Engineering Calcium-Bearing Mineral/Hydrogel Composites for Effective Phosphate Recovery

Abstract

Effectively recovering phosphate from wastewater streams and reutilizing it as a nutrient will critically support sustainability. Here, to capture aqueous phosphate, we developed novel mineral–hydrogel composites composed of calcium alginate, calcium phosphate (CaP), and calcium silicate hydrate (CSH) (CaP + CSH/Ca-Alg). The CaP + CSH/Ca-Alg composites were synthesized by dripping a sodium alginate (Na-Alg) solution with ionic precursors into a calcium chloride bath. To change the mineral seed’s properties, we varied the calcium bath concentrations and the ionic precursor (sodium dibasic phosphate (NaH2PO4) and/or sodium silicate (Na2SiO3)) amounts and their ratios. The added CSH in the mineral–hydrogel composites resulted in the release of calcium and silicate ions in phosphate-rich solutions, increasing the saturation ratio with respect to calcium phosphate within the mineral–hydrogel composites. The CSH addition to the mineral–hydrogel composites doubled the phosphate removal rate while requiring lesser initial amounts of Ca and P materials for synthesis. By incorporating both CSH and CaP mineral seeds in composites, we achieved a final concentration of 0.25 mg-P/L from an initial 6.20 mg-P/L. Moreover, the mineral–hydrogel composites can remove phosphate even under CaP undersaturated conditions. This suggests their potential to be a widely applicable and environmentally sustainable treatment and recovery method for nutrient-rich wastewater.

Oxide– and Silicate–Water Interfaces and Their Roles in Technology and the Environment

“…831 Struvite formation is closely related to ionic composition, temperature, pH, and ion speciation. 832 In addition to struvite, hydroxyapatite-nucleated calcium alginate hydrogels have been used for effective recovery of phosphate, 833,834 which can be utilized as fertilizer, enabling a circular economy for the phosphorous chemistry. In this process, in situ nucleation of calcium phosphate (hydroxyapatite) in hydrogels decreased the energy barriers to subsequent calcium phosphate nucleation.…”

Nanoscale Zero-Valent Iron Confined in Anion Exchange Resins to Enhance Selective Adsorption of Phosphate from Wastewater

“…The adsorption method has been widely adopted as a phosphorus capture process because of its effectiveness in phosphorus removal and low cost as well as its applicability and scalability as a phosphorus recovery process. − As one of the preferred adsorbents, iron-based materials have attracted extensive attention because of the selective adsorption of phosphorus. − Nanoscale zero-valent iron (nZVI), for example, has high affinity for phosphate with a maximum adsorption capacity of 245 mg P/g . Due to its magnetic properties, nZVI can be easily separated from the solution using an external magnetic field, eliminating the problem of difficult recovery and secondary pollution. , However, the poor chemical and mechanical properties of nZVI, such as easy oxidation and aggregation, have limited its application in aqueous media for phosphorus removal. − …”