Phosphate recovery from wastewater via vivianite crystallization using separable ferrous modified biochar beads

Ferrous modified biochar beads (Fe/SABC) with three-dimensional reed skeletal network uniformly dispersing Fe particles (FeO, Fe₃O₄ and Fe⁰) were developed by gelation-calcination method. Batch experiment results showed Fe/SABC could reach a phosphate recovery capacity of 52.92 mg/g, about 12 times that of unmodified biochar (BC) and twice that of ferrous modified biochar powder (Fe/BC). Fe/SABC exhibited better mass transfer performance than its powder form and maintained great phosphate recovery capacity within a wide initial pH range. Most co-existing metal ions enhanced phosphate recovery, while organic matters had little impacts. Characterization results indicated the main recovery mechanism was vivianite crystallization, with flower-like vivianite crystals (40–50 μm in diameter) forming on both the surface and inside of Fe/SABC. During the phosphate recovery process, reductive Fe on Fe/SABC generated Fe²⁺ in situ and form vivianite nuclei with phosphate. Fe/SABC provided attachment sites for vivianite nuclei and induced heterogeneous nucleation, consequently promoting vivianite auto-nucleation and growth. Furthermore, three-dimensional reed skeletal network within Fe/SABC could create microdomain inert atmosphere to slow down reductive Fe and vivianite oxidation. The recovered products could be separated magnetically and had potential as slow-release fertilizers. In summary, this study confirms the feasibility of using Fe/SABC to recover phosphate via vivianite, providing new insights into phosphate recovery and future applications.