Elucidating the mechanism of seed-mediated vivianite crystallization for enhanced phosphorus recovery from wastewater

Seed-mediated vivianite crystallization offers a sustainable route for recovering phosphorus (P) from wastewater while producing a value-added ferrous-phosphate product. However, fines formation and the absence of mechanism-based design strategies still constrain its application. Herein, a seed-mediated approach was developed to control the crystallization process and simultaneously elucidate the coupled nucleation–growth mechanism. At a supersaturation index (SI) of 10.83 with 0–50 μm seeds at 6 g/L, P recovery efficiency reached 96.32%, and the mean particle size increased 4.6-fold to 65.83 μm. A low-hydration amorphous ferrous phosphate (AM-FP) initially forms on seed surfaces, and then undergoes solid-state transformation into crystalline vivianite driven by hydration. As crystalline facets develop, solution-mediated ion attachment dominates, promoting crystal coarsening while suppressing new nucleation. This multi-stage process explains the marked reduction of fines and the generation of large, well-ordered crystals suitable for efficient P recovery and separation. The estimated reagent cost of $3.41 per cubic meter is considerably lower than that of struvite recovery ($11.14 per cubic meter), underscoring the economic and technical feasibility of seed-mediated vivianite crystallization for scalable circular-P applications.