Isotopic and geochemical modeling approach to evaluate abiotic nitrite reduction by ferrous iron

Abstract

Nitrite reduction has often been treated as a biotic process in water treatment systems, but it also occurs abiotically, and it is difficult to distinguish both reactions as they can co-occur at field-scale. The potential reduction of NO2− was tested in 3 anaerobic experimental scenarios using a NO2− bearing solution amended with: i) siderite (FeCO3) (Sid experiment); ii) Fe(II) solution from FeCl2.4H2O(s) (DFe experiment); and iii) siderite mixed with Fe(II) solution (Sid+DFe experiment). Non-sterilized batch experiments were carried out in anaerobic conditions with an initial ratio of nitrogen to dissolved iron of 5 in DFe and Sid+DFe (Sid had no initially dissolved Fe(II)) and 1000 mg L−1 of siderite in Sid and Sid+DFe experiments. At the end of the experiments, the NO2− removed was 3% for Sid, 54% for DFe and 84% for Sid+DFe. The NO2− concentration decrease over time was characterized by an enrichment in the δ15NNO2 of the unreacted NO2−, increasing from −26.9 ‰ to −26.4 ‰ (Sid), −18.0 ‰ (DFe) and −15.9 ‰ (Sid+DFe). The calculated ε15NNO2 for Sid was −11.8 ‰, whereas for DFe was −12.0 ‰ and Sid+DFe was −13.0 ‰, suggesting a common NO2− degradation mechanism in all experiments. The Rayleigh distillation equation showed that the generated N2O was the final product of the abiotic nitrite reduction reaction, and the calculated N2O site preference (SP) was 22.5 ± 0.7 ‰ for DFe and 23.5 ± 0.5 ‰ for Sid+DFe. The continuous N2O measurement showed that only 25.3 ± 5.1% in DFe and 31.0 ± 6.3% in Sid+DFe of the generated N2O in water was recovered in the headspace vials, suggesting that a large portion of the produced N2O(aq) in solution did not diffuse from water. The coupled NO2− reduction and Fe(II) oxidation followed a second-order kinetic reaction with a rate equal to (9.39 ± 0.36)·10−4·[NO2−]·[Fe(II)] (mol L−1 s−1) in all experiments. The experimental conditions supported by the Rayleigh distillation equation using the experimentally calculated ε15N values, coupled with NO2− isotopic data and N2O SP values, showed that biological denitrification had a negligible influence on nitrite reduction and that chemodenitrification was the main NO2− attenuation pathway. A geochemical model coupling the kinetic chemodenitrification, isotope fractionation, aqueous speciation in equilibrium and precipitation and dissolution of calcite has been implemented and reproduced the experimental results. The geochemical model developed in our study can be applied to similar experimental studies and to field-scale studies to predict the efficiency of abiotic nitrite reduction treatments using Fe(II).