| Biosorption, a process of passive metal binding to biomass, may be used for purification of metal bearing effluents. This work investigates the binding of heavy metal ions (Cd, Cu, Zn), light metal ions (Ca, Na) and protons to biomass of the brown alga Sargassum.;The mechanism of metal binding was confirmed to be ion exchange. A novel multi-component sorption isotherm model for cation binding was derived to aid in predicting the biosorption performance of the new biosorbent. This model considers chemical binding to free sites on the biomass and ion exchange (1:2 stoichiometry for divalent ions). It is based on chemical equilibrium constants and assumes competition of all cations for the same binding sites. Two main binding sites (carboxyl and sulfate) were characterized in terms of their respective quantities and pK;The two-site model successfully described metal and proton binding at different pH in mono- and di-metal systems. It was possible to predict the complete equilibrium sorption state, residual metal concentration in solution and metal uptake by the biosorbent, from the known initial state for varying amounts of biomass and different initial pH values. In order to account for the effect of ionic strength and electrostatic attraction, the above mentioned biosorption model was expanded by incorporating a version of the Donnan model and/or a Gouy-Chapman double layer model. The charge density of the biomass was characterized and intrinsic binding constants were derived. Correlations to account for biosorbent particle swelling were established. For the specific case of linear increase of swelling with the number of free sites, an explicit sorption isotherm equation was derived that includes the Donnan model in an easy-to use-way.;Using the parameters obtained from pH titrations at different ionic strength, it was possible to predict the effect of ionic strength on Cd binding. The influence of Ca on Cd binding was predicted from experiments with Cd and Ca, respectively, in mono-metal systems. |
