Abatement of hydrated silica and simultaneous removal of coexisting ions from deep well water by electrocoagulation using an up-flow reactor

Abstract

The removal of hydrated silica and coexisting ions from groundwater (hydrated silica 42 mg L−1, fluoride 7.3 mg L−1, arsenic 40 μg L−1, sulfate 57 mg L−1, phosphate 0.26 mg L−1, pH = 8.02 and conductivity 605 μS cm-1) by electrocoagulation (EC) was examined. The EC was carried out in a novel up-flow reactor with a six-cell stack, in a serpentine array opened to the atmosphere, using aluminum plates as electrodes. The influence of current density (10 ≤ j ≤ 16 mA cm−2) and mean linear flow rate in the EC reactor (1.16 ≤ u ≤ 4.67 cm s−1), corresponding to retention times between 13.9 ≤ τ ≤ 55.9 s, on the hydrated silica, fluoride, arsenic, sulfate and phosphate removal was investigated. The best removal of hydrated silica based on energy consumption (0.98 KWh m−3) and overall cost of EC (0.274 USD m−3) was obtained at 12 mA cm−2 and u= 2.33 cm s−1, giving a remaining concentration of silica of 7 mg L−1, while the residual concentrations of fluoride (1.4 mg L−1) and arsenic (1.88 μg L−1) met the WHO guidelines in human drinking water. The characterization of the flocs by SEM-EDS, XRF, XRD and FTIR indicated that the coagulant reacts with silica to yield aluminum silicates, fluoride substitutes a hydroxyl group from flocs, while arsenates, sulfates, and phosphates are adsorbed on aluminum aggregates.