Enhancing phosphate removal through defect and pore engineering in FeZr bimetallic Fe-HPUiO-66

The microporous nature of MOF adsorbents has limited phosphate adsorption capacity due to their poor mass transfer and shielded active sites. Here, we report a Fe-doped Zr-MOF (Fe-HPUiO-66) with hierarchical porosity and abundant defects, synthesized via a combined strategy of Fe doping and selective pyrolysis of thermally labile ligands. The obtained adsorbents retain the UiO-66 topology while exhibiting a micro-to-mesoporous transition, which dramatically enhances phosphate diffusion kinetics and site accessibility. Fe-HPUiO-66 achieves a high adsorption capacity of 134 mg P/g, outperforming many reported adsorbents, along with excellent stability and reusability. Mechanism studies reveal that phosphate uptake is governed primarily by bidentate binuclear inner-sphere complexation with unsaturated Zr sites, supplemented by electrostatic attraction with protonated hydroxyl groups. Furthermore, oxygen vacancies serve as key active centers that enhance the adsorption affinity by inducing electron localization. This work demonstrates that synergistic engineering of metal nodes and pore architecture is an effective pathway to boost the overall performance of MOF adsorbents for water remediation.