Determination of the Boom Clay size cut-off in synthetic seawater

Depending on the site and on the considered evolution scenario, saline conditions could prevail in Boom Clay (BC). In comparison to the present-day conditions found at the Mol site with a fresh-type pore water, the higher salinity could modify the physico-chemical properties of BC. In present day conditions, dissolved organic matter (DOM) is known to play an important role in the transport of radionuclides and it is important to evaluate to which extent this role is maintained at higher salinity. One of the key parameters controlling the transport of DOM colloids in low permeable media such as BC is the size. The size of the pores of BC and the size of the DOM species. In this work, percolation experiments were performed with BC samples pre-equilibrated with synthetic seawater. The transport of BC OM dissolved in synthetic seawater was followed over time and the evolution of the molecular weight/size distribution and concentration of DOM at the outlet of two BC cores was measured by a combination of size exclusion chromatography and UV spectrophotometry. The experimental results showed that DOM species larger than a log molecular weight of 3.65-3.80 and a diameter of~ 2.0-2.5 nm were trapped along their transport in BC and could be expected hardly mobile in synthetic seawater. This was a factor 2.6 to 3.8 smaller compared to what was determined for present day (“fresh”water) BC conditions and the decrease of mobility was assumed to be mainly due to DOM aggregation in the conditions imposed by synthetic seawater. The modelling of the experimental breakthroughs with the integration of a reaction rate accounting for physical or chemical irreversible immobilization, revealed that in saline conditions as already observed in present day conditions, the transport of DOM was size dependent. The effect of size was clearly visible for species with log MW > ~3 for which the retardation and irreversible immobilization increased with the size. Moreover, it was also observed that a significant fraction (from 25 to 75 %) of species with log MW < 3 was also lost during transport highlighting the reactivity and instability of BC DOM during transport in BC conditioned with synthetic seawater.