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

Abstract

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$/m3.