Ammonium Disrupts Vivianite Crystal Hydration to Enhance Crystallization Rate for Phosphorus Recovery

High-phosphorus wastewater often contains high concentrations of ammonium (NH4+), potentially influencing phosphate recovery through the vivianite crystallization method. This study comprehensively investigated the impact and underlying mechanisms of NH4+ on the kinetics and product properties of vivianite crystallization for phosphate recovery. The findings revealed that the presence of NH4+ resulted in a reduction in vivianite crystallinity, while remarkably augmenting the average crystal size and enhancing the structural strength of vivianite crystal aggregates. Mechanistic studies such as crystallization rate simulations, DFT calculations, and corresponding experiments indicated that NH4+ promoted the exposure of binding sites by diminishing the dissociation energy of surface-bound H2O molecules on vivianite, facilitating lattice ion assembly, and consequently accelerating crystallization. Upon the introduction of 600 mg-N/L NH4+, the vivianite crystallization rate constant surged from 28.79 to 39.11 h–1. On the contrary, NH4+ reduced the surface potential of vivianite, fostering aggregation and fusion among vivianite crystals and resulting in larger crystallization products. This study offers new insights into the recovery of phosphate from complex wastewaters through vivianite crystallization.