Spontaneous Abiotic Reduction of Arsenate to Arsenite Mediated by Structural Fe(II) Resulting from Abundant Oxygen Vacancy Clusters in Poorly Crystalline Ferrihydrite in Drought Environments

The reduction of As(V) to As(III) has been proposed as an undesirable process, increasing the mobility and toxicity of arsenic. Although most studies revealed that As(V) reduction occurs in the aqueous phase, it remains unclear whether abiotic As(V) reduction driven by minerals in drought environments also exists. In this study, we examined the transformation of As(V) to As(III) mediated by ferrihydrite during drying processes using high-resolution X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy analyses. The results revealed that nearly 40.8% of ferrihydrite-sorbed As(V) was transformed to As(III) after placing the As(V)-adsorbed ferrihydrite solids in a drought-tolerant environment for 7 days. As(V) reduction occurred under both oxic and anoxic conditions, with the reduction rate being higher in an anoxic atmosphere than in oxygen and air. Chemical analysis revealed the presence of structural Fe(II) in ferrihydrite, which was attributed to the abundance of oxygen vacancy clusters, as evidenced by positron annihilation lifetime (PAL) analysis. Fe L-edge XANES analysis and DFT calculations demonstrated that structural Fe(II) in dried ferrihydrite played a vital role in As(V) reduction, inducing electron transfer from Fe to As atoms. The findings of this study highlight a potentially important but long-overlooked As(V) reduction pathway at mineral surfaces under drought conditions in dried soils.