t1/2 gave straight line and the values of kid were calculated from the slopes of the plots. Values of C gave an idea about the thickness of boundary layer, i.e., the larger the intercept, greater the contribution of the surface sorption in the rate controlling step. The data for the adsorption of Cr(VI) on to oxidized and unoxidized activated carbons applied to intraparticle diffusion model is shown in Fig. 6(a) and (b) and the results are given in Table 3.

Table 3 shows the values of the correlation coefficient (R2 ) of pseudo-first order, pseudo-second order and intraparticle diffusion kinetic models. The results demonstrated that among these three models, pseudo-first order and pseudo-second order kinetic equations had high R2 values. So, both theses kinetic models were taken as the best fit equations for the description of the mechanism of sorption of Cr(VI) ions. Therefore, the sorption of Cr(VI) ions from aqueous solution onto oxidized and unoxidized activated carbons were found to follow both pseudo-first order and pseudo-second order kinetic equations. 3.6. Adsorption isotherms The equilibrium isotherms for the adsorption of Cr(VI) onto oxidized and unoxidized samples over a wide range of concentration (10–100 mg/L), optimum pH of adsorption and different temperatures are shown in Fig. 7(a) and (b), respectively. These isotherms showed the relationship between the amounts of Cr(VI) adsorbed

a a

10

14 12

9

10 qe (mg/g)

8 qt (mg/g)

7 6 5

8 6

20 ⁰C

4

30 ⁰C

2

40 ⁰C

30 mg/l

4

40 mg/l

3 2

50 mg/l

1

0

0

0 0

0.5

1

1.5

2

2.5

3

3.5

4

t½ (h½)

b

20

30

7

40

50

60

70

80

Ce (mg/l)

b

12 10

6

8 qe (mg/g)

5 qt (mg/g)

10

4.5

4 3

30 mg/l

2

40 mg/l

1

50 mg/l

6 4

20 ⁰C 30 ⁰C

2

40 ⁰C

0

0 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

t½ (h½) Fig. 6. Intraparticle diffusion kinetic plot for Cr(VI) adsorption on (a) oxidized carbon, (b) unoxidized carbon.

0

10

20

30

40

50

60

70

80

90

Ce (mg/l) Fig. 7. Isotherm study of Cr(VI) adsorption on (a) oxidized carbon, (b) unoxidized carbon at different temperatures.


1 RL = 1 + bCi

(10)

where b is the Langmuir constant and Ci is the lowest initial Cr(VI) concentration (mg/L), RL values indicate the type of isotherm. The average values of RL for each of the different initial concentration and temperatures used, was between 0 and 1, which indicated the favorable adsorption of Cr(VI) on both oxidized and unoxidized samples. The linear form of Freundlich isotherm as expressed by Eq. (11), was also applied to the adsorption data of Cr(VI) 1 ln qe = ln K1 + ln Ce n

Ce/qe (g/l)

7

(11)

where K1 (mg/g) and 1/n (g/l) are Freundlich adsorption constants, indicating the adsorption capacity and adsorption intensity, respectively. Straight lines were obtained by plotting ln qe against ln Ce as shown by Fig. 9(a) and (b) for the adsorption on oxidized and unoxidized carbon samples, respectively, which showed that adsorption

6 5

20 ⁰C

4 3

30 ⁰C

2

40 ⁰C

1 0 0

b

(9)

where qe was the quantity of Cr(VI) adsorbed per unit weight of adsorbent (mg/g) at equilibrium, Ce was the equilibrium concentration (mg/L) of Cr(VI) in solution. The constant Qo gives the theoretical monolayer adsorption capacity (mg/g) and b is related to the energy of adsorption (L/mg). Straight lines were obtained by plotting Ce /qe against Ce as shown in Fig. 8(a) and (b) for oxidized and unoxidized samples, respectively. The linear plot of Ce /qe against Ce indicated the applicability of Langmuir adsorption isotherm. Consequently, suggested the formation of monolayer coverage of the adsorbate on the surface of the adsorbent. Langmuir constants, Qo and b were calculated from the slopes and intercepts of plots of Ce /qe versus Ce , respectively, and are given in Table 4 along with correlation coefficients (R2 ). It is clear from Table 4 that the b values were higher at higher temperatures, showed endothermic nature of Cr(VI) adsorption. The essential characteristics of the Langmuir isotherm can be expressed by a dimensionless equilibrium parameter, RL, also known as the separation factor, defined by Weber and Chackravorti [55].

9 8

10

20

30

40 50 Ce (mg/l)

60

70

10 9 8 7 6 5 4 3 2 1 0

80

20 ⁰C 30 ⁰C 40 ⁰C 0

10

20

30

40 50 Ce (mg/l)

60

70

80

90

Fig. 8. Langmuir adsorption isotherms for Cr(VI) adsorption on (a) oxidized carbon, (b) unoxidized at different temperatures.

a

3 2.5 2

ln qe

Ce 1 Ce + = qe Qo Qo b

a

Ce/qe (g/l)

(qe ) and its equilibrium concentration (Ce ) in solution. It is also clear from these figures that the adsorptivity of Cr(VI) increased with increase in temperature. This suggested that Cr(VI) adsorption from aqueous solutions on oxidized and unoxidized carbon was endothermic process. The increase in the adsorption capacity may be due to the chemical interaction between adsorbate and adsorbent, creation of some new adsorption sites or the increased rate of intra particle diffusion of Cr(VI) ions into the pores of the adsorbent at higher temperature [52,53]. To examine the relationship between sorbed (qe ) and aqueous concentration (Ce ) at equilibrium, sorption isotherm models are widely employed for fitting the data, of which Langmuir and Freundlich equations are most widely used. The Langmuir model assumes that the uptake of adsorbate molecules occurs on a homogenous surface by monolayer adsorption without any interaction between adsorbed molecules [54]. Freundlich model is suitable for non-ideal adsorption on heterogeneous surfaces. The heterogeneity is caused by the presence of different functional groups on the surface, and various adsorbent–adsorbate interactions [54]. To get the equilibrium data, initial Cr(VI) concentrations were varied while the adsorbent mass for both samples were kept constant and equilibrium time 24 h, were used for sorption experiments on both oxidized and unoxidized samples. To ensure equilibrium conditions, the linear form of the Langmuir equation was applied to the experimental data.

1.5 1

20 ⁰C

0.5

30 ⁰C 40 ⁰C

0 1

1.5

2

2.5

3

3.5

4

4.5

ln Ce

b

3 2.5 2

ln qe

244

1.5 20 ⁰C

1

30 ⁰C 0.5

40 ⁰C

0 1

1.5

2

2.5

3

3.5

4

4.5

ln Ce Fig. 9. Freundlich adsorption isotherms for Cr(VI) adsorption on (a) oxidized carbon, (b) unoxidized at different temperatures.


245

Table 4 Langmuir and Freundlich isotherm constants for Cr(VI) adsorption on oxidized and unoxidized carbons. Temperature (K)

Oxidized carbon

Unoxidized carbon

Langmuir constants

293 303 313

Freundlich constants 2

Langmuir constants

b (L/mg)

R

1/n

n

K1

R

Qo (mg/g)

b (L/mg)

R

1/n

n

K1

R2

16.26 13.48 13.89

0.023 0.049 0.062

0.93 0.96 0.96

0.64 0.55 0.51

1.56 1.82 1.96

0.70 1.13 1.46

0.98 0.91 0.91

13.68 13.28 14.31

0.02 0.03 0.04

0.98 0.99 0.96

0.65 0.62 0.57

1.54 1.61 1.75

0.57 0.68 1.05

0.98 0.97 0.94

3.7. Adsorption thermodynamics The thermodynamic parameters such as H (enthalpy change) and S (entropy change) were calculated from the slopes and intercepts of the plots of ln Kc versus 1/T as shown in Fig. 10(a) and (b),

0 -0.5

ln Kc

-1 -1.5 -2 -2.5 -3 -3.5 -4 3.15

3.2

3.25

1/T

b

3.3

3.35

3.4

3.45

(Kx10-3)

-3.1 -3.2

ln Kc

-3.3

2

Table 5 Thermodynamic parameters for Cr(VI) adsorption on oxidized and unoxidized carbons. Adsorbent

Thermodynamic parameters

Oxidized

Unoxidized

T (K)

G◦

293 303 313 293 303 313

−28.38 −29.39 −30.32 −11.73 −11.13 −12.53

H◦

S◦

0.04

0.10

0.02

0.04

for adsorption on oxidized and unoxidized carbons, respectively, by using the following relation. ln Kc = −

a

Freundlich constants

Qo (mg/g)

of Cr(VI) obeyed Freundlich isotherm not very well. Values of Freundlich constants and correlation coefficient (R2 ) are given in Table 4. It is also evident from the correlation coefficient (R2 ) values that the Freundlich isotherm did not fitted the experimental data very well. The values of K1 and n changed with the rise in temperature. The value of n showed an indication of the favorability of adsorption. Values of n larger than 1, showed the favorable nature of adsorption [56,57]. The value of n suggested that Cr(VI) are favorably adsorbed by the activated carbon prepared from peanut shells. The values of Qo (monolayer adsorption capacity), as calculated from Langmuir adsorption isotherms for Cr(VI) was found to be higher for oxidized carbon than unoxidized carbon which was in agreement with the high surface area, high carbon content and micropore volume of oxidized carbons.

2

H ◦ S ◦ + RT R

(12)

The G◦ (free energy change) was calculated from the following relation: G◦ = H ◦ − TS ◦

(13)

where R (8.314 J/mol K) is the gas constant, T (K), absolute temperature and Kc (L/mg), standard thermodynamic equilibrium constant defined by qe /Ce . The values of H◦ , S◦ , and G◦ for Cr(VI) adsorption on oxidized and unoxidized carbons are given in Table 5. It may be observed from Table 5 that the values of H◦ was positive indicated the endothermic process of adsorption and the values of S◦ were positive which showed that Cr(VI) adsorption caused disorderness in the system. The value of G◦ indicated the degree of spontaneity of the adsorption process and a more negative value showed an adsorption process which was favorable energetically. The increase in G◦ with increasing temperature showed that the adsorption was more favorable at high temperature. Other workers [58,59] have also been reported similar results for the adsorption of Cr(VI). The value of G◦ was found negative in the adsorption of Cr(VI) at all temperatures but more negative values were obtained in case of oxidized carbon, which confirmed the feasibility of this adsorbent and spontaneity of the adsorption process. So, the capacity of the oxidized carbon for the removal of Cr(VI) was higher than unoxidized carbon. 4. Conclusion

-3.4 -3.5 -3.6 -3.7 -3.8 -3.9 3.15

3.2

3.25

3.3

3.35

3.4

3.45

1/T (Kx10-3) Fig. 10. Plot of ln Kc versus 1/T for Cr(VI) adsorption on (a) oxidized carbon, (b) unoxidized carbon.

Activated carbon was prepared from peanut shell by chemical activation with KOH, characterized and utilized for the removal of Cr(VI) from aqueous solutions in the concentration range of 10–100 mg/L. The oxidized carbon had high surface area and pore volume and higher capacity for Cr(VI) adsorption than the unoxidized carbon. Cr(VI) adsorption was found to be pH dependent. Effective adsorption was occurred in the pH range of 2–4. The kinetics of Cr(VI) followed both pseudo first order and pseudosecond order rate expressions. The removal of Cr(VI) was found to be dependent on the initial concentration. The percentage removal was decreased with increase in initial concentration. Isotherm

246


data were treated according to Langmuir and Freundlich models but the Langmuir adsorption isotherm model fitted well as compared to Freundlich model in the temperature range studied. Cr(VI) adsorption on both oxidized and unoxidized samples was found to be endothermic as adsorption capacity Qo and b values were higher at higher temperatures. The values of Qo (monolayer adsorption capacity), as calculated from Langmuir adsorption isotherms for Cr(VI) was found to be higher for oxidized carbon than the unoxidized one which was in agreement with the high surface area, high carbon content and micropore volume of oxidized sample. The value of G◦ was found negative in the adsorption of Cr(VI) on both carbon samples but more negative values were obtained in case of oxidized sample which confirmed the feasibility of these adsorbent and spontaneity of the adsorption process. The positive value of S◦ showed that Cr(VI) adsorption caused disorderness in the system. Acknowledgment The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding the work through the research group project no. RGP-VPP-043. References [1] S.A. Nabi, R. Bushra, Z.A. Al-Othman, Mu. Naushad, Synthesis, characterization and analytical applications of a new composite cation exchange material acetonitrile stannic(IV) selenite: adsorption behavior of toxic metal ions in nonionic surfactant medium, Sep. Sci. Technol. 46 (2011) 847–857. [2] Z.A. Al-Othman, Mu. Naushad, Inamuddin, Organic–inorganic type composite cation exchanger poly-o-toluidine Zr(IV)tungstate: preparation, physicochemical characterization and its analytical application in separation of heavy metals, Chem. Eng. J. 172 (2011) 369–375. [3] F.C. Richard, A.C.M. Bourg, Aqueous geochemistry of chromium: a review, Water Res. 25 (1991) 807–816. [4] K. Mohanty, M. Jha, B.C. Meikap, M.N. Biswas, Removal of chromium (VI) from dilute aqueous solutions by activated carbon developed from Terminalia arjuna nuts activated with zinc chloride, Chem. Eng. Sci. 60 (2005) 3049–3059. [5] G. Tiravanti, D. Petruzzelli, R. Passino, Pretreatment of tannery wastewaters by an ion exchange process for Cr(III) removal and recovery, Water Sci. Technol. 36 (1997) 197–207. [6] Z. Song, C.J. Williamsm, R.G.J. Edyvean, Sedimentation of tannery wastewater, Water Res. 34 (2000) 2171–2176. [7] H. Oda, Y. Nakagawa, Removal of ionic substances from dilute solution using activated carbon electrodes, Carbon 41 (2003) 1037–1047. [8] G.A. Vlyssides, C.J. Israelites, Detoxification of tannery waste liquors with an electrolysis system, Environ. Pollut. 97 (1997) 147–152. [9] A. Filibeli, N. Buyukkamaci, H. Senol, Solidification of tannery wastes, Resour. Conserv. Recy. 29 (2000) 251–261. [10] A. Kapoor, T. Viraraghavan, D.R. Cullimore, Removal of heavy metals using the fungus Aspergillus niger, Bioresour. Technol. 70 (1999) 95–104. [11] Z. Song, C.J. Williams, R.G.J. Edyvean, Treatment of tannery wastewater by chemical coagulation, Desalination 164 (2004) 249–259. [12] C. Fabianil, F. Rusciol, M. Spadonil, M. Pizzichini, Chromium(III) salts recovery process from tannery wastewaters, Desalination 108 (1996) 183–191. [13] A.I. Hafez, M.S. El-Manharawy, M.A. Khedr, RO membrane removal of unreacted chromium from spent tanning effluent. A pilot-scale study: part 2, Desalination 144 (2002) 237–242. [14] L.K. Wang, D.B. Dahm, R.E. Baier, Treatment of tannery effluents by surface adsorption, J. Appl. Chem. Biotechnol. 25 (1975) 475–490. [15] C.S. Brooks, Metal Recovery from Industrial Waste, Lewis Publishers, Ins., MI, USA, 1991, p. 27. [16] G. Macchi, M. Pagano, M. Pettine, M. Santrori, G. Tiravanti, A bench study on chromium recovery from tannery sludge, Water Res. 25 (1991) 1010–1026. [17] D.C. Sharma, C.F. Forster, A preliminary examination into the adsorption of hexavalent chromium using low-cost adsorbents, Biores. Technol. 47 (1994) 257–264. [18] J.A. Laszlo, F.R. Dintzis, Crop residues as ion exchange materials. Treatment of soybean hull and sugar beet fiber (pulp) with epichlorohydrin to improve cation-exchange capacity and physical stability, J. Appl. Polym. Sci. 52 (1994) 531–538. [19] S. Doyurum, A.C. Elik, Pb(II) and Cd(II) removal from aqueous solutions by olive cake, J. Hazard. Mater. 138 (2006) 22–28. [20] A. Kumar, N.N. Rao, S.N. Kaul, Alkali treated straw and insoluble straw xanthate as low cost adsorbents for heavy metal removal-preparation, characterization and application, Bioresour. Technol. 71 (2000) 133–142.

[21] A.A. Abia, J.C. Igwe, Sorption kinetics and intraparticulate diffusivities of Cd, Pb, and Zn ions on maize cob, Afr. J. Biotechnol. 4 (2005) 509–512. [22] K.K. Wong, C.K. Lee, K.S. Low, M.J. Haron, Removal of Cu and Pb by tartaric acid modified rice husk from aqueous solutions, Chemosphere 50 (2003) 23–28. [23] E. Pehlivan, T. Altun, S. Parlay, Utilization of barley straws as biosorbents for Cu2+ and Pb2+ ions, J. Hazard. Mater. 164 (2009) 982–986. [24] K.A. Krishnan, T.S. Anirudhan, Removal of cadmium(II) from aqueous solutions by steam-activated sulphurised carbon prepared from sugar cane bagasse pith: kinetics and equilibrium studies, Water SA 29 (2003) 147–156. [25] N.A.A. Babarinde, Adsorption of zinc (II) and cadmium (II) by coconut husk and goat hair, J. Pure Appl. Sci. 5 (2002) 81–85. [26] N. Meunier, J. Laroulandie, J.F. Blais, R.D. Tyagi, Cocoa shells for heavy metal removal from acidic solutions, Bioresour. Technol. 90 (2003) 255–263. [27] T.W. Tee, R.M. Khan, Removal of lead, cadmium and zinc by waste tea leaves, Environ. Technol. Lett. 9 (1988) 1223–1232. [28] G. Annadurai, R.S. Juang, D.L. Lee, Adsorption of heavy metals from water using banana and orange peels, Water Sci. Technol. 47 (2002) 185–190. [29] S. Valdimir, J.M. Danish, Characterization and metal sorptive properties of oxidized active carbon, J. Colloid Interface Sci. 250 (2002) 213–220. [30] G.J. Alaerts, V. Jitjaturant, P. Kelderman, Use of coconut shell based activated carbon for chromium(VI) removal, Water Sci. Technol. 21 (1989) 1701–1704. [31] C. Selomulya, V. Meeyoo, R. Amal, Mechanisms of Cr(VI) removal from water by various types of activated carbons, J. Chem. Technol. Biotechnol. 74 (1999) 111–122. [32] M. Kobya, Adsorption kinetic and equilibrium studies of Cr(VI) by hazelnut shell activated carbon, Adsorpt. Sci. Technol. 22 (2004) 51–64. [33] N.K. Hamadi, X.D. Chen, M.M. Farid, M.G.Q. Lu, Adsorption kinetics for the removal of chromium(VI) from aqueous solution by adsorbents derived from used tyres and sawdust, Chem. Eng. J. 81 (2001) 95–105. [34] X.U. Tao, X.Q. Liu, Peanut shell activated carbon: characterization, surface modification and adsorption of Pb2+ from aqueous solution, Chin. J. Chem. Eng. 16 (2008) 401–406. [35] Agricultural Statistics, USDA, National Agricultural Statistics Service, US Government Printing Office, Washington, DC, 2003. [36] C. Namasivayam, K. Periasamy, Bicarbonate-treated peanut hull carbon for mercury (II) removal from aqueous solution, Water Res. 27 (1993) 1663–1668. [37] K. Periasamy, C. Namasivayam, Adsorption of Pb(II) by peanut hull carbon from aqueous solution, Sep. Sci. Technol. 30 (1995) 2223–2237. [38] K. Periasamy, C. Namasivayam, Removal of nickel plating industry wastewater using an agriculture waste: peanut hulls, Waste Manag. 15 (1995) 63–68. [39] K. Periasamy, C. Namasivayam, Removal of Cu (II) by adsorption onto peanut hull carbon from water and copper plating industry wastewater, Chemosphere 32 (1996) 769–789. [40] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 1982. [41] E.P. Barrett, L.C. Joyner, P.H. Halenda, The determination of pore volume and area distribution in porous substances, J. Am. Chem. Soc. 73 (1951) 373–380. [42] M.M. Dubinin, Progress in Surface and Membrane, vol. 9, Academic Press, New York, 1975. [43] F.R. Reinoso, M. Molina-Sabio, M.T. Gonzalez, The use of steam and CO2 as activating agents in the preparation of activated carbons, Carbon 33 (1995) 15–23. ˘ [44] H. Demiral, I. Demiral, F. Tumsek, B. Karabacakoglu, Adsorption of chromium(VI) from aqueous solution by activated carbon derived from olive bagasse and applicability of different adsorption models, Chem. Eng. J. 144 (2008) 188–196. [45] A. Sharma, K.G. Bhattacharyya, Adsorption of chromium (VI) on Azadirachta indica (neem) leaf powder, Adsorption 10 (2004) 327–338. [46] V.K. Singh, P.N. Tiwari, Removal and recovery of chromium(VI) from industrial waste water, J. Chem. Technol. Biotechnol. 69 (1997) 376–382. [47] Y.S. HO, G. Mckay, D.A.J. Wase, C.F. Foster, Study of the sorption of divalent metal ions on to peat, Adsorpt. Sci. Technol. 18 (2000) 639–650. [48] S.K. Srivastava, R. Tyagi, N. Pant, Adsorption of heavy metal ions on carbonaceous material developed from the waste slurry generated in local fertilizer plants, Water Res. 23 (1989) 1161–1165. [49] W.J. Weber, R.K. Chakravarthi, Pore and solid diffusion modules for fixed bed adsorbent, J. Am. Inst. Chem. Eng. 22 (1974) 228–238. [50] M. Rao, A.V. Parwate, A.G. Bhole, Removal of Cr6+ and Ni2+ from aqueous solution using bagasse and fly ash, Waste Manag. 22 (2002) 821–830. [51] C. Namasivayam, D. Sangeetha, Recycling of agricultural solid waste coir pith: removal of anions, heavy metals, organics and dyes from water by adsorption onto ZnCl2 activated coir pith carbon, J. Hazard. Mater. 135 (2006) 449–452. [52] T. Karthikeyan, S. Rajgopal, L.R. Miranda, Chromium(VI) adsorption from aqueous solution by Hevea brasilinesis sawdust activated carbon, J. Hazard. Mater. 124 (2005) 192–199. [53] C. Namasivayam, R.T. Yamuna, Adsorption of chromium (VI) by a low-cost adsorbent: biogas residual slurry, Chemosphere 30 (1995) 561–578. [54] H. Deng, L. Yang, G. Tao, J. Dai, Preparation and characterization of activated carbon from cotton stalk by microwave assisted chemical activation – application in methylene blue adsorption from aqueous solution, J. Hazard. Mater. 166 (2009) 1514–1521. [55] T.W. Weber, R.K. Chackravorti, Pore and solid diffusion models for fixed bed adsorber, J. Am. Inst. Chem. Eng. 20 (1974) 228–238.

Z.A. AL-Othman et al. / Chemical Engineering Journal 184 (2012) 238–247 [56] N. Daneshvar, D. Salari, S. Aber, Chromium adsorption and Cr(VI) reduction to 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 developed from sawdust: adsorption equilibrium and kinetics, J. Hazard. Mater. 113 (2004) 81–88.

247

[58] J. Romero-Gonzalez, J.R. Peralta-Videa, E. Rodriguez, S.L. Ramirez, J.L. Gardeaorresdey, Determination of thermodynamic parameters of Cr(VI) adsorption from 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 Thuja oriantalis, Colloids Surf. 252 (2005) 121–128.