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ISSN Druckformat: 1091-028X
ISSN Online: 1934-0508
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Solute Exclusionary Properties of Porous Shale Wafers
ABSTRAKT
Solute sieving properties of porous shale result from its membrane-like properties, which give rise to a significantly induced osmotic pressure across relatively thin wafers. Osmotic movement across shales can be observed as reverse or anomalous osmosis and may arise due to diffusion-induced electro-osmosis. Shale wafers of 0.95 and 1.50 mm thickness and 2.5 cm diameter were made from unexposed Mancos shale. The wafers were thin enough to allow rapid osmotic pressure buildup and thick enough to reduce pore connectivity. Wafers (≈ 1.00 mm thick) with trace amounts of kaolinite were found to display anomalously high selectivity coefficients that were between 33 and 95%. These high selectivity coefficients emphasized either a size-exclusion-based osmotic pressure-driven separation process, or perhaps reflected a maximal contribution of anion exclusion from trace argillaceous minerals within the matrix. This discrepancy may be explained, in part, by the effect of nonclay minerals on dry density, porosity, and/or compaction pressures post-diagenesis. The contribution of trace argillaceous minerals coupled with porosity-driven processes may also further enhance the ideality of Mancos shale. The osmotic pressure generated across the shale wafers was dependent on phenomenological coefficients like coefficient of osmotic flow, LPD. Values of LPD ranged from 2.60 × 10−8 to 4.06 × 10−8 m3/Ns for the wafers, whereas the mechanical filtration coefficient, Lp, values ranged from 4.27 × 10−8 to 6.64 × 10−8 m3/Ns. These were atypical values that were dependent on wafer thickness and osmotic efficiency.
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