Crystalline phase-dependent hydrolysis of organophosphate esters by iron oxyhydroxides: implications for nanomineral-mediated transformation of plastic additives

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A key risk factor associated with pervasive plastic pollution is the release of additives, including organophosphate esters (OPEs). Metal-bearing nanoparticles, such as iron oxides, can significantly affect the transformation of these plastic additives. However, the mechanisms underlying how intrinsic properties of iron oxide minerals determine their capability to mediate the transformation of OPEs are still obscure. Here, it is demonstrated that iron oxyhydroxide nanoparticles (goethite, akaganeite, and lepidocrocite) can effectively catalyze the hydrolysis of 4-nitrophenyl phosphate (pNPP), a model OPE contaminant, under environmentally relevant pH conditions (pH 6.0 to 8.0). The catalytic efficiency exhibits a pronounced dependence on the crystalline phase of the iron oxyhydroxides, with surface area-normalized rate constant following the order: lepidocrocite > akaganeite > goethite. The crystalline phase-dependent catalytic performance is jointly governed by two key factors, i.e., the adsorption affinity toward pNPP of the iron oxyhydroxide nanoparticles and the reactivity of adsorbed pNPP at the surface. Specifically, differences in surface charge distribution among the crystalline phases lead to variations in electronic interactions with the phosphate groups in pNPP, affecting their adsorption affinities (akaganeite > goethite > lepidocrocite). Concurrently, the intrinsic catalytic reactivity (lepidocrocite > goethite > akaganeite) is governed by variations in Lewis acid sites and their ability to induce charge redistribution at the central P atom in adsorbed pNPP. These findings underscore the critical role of the mineral crystalline phase in controlling the environmental fate of OPEs and other plastic-derived additives, and can guide the accurate evaluation of the ecological risks of these emerging pollutants.