EFFECTS OF HYPERACIDIC H2SO4 MIGRATION THROUGH CLAY MEDIA UNDERLYING SULFUR STORAGE BLOCKS IN THE ALBERTA OIL SANDS

Elemental sulfur (S⁰) is recovered as a by-product of oil sands production and stored as above ground blocks (300 m x 150 m x 25 high) underlain by compacted clay barriers. Due to the projected long-term excess of S⁰ on the world market, oil sand operators, including Syncrude Canada Limited (SCL), are assessing the viability of storing their S⁰ blocks for upwards of 500 years. The infiltration of precipitation and atmospheric oxygen into the blocks leads to oxidative weathering, which results in the production of sulfuric acid (H₂SO₄). The resulting leachate is characterized by elevated SO₄^2- concentrations (> 18 g/L) and hyper-acidic pH values (pH < 1.0 and possibly as low as 0.3). As a result, the impact of hyperacidic leachate on the long-term performance of clay barriers is of environmental concern. To assess the geochemical and mineralogical effects of hyper-acidity that occur in clay barriers, a series of batch experiments were conducted on three mineralogically distinct clay media. For each clay media, experiments were conducted over a pH range of 5.0 to -3.0 (to expand the research scope beyond that of S⁰ storage) and for time periods of 0.5, 3, 6 and 12 months (to determine kinetic effects). Aqueous Al, Fe, Si and Ca concentrations attain peak values of 1.8, 1.5, 0.2 and 0.7 g/L, between pH = 1.0 and -3.0, suggesting significant mineral dissolution for all three clay media. Furthermore, results suggest that the amount and rate of dissolution increases with increasing H₂SO₄ concentration. However, mineralogical analyses (XRD and SEM) demonstrate the occurrence of secondary precipitation in all three samples, including the formation of a Si-Ca rich amorphous phase. The batch test results are currently being geochemically modeled to provide insights into the possible evolution of hyperacidic H₂SO₄ migration through clay barriers of varying mineralogical composition. These results will then be used in conjunction with on-going diffusion cell experiments to characterize the diffusive transport of hyperacidic H₂SO₄, and the associated geochemical reactions, through clay barriers.