Biosorption/desorption of cadmium(ii) and zinc (ii) from aqueous solutions by activated sludge immobilized onto chitosan beads

In this study, we evaluated the effectiveness of cadmium and zinc adsorption and desorption from solutions containing single metals and a mixture of metals in the ratio of 1:2 by activated sludge immobilized onto chitosan (ASC). The optimal pH value determined for metals adsorption ranged from pH 5 to pH 6, whereas that established for desorption reached pH 2. In the case of individual metals, the state of adsorption equilibrium in the solution was achieved after 180 min, whereas in the case of a metal mixture – after 270 min. In the case of desorption, the state of equilibrium was achieved after 45 min. It was stated that both adsorption and desorption proceeded according to the pseudo-second order reaction. The study enabled determining the maximum adsorption capacity based on Langmuir, Freundlich and Sips models. The Sips model was found suitable for the description of adsorption of single metals onto ASC, whereas both Sips and Freundlich models – for description of the adsorption of a metal mixture, which was indicated by the determined values of R2 coefficient. The adsorption capacity of ASC determined from Sips model for individual solutions of cadmium and zinc reached 216 and 188.3 mg/g d.m., respectively, whereas that determined for their mixture reached 106 mg/g d.m. for both metals.