Removal of hydrated silica, fluoride and arsenic from groundwater by electrocoagulation using a continuous reactor with a twelve-cell stack

The simultaneous removal of hydrated silica, fluoride and arsenic from deep well water (hydrated silica 72 mgL^-1, fluoride 4.4 mgL^-1, arsenic 106.2 μgL^-1, sulfate 50 mgL^-1, phosphate 0.99 mgL^-1, pH = 8.2 and conductivity 659 μScm^-1) by electrocoagulation (EC) was investigated. The EC was performed in a continuous electrochemical reactor using aluminum plates as sacrificial anodes coupled directly to a jar test device. The effect of current density (4 ≤ j ≤ 8 mA cm^-2) and mean linear flow rates in the EC reactor (0.057 ≤ u ≤ 0.57 cm s^-1) on the hydrated silica, fluoride, and arsenic removal efficiencies was analyzed. The abatement of hydrated silica was obtained at 8 mA cm^-2 and 0.057 cm s^-1, while the residual concentrations of F^- and As after the same electrolysis were 0.19 mg L^-1 and 9.8 μg L^-1, satisfying the WHO guidelines for F^- (≤1.5 mg L^-1) and As (≤10 μg L^-1). Spectroscopic analyses on aluminum flocs revealed that they are predominantly composed of aluminum silicates. Arsenates adsorb on aluminum flocs and fluoride replaces a hydroxyl group from aluminum aggregates.