RESUMO
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.
