Role of extracellular polymers in cadmium adsorption by activated sludges

The heavy metal uptake in activated sludge process is generally explained by a phenomenon of biosorption which largely implicates the extracellular polymers (ECP). In order to confirm such a specific mechanism, cadmium adsorption has been determined onto raw activated sludges, activated sludges inactivated by azide and dinibrophenol, washed activated sludges, solutions of extracted polymers and activated sludges enriched with extracted polymers. The Freundlich equation allows description of the adsorption isotherm of Cd2+ onto activated sludges as onto extracted extracellular polymers for the range of residual cadmium concentration studied. The adsorption of cadmium is a physicochemical phenomenon. A competitive binding of cadmium appears between the ECP mainly free in supernatant and the ECP so-called 'strongly attached to solids'. A speciation study of cadmium by polarography confirmed that the cadmium is entirely complexed by PEC in solution. This study underlines the complexity of interactions between cadmium and the components of the activated sludge.