Technical Report NTB 01-08

Porewater chemistry in compacted re-saturated MX-80 bentonite:Physico-chemical characterisation and geochemical modelling

Bentonites of various types are being investigated in many countries as backfill materials in high-level radioactive waste disposal concepts. Being able to understand the chemistry of the porewater in compacted bentonite, and the factors which influence it, is critical to the synthesis of sorption data bases and to predicting radionuclide solubilities, and hence to repository safety studies. However, quantification of the water chemistry in compacted bentonite is difficult because reliable samples for chemical analysis cannot be obtained even by squeezing at exceedingly high pressures.

In this report concepts are developed which are somewhat different from those used in previously published works on bentonite porewater. Considerations of the swelling properties of montmorillonite led to the proposition that there were, generally speaking, three types of water associated with re-saturated compacted bentonite. The water defined as the porewater is only a small fraction of the total. The porewater volume present in re-saturated bentonites having different initial dry densities was quantified using CI- "through diffusion" data.

Highly compacted bentonite is considered to function as an efficient semi-permeable membrane so that re-saturation involves predominantly the movement of water molecules and not solute molecules. This implies that the composition of the external saturating aqueous phase is a second order effect. Consequently CI- concentrations in the porewater could be calculated from the deduced porewater volume values and the measured CI- inventory.

The pH of the porewater of a compacted bentonite is an extremely important parameter because of its influence on radionuclide solubility and sorption. Arguments are presented in support of the thesis that the initial pH is fixed by the high buffering capacity afforded by the amphoteric ≡SOH sites. The pH of the porewater depends directly on the speciation of these sites i.e. the proportions of sites present as ≡SOH, ≡SOH2+ and ≡SO-. In the report it is explained how this speciation is determined by the preparation process in the "as received" powder.

As a consequence of the high cation exchange capacity of montmorillonite, the large mass of montmorillonite in relation to the small porewater volumes in a highly compacted re-saturated bentonite, the major ion composition in the porewater will be controlled by the montmorillonite and the other solid phases present and will be very strongly buffered.

The above considerations are used in conjunction with detailed physico-chemical characterisation studies on MX-80 (Appendix) to calculate initial porewater compositions in compacted bentonites.

For the MX-80 material specified, the porewaters calculated for initial dry densities between 1200 and 1600 kg m-3 had relatively high ionic strengths (0.3 to 0.33 M), similar cation concentrations and a pH equal to 8.0. The porewaters changed from being Na2SO4 rich at 1200 kg m-3 to a NaCI/ Na2SO4 type water at 1600 kg m-3.