Cellulose degradation products (CDPs) can complex with radioelements causing solubility enhancement and sorption reduction, effects which are detrimental to the containment of radionuclides in the near field of a geological disposal facility and surrounding geosphere. Isosaccharinic acid (ISA) is the principal component of CDPs formed under the alkaline anaerobic conditions of a cement-based near field and appears to be largely responsible for the impact of CDPs on radionuclide solubility and sorption under near-field conditions. However, the situation appears to be more complicated under near-neutral pH geosphere conditions.
A combined experimental and modelling study was undertaken to compare the impact of a CDP leachate to ISA in a simple model ternary sorption system consisting of hematite as a single mineral substrate, thorium as the radioelement and ISA or a CDP leachate as the complexant. Thorium sorbs strongly to hematite. A triple layer model for thorium sorption to hematite was refined to fit to the experimental data in the absence of ISA or CDP leachate; the effect of ISA on thorium sorption was then predicted.
In the presence of CDP leachate, a significant reduction in thorium sorption was observed from pH 6 to 12 in good agreement with model predictions based on a high concentration of ISA. However, only a limited impact of ISA on thorium sorption was observed at pH 6 to 12, in contrast to predictions. The effects of ISA could be accounted for by assuming the formation of a ternary thorium–ISA–surface complex. The model has yet to be extended to the more complex CDP systems. Differences in the thorium speciation in solution due to the formation of a ternary calcium–thorium–ISA complex in the CDP leachate, which is absent from solutions with ISA only, provides the most likely explanation for the differences observed experimentally.