Magnetic iron-based waterworks sludge modified by chitosan and FeS for aqueous Cr(vi) adsorption and reduction

Abstract

Heavy metals have been considered an evolving environmental concern due to their harmful and long-lasting impacts. We synthesized a composite of FeS/CS@MIBWS for aqueous Cr(VI) adsorption and reduction utilizing the iron-based waterworks sludge modified by chitosan and FeS. After determining the optimal conditions for the FeS/CS@MIBWS preparation, its Cr(VI) removal capability was evaluated using material characterisation and static Cr(VI) adsorption assays. Cr(VI) elimination by the composite was a pH-dependent process, with pH 2 being the optimum in the range of 2–10. The adsorption process was befitted a pseudo-second-order model, and the equilibrium results agreed well with the Langmuir model. The thermodynamics investigation showed that Cr(VI) removal by the composite has both spontaneous and endothermic nature. Considering the ionic effects, Cl⁻, SO₄²⁻ and PO₄³⁻ decreased Cr(VI) elimination in the sequence of Cl⁻ < SO₄²⁻ < PO₄³⁻. The key mechanisms for Cr(VI) elimination were physical and chemical adsorption, chelation, and Cr(VI) reduction into Cr(III). Furthermore, FeS/CS@MIBWS demonstrated steady reusability (removal effectiveness of 70% after 5 cycles). FeS/CS@MIBWS's rapid, high-performance, reusable, and easily separable adsorption properties make it a promising choice for heavy metal environmental cleaning.