Nanoconfinement engineering enhances HFO stability: Dual proton-sponge and size-exclusion strategy for robust heavy-metal sequestration in real electroplating effluents.

Hydrous ferric oxide (HFO) based hybrid adsorbents are extensively used for heavy metal removal due to their high sorption efficiency and operational durability. However, the complexity of real electroplating wastewater, with high loads of organics, salts, and acid, critically challenges their stability, resulting in short service life. Here, we developed a polydopamine (PDA)-confined strategy to enhance the stability of HFO, using a commercial HFO-polystyrene nanocomposite (PS-Fe) as model. We unveil a collaborative "Proton-Sponge and Size-Exclusion" underpinning the enhanced stability. The catechol-rich PDA matrix acts as an effective proton buffer to regulate the local chemical environment, while its crosslinked network selectively excludes macromolecular organic ligands, thereby protecting the embedded HFO nanoparticles from complexation and dissolution. Under aggressive conditions-including extreme pH, high salinity, mixed organic acids, elevated temperature, and prolonged operation-the PDA-confined PSP-Fe demonstrated 2 to 7-fold reduction in Fe leaching compared to bulk HFO and the unmodified PS-Fe. The improved structural integrity further conferred exceptional selectivity in multicomponent competing systems, achieving a distribution coefficient (Kd) of 10271 mL/g, 5 to 41 times higher than reference materials. Multi-site field-scale tests validated its engineering applicability, yielding the treatment capacity of 3600-4300 L water/kg with effluent Pb(II) level below 10 μg/L. Our work provides a generalized materials strategy to enhance the durability and efficiency of HFO-based adsorbents, advancing sustainable heavy metal removal.