Hydroxyl Radical-Induced Calcium Phosphate Crystallization to Gain Phosphorus Fertilizers from Circulating Water

Calcium phosphate (CaP) crystallization enables simultaneous calcium removal and phosphorus recovery during circulating water (CW) treatment. Adjusting the thermodynamic process is crucial for accelerating CaP crystallization and enhancing the energy efficiency. In this study, hydroxyl radicals (<middle dot>OH) generated via an electro-Fenton-like system were employed to induce CaP crystallization. Experimental investigation and theoretical calculations revealed <middle dot>OH to be key for phosphate release and CaP crystallization. The interactions between ions and NTMP are effectively strengthened by <middle dot>OH-induced multiple hydrogen bonds, leading to intermolecular aggregation and the formation of amorphous clusters. These clusters can create thermodynamic conditions that are more conducive to crystallization. Therefore, <middle dot>OH-induced CaP crystallization predominantly proceeded through the formation of amorphous clusters as the intermediate state, representing a nonclassical crystallization process. The issue of electrode deactivation caused by surface scaling was effectively avoided when crystallization primarily occurred within the amorphous clusters, resulting in the formation of CaP crystals in bulk solution. The recovered CaP crystals showed the possibility of being used as phosphorus fertilizers based on plant experiments. Finally, <middle dot>OH-induced CaP crystallization was achieved during the authentic CW treatment. This research expanded the application of <middle dot>OH to CaP crystallization and provided a sustainable strategy for CW treatment.