Geochemical and Geophysical Changes during Ammonia Gas Treatment of Vadose Zone Sediments for Uranium Remediation

Abstract

Low water content sediments were treated with NH3 gas to evaluate changes in U mobility as a potential field remediation method for vadose zone contamination. Injection of NH3 gas created high dissolved NH3 concentrations that followed equilibrium behavior. High NH3 concentration led to an increase in pH from 8.0 to 11 to 13, depending on the water content and NH3 concentration. The increase in pore water pH resulted in a large increase in pore water cations and anions from mineral‐phase dissolution. Minerals showing the greatest dissolution included montmorillonite, muscovite, and kaolinite. Pore water ion concentrations then decreased with time. Simulations based on initial pore water ion concentrations indicated that quartz, chrysotile, calcite, diaspore, hematite, and Na‐boltwoodite (hydrous U silicate) should precipitate. Electrical resistivity and induced polarization tomography (ERT/IP) was able to nonintrusively track these NH3 partitioning, dissolution, and precipitations processes through changes in conductivity and chargeability. Ammonia treatment significantly decreases the amount of U present as adsorbed and aqueous species in field‐contaminated sediments. In contrast, sediments containing a large fraction of U associated with carbonates generally showed little change. Uranium leaching from sediments containing high Na‐boltwoodite decreased significantly by NH3 treatment, but x‐ray absorption near‐edge structure/extended x‐ray absorption fine structure showed no change in the Na‐boltwoodite concentration. Therefore, NH3 treatment of contaminated sediment acts to decrease the highly mobile aqueous and adsorbed U by incorporation into precipitates and appears to decrease mobility of some existing U precipitates (Na‐boltwoodite) as a result of mineral coating.

Ammonia gas transport and reactions in unsaturated sediments: Implications for use as an amendment to immobilize inorganic contaminants

“…Self-dissociation of ammonia at this concentration results in an increase in the pore water pH from initially around pH 8 to about pH 11.5 [5,6]. Ion exchange and mineral dissolution (including silicate dissolution) is caused by the caustic pH (Step 2, Fig.…”

“…Ion exchange and mineral dissolution (including silicate dissolution) is caused by the caustic pH (Step 2, Fig. 1) [5,6]. With high total dissolved solids, precipitates start to form, especially as the pH is buffered back toward neutral.…”

“…The precipitates may incorporate uranium (e.g., forming sodium boltwoodite) or may form compounds, such as quartz, chrysotile, calcite, diaspore, and hematite that could coat uranium already precipitated or adsorbed on the sediment surface (Step 3, Fig. 1) [5][6][7]. The goal of the dissolution and re-precipitation process is to create uranium precipitates or coatings that render uranium less mobile than before treatment.…”

“…[4,1]. Treatment with ammonia (NH 3 ) gas has a potential for use in treating inorganic contaminants (such as uranium) by creating stable precipitates that limit the mobility of inorganic contaminants [5]. Using uranium as a specific example, Fig.…”

Experimental and Numerical Analysis of Parallel Reactant Flow and Transverse Mixing with Mineral Precipitation in Homogeneous and Heterogeneous Porous Media

“…Engineered mineral precipitation in the subsurface has been proposed as a method to modify contaminant mobility (Fujita et al 2008;Moore et al 2007;Szecsody et al 2012;Warren et al 2001), fluid flow paths (Ferris et al 1996;Taron and Elsworth 2009) and geotechnical properties of consolidated media (Muynck et al 2010;DeJong et al 2006). Some engineering objectives, such as immobilizing contaminants in situ by co-precipitation or encapsulation, involve forming precipitates at a distance from where reactant solutions can be injected.…”