Sulfate-regulated crystallization mechanism of LaF₃ for efficient defluorination in high-sulfate fluoride wastewater

High-sulfate industrial wastewater presents a critical challenge for defluorination because SO₄^2- severely disrupts conventional precipitation pathways and hinders high-purity fluoride recovery. In this study, the influence of coexisting sulfate ions on lanthanum-fluoride crystallization process was systematically evaluated, and a sulfate regulated LaF₃ crystal route was established. A distinct "induction-promotion-competition" zoning mechanism governed by the SO₄^2-/F^- ratio was found through a comprehensive combination of speciation modeling, molecular dynamics simulation crystallization kinetics, and multi-scale characterization. At low SO₄^2- ratio (3:1), limited formation of LaSO₄⁺ provided additional nucleation sites, increasing the proportion of small-sized LaF₃ crystals. Moderate SO₄^2- ratios (10:1-20:1) facilitated dynamic La³⁺-SO₄^2- complexation, sustained a controlled release of La³⁺, promoting ordered lattice development and yielding LaF₃ with the highest crystallinity. However, excessive SO₄^2- induced competitive coordination and surface adsorption, markedly inhibiting crystal growth and defluorination. These mechanistic insights lead to a ratio-tuning strategy (optimal SO₄^2-:F^- ≈ 10:1 at pH = 5; La³⁺:F^- = 1.2:3), achieving 96% defluorination and high-purity LaF₃ formation. Validation with actual industrial fluoride wastewater confirmed that sulfate-controlled precipitation enhanced La³⁺ utilization (reduced reagent consumption by 15-20%) and improved product purity to 90.78%. This study provides a mechanistic basis and practical pathway for fluoride control and recovery from complex, high-salinity industrial effluents.