Adsorption and reduction of Cr(VI): mechanistic investigations of magnetite-fulvic acid complexes

Iron oxides and natural organic matter are widely recognised for their roles in mitigating heavy metal contamination due to their surface reactivity. Magnetite, featuring Fe(II), is of particular interest for its reductive properties. However, limited studies have explored its synergistic interaction with organic matter in removing anionic contaminants like Cr(VI). In this study, magnetite–fulvic acid (Mt–FA) complexes with varying C/Fe molar ratios were synthesised and tested for Cr(VI) removal through batch experiments and spectroscopic analyses. Fulvic acid decreased the specific surface area of magnetite and partially blocked adsorption sites, leading to reduced adsorption capacity with increasing FA content. Under acidic conditions (pH 3), Mt–FA with a C/Fe ratio of 0.5 exhibited the highest Cr(VI) adsorption capacity (6.38 mg/g). FT-IR and XPS analyses confirmed that both FA and magnetite were involved in Cr(VI) adsorption and reduction, with Fe(II) contributing to redox reactions. Additional tests with FA alone revealed its inherent reductive capacity (1.12–1.59 mg/g), while magnetite alone contributed ∼1.34 mg/g. The combined Mt–FA complexes exhibited higher reduction capacity (1.57–2.11 mg/g), indicating a synergistic effect. FA not only provides redox-active groups but also facilitates electron transfer from magnetite to Cr(VI), enhancing Cr(VI) reduction. This dual-function material offers a promising approach for remediation of Cr(VI)-contaminated environments and highlights the importance of interfacial interactions between iron oxides and natural organic matter.