pH-related equilibria models for biosorption in single metal systems

Biosorption is an innovative technology used to remove heavy metals from aqueous solutions. A wider application of this alternative process is strictly related to the understanding of the chemico-physical aspects involved, in order to optimize the operative conditions. Mechanistic models are the most useful tools used for understanding purposes, even if the empirical models are still widely applied for their simplicity. In this paper, two original models were used to represent the experimental data of copper and cadmium biosorption onto Sphaerotilus natans in different operative conditions of pH and biomass concentration. Both models can represent the effect of pH onto biosorption performances using two different approaches. The first model is empirical, based on the observation that the maximal specific metal uptake courses vs. pH presents a logistic pattern. The second model is based on the non-competitive mechanism between heavy metals and H + protons. Both models can represent adequately the experimental data, but the non-competitive model also gives a realistic description of the mechanism operating in the system according to a preliminary biomass characterization.