Abstract
The coexistence of hexavalent chromium (Cr(VI)) and nitrate (NO3−) in groundwater and surface water presents a considerable challenge for the natural attenuation of these two contaminants because their interactions in nature remain contentious. This study investigated the interplay between Cr(VI) and NO3− in hyporheic zone (HZ) sediments by integrating Cr(VI) reduction kinetics, NO3− transformation, microbial community structure, and a three-rate model. The concurrent natural attenuation of Cr(VI) and NO3− in the sediments was significantly influenced by their initial concentrations and redox conditions. The reduction of low concentrations of Cr(VI) (37.1 and 96.2 μM) was slightly enhanced by NO3−, while inhibitory effects were observed at high concentrations of Cr(VI) (200.0 μM). However, except for an initial low concentration of Cr(VI) (37.1 μM) and NO3− (450 μM), the reduction of NO3− was adversely affected by Cr(VI). The reduction rates and efficiencies of Cr(VI) and NO3− were noticeably lower under aerobic conditions than under anaerobic conditions. This phenomenon can be attributed to the presence of O2, which decreased the selectivity of sediments-associated Fe(II) towards Cr(VI) and NO3− and induced alterations in the microbial community structure, leading to subsequent changes in NO3− transformation. Furthermore, the three-rate model represents a robust approach for elucidating the reduction of Cr(VI) in the presence of co-contaminants, such as NO3− contamination under diverse redox conditions. This study provides further insights into the interaction mechanism between Cr(VI) and NO3− within the HZ, necessitating the consideration of the microbial toxicity of Cr(VI) and electron competition among Cr(VI), NO3−, and O2.
Published Version
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