Nitrate Chemodenitrification by Iron Sulfides to Ammonium under Mild Conditions and Transformation Mechanism

Autotrophic denitrification utilizing iron sulfides as electron donors has been well studied, but the occurrence and mechanism of abiotic nitrate (NO₃^-) chemodenitrification by iron sulfides have not yet been thoroughly investigated. In this study, NO₃^- chemodenitrification by three types of iron sulfides (FeS, FeS₂, and pyrrhotite) at pH 6.37 and ambient temperature of 30 °C was investigated. FeS chemically reduced NO₃^- to ammonium (NH₄⁺), with a high reduction efficiency of 97.5% and NH₄⁺ formation selectivity of 82.6%, but FeS₂ and pyrrhotite did not reduce NO₃^- abiotically. Electrochemical Tafel characterization confirmed that the electron release rate from FeS was higher than that from FeS₂ and pyrrhotite. Quenching experiments and density functional theory calculations further elucidated the heterogeneous chemodenitrification mechanism of NO₃^- by FeS. Fe(II) on the FeS surface was the primary site for NO₃^- reduction. FeS possessing sulfur vacancies can selectively adsorb oxygen atoms from NO₃^- and water molecules and promote water dissociation to form adsorbed hydrogen, thereby forming NH₄⁺. Collectively, these findings suggest that the NO₃^- chemodenitrification by iron sulfides cannot be ignored, which has great implications for the nitrogen, sulfur, and iron cycles in soil and water ecosystems.