Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The ingress of water into an underground nuclear repository, described as a worst-case scenario, can lead to the degradation of cement-based engineered barriers and thus to the release of organic cement additives that can affect radionuclide immobilisation. The additive 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) is one of the most commonly used long-term retarders in cement, and also used as a corrosion inhibitor in reinforced concrete and steel. PBTC
is an organophosphonate ligand with one phosphonate and three carboxyl groups [1]. These functional groups make PBTC an effective dispersant and strong complexing agent for various metal ions (e.g. Ca2+, Al3+, Fe3+). However, neither the complexation of radionuclides by PBTC nor the influence of PBTC on radionuclide retention in cement phases has been investigated.
Therefore, both the complexation of U(VI) with PBTC in solution (binary system) and the influence of PBTC on the U(VI) retention by cementitious materials (ternary system) were investigated for the first time. The U(VI) complexation studies were performed by different series varying the pH from 2 to 11 and/or the U(VI) to PBTC ratio. The structure-sensitive methods NMR, IR and Raman spectroscopy were used to characterize the complex structure. Complementary DFT calculations were carried out. The U(VI) speciation in presence of PBTC was determined by UV-Vis and TRLFS spectroscopy. In the case of PBTC excess, soluble complex species are formed up to pH >10, which is relevant for cementitious systems due to degradation processes. For the U(VI) retention studies both calcium (aluminate) silicate hydrate (C-(A-)S-H) phases of different compositions, representing different cement degradation stages, as well as hardened cement paste were applied. TRLFS was applied to characterize the U(VI) binding. The PBTC retention was quantified by 1H and 31P solution NMR.