Spin-State Engineering at the FeS₂ Interface via CO₂: Dual-Channel Activation of Fe(II) for Enhanced ^•OH Generation

The low-spin electronic structure of pyrite (FeS₂) severely restricts its activity in generating hydroxyl radical (^•OH) through green oxidation. Herein, we report a dual-channel mechanism for in situ activation of natural FeS₂ by greenhouse gas CO₂ to overcome these limitations, with demonstrated potential for environmental remediation applications. Dissolution of CO₂ generates H₂CO₃, which acidifies the aqueous environment and promotes Fe(II) release, exposing reactive surface sites. Concurrently, HCO₃^-/CO₃^2- ligands coordinate with surface Fe(II) to form interfacial Fe-carbonate complexes. This coordination distorts the Fe-S lattice and triggers reconfiguration of the water-mineral interface, collectively boosting ^•OH generation. X-ray absorption fine-structure spectra (XAFS), Mössbauer spectra and density functional theory (DFT) calculations confirm that this reconstructed interface induces a transition of the Fe(II) center from low-spin (t_2g⁶e_g⁰) to intermediate-spin (t_2g⁵e_g¹), narrows the bandgap, lowers the activation free-energy barrier, and enhances the electron transfer with O₂, resulting in an approximately 90% increase in ^•OH yield. In batch experiments, degradation efficiencies of sulfamethoxazole, florfenicol, and 2,4,6-tribromophenol reached 66%, 54%, and 58%, respectively, with a significant reduction in the ecotoxicity of transformation products. Beyond batches, a two-dimensional sandbox aquifer model was conducted; after 5 days of continuous flow, degradation efficiencies were 73%, 48%, and 41%, respectively. This work advances a novel mechanism for enhanced ^•OH generation by CO₂ and FeS₂, converting low-value minerals and waste gas into effective resources for groundwater purification, thereby achieving a win-win situation of resource utilization and environmental protection.