Chemodenitrification by Fe(II) and nitrite: pH effect, mineralization and kinetic modeling

Despite the biogeochemical significance of redox interactions between nitrite (NO2-) and Fe(II) species, considerable uncertainty still remains as to the exact processes contributing to the rates and extents of such chemodenitrification processes and its associated importance to the global N2O flux. In this study, pH effect and mineral formation on the redox interactions between NO2- and Fe(II) under anaerobic conditions over the pH range 5.5-7.0 were investigated, with a detailed kinetic model developed to describe the predominant mechanisms operating in the system. Our results demonstrated that chemodenitrification proceeds without an initial solid phase and both Fe(II) oxidation and NO2- reduction rates increased with pH increasing from 5.5 to 7.0, with concomitant generation of N2O. While the oxidation of Fe(II) leads to a rapid formation of goethite and shifts the reaction into a heterogeneous mechanism, N2O detected in this study only partially account for the NO2- consumed throughout the entire examined pH range. The developed model indicated that both the dissolved and surface-associated Fe(II) species were kinetically active toward nitrite reduction, indicating the presence of Fe(III) minerals acting as a positive feedback and auto-catalytic pathway for the reaction. The kinetic model may provide critical insight into the underlying mechanisms and relative contribution of chemodenitrification to biological nitrite reduction and assist in understanding and prediction on the chemical aspects controlling iron and nitrite transformation in environments exhibiting rapidly fluctuating redox conditions.