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Clay Barriers Restrict Ionic Transport

Source:  Carl Steefel and Dan Hawkes

Steefel_bourgHow do the properties of water and ions confined in clay nanopores differ from those of liquid water? The answers to this question have profound implications for the performance of waste storage facilities, CO2 geological sequestration sites, and other large-scale confinement applications for which clay materials are used or envisioned as effective engineered or natural barriers.

Industrial companies and public waste management agencies envision clay-rich materials as efficient barriers for large-scale confinement of nuclear waste and subsurface CO2. In clays, small pores hinder water flow and increase ion sorption, but also limit transport of negatively charged ions (anions) compared to those of water and cations, as a result of the negative charge on clay.

Enviro_coverIn a paper featured on the cover of the May 12, 2015 issue of Environmental Science & Technology Letters, a team of scientists (including ESD’s Carl Steefel and Ian Bourg) present experimental data that demonstrate, for the first time, that anions can be completely excluded from the smallest pores within a compacted illitic clay material, an observation that has important implications for the semi-permeable membrane properties of clays.  These special properties impact the ability to accurately predict the containment capacity of clay-based barriers.

In a series of multitracer diffusion experiments performed with the clay mineral illite, the precipitation of the mineral celestite (SrSO4) blocked the macropores within the clay, leaving only the negatively charged nanopores available for transport.  This was confirmed by experiments that showed that uncharged species could still migrate through the clay, while the chloride anion could not.  This result demonstrates that anions can be completely excluded from the smallest pores within a compacted clay material.

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On the cover:

Citation: Chagneau, A., C. Tournassat, C.I. Steefel, I.C. Bourg, S. Gaboreau, I. Esteve, T. Kupcik, F. Claret, and T. Schaefer (2015), Complete restriction of 36Cl– diffusion by celestite precipitation in densely compacted illite. Environmental Science & Technology, 2 (5), 139–143; DOI: 10.1021/acs.estlett.5b00080