Prediction of diffusion coefficients in porous media using tortuosity factors based on interfacial areas

Determination of aqueous phase diffusion coefficients of solutes through porous media is essential for understanding and modeling contaminant transport. Prediction of diffusion coefficients in both saturated and unsaturated zones requires knowledge of tortuosity and constrictivity factors. No methods are available for the direct measurement of these factors, which are empirical in their definition. In this paper, a new definition for the tortuosity factor is proposed, as the real to ideal interfacial area ratio. We define the tortuosity factor for saturated porous media (tau5) as the ratio S/S(o) (specific surface of real porous medium to that of an idealized capillary bundle). For unsaturated media, tortuosity factor (tau(a)) is defined as a(aw)/a(aw),o (ratio of the specific air-water interfacial area of real and the corresponding idealized porous medium). This tortuosity factor is suitably measured using sorptive tracers (e.g., nitrogen adsorption method) for saturated media and interfacial tracers for unsaturated media. A model based on this new definition of tortuosity factors, termed the interfacial area ratio (IAR) model, is presented for the prediction of diffusion coefficients as a function of the degree of water saturation. Diffusion coefficients and diffusive resistances measured in a number of saturated and unsaturated granular porous media, for solutes in dilute aqueous solutions, agree well with the predictions of the IAR model. A comparison of permeability of saturated sands estimated based on tau(s) and the same based on the Kozeny-Carman equation confirm the usefulness of the tau(s) parameter as a measure of tortuosity.     

中温热液脉系中胶态金和二氧化硅的关系

后来结晶成石英的非晶质的硅胶（胶态）的最初沉淀，呆以很好地解释中温热液含金石英脉的一些结构特征，它可以生发于粘－滑破裂导致流体压力骤然下降的脆－韧性剪切带中，这个过程使热流体迅速过饱和由于有利的运动学条件，使非晶质的二氧化硅沉淀而不形成石英，减压作用一般也导致流体不混合以及可深的金的络合物不稳定，介是，胶态二氧化硅的存在可以稳定流体中的胶态金，并使微粒金呈悬浮状态作进一步的迁移，在中温热液条件下硅