Effects of the molar ratio of hydroxide and fluoride to Al(III) on fluoride removal by coagulation and electrocoagulation

The effect of the molar ratio of hydroxide and fluoride ions to Al(III) ions (γOH and γF) on coagulation and electrocoagulation (EC) was studied to solve the problem of the over addition of acid or base. The efficiency of defluoridation was approximately 100% when the sum of γOH and γF (γOH+F) was close to 3. This finding reveals that the fluoride ions and the hydroxide ions can co-precipitate with Al(III) ions and the formula of the precipitate is AlnFm(OH)3n−m. However, when γOH was less than 2.4, the defluoridation efficiency, given that γOH+F=3, dropped as γOH fell, because the amount of aluminum polymer formed dropped. The efficiency of defluoridation of EC exceeded that of coagulation for equal γOH and γOH+F, when γOH+F>3, proving the existence of an electrocondensation effect.