Defluoridation of tap water by electrocoagulation and fluoride adsorption on aluminum hydroxide flocs

Overload of fluoride ions in water is observed in several regions of southern Tunisia, mainly the regions close to the mining basin of Gafsa: Metlaoui, Omlarayes and Redayef. This study concerns fluoride removal from Metlaoui's tap water by electrocoagulation (EC) using aluminum electrodes in a batch reactor. This water contains 3.5 mg·L−1 of fluoride, the highest concentration observed in these basins. The effect of the operating conditions of EC treatment on tap water defluoridation was analyzed, namely, current density, temperature and interelectrode distance. Hydroxide aluminum flocs, formed with different durations of EC, were used as sorbents in fluorinated deionized water ([F−] = 3.5 mg·L−1) and Metlaoui's tap water. Flocs formed after 30 min of EC, produced by dissolving (79 ± 1) mg·L−1 of aluminum and adjusted at pH = 6.5, allowed the adsorption of (98 ± 1)% of fluoride ions from deionized fluorinated water. Flocs adjusted at different pH, from 5 to 9, were used as sorbents in fluorinated deionized water and Metlaoui's tap water. Acidic and neutral flocs allowed the best yields of fluoride adsorption. Contrary to the literature, this work highlighted the absence of ion exchange of hydroxide anion by fluoride anion in water, highlighting a mechanism of physical adsorption on aluminum hydroxide flocs.