Alkaline Degradation of Cellulose: Estimation of the Concentration of Isosaccharinic Acid in Cement Porewater

Abstract

ABSTRACTThe sorption of radionuclides on repository components (e.g. cement) is an important process since it controls the release of radionuclides from the repository [1]. A strong sorption of radionuclides is desirable since it will allow only a small release of radionuclides to the geo-and biosphere. The strong sorption behaviour of radionuclides, however, could possibly be decreased by several orders of magnitude by the presence of organic ligands. Ligands such as EDTA, NTA, citric acid etc. are inherent components of radioactive waste since these complexing agents are used in nuclear power stations for decontaminating purposes. Other ligands might be formed by degrading organic polymers present in low and intermediate level radioactive waste [2, 3]. Cellulose materials such as cotton, paper and wood form a substantial part (ca. 50 %) of the organic waste [1]. The use of large amounts of cement for constructing a repository causes alkaline environments in which the pH of the pore solution will remain above 12.5 for periods of the order of 105years [4]. It is well known from the literature that cellulose is unstable under alkaline conditions and will degrade to water soluble, low molecular weight compounds by the peeling-off reaction [5]. The main degradation product of cellulose is isosaccharinic acid (ISA), which is stable under alkaline conditions [5-10]. ISA enhances the solubility of Pu(IV) [7, 8] and has an adverse effect on the sorption of Eu(III), Th(IV) and Ni(II) [6]. For instance, in a solution of 10-3M ISA, the solubility of Pu(IV) at pH 12 increases by a factor of 20000 [11]. The sorption of Pu(IV) [8], Eu(III), Th(IV) and Ni(II) [6], however, was affected to only a minor extent. The observed effects were - by analogy with gluconic acid [12] - interpreted to be due to a strong complexation of these metals. The concentration of ISA in the pore water is the key parameter for evaluating its effect on radionuclide sorption [13].A full assessment of the effect of cellulose degradation on the sorption requires a detailed understanding of the mechanisms involved. The present study gives an overview of the different processes involved and describes how to quantify the concentration of isosaccharinic acid in the pore water of a repository.