Reactivating secondary pyrite as a solar-driven Fe-S-O interface for simultaneous nitrate removal and chlorophenol degradation through mineral-microbe coupling.

Nitrate contamination often co-occurs with toxic chlorophenolic micropollutants, such as 2,4-dichlorophenol (2,4-DCP), in agricultural drainage and wastewater-impacted waters. However, simultaneous removal is limited by scarce electron donors and pollutant toxicity. Unlike natural pyrite, secondary pyrite (SP), formed during Fe-S-mediated water treatment, is commonly considered a passivated residue because surface oxidation products suppress its reactivity. This study demonstrates that recycled SP serves as a defect-rich, solar-responsive mineral mediator for mineral-microbe-coupled remediation. Under simulated solar irradiation with an initial 2,4-dichlorophenol (2,4-DCP) concentration of 5 mg L-1, coupling SP with the denitrifying bacterium achieved 96.98% nitrate removal and 92.55% 2,4-DCP degradation, outperforming natural pyrite. The enhanced activity was associated with sulfur vacancies, mixed-valence Fe species, and reconstructed surface domains. These features narrowed the band gap, promoted visible-light harvesting, accelerated charge separation, and facilitated interfacial Fe-S redox cycling. Respiratory-chain inhibitor assays indicated a solar-dependent shift away from reliance on NADH-dependent Complex I toward quinone-associated electron transport, which was accompanied by accelerated denitrification. Meanwhile, photogenerated holes and reactive oxygen species promoted stepwise dechlorination and ring-opening of 2,4-DCP. These findings reposition SP as a reusable solar-active mineral interface, rather than an inert byproduct, and suggest a sustainable route for simultaneous nitrogen removal and organic contaminant remediation.