Uranium Partitioning upon Ammonia Gas Treatment on Phyllosilicate Minerals
In the past, > 200,000 kg of U was released to the Hanford Site vadose zone from disposal and leakage of waste byproducts from plutonium production during World War II and the Cold War (Zachara et al., 2007). U is highly mobile due to the presence of carbonates and oxidizing conditions, with K{sub d} of 0.1 - 8 mL/g at the site (Zachara et al., 2007; Szecsody et al., 2013; Serne et al., 2008). NH{sub 3} (gas) injection is being investigated as a remediation technique as it is an in situ method that will not introduce additional liquid that could drive contaminants deeper into vadose zone towards groundwater. Objectives: To understand the impact of alkaline injection on geochemical processes in the Hanford Site vadose zone, including: 1)Long-term fate of uranium (U); 2)Dissolution and re-precipitation of secondary minerals. Apparent K{sub d}'s increase significantly with NH{sub 3} gas treatment, especially for six-month aging. - K{sub d} decreases after aeration, primarily due to U desorption at lower pH. U removal is still significantly greater than natural conditions (K{sub d} < 8 vs. > 100 mL/g). - Three-day aeration has higher loading than six-month after ammonia gas treatment and aeration. However, significantly greater desorption of U occurs with aeration in the six-month aged sample potentially due to transformation into different, more soluble solid phases with longer term aging. - U removal is primarily due to (1) co-precipitation of U with secondary minerals and (2) adsorption to minerals' surface, although further research is required to identify dominant mechanisms. Future experiments will focus on garnering a better understanding of the solid phase U speciation. Solids will be characterized using SEM-EDS, EMPA, XPS, XAFS, and TEM to better understand the effects of basic treatments on initial minerals and formation of secondary precipitates.