Sorption of Isosaccharinic Acid, a Cellulose Degradation Product, on Cement

Abstract

Low- and intermediate-level radioactive waste (L/ILW) contains substantial amounts of cellulosic materials. In Switzerland ~50% of the organic waste is cellulosic (<em>1</em>). In a cementitious L/ILW repository, the large amounts of cement produce alkaline environments in which the pH of the cement pore water remains above 12.5 for periods of the order of 10⁵ years (<em>2</em>). It is well known in the literature that cellulose is unstable under alkaline conditions and will degrade to water-soluble, low molecular weight compounds (<em>3</em>). The type of degradation products formed depends strongly on the composition of the solution in contact with the cellulose. In the presence of Ca²⁺, as is the case for pore waters of cement-stabilized waste forms, isosaccharinic acid (ISA, see Figure 1) is the main degradation product (<em>4, 5</em>). The importance of ISA in the context of radwaste disposal is that it can form – in analogy with gluconic acid (see Figure 1) – stable complexes with tri- and tetravalent radionuclides such as Am³⁺ and Pu⁴⁺ (<em>6−8</em>), which adversely influences their sorption on the cement phase (<em>9</em>). This could lead to an enhanced release of such radionuclides to the geo- and biosphere.<br /> A key factor for complexation is the concentration of ISA in the cement pore water. The concentration of ISA in the pore water is essentially governed by the quantity of the cellulose present (cellulose loading), its degradation rate, and the cement porosity. Conservative estimates indicate that concentrations up to 0.1 M can be achieved, which would have severe consequences for the sorption of radionuclides (<em>9</em>). A few processes, however, can reduce the concentration of ISA in the pore water, i.e., the degradation of ISA, the formation of sparingly soluble ISA compounds, and/or the sorption of ISA on the cement. Stability experiments showed that ISA is stable under alkaline conditions, and consequently, a decrease of the ISA concentration by a degradation process is unlikely (<em>10</em>). The only sparingly soluble salt that can be formed in the repository and can potentially bound the upper limit of the ISA concentration is Ca(ISA)₂. On the basis of the solubility product of Ca(ISA)₂, a maximum concentration of ISA of ~5 × 10^–2 M could be calculated (<em>10</em>). The only process capable of significantly reducing the ISA concentration in the cement pore water is its sorption on the cement. No studies on the sorption of ISA are known in the literature. Consequently, we started an investigation on the sorption of ISA on cement under repository conditions, i.e., a (Na,K)OH solution saturated with respect to Ca(OH)₂ (Portlandite) and having a pH value of 13.3. In this study, we focus on measuring sorption isotherms for ISA at 25 °C and on determining the sorption capacity and the ISA – cement interaction constant.