Removal of Zn²⁺ and Pb²⁺ ions from aqueous solution using sulphonated waste polystyrene

We report the use of sulphonated waste polystyrene (SWPS) in the removal of heavy metal ions from water. Waste polystyrene (WPS) comprising of high impact polystyrene (HIPS) and expanded polystyrene (EPS) collected from dump sites in Harare were activated through sulphonation to produce a cation-exchange resin. The presence of the sulphonic group was studied with fourier transform infrared spectroscopy (FTIR). Flame atomic absorption spectrometry (FAAS) was used to determine the concentrations of residual metals (Zn2+, Pb2+) after batch adsorption experiments. Sulphonated HIPS reduced Zn2+ from 80 to 38.3 mg/L compared to 10-1.6 mg/L for sulphonated EPS. Similarly, sulphonated HIPS reduced Pb2+ from 100 to 33 mg/L compared to 80-50.3 mg/L for sulphonated EPS. The adsorption data followed both the Langmuir and Freundlich isotherms and pseudo-second order kinetics. Maximum adsorption capacities as quantified by the Langmuir parameter qmax for HIPS was 5.01 mg/g, EPS 0.38 mg/g for Zn2+ and HIPS 6.80 mg/g, EPS 0.68 mg/g for Pb2+. The data were analysed using pseudo first order and pseudo second order Lagergren equation and the adsorption kinetics of the metals Pb2+ and Zn2+ was found to follow the pseudo second order kinetic model. Interpretation of the sorption data in terms of separation factor (SF) suggested that the removal of Pb2+ and Zn2+ from water mainly occurred through chemisorption.