Polystyrene nanoplastics facilitate the Fe(II)-catalyzed ferrihydrite transformation into goethite and hematite under sedimentary conditions

Ferrihydrite (Fh) commonly interacts and efficiently captures nanoplastics (NPs) at the water-sediment interface, promoting NP immobilization in sediments. However, the reciprocal impact of NPs on Fh transformation, a critical geochemical process, remains poorly understood. Here, we investigated the effects of polystyrene NPs on the rate, mineral phase, and quantitative yield of Fe(II)-catalyzed Fh transformation under simulated sedimentary conditions. The results demonstrated that NPs significantly accelerated Fh transformation into more crystalline phases, namely goethite (Gt) and hematite (Hm), and markedly increased their proportions in solid products. In the presence of NPs, Gt formation was detectable within 24 h under both dark and light conditions, which was considerably faster than in NP-free controls. NPs also elevated the proportion of Gt in products to up to 15.3% and 12.3% under dark and light conditions, compared to 5.0% and 2.7% in control treatments, respectively. These findings can be primarily explained by the reduction of surface positive charge on Fh upon contact with NPs, which likely enhanced the interaction and electron transfer between Fe(II) and Fh. Furthermore, by enhancing the aggregation of Fe(III) sublattices formed by hydroxyl radical-mediated Fe(II) oxidation, NPs increased Hm proportion in the products from 3.9% to a maximum of 43.7% under light conditions. Our study thereby fills a critical knowledge gap in the NP-Fh interplay, revealing the overlooked impact of NPs through regulating iron oxide phases, which provides a novel perspective for understanding NP behavior in sediments.