Application of reactive gases including ammonia (NH3) is being considered for remediation of radionuclides by pH manipulation in unsaturated zones. Ammonia gas injections may decrease radionuclide mobility without addition of liquid amendments into the vadose zone and quickly elevate the pH inducing soil mineral dissolution. As NH3 dissipates and the pH returns to neutral, co-precipitation processes are expected to occur between radionuclides and dissolved cations. However, the mineral dissolution and secondary precipitate formation processes are not well understood in these conditions. This research is focused on the dissolution of pure minerals (illite, montmorillonite, and muscovite) in synthetic groundwater under anaerobic (oxygen-free) and aerobic conditions. Experiments were conducted using 3.1 M NH4OH solutions as well as NaOH as comparable alkaline treatments while measuring pH and oxidation reduction potential (ORP) changes. Major cations (Si 4+, Al 3+, Ca2+, etc.) in the aqueous phase were monitored for over 60 days to determine the dissolved concentration from the solid phase with inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectroscopy (ICP-MS). Throughout the experiments, the pH remained highly elevated (11.5 - 12.3) for both treatments and conditions. ORP measurements were significantly reduced under anaerobic conditions with the NaOH treatment reaching significantly lower ORP values than NH4OH. Further, under aerobic conditions, the ORP values were significantly greater for NaOH treatment than NH4OH. Dissolution results demonstrate that there is a rapid decrease in aluminum and slow increase in silica over time. This trend was particularly strong for montmorillonite although similar results were observed for illite and muscovite. Further, the comparative alkaline treatments indicate that there is slightly more dissolution with NaOH than NH4OH. Lastly, data shows that the geological phenomenon known as incongruent dissolution is occurring in batch experiments which suggests formation of secondary precipitates upon alkaline treatment.

Phyllosilicate mineral dissolution upon variable alkaline treatment and redox conditions

2019 Fall National Meeting

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