Laboratory-scale water-soil systems

Most investigators agree on the necessity of avoiding the development of an alkaline front from the cathode compartment into the soil since most heavy metals precipitate/adsorb to the soil in the alkaline environment, and the result is that the remediation process ceases, as shown in many of the early works on EK soil remediation (Kim and Kim, 2001). Most commonly, pH control maintains acidic conditions in the cathode compartment, and thus efficiently prevents the alkaline front from developing into the soil (Hicks and Tondorf, 1994 Wieczorek et al, 2005). Implementation of ion exchange membranes as mentioned above is another way to prevent the alkaline front (Hansen et ai, 1999). In laboratory experiments, the soil is commonly homogenized and the water is saturated. In full-scale experiments, the situation is different Here the electrode compartments are placed directly into the inhomogeneous soil, whose humidity is decided by the natural precipitation, as these systems are open and most often in situ. This could be one major reason why the scale up from laboratory cell experiments to full-scale remediation is so difficult (Hansen et al, 1997 Ottosen et al, 1997). 

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. 

Microcosms are laboratory systems generally consisting of tanks such as fish aquaria containing natural sediment and water, or soil. In those which have been most extensively evaluated for aquatic systems, continuous-flow systems are used. In all of them, continuous measurement of 14C02 evolved from 14C-labeled substrates may be incorporated, and recovery of both volatile and nonvolatile metabolites is possible so that a material balance may be constructed (Huckins et al. 1984). It should be pointed out that the term microcosm has also been used to cover much smaller scale experiments that have been carried out in flasks under anaerobic conditions (Edwards et al. 1992), and to systems for evaluating the effect of toxicants on biota (Section 7.4.2). Some examples are given to illustrate different facets of the application of microcosms to study various aspects of biodegradation. 

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