Biosorption properties of extracellular polymeric substances (EPS) towards Cd, Cu and Pb for different pH values

The aim of this study was to assess the influence of pH on the metal biosorption of extracellular polymeric substances (EPS) extracted from two different activated sludges called A and B. The composition and physico-chemical characteristics of EPS were determined. The biosorption capacities of the EPS were examined at pH 4, 6, 7 and 8 successively with three metals Cu, Pb and Cd using differential pulse polarography (DPP) as an investigation tool and Ruzic's model was used to produce polarographic titration curves. Two apparent pKa were obtained, the first were 6.6 (EPS A) and 5.7 (EPS B), attributed to carboxylic and phosphoric groups whereas the second was 8.7 for EPS A and 9.4 for B and these were attributed to phenolic and amino functional groups. Whatever the EPS and the metal considered, the conditional binding constant did not show significant differences in the strength of complex formed between the EPS and metals. But for all metals, the number of EPS binding sites was significantly lowered by a decrease in the pH of the medium. At pH 4, the metal biosorption capacity of EPS is very low. At pH 6, the number of EPS binding sites increased in the following order: Pb>Cu>Cd whereas at pH 7 and 8, this order changed and was: Cu>Pb>>Cd. Simulations of the speciation states of Cu, Pb and Cd at the different pH values in ultra-pure water (25 degrees C, ionic strength 0.045 M) were performed with MINEQL 4.5 software and indicated the presence of hydroxylated forms and sometimes solid forms for Pb and Cu. But the polarographic titration curves revealed precipitation of Cu only at the end of the experiments at pH 8.