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

Abstract: La-loaded absorbents have been widely reported for fluoride removal due to the strong affinity of La(3+) 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( x ) (3−x ) complexes, leading to the failure of fluoride removal. At 1:3.20 ≤ La/F molar ratio ≤1:3.10, the formation of LaF(3) is facilitated. However, co-existing LaF( x ) (3−x ) tends to absorb on the surface of LaF(3) particles, leading to the formation of colloidal solution with large numbers of LaF(3)·LaF( x ) (3−x ) 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( x ) (3−x ) and LaF(3)·LaF( x ) (3−x ), coagulation removal of fluoride is proposed and investigated using lanthanum salts and negatively charged SiO(2)·nH(2)O colloidal particles, which is in-situ provided via Na(2)SiO(3) hydrolysis at pH near 5.5. At a La/F molar ratio of 1:3.00 and Na(2)SiO(3) 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 Na(2)SiO(3) dose increases to 1.00 g L(−1), the retaining fluorine concentration could be further reduced to 0.80 mg L(−1).