Contaminants electrokinetic migration

Electrokinetic method Refers to an in situ, and sometimes an ex situ, technology that removes contaminants from wet soils and sediments by passing electric currents through them. Unlike in situ vitrification, the currents in electrokinetic methods are too low to melt the materials. Instead, the electric currents cause contaminant ions and charged particles in aqueous solutions within the solid materials to migrate towards the electrodes, where they may be collected or otherwise treated. 

The technology involves the application of low-intensity, direct electrical current across electrode pairs that have been implanted on each side of the contaminated soil. The electrical current induces electroosmosis and ion migration between the two implanted electrodes. Depending on their charge, the contaminants accumulate on one of the electrodes and are extracted to a recovery system (Fig. 14.4). Improved performance of electrokinetics could be attained by the introduction of surfactants. 

One of the more recent technologies in pollution treatment and remediation is based on the electrokinetic decontamination of soils [126-128], in which a dc potential (a few volts per centimeter) is applied across two inert electrodes embedded in a soil mass. This applied potential causes decomposition of the soil water to occur at the two electrodes. The migration of contaminants in the electric field, water transport, and reactions at the electrodes, as well as reactions caused by the induced pH gradient, can effectively clean soils. Acar et al. [127] reviewed electrokinetic remediation for the removal of metals and other inorganic contaminants from soil as well as its use in the extraction of organics from contaminated soils. 

The basic purpose of electrokinetic barriers is to prevent the migration of contaminants from its current location. These barriers are similar to traditional passive... 

Faulkner, Hopkinson, and Cundy, 2005). Because of the adverse effect of OH on soil remediation, due to the immobilization of many metal ions by precipitation in alkalinized soils, and the reduced efficiency of electrokinetic remediation when sacrificial iron-rich electrodes are employed (e.g. Leinz, Hoover, and Meier, 1998), noncorrosive electrodes and techniques to minimize soil alkalinization are generally employed for electrokinetic remediation (e.g. Rohrs, Ludwig, and Rahner, 2002 Virkutyte, Sillanpaa, and Latostenmaa, 2002). However, low adsorption of Cr(VI) in soils occurs in alkaline conditions, whereas high adsorption of Cr(VI) is favored in acidic conditions (Reddy et al, 1997). Furthermore, the reduction of Cr(VI) to Cr(III) by the delivery of iron (Fe°, Fe " ) is fairly well documented (Rai, Sass, and Moore, 1987 Eary and Rai, 1991 Haran et aL, 1995 Powell et aL, 1995 Pamukcu, Weeks, and Wittle, 1997 Batchelor et al., 1998 Reddy et /., 2003). Accordingly,under an applied direct current (DC) electric field, stabilization of Cr(VI)-contaminated soils may potentially be achieved where oxidative dissolution of iron-rich anodic electrodes provides Fe(j,q) to react with the anode-bound migration of Cr(VI). Hence, the use of iron-rich sacrificial electrodes and soil alkalinization may find application in the electrokinetic stabilization of Cr(VI)-contaminated soils. This concept is explained in this chapter based on the results of laboratory stabilization experiments on three Cr(VI)-impacted soils taken from three sites within the UK. 

A recent study on electrokinetic removal of ethyl benzene from contaminated clay showed a promising use of anionic-nonionic mixed surfactants. Surfactant addition resulted in 1.6-2.4-fold more removal than afforded by EK alone, and a mixed surfactant system, including 0.5% SDS and 2.0% PANNOX 110 (nonyl phenol polyethylene glycol ether) permitted optimal ethyl benzene removal (98%). This indicated that, in the presence of mixed surfactant micelles, the zeta potential of the soil particles significantly increased compared with that seen when anionic surfactant micelles were formed, and electrolytic mobility was thus enhanced (Yuan and Weng, 2004). The use of anionic-nonionic mixed surfactants in EK will also improve the desorption and migration of PAHs. 

The electrokinetic remediation of Hg from contaminated soils is notoriously very difficult due to its low solubility, as stated in the previous chapter. Moreover, the electrokinetics of Hg mixed with heavy metals has not been extensively studied. The most efficient removal of Hg in soils was conducted by the oxidation of reduced insoluble Hg(l) to Hg(II) using I2 (Cox, Shoesmith, and Ghosh, 1996). Here, an anionic complex is formed, where Hg(II) ions are available to migrate through the soil toward the anode. In a recent investigation of the decontamination of mixed heavy metals from contaminated field soils, only Hg was observed to have a different removal property from more than the 10 other metal contaminants (Reddy and Ala, 2005). The system where EDTA solution was applied as the electrolyte was... 

Narasimhan B, Sri Ranjan R. (2000). Electrokinetic barrier to prevent subsurface contaminant migration Theoretical model development and validation. Journal of Contaminant Hydrology 42(1) 1-17. 

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