Determination of model parameters for zinc (II) ion biosorption onto powdered waste sludge (PWS) in a fed–batch system

Biosorption of zinc (II) ions onto pre-treated powdered waste sludge (PWS) was investigated using a completely mixed tank operating in fed–batch mode instead of an adsorption column. Experiments with variable feed flow rate (0.05–0.5 L h−1), feed Zn(II) ion concentrations (37.5–275 mg L−1) and amount of adsorbent (1–6 g PWS) were performed using fed–batch operation at pH 5 and room temperature (20–25 °C). Break-through curves describing variations of aqueous (effluent) zinc ion concentrations with time were determined for different operating conditions. Percent zinc removal from the aqueous phase decreased, but the biosorbed (solid phase) zinc ion concentration increased with increasing feed flow rate and zinc concentration. A modified Bohart–Adams equation was used to determine the biosorption capacity of PWS (q′s) and the rate constant (K) for zinc ion biosorption. Biosorption capacity (q′s=57.7 g Zn kg−1 PWS) of PWS in fed–batch operation was found to be comparable with powdered activated carbon (PAC) in column operations. However, the adsorption rate constant (K=9.17 m3 kg−1 h−1) in fed–batch operation was an order of magnitude larger than those obtained in adsorption columns because of elimination of mass transfer limitations encountered in the column operations. Therefore, a completely mixed tank operated in fed–batch mode was proven to be more advantageous as compared to adsorption columns due to better contact between the phases yielding faster adsorption rates.