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

The removal of hydrated silica and coexisting ions from groundwater (hydrated silica 42 mg L-1, fluoride 7.3 mg L-1, arsenic 40 mu g L-1, sulfate 57 mg L-1, phosphate 0.26 mg L-1, pH = 8.02 and conductivity 605 mu S 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 <= tau <= 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 mu g 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.