M at 6.5 pH, 100 rpm and 0.01 M NaClO4 ionic strength. The pH of the solution plays an important role in the removal of cadmium. Rate of the removal was ...
Indian Journal of Chemical Technology Vol. 13, May 2006, pp. 218-221
Adsorption of cadmium(II) from aqueous solutions by an indigenous clay mineral Y C Sharma* & V Srivastava Environmental Engineering and Research Laboratories, Department of Applied Chemistry, Banaras Hindu University, Varanasi 221 005, India Email: Received 21 July 2005; revised received 7 March 2006; accepted 9 March 2006 Removal of cadmium, a priority pollutant, has been investigated by a locally available clay mineral, China clay. The removal depends on initial concentration of cadmium in solution and higher removal has been obtained in lower concentration ranges. The removal increased from 41.0 to 80.3% by decreasing the concentration of cadmium in solution from 2.0×10−4 to 0.5×10−4 M at 6.5 pH, 100 rpm and 0.01 M NaClO4 ionic strength. The pH of the solution plays an important role in the removal of cadmium. Rate of the removal was calculated by Lagergren’s model and was found to be 5.10×10−2 min−1 in optimum conditions. The process of removal proceeds with intraparticle diffusion and the value of the coefficient of intraparticle diffusion was found to be 3.25×10−10 cm2 s−1. The parameters can be used for designing a plant for treatment of Cd(II) rich water and wastewater economically. Keywords: Cadmium(II), Adsorption, China clay IPC Code: B01D21/00
Metals and metallic compounds are important for industrial development of any country. Like other metals, cadmium occurs in nature and is found in association with zinc minerals1,2. Cadmium has been reported to be highly toxic because there does not exist a homeostatic control in human body for this metal. Chronic exposures to cadmium through air, food or aquatic sources result in its steady accumulation in the body. About 1-2% of ingested cadmium, if retained in body is hazardous and poses health problems. It has been reported to be a potent enzyme inhibitor and it has also been reported to damage liver and kidney3 of animals and human beings. Animal species have shown cadmium to be teratogenic and crustaceans are more sensitive to the toxic effects of cadmium. Precipitation, ion exchange, solvent extraction, etc. are the well-documented technologies for the treatment of water and wastewater rich in different metallic species and cadmium4-8. Adsorption on activated carbon and activated charcoal has been recognized as popular choice for treatment of cadmium rich wastewater and also for other metallic species9-11 but all these processes are cost intensive inhibiting their largescale application to the developing nations. Because of the high treatment cost of water and wastewater by activated carbon etc. scientists world-over are engaged in the search of economically viable
alternatives12-14. Recently some scientific workers have used various low cost adsorbents15-18 for removal of cadmium from wastewaters. Emphasizing on the economic aspects of treatment of Cd(II) rich waters and wastewaters, the present communication aims at application of China clay, a clay mineral as an alternate to the otherwise costly activated carbon. Experimental Procedure The adsorbent, was procured from Patharghat, Bihar, India. In order to keep the process cost of cadmium removal low, it was used as such without any pretreatment in the experiments after crushing and passing through a sieve of 100 μm. The average particle size of the adsorbent was measured by particle size analyzer, model HIAC-320 (ROYCO Instruments Division, USA) and the surface charge by Lazar Zee Meter, model 1-500 (Penkem Inc., NY, USA). The surface area of the adsorbent was determined by a ‘three point’ N2 gas adsorption method using Quantasorb Surface Area Analyzer, model QS/7 (Quantachrome Corp., USA) and porosity by a mercury porosimeter. The chemical analysis of the adsorbent was carried out as per Indian Standard Methods19. Batch adsorption experiments were carried out by agitating 1.0 g of adsorbent with cadmium solutions of desired strength, temperature and pH in different
SHARMA & SRIVASTAVA: ADSORPTION OF CADMIUM(II) FROM AQUEOUS SOLUTIONS
glass bottles in a shaking thermostat at 100 rpm. Equilibrium time was determined by shaking the cadmium solutions with adsorbent for different time intervals. At the end of equilibrium time the adsorbent was separated from solutions of cadmium by filtration and the progress of removal was assessed by determining the amount of cadmium in the supernatant spectrophotometrically20 using a spectrophotometer, model UV 2100 (Shimadzu, Japan). The pH of the solutions was maintained by 1 M NaOH/HCl. Results and Discussion
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Table 1—Characterization of China clay Constituents
% by weight
SiO2 Al2O3 CaO Fe2O3 MgO Loss on ignition Mean diameter (μm)
46.22 38.40 0.86 0.68 0.37 13.47 100. 00
Surface area (m2 g−1)
13. 52
−3
Density (g cm )
2.69
Chemical characterization of China clay
The chemical characterization of China clay was carried out by Indian Standard Methods19 and the results of analysis are given in Table 1. The analysis shows that SiO2 and Al2O3 are its main constituents. Oxides of other metals are present in traces. Determination of time of equilibrium and optimum cadmium concentration
Study of the effect of contact time and concentration on adsorption of Cd(II) on China clay shows that removal increased from 41.0 to 80.3% (Fig. 1) by decreasing the concentration of cadmium in solution from 2.0×10−4 to 0.5×10−4 M at pH 6.5, temperature 30°C and 0.01 M NaClO4 ionic strength. The removal is rapid in initial stages and then gradually decreases and acquires equilibrium in 80 min. This finding reveals two important features: the process is highly dependent on initial concentration of cadmium and the time of equilibrium is independent of the initial solute concentration. These findings have lot of practical importance. The contact of the solute and the adsorbent must be eliminated after the equilibrium time. Effect of pH on the removal of cadmium
The pH plays an important role in most adsorption processes and in the present system also the effect of this parameter was studied on the process of removal. The adsorption of cadmium was found to be maximum (86.6%) at pH 9.5 (Fig. 2) out of the different values of pH viz. 2.5, 4.5, 6.5 and 9.5. It was, however, found to be minimum (9.3%) at pH 2.5. It is expected that the adsorbent surface in the alkaline range would favour the uptake of Cd(II) and the same has been found to be applicable in the present studies. In aqueous environments, oxides and oxide minerals
Fig. 1—Removal of Cd(II) by adsorption on an indigenous mineral, pH 6.5, temperature 30°C, ionic strength 0.01 M NaClO4
Fig. 2—Effect of pH on adsorption of Cd(II) on China clay, concentration 0.5×10−4 M, temperature 30°C, ionic strength 0.01 M NaClO4
are covered with hydroxyl groups, S-OH (‘S’ denotes the surface), which are amphoteric in nature. Adsorption at H+ and OH− is thus based on the protonation and deprotonation of surface hydroxyls21 : SOH2+ → Protonation
SOH + H+ → SO− + H+ Neutral surface Deprotonated
…(1)
The variation of adsorption with change in pH shows two distinct regions of interest:
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INDIAN J. CHEM. TECHNOL., MAY 2006
(i) Between pH 2.0 and 4.0, the variation in adsorption is quite small. (ii) Beyond pH 4.0, there is a significant increase in the removal and an adsorption edge can be noticed with a maximum at around pH 9.5. In the first region, ion exchange may be operative due to interactions at constant charge sites (planer sites): Cd2++2(Al,SiO)−Na+↔(Al,SiO)2Cd+2Na+
…(2)
This type of mechanism has been suggested5 earlier also. The second region which is greater than pHzpc of the adsorbent is very much influenced by pH. This region involves the interaction of solute with constant potential sites (edge sites) of the adsorbent.
Fig. 3—Lagergren’s plot for dynamic modelling for removal of Cd(II) by adsorption on China clay, concentration 0.5×10−4 M, pH 6.5, ionic strength 0.01 M NaClO4 and temperature 30°C
Dynamics of removal
The study of the dynamics of the process of removal of cadmium by adsorption on China clay was carried out and well known Lagergren’s model22 was used to determine rate constant of adsorption: log(qe – q) = logqe − (kad/2.303).t
…(3)
where qe and q (both in mgg−1) are the amounts of cadmium adsorbed at equilibrium and at time t, respectively, kad (min−1) is the rate constant of adsorption. The plots of log (qe – q) versus t (Fig. 3) are linear and indicate fitness of the model for the present system. The values of kad were calculated graphically and were found to be 5.10×10−2, 4.82×10−2, and 4.12×10−2 min−1 at 30, 40 and 50°C, respectively. Intraparticle diffusion studies
Almost in all rapidly stirred batch reactors, there always exists a possibility of intraparticle diffusion23−25. This process of intraparticle diffusion is rate limiting in many adsorption processes. For the present system, this possibility was studied and the values of the constants of intraparticle diffusion were determined as follows: D = 0.03 ro2/t1/2
…(4)
where D (cm2s−1) is coefficient of intraparticle diffusion, ro (cm) is the radius of adsorbent particles, and t1/2 (min) is the time for half adsorption of
Fig. 4—Removal of Cd(II) at different temperatures by adsorption on China clay, pH 6.5, ionic strength 0.01 M NaClO4
cadmium on China clay. The value of D as calculated from the above expression was found to be 3.25×10−10 cm2 s−1 and this value of D indicates intraparticle diffusion to be rate controlling step24. Effect of temperature
The removal of cadmium decreased from 80.3 to 51.3% (Fig. 4) by increasing the temperature of the process from 30 to 50°C at 0.5×10−4 M initial concentration of cadmium, pH, 6.5, particle diameter, 100 μm and 0.01 M NaClO4 ionic strength. Decreasing pattern of adsorption with increasing values of temperature reveals the exothermic nature of cadmium adsorption on China clay. Moreover, the decreasing values of kad at increasing temperature also prove the exothermic nature of the present process. The increased escaping tendency of the cadmium at elevated temperatures may be another explanation to this finding. Conclusion From the above studies the following conclusions may be drawn: (i) Higher removal (%) is observed at low concentrations. (ii) The pH has a pronounced effect on the removal of cadmium by adsorption on China clay with maximum removal (86.6%) at pH 9.5.
SHARMA & SRIVASTAVA: ADSORPTION OF CADMIUM(II) FROM AQUEOUS SOLUTIONS
(iii) Values of the rate constant of removal process are determined by Lagergren’s model. The values of kad indicate the exothermic nature of the process. (iv) Intraparticle diffusion plays important role during the removal of cadmium in the present system. Acknowledgements The author is thankful to Prof C H Weng, for useful suggestions. He is also thankful to Prof Huang, Prof McKay and Prof Pfafflin for encouragement from their inspiring work. References 1 Chu K H, Hasim M A, Shang S M & Sammel V B, Water Sci Technol, 35 (1997) 115. 2 Huang C P & Ostovic F J, Environ Eng Div ASCE, 104 (1998) 863. 3 Huang C P & Rhoads E A J, Colloid Interface Sci, 131 (1989) 230, 236. 4 Forstner U & Wittmann G T W, Metals in Aquatic Environment (Springer-Verlag, NY), 1985. 5 Kannan K, Fundamentals of Environmental Pollution, 1st edn (S Chand & Co. Ltd., India), 1985. 6 Srivastava S K, Tyagi R & Pant N, Water Res, 23 (1989) 1161. 7 Quan C, Khoe G & Bagster D, Water Res, 35 (2001) 478.
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