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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.

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There is a critical need for highly sensitive, cost-effective sensors to conduct ecological analyses for environmental and homeland security-related applications. Enzyme biosensors, which are currently gaining acceptance for environmental monitoring applications, need improvements to deliver faster measurements with stabilized sensing elements, e.g., enzymes. We report here on a method which significantly overcomes this difficulty, and demonstrate its application in a biosensor for aquatic environmental applications. A fast-responding and stable biosensor was developed via immobilization of organophosphorus hydrolase (OPH) in functionalized mesoporous silica (FMS) with pore sizes in tens of nanometers. The OPH-FMS composite was held on glassy carbon electrode by a dried Nafion gel and FMS protected OPH from Nafion-resulted activity loss. The resulting enzyme biosensor, when integrated with an electrochemical instrument, responded rapidly to low paraoxon concentration and achieved steady-state current in less than 10 s, with a detection limit of 4.0x10(-7) M paraoxon. The biosensor was tested for detection of paraoxon in simulated environmental samples, under wide-ranging physicochemical conditions. Results clearly indicate high recovery efficiencies in aqueous solutions (96 to 101%) at different pH, total organic carbon, total dissolved solids, and total suspended solids, and demonstrate the ability of the biosensor unit to continuously monitor paraoxon in aqueous conditions similar to those found in river and lake systems.

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A new variable tortuosity definition is introduced that is based on the immiscible fluid (air–water) interfacial area. Unsaturated media tortuosity (τa) is defined as the ratio of aaw to aaw,o where aaw is the estimated air–water interfacial area in a real unsaturated medium (i.e., a soil sample), and aaw,o is the same variable for the corresponding, idealized capillary bundle. The air–water interfacial area for both real and idealized media is directly proportional to the area under their respective retention curves. With τ being the saturated tortuosity, we relate the variable tortuosity ratio (τ/τa) to the Seε term in Mualem's (ε = 0.5) and Burdine's (ε = 2) pore‐size distribution models. Thus, instead of using tortuosity and pore connectivity formulations, which have empirical exponents of either 0.5 or 2, the new model depends on a variable interfacial area for varying saturation and soil texture, as reflected in the measured retention data. We tested the new definition of tortuosity to predict unsaturated hydraulic conductivity, K, as a function of volumetric moisture content, θ, for 22 repacked Hanford sediments that are comprised of mostly coarse and fine sands but some also contain a sizeable fraction (as high as 27%) of fines (silt and clay). Replacing the Seε term in van Genuchten–Mualem (VGM) model by the tortuosity ratio τ/τa, and still using saturated hydraulic conductivity and moisture retention parameters as used in the conventional approach, we obtained τa‐based K(θ) predictions that are nearly identical to the conventional VGM model predictions. We also compared the τa‐based K(θ) predictions with the standard Brooks–Corey–Burdine (BCB) model predictions. In comparison to the VGM model predictions, τa‐based BCB K(θ) predictions appear to be less biased relative to the measured K for the coarse‐textured samples.