Application of oily sludge-derived char for lead and cadmium removal from aqueous solution

Abstract

The objective of this study was to investigate the physicochemical properties of oily sludge-derived char produced from pyrolysis at 500 °C (OS500) and its sorption behavior towards lead (Pb2+) and cadmium (Cd2+) in aqueous solution. The sorption kinetics and isotherm of Pb2+ and Cd2+ sorption on OS500 were determined. The results were fitted with four kinetic (pseudo first order, pseudo second order, Elovich equation, and intraparticle diffusion) and six isotherm (Langmuir, Freundlich, Sips, Redlich-Peterson, Temkin, and Dubinin-Radushkevich) models. The maximum sorption capacity (QL) of Pb2+ obtained from the Langmuir model was 373.2 mg/g, while QL of Cd2+ was 23.19 mg/g. The mechanisms of lead and cadmium sorption on OS500 and their quantitative contributions were further studied by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and sequential extraction test. The results indicated the chemical characteristics (e.g., alkaline minerals and effective cation exchange capacity (CEC)) but not microstructure of OS500 controlled its Pb2+ and Cd2+ sorption. New mineral precipitates (i.e., hydrocerussite (Pb3(CO3)2(OH)2) and cerussite (PbCO3)) formed during Pb2+ sorption. The Pb2+-π interaction and complexation of Cd2+ with hydroxyl functional groups were evidenced by FTIR. Mineral precipitation with Pb2+ was the predominant mechanism for Pb2+ sorption on OS500 (accounted for 93.79%), whereas the complexation dominated Cd2+ sorption (accounted for 84.15%). The OS500 shows application potential in removing heavy metal contaminants from solution, especially Pb2+.