Role of Cr(VI) in the efficient removal of Cd(II) from aqueous solutions using Fe₃S₄ in a Cd(II)/Cr(VI) binary system

In this study, the synthesized magnetic greigite (Fe₃S₄) was utilized to simultaneously eliminate Cd(II) and Cr(VI) from aqueous solution. The removal efficiency (96.6 %) of Cd(II) in the binary (Cd(II)/Cr(VI)) system was superior to that (79.2 %) of the single (Cd(II)) system, accompanied by a negligible difference in the removal efficiency of Cr(VI) (∼100 %) between the single Cr(VI) and binary systems. The adsorption process of Cd(II) onto Fe₃S₄ closely followed the pseudo-first order kinetic model and Freundlich isotherm, suggesting that the adsorption of Cd(II) primarily involved multi-molecular layer physical adsorption. Characterization results confirmed that the surface hydroxyl groups on Fe₃S₄ played a significant role in the Cd(II) adsorption in the single system. In contrast, in the binary system, the adsorption of Cd(II) was predominantly attributed to surface sulfide groups at pH 3.8, while hydroxyl groups were the primary contributors to Cd(II) adsorption at pH 6.8. Additionally, at pH 5.3, both sulfide and hydroxyl groups contributed equally to Cd(II) adsorption. This pH-dependent mechanism for Cd(II) removal was resulted from the fact that Cr(VI) could not only influence the solution pH and adjust the isoelectric point of Fe₃S₄ to enhance the electrostatic attraction between Cd(II) and Fe₃S₄, but also diminish the concentrations of Fe species to eliminate their competitive adsorption with Cd(II). This study is significantly important for understanding the enhanced removal mechanisms of Cd(II) in the presence of Fe₃S₄, particularly with the coexistence of Cr(VI), as well as for the simultaneous remediation of both cationic and anionic pollutants from wastewater using iron sulfides.