Efficient phosphorus sequestration by metastable magnesium-bearing amorphous calcium carbonate via dissolution-coupled precipitation.

Phosphorus removal from low-concentration wastewater remains challenging. Conventional technologies often require high chemical dosages, exhibit poor settling behavior, and show reduced effectiveness under chemically complex conditions. Herein, a metastable magnesium-bearing amorphous calcium carbonate (Mg-ACC) was developed for efficient phosphorus sequestration from aquaculture tailwater. To enable practical implementation, Mg-ACC was synthesized through a scalable waste-liquid-recycling (WLR) process, reducing the production cost to ∼0.42 USD/kg while maintaining excellent storage stability for at least 16 months. Mg-ACC exhibited a high phosphate capture capacity (qm = 128.94 mg/g and q0.1 = 3.39 mg/g) and reduced total phosphorus in real aquaculture tailwater from 13.25 to <0.5 mg/L without requiring additional flocculants or pH adjustment. Time-resolved characterization revealed that Mg-ACC predominantly underwent dissolution-recrystallization under low-phosphate conditions, whereas under high-phosphate conditions it followed a dissolution-reprecipitation pathway toward Mg-containing amorphous calcium phosphate. Despite these different evolution pathways, phosphate sequestration under both conditions shared a common dissolution-coupled phosphate precipitation process. Control experiments further showed that homogeneous bulk-solution precipitation and heterogeneous nucleation on crystalline particles alone could not fully account for the high phosphate removal by Mg-ACC. These results support a proposed interfacial microenvironment-mediated precipitation pathway, in which continuous Mg-ACC dissolution may create locally favorable conditions for phosphate precipitation near the particle surface. In addition, the resulting phosphate-containing solids exhibited favorable settling behavior and showed potential for phosphorus recovery and reuse as a fertilizer source. This work demonstrates the potential of metastable Mg-ACC as a low-cost and scalable phosphorus-removal material and provides new insights into dissolution-coupled phosphate precipitation by metastable amorphous carbonate materials.