Removal of fluoride and hydrated silica from underground water by electrocoagulation in a flow channel reactor

This paper concerns simultaneous removal of fluoride and hydrated silica from groundwater (4.08 mg L-1 fluoride, 90 mg L-1 hydrated silica, 50 mg L-1 sulfate, 0.23 mg L-1 phosphate, pH 7.38 and 450 mu S cm(-1) conductivity) by electrocoagulation (EC), using an up-flow EC reactor, with a six-cell stack in a serpentine array, opened at the top of the cell to favor gas release. Aluminum plates were used as sacrificial electrodes. The effect of current density (4 <= j <= 7 mA cm(-2)) and mean linear flow rate (1.2 <= u <= 4.8 cm s(-1)), applied to the EC reactor, on the elimination of fluoride and hydrated silica was analyzed. The removal of fluoride followed the WHO guideline (<1.5 mg L-1), while the hydrated silica was abated at 7 mA cm(-2) and 1.2 cm s(-1), with energy consumption of 2.48 kWh m(-3) and an overall operational cost of 0.441 USD m(-3). Spectroscopic analyses of the flocs by XRD, XRF-EDS, SEM-EDS, and FTIR indicated that hydrated silica reacted with the coagulant forming aluminosilicates, and fluoride replaced a hydroxide from aluminum aggregates, while sulfates and phosphates were removed by adsorption process onto the flocs. The well-engineered EC reactor allowed the simultaneous removal of fluoride and hydrated silica. (C) 2019 Elsevier Ltd. All rights reserved.