Sequestering Uranium at the Hanford 200 Area Vadose Zone by in Situ Subsurface pH Manipulation Using NH3

Characterization of vadose zone (VZ) soil at the 200 Area has identified a number of radiological and hazardous contaminants, including technetium and uranium. This work is focusing on long-lived uranium contamination, which is one of the key contaminants of concern that needs to be reduced to below levels that can cause harm to human health and the environment. Injection of reactive gases such as NH3, is an innovative remediation technology shown to mitigate uranium contamination in soil. The injection of NH3 gas causes ammonia gas dissolution in soil moisture, with the formation of NH4OH and a subsequent increase in pH. This manipulation will significantly alter the pore water chemistry and affect the dissolution of silica and aluminosilicate from soil minerals, followed by co-precipitation of U(VI) [as uranyl (UO22+)] and Al at higher pH conditions. The main objective of the project is to evaluate the role of major pore water constituents such as Al, Si, bicarbonate and Ca on the formation of precipitates after NH3 injection and to examine the solubility of formed minerals under environmental conditions relevant to the Hanford vadose zone. The batch experiments include preparation of a series of vials on synthetic fluids imitating contaminated pore water. The concentrations of major constituents including silicate, aluminate and bicarbonate are in the range reported for the Hanford VZ.