Patterns and mechanisms of sediment charging and discharging driven by groundwater level fluctuations

ObjectiveElectron transfer is fundamental to biogeochemical reactions in subsurface environments. Sediments act as key electron reservoirs capable of cyclic electron storage and release under groundwater level fluctuations, thereby significantly influencing material transformation and elemental cycling. However, the patterns and mechanisms governing sediment charging and discharging driven by groundwater level fluctuations remain poorly understood. MethodsIn this study, a one-dimensional soil column system was developed to simulate the groundwater fluctuation zone. The patterns and mechanisms of sediment charging and discharging driven by groundwater level fluctuations were investigated using a combination of chemical analyses, iron mineral speciation, and molecular biological techniques. ResultsThe results showed that under short-cycle fluctuation conditions, sediments completed two charging-discharging cycles, with maximum charge/discharge capacities of 2.3 and 8 μmol e−·g−1, and peak rates of 0.577 and 2.012 μmol e−·g−1·d−1, respectively. The electron-donating capacity (EDC) of the sediments was mainly contributed by adsorbed, ion-exchangeable, and highly reactive weakly crystalline Fe(Ⅱ). Ground water level fluctuations drove the microbial Fe(Ⅲ) reduction followed by the chemical Fe(Ⅱ) oxidation, thereby enabling the cyclic charging and discharging of sediment. However, repeated redox cycles reduced the bioavailability of iron oxides, ultimately hindering their sustained cyclic electron storage and release. The addition of the electron shuttle anthraquinone-2,6-disulfonate (AQDS) significantly increased the initial charging and discharging rates but also accelerated the decline in Fe(Ⅲ) bioavailability, resulting in a gradual decrease in the charging and discharging rates and the termination of cycling after the third one. In contrast, the addition of sodium lactate (an electron donor) significantly enriched the iron-reducing bacterium Anaeromyxobacter, maintained high Fe(Ⅲ) bioavailability, and markedly enhanced the charging and discharging rates, thus enabling its sustained cyclic charging and discharging under groundwater level fluctuations. ConclusionThis study clarifies the variation patterns and regulatory mechanisms charging and discharging behaviors of underlying the sediment under different groundwater level fluctuation conditions, and provides new strategies for the prevention and control of groundwater pollution.