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-x)F-x 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. (C) 2011 Elsevier B.V. All rights reserved.