Studies on interactions and reactive transport of higher-valent metals in clay, considering organic carriers and high ionic strengths

Colloid-borne transport of radiotoxic metals in the subsurface hydrosphere is a topic of major interest in view of long-term risk assessments for nuclear waste repositories. In particular, interaction with natural organic complexants such as humic substances can be decisive for the mobility of higher-valent metals. 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, total metal adsorption in the presence of humic matter can be calculated by linking parameters for adsorption of both components and for their interaction with each other. The basics of this approach are also implicit in reactive transport models where humic carriers are considered. The applicability is, however, not unanimously accepted, and modelling results are in part not satisfactory.

Clay rock is being discussed as a potential host formation for a final repository, mainly because of its high adsorption capacity. This barrier function may, however, be subverted by charge screening or competition effects due to the high salt contents of pore waters. Complexation of radionuclides with humic-like clay organics could cause an additional mobilisation. So far, the effect of high ionic strengths on interactions of such organic carriers is unknown.

Within the framework of the planned PhD thesis, these processes will be investigated in detail, using radioactive tracers for interaction studies at high salt contents and for 3D imaging of transport processes by means of Positron Emission Tomography. The Linear Additive Model will be tested for static and dynamic systems, aiming at a specification of criteria for its applicability. To describe the effects of high salinities on a molecular level, mechanistic models for metal-humic interaction and surface complexation will be employed as well. An outline of this topic shall be presented at the meeting.