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 Ca(2+) and Mg(2+) were investigated. The behaviors and mechanisms of EC defluoridation in Ca(2+)-containing systems were different from those in Mg(2+)-containing systems. An increase in Ca(2+) concentration improved the defluoridation efficiency (ε(F)), but it could not change the optimal molar ratio of OH(-) and F(-) to Al(3+) (r(OH+F)). The highest ε(F) can usually be obtained at r(OH+F) = 3 for defluoridation. Only a small portion of Ca(2+) entered into the flocs, and Ca(2+) could not influence the mechanism of EC defluoridation. For the Mg(2+)-containing system, the optimal r(OH+F) increased with increasing Mg(2+) concentration. The optimal r(OH+F) was maintained at 3 after the Mg(2+) concentration was corrected using the obtained correction coefficient of 0.3435. About 50% to 70% of the total Mg(2+) entered into the flocs. From the XRD analysis, it was found that some Mg-Al-F layered double hydroxides (LDHs) were formed by Mg(2+), F(-), and Al(3+) 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(2+).
Department of Environmental Engineering, Peking University, Beijing 100871, People's Republic of China. zhaohuazhang@pku.edu.cn
Huazhang
H
Zhao
Bin
B
Zhao
Wei
W
Yang
Tianhong
T
Li
D000327:Adsorption; D002118:Calcium / Q000737:chemistry*; D002413:Cations, Divalent / Q000737:chemistry; D004202:Disinfectants / Q000737:chemistry*; D055664:Electrochemical Techniques; D005457:Fluoridation; D005459:Fluorides / Q000737:chemistry*; D008274:Magnesium / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000737:chemistry; D018508:Water Purification / Q000379:methods*; D014961:X-Ray Diffraction
D002413:Cations, Divalent; D004202:Disinfectants; D014874:Water Pollutants, Chemical; D008274:Magnesium; D005459:Fluorides; D002118:Calcium