The regulatory role of dissolved oxygen in N-doped biochar-driven nonradical oxidation

Groundwater remediation via electron transfer oxidation-dominated peroxydisulfate (PDS)-based in-situ chemical oxidation (ISCO) is appealing due to its high selectivity in pollutant removal and the efficient utilization of oxidants. However, the electron transfer oxidation pathway (ETP) mechanism involving dissolved oxygen (DO) remains ambiguous in aquifers, resulting in the regulatory role of groundwater fluctuations for ETP being typically overlooked. This study revealed the novel mechanism of DO involvement in the N-doped biochar-mediated PDS ETP activation. 2,4,6-trichlorophenol (TCP) was effectively removed by the N-doped biochar/PDS system through the ETP within 120 min, with the degradation efficiency improving from 5 % to 100 % in the presence of DO. Electrochemical tests and density functional theory (DFT) calculations identified that pyridinic N was the dominant active site for harnessing O₂ as an electron acceptor. The redox potential of the metastable complex formed by PDS adsorption on N-biochar under aerobic conditions was significantly higher than that under anaerobic conditions, which enhanced the electron transfer efficiency between the pollutant and PDS mediated by N-biochar. Continuous-flow experiments demonstrated TCP effluent being completely removed until 57 (anaerobic) and 81(aerobic) pore volumes (PV) at 0.5 mM PDS and a 1 % mass ratio of biochar (Mass_biochar/Mass_sand). This novel mechanism provided valuable insights into the DO-enhanced ISCO groundwater remediation process.