Humic acid-mediated oxidation of As(III) by magnetic iron oxides derived from iron sludge: Insights into electron...

Arsenic contamination in groundwater poses a serious global health risk, and efficient oxidation of As(III) to the less toxic and more easily removable As(V) remains a key challenge. High-arsenic groundwater is often accompanied by elevated dissolved organic matter (DOM), particularly humic substances. Here, a composite system of iron-sludge-derived magnetic iron oxide (MIO_500-2) and humic acid (HA) was investigated as a green strategy for As(III) transformation without added chemical oxidants under conditions relevant to groundwater systems. Under typical conditions (C_0As(III) = 200 μg L^-1, C_0HA = 20 mg L^-1, pH ≈ 7), As(III) decreased to 13 μg L^-1 with concurrent formation of 23 μg L^-1 As(V), suggesting that oxidative transformation occurred together with adsorption. Spectroscopic analyses (UV-Vis, FTIR, XPS) suggest that quinone/semiquinone moieties in HA may mediate interfacial electron transfer and participate in Fe(III)/Fe(II) redox cycling. ESR and quenching experiments further suggest the involvement of reactive oxygen species (ROS), with •O₂^- likely playing an important role under the tested conditions. Oxygen availability is critical, as oxygen-limited conditions increased residual As(III) to 53 μg L^-1, indicating suppressed oxidation. These results indicate that As(III) transformation in the HA + MIO_500-2 system is likely governed by a coupled mechanism involving HA adsorption, interfacial electron transfer, Fe redox cycling, dissolved oxygen activation, and ROS generation. This study provides new insights into arsenic transformation in Fe-organic matter systems and highlights the potential of waste-derived magnetic iron oxides for low-cost groundwater remediation.