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 (O 2 ) 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 O 2, triggering concurrent oxidative degradation by hydroxyl radical ( • OH), high-valent iron (Fe(IV)), and singlet oxygen ( 1 O 2 ). 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.