Groundwater contamination prevention using an electrokinetic barrier-cobalt trapping

Abstract

The quality of groundwater in many regions around the world has been compromised due to pollution from anthropogenic activities such as deposition of solid wastes containing heavy metals in landfills, industrial spills and leaching from mine tailings. Inorganic contaminants such as lead, cobalt, and cadmium have been a growing concern owing to their detrimental environmental and health effects. Several methods have been used to remediate contaminated soils and groundwater. These methods include phytoremedation, biosorption, neutralization, and chemical oxidation. These methods, however, have several limitations and disadvantages including quality control, lack of selectivity and generation of additional contamination and waste sludge. Electrokinetic remediation is a growing technology used largely to remediate and restore soils and groundwater affected by organic and inorganic contamination. Electrokinetics is the application of a direct current to a wet soil to transport or remove water and/or ions via the soil pores. Two main phenomena are observed when a voltage gradient is applied to a wet and compacted soil. The first is the movement of the soil's pore fluid towards the cathode, known as electro–osmosis. The second is electro–migration which is the movement of ions towards the oppositely charged electrode. Though much work reported in the literature has focused on the application of electrokinetic phenomena to remediate contaminated soils, this thesis focuses on utilizing electrokinetic phenomena to prevent groundwater contamination at an industrial mine site. The first objective of this work was to investigate the applicability of an electrokinetic barrier to prevent groundwater contamination downstream of a tailings management area at a Canadian mine site by trapping cobalt near the cathode. To meet the first objective, direct current was applied either continuously or intermittently. The second objective was to examine the fate of the cobalt accumulated in the soil after the termination of the electrokinetic barrier. The experimental studies showed the effectiveness of the applied voltage gradient in reducing the net flow of water downstream of the barrier and diminishing cobalt concentration and mass in the effluent. Furthermore, the influences of electro–osmosis, electro–migration, and the sharp pH gradient which favored cobalt adsorption/precipitation near the cathode resulted in trapping cobalt in the cathode region near the inlet of the cell. Additionally, using intermittent current to power the electrokinetic barrier yielded results that were generally comparable to those obtained with continuous current. This finding offered important insights for future work on the use of solar powered electrokinetic barriers. Wash out tests in which the potential gradient was stopped midway through the test were conducted to address the second objective. The concentration of cobalt in the effluent did not increase after the termination of the potential gradient, for the duration of the test. The accumulated cobalt was retained in the soil and was not washed out of the cells. In summary, this study demonstrated that an electrokinetic barrier can be successfully used to prevent groundwater contamination due to cobalt by taking advantage of the coupled effects of electro–osmosis, electro–migration and water electrolysis at the electrodes.