Effect of organic ligands on the sorption of europium on TiO2 and cement at high pH

Cement and cement-based materials are used as matrices and as containers for various categories of low-level and medium-level waste. The cement generally contains additives of various kinds, usually organic polymers that will act as plasticisers (up to a few percent). The degradation of these agents will, with time, generate low molecular organics and, eventually, carbon dioxide. The fraction of organic matter (ion exchange resins, filter materials, cellulose, cleaning agents etc) in the cement matrix will be therefore substantial. It can not be ruled out that these agents may act as metal complexing agents with ability to enhance the solubility and mobility of radionuclides from the radioactive waste within or in contact with the cement. Laboratory studies were performed to assess the potential effects of cement additives, spent organic adsorbent resins and some organic cleaning agents in a cement/concrete system on the mobility and distribution of radionuclides using Eu(III) as a model element (for lanthanides as well as actinides in the trivalent state). Batch distribution studies were conducted in the following systems: Solid adsorbents: Standard Portland cement, TiO2. Water phase: 0.3 M NaCl equilibrated with cement or TiO2; 0 or 2 mM Ca (for the TiO2-systems); pH 12.5. Organic ligands: EDTA, DTPA, NTA, citric acid, D-gluconic acid, oxalic acid, fulvic acid, isosaccharinic acid, acetyl acetone, TTA. Cement additives: Sikament 10, Sikament 210, Peramin Conpac 30, Peramin F, Glenium 51, Cementa Melcrete, Mighty 150. Cleaning agents: Clax Delta Balans, Industrikombi, Prefect Citron. Organics: Dissolved organic matter from the degradation of two solid organic components: an ion exchange resin and a filter aid (Acrisorb LSR 33 and Ricem UP2). Element: Eu (10-8 M). The distribution of Eu was measured radiometrically (constant solid/liquid ratio, 1g/L) as a function of time (up to 420 d) and additive concentration (0.001-10% of water phase) or as TOC concentration (0.3-8 mM). Distribution measurements were also performed using additives that had been stored at pH 12.5, 60 deg C, up to 107 d prior to the addition of solid adsorbent and Eu. Generally, there is a significant reduction of the sorption (expressed as the distribution coefficient) of Eu on cement as well as on TiO2 due to the presence of cement additives at concentration levels expected in fresh cement and concrete of technical quality. Most of the liquid agents had some effect on the adsorption. The effects of the presence of these organic agents are compared and the presence of additives at various concentrations on the distribution of Eu is assessed. Possible consequences for the safety and performance of a cement-based waste deposition concept are analysed and discussed