Production of Surface-Bound Hydroxyl Radicals on Sediments for Adsorbed Contaminant Transformation at Oxic–Anoxic Interfaces

Organic contaminants in groundwater can be effectively degraded by aqueous free hydroxyl radicals (•OH_free) generated from Fe(II)-bearing sediment oxygenation. However, how contaminants adsorbed on sediment surfaces are degraded at oxic-anoxic interfaces remains unclear. Here we investigated the degradation of tetracycline (TC), a model adsorbed contaminant, during the oxygenation of three Fe(II)-bearing sediments. Quenching experiment results, in situ Raman spectra, and fluorescence microscope images revealed the formation of surface-bound •OH (•OH_surface). •OH_surface, rather than •OH_free, was primarily responsible for the rapid degradation of adsorbed TC. Sediments with moderate Fe(II) oxidation rates and electron release exhibited the highest •OH_surface production and TC degradation, due to competition between the pathways of •OH_surface generation and Fe(IV) formation. Density functional theory (DFT) calculations indicate a facet-dependent mechanism for the activation of adsorbed O₂ on Fe(II)-bearing clay minerals in sediments and the subsequent production of •OH_surface via surface electron transfer processes. The degradation of adsorbed TC via •OH_surface oxidation followed a distinct pathway compared to homogeneous reactions, generating intermediates with larger molecular weights. Overall, our findings expand the understanding of •OH formation from aqueous phases to sediment surfaces, offering a novel but overlooked pathway for the natural attenuation of adsorbed organic contaminants at oxic-anoxic interfaces.