Selectivity coefficient for Ca/Na ion exchange in highly compacted bentonite Ola Karnland ⇑ , Martin Birgersson, Magnus Hedström Clay Technology AB, IDEON Science Park, SE-223 70 Lund, Sweden article info Article history: Available online 16 August 2011 Keywords: Bentonite Ca/Na selectivity Ion exchange Montmorillonite abstract Bentonite clay is proposed as buffer material around the waste canisters and as tunnel backfill material in several concepts for disposal of radioactive waste. The distribution of charge compensating cations in the bentonite is of interest for several reasons, one being possible release of colloid particles from the ben- tonite to groundwater with very low ionic strength. The cation distribution at equilibrium may be calcu- lated for various relevant groundwater compositions by use of selectivity coefficients. However, present literature data generally concerns coefficients measured in batch experiments with high water-to-solid ratios. The basic aim with the present work was therefore to determine selectivity coefficients for sodium/calcium exchange in bentonite with low water-to-solid ratios, and thereby give a reliable base for calculating the cation distribution in a confined bentonite buffer with a relatively high density. In total, six tests with homo-ionic Na- and Ca-montmorillonite, prepared to three material densities, were equilibrated with test solutions of successively increasing concentration. The distribution of cations at equilibrium was measured by use of ion selective electrodes and ICP/AES, and selectivity coefficients were calculated according to the Gaines–Thomas convention. The obtained selectivity coefficient was found to be in the range of 3.8–7.8, which is similar to those previously reported for high water-to-solid ratios. Ó 2011 Elsevier Ltd. All rights reserved. 1. Background The most abundant natural occurring cations in Swedish groundwater are sodium and calcium ions. In the Swedish KBS-3 concept (SKB, 2011), the saturated density of the buffer material is defined to be between 1950 and 2050 kg/m 3 , corresponding to a dry density of about 1490–1650 kg/m 3 . The sealing properties are in principle the same for Na- and Ca-bentonites at this relatively high density range. At lower densities, however, the properties dif- fer quite substantially, with a more reduced sealing capacity in Ca- bentonites (Karnland et al., 2006). The type of dominating cation is of special interest for repositories of KBS-3 type, where the benton- ite is in direct contact with fractures in the crystalline rock. Na- bentonite may release colloidal montmorillonite particles into low salinity groundwater, which may prevail during a glaciation. Homo-ionic Ca-montmorillonite does not form sol but relatively small amounts of Na + (>20%) in exchange position, makes the mont- morillonite release colloidal particles in pure water, i.e. (Birgersson et al., 2009; Shainberg and Kaiserman, 1969; Hedström et al., 2011). A selectivity coefficient for Ca 2+ /Na + exchange is needed to cal- culate the distribution of these cations in the bentonite buffer for various relevant groundwater compositions. Several investigators have reported selectivity coefficients from batch experiments, i.e. in high water to solid ratios e.g. Bruggenwert and Kamphorst (1982), Sposito et al. (1983), Bradbury and Baeyens (2002) and Missana and García-Gutiérrez (2007). The conditions in compacted bentonite buffer with low water-to-solid ratio are, however, radi- cally different in many respects, e.g. the interlayer space is in the nanometer scale with strongly overlapping electrical double layers and the counter-ion concentration in the molar range, and it has been discussed if these conditions may affect the selectivity coeffi- cient. The basic aim with the present work was therefore to deter- mine selectivity coefficients for sodium/calcium exchange at low water-to-solid ratio, and thereby give a relevant basis for calculat- ing the long-term cation distribution in a KBS-3 bentonite buffer at various possible groundwater compositions. 2. Experimental Montmorillonite is the dominating mineral in commercial bent- onites, and the exchange capacity of montmorillonite is orders of magnitude larger than the exchange capacity of most accessory minerals in the bentonite. All tests in this study were therefore performed with homo-ionic Na- and Ca-montmorillonite in con- tact with Ca/Na-chloride solutions in order to avoid interference from dissolving accessory minerals. The Ca/Na exchange reaction in the montmorillonite can be written as: Ca 2þ ðaqÞþ 2Na þ ðclayÞ$ Ca 2þ ðclayÞþ 2Na þ ðaqÞ ð1Þ 1474-7065/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2011.07.023 ⇑ Corresponding author. E-mail address: ok@claytech.se (O. Karnland). Physics and Chemistry of the Earth 36 (2011) 1554–1558 Contents lists available at SciVerse ScienceDirect Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce