Construction of a cellulose nanofiber-based magnetic hydrous zirconium oxide nanocomposite for phosphate adsorption and struvite recovery.

Utilizing sustainable cellulose nanofibers (CNF) as a substrate, this study developed a magnetic nanocomposite for efficient phosphate adsorption and recovery. Through a facile synthetic method, nanoscale ferroferric oxide (Fe₃O₄) and hydrous zirconium oxide (ZrO2·nH2O) were anchored onto polyethyleneimine (PEI)-functionalized CNF to fabricate magnetic zirconium-functionalized aminated nanocellulose composites. The optimal composite (CNF-PEI@Fe₃O₄-Zr4) exhibited a high specific surface area and delivered an exceptional phosphate adsorption capacity of 78.13 mg P/g, surpassing some zirconium-based adsorbents and significantly outperforming pristine hydrous zirconium oxide (48.08 mg P/g). Phosphate adsorption followed pseudo-second-order kinetics and the Langmuir isotherm, and the thermodynamic analysis indicated an endothermic and spontaneous process. The composite maintained highly suitable for phosphate removal from wastewater containing high levels of sulfate, nitrate, or chloride. After seven consecutive adsorption-desorption cycles, the removal rate underwent only a marginal decrease, retaining over 96% of its initial efficiency. Furthermore, phosphate was successfully recovered from the spent adsorbent as crystalline struvite (MgNH₄PO₄·6H₂O), as confirmed by XRD, FTIR, and SEM-EDX analyses. The recovered product exhibited characteristic orthorhombic morphology with an Mg:P molar ratio of 0.96:1, and its XRD pattern matched the commercial struvite standard. The adsorption mechanism was attributed to a synergistic effect of electrostatic attraction and ligand exchange. This work presents a facile strategy to transform renewable cellulose into a high-performance adsorbent for advanced phosphate remediation and resource recovery.