Analysis and the understanding of fluoride removal mechanisms by an electrocoagulation/flotation (ECF) process

Electrocoagulation is a method of applying direct current to sacrificial electrodes that are submerged in an aqueous solution. Dissolving aluminum (Al3+) is predominant in the acidic condition and aluminum hydroxide has tendency soluble. The defluoridation process was found to be efficient for a pH ranging from 6 to 8. The fluoride removal mechanisms are investigated based on the solution speciation (Al and Al–F complexes) and dried sludge characteristics in the electrocoagulator. The XRD analysis of the composition of the dried sludge shows the formation of Al(OH)3 − xFx and provides confirmation for the main mechanism for fluoride removal. The mechanism of the fluoride removal was confirmed to be not only the competitive adsorption between OH– and F− but also the formation of solid cryolite in pH range of 5–8.

Research highlights
► The understanding of the mechanisms of speciation and its impact on fluoride removal.
► The characteristics of the sludge and initial pH effects were studied in this research.
► The XRD analysis of the composition of the dried sludge.
► The formation of Al(OH)3-xFx and provides confirmation for the main mechanism for fluoride removal.
► The mechanism of the fluoride removal was confirmed to be not only the competitive adsorption between OH− and F− but also formation of solid cryolite.