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

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‐ netic remediation is also useful for the removal or elimination of organic contaminants [28]. Considering the different physico-chemical properties of the organic contaminants compared to the properties of heavy metals, the operating conditions of the electrokinetic treatment and the enhancing chemicals will be rather different than those used for heavy metal polluted soils. The main transportation mechanisms in electrokinetic remediation are: electromigration and electro-osmosis. In general, the more dangerous and persistent organic contaminants are not soluble in water (which is the interstitial fluid in natural soils) and are neither ionic nor ioniza‐ ble molecules. Therefore, electromigration cannot be considered as the transport mechanisms for organic contaminants. Electro-osmosis is the net flux of water in the soil matrix that flows through the soil from one electrode to the other due to the effect of the electric field. Electro-os‐ motic flow moves towards the cathode in electronegatively charged soils, which is the most common case. Again, organic contaminants are not soluble in water and therefore their elimi‐ nation from soils cannot be achieved in an unenhanced electrokinetic treatment. In order to achieve an effective removal or elimination of organic contaminants from soils, their solubility in has to be enhanced with the use of co-solvents, surfactants or any other chemical agent. Al‐ ternatively, the removal or elimination of organic contaminants can be achieved by the combi‐ nation of electrokinetics and other remediation techniques such as chemical oxidation/ reduction, permeable reactive barriers, electrolytic reactive barriers or thermal treatment. For the removal of organic contaminants, both solubilization of the contaminants and adequate electro-osmotic flow are required, which appear to be quite challenging to accomplish simulta‐ neously. The electro-osmotic flow is found to be dependent on the magnitude and mode of electric potential application. The electro-osmotic flow is higher initially under higher electric potential, but it reduces rapidly in a short period of time. Interestingly, the use of effective solu‐ bilizing agent (surfactant) and periodic voltage application was found to achieve the dual ob‐ jectives of generating high and sustained electro-osmotic flow and at the same time induce adequate mass transfer into aqueous phase and subsequent removal. Periodic voltage applica‐ tion consists of a cycle of continuous voltage application followed by a period of "down time" where the voltage was not applied was found to allow time for the mass transfer, or the diffu‐ sion of the contaminant from the soil matrix, to occur and also polarize the soil particles. Sever‐ al laboratory studies have demonstrated such desirable electro-osmotic flow behavior in a consistent manner, but field demonstration projects are needed to validate these results under scale-up field conditions [29, 30].

**3.1. Electrokinetic removal of soluble organics**

pH into the soil was not alkaline.

**3.2. Co-solvents**

Although most dangerous organic contaminants in soils, sediments and sludges are persis‐ tent hydrophobic organics, several works in literature focused on the treatment of soils with soluble organics. Thus, reactive black 5 is a common dye used in the industry. Reactive black 5 is a complex organic molecule difficult to biodegrade in the environment and shows a significant toxicity for living organisms in soils and water. Reactive black 5 is soluble in water, but it can be retained in soils and sediments adsorbed upon the surface of mineral particles and organic matter. Considering the chemical structure of the reactive black 5 (fig‐ ure 3), the molecule can be ionized at alkaline pH when the sulfonic groups are neutralized forming an anion with 4 negative charges. In this conditions, reactive black 5 can be trans‐ ported by electrokinetics toward the anode, but only if the molecule is in solution. The de‐ sorption of the molecule can be achieved using potassium sulfate as flushing solution in the anode and cathode chambers. Figure 4 shows the advance of the Reactive Black 5 toward the anode by electromigration. The advance of the day is evident in the 4th day of treatment, and it is completely removed from the soil in 5 days. The removal of Reactive black 5 is only pos‐ sible when the molecule is desorbed from the soil particles but the electromigration was on‐ ly possible when the pH into the soil was alkaline [31]. The pH was controlled in the anode (the left hand side in figure 4) at a value below 7 and the alkaline front electrogenerated at the cathode (the right hand side in figure 4) advanced through the soil favoring the dissolu‐ tion and electromigration of reactive black 5. Negligible Reactive Black 5 was observed if the

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

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

215

Most of organic contaminants of environmental concern are practically insoluble in water but they can be dissolved in other organic solvents. Thus, the use of other processing fluid than wa‐ ter may help in the desorption and dissolution of the organic contaminants in soils, sediments and sludges. Electrokinetic remediation is an in situ technique, and water is always present in soils. So, the organic solvent will not be used alone but in combination with water as a co-sol‐ vent. Thus, the possible organic solvents to be used are now reduced to those miscible with wa‐ ter. But this is not the unique condition a co-solvent has to meet. Organic co-solvents have to be safe for the environment or with a minor environmental impact, and it has to be easy to recover from soil after the treatment. Apart from the environmental limitations in the selection of the co-solvents, there are also some technical aspects to take into account. The use of co-solvents mixed with water decreases the conductivity of the processing fluid due to the decrease of salts solubility in the organic co-solvent. It decreases the current intensity through the soil. The pres‐ ence of an organic co-solvent will also affects the viscosity of the processing fluid and change the interaction between the processing fluid and the soil particles. Those alterations will im‐ pact directly the evolution of the electro-osmotic flow which is the main transportation mecha‐ nism for the removal of organic contaminants. Any rate, the increase in the contaminant solubility due to the use of the co-solvent may largely compensate the decrease in the electroos‐ motic flow, being the result positive for the removal of the organic contaminants. Some of the co-solvents used in literature are: ethanol, n-butanol, n-butylamine, tetrahydrofuran, or ace‐

**Figure 3.** Chemical structure of reactive black 5

#### **3.1. Electrokinetic removal of soluble organics**

Although most dangerous organic contaminants in soils, sediments and sludges are persis‐ tent hydrophobic organics, several works in literature focused on the treatment of soils with soluble organics. Thus, reactive black 5 is a common dye used in the industry. Reactive black 5 is a complex organic molecule difficult to biodegrade in the environment and shows a significant toxicity for living organisms in soils and water. Reactive black 5 is soluble in water, but it can be retained in soils and sediments adsorbed upon the surface of mineral particles and organic matter. Considering the chemical structure of the reactive black 5 (fig‐ ure 3), the molecule can be ionized at alkaline pH when the sulfonic groups are neutralized forming an anion with 4 negative charges. In this conditions, reactive black 5 can be trans‐ ported by electrokinetics toward the anode, but only if the molecule is in solution. The de‐ sorption of the molecule can be achieved using potassium sulfate as flushing solution in the anode and cathode chambers. Figure 4 shows the advance of the Reactive Black 5 toward the anode by electromigration. The advance of the day is evident in the 4th day of treatment, and it is completely removed from the soil in 5 days. The removal of Reactive black 5 is only pos‐ sible when the molecule is desorbed from the soil particles but the electromigration was on‐ ly possible when the pH into the soil was alkaline [31]. The pH was controlled in the anode (the left hand side in figure 4) at a value below 7 and the alkaline front electrogenerated at the cathode (the right hand side in figure 4) advanced through the soil favoring the dissolu‐ tion and electromigration of reactive black 5. Negligible Reactive Black 5 was observed if the pH into the soil was not alkaline.

#### **3.2. Co-solvents**

netic remediation is also useful for the removal or elimination of organic contaminants [28]. Considering the different physico-chemical properties of the organic contaminants compared to the properties of heavy metals, the operating conditions of the electrokinetic treatment and the enhancing chemicals will be rather different than those used for heavy metal polluted soils. The main transportation mechanisms in electrokinetic remediation are: electromigration and electro-osmosis. In general, the more dangerous and persistent organic contaminants are not soluble in water (which is the interstitial fluid in natural soils) and are neither ionic nor ioniza‐ ble molecules. Therefore, electromigration cannot be considered as the transport mechanisms for organic contaminants. Electro-osmosis is the net flux of water in the soil matrix that flows through the soil from one electrode to the other due to the effect of the electric field. Electro-os‐ motic flow moves towards the cathode in electronegatively charged soils, which is the most common case. Again, organic contaminants are not soluble in water and therefore their elimi‐ nation from soils cannot be achieved in an unenhanced electrokinetic treatment. In order to achieve an effective removal or elimination of organic contaminants from soils, their solubility in has to be enhanced with the use of co-solvents, surfactants or any other chemical agent. Al‐ ternatively, the removal or elimination of organic contaminants can be achieved by the combi‐ nation of electrokinetics and other remediation techniques such as chemical oxidation/ reduction, permeable reactive barriers, electrolytic reactive barriers or thermal treatment. For the removal of organic contaminants, both solubilization of the contaminants and adequate electro-osmotic flow are required, which appear to be quite challenging to accomplish simulta‐ neously. The electro-osmotic flow is found to be dependent on the magnitude and mode of electric potential application. The electro-osmotic flow is higher initially under higher electric potential, but it reduces rapidly in a short period of time. Interestingly, the use of effective solu‐ bilizing agent (surfactant) and periodic voltage application was found to achieve the dual ob‐ jectives of generating high and sustained electro-osmotic flow and at the same time induce adequate mass transfer into aqueous phase and subsequent removal. Periodic voltage applica‐ tion consists of a cycle of continuous voltage application followed by a period of "down time" where the voltage was not applied was found to allow time for the mass transfer, or the diffu‐ sion of the contaminant from the soil matrix, to occur and also polarize the soil particles. Sever‐ al laboratory studies have demonstrated such desirable electro-osmotic flow behavior in a consistent manner, but field demonstration projects are needed to validate these results under

scale-up field conditions [29, 30].

214 Organic Pollutants - Monitoring, Risk and Treatment

**Figure 3.** Chemical structure of reactive black 5

Most of organic contaminants of environmental concern are practically insoluble in water but they can be dissolved in other organic solvents. Thus, the use of other processing fluid than wa‐ ter may help in the desorption and dissolution of the organic contaminants in soils, sediments and sludges. Electrokinetic remediation is an in situ technique, and water is always present in soils. So, the organic solvent will not be used alone but in combination with water as a co-sol‐ vent. Thus, the possible organic solvents to be used are now reduced to those miscible with wa‐ ter. But this is not the unique condition a co-solvent has to meet. Organic co-solvents have to be safe for the environment or with a minor environmental impact, and it has to be easy to recover from soil after the treatment. Apart from the environmental limitations in the selection of the co-solvents, there are also some technical aspects to take into account. The use of co-solvents mixed with water decreases the conductivity of the processing fluid due to the decrease of salts solubility in the organic co-solvent. It decreases the current intensity through the soil. The pres‐ ence of an organic co-solvent will also affects the viscosity of the processing fluid and change the interaction between the processing fluid and the soil particles. Those alterations will im‐ pact directly the evolution of the electro-osmotic flow which is the main transportation mecha‐ nism for the removal of organic contaminants. Any rate, the increase in the contaminant solubility due to the use of the co-solvent may largely compensate the decrease in the electroos‐ motic flow, being the result positive for the removal of the organic contaminants. Some of the co-solvents used in literature are: ethanol, n-butanol, n-butylamine, tetrahydrofuran, or ace‐ tone [26, 32-34]. Phenanthrene was the target contaminant in the studies with co-solvents. The removal of phenanthene was negligible when water was used as flushing solution but the re‐ moved fraction of phenanthrene clearly increased with the use of co-solvents, especially n-bu‐ tylamine which resulted in a removal of 43% in 127 days in a lab test with a soil specimen of 20 cm long. The removal can be enhanced controlling other variables such as the pH into the soil and improving the electro-osmotic flow operating at higher voltage gradient or with periodic voltage application [34].

face properties of the solution when they are present. In environmental applications, the inter‐ est of surfactants is their ability to lower the surface and interfacial tension of water improving the solubility of hydrophobic organics. There is a wide variety of chemical structures and fami‐ lies that fits in the definition of surfactant. Basically a surfactant is a chemical compound whose molecule includes a hydrophilic group in one side of the molecule and in the opposite side a hy‐ drophobic group or chain. The interaction of the hydrophilic group with water assures its solu‐ bility whereas the interaction of the hydrophobic group with the organic contaminants assures the solubilization of hydrophobic organics. The hydrophobic group or chain in the surfactant molecule is repelled by water, so the surfactant molecules tend to form spherical structures with the hydrophilic group outside and the hydrophobic chains inside. These spherical struc‐ tures are called micelles. Thus, the surfactant creates a hydrophobic environment very appro‐ priate for the solubilization of organic compounds. The formation of micelles depends on the surfactant concentration and the micelle formation reach a maximum for a surfactant concen‐

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

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

217

There is a wide variety of chemical structures in the surfactants, but usually they are classi‐ fied by the electric charge in the molecule in 4 groups: cationic, anionic, neutral and zwitter‐ ionic (includes positive and negative charges in the same molecule). In environmental applications, neutral or anionic surfactants are preferred because cationic surfactants tend to interact with the soil particles, retarding their advance and reducing their effectiveness [26]. The toxicity of surfactants to the soil microorganisms it is also very important for the reme‐ diation and restoration of soils. That is why in recent years the research was redirected to

A wide variety of surfactants have been used in electrokinetic remediation for the removal of organic contaminants: Sodium dodecyl sulfate (SDS), Brij 35, Tween 80, Igepal CA-720, Tergitol and other. Target contaminant in these studies includes hydrophobic and persistent organics such as: phenanthrene, DDT, diesel, dinitrotoluene, hexachlorobenzene and others. In general, it can be conclude that the reported results in literature are quite good reaching removal efficiencies over 80% in many studies, at least in bench scale laboratory test with both model and real contaminated soils [36, 37]. Reddy et al. demonstrated the removal of phenanthrene by electrokinetics using surfactants as an enhanced flushing solution in the electrode chambers. Different types of soils, commonly kaolin and glacial till, were used in this study. In general, there is no elimination of phenanthrene when water was used as flushing solution despite the large electro-osmotic flow registered in these experiments. The use of surfactants such as Igepal CA-720, Tween 80 or Witconol tend to decrease the electroosmotic flow due to the changes in the interaction of the flushing solution with the soil par‐ ticle surface, the decreasing in the electric conductivity of the system, and the increase of the viscosity of the flushing solution. Despite the decreasing of the electro-osmotic flow, the in‐ crease of phenanthrene solubility in the surfactant flushing solution resulted in a very im‐ portant transportation and removal of phenanthrene in the fluid collected on the cathode side. The specific removal results did not only depend on the type and concentration of sur‐ factant but also in the pH evolution into the soil, the type of soil and the ionic strength in the processing fluid. Those variables affect the solubilization of the organic contaminants by the

tration called CMC "critical micelle concentration" [26].

the use of natural surfactants or biosurfactants [35].

**Figure 4.** Removal of Reactive Black 5 from a kaolin specimen by electrokinetic remediation

#### **3.3. Surfactants**

The name surfactant is the short version of "surface-active agent". It means that the so-called surfactants are a group of substances that has in common a special capacity to change the sur‐ face properties of the solution when they are present. In environmental applications, the inter‐ est of surfactants is their ability to lower the surface and interfacial tension of water improving the solubility of hydrophobic organics. There is a wide variety of chemical structures and fami‐ lies that fits in the definition of surfactant. Basically a surfactant is a chemical compound whose molecule includes a hydrophilic group in one side of the molecule and in the opposite side a hy‐ drophobic group or chain. The interaction of the hydrophilic group with water assures its solu‐ bility whereas the interaction of the hydrophobic group with the organic contaminants assures the solubilization of hydrophobic organics. The hydrophobic group or chain in the surfactant molecule is repelled by water, so the surfactant molecules tend to form spherical structures with the hydrophilic group outside and the hydrophobic chains inside. These spherical struc‐ tures are called micelles. Thus, the surfactant creates a hydrophobic environment very appro‐ priate for the solubilization of organic compounds. The formation of micelles depends on the surfactant concentration and the micelle formation reach a maximum for a surfactant concen‐ tration called CMC "critical micelle concentration" [26].

tone [26, 32-34]. Phenanthrene was the target contaminant in the studies with co-solvents. The removal of phenanthene was negligible when water was used as flushing solution but the re‐ moved fraction of phenanthrene clearly increased with the use of co-solvents, especially n-bu‐ tylamine which resulted in a removal of 43% in 127 days in a lab test with a soil specimen of 20 cm long. The removal can be enhanced controlling other variables such as the pH into the soil and improving the electro-osmotic flow operating at higher voltage gradient or with periodic

**Figure 4.** Removal of Reactive Black 5 from a kaolin specimen by electrokinetic remediation

The name surfactant is the short version of "surface-active agent". It means that the so-called surfactants are a group of substances that has in common a special capacity to change the sur‐

voltage application [34].

216 Organic Pollutants - Monitoring, Risk and Treatment

**3.3. Surfactants**

There is a wide variety of chemical structures in the surfactants, but usually they are classi‐ fied by the electric charge in the molecule in 4 groups: cationic, anionic, neutral and zwitter‐ ionic (includes positive and negative charges in the same molecule). In environmental applications, neutral or anionic surfactants are preferred because cationic surfactants tend to interact with the soil particles, retarding their advance and reducing their effectiveness [26]. The toxicity of surfactants to the soil microorganisms it is also very important for the reme‐ diation and restoration of soils. That is why in recent years the research was redirected to the use of natural surfactants or biosurfactants [35].

A wide variety of surfactants have been used in electrokinetic remediation for the removal of organic contaminants: Sodium dodecyl sulfate (SDS), Brij 35, Tween 80, Igepal CA-720, Tergitol and other. Target contaminant in these studies includes hydrophobic and persistent organics such as: phenanthrene, DDT, diesel, dinitrotoluene, hexachlorobenzene and others. In general, it can be conclude that the reported results in literature are quite good reaching removal efficiencies over 80% in many studies, at least in bench scale laboratory test with both model and real contaminated soils [36, 37]. Reddy et al. demonstrated the removal of phenanthrene by electrokinetics using surfactants as an enhanced flushing solution in the electrode chambers. Different types of soils, commonly kaolin and glacial till, were used in this study. In general, there is no elimination of phenanthrene when water was used as flushing solution despite the large electro-osmotic flow registered in these experiments. The use of surfactants such as Igepal CA-720, Tween 80 or Witconol tend to decrease the electroosmotic flow due to the changes in the interaction of the flushing solution with the soil par‐ ticle surface, the decreasing in the electric conductivity of the system, and the increase of the viscosity of the flushing solution. Despite the decreasing of the electro-osmotic flow, the in‐ crease of phenanthrene solubility in the surfactant flushing solution resulted in a very im‐ portant transportation and removal of phenanthrene in the fluid collected on the cathode side. The specific removal results did not only depend on the type and concentration of sur‐ factant but also in the pH evolution into the soil, the type of soil and the ionic strength in the processing fluid. Those variables affect the solubilization of the organic contaminants by the surfactant, but the main influence is in the develonment and evolution of the electro-osmotic flow. Thus, the limitation of very acidic environments into the soil avoids a sharp reduction of the electro-osmotic flow. This can be achieved controlling the pH on the anode or using a buffering solution with the flushing surfactant solution. The buffering capacity of the soil al‐ so contributes to avoid the acidification of the interstitial fluid [26, 33, 38, 39]. However, the only use of surfactants seems to be not enough to get a complete removal of phenanthrene from polluted soils, and it is necessary to enhance the electro-osmotic flow using high volt‐ age gradients (2 V/cm or higher) and even the use of periodic voltage applications operating with a constant voltage drop intermittently. The periodic voltage application resulted in about 90% of the phenanthrene removed on the cathode solution [40].

taminants are difficult to remove from soils, mainly due to the low solubility in water, and their strong adsorption to organic matter and soil particles. There are some other ways to look at the problem of organic contaminants in soil. One possibility is to degrade the con‐ taminants in situ. To achieve such degradation, it is necessary to create the adequate condi‐ tions into the soil supplying strong oxidizing chemicals to the soil pores to perform the degradation in situ. Oxidants such as ozone, hydrogen peroxide or persulfate can be trans‐ ported into de soil by electromigration and/or electro-osmosis. As the oxidants advance through the soil, they react with the organic contaminants resulting in smaller molecules usually less toxic that the original ones. The objective is to be able to completely oxidize the organic contaminants to carbon dioxide and water. If such complete degradation is not pos‐ sible under the operating conditions into the soil, the formation of simpler molecules are considered enough, because small and simpler molecules can be degraded easily by the mi‐ croorganisms into the soil. Thus, this technology can be a very attractive solution for the degradation of complex organic contaminants into soil. This technology does not generate waste effluents with harmful compounds because they are destroyed into the soil. More‐ over, the contact of the workers with the contaminants and contaminated soil particles are

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

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

219

reduced to a minimum, which is a very important point in the field operation.

77.9% of removal when temperature was used as activator of the persulfate.

the microorganisms into the soil.

On the other hand, the chemical destruction of organic contaminants can be carried out by chemical reduction, when a reductive chemical process results in less toxic compounds. Thus, organochlorine pesticides can be degraded by reductive dechlorination. The result is the organic molecule without chlorine atoms in its structure. Thus, the resulted organic com‐ pounds are much less toxic than the original compound and they can be easily degraded by

There are several applications of chemical oxidation combined with electrokinetics in litera‐ ture. Yukselen-Aksoy and Reddy [47] have tested the degradation of PCB in contaminated soils by persulfate. Sodium persulfate is a strong oxidizing agent with a standard reduction potential of 2.7 V which assures the effective oxidation of most of the organic contaminants. Persulfate is firstly transported into the soil by electromigration and/or electro-osmosis, and then it is activated by pH or temperature. To active the persulfate, it is necessary to achieve over 45ºC or acidify the soil below 4. Both conditions can be reached with the electric field. High voltage gradient results in the heating of soil; and the acid front electrogenerated at the anode can acidify the soil. So, in this case the application of the electric field not only was used as a transportation mechanism but as a tool to control the key variables of the process. In this work [47], the highest degradation of PCBs was achieved in a kaolin specimen with a

The combination of electrokinetics and chemical oxidation was tested in a contaminated soil with hexachlorobenzene [46, 48]. Hydrogen peroxide was supplied to the soil from the anode in a Fenton-like process where the iron content in the soil was sufficient to activate the descomposition of H2O2 for the generation of hydroxyl radicals ( OH). 60% of HCB was eliminated from the soil in 10 days of treatment avoiding the deactivation of the Fenton re‐ agent at high pH values. Higher removal can be achieved at longer treatment time, control‐

#### **3.4. Cyclodextrins**

Glucose may form cyclic structures with 6, 7 or 8 molecules called cyclodextrins. The result‐ ing molecule has the structure of a truncated cone. The internal cavity has different size de‐ pending on the number of glucose units. The inner diameter of the molecule ranged from 0.45-0.53 nm for α-cyclodextrin (ring of 6 glucose molecules); 0.60-0.65 nm for β-cyclodextrin (ring of 7 glucose molecules); and 0.75-0.85 nm for γ-cyclodextrin (ring of 8 glucose mole‐ cules). Cyclodextrin shows an amphiphilic behavior due to the rings of –OH groups present at the both ends of the molecule. The hydroxyl groups are polar and confer to the cyclodex‐ trin the solubility in water. However, the inner surface of the molecule is hydrophobic and cyclodextrins can accommodate different non-polar, hydrophobic molecules such as aliphat‐ ic, aromatic or lipophilic compounds. Moreover, the different size of the inner cavity of the ciclodextrin molecules can be used as a select the molecules to be trapped inside, and there‐ fore, transported and removed.

Cyclodextrins have been used to enhance the removal of hydrophobic organics such as phe‐ nanthrene [41], dinitrotoluene [42], the herbicide atrazine [43], and other contaminants [44] in real and model soils. In general, cyclodextrins are facilitating agents that improve the re‐ moval of organic contaminants from soil compared to other experiments with unenhanced electrokinetics, but results from cyclodextrin tests are usually less effective than test with surfactants, iron nanoparticles or with chemical oxidants. The efficiency of the removal can be enhanced combining more than one facilitating agent. Thus, Pham et al. [45] and Oonnit‐ tan et al. [46] used the electrokinetic treatment with a cyclodrextring flushing solution, com‐ bined with ultrasounds or chemical oxidation with hydrogen peroxide. Anyway, the use of cyclodextrins may enhance the removal of the hydrophobic contaminants but the results are usually lower than that found with surfactants.
