Electrolytic oxygen generation for subsurface delivery: effects of precipitation at the cathode and an assessment of side reactions.

This research investigated the oxygen-generating characteristics and side reactions of an electrolytic cell assembly that could be used to remediate sites with contaminants that are amenable to aerobic biodegradation. The oxygen-generating capabilities of new electrolytic cells and cells with light and heavy calcium carbonate precipitates on the cathode were evaluated in the laboratory under current densities ranging from 0.5 to 5.0 mA/cm2. Higher current densities resulted in higher mass transfer coefficients (K(L)a) and greater saturation oxygen concentrations (Csat). As the cathodic deposits increased, the K(L)a tended to decrease and the Csat tended to increase. The oxygen transfer efficiency (OTE) did not vary as a function of current density or cathode coating, while the average OTE for all the tests was 67%. Laboratory column tests showed that chlorine production increased with current density and depended on chloride levels in the water. Hydrogen peroxide was generated at low concentrations (< 1 mg/L) and at higher levels when chloride was absent in the feed solution. Calcium removal from solution increased with current density and resulted in a decrease in solution pH. Tests at a field monitoring well showed average Csat levels of 16.9 mg/L after 14 days of operation, no chlorine production because of low chloride levels in the well, artificially elevated hydrogen peroxide levels because of background interferences, and a pH decrease of 2.4 units. With passive venting, the average hydrogen gas levels at the headspace of the well were less than 1%.