Copper-Enhanced Ferrihydrite Fenton Generates Dual Oxidants for Pollutant Degradation via Surface Affinity Differentiated Pathways

Ferrihydrite (Fh) and transition metal ions such as Cu(II) are ubiquitous in natural and engineered aquatic systems, influencing pollutant degradation through Fenton-like processes. In contrast to previous studies focusing exclusively on either Cu(II)-catalyzed homogeneous reactions or iron oxide-mediated heterogeneous reactions, this work presents an integrated view of Cu(II)-Fh interactions under environmentally relevant pH conditions. We demonstrate that Cu(II) and Fh jointly establish a dual-oxidant system capable of generating both hydroxyl radicals (•OH) and Cu(III). These reactive species operate through spatially distinct mechanisms, leading to pollutant-specific degradation behaviors. Five organic compounds, including formate (FA), benzoic acid (BA), bisphenol A (BPA), dimethyl phthalate (DMP), and hydroxyethylidene diphosphonic acid (HEDP), were examined to elucidate the roles of mineral surface affinity and reactive species distribution. Adsorbed compounds (FA, BA and BPA) were primarily degraded via nonradical inner-sphere electron transfer at the Fh surface, with limited •OH involvement. In contrast, weakly adsorbing pollutant, HEDP, degradation was governed by •OH and Cu(III), while DMP, which exhibited negligible adsorption and minimal Cu(III) reactivity, proceeded mainly through •OH generated with the presence of adsorbed surface Cu(II). This affinity-differentiated oxidation paradigm highlights how pollutant structure, coordination chemistry, and interfacial redox dynamics jointly control degradation pathways in mineral-water systems. Cu(II) plays a dual catalytic role: enhancing radical formation and acting as a selective oxidant via Cu(III) for strongly complexing ligands. These findings advance the mechanistic understanding of Cu-enhanced heterogeneous Fenton systems and provide new insight into contaminant fate and transformation in redox-active aquatic environments.