Technical Report NTB 97-04

Experimental and Theoretical Studies on Alkaline Degradation of Cellulose and its Impact on the Sorption of Radionuclides

For more than ten years, cellulose degradation has been regarded as an important process which can adversely effect the sorption of radionuclides on cement in a radioactive waste repository. However, so far, it was not possible to quantify this effect. This study reports new experimental data on alkaline degradation of cellulose, together with a re-evaluation of old literature data. For the first time now, it becomes possible to quantitatively estimate the potential role of cellulose degradation in performance assessment studies.

In the first part of this study (chapters 1-4), a literature overview of other studies on alkaline degradation of cellulose is given, together with a general discussion on the effect of organic ligands on the sorption of radionuclides. Further, an overview of the important mechanisms of alkaline degradation of cellulose and some kinetic aspects of the main reactions taking place is presented. The relevance of the processes for performance assessment is explained in detail. The discussion forms the starting-point for a detailed experimental program for evaluating the role of alkaline degradation of cellulose in performance assessment.

In the second part (chapters 5-8), experimental studies on alkaline degradation are presented. Different cellulosic materials were degraded in an artificial cement pore water, representing the first stage of cement degradation. The most important degradation products (α- and β-isosaccharinic acid) were characterised and the results compared with other studies. Kinetic parameters for the main reactions were measured and discussed. A good agreement was found between the measured values and values extrapolated from the literature. The solubility of the sparingly soluble Ca-salt of α-isosaccharinic acid (ISA) was studied as well as the interaction of ISA with cement. Sorption of ISA on cement can keep the ISA concentration in the pore water of a repository at a low level. The effect of pure ISA and degradation products on the sorption of radionuclides on feldspar at pH 13.3 was investigated. It was shown that both α-isosaccharinic acid and cellulose degradation products have an adverse effect on the sorption of Eu-152, Th-234 and Ni-63. The effect observed for degradation products could be satisfactorily explained by the presence of α-ISA. For Eu(lIl) and Th(IV), the concentration of α-ISA must be larger than 10-4-10-3 M in order to significantly reduce sorption. For Ni(II), the concentration has to be larger than 10-2 M. β-ISA was shown to affect the sorption of Eu(III) at concentrations >10-2 M only.

The last part of the report (chapter 9) deals with the application of the theoretical and experimental studies in performance assessment. It shows how a combination of the different parameters and processes discussed (cellulose loading, kinetics of degradation, sorption of ISA, sorption of metals, etc.) can be used in performance assessment studies to quantitatively evaluate the role of alkaline cellulose degradation on radionuclide mobility.