BackgroundIn geochemically perturbed systems where porewater and mineral assemblages are unequilibrated the processes of mineral precipitation and dissolution may change important transport properties such as porosity and pore diffusion coefficients. These reactions might alter the sealing capabilities of the rock by complete pore-scale precipitation (cementation) of the system or by opening new migration pathways through mineral dissolution. In actual 1D continuum reactive transport codes the coupling of transport and porosity is generally accomplished through the empirical Archie’s law. There is very little reported data on systems with changing porosity under well controlled conditions to constrain model input parameters. In this study celestite (SrSO4) was precipitated in the pore space of a compacted sand column under diffusion controlled conditions and the effect on the fluid migration properties was investigated by means of three complementary experimental approaches: (1) tritiated water (HTO) tracer through diffusion, (2) computed micro-tomography (µ-CT) imaging and (3) post-mortem analysis of the precipitate (selective dissolution, SEM/EDX).ResultsThe through-diffusion experiments reached steady state after 15 days, at which point celestite precipitation ceased and the non-reactive HTO flux became constant. The pore space in the precipitation zone remained fully connected using a 6 µm µ-CT spatial resolution with 25 % porosity reduction in the approx. 0.35 mm thick dense precipitation zone. The porosity and transport parameters prior to pore-scale precipitation were in good agreement with a porosity of 0.42 ± 0.09 (HTO) and 0.40 ± 0.03 (µ-CT), as was the mass of SrSO4 precipitate estimated by µ-CT at 25 ± 5 mg and selective dissolution 21.7 ± 0.4 mg, respectively. However, using this data as input parameters the 1D single continuum reactive transport model was not able to accurately reproduce both the celestite precipitation front and the remaining connected porosity. The model assumed there was a direct linkage of porosity to the effective diffusivity using only one cementation value over the whole porosity range of the system investigated.ConclusionsThe 1D single continuous model either underestimated the remaining connected porosity in the precipitation zone, or overestimated the amount of precipitate. These findings support the need to implement a modified, extended Archie’s law to the reactive transport model and show that pore-scale precipitation transforms a system (following Archie’s simple power law with only micropores present) towards a system similar to clays with micro- and nanoporosity.Graphical abstract:Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-015-0027-z) contains supplementary material, which is available to authorized users.
Industrial companies and public waste management agencies envision clay-rich materials as efficient barriers for large-scale confinement of nuclear waste and subsurface CO 2 . In clays, small pores hinder water flow and make diffusion the dominant solute-transport mechanism. Most clay mineral structures exhibit a negative charge that is balanced by an electrical double layer at the mineral water interface. This clay mineral property delays cation migration through adsorption processes, decreases the accessible porosity and diffusion fluxes for anions compared to those of water and cations, and gives rise to semipermeable membrane properties. Here we 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 ability to accurately predict the containment capacity of clay-based barriers. In a series of multitracer diffusion experiments, celestite (SrSO 4 ) precipitation reduced the porosity of compacted illite to the point where the water tracer diffusion flux decreased by half, while the chloride diffusion flux decreased to zero. This result demonstrates that anions can be completely excluded from the smallest pores within a compacted clay material. ■ INTRODUCTIONHow do the properties of water and ions confined in clay nanopores differ from those of bulk liquid water? Answers to this question have profound implications in performance predictions for waste storage facilities, CO 2 geological sequestration sites, and other large-scale confinement applications for which clay materials are used or envisioned as effective engineered or natural barriers. 1−4 For these applications, the main clay minerals in targeted indurated clayey rocks are smectite, illite, and mixed layer illite/smectite. 5−7 These minerals exhibit a small particle size and a large aspect ratio because of their layered structure (each phyllosilicate layer, TOT layer, is ∼1 nm thick and up to 2 μm wide; clay mineral particles consist of 1−20 stacked TOT layers) and are among the natural minerals with the largest specific surface areas (∼750 and ∼100 m 2 g −1 for smectite and illite, respectively). Because of this large specific surface area, clayey media have small median pore sizes (on the order of 10 0 −10 2 nm), very low hydraulic conductivities (∼10 6 times lower than that of sandstone), and macroscopic properties that are strongly influenced by physical and chemical processes taking place at clay mineral surfaces. 8 In particular, molecular diffusion is the main mass transfer mechanism in clayey media under most conditions. In addition, TOT layers bear a negative structural charge that is compensated by cation accumulation and anion depletion near their surfaces in a region known as the electrical double layer (EDL). This property gives clay materials their semipermeable membrane properties: ion transport in the clay material is hindered by electrostatic repulsion of anions from the EDL...
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