Understanding the relation between osmotic pressure and structure of calcium montmorillonite

Purpose and goal:

Bentonite clay with high montmorillonite (Mt) content is proposed as a barrier material for nuclear waste disposal concepts in copper canisters. Mt is a layered mineral with negatively charged 10 Å thick lamellae, charge compensated by cations. These ions attract water to the space between the lamellae and thereby build up an osmotic pressure, which suppresses sulphate-reducing bacteria that might otherwise contribute to copper corrosion.

To enhance the understanding between the structure of water-saturated Mt and osmotic pressure, synchrotron light at MAX IV was used. Expected results and effects:

Depending on the water content, the expected (ideal) thickness of the water film between the Mt layers was 6-100 Å. Thanks to the fact that synchrotron SAXS made it possible to detect structures up to 1,000 Å, a unique opportunity was provided to unambiguously demonstrate structural differences between Na-, K- and Ca-Mt, establishing that at high water content Ca-Mt has water in both interlamellar and non-lamellar regions.

Differences in osmotic pressure were partially correlated to structural differences. The insights are important for the final repository of nuclear waste. Approach and implementation:

The CoSAXS beamline at MAX IV is a suitable research facility to perform small-angle X-ray scattering (SAXS) measurements in the q-range 5 10-3 q 1 Å-1 required for the detection of the expected montmorillonite structures, and to ensure robust information on how the structures vary depending on cation (Na, K, Ca) and water content. The measurements were performed on 8 Mt samples from osmotic pressure tests as well as 16 reference samples with different water content and different cations.