Effects of Ca and Mg on defluoridation in the electrocoagulation process

Because aqueous ions can influence the defluoridation of the electrocoagulation (EC) process, the effects of Ca and Mg were investigated. The behaviors and mechanisms of EC defluoridation in Ca-containing systems were different from those in Mg-containing systems. An increase in Ca concentration improved the defluoridation efficiency (ε), but it could not change the optimal molar ratio of OH and F to Al (r). The highest ε can usually be obtained at r = 3 for defluoridation. Only a small portion of Ca entered into the flocs, and Ca could not influence the mechanism of EC defluoridation. For the Mg- containing system, the optimal r increased with increasing Mg concentration. The optimal r was maintained at 3 after the Mg concentration was corrected using the obtained correction coefficient of 0.3435. About 50% to 70% of the total Mg entered into the flocs. From the XRD analysis, it was found that some Mg-Al-F layered double hydroxides (LDHs) were formed by Mg, F, and Al 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 Mg. © 2010 American Chemical Society.