Phosphate (P) is a key pollutant that contributes to eutrophication, which can lead to harmful algal blooms. Removal and control of phosphate in effluents and surface water are necessary to maintain healthy ecosystems and high water quality. Herein, to capture aqueous P, we developed novel mineral-hydrogel composites composed of calcium alginate, calcium phosphate (CaP), and calcium wollastonite (Woll) (CaP + Woll/Ca-Alg), a naturally occurring calcium silicate mineral. The mineral-hydrogel composites were synthesized by dripping a sodium alginate (Na-Alg) solution with ionic precursors and suspended wollastonite particles (&lt; 2.5 µm) into a calcium chloride bath. We found that the wollastonite slowly releases of calcium and silicate into solution. Combined with the CaP mineral seed, this mineral-hydrogel composite promotes removal of P through facilitated CaP heterogeneous nucleation and growth by maintaining a favorable aqueous chemistry over time. By adding wollastonite, the stability of the mineral-hydrogel composite was enhanced, as well as its P removal performance. The synergistic performance of the CaP and Wollastonite for P removal under undersaturated conditions with respect to hydroxyapatite (HA) was demonstrated through comparison of CaP and CaP/Woll. seeded mineral-hydrogel composites. Mineral-hydrogel composites containing only CaP removed 30% of P from an undersaturated solution with respect to HA, whereas a mineral-hydrogel composites containing both CaP and wollastonite removed 75% of P in the same undersaturated solution, showing a marked increase in heterogeneous nucleation and growth of CaP mineral in the presence of wollastonite by controlling local aqueous chemistry. Furthermore, the CaP mineral seed’s solubility and elemental composition changed as the removal of phosphate by the composites occurs. This finding provides a better understanding of the coupled dissolution/precipitation process occurring in local environments of the mineral-hydrogel composites and the change in bulk P concentration. Overall, the mineral-hydrogel composites seeded with CaP and wollastonite exhibits a good promise to polish P from nutrient-enriched water and to control harmful algal blooms.

Synergistic effects of wollastonite and calcium phosphate mineral seeds in alginate hydrogels on phosphate removal and control

ACS Fall 2021

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