Electron exchange capacities of sediments as chemical controls on natural attenuation and engineered remediation of contaminated aquifers

Natural attenuation and remediation of contaminants in aquifers are strongly constrained by both structural and chemical heterogeneity. While structural heterogeneity has been extensively characterized, chemical heterogeneity remains poorly quantified and insufficiently understood. In this review, we use electron exchange capacities (EEC) as a proxy for chemical heterogeneity and demonstrate how sediment EEC influences contaminant transformation and remediation performance. The evolution of EEC measurement approaches, including chemical probe and mediated electrochemical methods, is summarized. Sediment EEC is linked to the transport and transformation of groundwater contaminants through EEC-driven processes such as anaerobic microbial respiration, mineral dissolution, direct contaminant reduction, and reactive oxygen species-mediated oxidation. In terms of remediation, sediment EEC quantitatively governs the consumption and activation of injected oxidants during in situ chemical oxidation and serves as a robust parameter for evaluating bioremediation performance. Overall, EEC provides a quantitative measure of aquifer chemical heterogeneity, offering mechanistic insights into natural attenuation and guidance for green and sustainable remediation strategies. Future research should focus on methodological improvements, mechanistic elucidation of EEC composition and its environmental significance, and the development of predictive models incorporating EEC.