Investigation of acidic dissolution of mixed clays between pH 1.0 and −3.0 using Si and Al X-ray absorption near edge structure

Although widely investigated in relation to acid mine drainage systems at pH > 1.0, we know little about the impact of sulfuric acid (H2SO4) on the geochemistry and mineralogy of clays at pH < 1.0 (including negative pH values). Thus, laboratory batch experiments were conducted on three mixed clay samples with different mass ratios of phyllosilicates (smectite, illite, and kaolinite) to investigate the impact of H2SO4 from pH 1.0 to −3.0 for exposure periods of 14, 90, 180, and 365 days. Si and Al K- and L2,3-edge X-ray absorption near edge structure (XANES) spectroscopy were employed on these samples to determine the chemical and structural changes that occur during acidic dissolution of phyllosilicates that cannot be distinguished using X-ray diffraction analyses. A series of silicate, phyllosilicate, and Al-bearing standard compounds were also studied to provide an explanation for the observed changes in the clay samples. The Si XANES results indicated the preferential dissolution of the phyllosilicates (pH ⩽ 1.0, t ⩾ 14 d), the persistence of quartz even at pH ⩾ −3.0 and t ⩾ 365 d, and the 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). Al XANES results demonstrated dissolution of Al-octahedral layers (pH ⩽ 1.0, t ⩾ 14 d), the persistence of four-fold relative to six-fold coordinated Al, and the precipitation of an Al-SO4-rich phase (pH ⩽ −1.0, t ⩾ 90 d). An existing conceptual model of phyllosilicate dissolution under extremely acidic conditions was modified to include the results of this study.