Review on cation exchange selectivity coefficients for MX-80 bentonite

Full text of publication follows: Bentonite is considered as engineered barrier in the near field of a nuclear waste repository due to its low permeability, what impedes groundwater flow to the nuclear waste, and its high retention capacity (sorption) of radionuclides in the eventuality of groundwater intrusion. One of the main retention processes occurring at the bentonite surface is ion exchange. This process may exert a strong control on the mobility of major pore water cations. Changes in major cation concentration, especially calcium, can affect the dissolution-precipitation of calcite, which in turn controls one of the key parameters in the system: pH. The cation exchange process is usually described according to the Gaines-Thomas convention: Ca2+ + 2 NaX = CaX2 + 2 Na+, KCa = (NCa x a2Na+)/(N2Na x aCa2+) where KCa is the selectivity coefficient for the Ca by Na exchange, ai is the activity of cation 'i' in solution and NJ the equivalent fractional occupancy of cation 'J' in bentonite. Parameters such as solid to liquid (S:L) ratio and dry density of the solid have an important influence on the value of selectivity coefficients (Kex). Although in most geochemical modelling works, Kex values are directly taken from experiments conducted at low S:L ratios and low dry densities, the expected conditions in a deep geological nuclear waste repository are higher S:L and higher bentonite density (1.6 g.cm-3 in the SKB design to obtain a fully water saturated density of around 2.0 g.cm-3). Experiments focused at obtaining selectivity coefficients under the conditions of interest face the difficulty of achieving a proper extraction and analyses of pore water without disturbing the system by the sampling method itself. In this work we have conducted a complete analyses of published data on MX-80 bentonite cationic exchange in order to assess the effect of the S:L ratio and dry density on the value of the selectivity coefficients determined so far. From this analyses, it can be concluded that the calculated selectivity coefficients for Ca and Mg (KCa and KMg) increase linearly with the S:L ratio, following an equation of the type Kex = A x (S:L) (g dm-3) for a fixed solid dry density. Besides, the value of Kex decreases when increasing the dry density of bentonite. These effects can be related to osmosis. Indeed, the higher the density, the lower the pore size in the interlaminar space. This may increase anion exclusion effects and, therefore, difficult cation diffusion given that the solution must be kept electrically neutral. It must be also highlighted that ionic strength effects can be relevant in any cation exchange process and, therefore, the influence of this parameter on the value of Kex must also be assessed. (authors)