Binding sites and mechanisms of cadmium to the dried sewage sludge biomass

• The main functional groups of sewage sludge biomass were identified.
• Carboxyl and phosphonate groups played a role as binding sites for Cd.
• The binding mechanism was established to be complexation with binding groups.
• The contribution of functional groups on Cd binding was predicted and visualized.

The Cd biosorption on the dried sewage sludge biomass were experimentally evaluated and mathematically modeled at different pH values. The potentiometric titration of the biomass was well fitted by the four-site model, which consists of three-negative and one-positive sites. The main functional groups were identified through the FTIR study. The pH edge study showed that both the carboxyl and phosphonate groups played an important role in the binding of Cd. From the dynamic biosorption experiments, the H+/Cd2+ exchange ratios at pH 4, 5 and 6 were estimated; thereby the binding mechanisms were established to be complexation with carboxyl and phosphonate groups. Finally, biosorption model was developed based upon the binding mechanism, which was successfully applied for predicting the isotherms and pH edges. Using the developed model equation, the contribution of each functional group on Cd binding could be predicted and visualized.