Stability of bentonites in salt solutions | sodium chloride

Abstract Highly saline solutions are expected to play an important role in long term safety assessment of bentonite buffer for storage of radioactive waste. Montmorillonites being the main components of bentonites are suspected to undergo mineral alteration (particularly illitization or related mineral alteration processes) upon reaction with highly saline solutions. Literature data, however, do not provide a homogenous picture, yet. Possible alteration mechanisms, therefore, are still under discussion. In the present study 36 bentonites from different deposit were reacted in a closed system with a 6 M NaCl solution at 60 °C for 5 months, respectively. Run products were washed and dialysed and finally analyzed with respect to chemical and mineralogical changes by XRF, XRD, CEC, water uptake capacity, and amount of soluble silica. All significant chemical changes could be explained by the expected cation exchange (Na+ for Ca2+/Mg2+). After the experiment, however, the exchange sites were not completely occupied by Na+ despite a 100 fold excess of Na+ compared to the CEC. The presence of carbonates, obviously, interferes with the exchange of Na+ for Ca2+/Mg2+. The presence of gypsum proved to be even more effective and caused the opposite trend (Ca2+ was exchanged for Na+). The cation exchange buffer capacity of carbonates is believed to be particularly effective during dialysis, where the Na+ excess is significantly reduced while carbonate/gypsum dissolution proceeds. The changes of the XRD patterns, particularly the position, shape, and intensity (area) of the basal reflections, could not be attributed to mineral alteration but can be explained by cation exchange, differences of the hydration state, and/or different degrees of preferred orientation, the latter can be explained by particle arrangement. The results of the present study indicate that montmorillonites are stable in NaCl solutions of moderate pH up to 60 °C which is in agreement with most of the available studies. As expected there are no indications for illitization. In addition the study clearly shows the importance of using different chemical, mineralogical, and physical methods in order to unambiguously prove a process as illitization. Literature data indicate that temperatures of > 100 °C can lead to irreversible montmorillonite alteration. The same, of course, holds true for extreme pH values, drying, or the presence of K+. These alteration processes have to be considered specifically.