Supersaturation-driven seed optimization and nucleation-regime transition in Vivianite crystallization

To address the prolonged induction time and unstable phase selectivity associated with the nonclassical nucleation of vivianite, this study established a supersaturation-driven comparative framework to elucidate seed-dependent dominant nucleation mechanisms. This framework enables controllable formation of the target vivianite phase. The phosphorus recovery results showed that graphite performed best at 4 g L^-1 and 100 μm, achieving 85.5% PO₄^3- removal at 0.5 h, yet with relatively low Fe utilization (consumed Fe/P = 1.30-1.35). In contrast, the optimal combinations for sponge iron and vivianite seeds were 3 g L^-1 with 200 μm and 4 g L^-1 with 200 μm, respectively; both exhibited markedly higher PO₄^3- removal than the graphite system and near-stoichiometric Fe consumption (Fe/P ≈ 1.5). FTIR, XRD, XPS, and SEM revealed distinct interfacial pathways. Graphite was dominated by weak surface loading; sponge iron developed a pronounced deposition layer with an Fe(II)-enriched surface chemical state, whereas vivianite seeds showed clearer signatures of the target mineral phase accompanied by secondary growth features. Across a continuous SI range of 2-11, the no-seed system exhibited limited self-nucleation at SI = 2-4, yielding only 18.53-47.66% PO₄^3- removal at 120 min. Phosphate removal rapidly reached a plateau within the first 10-20 min as SI increased. Seeding substantially advanced the kinetics in SI = 2-4 and increased PO₄^3- removal to above 80%. Overall, under low supersaturation, the controlling factor shifts from the mere presence of an interface to whether the interface can sustain a growable Fe-P deposit/epitaxial layer. This critical-SI-based seed-selection framework enables joint regulation of induction-time variability, Fe utilization efficiency, and target-phase quality, and provides reproducible engineering decision criteria for vivianite recovery from sludge sidestreams and high-phosphorus industrial wastewaters.