This study is concerned with passive biosorption of metal ions by biomass of the brown alga Sargassum, a process that may eventually be used for the treatment of industrial wastewaters, This work provides a mathematical model for predicting the equilibrium of proton and metal ion binding as a function of metal ion concentration, pH, and ionic strength. Since the presence of sodium significantly influenced Cd binding, it is recommended to use models that incorporate ionic strength effects. Although swelling of the biomass particle was observed, a simple Donnan model that assumed a rigid particle already yielded good prediction of the experimental data. A combined Donnan-Biosorption-Isotherm equation was derived that allowed for direct calculation of cation binding without iterations. Only one parameter, the binding constant, had to be determined each for Cd and Ca. The intrinsic binding constant was several orders of magnitude lower than the apparent one. Using the parameters (amount of binding sites, proton binding constant, and specific particle volume) obtained from pH titrations in an earlier study, it was possible to predict the effect of pH, ionic strength, and Ca concentration on Cd binding.
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