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 (NO) 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 NO in hyporheic zone (HZ) sediments by integrating Cr(VI) reduction kinetics, NO transformation, microbial community structure, and a three-rate model. The concurrent natural attenuation of Cr(VI) and NO 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 NO, 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 NO (450 μM), the reduction of NO was adversely affected by Cr(VI). The reduction rates and efficiencies of Cr(VI) and NO were noticeably lower under aerobic conditions than under anaerobic conditions. This phenomenon can be attributed to the presence of O, which decreased the selectivity of sediments-associated Fe(II) towards Cr(VI) and NO and induced alterations in the microbial community structure, leading to subsequent changes in NO 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 NO contamination under diverse redox conditions. This study provides further insights into the interaction mechanism between Cr(VI) and NO within the HZ, necessitating the consideration of the microbial toxicity of Cr(VI) and electron competition among Cr(VI), NO, and O.