By comparing three common iron oxide polymorphs, researchers discovered that the crystalline phase of these minerals determines how effectively they catalyze the hydrolysis of organophosphate ester (OPE) pollutants. The findings clarify why some iron nanoparticles accelerate contaminant breakdown while others lag behind, offering a deeper understanding of how OPEs transform in real environments.
As plastics degrade into micro- and nanoscale fragments, they increasingly release chemical additives such as phthalates and OPEs—compounds linked to neurological, reproductive, developmental, and respiratory toxicities. These pollutants accumulate widely in soils and waterways, where their environmental fate depends on both biological and chemical transformation pathways. Metal-bearing nanoparticles, particularly iron oxides, are abundant in natural systems and are known to adsorb, bind, and catalyze reactions involving organic contaminants. Yet despite the prevalence of iron nanominerals, the influence of their crystalline phase on OPE degradation remains poorly understood.
A study (DOI:10.48130/ebp-0025-0008) published in Environmental and Biogeochemical Processes on 21 October 2025 by Chuanjia Jiang’s team, Rice University, reveals how the crystalline phase of iron oxyhydroxide nanoparticles fundamentally controls their ability to catalyze the breakdown of organophosphate ester pollutants, providing essential mechanistic insight for predicting the environmental fate and risks of plastic-derived contaminants.
The researchers first synthesized three iron oxyhydroxide nanoparticles—goethite, akaganeite, and lepidocrocite—and systematically characterized their physicochemical properties using X-ray diffraction (to confirm distinct crystalline phases), N₂ adsorption–desorption isotherms (to determine specific surface area and pore structure), and SEM/TEM/HRTEM imaging (to resolve morphology and lattice spacings). Structural modeling and electron localization function analysis were then applied to quantify the density of unsaturated Fe sites, while catalytic tests were conducted by monitoring the hydrolysis of p-nitrophenyl phosphate (pNPP) at environmentally relevant pH values (6.0–8.0). Kinetics were analyzed using pseudo-first-order and Langmuir–Hinshelwood models to separate adsorption affinity from surface reaction rates. In situ ATR-FTIR spectroscopy and DFT calculations were used to probe adsorption configurations and energies, supplemented by PDOS and COHP analyses, and Py-IR and Bader charge analyses were employed to evaluate Lewis acidity and charge redistribution during catalysis. These methods revealed that lepidocrocite has the highest specific surface area and nanofiber morphology, while goethite and akaganeite form nanorods and spindle-like particles; ELF analysis showed phase-dependent densities and coordination environments of unsaturated Fe sites. Catalytic tests demonstrated that all three phases strongly accelerated pNPP hydrolysis, with lepidocrocite consistently showing the highest activity and surface area–normalized rate constants, despite having the weakest adsorption affinity. ATR-FTIR and DFT confirmed that pNPP binds mainly via inner-sphere complexation through its –PO₄ group, with akaganeite exhibiting the strongest adsorption due to more favorable Fe 3d–O 2p interactions. However, Py-IR, Bader, PDOS, and COHP analyses showed that lepidocrocite has the highest density of Lewis acid sites and induces the greatest charge polarization and P–O bond weakening in adsorbed pNPP, explaining its superior surface reactivity and overall hydrolytic efficiency.
These findings carry broad implications for understanding and predicting the environmental fate of plastic-derived contaminants. Because iron oxyhydroxide nanoparticles are ubiquitous in soils, sediments, and aquatic systems, their crystalline makeup will directly influence the transformation, persistence, and ecological risks of OPE additives. The work also provides a mechanistic framework for environmental models that assess pollutant degradation pathways, highlighting the need to account for mineralogical diversity rather than treat iron oxides as a uniform category. Moreover, the insights may inform strategies for designing mineral-based remediation materials that exploit high-density Lewis acid sites to accelerate contaminant breakdown.
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References
DOI
10.48130/ebp-0025-0008
Original Source URL
https://doi.org/10.48130/ebp-0025-0008
Funding information
This work was supported by the National Natural Science Foundation of China (22125603, 22241602, and 22020102004), Starting Research Grant for High-level Talents and Innovative Foundation from Ankang University (2023AYQDZR21, 2025AYHX008, and 2024AKHX009), Tianjin Municipal Science and Technology Bureau (23JCZDJC00740), the Fundamental Research Funds for the Central Universities (63253200 and 63251028), and the Ministry of Education of China (B17025).
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