Ionic strength and electrostatic effects in biosorption of protons

New biosorbent materials are being developed for removal of heavy metals from effluents. This study investigates the effect of ionic strength on titrations of protonated Sargassum seaweed biomass and of alginate, a constituent molecule mainly responsible for the binding of cations in Sargassum. The results indicate a significant influence of electrostatic effects on the protonation behavior. The main objective of this work was to examine the capability of an adapted Donnan model to predict pH titrations of negatively charged biomass particles at different ionic strengths. The model, applicable for wider ranges of ionic strength and charge density in the biomass than other Donnan models in the recent literature, yielded good agreement of model predictions and experimental data. Swelling of the sorbent was found to increase with pH. While incorporating a linear relationship between swelling and pH improved the model predictions, it was not necessary in order to obtain reasonable predictions. The model required only three parameters: the total number of binding sites, the equilibrium constant K-CH for proton binding, and only one additional parameter to model the influence of ionic strength, even when swelling was considered. Plots of the degree of dissociation versus (pH-pNa) as a criterion for the rigidity of the particle were found to be valid only under conditions where the concentration factor lambda is high.