Rapid, Abiotic Nitrous Oxide Production from Fe(II)-Driven Nitrate Reduction Governed by pH Under Acidic Conditions

Denitrification is conventionally viewed as a microbially mediated process driven by organic carbon, but the coupling of iron and nitrogen cycles represents a significant alternative pathway. Previous research focused on alkaline environments, leaving the direct reaction between ferrous iron (Fe²⁺) and nitrate (NO₃^-) in acidic conditions poorly understood. This study investigated this process using in situ spectroscopy, examining the effects of the reactant ratio, time, temperature, and initial pH while monitoring nitrous oxide (N₂O) production. Results showed that the reaction was rapid, with most nitrate reduction within 5 min. The Fe²⁺ to NO₃^- molar ratio had a limited influence on efficiency. Temperature had a non-monotonic effect, optimal at 25 °C. The initial pH was the dominant control, with lower pH (e.g., 4.6) essential for high efficiency. Crucially, the process was confirmed as a significant source of N₂O under anoxic conditions. This work confirms Fe²⁺-driven nitrate reduction is a fast, acid-dependent process governed by pH and modulated by temperature. These findings revise our understanding of nitrogen fate and N₂O emissions and warn of potential underestimation of nitrate in samples containing Fe²⁺.