Biosorption of cadmium and zinc by activated sludge from single and binary solutions: Mechanism, equilibrium and experimental design study

摘要

Dried activated sludge (DAS) has been used for obtaining quantitative data of the cadmium and zinc biosorption from single and binary aqueous CdCl _2 and ZnCl _2 solutions in batch experiments using radiotracer technique. It was shown that the metal removal is a rapid process significantly influenced by solution pH. The mechanisms of biosorption of Cd and Zn by DAS were examined by FTIR, SEM-EDX analysis and chemical blocking of functional groups. Results revealed the dominant role of the carboxyl group in Cd and Zn ion-binding by DAS and participation of the ion-exchange mechanism in metal ion biosorption. The maximum uptake capacity Q _(max) at pH 6.0 calculated from Langmuir isotherm was 540±16μmol/g for Zn ~(2+) and 510±17μmol/g for Cd ~(2+) ions. The response surface methodology was used for investigation of interactions and competitive effects in binary metal system Cd ~(2+)-Zn ~(2+). Interaction plots revealed strong interactions between the initial concentration of co-ion, solution pH and sorption capacity of the primary ion. Maximum sorption capacities of DAS in the binary system Cd ~(2+)-Zn ~(2+) were 321μmol Cd ~(2+)/g and 312μmol Zn ~(2+)/g, indicating a higher affinity for Cd ~(2+) compared with Zn ~(2+) ions.