Insight into the Photodegradation of Microplastics Boosted by Iron (Hydr)oxides

Iron (hydr)oxides as a kind of natural mineral actively participate in the transformation of organic pollutants, but there is a large knowledge gap in their impacts on photochemical processes of microplastics (MPs). This study is the first to examine the degradation of two ordinary plastic materials, polyethylene (PE) and polypropylene (PP), mediated by iron (hydr)oxides (goethite and hematite) under simulated solar light irradiation. Both iron (hydr)oxides significantly promoted the degradation of MPs (particularly PP) with a greater effect by goethite than hematite, related to hydroxyl radical (^•OH) produced by iron (hydr)oxides. Under light irradiation, the surface Fe(II) phase catalyzed the production of H₂O₂ and promoted the release of Fe²⁺, leading to the subsequent light-driven Fenton reaction which produced a large amount of ^•OH. As the iron (hydr)oxides were modified with NaF at various concentrations, the activity of the surface Fe(II) as well as the release of Fe²⁺ were greatly reduced, and thus the ^•OH formation and MP degradation were depressed remarkably. It is worth noting that the surface hydroxyl groups (especially ≡FeOH) affected the reaction kinetics of ^•OH by regulating the activity of Fe species. These findings unveil the distinct impacts and intrinsic mechanisms of iron (hydr)oxides in influencing the photodegradation of MPs.