## 5 Discussion
### Differences between Single- and Multi-Component Diffusion Approach
Comparing the \(K_{d}\) values of both concepts (MC\({}_{4 Clay}\) and MC\({}_{2 Clay}\), Table 3), it becomes clear that the contribution of chlorite and kaolinite to the uranium sorption is only about 10% in the carbonate-rich as well as in the shaly facies and about 30% in the sandy facies. Consequently, the majority of uranium is retained by illite and montmorillonite. From this point of view, one could say that illite and montmorillonite are sufficient to be considered to describe the sorption of uranium with a respective uncertainty for the shaly and carbonate-rich facies. However, this does not apply for the sandy facies as the contribution of chlorite and kaolinite to uranium sorption is in that case much larger with 30%. As already discussed by [PERSON] et al. [11], sorption in the sandy facies is only slightly larger than in the carbonate-rich due to the geochemistry of the porewater (e.g., pH and Ca\({}^{2+}\) concentration), although in the sandy facies are about 20 wt.% more clay minerals present. This also applies to our calculated \(K_{d}\) values of both facies (Table 3) and indicates that the contribution of kaolinite and chlorite to uranium sorption in the sandy facies is increased due to the porewater composition. Thus, we can confirm the findings of [PERSON] et al. [11] on the facies-dependence of uranium sorption in the Opalinus Clay and the governing role of the porewater geochemistry on sorption processes. Chlorite plays a geochemical key role for the diffusive uranium transport using the MC diffusion approach. Due to the determined point of zero charge (pzc) of 9.5 for the chlorite used here [47] and the corresponding SCM data for uranium [48], the mineral surface is positively charged in the pH range present in the facies of the Opalinus Clay as a result of the interaction between porewater and prevailing minerals for the applied pCO\({}_{2}\)[11]. This leads to an enrichment of anionic species, such as the predominant anionic, ternary uranyl complex, in the DDL to compensate the remaining net surface charge and thus the highest uranium concentrations could be observed in the DDL of chlorite. This in turn affects the migration behaviour of uranium as the accessible pore space is increased instead of decreased compared to the clay minerals with a negative surface charge. The contribution of chlorite to the total transport of uranium depends on the thickness of the DDL and thus on the ionic strength of the porewater. A lower ionic strength means a thicker DDL and in the case of the chlorite used here an enhanced diffusion of uranium instead of an intensified effect of anion exclusion as it would be the case for the clay minerals with negatively charged surfaces. Therefore, a farther uranium migration into the formation was observed in the sandy facies for the MC\({}_{4 Clay}\) simulation compared to SC and hence counteracts anion exclusion (Figure 3d). Without chlorite (MC\({}_{2 Clay}\), Figure 3d), the anion exclusion effect is fully effective. This emphasizes the important role of chlorite within the system, especially for the migration of uranium or other anionic radionuclides such as selenite in the sandy facies. However, the thermodynamic data used here are based on investigations of an almost pure chlorite sample taken from a granite fracture in the rock laboratory Grimsel (Switzerland) [47,48]. It serves for us as an analogue, because chlorite from the Opalinus Clay has not yet been studied in detail. It therefore remains unclear, whether chlorite from Grimsel and Mont Terri actually correspond in their chemical properties. For instance, a pzc \(<\) 7 would result in a negatively charged surface for the geochemical conditions and turn around the behaviour of the system. In this case, the determined migration lengths would correspond to the MC\({}_{2 Clay}\) scenarios. Accordingly, a detailed characterization of the clay minerals in the Opalinus Clay and of their individual properties is essential and indicates the need for further research.