Sludge-derived biopolymers for in-situ synthesis of magnetic ALE-Fe-Zr composites for phosphate removal

Resource recovery of excess sludge has been important to achieve the sustainability of wastewater treatment, thereinto alginate-like exopolysaccharides (ALE) are considered as valuable bio-products with broad application potential. In this study, ALE were firstly employed as the biotemplates for fabricating controllable nanostructures to remove phosphate, the binding ability of ALE to metal ions (Fe³⁺ and Zr⁴⁺) effectively confined the in-situ growth of metal nanoparticles under calcination. The ALE-metal composites with a Fe/Zr molar ratio of 5 (ALE-Fe₅-Zr) showed high phosphate removal efficiencies of 96.1% and 98.1% at 1 and 10 mg-P L⁻¹, respectively, which was largely attributed to its homogeneity, high specific surface area, and mesoporous structure. The phosphate adsorption by ALE-Fe₅-Zr composites fitted well with pseudo-second-order kinetics and Freundlich isotherm model. Meanwhile, the composites achieved stable phosphate removal performance under a wide pH range of 4–8, and showed a high selectivity to phosphate in the presence of Cl⁻, SO₄²⁻, NO₃⁻, HCO₃⁻, and F⁻. Moreover, ALE-Fe₅-Zr composites maintained the favorable performance in treating real wastewater containing 2.47 mg-P L⁻¹, and the phosphate concentration reached target discharge limit of 0.5 mg-P L⁻¹ within 20 min. The reusability tests demonstrated that ALE-Fe₅-Zr composites could be recovered by magnetic separation and kept 83.8% of original adsorption capacity after five cycles. The possible adsorption mechanisms involving electrostatic interaction, ligand exchange, and Lewis acid-base interaction were supported by batch experiments, FTIR, and XPS analyses. This work provided a facile route for synthesizing ALE-mediated magnetic composites with high efficiency, selectivity, and stability for phosphate removal.