Oxygen regulates Sb(III) immobilization on schwertmannite via surface precipitation and nanotunnel-confined electron transfer

Antimony (Sb) contamination from mining activities poses global risks, particularly in acid mine drainage environments where schwertmannite serves as a key sink for Sb. However, how oxygen regulates Sb(III) immobilization pathways and electron transfer mechanisms, particularly via the nanotunnel structure of schwertmannite, remains poorly understood. This study aims to elucidate the oxygen-dependent immobilization and electron transfer of Sb(III) on schwertmannite. Results demonstrated that schwertmannite exhibited observed Sb(III) retention capacities of 328.3 ± 8.5 mg/g and 304.8 ± 5.7 mg/g under the tested aerobic and anaerobic conditions (pH 3.0), respectively. EXAFS and DFT analyses revealed that aerobic conditions favored edge-sharing Sb-Fe complexes accompanied by significant surface precipitation, whereas anaerobic conditions favored corner-sharing configurations with only minor surface precipitation, highlighting oxygen as a key regulatory factor. Under aerobic conditions, dissolved oxygen promoted an oxidation-first pathway, converting part of aqueous Sb(III) to Sb(V), which competed for reactive sites within schwertmannite nanotunnels and partially restricted subsequent Sb(III) diffusion into the nanotunnel structure, thereby promoting external Sb(III) accumulation and valentinite-like precipitation. In contrast, under anaerobic conditions, the absence of oxygen-driven Sb(V) formation minimized competition for nanotunnel reactive sites, allowing aqueous Sb(III) to preferentially access schwertmannite nanotunnels and become immobilized through inner-sphere complexation. Compared with aerobic conditions, surface precipitation was suppressed, while up to ∼20% solid-phase Sb(III) oxidation was observed under anaerobic conditions, consistent with a possible contribution from nanotunnel-confined electron transfer. This oxygen-dependent reaction sequence effect highlights that oxygen regulates Sb transformation pathways rather than simply oxidation extent, providing molecular insights into schwertmannite-based Sb remediation strategies.