Comparison and understanding of fluoride removal mechanism in Ca²⁺, Mg²⁺ and Al³⁺ ion assisted electrocoagulation process using Fe and Al electrodes

The present study is to scrutinize the effect of Ca²⁺, Mg²⁺ and Al³⁺ ion on fluoride removal by electrocoagulation (EC) using Fe and Al electrode. The effect of charge loading (Q_e), concentration of cations (Mⁿ⁺ = Ca²⁺, Mg²⁺ and Al³⁺) and anions (X^n- = NO₃^-, SO₄^2- and Cl^-) on fluoride removal is analysed in detail. Experimental results ensure that, in absence of Mⁿ⁺, the maximum fluoride removal efficiency up to 60% for Al and 18% for Fe is achieved by EC at Q_e 1620 C L^-1 when the initial fluoride concentration is 0.53 mM. Nonetheless, the initial fluoride concentration (0.53 mM) is reduced to 0.06 mM (WHO recommended limit of fluoride) with the removal of efficiency up to 87% by the addition of 0.16 and 0.25 mM of Al³⁺ (as AlCl₃) in EC with Al and Fe electrodes, respectively. The maximum fluoride removal efficiency of 90% for Fe electrodes and 99% for Al electrodes is obtained through Al³⁺ assisted EC and Mg²⁺ assisted EC, respectively. The effect of co-existing anions on fluoride removal has emphasized that SO₄^2- ions show pronounced effect compared to NO₃^- and Cl^-. In addition to that, to elucidate the fluoride removal mechanism, crystalline nature and composition of the sludge generated in defluoridation processes by Mⁿ⁺ assisted EC are also been characterized using XRD, SEM and FT-IR.