Modeling multi-metal ion exchange in biosorption

Some types of dead microbial or seaweed biomass are capable of effectively binding heavy metals often through ion exchange. This biosorption can be used for purification of metal-bearing effluents. A recently developed two-site model for the ion exchange between divalent metal ions and protons in biosorption was extended to describe multi-site and multi-ion system behavior. A model case for two ion exchange sites and three ions was compared to the experimental data for the sorption of Cd, Cu, and Zn by protonated Sargassum fluitans biomass in systems containing two metal ions and protons at different pH values. The equilibrium constants were comparable to those obtained for systems where, in addition to protons, only one metal ion was present. This opens the possiblity of predicting the sorption performance for three-cation systems from equilibrium constants determined in two-cation systems. There was no interference between the sorbed species except for competition for the same sorption sites. The model was able to predict the equilibrium binding of all three ions as a function of pH and of the final metal concentrations. Only two equilibrium constants, which were valid for any pH and in the presence of any competing ion, were necessary for predicting the binding of each metal ion. The complete equilibrium state (final concentration and binding) can be predicted as a function of the initial state for a pH titration in the simultaneous presence of two metal ions when the equilibrium constants are used in the chemical equilibrium program MINEQL+.