Influence of some parameters on adsorption rate and toxicity of cadmium in activated sludge

The influence of pH, competitor ions (NaNO₃) and aerobic and anaerobic stabilization of activated sludge on the cadmium uptake by activated sludge solids was investigated. Above 0.08 mg L^‐1 cadmium in solution, biosorption was found to follow the Freundlich isotherm. Active cellular uptake of soluble cadmium does not appear to be a significant mechanism of the biosorption. In addition, the cadmium uptake is not completely reversible. The adsorption of cadmium by activated sludge seems to involve a physico‐chemical mechanism with especially weak electrostatic interactions with ion‐exchange reaction. The optimum adsorption pH was 7.5. Adsorption is influenced by sodium ion concentrations up to an equivalent conductivity of 10 000 μS cm^‐1. Aerobic and anaerobic stabilization of activated sludge increase systematically the initial adsorption capacities. Respirometric measurements were done to evaluate the inhibitive effects of cadmium on activated sludge. Monod's equation and the equation of non‐competitive inhibition were used to describe the toxicity related to cadmium uptake. These two equations appear to be complementary.