Efficient Coagulation Removal of Fluoride Using Lanthanum Salts: Distribution and Chemical Behavior of Fluorine

La-loaded absorbents have been widely reported for fluoride removal due to the strong affinity of La³⁺ towards fluoride ion. Herein, chemical removal of fluoride from flue gas scrubbing wastewater using lanthanum salt is investigated. The retaining free F^- concentration, phase composition and morphology of filtration residues, and the distribution of fluorine have been investigated using ion-selective electrode, analytical balance, scanning electron microscopy, and X-ray diffractor. The results show that at La/F molar ratio ≥1:3.05, the majority of fluorine exists as LaF ^3- complexes, leading to the failure of fluoride removal. At 1:3.20 ≤ La/F molar ratio ≤1:3.10, the formation of LaF₃ is facilitated. However, co-existing LaF ^3- tends to absorb on the surface of LaF₃ particles, leading to the formation of colloidal solution with large numbers of LaF₃·LaF ^3- suspended solids. At an optimized La/F molar ratio of 1:3.10, a fluoride removal of 97.86% is obtained with retaining fluorine concentration of 6.42 mg L^-1. Considering the existing of positively charged LaF ^3- and LaF₃·LaF ^3- , coagulation removal of fluoride is proposed and investigated using lanthanum salts and negatively charged SiO₂·nH₂O colloidal particles, which is in-situ provided via Na₂SiO₃ hydrolysis at pH near 5.5. At a La/F molar ratio of 1:3.00 and Na₂SiO₃ dose of 0.50 g L^-1, a fluoride removal of 99.25% is obtained with retaining fluorine concentration of 2.24 mg L^-1. When Na₂SiO₃ dose increases to 1.00 g L^-1, the retaining fluorine concentration could be further reduced to 0.80 mg L^-1.