Ind. Eng. Chem. Res. 2006, 45, 6531-6537
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Cadmium(II) Uptake from Aqueous Solution by Adsorption onto Carbon Aerogel Using a Response Surface Methodological Approach† Jyotsna Goel,‡,§ K. Kadirvelu,*,‡ C. Rajagopal,‡ and V. K. Garg§ Centre for Fire, ExplosiVes and EnVironment Safety (CFEES), Defence Research and DeVelopment Organisation (DRDO), Timarpur, Delhi 110054, India, and Department of EnVironmental Science and Engineering, Guru Jambheshwar UniVersity, Hisar, Haryana 125001, India
Recently a new form of activated carbon has appeared: carbon aerogel. Use of carbon aerogel for the adsorptive removal of inorganic compounds (and especially metal ions) has not been investigated. The purpose of this study is to investigate feasibility of cadmium(II) adsorption on carbon aerogel from aqueous solution. The physicochemical properties of carbon aerogel were analyzed to obtain a better understanding of the adsorption mechanism of Cd(II) onto carbon aerogel in aqueous solution. The surface structure of carbon aerogel was analyzed by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray analysis (EDAX). Batch mode adsorption experiments were carried out to assess the adsorption behavior of Cd(II) in aqueous solution. The maximum adsorption capacity (Q0) was calculated by applying the Langmuir equation to Cd(II) in the adsorption isotherm and was found to be 15.53 mg g-1. The influence of three-process variables, namely, adsorbent concentration (0.02-0.1 g) per 50 mL of metal solution, solution pH (2.0-10.0), and temperature (20-70 °C), on the percentage removal of Cd(II) was also examined, using a response surface methodological (RSM) approach. The Box-Behnken model was used as an experimental design, and a statistically practicable second-order polynomial equation was fitted to the model exhibiting a responsevariable relationship. This was evidenced by a high R2 value of 0.9602. Response surfaces were plotted on the basis of the fitted second-order polynomial equation. The optimum conditions for maximum adsorption of Cd(II) were found to be as follows: adsorbent concentration of 0.1 g per 50 mL of metal solution, pH range 6.0-7.0, and temperature of 70 °C. Effect of initial solution pH on Cd(II) removal was carried out to assess the adsorption behavior at different pH values. Adsorption of cadmium(II) increases with increasing initial pH from 2.0 to 10.0. 1. Introduction Cadmium is a highly toxic heavy metal ion, which is found both naturally and as an introduced contaminant in the environment. Cadmium is introduced into water from smelting, metal plating, cadmium and nickel batteries, phosphate fertilizers, mining, pigments, stabilizers, alloy industries, and sewage sludge.1-3 In humans, cadmium is accumulated in the kidneys, which will begin to malfunction at overdoses splitting proteins in urine and disturbing protein metabolism.4 It is well-known that chronic cadmium toxicity has been the cause of Japan ItaiItai disease.5 The harmful effects of cadmium also include the number of acute and chronic disorders, such as renal damage, emphysema, hypertension, testicular atrophy, and skeletal malformation in fetus.6 In India, the tolerance limit for cadmium for discharge into inland surface waters is 2.0 mg L-1 7 and for drinking water is 0.01 mg L-1.8 In this context, several treatment processes for the removal of cadmium from aqueous solutions have been reported, mainly ion exchange, solvent extraction, reverse osmosis, chemical precipitation, membrane filtration, and adsorption. Out of these adsorption has been shown to be a simple, very quick, and economical alternative for removing trace metals. This is due to the ease and efficiencies with which it can be applied in the treatment of wastewater containing heavy metal. * Authors to whom correspondence should be addressed. Tel.: +9111-23907278. Fax: +91-11-23819547. E-mail:
[email protected] (K.K.);
[email protected]. † Abstract submitted to International Workshop on Carbon Materials for Energy Applications at National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India. ‡ Defence Research and Development Organisation (DRDO). § Guru Jambheshwar University.
Recently a new form of activated carbon has been appeared, carbon aerogel, but its use for the removal of inorganic (and specifically metal ions) has not yet been studied. Studies are available which have investigated its application in electrosorption of metal ions from an aqueous medium.9 This paper is mainly concerned with assessing adsorptive properties of carbon aerogel for the removal Cd(II) from aqueous solution. The study also reports the maximum adsorption capacity and kinetic parameters. The important parameters that effect the adsorption, such as adsorbent concentration, pH of solution, and temperature (°C), have also been explored using the response surface methodology (RSM) approach. The BoxBhenken model was used as the experimental design matrix. This approach has limited the number of actual experiments performed while allowing probing into possible interaction between these parameters studied and their effect on percentage removal of Cd(II). 2. Experimental Section 2.1. Materials (Adsorbent). The carbon aerogel used in the study is a commercial product from Lawrence Livermore National Laboratory, CA, U.S.A. Aerogels are microcellular foam materials which are composed of interconnected particles with microscopic interstitial pores; they are quite porous and have an area-to-mass ratio of 700 m2 g-1 and a controllable pore size distribution (e50 nm).10 In the present study, particle sizes of less than 500 µm have been used. The cadmium(II) concentration in the solution after and before adsorption was analyzed using atomic absorption spectrophotometery (AAS: model GBC 935). The surface morphology and metal ion distribution on the surface of activated carbon samples equili-
10.1021/ie060010u CCC: $33.50 © 2006 American Chemical Society Published on Web 08/11/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006 Table 1. Experimental Range and Level of Variables variable
low level
middle level
high level
adsorbent concentration (g), X1 pH, X2 temperature (°C), X3
0.02 (-1) 2 (-1) 20 (-1)
0.06 (0) 6.0 (0) 45 (0)
0.1 (+1) 10 (1) 70 (+1)
and obtained from SD’s Fine Chemicals, India. All the experiments were carried out in duplicate, mean values were used for analysis of results, and maximum deviation was 3%. 2.3. RSM and Variables Optimization. RSM is a collection of statistical and mathematical techniques used for developing, improving, and optimizing any process, product design, system, or for that matter, any experiment under study.11 Percentage removal of cadmium(II) was taken as the response of the system while the three process parameters, adsorbent concentration (0.02-0.1 g), pH (2.0-10.0), and temperature (20-70 °C), were taken as input independent variables. The three-process variables, namely, adsorbent concentration, pH, and temperature, are represented by X1, X2, and X3, respectively. These original variables have been transformed as coded variables x1, x2, and x3, which are usually defined to be dimensionless with mean zero and the same standard deviation.12 This transformation is done by the following formula:
xij )
Figure 1. (a) SEM micrograph of carbon aerogel. (b) SEM micrograph of carbon aerogel equilibrated with Cd(II) solution.
brated with Cd(II) ions at pH 6.0 were visualized via scanning electron microscopy (SEM; model JSM-840 JEOL Techniques, Ltd., Japan) at 200 µm magnification (Figure 1a,b). 2.2. Batch Mode Adsorption Experiments. Batch mode adsorption studies were conducted to determine the equilibrium time needed to reach saturation. Adsorption kinetics were carried out using 50 mL of metal ion solution containing the desired concentration at initial pH 6.0 and 100 mg of adsorbent in a 100 mL conical flask (agitation speed 120 rpm). At a predetermined time interval, samples were separated by centrifugation and analyzed as above. All experiments were carried out at an initial pH of 6.0 where the adsorption was quite significant but below the pH where metal hydroxide precipitation occurs. Stock solution of Cd(II) was prepared (1000 mg L-1) by dissolving 1.1416 g of cadmium sulfate [3CdSO4‚6H2O] in 1 L of slightly acidified distilled water. The stock solution was diluted with distilled water to obtain the desired concentration range of Cd(II) solutions. The adsorption isotherm analyses were carried out with different initial Cd(II) metal ion concentrations (5-70 mg L-1) and a fixed adsorbent concentration of 100 mg for 50 mL of solution at a temperature of 70 °C. The effect was competing ions on the adsorption of cadmium onto the carbon aerogel in the presence of other inorganic metal ions such as mercury and lead in binary and tertiary systems with an equivalent concentration in the same range. HCl and NaOH (0.1 M) was used to adjust the pH. The salts used for lead and mercury were lead nitrate [Pb(NO3)2] and mercuric chloride (HgCl2), respectively. All the chemicals used were extra pure
Xij - [(max Xij + min Xij)/2] (max Xij - min Xij)/2
(1)
where i ) (1, 2, 3, ..., n) variables, and j ) (1, 2, 3, ..., N) number of experiments. It results in all the values of x1, x2, and x3 falling between -1 and +1. As shown in Table 1, the low, middle, and high levels of each variable are designated as -1, 0, and +1. A second-order polynomial model with interaction terms fitted to the experimental data obtained from the experimental runs conducted on the basis of Box-Behnken experimental design model13 is presented. The system was discribed by the following equation:
Y ) β0 + β1x1 + β2x2 + β3x3 + β11x12 + β22x22 + β33x32 + β12x1x2 + β13x1x3 + β23x2x3 (2) where Y is the percentage adsorption of cadmium(II), β0 is the offset term, β1 is the first-order main effect, βii is the secondorder main effect, and βij is the interaction effect. In total 15 experiments are needed to calculate 10 coefficients of the second-order polynomial equation which was fitted on the experimental data. The actual experimental design matrix, based on the Box-Behnken design, is given in Table 2. Each experimental run was carried out with the maintained process variable conditions as given in the design matrix with an initial metal ion concentration of 20 mg L-1. 3. Results and Discussion 3.1. Analysis of Carbon Aerogel by SEM and EnergyDispersive X-ray Analysis (EDAX). The SEM micrographs of the carbon aerogel unabsorbed and loaded with 100 mg L-1 of solution, which contains Cd(II), are shown in Figure 1a,b. These SEM micrographs show clearly the presence of Cd(II) ions on the carbon aerogel particles, whereas Cd(II) was absent on the carbon aerogel before loading it with Cd(II) solution. Some surface modification can be observed on the carbon
Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006 6533 Table 2. Box-Behnken Design Matrix for the Three Variables Together with the Observed Response variable
response
experimental run
adsorbent concentration (g), X1
pH, X2
temperature (°C), X3
% removal of Cd(II), Y
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0.02 0.1 0.02 0.1 0.02 0.1 0.02 0.1 0.06 0.06 0.06 0.06 0.06 0.06 0.06
2 2 10 10 6.5 6.5 6.5 6.5 2 10 2 10 6.5 6.5 6.5
45 45 45 45 20 20 70 70 20 20 70 70 45 45 45
68.49 80.23 75.85 90.63 68.81 76.64 65.93 96.74 63.99 79.86 69.68 84.65 75.64 75.64 75.64
aerogel particle adsorption with (i) surface chemistry of pellets on the surface of the carbon aerogel and (ii) inside the layers of the carbon aerogel. 3.2. Adsorption Kinetics. 3.2.1. Effect of Contact Time on Cd(II) Adsorption. Figure 2 shows the effect of contact time and initial metal ion concentration on its adsorption onto carbon aerogel. As can be depicted from Figure 2, adsorption dynamics curves for initial concentrations of 15-40 mg L-1 are increasing the removal (mg/g) with contact time and attained equilibrium at about 240 min for carbon aerogel. The equilibrium time is one of the important parameters for economical wastewater treatment application. Therefore, an optimum agitation period of 10 h was selected for their experiments to make sure equilibrium is reached. The curves shown in Figure 2 present a double nature; the initial portion of the curve rises linearly, is changed into a curve, and levels after 4 h of agitation time. The plateau portion of the curve corresponds to pore diffusion, and the linear portion of the curve reflects surface layer diffusion. The removal curves were single, smooth, and continuous, indicating the monolayer coverage of the metal ion on the carbon aerogel surface.3 The equilibrium sorption capacity of the carbon aerogel for the Cd(II) ion increased from 5.26 to 9.39 mg g-1 with a rise in the initial metal ions concentrations of 15 to 40 mg L-1. The cadmium(II) ion removal is highly concentration dependent (Figure 3). This increase in metal ion uptake capacity of the carbon aerogel with relation to Cd(II)
Figure 2. Effect of agitation time and initial concentration of Cd(II) on equilibrium concentration qt (mg/g). Conditions: initial pH, 6.0; adsorbent concentration, 80 mg/50 mL.
Figure 3. Cadmium(II) adsorption onto carbon aerogel in single, binary, and tertiary systems with mercury and lead.
ion initial concentration is probably due to the high driving force for mass transfer. In fact, the higher the concentration of the solution, the better the adsorption. Similar results were found by Burns et al., Saucedo et al., and Ho and McKay.14-16 The rate constant of Cd(II) adsorption on carbon aerogel was determined using pseudo-first-order (Lagergren rate equation) and pseudo-second-order rate equations17,18 shown below as eqs 3 and 4, respectively:
log(qe - qt) ) log qe -
k1 t 2.303
t 1 1 ) + t qt k q 2 qe
(3) (4)
2 e
where k1 is the Lagergren rate constant of adsorption (min-1); k2 is the pseudo-second-order rate constant (g mg-1 min-1); and qe and qt are the amounts of metal ion sorbed (mg g-1) at equilibrium and at time t, respectively. The constant values obtained from the two kinetics models are presented in Table 3. The first order equation of Lagergren deviated considerably from the theoretical calculation data of qe. It was clearly found that the pseudo-second-order equation was able to better describe the adsorption of the cadmium ions as evidenced from the better correlation coefficient values as well as the better-predicted value for qe than in the case of the first-order one. Hence, the proves the validity of second-order kinetic model for cadmium adsorption systems. Similar reporting is presented for the cadmium adsorption on chitin19 and on sawdust of Pinus sylVestris.20 3.3. Adsorption Isotherms. The linearized Langmuir21 and Freundlich22 models were applied to adsorption isotherm data. Model equations were as follows:
Ce Ce 1 + ) qe Q0b Q0
(5)
ln(qe) ) ln Kf + 1/n(ln Ce)
(6)
where Ce is the equilibrium concentration (mg L-1), qe is the amount of metal adsorbed at equilibrium (mg g-1), and Q0 (mg g-1) and b (l mg-1) are Langmuir constants related to adsorption capacity and energy of adsorption, respectively, while in case of the Freundlich isotherm, Kf and n are constants incorporating all factors affecting the adsorption capacity and an indication of the favorability of Cd(II) adsorption onto carbon, respectively. Out of the two models applied, the Langmuir isotherm followed better with a coefficient of determination, R2 value of
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Table 3. Kinetic Model Parameters for Cadmium Adsorption onto Carbon Aerogel first-order model metal ion
initial metal ion concn (mg L-1)
qe (mg/g) exptl
Cd(II)
40 30 15
9.26 8.16 5.39
g-1)
qe (mg calcd 2.01 2.07 2.13
0.956. The values of Q0 and b were found to be 15.53 mg g-1 and 0.164 l mg-1, respectively, which were determined from the slope and intercept of the linear plot. Other authors have reported a lower adsorption capacity by other adsorbents, such as Perlite (0.64 mg g-1), Aspergillus Niger (4 mg g-1), furnace sludge (7.39 mg g-1), almond shell carbon (2.7 mg g-1), and granular activated carbon (11.1 mg g-1).23-27 A comparison of these values with the one obtained in this study (15.53 mg g-1) showed that the carbon aerogel used in this study exhibited a comparable capacity of cadmium adsorption from aqueous solution. 3.4. Effect of Competing Ions. The purpose of these experiments has been to investigate the adsorption characteristics of carbon aerogel with respect to cadmium(II) in a competitive adsorption environment as posed by other inorganic ions such as mercury(II) and lead(II) onto carbon aerogel in single, binary, and tertiary systems. Figure 3 represents the equilibrium adsorption curves for Cd(II) in binary and tertiary systems composed of Hg(II) and Pb(II) to demonstrate the change in performance of the adsorbent with respect to cadmium in different multi-metal systems. The Langmuir equation was also applied with the respective cadmium isotherm in the presence of mercury and lead for binary and tertiary systems. Q0 maximum adsorption capacity of Cd(II) was found to be 15.43, 13.30, and 11.57 mg/g for the binary systems Cd(II) + Hg(II) and Cd(II) +Pb(II) and the tertiary system Pb(II) + Hg(II) + Cd(II), respectively. It can be easily observed that sorption is decreased in the presence of other bivalent metal ions. This is due to the screening effect of the surface charge produced by the added metal ions.28,29 3.5. RSM Approach and Statistical Analysis. The BoxBehnken model was used as the experimental design, and a statistically practicable second-order polynomial equation was fitted to model the exhibited response-variable relationship. Response surfaces were plotted based on the fitted second-order polynomial equation. The optimum conditions for maximum removal were found to be as follows: adsorbent concentration of 0.1 g per 50 mL of 20 mg L-1 metal solution, pH range 6.0-7.0, and temperature of 70 °C. An empirical relationship between the response and the input variables expressed by the following fitted second-order polynomial equation is as follows:
Y ) 75.64 + 8.145X1 + 6.075X2 + 0.91813X3 + 2.82250X12 + 0.3375X22 - 1.4325X32 + 0.76X1X2 + 5.745X1X3 - 0.22500X2X3 (7) The significance of each parameter was determined via p values and the student’s t test. A larger t value and smaller p values identifies the effect that appears to be very important. As depicted from Figure 4, it was observed that the quantities X1, X2, and X3 have positive influence while X23 and X32 had a negative influence on adsorption. To determine whether the second-order polynomial equation was significant to fit with the experimental result, it was
second-order model g-1)
k1 (L min-1)
R2
0.007 0.004 0.0003
0.948 0.959 0.975
qe (mg calcd
k2 (L min-1)
R2
0.0059 0.0047 0.0639
0.985 0.988 0.995
8.89 7.87 4.83
Figure 4. Significant main and interactive terms for Cd(II) adsorption. Table 4. ANOVA for the Fitted Second-Order Polynomial Equationa sources of variation
sum of squares
degree of freedom
mean square
F value
probability >F
model error cor total root MSEb dep meanc
1096.152 45.426 1141.578 3.01417 76.56133
9 5 14
121.79461 9.08521
13.41
0.0001
a R2 ) 0.9602. b Root MSE ) square root of mean square error. c Dep mean ) dependent mean (overall mean of the response).
necessary to conduct an analysis of variance (ANOVA). The ANOVA for the second-order equation is presented in Table 4. The ANOVA indicated that the equation represented adequately the actual relationship between the response (the percent removal of cadmium) and the significant variables. It was found that the coefficient of determination (R2) was 0.9602 which is very high and has advocated high correlation between the observed and the predicted value. Moreover, the constant variance between the measured and modeled results is only 2.41%. 3.6. Response Surface Estimations for Maximum Removal of Cadmium. As discussed in the previous sections, the experimental design model, that is, the Box-Behnken model, and RSM were used with three important variables to evaluate their effect on the Cd(II) adsorption process. The response equation was obtained (as given by eq 7) for the percentage removal of cadmium. From the trace plot, Figure 5, it can be depicted that each of the process variables investigated in the study has its individual effect on percentage removal of cadmium(II) by adsorption onto carbon aerogel. The gradual increase in percentage removal of cadmium(II) to maximum is seen with increasing pH from 6.0 (coded value ) 0) to 7.14 (coded value ) 0.38); adsorbent concentration 0.1 g/50 mL (coded value ) 1), and temperature 70 °C (coded value ) 1). The inferences obtained from the response surfaces to estimate maximum removal of cadmium, with respect to each variable and their effects on adsorption, are discussed below. 3.6.1. Effect of Adsorbent Concentration on Cd(II) Adsorption. Figure 6 showed the effect of adsorbent mass on
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Figure 5. Trace plot representing the individual variable effect on percentage removal of Cd(II).
adsorption. It can be depicted that the removal of cadmium increases with increasing adsorbent concentration. It has shown a maximum removal at adsorbent concentration of 0.1 g per 50 mL of metal ion solution. The increase in the percent removal of Cd(II) with the increasing carbon concentration is due to the increase in the surface area consequent to the increase in the number of carbon particles with a larger number of active surface sites for the adsorption, and the saturation occurs as a result of nonavailability of Cd(II) molecules for adsorption.3,28-31 The removal efficiency of metal ions is affected by the initial metal ion concentration with the removal decreasing as the concentration increases at constant pH 6.0. This can be explained as, at low metal/carbon ratio, the metal ion adsorption involves the high energy sites, and as the metal/carbon ratio increases, the higher energy sites are saturated and adsorption begins on the lower sites, resulting in a decrease of adsorption efficiencies.3,28-31 3.6.2. Effect of pH on Cd(II) Adsorption. Figures 6 and 7 show the effect of solution initial pH on percentage removal of cadmium by carbon aerogel. Increasing the initial solution pH from 2.0 to 10.0 increases the percent removal of Cd(II) ion. The adsorption of metal ion depends on the nature of the adsorbent surface and adsorbate species distribution of the ions in aqueous solution, which in turn mainly depends on the pH
of the solution. Thus, at lower pH, both the adsorbent surface and adsorbate species distribution changes are positively charged (H+ and M2+, respectively), and, hence, the net interaction is the electrostatic repulsion between the positively charged adsorbent surface and the cation. The pHZPC value of the carbon aerogel is found to be 4.87. According to these results, when the Cd(II) is adsorbed on the carbon aerogel, the pH range of the solution media for which the carbon aerogel has a negative charge increases (i.e., pH 4.87 and above). In addition, the wiser concentration of the H+ ions present in the aqueous system competes with the positively charged metal ion for the adsorbing sites on the surface of the carbon aerogel. It is very clear that carbon aerogel is effective for the quantitative removal of cadmium from the solution over the pH range 6.0-7.0. Perusal of literature on speciation shows that in the presence of Cl- ion, the dominant species at pH > 4.0 is Cd(OH)2.29 The species such as CdCl2 or (CdCl2)2, Cd(OH)+, and CdOCl are also present in small concentrations. The presence of dimeric CdCl2 in solution has been reported by several authors.29,30 The increase in Cd(II) removal above pH 4.0 for carbon aerogel may be due to the retention of the Cd (OH)2 species into pores of the carbon aerogel particles. The surface functional groups can also contribute the adsorption process. In an acidic pH solution medium, the positively charged Cd2+ or Cd(OH)+ species present in the solution may exchange with H+ from -COOH groups of the carbon aerogel. When cadmium is present in the solution the following surface reaction may be possible:
2CxOH+ + Cd2+ f (CxO)2Cd2+ + 2H+
(8)
It has been shown that the final pH of the solution is always less than the initial pH. Similar results were reported for metal ion removal by activated carbon cloths.32,33 3.6.3. Effect of Temperature on Cd(II) Adsorption. The effect of temperature on Cd(II) adsorption can be inferred from Figure 6. Temperature has a direct influence on the amount of the adsorbed substance. In the present investigation the adsorption experiments were performed in the temperature range of 20-70 °C. It was noticed that, according to the adsorption
Figure 6. Response surface to estimate percentage removal of Cd(II) over the variables adsorbent concentration and pH.
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Figure 7. Response surface plot for percent removal of Cd(II) over the variables temperature and pH.
isotherm, the amount of cadmium adsorbed by the carbon aerogel increases from 75 to 100 percent with increasing temperature from 20 to 70 °C. The increase in percentage of adsorption with respect to temperature indicated that some kind of chemical interaction may be taking place during the adsorption process. Because diffusion is an endothermic process, it would be expected that an increase in the solution temperature would result in an increase in the adsorption capacity. The results of the investigation on the temperature effect also support the conclusion that Cd(II) is controlled by pore diffusion. Values of the thermodynamic parameters such as ∆G°, ∆H°, and ∆S°, describing cadmium uptake by the carbon aerogel, were calculated using following thermodynamic equations as used by Panayatova.33
∆G° ) -RT ln K
(9)
∆H° ) {RT1[T2/(T2 - T1)]} ln(K2/K1)
(10)
∆S° ) (∆H° - ∆G°)/T
(11)
where R is the gas constant and K, K1, and K2 are the equilibrium constants at temperatures T, T1, and T2, respectively. The equilibrium constants were calculated from
K ) Ceq,s/Ceq,1
(12)
where Ceq,s and Ceq,1 are the equilibrium concentration of Cd(II) in solution and on the adsorbent, respectively. Thermodynamic parameters were obtained by varying the temperature conditions over the range 20-70 °C by keeping other variables constant. Values found for ∆G°, ∆H°, and ∆S°, as presented in Table 5, are indicative of the spontaneous nature of the uptake process. The positive ∆H° value confirms the endothermic nature of the sorption process. Because diffusion is an endothermic process, it would be expected that increased solution temperature would result in an increased uptake of cadmium ions from aqueous solution.
Table 5. Thermodynamic Parameters for the Cadmium Adsorption on Carbon Aerogel temperature (K)
% removal
∆G° (J mol -1)
∆H° (J mol-1)
∆S° (J mol-1 K-1)
293 303 313 323 333 343
75 82 89 95 97 100
-987.714 -2073.76 -3636.93 -6045.67 -7704.77
30 833.39 45 290.44 71 756.47 47 543.43
108.6045 156.3175 240.8735 165.9105
The results obtained in our experiments generally agree with the results previously found for the uptake of Cd(II) by various adsorbents. Vinod and Sharma34 and Ferro-Garcia et al.35 have also presented similar thermodynamics for the uptake of cadmium by red mud and activated carbon. 4. Conclusion The objective of this study was to investigate the feasibility of using carbon aerogel for the removal of cadmium metal from aqueous solution. The adsorption equilibrium was reached within 240 min for all the Cd(II) concentrations. The adsorption kinetics studies show that the first order equation of Lagergren deviated considerably from the theoretical calculation data of qe. It was clearly found that the pseudo-second-order equation was able to better describe the adsorption of the cadmium ion as evidenced from the better correlation coefficient values as well as better-predicted value for qe than in case of the first-order one. Adsorption of Cd(II) followed the Langmuir model as evidenced from better coefficient of the correlation value (0.956). The adsorption capacity (Q0) from the Langmuir model was found to be 15.53 mg g-1 for Cd(II) at an initial solution pH of 5.0. The positive ∆H° value confirms the endothermic nature of the sorption process. The effect of the initial solution pH on Cd(II) removal shows that increasing the pH from 2.0 to 10.0 increases the percent adsorption. This paper is also an attempt to demonstrate the response surface methodological approach for and assessing the effects
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of process variables, thereby obtaining the optimal conditions for maximum removal of the cadmium by statistical design of experiments. A Box-Behnken design model for developing the experimental design matrix for three variables, adsorbent mass, pH, and temperature, and then the subsequent fitting of the second-order polynomial equation have revealed the following optimal conditions: pH ) 6.0-7.0, adsorbent mass ) 0.1 g, and temperature ) 70 °C. The whole process has facilitated the optimum conditions in the minimum number of experiments while allowing possible interaction between these variables to be probed. Adsorption of cadmium(II) onto carbon aerogel is viable and technically feasible; moreover, the rapid uptake allows carrying out the adsorption of heavy metals on a column filled with this carbon aerogel to be considered because the contact time between the metal solution and the adsorbent is generally short in this process. These results were confirmed working with some industrial wastewater, and development on the pilot scale is now being studied. Acknowledgment The authors are grateful to Director CFEES, for his encouragement and providing facilities for carrying out the work. J.G. is grateful for extension of Senior Research Fellowship (SRF) at CFEES, DRDO, Delhi. Literature Cited (1) Buchaver, M. Contamination of soil and vegetation near a zinc smelter by zinc, cadmium, copper and lead. EnViron. Sci. Technol. 1973, 7. (2) Plunkett, E. R. Handbook of Industrial Toxicology; Chemical Publishing Company, Inc.: New York, 1987. (3) Kadirvelu, K. Preparation and characterization of coir pith carbon and its utilization in the treatment of metal bearing wastewater. Ph.D. Thesis, Bharthiar University, Coimbatore, India, 1998. (4) Patterson, J. W.; Passino, R. Metals speciation, separation, and recoVery; Lewis Publishers: Chelsea, MI, 1987. (5) Friberg, L.; Piscato, M.; Nordbert, C. G.; Kjellstrom, T. Cadmium in the enVironment; Springer: Berlin, Germany, 1979. (6) Oehme, F. W. Toxicity of heaVy metals in the enVironment; Marcel Dekker, Inc.: New York, 1979. (7) The Indian Standard for Drinking Water Specifications; IS 10500; Bureau of Indian Standards: New Delhi, 1991. (8) Tolerance limits for industrial effluents prescribed by Bureau of Indian Standards; IS 2490 (Part I); Bureau of Indian Standards: New Delhi, 1981. (9) Farmer, J. C. Electrosorption of chromium ions on carbon aerogel electrodes as a means of remediating groundwater. Energy Fuels 1997, 11, 337. (10) Pekala, R. W. Organic aerogels from the polycondensation of resorcinol with formaldehyde. J. Mater. Sci. 1989, 24, 3221. (11) Ozturk, N.; Duygu, K. Boron Removal from Aqueous Solutions by Adsorption on Waste Sepiolite and Activated Waste Sepiolite Using Full Factorial Design Adsorption. 2004, 10, 245. (12) Box, G. E. P.; Draper, N. R. Empirical model building and response surfaces; John Wiley & Sons: New York, 1987. (13) Singh, B. P.; Besra, L.; Bhattacharjee, S. Factorial design of experiments on the effect of surface charges on stability of aqueous colloidal ceramic suspension. Colloids Surf., A 2002, 175, 204. (14) Burns, C. A.; Boily, J.-F.; Crawford, R. J.; Harding, I. H. Cd(II) sorption onto chemically modified Australian coals. Fuel 2005, 84, 1653.
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ReceiVed for reView January 4, 2006 ReVised manuscript receiVed May 28, 2006 Accepted July 5, 2006 IE060010U
