Fe³⁺ Addition Promotes Arsenopyrite Dissolution and Iron(III) (Hydr)oxide Formation and Phase Transformation

The oxidative dissolution of arsenic-containing pyrite is an important process controlling arsenic fate and transport in groundwater aquifers. This process is further complicated by iron(III) (hydr)oxide formation during pyrite oxidation, which can serve as a crucial sink for mobilized arsenic. This study examines the oxidative dissolution of arsenopyrite in the presence of Fe³⁺ at circumneutral pH. We show for the first time that despite their low solubility, small quantities of additional Fe³⁺ trigger electron transfer between Fe³⁺ and Fe(II) in arsenopyrite, resulting in higher extents of secondary mineral formation and faster phase transformation. In addition, dissolved arsenic concentrations are elevated in these systems because of faster dissolution and faster phase transformation. These findings have significant environmental implications for arsenic transport under dynamic redox conditions, where interactions between Fe³⁺ and arsenopyrite can dominate arsenic-bearing pyrite oxidation as well as iron(III) (hydr)oxide formation and stability.