Sorption and diffusion of organic acids through clayrock: Comparison with inorganic anions

- Transport of a family of 14C-labelled organic acids was studied in COx clayrock.
- Sorption and diffusion of EDTA, ISA, Phthalate and oxalate were quantified.
- Small organic anions display higher affinity for clayrock than inorganic anions.
- Sorption is significantly higher in batch than in diffusion experiments.
- Effective diffusion of acids is mainly driven in clayrock by anionic exclusion and size effects.

SummaryOrganic complexing species are known to affect radionuclide mobility in the environment. The migration behaviour of several organic ligands was evaluated in the context of a proposed French radioactive waste repository in the Callovo-Oxfordian clayrock formation (COx). This study focuses on four anthropogenic acids (ethylenediaminetetraacetate, isosaccharinate, phthalate, oxalate) that are used in the nuclear fuel cycle or that occur as hydrosoluble degradation products of waste materials. Batch sorption and diffusion experiments were performed with COx clayrock samples using 14C-labelled radiotracers. The observed effective diffusion coefficients were low (De ∼ 1-6 × 10−12 m2 s−1), an order of magnitude lower than that of tritiated water in the same material, and roughly the same as values for inorganic anions such as I−, Cl− and SO42-. The observed correlation of De with molecular mass, M−1/3, differs significantly from that observed for cations. The organic ligands displayed significant affinity for the COx clayrock, with distribution ratios measured in batch experiments, Rd = 1-30 L kg−1, which are much higher than usually observed for anionic species. While this result was confirmed by through-diffusion experiments, the Kd values obtained by fitting diffusion modelling were significantly lower than those measured in the batch experiments.