Alkali resistant nanocomposite gel beads as renewable adsorbents for water phosphate recovery

Hydrous zirconium oxide (HZO) encapsulated alginate gel beads were synthesized for phosphate recovery from water. Importantly, we find that HZO/alginate gel beads (ZrA) crosslinked with Ca²⁺, Mg²⁺, Fe³⁺, Al³⁺, and Zr⁴⁺ are unstable under an intense alkali regeneration condition. Only Sr²⁺-crosslinked ZrA can endure a high alkali solution. ZrA possesses a high specific surface area (80.84 m²·g^-1) and a mesoporous structure (15.3 nm and 0.196 cm³·g^-1), which endow them with a high Langmuir adsorption capacity of 52.5 mg-P/g. ZrA can be easily recycled, and the mass loss of HZO is prevented. Furthermore, the strontium alginate gel framework protects the encapsulated HZO nanoparticles from adverse humic acid contamination. ZrA can be regenerated for at least 5 adsorption/desorption cycles. Cost analysis indicates the potential scale application feasibility for ZrA. This study provides a novel, simple, and environmentally benign solution to immobilize HZO for efficient phosphate recovery.