AN EQUILIBRIUM MODEL FOR ABSORPTION OF MULTIPLE DIVALENT METALS BY ALGINATE GEL UNDER ACIDIC CONDITIONS LARRY K. JANG 1 *, DUY NGUYEN 2 and GILL G. GEESEY 2 1 Department of Chemical Engineering, California State University, Long Beach, CA 90840, USA; 2 Center for Bio®lm Engineering and Department of Microbiology, Montana State University, Bozeman, MT 59717, USA (First received October 1995; accepted in revised form August 1998) AbstractÐAn extended Langmuir model was proposed in this work to describe the simultaneous absorption of multiple divalent metal ions and hydrogen ions from acidic metal-containing aqueous media by an absorbent having one kind of active functional group. A viscous Na±alginate solution was directly dispensed dropwise into the acidic aqueous media containing dissolved copper and zinc to form spherical Cu±Zn±H±alginate gels which subsequently absorb these ions until ®nal equilibrium was reached in batch experiments. Initial pH of the aqueous media was in the range of 2.6±5.4. Binding group density and binding stability constants of alginic acid, cupric alginate and zinc alginate were obtained simultaneously by ®tting data to the proposed model developed in this work. Application of the constants obtained to the prediction of the extent of absorption of copper and zinc from an aqu- eous medium having an initial pH in the range of 2.6±5.4 is illustrated and compared with experimental results. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐcopper, zinc, alginate, selectivity, metal recovery, Langmuir model INTRODUCTION Many aqueous media such as mine drainage and metal ore leachates contain dissolved metal ions and have pH in the range 2±5. It is desired to recover or remove these metals from the aqueous media, so that water can be safely discharged or reused and valuable metals can be recovered for industrial use. One of the feasible approaches is the use of biopolymer (such as alginate from seaweed) which bear metal-binding functional groups to absorb metals. In our previous work (Jang et al., 1995a), a method of generating spherical gels of Cu±alginate in an acidic aqueous media containing dissolved copper was described. In this method, a viscous Na±alginate solution (3.2% by weight) was dis- pensed dropwise into the aqueous media to form Cu±alginate gels in situ which subsequently absorbed copper until ®nal equilibrium was reached. The gelation phenomenon can be explained by the egg-box model (Rees and Welsh, 1977) in which each divalent metal ion binds to two carboxyl groups on adjacent alginate molecules. In the ®rst paper of this series (Jang et al., 1999), a model developed earlier (Jang et al., 1995b) was used to treat data of simultaneous absorption of multiple divalent metal species (copper and zinc) by alginate gel to yield conditional metal-binding ca- pacity and stability constants under a given neutral or weakly acidic pH. Experimental data from Jang et al. (1999) were treated with an extended Langmuir model developed in this work to describe the simultaneous absorption of hydrogen ions and n divalent metal ions by carboxyl groups on the algi- nate molecules. The pH-independent binding stab- ility constants of alginic acid, cupric alginate and zinc alginate and density of carboxyl groups in the alginate gel were calculated from batch absorption data. Under the assumption that hydrogen ions and all divalent metal species compete for the same binding groups (i.e. carboxyl groups in the case of alginic acid), X  , and the major form of metal±alginate complex is bi-dentate, the binding equilibria can be de®ned as: H   X   * ) HX; K H 1 M 2 i  2X   * ) M i X 2 ; K i i  1, 2, ... , n 2 where M i 2+ denotes the ith divalent metal species in the aqueous media and the equilibrium binding Wat. Res. Vol. 33, No. 12, pp. 2826±2832, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter PII: S0043-1354(98)00373-X www.elsevier.com/locate/watres *Author to whom all correspondence should be addressed. [Tel.: +1-562-985-7533; fax: +1-562-985-8887]. E-mail: jang@engr/csulb.edu. 2826