**1. Introduction**

Procion H-exldyes. Comparison of H2O2/UV, Fenton, UV/ Fenton, TiO2/UV and

[49] Rincón A.G., Pulgarin C. Effect of pH, Inorganic Ions, Organic Matter and H2O2 on E. coli K12 Photocatalytic Inactivation by TiO2-implications in Solar Water Disinfection.

[50] Schmelling D.C., Gray K.A., Vamat P.V. The Influence of Solution Matrix on the Pho‐ tocatalytic Degradation of TNT in TiO2 slurries. Water Resources 1997 (31) 1439-1447

[51] Wang K., Zhang J., Lou L., Yang S., Chen Y. UV or Visible Light Induced Photode‐ gradation of AO7 on TiO2 Particles: The Influence of Inorganic Anions. Journal of

[52] Diebold U. The Surface Science of Titanium Dioxide. Surface Science Reports 2003

[53] Matthews R.W., McEnvoy S.R. Photocatalytic Degradation of Phenol in the Presence of near-UV Illuminated Titanium Dioxide. Journal of Photochemistry and Photobiol‐

[54] Burns, R., Crittenden, J.C., Hand, D.W., Sutter, L.L., Salman, S.R., 1999. Effect of inor‐

ganic ions in heterogeneous photocatalysis. J. Environ. Eng. 125, 77-85.

TiO2/UV/H2O2 Processes. Desalination 2007 (211) 72-86.

Applied Catalysis B: Environmental 2004 (51) 283-302.

Photochemistry Photobiology A: Chemical 165 (2004) 201-207.

(48) 53-229.

208 Organic Pollutants - Monitoring, Risk and Treatment

ogy A: Chemical 1992 (64) 231.

Soil contamination is associated to industrial activities, mining exploitations and waste dumping. It is considered a serious problem since it affects not only the environment, living organisms and human health, but also the economic activities associated with the use of soil [1]. The risks associated with soil contamination and soil remediation are important points in the agenda of politicians, technicians and scientific community. The present legislation es‐ tablishes a legal frame to protect the soil from potentially contaminant activities; however, the present situation of soil contamination is the result of bad practices in the past, especially related to bad waste management [2-3].

Soil contamination affects living organisms in the subsurface but also affects the plants that accumulate contaminants as they grow. Thus, contaminants enter the food chain with a po‐ tential impact in public health [4]. On the other hand, contaminants can be washed out the soil by rain and groundwater, resulting in the dissemination of the contamination. This process is not desirable because the area affected by the contaminants is bigger and bigger and the possible remediation is more difficult and costly as the affected area grows [5]. Therefore, soil contamination is a serious problem that requires a rapid solution in order to prevent more environmental damages. Prevention is the best "technology" to save our soils from the contamination. A strict management of the wastes and good environmental practi‐ ces associated to industrial activities, mining, transportation and dumping management are required to prevent the contamination of the environment. However, many sites have been identified as contaminated sites. The European Union, USA, Canada, Japan and South Korea made a lot of efforts in recent years to identify the contaminated sites in each country. The new legislation, especially in the European Union, forces the administration to identify the

© 2013 Cameselle et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Cameselle et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

contaminated sites and evaluate the risks associated to the environment and public health. Then, the remediation of those sites must be carried out, starting with the riskier sites for humans and living organisms [6]. This is the aim of the present legislation in Spain about the management of wastes and soil contamination [7, 8] which is the transposition of the Eu‐ ropean Directive 2008/98/CE [9].

which is fill with water or the appropriate solution to enhance the removal of contaminants

Advances in Electrokinetic Remediation for the Removal of Organic Contaminants in Soils

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Contaminants are transported out of the soil due several transportation mechanisms in‐

**•** Electromigration is defined as the transportation of ions in solution in the interstitial fluid in the soil matrix towards the electrode of the opposite charge (Figure 2). Cations move toward the cathode (negative electrode), and anions move toward the anode (positive electrode). The ionic migration or electromigration depends on the size and charge of the

**•** Electro-osmosis is the net flux of water or interstitial fluid induced by the electric field (Figure 2). Electro-osmosis is a complex transport mechanism that depends on the electric characteristics of the solid surface, the properties of the interstitial fluid and the interac‐ tion between the solid surface and the components in solution. The electro-osmotic flow transports out of the porous matrix any chemical species in solution. Soils and sediments are usually electronegative (solid particles are negatively charged), so the electro-osmotic flow moves toward the cathode. In the case of electropositive solid matrixes, the electroosmotic flow moves toward the anode. Detailed information about electro-osmosis can be

(Figure 1). Typically, a voltage drop of 1 VDC/cm is applied to the main electrodes.

**Figure 1.** Application of the electrokinetic remediation in a contaminated site.

duced by the electric field [22, 23]:

found in literature [24].

ion and the strength of the electric field.

Soil remediation implies the application of a technology able to remove or eliminate the con‐ taminants following by the restoration of the site to the original state. So far, it sounds easy to do. However, there is not a technology able to remove any kind of contaminants in any kind of soil. Moreover, the restoration of the site to the original state is not always possible due to the characteristics of the soil and/or the remediation technology. Thus, the common objective in soil remediation is to remove the contaminants to a safe level for humans and the environment, and restore the properties of the soil to a state appropriate for the common soil uses [10-12]. So, the final target concentration to consider the soil non-contaminated will be different depending on the future use of the soil: urban, agriculture or industrial.

During the last 20 years, scientist and technicians spent a lot of efforts in the developing of innovative technologies for soil remediation [13]. Those technologies use the physical, chem‐ ical and biological principles to remove and/or eliminate the contaminants from soil. Thus, for instance, bioremediation uses the capacity of soil microorganisms to degrade organic contaminants into the soil [14]. Thermal desorption was designed to remove volatile and semi-volatile organics. Gasoline, BETX, chlorinated organics can be removed by thermal de‐ sorption, but also PAHs or PCBs [15]. Soil washing uses a solution in water to dissolve the contaminants from soil. Once the soil is clean, it can be stored in the same place and the con‐ taminants will undergo a stabilization process [16]. Soil remediation technologies can be ap‐ plied in situ, i.e. in the contaminated site without excavation, or ex-situ: the soil is excavated and it is treated in a facility specifically designed for the remediation process. In situ tech‐ nologies are preferred because they results in lower costs, less exposition to the contami‐ nants and less disruption of the environment. However, the control of the operation is more difficult and depending on the permeability of the soil and soil stratification, the operation may results in very poor results. On the other hand, ex-situ technologies permit a better con‐ trol of the operation, and the remediation results are not very affected by some soil charac‐ teristics as permeability and stratification [17-19].

### **2. Electrokinetic remediation: Basis and applications**

Electrokinetic remediation is an environmental technique especially developed for the re‐ moval of contaminants in soil, sediments and sludge, although it can be applied to any solid porous material [20]. Electrokinetic remediation is based in the application of a direct elec‐ tric current of low intensity to the porous matrix to be decontaminated [21]. The effect of the electric field induces the mobilization and transportation of contaminants through the po‐ rous matrix towards the electrodes, where they are collected, pumped out and treated. Main electrodes, anode and cathode, are inserted into the soil matrix, normally inside a chamber which is fill with water or the appropriate solution to enhance the removal of contaminants (Figure 1). Typically, a voltage drop of 1 VDC/cm is applied to the main electrodes.

**Figure 1.** Application of the electrokinetic remediation in a contaminated site.

contaminated sites and evaluate the risks associated to the environment and public health. Then, the remediation of those sites must be carried out, starting with the riskier sites for humans and living organisms [6]. This is the aim of the present legislation in Spain about the management of wastes and soil contamination [7, 8] which is the transposition of the Eu‐

Soil remediation implies the application of a technology able to remove or eliminate the con‐ taminants following by the restoration of the site to the original state. So far, it sounds easy to do. However, there is not a technology able to remove any kind of contaminants in any kind of soil. Moreover, the restoration of the site to the original state is not always possible due to the characteristics of the soil and/or the remediation technology. Thus, the common objective in soil remediation is to remove the contaminants to a safe level for humans and the environment, and restore the properties of the soil to a state appropriate for the common soil uses [10-12]. So, the final target concentration to consider the soil non-contaminated will

be different depending on the future use of the soil: urban, agriculture or industrial.

During the last 20 years, scientist and technicians spent a lot of efforts in the developing of innovative technologies for soil remediation [13]. Those technologies use the physical, chem‐ ical and biological principles to remove and/or eliminate the contaminants from soil. Thus, for instance, bioremediation uses the capacity of soil microorganisms to degrade organic contaminants into the soil [14]. Thermal desorption was designed to remove volatile and semi-volatile organics. Gasoline, BETX, chlorinated organics can be removed by thermal de‐ sorption, but also PAHs or PCBs [15]. Soil washing uses a solution in water to dissolve the contaminants from soil. Once the soil is clean, it can be stored in the same place and the con‐ taminants will undergo a stabilization process [16]. Soil remediation technologies can be ap‐ plied in situ, i.e. in the contaminated site without excavation, or ex-situ: the soil is excavated and it is treated in a facility specifically designed for the remediation process. In situ tech‐ nologies are preferred because they results in lower costs, less exposition to the contami‐ nants and less disruption of the environment. However, the control of the operation is more difficult and depending on the permeability of the soil and soil stratification, the operation may results in very poor results. On the other hand, ex-situ technologies permit a better con‐ trol of the operation, and the remediation results are not very affected by some soil charac‐

Electrokinetic remediation is an environmental technique especially developed for the re‐ moval of contaminants in soil, sediments and sludge, although it can be applied to any solid porous material [20]. Electrokinetic remediation is based in the application of a direct elec‐ tric current of low intensity to the porous matrix to be decontaminated [21]. The effect of the electric field induces the mobilization and transportation of contaminants through the po‐ rous matrix towards the electrodes, where they are collected, pumped out and treated. Main electrodes, anode and cathode, are inserted into the soil matrix, normally inside a chamber

ropean Directive 2008/98/CE [9].

210 Organic Pollutants - Monitoring, Risk and Treatment

teristics as permeability and stratification [17-19].

**2. Electrokinetic remediation: Basis and applications**

Contaminants are transported out of the soil due several transportation mechanisms in‐ duced by the electric field [22, 23]:


**•** Electrophoresis is the transport of charged particles of colloidal size and bound contami‐ nants due to the application of a low direct current or voltage gradient relative to the sta‐ tionary pore fluid. Compared to ionic migration and electro-osmosis, mass transport by electrophoresis is negligible in low permeability soil systems. However, mass transport by electrophoresis may become significant in soil suspension systems and it is the mecha‐ nism for the transportation of colloids (including bacteria) and micelles.

nants and therefore affect the transportation and contaminant removal efficiency [27]. The main reaction in the electrochemical/electrokinetic systems is the decomposition of water that occurs at the electrodes. The electrolytic decomposition of water reactions generates

) ions due to reduction at the cathode as shown in equations 1 and 2.

() (

( ) ()

Essentially, acid is produced at the anode and alkaline solution is produced at the cathode, therefore, pH in the cathode is increased, while pH at the anode is decreased. The migration

acid front moves across the soil until it meets the hydroxyl front in a zone near the cathode where the ions may recombine to generate water. Thus, the soil is divided in tow zones with a sharp pH jump in between: a high pH zone close to the cathode, and a low pH zone on the

ions and the geochemical characteristics of the soil. The implications of these electrolysis re‐ actions are enormous in the electrokinetic treatment since they impact the absorption/ desorption of the contaminants, the dissolution/precipitation reactions, chemical speciation and the degradation of the contaminants. Moreover, pH changes into the soil affects the con‐ taminant migration, and the evolution of the electro-osmotic flow which is decisive in the removal of non-charged organic contaminants [20]. In electrokinetic remediation, it is also common the use of chemical to enhance the dissolution and the transportation of the con‐ taminants. The enhancing chemical are going to interact with the soil and the contaminants, therefore it is necessary to evaluate the geochemistry of the soil and the possible reactions with the enhancing chemicals, considering at the same time the effect of the pH, in order to design a satisfactory technique that removes or eliminates the contaminants keeping the nat‐

**3. Removal of organic contaminants by electrokinetics: Limitations and**

Electrokinetic remediation was first proposed and tested for the removal of heavy metals and other charged inorganic contaminants in soils, sediments and sludges. However, the electroki‐

anode side. The actual soil pH values will depend on the extent of transport of H+

0 <sup>2</sup> aq 2 gas) 2 H O 4 e- + 4H+ + O E =-1.229 V ® (1)

Advances in Electrokinetic Remediation for the Removal of Organic Contaminants in Soils


) due to oxidation at the anode and hydrogen gas and hy‐

http://dx.doi.org/10.5772/54334

213

from the cathode into the soil leads to dynamic changes

, so the protons dominate the system and an

and OH<sup>−</sup>

oxygen gas and hydrogen ions (H+

ions from the anode and OH<sup>−</sup>

is about twice as mobile as OH<sup>−</sup>

ural properties of the soil for its use after the remediation process.

droxyl (OH<sup>−</sup>

of H+

in soil pH. H+

**enhancements**

At Anode (Oxidation):

At Cathode (Reduction):

**•** Diffusion refers to the mass transport due to a concentration gradient, not to a voltage gradi‐ ent as the previous transport mechanisms. During the electrokinetic treatment of contami‐ nated soils, diffusion will appear as a result of the concentration gradients generated by the electromigration and electro-osmosis of contaminants. Diffusive transport is often neglect‐ ed considering its lower velocity compared to electromigration and electro-osmosis.

**Figure 2.** Transport mechanisms in electrokinetic remediation

The two main transport mechanisms in electrokinetic remediation are electromigration and electro-osmosis [25]. The extent of electromigration of a given ion depends on the conductiv‐ ity of the soil, soil porosity, pH gradient, applied electric potential, initial concentration of the specific ion and the presence of competitive ions. Electromigration is the major transport processes for ionic metals, polar organic molecules, ionic micelles and colloidal electrolytes.

The electro-osmotic flow depends on the dielectric constant and viscosity of pore fluid as well as the surface charge of the solid matrix represented by zeta potential. The zeta potential is a function of many parameters including the types of clay minerals and ionic species that are present as well as the pH, ionic strength, and temperature. Electro-osmosis is considered the dominant transport process for both organic and inorganic contaminants that are in dissolved, suspended, emulsified or such similar forms. Besides, electro-osmotic flow though low perme‐ ability regions is significantly greater than the flow achieved by an ordinary hydraulic gradi‐ ent, so the electro-osmotic flow is much more efficient in low permeability soils [26].

The application of an electric field to a moisten porous matrix also induces chemical reac‐ tions into the soil and upon the electrodes that decisively influences the chemical transporta‐ tion and speciation of the contaminants and other constituents of the soil. Chemical reactions include acid-alkaline reactions, redox reaction, adsorption-desorption and dissolu‐ tion-precipitation reactions. Such reactions dramatically affect the speciation of the contami‐ nants and therefore affect the transportation and contaminant removal efficiency [27]. The main reaction in the electrochemical/electrokinetic systems is the decomposition of water that occurs at the electrodes. The electrolytic decomposition of water reactions generates oxygen gas and hydrogen ions (H+ ) due to oxidation at the anode and hydrogen gas and hy‐ droxyl (OH<sup>−</sup> ) ions due to reduction at the cathode as shown in equations 1 and 2.

At Anode (Oxidation):

**•** Electrophoresis is the transport of charged particles of colloidal size and bound contami‐ nants due to the application of a low direct current or voltage gradient relative to the sta‐ tionary pore fluid. Compared to ionic migration and electro-osmosis, mass transport by electrophoresis is negligible in low permeability soil systems. However, mass transport by electrophoresis may become significant in soil suspension systems and it is the mecha‐

**•** Diffusion refers to the mass transport due to a concentration gradient, not to a voltage gradi‐ ent as the previous transport mechanisms. During the electrokinetic treatment of contami‐ nated soils, diffusion will appear as a result of the concentration gradients generated by the electromigration and electro-osmosis of contaminants. Diffusive transport is often neglect‐

The two main transport mechanisms in electrokinetic remediation are electromigration and electro-osmosis [25]. The extent of electromigration of a given ion depends on the conductiv‐ ity of the soil, soil porosity, pH gradient, applied electric potential, initial concentration of the specific ion and the presence of competitive ions. Electromigration is the major transport processes for ionic metals, polar organic molecules, ionic micelles and colloidal electrolytes.

The electro-osmotic flow depends on the dielectric constant and viscosity of pore fluid as well as the surface charge of the solid matrix represented by zeta potential. The zeta potential is a function of many parameters including the types of clay minerals and ionic species that are present as well as the pH, ionic strength, and temperature. Electro-osmosis is considered the dominant transport process for both organic and inorganic contaminants that are in dissolved, suspended, emulsified or such similar forms. Besides, electro-osmotic flow though low perme‐ ability regions is significantly greater than the flow achieved by an ordinary hydraulic gradi‐

The application of an electric field to a moisten porous matrix also induces chemical reac‐ tions into the soil and upon the electrodes that decisively influences the chemical transporta‐ tion and speciation of the contaminants and other constituents of the soil. Chemical reactions include acid-alkaline reactions, redox reaction, adsorption-desorption and dissolu‐ tion-precipitation reactions. Such reactions dramatically affect the speciation of the contami‐

ent, so the electro-osmotic flow is much more efficient in low permeability soils [26].

ed considering its lower velocity compared to electromigration and electro-osmosis.

nism for the transportation of colloids (including bacteria) and micelles.

**Figure 2.** Transport mechanisms in electrokinetic remediation

212 Organic Pollutants - Monitoring, Risk and Treatment

$$2\text{ H}\_2\text{O} \rightarrow 4\text{ e-} + 4\text{H} + \_{\text{(aq)}} + \text{O}\_{2\text{(gas)}} \quad \text{E}^0 = 1.229\text{ V} \tag{1}$$

At Cathode (Reduction):

$$4\text{ H}\_2\text{O} + 4\text{ e- }\rightarrow 2\text{ H}\_{2\text{(gas)}} + 4\text{ OH}^-\_{\text{(aq)}}\text{ E}^0 = 0.828\text{ V}\tag{2}$$

Essentially, acid is produced at the anode and alkaline solution is produced at the cathode, therefore, pH in the cathode is increased, while pH at the anode is decreased. The migration of H+ ions from the anode and OH<sup>−</sup> from the cathode into the soil leads to dynamic changes in soil pH. H+ is about twice as mobile as OH<sup>−</sup> , so the protons dominate the system and an acid front moves across the soil until it meets the hydroxyl front in a zone near the cathode where the ions may recombine to generate water. Thus, the soil is divided in tow zones with a sharp pH jump in between: a high pH zone close to the cathode, and a low pH zone on the anode side. The actual soil pH values will depend on the extent of transport of H+ and OH<sup>−</sup> ions and the geochemical characteristics of the soil. The implications of these electrolysis re‐ actions are enormous in the electrokinetic treatment since they impact the absorption/ desorption of the contaminants, the dissolution/precipitation reactions, chemical speciation and the degradation of the contaminants. Moreover, pH changes into the soil affects the con‐ taminant migration, and the evolution of the electro-osmotic flow which is decisive in the removal of non-charged organic contaminants [20]. In electrokinetic remediation, it is also common the use of chemical to enhance the dissolution and the transportation of the con‐ taminants. The enhancing chemical are going to interact with the soil and the contaminants, therefore it is necessary to evaluate the geochemistry of the soil and the possible reactions with the enhancing chemicals, considering at the same time the effect of the pH, in order to design a satisfactory technique that removes or eliminates the contaminants keeping the nat‐ ural properties of the soil for its use after the remediation process.
