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+.