In-situ and long-enduring oxidation of SMX by Fe-modified biochar activated O2 in soil: bridging Fe-redox cycling and electron transfer modulation

Abstract Hydroxyl radicals (·OH) generated from endogenous Fe(II)/O 2 catalytic system hold substantial potential for the in situ remediation of contaminated farmland, but are substantially constrained by the insufficient Fe-redox cycling. In this study, we designed a Fe-loaded biochar (BC-Fe) that acts as an “electron highway” and a “Fe-redox modulator,” enabling the in situ oxidative degradation of sulfamethoxazole (SMX) through the synergistic enhancement of Fe(II)/·OH activation achieved by regulating Fe speciation and electron exchange capacity. Mechanistically, the coexistence of highly reactive surface Fe(II) and optimized electron storage and conductivity establishes a sustainable redox system. This system enables spatiotemporally coupled “charging” (0.5 and 5 M HCl Fe(II) formation and microbial Fe(III)-reduction) and “discharging” (O 2 activation) processes, which collectively promote soil Fe(II) production and Fe phase transformation to drive sustained ·OH production efficiently. Notably, HBC-Fe400 with optimized Fe loading not only minimized the depletion of crystalline Fe(II) in soil and markedly enriched functional genes associated with Fe-redox, but also enabled the synchronized activation of both the direct (BC-Fe-catalyzed) and indirect (soil Fe-redox cycling-amplified) Fenton-like pathways. This dual coordination led to a dramatic 4.2-fold enhancement in ·OH production (881.6 μM), and maintained a 3.58-fold increase under field conditions. Finally, SMX was degraded through three degradation pathways, namely the ring-opening reaction of the isoxazole ring, hydroxylation, and S–N bond cleavage, generating intermediates that contributed to toxicity attenuation. This study provides a sustainable pathway for pollutant degradation by achieving O 2 activation and offers valuable insights for designing advanced Fe-based biochar catalysts in green oxidation processes and environmental remediation.