Effects of Ca2+ and Mg2+ on Defluoridation in the Electrocoagulation Process

Because aqueous ions can influence the defluoridation of the electrocoagulation (EC) process, the effects of Ca2+ and Mg2+ were investigated. The behaviors and mechanisms of EC defluoridation in Ca2+-containing systems were different from those in Mg2+-containing systems. An increase in Ca2+ concentration improved the defluoridation efficiency (εF), but it could not change the optimal molar ratio of OH− and F− to Al3+ (rOH+F). The highest εF can usually be obtained at rOH+F = 3 for defluoridation. Only a small portion of Ca2+ entered into the flocs, and Ca2+ could not influence the mechanism of EC defluoridation. For the Mg2+-containing system, the optimal rOH+F increased with increasing Mg2+ concentration. The optimal rOH+F was maintained at 3 after the Mg2+ concentration was corrected using the obtained correction coefficient of 0.3435. About 50% to 70% of the total Mg2+ entered into the flocs. From the XRD analysis, it was found that some Mg−Al−F layered double hydroxides (LDHs) were formed by Mg2+, F−, and Al3+ during electrolysis. It is proposed for the first time that the formation of Mg−Al−F LDH is one of the mechanisms for EC defluoridation in systems containing both F− and Mg2+.