U(VI) retention on bentonite and cementitious materials: Effect of increased ionic strengths and presence of organics

The safe disposal of radioactive waste from operation and decommissioning of nuclear power plants in geological repositories requires the application of multiple barriers to isolate the waste from the biosphere. Most disposal concepts consider the extensive use of bentonite and cementitious materials in the geoengineered barrier as buffer and borehole sealing material as well as to enforce mechanical stability of disposal facilities. To evaluate the radionuclide retention potential of these barrier materials, it is necessary to examine the effects of various repository-relevant conditions that are expected to develop over time, such as altered pH, increased ionic strengths or temperatures, or the release of organic constituents.
Pore waters of the North German clay deposits are characterized by high ionic strengths up to 4 M [1,2]. The contact of such saline formation waters with concrete will result in an enhanced corrosion of concrete and to the evolution of highly alkaline cement pore waters (10 < pH < 13), which in turn, can react with the bentonite buffer as well as with the clay host rock, changing their retention potential towards radionuclides. Moreover, the role of organics (as admixtures in cement-based materials or waste constituents [3]) on actinide retention has to be studied.
The U(VI) retention on Ca-bentonite in mixed electrolyte solutions (‘diluted Gipshut solution’, I = 2.6 M) was found to be very effective at pH>10, even in the presence of carbonate and despite the prevalence of anionic aqueous uranyl species [4]. By means of luminescence and X-ray absorption spectroscopy, two dominating U(VI) surface species at hyperalkaline conditions were identified: (i) a ternary U(VI) complex, where U(VI) is bound to the surface via bridging Ca cations with the configuration surface ≡Ca–OH–U(VI) and, (ii) U(VI) sorption into the interlayer space of calcium (aluminum) silicate hydrates (C-(A-)S-H), which form as secondary phases in the presence of Ca due to partial dissolution of alumosilicates at hyperalkaline conditions (Figs. 1 and 2) [5]. Citrate and 2 phosphonobutane-1,2,4,-tricarboxylate (PBTC) were found to reduce U(VI) retention only when present at high concentrations.
The U(VI) retention by C-A-S-H, formed due to Al-rich additives in cement formulations, was studied applying samples with Ca/Si molar ratios of 0.8, 1.2 and 1.6, representing different alteration stages of concrete, and with increasing Al/Si molar ratios of 0, 0.06 and 0.18 in each series. Furthermore, the impact of temperature (25°C, 100°C, 200°C) on both the C-A-S-H structure and the U(VI) retention mechanism was studied. Solid-state 27Al and 29Si NMR spectroscopy along with XRD revealed that enhanced temperatures increase the crystallinity of the material with the appearance of neoformed crystalline phases. Surface-sorbed and interlayer-sorbed U(VI) species were detected by luminescence spectroscopy. U(VI) mobilization due to high ionic strengths or presence of organics (gluconate or PBTC) was very low [6,7].
The results show that both bentonite and cementitious material constitute an important geoengineered retention barrier for U(VI) under hyperalkaline conditions at increased ionic strengths and in presence of organics. Thus, both bentonite and cementitious material strongly contribute to the safe confinement of radionuclides in a repository to isolate radiotoxic contaminants from the hydrosphere and biosphere.