Oxalic Acid-Promoted Pollutant Attenuation by Hematite via Hydrogen Radical Generation under Aerobic Conditions

Despite oxalic acid (OA) is widely recognized for enhancing pollutant degradation by minerals via ligand-promoted electron transfer and molecular oxygen (O₂) activation, its comprehensive mechanistic contributions are not yet fully understood. Herein, we uncover a novel and pivotal role of OA as a proton donor, which triggers a potent reducing force mediated by H^• under dark oxic conditions. In a hematite system rich in oxygen vacancies (OVs), the introduction of OA significantly enhanced the removal of bisphenol A and p-nitrophenol from 0% to 62.3% and 21.3% to 64.1%, respectively. Further investigations identified the formation of H^• through the transfer of OV electrons to OA-supplied protons as the rate-limiting step in pollutant degradation via a series of analyses including electron paramagnetic resonance spectroscopy, electrochemical analysis, scavenger tests, and theoretical calculations. Moreover, mineral speciation analysis reveals that H^• generated at the mineral-water interface directly drives the reductive transformation of pollutants. Simultaneously, H^• reduces surface Fe(III) to Fe(II). This newly formed Fe(II) then activates dissolved O₂, triggering concurrent oxidative degradation by hydroxyl radical (^•OH), high-valent iron (Fe(IV)), and singlet oxygen (¹O₂). Our findings emphasize the critical role of H^• in mineral-mediated biogeochemical processes and pollutant dynamics, and highlight the synergistic reductive-oxidative system driven by OA.