Pu distribution among mixed waste components at the Hanford legacy site, USA and implications to long-term migration

Subsurface Transport of Plutonium in Organic and Aqueous Acidic Processing Wastes at the Hanford Site, USA

“…The lower pH of the system would also favor less Pu hydrolysis and more positively charged Pu species resulting in less sorption and increased migration in the column. A pH gradient has been observed beneath the trench , with sediments as low as pH 2 observed directly beneath the trench and pH 3.9 sediments at approximately 20 m bgs . Given the high buffering capacity of Hanford sediments and the observed pHs below the Z-9 trench, it is likely that the pH of aqueous fluids released into the Z-9 trench were not consistently neutralized to pH ∼ 2.5 before disposal as documented in disposal records and that more acidic wastes were likely released in some cases.…”

“…Our experiments indicate that Pu migration at the Z-9 trench can be explained by the surprisingly fast transport of Pu in high ionic strength, organic fluids through acid treated Hanford sediments. However, neutralization of acidic waste streams (>pH 4) and infiltration of low ionic strength water will limit further Pu migration . The buffering capacity of the caliche layer that resides at 37 m beneath the Z-9 trench likely limited further downward migration of Pu at Hanford, as evidenced by limited detection of Pu contamination in the groundwater that resides beneath the caliche layer …”

“…25,26 At the Z-9 trench, previous studies have shown that Pu exists as both discrete Pu particles in the shallow sediments and diffuse low concentrations of Pu further beneath the trench. 25,27,28 Pu particles in the shallow sediments beneath the trench are likely undissolved Pu scrap metal, precipitated PuO 2 , and Pu iron phosphates. 3,12,19,27 Core samples show the presence of a pH gradient beneath the trench, with low pH (pH ∼ 2), acid-weathered sediments in the shallow region beneath the trench, and unaltered sediments (pH 7−9) deeper in the cores.…”

“…The column experiments used uncontaminated native Hanford sediments and Pu solutions in a variety of acidic aqueous and organic solvents. The column experiments provide important insights into Pu transport mechanisms in Hanford sediments that were not resolved in our previous batch experiments . We investigated two main disposal scenarios: Pu mobility in (1) low pH, high ionic strength aqueous fluids and (2) mixed low pH, high ionic strength aqueous fluids in combination with organic TBP-containing solvents.…”

Impact of a Biological Chelator, Lanmodulin, on Minor Actinide Aqueous Speciation and Transport in the Environment

“…Several physicochemical mechanisms have previously been considered and observed for the transport behavior of transuranic actinide isotopes in the environment. Such mechanisms include solution complexation to small inorganic ions, ,,,- complexation to small water-soluble organic chelators, - sorption onto mineral phases, ,, ion exchange with clay minerals, , as well as colloid-mediated transport. ,,, Given the growing inventory of spent nuclear fuel worldwide , and the necessity to understand, model, and predict the behavior of radiotoxic elements over thousands of years, it is imperative to be aware of the full range of mechanisms that may influence the environmental behavior of actinides. In this context, the recent discovery of the natural lanthanide-binding protein LanM in M.…”