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 La 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 complexes, leading to the failure of fluoride removal. At 1:3.20 ≤ La/F molar ratio ≤1:3.10, the formation of LaF is facilitated. However, co-existing LaF tends to absorb on the surface of LaF particles, leading to the formation of colloidal solution with large numbers of LaF·LaF 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. Considering the existing of positively charged LaF and LaF·LaF , coagulation removal of fluoride is proposed and investigated using lanthanum salts and negatively charged SiO·HO colloidal particles, which is provided NaSiO hydrolysis at pH near 5.5. At a La/F molar ratio of 1:3.00 and NaSiO dose of 0.50 g L, a fluoride removal of 99.25% is obtained with retaining fluorine concentration of 2.24 mg L. When NaSiO dose increases to 1.00 g L, the retaining fluorine concentration could be further reduced to 0.80 mg L.