Technical Report NTB 08-10
Chemical reactivity of alpha-isosaccharinic acid in heterogeneous alkaline systems
Cellulose degradation under alkaline conditions is of relevance for the mobility of many radionuclides in the near-field of a cementitious repository for radioactive waste, because metal-binding degradation products may be formed. Among these α-isosaccharinic acid (α-ISA) is the strongest complexant. The prediction of the equilibrium concentration of α-ISA in cement pore water is therefore an important step in the assessment of the influence of cellulose degradation products on the speciation of radionuclides in such environments.
The present report focuses on possible chemical transformation reactions of α-ISA in heterogeneous alkaline model systems containing either Ca(OH)2 or crushed hardened cement paste. The transformation reactions were monitored by measuring the concentration of α-ISA by high performance anion exchange chromatography and the formation of reaction products by high performance ion exclusion chromatography. The overall loss of organic species from solution was monitored by measuring the concentration of non-purgeable organic carbon. The reactions were examined in diluted and compacted suspensions, either at 25 °C or 90 °C, and under anaerobic atmospheres obtained by various methods. It was found that α-ISA was transformed under all conditions tested to some extent. Reaction products, such as glycolate, formate, lactate and acetate, all compounds with less complexing strength than α-ISA, were detected. The amount of reaction products identified by the chromatographic technique applied was ~50 % of the amount of α-ISA reacted. Sorption of α-ISA to Ca(OH)2 contributed only to a minor extent to the loss of α-ISA from the solution phase.
As the most important conclusion of the present work it was demonstrated that the presence of oxidising agents had a distinctive influence on the turnover of α-ISA. Under aerobic conditions α-ISA was quantitatively converted to reaction products, whereas under strict anaerobic conditions, only small amounts of α-ISA were transformed. It can be hypothesised that, under these conditions, either traces of oxygen remaining bound to Ca(OH)2 or unidentified impurities in Ca(OH)2 were responsible for the reactions observed. The involvement of microbially mediated processes can be excluded, because the reactions proceeded in a similar qualitative manner, however faster, at 90 °C than at room temperature.
The possible chemical degradation of α-ISA to organic compounds with less complexation capabilities under anaerobic repository conditions is therefore not supported by the experimental findings of the present study.
