Modeling of lithium interference in cadmium biosorption

Biosorption of Cd by the brown seaweed Sargassum polycystum biomass was experimentally investigated and mathematically modeled at different pH and ionic strength levels. From the potentiometric titration of the biomass, three types of functional groups were identified, and the dissociation constant and the numbers of these groups were determined. The carboxyl group (pK_H 3.70 ± 0.09) was found to play a major role in binding protons and Cd. The background ion, Li⁺, could interfere with the uptake of protons and Cd by competition for the carboxyl sites. Whereas the binding mechanism on the carboxyl group was established as an ion exchange process, the second functional group, phosphonate (pK_H 5.41 ± 0.31), most likely bound metals by a complexation reaction. A biosorption model was developed based upon the binding mechanisms and was successfully used for predicting the isotherm and pH edge experiments. In addition, the speciation of the binding sites as a function of the pH was simulated using the developed model in order to visualize the distribution of Cd on the binding sites.