Experimental Determination of Conditional Solubility Constants of Wyoming Bentonite (MX-80) at Elevated Temperatures

Abstract:

The Wyoming bentonite (MX-80) is the candidate buffer material in many nuclear waste programs in the world. When the buffer material is in contact with the groundwaters in various rock formations, it will be subject to dissolution. However, its solubility constants are not well known. In this study, we have experimentally determined its “conditional/empirical” solubility constants at elevated temperatures. By “conditional/empirical”, we mean the composite nature of the Wyoming bentonite. The Wyoming bentonite is predominantly composed of montmorillonite with minor amounts of impurities. We define this composite as a “conditional/empirical” thermodynamic entity.

In our experiments, the supporting solutions are NaCl solutions with the following concentrations: 0.10, 0.20 and 0.30 mol•kg^–1, and CaCl₂ solutions with the following concentrations: 0.040, 0.092 and 0.13 mol•kg^–1. The starting material is the Wyoming bentonite (MX-80) from the Source Clay Minerals Repository maintained by the Clay Minerals Society. The temperature in our experiments is at 60^oC. Our solubility experiments are considered as long-term experiments, as the experimental durations are at least 160 days. The solutions were sampled, and pH measurements were taken at the experimental temperature. The chemical compositions of the solution samples are determined by using various techniques including a Perkin Elmer Optima 8000 inductively coupled plasma atomic emission spectrometer (ICP-AES) and an ICP-MS. The “conditional” solubility constants for Wyoming bentonite at infinite dilution are calculated based on the solution chemistry including pH measurements according to the Al-Si thermodynamic model at elevated temperatures that we previously established, by using the computer code EQ3/6 Version 8.0a.

Acknowledgements: Sandia National Laboratories is a multi-mission laboratory operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. SAND2020-7313A. This research is funded by the SFWST programs administered by the Office of Nuclear Energy (NE) of the U.S. Department of Energy.