Abatement of hydrated silica, arsenic, and coexisting ions from groundwater by electrocoagulation using iron electrodes

Abstract

The paper deals with the removal of arsenic (As), hydrated silica (HS), and coexisting ions from groundwater by electrocoagulation (EC) using a laboratory-scale up-flow reactor with sacrificial iron anodes (1018 steel, >99% wt. Fe). Natural groundwater, taken in the northern region of Mexico, contained 25.7 μg L−1 As, 237.8 mg L−1 HS, 1.43 mg L−1 F−, 45.0 mg L−1 SO42−, 0.61 mg L−1 PO43−, pH 8.62, and 577 μS cm−1 conductivity. The effect of current densities (4≤j≤8 mA cm−2) and mean linear flow velocities (1.1≤u≤4.6 cm s−1) on the pollutant's removal was systematically addressed. The best EC trial that showed the lowest overall cost and complied with the WHO guideline (<10 μg L−1 As) was obtained at j = 6 mA cm−2 and u = 2.3 cm s−1, reaching residual concentrations of As and HS of 4.6 μg L−1 and 150.0 mg L−1, respectively. A large amount of HS was found after electrolysis; therefore, a second EC was applied to reduce the HS concentration further. This time, residual concentrations of HS and As of 37.0 mg L−1 and 1.2 μg L−1 were obtained, with electrolytic energy consumption and overall cost of EC of 0.872 kWh m−3 and 0.178 USD m−3, respectively. XRF, EDS, XRD, and FTIR analyzes on flocs indicate that hydrated silica reacts with iron, forming iron silicates with divalent cations as flocs. Arsenic and PO43− are abated by adsorption on flocs. The modest removal of F− and SO42− (44% and 12%, respectively) is due to its weak adsorption on flocs.