Ind. Eng. Chem. Res. 1994,33, 317-320
317
Process Development for Removal and Recovery of Cadmium from Wastewater by a Low-Cost Adsorbent: Adsorption Rates and Equilibrium Studies Karuppanna Periasamy and Chinnaiya Namasivayam’ Environmental Chemistry Division, Department of Environmental Sciences, Bharathiar University, Coimbatore-641 046, Tamil Nadu, India
Activated carbon prepared from peanut hulls (PHC), an agricultural waste by-product, has been used for the adsorption of Cd(I1) from synthetic wastewater. The adsorption data fit better with the Freundlich adsorption isotherm. The applicability of the Lagergren kinetic model has also been investigated. An almost quantitative removal of 20 mg/L Cd(I1) by 0.7 g of PHC/L of aqueous solution was observed in the p H range 3.5-9.5. A comparative study with a commercial granular activated carbon (CAC) showed that the adsorption capacity (Kf)of PHC was 3 1 times larger than that of CAC. 1. Introduction
Cadmium is introduced into bodies of water from smelting (Buchauer, 1973), metal plating, cadmium-nickel batteries, phosphate fertilizer, mining, pigments, stabilizers, alloy industries (Low and Lee, 1991), and sewage sludge (Bhattacharya and Venkobachar, 1984). The harmful effects of cadmium include a number of acute and chronic disorders, such as “itai-itai” disease, renal damage, emphysema, hypertension, and testicular atrophy (Huang and Ostovic, 1978). The tolerance limit for cadmium for discharge into inland surface waters is 2.0 mg/L (ISI, 1982) and in drinking water is 0.01 mg/L (ISI, 1991) according to the Indian StandardsInstitution. Both powdered activated carbon (Sorg et al., 1978) and granular activated carbon (Huang and Smith, 1981) have been examined for the removal of Cd(I1) and were found to show a removal efficiency of nearly 50% in the pH range 7.0-9.0. Many reports have appeared on the development of activated carbon from cheaper and readily available materials (Hassler, 1974). Activated carbon derived from rice husk (Srinivasan et al., 1988) and coconut shell (Arulanantham et al., 1989) have been successfully employed for the removal of heavy metals from aqueous solutions. We have recently reported that carbon derived from peanut hull, a waste agricultural by-product, removed Cr(V1) (Periasamy et al., 1991) and Hg(I1) (Namasivayam and Periasamy, 1993) efficiently from aqueous solutions. The investigation reported here deals with a comparative study of PHC and CAC for the removal of Cd(I1) from aqueous solutions.
2. Materials and Methods 2.1. Adsorbent. PHC was prepared as reported before (Namasivayam and Periasamy, 1993). The particle size of 0.575 mm (20-50 mesh ASTM) was used. The CAC obtained from M/s. Burbidges Company, Bombay, India, was ground and sieved to the same size. The characteristics of the carbons such as bulk density, ash content, moisture, solubility in water (inorganic matter) and 0.25 M HC1 (inorganic and organic matter), decolorizing power, iron content, and pH were determined accordingto IS1methods (ISI, 1977). Phenol number (Fairet al., 1971), ion exchange capacity (Vogel, 19691, surface area by the p-nitrophenol method (Giles and D’Silva, 1969), and porosity (Vomocil,
* Author
to whom correspondence should be addressed.
Table 1. Characteristics of the Carbons parameter PHC bulk density, g/mL 0.63 porosity, % 61.7 14.14 moisture, 5% 2.11 ash, % 0.74 solubility in water, % solubility in 0.25 M HC1, % 2.25 6.68 PH 36.00 decolorizingpower, mg/g phenol number 68.00 ion exchange capacity, mequiv/g 0.49 208.00 surface area, m2/g iron, % 0.27 ash analysis, % Si02 19.6 0.58 K2O 0.56 CaO 9.8 MgO 0.1 p206 78.7 Na2O 0.76 Fez03
CAC 0.60 60.4 6.79 3.84 1.42 1.56 8.18 77.00 20.00 nil 354.00 1.43 79.6 2.3 0.28 6.5 1.2 12.1 4.1
1965) were estimated according to published procedures. Ash analysis was carried out as per standard procedures (APHA, 1980; Vogel, 1969). The characteristics of PHC and CAC are summarized in Table 1. 2.2. Batch Mode Studies. A synthetic wastewater sample was prepared from cadmium sulfate [ 3 C d S 0 ~ 8H201 in water containing a few drops of nitric acid to prevent hydrolysis. The stock solution was diluted as required to obtain standard solutions containing 10-20 mg/L Cd(I1). One hundred milliters of Cd(I1) solution of a desired concentration, adjusted to a desired pH, was taken in reagent bottles of 300-mL capacity, and known amounts of PHC or CAC were added. The pH was adjusted using dilute nitric acid or sodium hydroxide solutions. All the chemicals used were of analytical reagent grade and were obtained from BDH, E. Merck, SD’s,and/or Ranbaxy. The solutions were agitated at 180rpm for a predetermined period at 30 f 1“C in a reciprocating shaker. The bottles were removed from the shaker, centrifuged at 10 OOO rpm, 8600Xg, and the supernatant was analyzed for Cd(I1) spectrophotometrically (Prasad Rao and Ramakrishna, 1982). Adsorption isotherm studies were carried out with different initial concentrations of Cd(I1) and fixed concentration of carbon. For pH effects, 20 mg/L Cd(I1) and a PHC concentration of 1.0 g/L or a CAC concentration of 5.0 g/L was used. In order to correct for any adsorption of Cd(I1) due to containers, control experiments were
0888-588519412633-0317$04.50/0 0 1994 American Chemical Society
318 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
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Figure 1. Effect of agitation time on adsorption of Cd(I1). Cd(I1) 15 mg/L, (A)10 mg/L; pH, 5.0. (A: concentration, (0)20 mg/L, (0) PHC) Carbon concentration, 1g/L; (B: CAC) carbon concentration, 5.0 g/L.
carried out without adsorbent and there was negligible adsorption by the container walls. Desorption studies were carried out as follows. After the adsorption experiment with 20 mg/L Cd(I1) and 1.0 g/L PHC or 5.0 g/L CAC, the cadmium-loaded carbons were separated and gently washed with distilled water to remove any unadsorbed Cd(I1). The carbons were then agitated with 100 mL of hydrochloric acid of various strengths for 3 h in the case of PHC and 7 h in the case of CAC, and the amount of desorbed cadmium was estimated as before.
A PHC
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3. Results and Discussion
3.1. Effect of Agitation Time and Initial Concentration. Figure 1presents the effect of agitation time on the removal of Cd(I1) by PHC and CAC. The removal (mg/g) increases with time and attains equilibrium at 60 min for PHC and 300 min for CAC for initial Cd(I1) concentration of 10,15, and 20 mg/L. It shows that the residence time required for maximum Cd(I1) removal by PHC would be 5 times less than that required by CAC. 3.2. Effect of Carbon Concentration. Figure 2 shows the removal of Cd(I1)as a function of carbon concentration by PHC and CAC. It is evident that for the maximum removal of 20 mg/L Cd(1I)in 100mL of solution aminimum PHC concentration of 0.7 g/L or CAC concentration of 12 g/L is required. 3.3. Adsorption Isotherms. The Langmuir equation was applied for adsorption equilibrium for both PHC and CAC CJq, = 1/(Qob) + (Ce/Qo)
(1)
where C e is the equilibrium concentration (mg/L), q e is the amount adsorbed at equilibrium (mg/g), and Qo and b are Langmuir constants related to adsorption capacity and energy of adsorption, respectively. The linear plots C,/qe vs C, show that the adsorption obeys Langmuir
l6
t C, (mg/L)
Figure 3. Langmuir plots of adsorption of Cd(I1). Cd(I1) concentration, 40-100 mg/L; pH,6.5; agitation time, 24 h. ( A PHC) Carbon concentration, 0.7 g/L; (B: CAC) carbon concentration, 1.2 g/L.
isotherm model for both PHC and CAC (Figure 3). QO and b, respectively, were determined from the Langmuir plots and found to be 89.29 mg of Cd/g of carbon and 0.37 L/mg of Cd for PHC; and 2.74 mg of Cd/g of carbon and 0.10 L/mg of Cd for CAC. The ratio of the QOvalue of PHC to that of CAC works out to be 32.54. The essential characteristics of Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter, RL,which is defined by RL = 1/(1+ bCo), where b is the Langmuir constant and COis the initial concentration of Cd(I1) (McKay et al., 1982). RL values between 0 and 1 indicate favorable adsorption of Cd(I1) on both PHC and CAC.
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 319 2.0r
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c, Figure 4. Freundlich plots for adsorption of Cd(I1). Cd(I1) concentration, 40-100 mg/L; pH, 6.5;agitation time, 24 h. ( A PHC) Carbon concentration, 0.7 g/L; (B: CAC) carbon concentration, 1.2 g/L.
0.010
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Figure 5. Lagergren plots for adsorption of Cd(I1). Cd(I1) concentration; (0) 20 mg/L, (0) 15 mg/L, (A)10mg/L. A,PHC; B, CAC.
The Freundlich isotherm is represented by the equation (McKay et al., 1982) log(x/m) = log Kf + (l/n) log c, (2) where C, is the equilibrium concentration (mg/L),x is the amount adsorbed (mg/L), and m is the weight of carbon (g/L). Plots of log(x/m) vs log Ce are linear for both PHC and CAC (Figure 4). The constants Kf (a measure of adsorption capacity) and n (a measure of adsorption intensity), respectively, were found to be 46.24 mg of Cd(II)/g of carbon and 6.00 for PHC and 1.48 mg of Cd(II)/g of carbon and 3.57 for CAC. Values of 1 < n < 10 show favorable adsorption of Cd(I1) on both PHC and CAC (McKay et al., 1982). Similar results have been observed in the adsorption of Cd(I1)on rice husk carbon (Srinivasan, 19861, where the Kfand n values were found to be 31.62 mg of Cd(II)/g of carbon and 5.00, respectively. The correlation coefficients for the linear regression fits of the Langmuir plots were found to be 0.9978 and 0.9989 for PHC and CAC, respectively. The corresponding values for the Freundlich plots were 0.9974 and 0.9834. The Langmuir model does not satisfactorily explain the effect of adsorbent concentration. For example, in the case of PHC, calculations using Langmuir and Freundlich models show a removal of 93.6% and 99.7 % ,respectively, for a Cd(I1) concentration of 20 mg/L and adsorbent concentration of 0.7 g/L, whereas Figure 2 shows nearly 100% removal for the same conditions. Hence the Freundlich model fits better with the data for PHC. This is true for CAC as well. From parameters given one would need to have 0.38 and 0.93 g of PHC/L, respectively, to reach the tolerance level of 2.0 mg/L for industrial discharge into inland surface waters and 0.01 mg/L for drinking water, if the initial water had 20 mg/L Cd(I1).Kf of 46.24 mg of Cd(II)/g of PHC (surface area 208 m2/g) and 1.48 mg of Cd(II)/g of CAC (surface area 354 m2/g), respectively, translate into an effective area of 83 and 4464 A2/Cd(II)ion. I t appears that Cd(I1) sites are more closely packed in PHC compared to CAC. 3.4. Adsorption Kinetics. The kinetics of Cd(I1) adsorption on both PHC and CAC follow the first-order
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Figure 6. Effect of pH on removal of Cd(I1). Cd(I1) concentration: 20 mg/L; (0) PHC; carbon concentration, 1 g/% agitation time, 3 h. (0) CAC; carbon concentration, 5.0 g/L; agitation time, 7 h. Table 2. A d s o r d o n Rate Constants kad
[Cd(II)I (mg/L) 10 15 20
PHC 5.36 X 6.03 X 4.48 X
(l/min) CAC 1.09 x 10-2 0.92 X 10-2 0.80x 10-2
rate expression given by Lagergren (Singh et al., 1989): lOg(q, - 4) = log 4, - kadt/2.303 (3) where q and qe are the amounts of Cd(I1) adsorbed (mg/g) at time t(min) and at equilibrium time, respectively, and kad is the rate constant of adsorption. Linear plots of log(9, - q ) vs t show the applicability of the above equation for both PHC and CAC (Figure 5). The kad values were calculated and are presented in Table 2. 3.5. Effect of pH. Figure 6 presents the effect of initial pH on the removal of Cd(I1) by PHC and CAC. The Cd(11)removal by both the carbons increases with increase in pH and attains 100% and 46.5% for PHC and CAC, respectively, at pH 3.0. It is evident that both the carbons are effective for the maximum removal of Cd(I1) over the pH range 3.0-9.5. A pure carbon surface is considered to
320 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
be nonpolar, but in actual practice some carbon-oxygen complexes are usually present, which render the surface slightly polar. Since there is no satisfactory method for determining the polar character of the surface quantitatively, the above statement is relative (Yenkie and Natarajan, 1991). The influence of pH on Cd(I1) removal may probably be explained as follows: Significant adsorption is observed below pH 3.0, at which both sorbent and sorbate species are positively charged and therefore the interaction is that of electrostatic repulsion. Besides this, a higher concentration of H+ ions present in the reaction mixture competes with Cd2+ions for the adsorption sites, resulting in the reduced uptake of Cd(I1). As the pH increases, the concentration of H+ ion decreases, whereas the concentration of Cd2+remains constant and therefore the uptake of Cd(I1) can be explained as an H+Cd2+exchange reaction. It appears that, in addition to electrostatic forces, specificchemical interaction also plays an important role in Cd(I1) adsorption. This is in accordance with the observations made by Bhattacharya and Venkobachar (1984)and Huang and Ostovic (1978). 3.6. Desorption Studies. Desorption studies help elucidate the nature of adsorption and to recover the precious metals from wastewaters and the adsorbent. Attempts were made to desorb Cd(I1) from the spent carbons using hydrochloricacid of various strengths (0.0250.25 M). With 0.05 M hydrochloric acid quantitative recovery of Cd(I1) was observed for both PHC and CAC. It may be concluded that ion exchange is involved in the adsorption mechanism. 4. Conclusion
Peanut hull carbon is an effective adsorbent for the removal and recovery of Cd(I1) from aqueous solutions. Its adsorption capacity is much superior to commercial activated carbon. It would be useful for the economic treatment of wastewater containing Cd(II), as the adsorbent is derived from an agricultural waste by-product, namely, peanut hull.
Acknowledgment
K.P. is grateful to Dr. M. Shanmugam, Principal, Institute of Road and Transport Technology,Erode, Tamil Nadu, for providing facilities and encouragement. Literature Cited APHA. Standard methods for the examination of water and wastewater; American Public Health Association: Washington DC, 1980. Arulanantham, A.; Balasubramanian, N.; Ramakrishna, T. V. Coconut shell Carbon for Treatment of Cadmium and Lead Containing Wastewater. Met. Finish. 1989,Nov, 51-55. Bhattacharya, K.; Venkobachar, C. Removal of Cadmium(I1)by Low Cost Adsorbents. J. Environ. Eng. Diu. ASCE 1984, 104, 110122.
Buchauer, M. J. Contamination of Soil and Vegetation Near a Zinc Smelter by Zinc, Cadmium, Copper and Lead. Environ. Sci. Technol. 1973,7, 131-135. Fair, G. M.; Geyer, J. C.; Okun, D. A. Elements of Water Supply and Wastewater Disposal; John Wiley and Sons Inc.: New York, NY, 1971;p 446. Giles, C. H.; D'Silva, A. P. Trans. Faraday SOC. 1969,65, 1943. Hassler, J. W. Purification with Activated Carbon; Chemical Publishing Co. Inc.: New York, NY, 1974; p 169. Huang, C. P.; Ostovic, F. B. Removal of Cadmium(I1) by Activated Carbon Adsorption. J. Environ. Eng. Diu. ASCE 1978,104,863878. Huang, C. P.; Smith, E. H. Removal of Cd(I1)from Plating Wastewater by an Activated Carbon Process. Chemistry in Water Reuse; Cooper, W. J., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981;Vol. 2. ISI. Methods of Sampling and Tests for Activated Carbon used for Decolorizing Vegetable Oils and Sugar Solutions; ISI, 1977;p 877. ISI. Tolerance limits for industrial effluents prescribed by Indian Standards Institution, IS:2490 (Part I), 1982. ISI. Drinking Water Specification,IS10500,1991.(Address: Bureau of Indian Standards, Manak Bhavan, 9 Bahadur Shah Zafar Marg, New Delhi 110 002, India.) Low, K. S.; Lee, C. K. Cadmium Uptake by the Moss Calympers Delesertii, Besch. Bioresour. Technol. 1991,38,1-6. McKay, G.; Blair, H. S.; Garden, J. R. Adsorption of Dyes on Chitin. 1. Equilibrium Studies. J. Appl. Polym. Sci. 1982,27,30433057. Namasivayam, C.; Periasamy, K. Bicarbonate-treated Peanut Hull Carbon for Mercury(I1) Removal from Aqueous solution. Water Res. 1993,27,1663-1668. Periasamy, K.;Srinivasan, K.; Murugan, P. K. Studies on Chromium(VI) Removal by Activated Groundnut Husk Carbon. Indian J. Environ. Health. 1991,33,433-439. Prasad Rao, T.;Ramakrishna, T.V. Spectrophotometric Determination of Trace Amounts of Cadmium in Pure Zinc Materials with Iodine and Pyronine G. Analyst 1982,107,704-707. Singh, D. B.; Prasad, G.; Rupainwar, D. C.; Singh, V. N. As(II1) Removal from Aqueous solution by Adsorption. Water, Air Soil Polht. 1989,42, 373-386. Sorg, T. J.; Casnady, M.; Logsdon, G. S. Treatment Technology to Meet Interim Primary Drinking Water Regulations for Inorganics. J. Am. Water Works Assoc. 1978,70, 680-691. Srinivasan, K. Evaluation of Rice Husk Carbon for the Removal of Trace Inorganicsfrom Water. Ph.D. Dissertation, Indian Institute of Technology, Madras, India, 1986. Srinivasan, K.; Balasubramanian, N.; Ramakrishna, T. V. Studies on Chromium Removal by Rice Husk Carbon. Indian J. Environ. Health 1988,30,376-387. Vogel, A. I.A Text Book of Quantitative Inorganic Analysis, 3rd ed.; ELBS Publication: London, 1969;p 714. Vomocil, J. A. Porosity. In Methods of Soil Analysis; Black, C. A.; Ed.; American Society of Agronomy, Inc.: Madison, WI, 1965;p 299. Yenkie, M. K. N.; Natarajan, G. S. Adsorption Equilibrium Studies of some Aqueous Aromatic Pollutants on Granular Activated Carbon Samples. Sep. Sci. Technol. 1991,26,661-674. Received for review June 22, 1993 Revised manuscript received October 7, 1993 Accepted October 31, 1993' ~
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* Abstract published in Advance ACS Abstracts, January 1, 1994.
