Data for: HTO and Selenate Diffusion Through Compacted Na-, Na-Ca-, and Ca-Montmorillonite

Description

This dataset contains raw data from through-diffusion experiments. Through-diffusion experiments with tritiated water (HTO, conservative tracer) and Se(VI) were conducted with a well-characterized, purified montmorillonite source clay (SWy-2) under a constant ionic strength (0.1 M) and three different electrolyte compositions: Na+, Ca2+, and a Na+-Ca2+ mixture at pH 6.5 in order to probe the effects of electrolyte composition and interlayer cation composition on clay microstructure, Se(VI) aqueous speciation, and ultimately diffusion. Four data files (.csv) are provided, including: dd.csv (data dictionary which provides definitions for the column headers); SeVI_diffusion_data.csv (Se diffusion data); HTO_diffusion_data.csv (tritiated water (HTO) diffusion data); and Se_clay_profiles.csv (Se concentrations in clay profiles).

Steps to reproduce

Selenium diffusion experiments were conducted using a well-characterized montmorillonite source clay (SWy-2) obtained from the Clay Minerals Society. The montmorillonite was purified in order to remove minor impurities (quartz, feldspars, and calcite) and pre-equilibrated with the appropriate background electrolyte prior to packing in diffusion cells. Through-diffusion experiments were conducted at a dry bulk density of approximately 1.3 kg dm-3. Custom-built PEEK diffusion cells were used for experiments. The clay plug inside the diffusion cells had a diameter of 9.5 mm and a length of 5 mm. Filters on each end of the cell were made of PEEK with a PCTFE ring. The filters had an overall diameter of 0.95 cm, filter diameter of 0.74 cm, thickness of 0.16 cm, 5 µm pore size, and porosity of 0.26. Experiments were conducted under a single ionic strength (0.1 M) and three different electrolyte compositions: 0.1 M NaCl, 0.033 M CaCl2, and 0.085 M NaCl + 0.005 M CaCl2, representing pure Na+, pure Ca2+, and a Na+-Ca2+ mixture, respectively. All experiments were performed at room temperature (23°C) at pH 6.5. Diffusion experiments were conducted by circulating a high concentration reservoir (~200 mL) containing background electrolyte spiked with 1mM Se(VI) or 30 µCuries/L HTO at one end of the cell and a low concentration reservoir (5-20 mL) containing only background electrolyte at the other end. The low concentration reservoir was changed at regular time intervals and analyzed for HTO by liquid scintillation counting or Se by ICP-MS. At the end of the experiment, the clay plug was extruded, sliced, dried, and extracted with 0.5 M HCl, and the Se concentration in each slice was determined by ICP-MS.

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