Engineering iron-polyphenol systems for priority pollutant removal: From coordination chemistry to water treatment applications.

Iron (Fe)-based materials are widely used in water pollution but often suffer from particle agglomeration, surface passivation, restricted Fe(III)/Fe(II) cycling, Fe leaching, and activity decline under neutral and complex conditions. Polyphenolic molecules, rich in coordinating/reducing groups such as catechol and galloyl groups, can coordinate Fe species, regulate Fe dissolution and stabilization, mediate interfacial electron transfer, and thereby alter reactive oxygen species. This review summarizes the molecular basis of polyphenol-regulated Fe chemistry by comparing single polyphenols with polyphenol-rich plant extracts. It examines Fe-polyphenol coordination, homogeneous and heterogeneous system design, fabrication strategies, and the use of advanced characterization and modelling to resolve Fe speciation and reaction pathways. Structure-activity relationships governing adsorption/complexation, reductive transformation, advanced oxidation, coagulation/coprecipitation, and cascade coupling processes are critically evaluated. Applications for metals/metalloids, nitrogen species, pharmaceuticals, antibiotics, endocrine-disruptors, dyes, industrial organic compounds, and microplastics are comparatively assessed. The effects of pH and coexisting water constituents are also discussed. Finally, key challenges in complex aqueous matrices, including polyphenol-source variability, stability, scale-up, byproducts risks, and life-cycle impacts, are identified. This review provides a framework linking polyphenol structure, Fe coordination chemistry, and pollutant-specific treatment pathways for the rational design of Fe-polyphenol water treatment technologies.