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Langmuir 2002, 18, 650-656
Removal of Cu(II) and Zn(II) Ions by Sorption onto Bone Char Using Batch Agitation C. W. Cheung, J. F. Porter, and G. McKay* Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong Received May 14, 2001. In Final Form: November 5, 2001 The sorption rate of copper and zinc ions onto bone char around pH 5 has been studied using a batch agitation system. The calcium hydroxyapatite, which is the main composition in bone char, removes the metal ions by means of adsorption and ion exchange from the solutions. When the pH values of solutions are adjusted to pH 4 or below, the X-ray desorption patterns show the degrees of the crystalline of hydroxyapatite in the bone char reducing. Therefore, the sorption process for the metal ions removal from effluent is recommended to control around pH 5 or above to reduce the loss of the sorbent. In addition, the experimental data from the adsorption isotherms and batch kinetics studied were correlated using the Langmuir equation and a film-pore diffusion mass transport model. The Langmuir parameters were incorporated into the film-pore diffusion model to correlate the batch kinetic data. The constant pore diffusivities for the sorption of copper and zinc ions onto bone char are equivalent to 6.67 × 10-7 and 6.32 × 10-7 cm2/s, respectively. The film-pore diffusion model shows slight deviation to the experimental data when the solution is in high concentration or when the volume-to-mass ratio is low.
Introduction Bone char is a mixed component adsorbent in which carbon is distributed throughout a porous structure of hydroxyapatite (Ca10(PO4)6(OH)2 or CaHAP). This adsorbent is extensively used in sugar refining as a decolorization material1 to remove the colorant from the sugar syrup. In addition, bone char was used as a defluoridating agent at the South Dakota drinking water plant which operated from 1948 until the city switched to a new source of supply in 1971.2 Recently, bone char was studied to use as a sorbent to remove the antimony and europium radioisotopes from radioactive wastes3 and hexavalent chromium ions from wastewaters.4 Although the use of bone char to remove metal ions from the effluents is still in the research stage, the potential use of bone char in wastewater treatment systems cannot be ignored. Bone char contains around 76 wt % of CaHAP, which is not only a main inorganic constituent of teeth and bones but also the major inorganic constituent of phosphate rock.5 Hence, the physical and chemical properties of the CaHAP have been widely reported6,7 in this form. The major compound CaHAP in bone char has been independently used to immobilize metal ions in aqueous solution.8-10 The removal mechanism was reported to be not merely an adsorption effect but a type of ion-exchange * Author for correspondence. Fax: (852) 2358 0054. Telephone: (852) 2358 7133. E-mail:
[email protected]. (1) Bennett, M. C.; Abram, J. C. J. Colloid Interface Sci. 1967, 23, 513. (2) Bhargave, D. S.; Killedar, D. J. Indian J. Eng. Mater. Sci. 1995, 2, 157. (3) Raouf, M. W. A.; Daifullah, A. A. M. Adsorpt. Sci. Technol. 1997, 15, 559. (4) Dahbi, S.; Azzi, M.; de la Guardia, M. Fresenius J. Anal. Chem. 1999, 363, 404. (5) Suzuki, T.; Ishigaki, K. Chem. Eng. Commun. 1985, 34, 143. (6) Elliott, J. C. Studies in Inorganic Chemistry 18: Structure and Chemistry of the Apatites and Other Calcium Orthophosphates; Elsevier: Amsterdam, 1994. (7) Amjad, Z. Calcium Phosphate in biological and industrial systems; Kluwer Academic Publishers: Boston, MA, 1998. (8) Suzuki, T.; Hatsushika, T.; Hayakawa, Y. J. Chem. Soc., Faraday Trans. 1 1981, 77, 1059. (9) Suzuki, Y.; Takeuchi, Y. J. Chem. Eng. Jpn. 1994, 27, 571.
reaction between the ions in solution and the calcium ions of the apatites.8 Since the CaHAP can effectively adsorb the metal ions, this compound was studied to immobilize the lead ions in contaminated soils and wastes11 and uranium in contaminated sediments.12 Recently, the ability of bonemeal additions (finely ground, poorly crystalline apatite) to immobilize pollutant metals in soils was also reported.13 Therefore, the CaHAP is a recognized active compound for the metal ion removal from wastewaters. In this research, bone char will be studied to remove the copper and zinc ions from effluent using an agitated adsorber. The pH values of solutions were adjusted to around 3, 4, and 4.8 to test the stability of the sorbent. The sorbent before and after sorption at different pH values will be analyzed using X-ray diffraction (XRD) to verify whether the adsorbed metal ion forms a new crystal phase on a sorbent surface or not. In addition, the removal rate of metal ions will be interpreted by a new mass transport diffusion model. The film-pore diffusion model will be used to correlate the batch kinetic data by incorporating the Langmuir-type equation and the mass balance equation into the shrinking core model14 (SCM) to calculate the pore diffusion coefficients. The SCM is based on the unreacted shrinking core where reaction starts at the particle surface forming a reacted zone which moves inward with a certain velocity. Hence during the whole reaction time there is an unreacted core shrinking in size as the sorption reaction proceeds. The SCM in sorption systems, using an analytical solution method, was developed by Spahn and Schlu¨nder.15 The authors (10) Fedoroff, M.; Jeanjean, J.; Rouchaud, J. C.; Mazerolles, L.; Trocellier, P.; Maireles-Torres, P.; Jones, D. J. Solid State Sci. 1999, 1, 71-83. (11) Ma, Q. Y.; Traina, S. J.; Logan, T. J.; Ryan, J. A. Environ. Sci. Technol. 1994, 28, 1219. (12) Arey, J. S.; Seaman, J. C.; Bertsch, P. M. Environ. Sci. Technol. 1999, 33, 337. (13) Hodson, M. E.; Valsami-Jones, E.; Cotter-Howells, J. D. Environ. Sci. Technol. 2000, 34, 3501. (14) Levenspiel, O. Chemical Reaction Engineering; John Wiley & Sons: New York, 1972; p 359. (15) Spahn, H.; Schlu¨nder, E. U. Chem. Eng. Sci. 1975, 30, 529.
10.1021/la010706m CCC: $22.00 © 2002 American Chemical Society Published on Web 01/10/2002
Sorption Rates of Cu and Zn Ions
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Figure 1. Pore volume distribution of bone char. Table 1. Physical and Chemical Properties of Bone Char chemical composites
physical properties
items
limits
items
limits
acid insoluble ash calcium carbonate calcium sulfate carbon content CaHAP iron as Fe2O3
3 wt % max 7-9 wt % 0.1-0.2 wt % 9-11 wt % 70-76 wt %
