Elucidating the Geochemical Dynamics of Arsenite and Pyrite in Aquatic Systems

Pyrite, a ubiquitous sulfide mineral, exerts a strong influence on the fate of coexisting As(III) and As(V) species in natural settings, such as gold deposits, sedimentary basins, and hydrothermal systems. However, the As(III) adsorption and oxidation mechanisms on pyrite at neutral pH remain contested. Through oxic and anoxic kinetic experiments using pyrite with varying oxidation degrees, we demonstrate that As(III) adsorbs preferentially to the Fe(III) (oxy)(hydr)oxide coatings rather than to pyrite sites. Contrary to prevailing assumptions, the HO^• radicals contribute minimally to As(III) oxidation at circumneutral pH. Spectroscopy and molecular simulations revealed that pyrite-generated H₂O₂ oxidizes As(III) via an inner-sphere electron transfer process. This heterogeneous oxidation likely proceeds through a ternary surface complexation involving arsenite and Fe sites. These findings challenge both the conventional radical-dominated pathway and the assumed mechanism of natural arsenopyrite formation. By elucidating the dominant As(III) sorption/oxidation pathway on pyrite surfaces, our findings reshape our current understanding of arsenic geochemistry. They also inform risk assessment and remediation strategies for arsenic-impacted environments.