Arsenic contamination in soil is one of the major issues faced globally. Arsenic (As(III)) and arsenate (As(V)) are the two common forms of arsenic present in soil, with the former being more mobile and toxic This study systematically investigated the synergistic oxidation and immobilization mechanisms of As(III) in systems combining fulvic acid (FA) with schwertmannite (Sch) under two distinct association modes: adsorption (FA + Sch) and co-precipitation (FA-Sch). The results indicated that Sch alone primarily removed As(III) via adsorption. However, in the presence of FA, Sch catalyzed the generation of reactive oxygen species (ROS), particularly hydroxyl radicals (•OH), leading to efficient oxidation of As(III) to As(V). The adsorbed FA system (FA + Sch) exhibited the superior oxidation and immobilization efficiency compared to the co-precipitated FA-Sch complex. Mechanistic investigations revealed that in the FA + Sch system, FA acted as a sacrificial electron shuttle, driving the surface Fe(III)/Fe(II) cycle on Sch and promoting •OH formation, while undergoing partial mineralization. In contrast, in the co-precipitated system, FA was encapsulated within the mineral structure, which restricted electron transfer and reduced oxidative activity but facilitated effective transfer and sequestration of carbon from the dissolved to the solid phase. These findings elucidated a nature-inspired pathway for As(III) oxidation mediated by mineral-organic synergy, providing new insights into arsenic biogeochemical cycling and a theoretical basis for developing efficient, sustainable remediation technologies.
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