Electroosmosis, scale

Most laboratory experiments demonstrating the utility of EO transport of organic compounds were conducted with kaolinite as the model clay-rich soil medium. Shapiro et al. (1989) used EO to transport phenol in kaolinite. Bruell et al. (1992) have shown that TCE can be transported down a slurry column by electroosmotic fluid flow, and more recently, Ho et al. (1995) demonstrated electroosmotic movement of p-nitrophenol in kaolinite. Kaolinite is a pure clay mineral, which has a very low cation exchange capacity and is generally a minor component of the silicate clay mineral fraction present in most natural soils. It is not, therefore, representative of most natural soil types, particularly those which are common in the midwestem United States. The clay content can impact the optimization and effectiveness of electroosmosis in field-scale applications, as has recently been discussed by Chen et al. (1999). 

Maini et al (2000) attempted to remediate historically contaminated soil from a former gasworks site in East London, UK. The organic content of the soil was not reported but the soil contained a range of heavy metals, PAHs, and BTEX. Although no significant metal removal was observed, PAHs were effectively (over 90%) removed by electroosmosis, with or without surfactant, in both small- and large-scale reactor studies. This shows that electrokinetic soil flushing shows promise for field application. 

In 1996, as part of the Superfund Innovative Technology Evaluation (SITE) Program, the United States Environmental Protection Agency (USEPA) demonstrated the ISEE system at the SNL chemical waste landfill site in Albuquerque, New Mexico (USEPA, 1998). The ISEE system was developed by SNL for removing hexavalent chromium from unsaturated soil. This was a cutting edge, since most of the laboratory-scale studies were carried out in saturated soil samples. In a saturated sample, the contact of the interstitial fluid with the solid particles is more effective and aids in the extraction and transportation of pollutants. The two primary transport mechanisms in electrokinetics (electromigration and electroosmosis) require a liquid medium (water), but in the unsaturated soil zone, the lack of water in the interstices makes the solubilization and transportation of the heavy metals precipitated or adsorbed on the solid particles surface more difficult. 

We consider the velocity field bounded by a cylindrical slip surface, which excludes a thin electric double layer (EDL) [16]. The velocity scale Ut includes the effects of local pressure gradients, electroosmosis, and electrophoresis and can be expressed as follows ... 

Pee is the electric Peclet number, expressed as the ratio of diffusion time to electromigration time a is the ratio of electroosmosis to electrophoretic mobility /S is the ratio of channel width to characteristic length scale for the initial sample ion concentration distribution and 5 is the ratio of the length scale of the initial BGE and sample ion concentration gradients. Here, d is the channel depth, ss and sb are the initial sample and BGE ion concentration gradients, and Ep is the nominal electric field. The function g x, t) in the advective dispersion term accounts for the axial variation in pressure-driven velocity profile (cf. Figure 38.2). 

The isotachophoretic boundary between two adjacent zones, under constant current condition and in the absence of bulk flow, assumes a constant width governed by the balance of electromigration and dispersion fluxes. For negligible electroosmosis (and negligible Taylor dispersion), the dispersion is determined by diffusion alone. Analytical solution to the concentration of the species in this diffused boundary, for a three-component fully ionized system, has been presented by Saville et al. [90]. The characteristic length-scale, S, of the ITP boundary in this case is given by... 

The mathematical theory of electroosmosis of the second kind is still in its infancy. Scaling... 

Electrokinetic stimulation may also be achieved by transport of immobilized indigenous or bioaugmented bacteria to biogeochemical niches of suitable chemical and environmental conditions (Fig. 2). Several studies have demonstrated centimeter- to meter-scale electrokinetic transport of bacteria and yeast cells through sand, soU, and aquifer sediments by either electrophoresis or electroosmosis. Transport direction and rates depend on the type of the subsurface matrix, the environmental conditions, and the size and... 

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