Radiotracer studies on interaction processes related to humic-bound transport of radionuclides

Colloid-borne transport of actinides in aquifer systems is a topic of major interest in view of long-term risk assessments for underground radwaste repositories. In particular, complexation with aquatic humic substances can be decisive for the mobility of radiotoxic metals [1,2]. Depending on geochemical parameters, migration can be both enhanced and reduced. The respective conditions need to be identified, and models must be able to describe such complex systems by few parameters. According to the Linear Additive Model [3], total metal adsorption in the presence of humic matter can be calculated by linking parameters for the adsorption of both components and their interaction with each other. The basics of this approach are also implicit in advanced transport models [4].
In our study, the influence of humic acid on metal adsorption onto three clay materials (montmorillonite, illite, Opalinus clay) as a function of pH was investigated for Tb(III) as an analogue of trivalent actinides. 160Tb and 131I-labelled humic acid were used as radiotracers, allowing experiments at very low concentrations to mimic realistic conditions. For all clay materials under study, the presence of humic acid caused an increase in metal adsorption at neutral and acidic pH, i.e., metal desorption from clay barriers in consequence of acidification processes is generally counteracted in the presence of humic matter. Based on the pH-dependences of humic acid adsorption and Tb-humate complexation, this can be qualitatively explained by co-adsorption of humic-bound Tb. Quantitative estimates by means of the Linear Additive Model were, however, not successful.
In equilibrium models (Kd models), it is presumed that reaction rates for adsorption and desorption are both high enough to ensure a steady local equilibrium under flow conditions. Regarding the adsorption of humic substances onto geological materials, however, there is a lack of clarity concerning the dynamic character of this process. Recoveries in column experiments suggest a limited reversibility. In order to gain direct insight into the dynamics of the adsorption-desorption equilibrium, we conducted tracer exchange experiments with 14C-labelled humic acid. A negligible amount of the radiotracer was contacted with equilibrated systems of kaolinite and non-labelled humic acid for different periods of time. Tracer exchange at surface saturation provided evidence of a reversible process, but the time needed until the dynamic equilibrium was quantitatively represented by the tracer turned out to be much longer than the time needed to attain the overall adsorption equilibrium. This discrepancy between exchange kinetics and adsorption kinetics, which is indicative of a very slow desorption rate, has to be taken into consideration when the equilibrium condition is assigned to a maximum flow rate in transport systems.