Phosphate removal, recovery, and upcycling by wollastonite and calcium phosphate mineral-hydrogel composites

Elevated phosphate (P) levels in surface waters caused by anthropogenic activities contribute to eutrophication, which threatens ecosystems and the communities relying on the surface water. To mitigate this threat, developing new P removal/recovery technologies is vital. To capture aqueous P, we created a novel mineral-hydrogel composite composed of calcium alginate (Ca-Alg), calcium phosphate (CaP), and wollastonite (CaSiO₃), a naturally occurring calcium silicate mineral. While CaP seeds decreased the nucleation energy barrier for new CaP formation, the steady dissolution of wollastonite released calcium and silicate, elevating the local calcium concentration and pH to promote heterogeneous CaP formation. Through the synergy between the two minerals, treated P can be reduced to 0.067 mg-P/L (an average of remaining P concentration from three batch tests; a dose of 0.3 Ca-Alg/L, 72-hour reaction) from a starting solution of 6.2 mg-P/L. The addition of wollastonite improved the composite's P removal ability even in multiple cycles of 24-hour batch reactions. P-recovered composites were further evaluated in plant growth tests as proof-of-concept demonstration of agronomic reuse potential, yielding >90% seed germination and enhanced early-stage stem and leaf growth relative to nutrient-free controls. Overall, these results demonstrate the potential of a circular approach for P capture and reuse, supporting water quality management and suggesting the possibility of beneficial agricultural applications.