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 (•OHfree) 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 (•OHsurface). •OHsurface, rather than •OHfree, was primarily responsible for the rapid degradation of adsorbed TC. Sediments with moderate Fe(II) oxidation rates and electron release exhibited the highest •OHsurface production and TC degradation, due to competition between the pathways of •OHsurface 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 •OHsurface via surface electron transfer processes. The degradation of adsorbed TC via •OHsurface 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.