U(VI) adsorption on mudstone in the presence of carbonate: Batch experiments, modeling, cryo-TRLFS and PARAFAC analysis

Dissolved inorganic carbon (DIC) present in groundwaters near geological disposal site may affect the migration and fate of released radionuclides (e.g., uranium). However, it is still unclear how DIC affects the sorption of U(VI) on mudstone, a potential choice for host rock formation barriers, and the characteristics of U(VI) adsorption species. In view of this, adsorption of U(VI) on mudstone as a function of pH or the DIC level was investigated through batch adsorption experiments, component additivity (CA) modeling, and parallel factor (PARAFAC) analysis of cryogenic time-resolved laser fluorescence spectroscopy (cryo-TRLFS). The inhibition effect of DIC on adsorption of U(VI) was deduced from the batch experimental results. The CA model considering montmorillonite and illite as the main phases responsible for adsorption reflected the variation trend but underestimated the experimental data. Three components were decomposed from the PARAFAC analysis of the cryo-TRLFS data of mudstone and adsorption samples. Based on the spectral shapes, lifetimes, and variation trends, two components were assigned to the naturally occurring U(VI) species in the mudstone matrix and the third component to a ternary uranyl‑carbonate surface species. The obtained results imply a significant influence of the DIC level on retention and fate of U(VI), which needs to be considered in safety assessment of potential deep geological disposal schemes.