Geochemical and mineralogical impacts of sulfuric acid on clays between pH 5.0 and -3.0

Abstract

Natural and constructed clay liners are routinely used to contain waste and wastewater. The impact of acidic solutions on the geochemistry and mineralogy of clays has been widely investigated in relation to acid mine drainage systems at pH > 1.0. The impact of sulfuric acid leachate characterized by pH < 1.0, including potentially negative pH values on the geochemistry and mineralogy of clays is, however, not clear. To address this deficiency a series of batch and diffusion cell studies, investigating the geochemical and mineralogical impacts of H2SO4 solutions (pH 5.0 to -3.0), were conducted on three mineralogically distinct clays (Kc, Km, and BK). Batch testing was conducted at seven pH treatments (5.0, 3.0, 1.0, 0.0, -1.0, -2.0 and -3.0) using standardized sulfuric acid solutions for four exposure periods (14, 90, 180, and 365 d). Aqueous geochemical, XRD, and Si and Al XANES analyses showed: increased dissolution of aluminosilicates with decreasing pH and increasing exposure period; preferential dissolution of aluminosilicate Al-octahedral layers relative to Si-tetrahedral layers; formation of an amorphous silica-like phase that was confined to the surface layer of the altered clay samples at pH ≤ 0.0 and t ≥ 90 d; and precipitation of anhydrite and a Al-SO4-rich phase (pH ≤ -1.0, t ≥ 90 d).

The diffusive transport of H2SO4 (pH =1.0, -1.0, and -3.0) through the Kc and Km clays for 216 d was examined using single reservoir, constant concentration, diffusion cells. The diffusive transport of H+ within the cells was modeled using 1-D transport models that assumed no absorption, linear absorption, and non-linear absorption of H+. The absorption isotherms were calculated from the pH 5.0, 3.0, and 1.0 batch experiment results, which were assumed representative of H+ absorption at pH < 1.0. However, model results indicated that the batch test results can not account for the observed H+ consumption in all cells and greatly underestimate the amount of H+ consumption in the pH -1.0 and -3.0. In the Kc and Km diffusion cells, above-background Ca, Al, Fe, and Si aqueous concentrations were associated with depth intervals characterized by decreased pH values. Respective peak concentrations of 325, 403, 176, 11.7, and 1.38 x 10³ μmol g-1 (Kc) and 32.4, 426, 199, 7.2, and 1.22 x 10³ μmol g-1 (Km) were measured in the pH -3.0 cells. XRD results showed that the elevated concentrations corresponded to the loss of carbonates and montmorillonite peaks and decreased peak intensities for illite and kaolinite in depth intervals with pH ≤ 1.0, in the Kc and Km pH -1.0 and -3.0 cells. The combined results of these studies indicated that the long-term diffusion of H2SO4 through clays at pH < 1.0 will result in a large amount of primary phase dissolution; however, this will be accompanied by precipitation of soluble Ca and Al sulfate salts and amorphous silica, especially at pH ≤ 0.0. Additionally, the presence of even a small amount of carbonate will serve to greatly buffer the diffusive transport of H2SO4 through clays, even at a source pH of -3.0.