Induced crystallization techniques for fluoride removal from wastewater: Current developments and applications

Abstract

Excessive fluoride in wastewater poses significant risks to ecosystems and human health. Crystallization methods, including cryolite, calcium fluoride, and fluorapatite, demonstrate high fluoride removal efficiency (>90 %) under optimized conditions and offer the added benefit of resource recovery. Maintaining an optimal pH range (5−7) after chemical dosing enhances removal efficiency, though sulfate interference remains a limiting factor. CaF₂ and FAP typically crystallize at ambient temperature, with molar ratios of Ca/F= 0.6–0.7 and Ca:P:F= 10:4:1, respectively. In contrast, Na₃AlF₆ crystallization performs best at 20–50°C with an Al/F ratio of 1:6. While the small-scale research on the crystallization methods of the three products is in full swing, only the CaF₂ crystallization has been comprehensively applied in pilot-scale trials and full-scale engineering. Na₃AlF₆ crystallization is currently at the pilot-scale stage, whereas FAP crystallization has made no progress during the small-scale stage. Each method faces specific challenges. CaF₂ crystallization requires improvements in product purity and effluent quality. FAP crystallization needs effective phosphate control and cost-efficient reactor design. Na₃AlF₆ must balance product purity with fluoride removal efficiency. Future research should prioritize the optimization of operational parameters, cost reduction, and the development of practical applications for novel approaches to facilitate large-scale implementation.