Pathways of fluoride removal and MgF₂ recovery from fluoride-containing wastewater using different magnesium sources

Removing fluoride from fluoride-containing wastewater while recovering fluorine resources is important for both environmental protection and resource utilization. Here, three magnesium sources (MgO, MgCl₂, and MgSO₄) were compared for fluoride removal and magnesium fluoride (MgF₂) recovery, and the roles of different Mg²⁺ supply pathways in MgF₂ recovery were clarified. At pH 1.6-2.4, the hydration products of MgO drive MgF₂ precipitation via acid-promoted dissolution and interfacial F^- enrichment, whereas the two soluble magnesium salts provide a stable supply of dissolved Mg²⁺ at pH 5-8, thus favoring MgF₂ nucleation and crystal growth. All three magnesium sources yielded rutile-type MgF₂ with purities of 88.3 %-90.5 %, as determined by chemical composition analysis, and all recovered products met the MF-2 grade requirement. Appropriate magnesium source selection, through distinct Mg²⁺ supply pathways, broadened the operable pH range for fluoride removal and MgF₂ recovery. Additionally, the anions introduced by the magnesium sources exerted stage-specific effects on MgF₂ crystallization, with Cl^- promoting nucleation and SO₄^2- facilitating early crystal growth and particle aggregation. These findings provide mechanistic insight into MgF₂ recovery from complex fluoride-containing wastewater matrices and practical guidance for magnesium source selection.