Quantification of the Interaction of Tc with Dissolved Boom Clay Humic Substances

Abstract

Technetium-99 (Tc), a fission product of uranium-238, is an important radionuclide because of its long half-life and its high yield in radioactive waste. To elucidate the Tc geochemical behavior in reducing environments relevant to geological disposal and in the presence of humic substances (HS), experiments were set up that resulted forthe first time in the determination of an interaction constant for Tc with dissolved humic substances. A number of lab-scale Boom Clay (a possible geological underground High-Level Radioactive Waste storage site in Mol, Belgium) batch experiments were set up, combining both different initial Tc(VII) concentrations and different solid/liquid ratios. On these batches several sequential extraction steps with HS-free synthetic Boom Clay water were performed. Equilibration times were fixed at 1 week for each extraction step. Tc(VII) was found to be readily reduced to Tc(IV) by the solid Boom Clay phase. This solid phase was able to sorb Tc(IV) to a very large extent (log Kd approximately 2.5-4.0), and two sorption sinks (one of which is humic substances) were detected. In solution, Tc(IV) was mainly associated with HS. Concentrations in solution were found up to the order of 2 x 10(-6) M. The results were quantitatively described as a competition for Tc(IV) between the solid phase and the dissolved HS (Schubert-like approach). It was concluded that a hydrophobic sorption of uncharged Tc(IV) species in solution would act as the dominating interaction mechanism with HS, with an interaction constant log K(HS) = 5.3 +/- 0.3.

Evidence for the Interaction of Technetium Colloids with Humic Substances by X-ray Absorption Spectroscopy

“…Judged from (i) the removal rate of Tc(VII) species in the GPC results and (ii) the change in the observed equilibrium solubility in the supernatant solutions, it is noticed that in the case of Gorleben sand/Gorleben groundwater systems, complete reduction and equilibrium were reached after 1 day of reaction, whereas in the synthetic systems with magnetite and pyrite, it took more than 200 days, similar to previous observations (4). Pertechnetate reduction in Boom clay was complete in less than 6 days in agreement with previous work (12). In the absence of humic substances (systems 1 and 3), the pertechnetate reduction took about 50 days for the pyrite-containing sample and more than 300 days for the magnetite-containing sample.…”

“…This sample therefore represents Tc species associated with the immobile HSs in Boom clay systems. It was previously shown (12) that these Tc species also interact with dissolved Boom clay HSs, and that this interaction could be described by a single constant. Sample 8 consisted of the concentrate of dissolved Gorleben HSs and Tc species in solution obtained from the precipitation with La 3+ of the GS + GW system.…”

Mycorrhizal association of maritime pine, Pinus pinaster, with Rhizopogon roseolus has contrasting effects on the uptake from soil and root-to-shoot transfer of 137Cs, 85Sr and 95mTc

“…Under reduced conditions, it may be reduced to sparingly soluble forms of Tc(IV) (Sheppard et al, 1990;Tagami and Uchida, 1999;Abdelouas et al, 2005;Burke et al, 2005). It is also immobilised by chemical reactions with soil organic matter, particularly humic substances (Rössler et al, 2000;Geraedts et al, 2002;Maes et al, 2003). Pertechnetate is readily taken up by plant roots and translocated within plants and Tc is found in a range of organic compounds in plants, particularly proteins (Murphy and Johnson, 1993;Echevarria et al, 1997;Krijger et al, 1999).…”

Dissolved Organic Matter (DOM) in Natural Environments

“…Practical interest was also given to the leaching of metals in coal mining areas because there is always washing out with soil seepage water (Suteerapataranon et al, 2006 ). In case of disposal of highlevel nuclear waste, the prediction of the behavior of radionuclides in aquifers is needed (e.g., Maes et al, 2003 ).…”