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

摘要

Since nitrate (NO_3 ~-) 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 (NO_2 ~-) or nitrous oxide (N_2O)). 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, NO_3 ~- polluted groundwater was employed as inoculum. In these experiments, NO_3 ~- reduction to nitrogen gas (N_2) 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 NO_3 ~- or NO_2 ~- and micro-sized magnetite, siderite or olivine, while NO_2 ~- was rapidly reduced when dissolved Fe~(2+) was added. These results point to the need of a certain amount of dissolved Fe~(2+) to stimulate the abiotic NO_2 ~- reduction by Fe(II) oxidation. For the microbial NO_3 ~- reduction by magnetite nanoparticles, the calculated ε~(15)NNO_3 was -33.1‰ (R~2 = 0.86), ε~(18)ONO_3 was -10.7‰ (R~2 = 0.74) and ε~(15)NNO_3/ε~(18)ONO_3 was 3.1. For the abiotic NO_2 ~- reduction by Fe~(2+), the ε~(15)N_(NO2) ranged from -14.1 to -17.8‰ (R~2 > 0.89). Considering the wide range of ε~(15)N_(NO2) reported in the literature, it is not likely that NO_2 ~- isotopic characterization can be useful at field-scale to distinguish abiotic from microbial NO_2 ~- reduction. Nevertheless, the measured δ~(15)N for N_2O 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 model