Upcycling titanium gypsum into pH-gated calcium peroxide hydrogel beads for integrated water remediation with phosphorus-reuse potential

Cleaner remediation of eutrophic waters requires technologies that simultaneously improve chemical-use efficiency and create value from industrial by-products. Here, titanium gypsum (TiG), a calcium-rich industrial by-product, was upcycled into CaO 2 nanoparticles (CaO 2 NPs) and encapsulated in a sodium alginate/carboxymethyl chitosan/gelatin matrix to fabricate CaO 2 hydrogel beads (CaO 2 HBs) enabling pH-responsive swelling-controlled release. The biopolymer network served as a structural gate: protonation-driven contraction under acidic or near-neutral conditions restricted water access to CaO 2 , whereas alkaline deprotonation induced swelling and promoted CaO 2 hydrolysis. This pH-responsive swelling-controlled response increased H 2 O 2 accumulation under alkaline conditions while moderating alkaline shock, thereby reducing premature CaO 2 consumption under non-target conditions. CaO 2 HBs enhanced oxygenation, model bacterial inactivation, and Ca 2+ -mediated phosphate mineralization, reducing phosphate from 20 to below 2.0 mg PO 4 3− L −1 in a high-phosphate model solution under alkaline conditions. In simulated complex waters, the beads retained better multifunctional performance than bare CaO 2 NPs. A 1 L high-load simulated black-odorous wastewater test increased DO to 4.15 mg L −1 and achieved 94.36% relative bacterial inactivation, while partially reducing phosphate in a chemically complex matrix. Preliminary cost analysis identified PEG-400 as the principal cost hotspot in the nano-CaO 2 route, while a proof-of-concept plant-growth assay provided preliminary evidence that P-loaded spent beads could potentially be reused as a soil phosphorus source. Overall, this study establishes a recovery-conversion-remediation-reuse pathway that couples titanium gypsum valorization with regulated CaO 2 utilization, integrated water treatment, and phosphorus-reuse potential.