Microbial regulation of arsenopyrite oxidation and arsenic transformation in pyrrhotite-mediated interactions

The coexistence of arsenopyrite (Asp) and pyrrhotite (Po) in sulfide deposits and acid mine drainage (AMD) systems significantly influences As/Fe redox dynamics through galvanic interactions. As arsenopyrite is both a primary gold carrier and a source of hazardous arsenic, clarifying its pyrrhotite-coupled oxidation behavior is critical for optimizing gold recovery and predicting environmental risks. Here, the interactions between Po and Asp were investigated under acidic abiotic conditions and in the presence of Acidithiobacillus ferrooxidans (A. ferrooxidans). In abiotic systems, pyrrhotite inhibited arsenopyrite oxidation and As mobilization in a dose-dependent manner, decreasing aqueous total arsenic (As(T)) accumulation by up to 80.73%. This inhibition was accompanied by an increased As(III)/As(T) ratio. Iron (hydr)oxides formed at higher pyrrhotite loadings may have limited hydroxyl radical (HȮ) accumulation, which coincided with reduced As(III) oxidation to As(V). Conversely, A. ferrooxidans partially weakened the protective effect of pyrrhotite by regenerating Fe³⁺ and oxidizing reduced sulfur species. The resulting acidification further destabilized surface passivation layers, promoting arsenopyrite dissolution and As mobilization. Arsenic speciation in biotic Po-Asp systems showed a stage-dependent transition from As(III) persistence under relatively low redox potential to increased dissolved As(V) likely associated with the partial dissolution of Fe-As secondary phases or desorption of surface-associated As(V). Overall, arsenic mobilization and speciation in Po-Asp systems were jointly influenced by galvanic effects, HȮ chemistry, microbially mediated Fe and S redox cycling and Fe-As secondary-phase dynamics, providing mechanistic insights into AMD arsenic management and sulfide mineral biooxidation.