Goethite reshapes photodegradation pathways and site selectivity of environmentally relevant polystyrene microplastics

Natural iron (oxyhydr)oxides are increasingly recognized as important regulators of microplastics (MPs) transformation, yet their mechanistic influence on polymer photodegradation remains poorly understood. Here, we investigated how goethite (α-FeOOH) alters the photodegradation of polystyrene microplastics derived from disposable plastic spoons (Spoon-MPs) under simulated sunlight. Goethite increased Spoon-MP mass loss from 14.41% under light-only conditions to 21.78% after 12 d and increased the apparent degradation rate constant. Photodegradation induced pronounced physicochemical changes, including surface erosion, loss of structural order, molecular-weight reduction, and enhanced surface oxidation. Time-resolved FTIR and 2D-COS revealed distinct transformation sequences: aromatic-associated bands responded earlier under light-only conditions, whereas backbone C–H-associated bands responded earlier in the presence of goethite. EPR and scavenger experiments further showed enhanced generation of multiple reactive species, particularly •OH, supporting a greater contribution of hydrogen-abstraction-driven oxidation in the goethite system. GC–MS revealed increased relative abundances of low-molecular-weight oxygenated products, including ketone- and ester-containing compounds. DFT calculations identified aromatic and benzylic regions as intrinsically reactive sites, complementing the experimentally observed transformation patterns. Collectively, these findings demonstrate that goethite not only accelerates PS photodegradation but also alters the temporal sequence and relative importance of oxidative pathways, providing mechanistic insight into mineral-mediated transformation of microplastics in aquatic environments.