Effective sequestration of phosphate by ultrasmall hydrated Zr(IV) oxide nanoparticles confined inside the PEI-crosslinked chitosan beads

Nanocomposites fabricated through the encapsulation of hydrated Zr(IV) oxide (HZO) nanoparticles within millimeter-scale hosts have emerged as promising candidates for phosphate sequestration from water. However, achieving ultrasmall nanoparticles within these hosts that maintain high decontamination reactivity is still a significant challenge. Herein, a novel nanocomposite (denoted HZO@CS-PEI) was obtained by confining the growth of HZO in dual crosslinked network hydrogels composed of interpenetrating chitosan (CS) and polyethyleneimine (PEI). Benefiting from the confinement effect of the dual cross-linked structure, the HZO within HZO@CS-PEI exhibited an ultrasmall size of approximately 2.16 nm. The incorporation of PEI enhanced both phosphate sequestration efficiency and mechanical stability of the nanocomposite, improving its practical applicability. Ultrafine dispersion of HZO nanoparticles within the composite matrix achieved a saturable phosphate adsorption capacity exceeding 64.41 mg/g at 298 K. Crucially, selective phosphate adsorption by HZO@CS-PEI remained effective in water containing high concentrations of coexisting anions and organic matter. The synergistic application of spectroscopic analyses and density functional theory (DFT) calculations revealed three distinct interaction mechanisms between HZO@CS-PEI and phosphate: inner-sphere complexation, hydrogen bonding, and electrostatic interactions. Furthermore, the used HZO@CS-PEI could be effectively regenerated with 0.1 mol/L NaOH for repeat use. In the column adsorption tests, phosphate-contaminated water was successfully treated by HZO@CS-PEI columns over 2750 bed volumes (BV), confirming operational stability under dynamic flow conditions.