Kinetics of Zinc(II) Ion Biosorption onto Powdered Waste Sludge (PWS) at Different Operating Conditions

Batch biosorption kinetics of zinc(II) ions onto powdered waste sludge (PWS) were investigated under different experimental conditions such as variable pH (3–6), temperature (30–50°C), Zinc ion (50–350 mg L⁻¹) and PWS (0.25–3 g L⁻¹) concentrations and PWS particle size (53–231 μm). The rate and the extent of zinc(II) biosorption increased with pH yielding pH 5 (q_m = 60 mg g⁻¹) as the optimal value without Zn(OH)₂ precipitation. Increases in temperature also resulted in increases in biosorption rate with an activation energy of 6 kcal mol⁻¹ according to the second-order kinetics. Maximum biosorbed Zn(II) concentration was 72 mg g⁻¹ at 50°C. Biosorbed zinc(II) concentration at the end of 6 h of incubation increased with increasing zinc(II) concentration (q_m = 95 mg g⁻¹ at 300 mg L⁻¹ Zn), but with decreasing particle size (q_m = 70 mg g⁻¹ with D_p = 53 μm) and concentration of PWS (q_m = 168 mg g⁻¹ with PWS of 0.25 g L⁻¹). Pseudofirst- and second-order kinetics were used for correlation of biosorption data and for determination of the rate constants. The pseudosecond-order kinetics was found to be more suitable for representation of the kinetic data for almost all cases due to similarity between the adsorption kinetics and bimolecular reactions. Kinetic constants increased with increasing pH, temperature, and PWS concentration, but decreased with increasing zinc(II) concentrations and the particle size of PWS. The highest rate constants for the pseudofirst- and second-order kinetics were 0.71 h⁻¹ and 0.024 (mg/g)⁻¹ h⁻¹ , respectively, for the smallest particle size of 53 μm. The highest Zn(II) biosorption capacity of 168 mg g⁻¹ was obtained with PWS and Zn(II) concentrations of 0.25 g L⁻¹ and 100 mg L⁻¹at the end of 6 h biosorption period.