Iron at the helm: Steering arsenic speciation through redox processes in soils

The toxicity and bioavailability of arsenic (As) in soils are largely determined by its speciation. Iron (Fe) is widely present in soils with a strong affinity for As, and therefore the environmental behaviors of As and Fe oxides (including oxides, hydrates and hydrated oxides) are closely correlated with each other. The redox fluctuations of Fe driven by changes in the environment can significantly affect As speciation and its fate in soils. The interaction between Fe and As has garnered widespread attention, and the adsorption mechanisms of As by Fe oxides have also been well-documented. However, there is still a lack of systematic understanding of how Fe redox dynamics affects As speciation depending on the soil environmental conditions. In this review, we summarize the mechanisms for As speciation transformation and redistribution, as well as the role of environmental factors in the main Fe redox processes in soils. These processes include the biotic Fe oxidation mediated by Fe-oxidizing bacteria, abiotic Fe oxidation by oxygen or manganese oxides, dissimilatory Fe reduction mediated by Fe-reducing bacteria, and Fe(II)-catalyzed transformation of Fe oxides. This review contributes to a deeper understanding of the environmental behaviors of Fe and As in soils, and provides theoretical guidance for the development of remediation strategies for As-contaminated soils.