DOI: 10.5061/dryad.x69p8czrt
title: Data from: Understanding the limits of binary diffusion for enhanced clay barrier design

description: Waste containment and isolation strategies often utilize bentonite as a
 buffer material due to its swelling capacity, sealing efficiency, low
 permeability and limited diffusive transport. However, previous
 experimental studies of ionic diffusion through bentonite have shown
 discrepancies with binary diffusion assumptions. Meticulous experiments
 and complementary analyses reveal that the migration of preexisting ions
 in the medium enables the differential flux of diffusing anions and
 cations, while maintaining local electro-neutrality in all cases. The
 separation between the cationic and anionic fronts is electrically tied to
 the motion of the preexisting ions, and reflects the interplay between
 valence, concentration, and self-diffusion coefficients of the ions
 involved. Imposing binary diffusion conditions forces the faster anions to
 diffuse at the same rate as cations. Therefore, effective barriers to
 mitigate both cation and anion transport should have low surface charge
 and low excess salts to minimize the preexisting ionic concentration.
description: # Data from: Understanding the limits of binary diffusion for enhanced
 clay barrier design
 [https://doi.org/10.5061/dryad.x69p8czrt](https://doi.org/10.5061/dryad.x69p8czrt) ## Description of the data and file structure Data describes 1-D diffusion of bentonite dosed with different salts. Two identical cylindrical molds of bentonite dosed samples are put in contact and diffusion is allowed to occur. After testing the samples are sampled at 0.5 mm intervals to measure the concentration of the diffusing ions. The exact testing conditions of each sample are on the top left of each tab. For more information on testing condition and apparatus see [https://doi.org/10.1093/pnasnexus/pgae366](https://doi.org/10.1093/pnasnexus/pgae366) Raw data are the % of the target elements measured directly from XRF at each location and testing condition. Water contents of each sample are measured by weighing the dry and wet weights of the samples before and after drying in an oven at 105 C over 24 hrs. ***Detailed Procedure*** Diffusion experiments using two contiguous cylindrical cells (d= 50 mm, h= 20 mm). Tests are designed to focus on differential diffusion rates. Bentonite with elemental mass fraction: 77% SiO2, 13% Al2O3, 3.8% Fe2O3, 1.4% MgO, 1.5% Na2O, 1.3% CaO, 0.5% K2O, 0.2% SO3 is used for all tests except 1 (CsCl kaolin) in which kaolinite with mass fraction: 67% SiO2, 29% Al2O3, 1.5% TiO2, 0.9% Fe2O3, 0.2% K2O, 0.07% CaO and 0.06% SO3 was used. Clay pastes are prepared by thoroughly mixing dry clay powder with the selected solution at 70% water content by weight. This results in an average dry density of 1.0 ± 0.08 g/cm³, an average degree of saturation of 97%, and an average porosity of 0.62 for the bentonite samples. Kaolin test (Test 9) has a dry density of 0.99 g/cm3, degree of saturation of 99%, and porosity of 0.62. The 70% water content is chosen to ensure that the bentonite paste prepared with high salinity solutions (up to 1M) does not exceed the Liquid Limit (LL), while still allowing sufficient homogeneous mixing of the bentonite with deionized water (Tests 1-4). The selected test conditions result in similar mineral "dry" densities across all specimens, help preserve the clay fabric during the test, and prevent fluid leakage. The clay powder and target solution are thoroughly mixed within a sealed bag to avoid moisture loss. The mixture is then used to fill cylinders, and pressed to reach the target density (the resulting specimens can be deformed with hand pressure). Each cylinder is individually sealed to prevent evaporation and left to homogenize at room temperature for 72 hours. After homogenization, two distinct cylinders are aligned and brought together; the tested clays are water-wet and displace the non-wetting air upon approach; a light load presses specimens together to ensure an adequate contact. Data repeatability and the evolving continuity of concentration profiles suggest that proper contact prevailed in all tests. Diffusion is allowed to progress at 20°C for 8 to 24 hours, ensuring that the diffusing ions remain within approximately 15 mm from the interface and away from the end boundaries. After the allocated diffusion time, we sample specimens by successively “shaving” 0.5 mm thick layers normal to the direction of diffusion, and analyze all samples using Wavelength Dispersive X-Ray Fluorescence (WD-XRF Bruker S8 Tiger Series 2). **Abbreviation** DI = Deionized water mixed with bentonite W/C = Water Content "Cation" "Anion" (i.e. CsCl) = salt composition used in tests ***Tab Information*** KCl-DI = Potassium Chloride dosed bentonite on left, Deionized water mixed bentonite on right (same for tabs 1-8 with different elements) CsCl kaolin = Cesium Chloride on left, DI on right, Soil is Kaolin not bentonite "Number" "Salt" (i.e. 0.175 Na2SO4) = the Number is the molality of the salt in the solution prior to mixing with the soil added to both the left and the right sides of the diffusion. Diffusing ions are in cell A1 (i.e. CsCl, Na2SO4 - Na2SO4) = Diffusing ions CsCl on the left with equal amounts of Na2SO4 added on both the left and the right