Electrokinetic Remediation of Mixed Metal Contaminants

Electrochemical Remediation Technologies for Polluted Soils, Sediments and Groundwater, Edited by Krishna R. Reddy and Claudio Cameselle Copyright 2009 John Wiley Sons, Inc. 

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In order to insure the sufficient efficiency of electrokinetic removal of multiple heavy metals from porous media, it is essential to understand the main parameters affecting the transport and electrokinetic phenomena. Such parameters can be summarized as (a) the theoretical ionic mobility related to the ionic valance and molecular diffusion coefficient of species, (b) the delaying or retardation effect caused by the affinity of heavy metals in solid matrix, and (c) the chemical forms of metal contaminants initially existing in soils. In addition, some unexpected effects especially brought about in the electrokinetic remediation of mixed metal contaminants should be considered. The electrokinetic remediation for mixed metal contaminants generally shows lower removal efficiency than that for individual metal contaminants. High concentrations of multiple metal contaminants can be related to other parameters, for example, transference number, zeta potential, electroosmotic flow, and so on, which are factors that should be taken into consideration with regard to the removal mechanisms. 

The potential of electrokinetic remediation technology has been demonstrated for the remediation of mixed metal-contaminated soils, sediments, and groundwater over the past decade. Various enhancement schemes have been developed by the laboratory experimental and field applications, such as electrolyte conditioning and electrodialytic remediation. Despite such advances in the technology, it still has some limitations on the removal of mixed metal contaminants, including several specific heavy metals such as Cr, As, and Hg. Finding a simultaneously applicable process for the remediation of mixed heavy metals wUl require further study. 

ELECTROKINETIC REMEDIATION OF MIXED METALS AND ORGANIC CONTAMINANTS... 

While electrokinetic treatment of soils looks promising, most of the work performed was bench-scale, under carefully controlled laboratory conditions. For electrokinetic remediation to be a viable alternative for in-situ cleanup of waste sites, a number of factors will have to be investigated. All of the work to date has dealt with uniformly contaminated soil samples. Studies performed on partially saturated soils will yield different results. Further studies on the removal of mixed metal contaminated soils, using different soil types, are needed. The presence of organic compounds in the soil will also influence successful treatment of real contaminated soils. The use of reagents which could increase desorption and/or solubilization (without further contaminating the soil matrix) may also be areas of future investigation. Finally, field tests need to be performed to substantiate studies accomplished on the bench scale. 

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... 

Reddy KR, Maturi K. (2005). Enhanced electrokinetic remediation of mixed heavy metal and organic contaminants in low permeability soils. 16th International Conference on Soil Mechanics and Geotechnical Engineering, September 12-16, Osaka, Japan. Rotterdam, the Netherlands Millpress Science Publishers, pp. 2429-2432. 

As pointed out in the previous section of this chapter, attention should be paid to electromigration rather than electroosmosis to establish the removal mechanisms of mixed heavy metals. However, high concentrations of multiple metal contaminants, especially divalent cations, can affect the electroosmotic flow in electrokinetic remediation, which is a factor that should be taken into consideration with regard to the removal mechanisms of mixed heavy metals. The electroosmotic velocity, V, is given by the Helmholtz-Smoluchowski equation (Acar and Alshawabkeh, 1993 Mitchell, 1993) ... 

Considering the effectiveness of the simultaneous heavy metals and PAH removal, it can be concluded that SEKRIOP might be used for an electrokinetic in situ remediation of mixed contaminated soils. The development of the above-described multifunctional method permits remediating the soils, particularly those characterized with low permeability. The results from the research can be applied to various municipal and industrial sites containing petroleum products and heavy metals. 

Reddy and Maturi (2005) examined the feasibility of using electrokinetic remediation for the removal of mixed contaminants (i.e. mixtures of heavy metals and PAHs) from kaolin (low permeability soU). Likewise, different types of flushing solution were evaluated by a laboratory experimental program, including a cosolvent (n-butylamine), surfactants (3% Tween 80 and 5% Igepal CA-720), and a cyclodextrin (10% hydroxypropyl-j8-cyclodextrin or HPCD). It was reported that... 

Numerous sites are contaminated with both inorganic (heavy metals) and organic contaminants (e.g. PAHs), and the technologies that treat such mixed contaminants are very limited. Electrokinetic technologies have the potential to remediate mixed contaminated soils however, the efficiency of this technology depends on... 

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