Simultaneous natural attenuation of Cr(VI) and nitrate in the hyporheic zone sediments from an upstream tributary of the Jinsha River in the Sichuan Basin

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.