The bentonite-based engineered bar rier or packing is a proposed component of several designs conceived to dispose of high-level nuclear waste in geologic reposi tories. Once radionulcides escape the waste package, they must first diffuse through the highly impermeable day-rich barrier before they reach the host repository. To deter mine the effectiveness of the packing as a sorption barrier in the transient release period and as a mass-transfer barrier in the steady release period over the geologic time scales involved in nuclear waste disposal, a fundamental understanding of the diffusion of electrolytes in compacted clays is required. We present, and compare with laboratory data, a model quantifying the diffusion rates of cationic cesium and uncharged tritium in compacted mont morillonite clay. Neutral tritium character izes the geometry (Le., tortuosity) of the particulate gel. After accounting for cation exchange, we find that surface diffusion is the-dominant mechanism of cation tran sport, with an approximate surface diffusion coefficient of 2xl0~6 cm 2 /s for cesium. This value increases slightly with increasing background ionic strength.The implications of this work for the packing as a migration barrier are twofold. During the transient release period, K4 values are of little importance in retarding ion migration. Tbii is because sorption also gives rise to a surface diffusion path, and it is surface diffusion which controls the diffusion rate of highly sorbing cations in compacted montmorillonite. During the steady release period, the presence of sur face diffusion leads to a flux through the packing which is greatly enhanced. In either case, if surface diffusion is neglected, the appropriate diffusion coefficient of ions in compacted packing will be in considerable error relative to current design recommen dations. IntroductionHigh-level nuclear wastes will be stared in underground repositories. To aid the host formation in protecting the bio sphere from the harmful, long-lived radioisotopes contained in the waste pack age, use of an engineered barrier or packing is being considered [6]. This barrier, based upon the day mineral montmorillonite, is envisaged as an impermeable barrier sur rounding the waste canisters which retards the migration of ions in the transient release period due to its high cation exchange capadty [3] and low hydraulic permeability [7]. As time passes and steady-state diffusion through the packing is established, it further serves as a masstransfer resistance retarding the escape of ions from the canister to the fluid percolat ing around the waste package. To deter mine the steady-state plume of radionu clides eminating from the waste repository in the steady-state period and the protec tion an engineered barrier affords the waste disposal scheme in the transient release period, the diffusion rate of ions in com pacted montmorillonite clay must be deter mined.
Nucleic acid aptamers have a number of advantages compared to antibodies, including greater ease of production and increased thermal stability. We hypothesized that aptamers may also be capable of functioning in the presence of high concentrations of surfactants, which readily denature antibodies and other protein-based affinity reagents. Here we report the first systematic investigation into the compatibility of DNA aptamers with surfactants. We find that neutral and anionic surfactants have only a minor impact on the ability of aptamers to fold and bind hydrophilic target molecules. Additionally, we demonstrate that surfactants can be utilized to modulate the substrate binding preferences of aptamers, likely due to the sequestration of hydrophobic target molecules within micelles. The compatibility of aptamers with commonly used surfactants is anticipated to expand their scope of potential applications, and the ability to modulate the substrate binding preferences of aptamers using a simple additive provides a novel route to increasing their selectivity in analytical applications.
A unique radially perfused cell designed specifically to measure the transient diffusion of species in compacted, highly adsorbing and swelling porous media is described in this paper. The perfused cell exhibits extremely high mass-transfer coefficients while resisting large swelling pressures. It also allows a simultaneous determination of the effective diffusion and sorption retardation coefficients of species from a single experiment. The concentration boundary layer resulting from the radial flow configuration is sufficiently uniform that one-dimensional, axial diffusion is studied in the sample medium. Over the range of Peclet numbers studied, from 103 to 3 X 105, overall Nusselt numbers range from 70 to 550, in excellent agreement with a theoretical convective dispersion prediction containing no adjustable parameters. Use of the cell is illustrated by an experiment in which cesium chloride diffuses into a moderately compacted sodium montmorillonite clay gel. We find that surface diffusion controls the migration of cesium in these clays.
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