Heterogeneous Photochemical Generation of Hydroxyl Radical in Mineral-Organics Systems: Dual Roles of Iron Oxides

Photosensitive organic molecules and semiconducting minerals have been widely reported to produce hydroxyl radicals (^•OH) in aquatic environments. However, studies employing multicomponent systems remain limited. This study investigates the role of minerals in ^•OH photochemical generation in the presence of organic compounds with distinct functional groups. Using model compounds to represent key moieties in dissolved organic matter (DOM), we examined the capacity of both single-component systems (organic matter and minerals) and composite systems (ferrihydrite-organic matter) to generate ^•OH. Our findings demonstrate that low-molecular-weight organics such as oxalate and citrate, rich in carboxyl groups, significantly enhance ^•OH production when combined with ferrihydrite. In contrast, organic molecules with aromatic or quinone-like structures (e.g., Aldrich humic acid (AHA) and 9,10-anthraquinone-2,6-disulfonic acid disodium salt (AQDS)) tend to suppress ^•OH production. Specifically, the ferrihydrite-citrate system shows a doubling of ^•OH production under solar light and a 20-fold increase under visible light at pH 3. Conversely, the addition of ferrihydrite significantly inhibited ^•OH production in the AQDS system, which by itself had a strong ability to generate ^•OH. Furthermore, mineral transformation and organic carbon loss suggest that ^•OH photochemical generation could accelerate the geochemical cycling of iron and carbon. These insights deepen our understanding of the mechanisms underlying ^•OH generation and the environmental implications of iron-carbon interactions at the irradiated interfaces.