Ion Diffusion Within Water Films in Unsaturated Porous Media

Journal Article · Tue Mar 28 00:00:00 EDT 2017 · Environmental Science and Technology

DOI:https://doi.org/10.1021/acs.est.6b05891· OSTI ID:1508044

Tokunaga, Tetsu K. ^[1]; Finsterle, Stefan ^[2]; Kim, Yongman ^[2]; Wan, Jiamin ^[2]; Lanzirotti, Antonio ^[3]; Newville, Matthew ^[3]

---

1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); The University of Chicago
2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
3. Univ. of Chicago, Chicago, IL (United States)

Diffusion is integral in controlling local solute transport and reactions in unsaturated soils and geologic formations. While it is commonly assumed that thinning of water films controls solute diffusion at low water contents, transport under these conditions is not well understood. These experiments conducted in quartz sands at low volumetric water contents (θ) were used to quantify ion diffusion within adsorbed films. At the lowest water contents, fixed relative humidities were employed to control water films at nm thicknesses. Diffusion profiles for Rb+ and Br- in unsaturated sand packs were calculated with a synchrotron X-ray microprobe, and inverse modeling was used to determine effective diffusion coefficients, D_e, as low as ~9 × 10^-15 m² s^-1 at θ = 1.0 × 10^-4 m³ m^-3, where the film thickness = 0.9 nm. Given that the diffusion coefficients (D_o) of Rb⁺ and Br^- in bulk water (30 °C) are both ~2.4 × 10^-9 m2 s^-1, we found the impedance factor f = D_e/ (θD_o) is equal to 0.03 ± 0.02 at this very low saturation, in agreement with the predicted influence of interface tortuosity (τ_a) for diffusion along grain surfaces. Thus, reduced cross-sectional area (θ) and tortuosity largely accounted for the more than 5 orders of magnitude decrease in D_e relative to D_o as desaturation progressed down to nanoscale films.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); Univ. of Chicago, Chicago, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
• Grant/Contract Number: AC02-05CH11231; FG02-94ER14466
• OSTI ID: 1508044
• Alternate ID(s): OSTI ID: 1532245; OSTI ID: 1355036
• Report Number(s): DOE-UCHICAGO-14466--8
• Journal Information: Environmental Science and Technology, Journal Name: Environmental Science and Technology Journal Issue: 8 Vol. 51; ISSN 0013-936X
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES