Spin-State Engineering at the FeS ₂ Interface via CO ₂ : 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–/CO32– 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 (t2g6eg0) to intermediate-spin (t2g5eg1), 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.