Hierarchical Magnetic Nanocomposite for the Efficient Sequestration of Perfluorooctanoic Acid and Toxic Metals: Unraveling Multimodal Adsorption Mechanisms

Abstract

Adsorbents that remove per- and polyfluoroalkyl substances (PFAS) alongside heavy metals typically fail because PFAS capture requires acidic pH while metal capture requires near-neutral pH, and materials optimized for one class perform poorly on the other. We report a magnetically recoverable nanocomposite, Fe3O4@SiO2@Fe-MOF@N-doped carbon quantum dots (Mag–MOF–CQD), that resolves this conflict by spatially segregating the two capture chemistries: a mesoporous Fe-MOF shell binds perfluorooctanoic acid (PFOA) through electrostatic and hydrogen-bonding interactions, while an outer nitrogen-doped carbon-dot layer binds Pb2+ and Hg2+ through inner-sphere coordination. A two-stage pH-swing protocol (utilizing sequential fresh doses of the same material platform) exploits this segregation to remove all three pollutants from a single ternary mixture at 94–99% efficiency, with magnetic recovery in under 30 seconds. In authentic tap water, the material lowers PFOA from 50 ng L–1 to 1.5 ng L–1, below the 4 ng L–1 US EPA drinking-water limit. The Fe-MOF shell also catalyzes in situ photo-Fenton-like defluorination of captured PFOA (72% efficiency under simulated solar light), partially degrading the pollutant rather than merely relocating it. Density functional theory models and spectroscopic analysis suggest that the two active-site classes operate independently, mitigating competitive interference. Mag–MOF–CQD thus combines regulatory-relevant trace removal, multi-pollutant selectivity, and integrated partial destruction in a single recoverable material.