Electrokinetic Removal of PAHs

Since the mid-1980s, electrokinetics (EK) has been widely used as a soil remediation method, especially for low-permeability soils. Numerous studies have demonstrated the effectiveness of EK in the removal of soil contaminants. At first, most investigations focused on metals and only a few targeted relatively soluble organic pollutants, such as gasoline hydrocarbons, phenols, and trichloroethylene. In the case of hydro-phobic organic compounds (HOCs) with low solubility in water, but with a high tendency to be adsorbed onto soil, electrokinetic remediation was previously considered as not applicable because transport by electroosmosis and/or electrophoresis was not to be expected (Acar et aL, 1995 Virkutyte, SiUanpaa, and Latostenmaa, 2002). Therefore, methods to increase the solubility of HOCs had to be coupled with electrokinetic remediation. 

Polycyclic aromatic hydrocarbons (PAHs), representative HOCs, are very hydro-phobic and have quite low aqueous solubUity. The solubilization/desorption and partitioning of PAHs in soil-water systems have been extensively studied using solubUity-enhancing solutions such as surfactants and cosolvents to achieve effective removal of PAHs from contaminated sites. Recently, a number of laboratory studies on the electrokinetic removal of PAHs have appeared, evaluating the effect of enhancing solution and electrokinetic variables. The field remains underresearched in comparison with metal removal studies. 

The electrokinetic process can assist in eUmination of PAHs from soil when combined with other technologies such as biodegradation (Kim et al, 2005 Niqui-Arroyo and Ortega-Calvo, 2007) and chemical oxidation (Kim et al, 2006 Park et al, 2005). However, the present chapter focuses on the electrokinetic removal of PAHs using solubiUty-enhancing agents. Other integrated electrokinetic 

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

In this chapter, background information on PAHs and EK-facilitating agents is briefly provided, and earher research papers are discussed, with reference to types of enhancing solutions and process variables. The chapter continues with a description of recent research trends and conclusions. 

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ELECTROKINETIC REMOVAL OF PAHs USING FACILITATING AGENTS... 

Below, laboratory studies on the electrokinetic removal of PAHs are discussed according to the types of solubility-enhancing agents used and the effects of electrokinetic variables on contaminant removal efficiency. 

Enhanced electrokinetic remediation has been studied in order to remove organic pollutants as well, PAHs in particular. Solubilization and removal of PAHs proved to be heavily hindered by high organic contents of sediment, as well as by the low liquid/solid ratio achievable as compared to chemical washing, as reported in Andreottola et al. (2008) who also underlined the negative influence exerted by the high viscosity of the chemicals adopted on the process yield. 

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. 

Previous reports have generally described laboratory-scale experiments and there were few field demonstrations or full-scale applications. Kaolin was used as a model soil in most studies. However, recent electrokinetic tests of PAH removal have been conducted with real heterogeneous contaminated soils and have demonstrated the potential of EK. 

Elektorowicz and Hakimipour (2001,2003a) presented a technology that permitted the simultaneous removal of heavy metals and PAHs from natural soil called Simultaneous Electrokinetic Removal of Inorganic and Organic Pollutants (SEKRIOP). This technology used EDTA for metal mobility and zwitterionic surfactants for hydrocarbon mobility. Furthermore, the application of cationic reactive membranes permitted capturing free metallic ions generated by electrokinetic phenomena before their precipitation in the cathode area. The capture of metal-EDTA complexes was done on anionic reactive membranes. 

On the other hand, tests with sodium dodecylbenzene sulfonate showed that PAH desorption could be accomplished by electromigration of anionic micelles in the direction of the anode the surfactant was injected at the cathodic side of the electrokinetic cell. PAH removal of 90% was seen in the cathodic region (Pamukcu, 1994). Consequently, it is apparent that anionic surfactants migrate against the electroosmotic flow, and hence, are less useful in EK than are nonionic surfactants, even though the anionic surfactants are less adsorbed onto soil than are nonionic materials. 

The chemical n-butylamine has been also used in electrokinetic tests employing contaminated sediment and gas plant soil. The cosolvent contributed to the solubi-lization/desorption of PAHs. In the sediment test, 20% n-butylamine was more effective for partial solubilization of PAHs than was 3% Tween 80, and PAH concentrations in the anodic region were reduced to levels equivalent to those seen when 5% Igepal CA-720 was employed (Reddy and Ala, 2006). Also, 20% n-butylamine did not effectively transport PAHs in gas plant soil, even though PAHs were solubilized in effluent samples. This low removal efficiency might be attributable to the high organic content of the soil (Reddy et /., 2006). 

Facilitating agents change solution properties, such as dielectric constant, pH, and viscosity, and influence electroosmotic flow velocity, thereby impacting on PAH removal efficiency. The effect of electrokinetic variables on PAH removal has therefore been investigated to optimize operating conditions. pH adjustment and periodic voltage applications positively affect PAH removal efficiency. 

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... 

Overall, it appears that PAH desorption in an electrokinetic cell could be enhanced by use of n-butylamine or ethanol as a cosolvent, but actual removal efficiencies are very low. Therefore, further study is required to achieve high removal efficiencies in cosolvent-enhanced systems. Moreover, the application of cosolvent requires care because cosolvents are flammable and explosive, which could pose particular dangers in electrokinetic applications (Saichek and Reddy, 2005a). 

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. 

Several studies were conducted on the application of electrokinetics in different types of soils (see other chapters). They showed that electrokinetic remediation has the potential to remove heavy metals or organic contaminants, notably PAHs from soils. Based on previous experience, the decontamination using the electrokinetic process appears to be the least damaging and most environmentally acceptable technique. 

The simultaneous introduction of several conditioning liquids presents a challenge, particularly in clayey soils. Elektorowicz and Hakimipour (2003a) designed a system for the simultaneous desorption of heavy metals and PAHs, their simultaneous transport through the clayey soU, and removal. The system included the simultaneous utilization of a surfactant, a chelating agent, and special electrode supply/removal systems in combination with electrokinetic transport. 

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. 

The applicability of solvent flushing, however, is often limited by the characteristics of the soil, especially the particle size distribution. While sandy soils may result in xmcon-trolled fluid migration, clayey soils with partieles size less than 60 pm are often considered unsuitable for in-situ solvent flushing due to low soil permeability. In an attempt to remove PAHs from poorly permeable soils, Li, et alP investigated the possibility of combining cosolvent flushing with the electrokinetie technique. Electrokinetic remediation... 

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