Electrocoagulation of tetrafluoroborate (BF₄⁻) and the derived boron and fluorine using aluminum electrodes

Tetrafluoroborate anion (BF₄⁻) is found in the streams of flue-gas desulfurization and borosilicate glasses etching which deteriorates water quality through slow hydrolysis into boric acid and fluoride. Decomposition and electrocoagulation (EC) of BF₄⁻ were studied using metallic aluminum as the sacrificial electrode. The dissolved Al(III) from the anode could efficiently decompose BF₄⁻ in forms of fluoroaluminate complexes, and the derived boric acid and fluoride ion were removed by sweep flocculation. Major variables were investigated to optimize EC, including the reaction pH, initial concentration of BF₄⁻, current density and electrolyte type. The mechanism of EC process was elucidated with the kinetics of consecutive reactions. Experimental results suggested that the removal of BF₄⁻ and total fluoride were less influenced by pH, and that of total boron reached a maximum at pH 8 which favored the surface complexation between borate species and EC precipitates. Under the conditions: [BF₄⁻]₀ = 9.3 mM, [NaCl] = 10 mM, pH = 8.0, current density = 5 mA/cm², 98.3% of BF₄⁻ was decomposed and the removal of total fluoride and boron attained 98.2% and 74.1%, respectively within 3 h. EC using the Al electrode outperformed the conventional chemical coagulation and reduced the levels of BF₄–], B(OH)₃ and F⁻ in aqueous solution synergically.