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

Abstract

This paper concerns simultaneous removal of fluoride and hydrated silica from groundwater (4.08mgL-1 fluoride, 90mgL-1 hydrated silica, 50mgL-1 sulfate, 0.23mgL-1 phosphate, pH 7.38 and 450μS 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≤7mAcm-2) and mean linear flow rate (1.2≤u≤4.8cms-1), applied to the EC reactor, on the elimination of fluoride and hydrated silica was analyzed. The removal of fluoride followed the WHO guideline (<1.5mgL-1), while the hydrated silica was abated at 7mAcm-2 and 1.2cms-1, with energy consumption of 2.48kWh 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.