Electrolysis removal of fluoride by magnesium ion-assisted sacrificial iron electrode and the effect of coexisting ions

A novel system for removing fluoride (F-) by electrolysis (ELC) was investigated as magnesium (Mg2+) ion assistive system with a sacrificial iron (Fe) electrode. To minimize and control the Fe leachate from the Fe electrode, ELC was performed in two stages. First stage: Fe dissociation (Fe/stainless steel (SS) electrodes); second stage: ELC electrocoagulation (SS/platinum (Pt) electrodes). The effect of initial ion concentrations on F-removal was investigated with different levels of Fe (0-486 mg/L), Mg2+(0-48 mg/L) and coexisting ions calcium (Ca2+), and carbonates (CO32-+HCO3-). Experimental results revealed that the proposed system's Fe (486 mg/L) alone may remove 17% of F-. However, incorporating Fe (52 mg/L) and Mg2+ (46 mg/L) significantly increased the removal of F- to 77% for 5 mg/L initial F- level and the molar ratio of (0.24-0.94):1 of the Fe:Mg mixture showed maximum F- removal properties. The F- was removed by both co-precipitation and by Coulomb forces. It was found that F- was removed by co-precipitating with a mixture of Mg(OH)(2) and Fe (OH)(3) as F- was metathesis with the OH- ion due to the similar radius of both ions. To achieve 1.5 mg/L of F- , the desirable level recommended by the World Health Organization, minimum Mg2+ and Fe concentration ratios of 5:20, 10:20, and 50:50 mg/L were required, respectively, for the initial 2, 3, and 5 mg/L F-concentrations. The presence of Ca2+ ions and the presence of CO32-+HCO3- inhibited the system's functionality significantly. The optimum operating cost was calculated as 0.56-1.50 US$/m(3).