Phyllosilicate mineral dissolution upon alkaline treatment under aerobic and anaerobic conditions

The dissolution of phyllosilicate minerals exposed to high-pH environments was studied to quantify the influence of alkaline treatments and variable redox conditions on clay dissolution including incongruent dissolution phenomena. The objective of this research was to systematically quantify mineral dissolution with variable alkaline treatments and redox conditions for the first time. This study is focused on the dissolution of phyllosilicate minerals (illite, muscovite, and montmorillonite) under anaerobic and aerobic conditions using comparative solutions (sodium hydroxide and ammonium hydroxide) at similar hydroxide concentration. Our batch data show that there is a rapid decrease in aluminum dissolution (<240 h) and slow increase in silica dissolution over time (up to 1440 h). This trend was particularly evident for montmorillonite for which the greatest dissolution was observed with ammonium hydroxide treatment, likely due to intercalation of the polyatomic cation ammonium into the mineral's expandable layers. When comparing alkaline treatments, the strong base sodium hydroxide dissolved more of the mica minerals, illite and muscovite, likely due to ion-pairing between the silicate tetrahedra [SiO4]n- and Na+ cations in solution compared with weak base NH4OH treatment. In addition, the decreasing redox changes in the absence of oxygen were similar, although the sodium hydroxide treatment had greater variability. For all investigated phyllosilicates, the calculated aqueous aluminum and silicon ratios over time were significantly different from the minerals' stoichiometric ratios. As a result, we conclude that incongruent dissolution occurred and suggest formation of secondary precipitates. Understanding the potential for clay mineral alterations from interaction with alkaline solutions has implications for in situ remediation, mining operations, and waste interactions within the subsurface. This research shows that incongruent dissolution in phyllosilicate minerals occurs and will likely lead to secondary precipitation which may have long term physical and chemical impacts in the subsurface.