Magnetic Fe₃O₄-NH₂/UiO-66/SiO₂ seed for interfacial CaF₂ crystallization

Construction of recoverable magnetic Fe₃O₄-NH₂/UiO-66/SiO₂–silica seed for CaF₂ crystallization. • Composite seed directs fluoride conversion from wastewater to CaF₂ solid. • UiO-66/SiO₂ interface promotes ion enrichment and heterogeneous nucleation. • Magnetic recovery reduces the loss of fine CaF₂-loaded seed particles. • Integration of fluoride removal, CaF₂ recovery and solid-liquid separation. Calcium-based precipitation is widely used for treating concentrated fluoride wastewater, but rapid homogeneous nucleation often generates fine CaF₂ particles that are difficult to recover. In this study, a silica-supported Fe₃O₄-NH₂/UiO-66 composite was developed as a magnetically recoverable crystallization seed for directing CaF₂ formation. At an initial fluoride concentration of 2000 mg/L, a Ca:F molar ratio of 0.8:1, 0.5 g seed dosage, pH 7–8, and 25 °C reduced residual fluoride to 2.56 mg/L, corresponding to 99.87% removal. Ca 2+ -free tests yielded equilibrium fluoride uptakes of 570 and 755 mg/g for UiO-66 and the composite, respectively, indicating that adsorption contributed to interfacial enrichment but was insufficient to account for the high fluoride conversion after Ca 2+ addition. Thermodynamic analysis showed that the initial saturation index increased from 3.1 to 7.5 and relative supersaturation from 28.9 to 288 as the initial fluoride concentration increased from 250 to 2000 mg/L. The crystallization rate followed a supersaturation-dependent power law with a kinetic exponent of 2.146 (R 2  = 0.993). After 8 cycles, fluoride-removal and magnetic-recovery efficiencies remained at 80.09% and 80.50%, respectively. In actual industrial wastewater, 98.82% fluoride removal was achieved after pH adjustment to 7. These results demonstrate the potential of Fe₃O₄-NH₂/UiO-66/SiO₂ as a reusable crystallization seed for fluoride recovery.