Dialysis Leaching Test Method for Measuring Solute Diffusion in Bentonites

Bentonite-based materials are used extensively in engineered containment barriers to limit leakage and advective flux of contaminants into the surrounding environment because of the high osmotic swell, low hydraulic conductivity (k), and absorption properties of the bentonite. Diffusion becomes an important, if not dominant, contaminant transport mechanism relative to advection in barriers with k < 10⁻⁹ m/s. However, measurement of bentonite diffusion coefficients often requires long test durations, limiting available data for modeling barrier performance. A new dialysis-leaching test (DLT) method was developed and refined for measuring diffusion properties of clayey materials. The method is relatively simple (e.g., can be performed independently by an undergraduate student), time efficient (<5-day duration), and may be applied for a range of clays/clay mixtures and temperatures. In addition to describing the DLT methodology, example results are provided herein for apparent diffusion coefficients measured for a range of bentonite-based barrier materials using potassium chloride and calcium chloride solutions. Experimental results were found to be reproducible between two independent laboratories and are compared with data measured with existing diffusion test methods.