Nitrate and nitrite reduction by ferrous iron minerals in polluted groundwater: Isotopic characterization of batch experiments

Abstract

Since nitrate (NO3-) has been related to human health and environmental problems, safe and sustainable strategies to remediate polluted water bodies must be investigated. This work aims to assess the feasibility of using ferrous iron (Fe(II))-containing minerals to stimulate microbial denitrification while avoiding pollution swapping (e.g. accumulation of the by-products nitrite (NO2-) or nitrous oxide (N2O)). To accomplish the objective, samples obtained from several batch experiments were characterized chemically and isotopically. Magnetite, siderite and olivine were tested micro-sized and magnetite was also tested nano-sized. In microbial experiments, NO3- polluted groundwater was employed as inoculum. In these experiments, NO3- reduction to nitrogen gas (N2) was only completed in microcosms containing magnetite nanoparticles, suggesting an increased Fe(II) availability from nano-sized compared to micro-sized magnetite. In abiotic experiments, no reactivity was observed between NO3- or NO2- and micro-sized magnetite, siderite or olivine, while NO2- was rapidly reduced when dissolved Fe2+ was added. These results point to the need of a certain amount of dissolved Fe2+ to stimulate the abiotic NO2- reduction by Fe(II) oxidation. For the microbial NO3- reduction by magnetite nanoparticles, the calculated e15NNO3 was -33.1‰ (R2 = 0.86), e18ONO3 was -10.7‰ (R2 = 0.74) and e15NNO3/e18ONO3 was 3.1. For the abiotic NO2- reduction by Fe2+, the e15NNO2 ranged from -14.1 to -17.8‰ (R2 > 0.89). Considering the wide range of e15NNO2 reported in the literature, it is not likely that NO2- isotopic characterization can be useful at field-scale to distinguish abiotic from microbial NO2- reduction. Nevertheless, the measured d15N for N2O in microbial and abiotic tests, allowed determining if it was an intermediate or a final product of the reactions by comparing these results with the modelled isotopic composition calculated using the e15N values determined for the substrates. Hence, isotopic data confirmed that the product of the microbial NO3- reduction was innocuous N2 while the product of the abiotic NO2- reduction was N2O. The latter reaction would be advantageous to avoid NO2- accumulation during denitrification only if the generated N2O is further reduced by microorganisms.