**4. Combined technologies**

#### **4.1. Electrokinetics and chemical oxidation/reduction**

Electrokinetic remediation is a technique that removes the contaminants from the contami‐ nated soil by transportation (electro-osmosis and electromigration). However, organic con‐ 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 reduced to a minimum, which is a very important point in the field operation.

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

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‐

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

Electrokinetic remediation is a technique that removes the contaminants from the contami‐ nated soil by transportation (electro-osmosis and electromigration). However, organic con‐

about 90% of the phenanthrene removed on the cathode solution [40].

**3.4. Cyclodextrins**

218 Organic Pollutants - Monitoring, Risk and Treatment

fore, transported and removed.

usually lower than that found with surfactants.

**4.1. Electrokinetics and chemical oxidation/reduction**

**4. Combined technologies**

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 the microorganisms into the soil.

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

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‐ ling the pH in the optimum range for Fenton reagent which is slightly acid environments. At alkaline pH, H2O2 decomposes in water and oxygen and do not form OH radicals.

ment and maintenance costs. Anyway, the limited results found in several application im‐ pulse the research in several directions in order to improve the removal of the contaminants

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The combination of electrokinetic remediation with PRB has been satisfactory used to re‐ mediate soils polluted with heavy metals such as chromium. The electric field transports the chromium towards the main electrodes, but in their way, the chromium ions pass through a PRB made of elemental iron. The chemical reduction of chromium takes places reacting with the elemental iron. The electric field also plays a role in the reduction of the chromium [54]. In the case of organic contaminants, Chang and Cheng [55] applied the combination of PRB with electrokinetics to remediate a soil specimen contaminated with perchloroethilene. The experiments were carried out at a constant voltage drop of 1 v/cm and sodium carbonate 0.01 M was used as processing fluid to avoid the formation of an acid front in the anode. It eliminates the acidification of the soil and the possible negative effects on the electro-osmot‐ ic flow. The PRB were made of nanoparticles of elemental iron and zinc. The perchloroethy‐ lene is dechlorinated upon the nanoparticles of iron and zinc. However, the formation of ferric oxide and ferric hydroxides limits the activity of the PRB and its operational life. The protons electrogenerated at the cathode can contribute in the solubilization and removal of the ferric hydroxides increasing the activity and duration of the PRB. Moreover, the proton also favors the dechlorination reaction. In conclusion, the formation of H+ ions in the anode favors the elimination of perchloroethylene. As the voltage drop applied to the system in‐ creases, the formation of H+ upon the anode also increases resulting in more and faster per‐ chloroethylene removal. Thus, the operation at 2 V/cm resulted in the removal of almos 99%

Chung and Lee [56] applied a combination of electrokinetics with PB for the remediation of the tetrachloroethylene contaminated soils and groundwater. The interest of this work is the media used in the PRB. The authors used a mixture of sand with a material they called atomizing slag (material patented) which is basically a mixture of oxides of Si, Fe, Ca and Al. The atomizing slag is mainly used as a construction material but it was selected for the PRB because is much cheaper than other materials reported in literature such as iron nano‐ particles. The operation of such system in situ resulted in the removal of 90% of the tetra‐ chloroethylene considering the concentrations measured before and after the system electrokinetic-PRB confirmeing the suitability of this technology for its application in situ to

The combination of electrokinetic remediation with bioremediation has shown some inter‐ esting results that promise this technology a good development in the near future. Basically, the application of an electric field to a polluted site may help in the mobilization of the con‐ taminants. That mobilization makes the contaminants available for the microorganisms. At the same time, soil bacteria are like a colloid with a surface charge. So, they can be moved under the effect of the electric field. The transport of bacteria, even in small distances, may help in the interaction between the bacteria and the contaminants. Finally, the electric field

[53]. One possibility is the combination of the PRB with electrokinetic remediation.

of the initial perchloroethylene in only 10 days of operation.

contaminated soils.

**4.3. Bioelectroremediation**

The use of Fe0 for the remediation of soils has been used recently for the ability of the native iron to catalyze the reductive dechlorination of organic compounds such as pentachlorophe‐ nol, trichloroethylene, hexachlorobenzene and others. In this technology, the electric field can be used as a driving force to transport the nanoparticles into the soil. Reddy and Karri [49] found that the combination of electrokinetic remediation and Fe0 nanoparticles can be applied for the removal of pentachlorophenol from soil. The transportation of Fe0 nanoparti‐ cles was determined by the iron concentration into the soil at the end of the experiments. Iron concentration at the end of the experiments increased with the initial Fe0 concentration used in the anode and with the voltage gradient. However, the transport of nanoparticles was limited by their aggregation, settlement and partial oxidation within the anode. Penta‐ chlorophenol was partially reduced (40-50%) into the soil, but a complete PCP elimination was found near the cathode due to the combination of Fe0 and the reductive dechlorination within the cathode. In order to favor the transportation of nanoparticles into the soil, new strategies are needed to prevent aggregation, settlement and oxidation of iron nanoparticles for enhanced remediation of soils. Cameselle et al. [50] studied the surface characteristics of the iron nanoparticles and proposed several dispersant to favor the transportation and avoid aggregation and settlement. Among the dispersants proposed aluminum lactate presents good characteristics to be used in large scale application. Other metallic catalysts such as Cu/Fe or Pd/Fe bimetal microscale particles were satisfactorily used for the remedia‐ tion of soils with organochlorines.. Dechlorination of hexachlorobenzene up to 98% was ach‐ ieved with Cu/Fe [51] and only 60% with Pd/Fe [52].

#### **4.2. Electrokinetics and permeable reactive barriers**

Permeable reactive barriers (PRB)are passive remediation systems especially designed for the remediation of contaminated ground water. PRBs consist of digging a trench in the path of flowing groundwater and then filling it with a selected permeable reactive material. As the contaminated groundwater passes through the PRB, contaminants react with the active material in the PRB being absorbed, precipitated or degraded. Clean groundwater exits the PRB. In the design of a PRB several factors have to be taking into account. First, the nature and the chemical properties of the contaminants have to be considered for the selection of the reactive material. For organic contaminants, materials such as active carbon or Fe0 were used. Organic contaminants can be retained in in the porous structure of the active carbon. Native iron has been used for the reductive dechlorination of pesticides and other organo‐ chlorines. The flow rate of groundwater and the reaction rate of the contaminants with the active material in the PRB are used to define the width of the barrier. The resident time of the groundwater in the barrier has to be enough to reach a complete removal or degradation of the contaminants. Finally, the porous structure of the barrier has to confer the barrier it‐ self a permeability value higher than the surrounding soil, to assure that all the groundwa‐ ter pass through the barrier and there will not be bypass. The main advantages of the PRB are the stable operation for long treatment time, even several years, with very low invest‐ ment and maintenance costs. Anyway, the limited results found in several application im‐ pulse the research in several directions in order to improve the removal of the contaminants [53]. One possibility is the combination of the PRB with electrokinetic remediation.

The combination of electrokinetic remediation with PRB has been satisfactory used to re‐ mediate soils polluted with heavy metals such as chromium. The electric field transports the chromium towards the main electrodes, but in their way, the chromium ions pass through a PRB made of elemental iron. The chemical reduction of chromium takes places reacting with the elemental iron. The electric field also plays a role in the reduction of the chromium [54]. In the case of organic contaminants, Chang and Cheng [55] applied the combination of PRB with electrokinetics to remediate a soil specimen contaminated with perchloroethilene. The experiments were carried out at a constant voltage drop of 1 v/cm and sodium carbonate 0.01 M was used as processing fluid to avoid the formation of an acid front in the anode. It eliminates the acidification of the soil and the possible negative effects on the electro-osmot‐ ic flow. The PRB were made of nanoparticles of elemental iron and zinc. The perchloroethy‐ lene is dechlorinated upon the nanoparticles of iron and zinc. However, the formation of ferric oxide and ferric hydroxides limits the activity of the PRB and its operational life. The protons electrogenerated at the cathode can contribute in the solubilization and removal of the ferric hydroxides increasing the activity and duration of the PRB. Moreover, the proton also favors the dechlorination reaction. In conclusion, the formation of H+ ions in the anode favors the elimination of perchloroethylene. As the voltage drop applied to the system in‐ creases, the formation of H+ upon the anode also increases resulting in more and faster per‐ chloroethylene removal. Thus, the operation at 2 V/cm resulted in the removal of almos 99% of the initial perchloroethylene in only 10 days of operation.

Chung and Lee [56] applied a combination of electrokinetics with PB for the remediation of the tetrachloroethylene contaminated soils and groundwater. The interest of this work is the media used in the PRB. The authors used a mixture of sand with a material they called atomizing slag (material patented) which is basically a mixture of oxides of Si, Fe, Ca and Al. The atomizing slag is mainly used as a construction material but it was selected for the PRB because is much cheaper than other materials reported in literature such as iron nano‐ particles. The operation of such system in situ resulted in the removal of 90% of the tetra‐ chloroethylene considering the concentrations measured before and after the system electrokinetic-PRB confirmeing the suitability of this technology for its application in situ to contaminated soils.

#### **4.3. Bioelectroremediation**

ling the pH in the optimum range for Fenton reagent which is slightly acid environments.

The use of Fe0 for the remediation of soils has been used recently for the ability of the native iron to catalyze the reductive dechlorination of organic compounds such as pentachlorophe‐ nol, trichloroethylene, hexachlorobenzene and others. In this technology, the electric field can be used as a driving force to transport the nanoparticles into the soil. Reddy and Karri [49] found that the combination of electrokinetic remediation and Fe0 nanoparticles can be

cles was determined by the iron concentration into the soil at the end of the experiments. Iron concentration at the end of the experiments increased with the initial Fe0 concentration used in the anode and with the voltage gradient. However, the transport of nanoparticles was limited by their aggregation, settlement and partial oxidation within the anode. Penta‐ chlorophenol was partially reduced (40-50%) into the soil, but a complete PCP elimination

within the cathode. In order to favor the transportation of nanoparticles into the soil, new strategies are needed to prevent aggregation, settlement and oxidation of iron nanoparticles for enhanced remediation of soils. Cameselle et al. [50] studied the surface characteristics of the iron nanoparticles and proposed several dispersant to favor the transportation and avoid aggregation and settlement. Among the dispersants proposed aluminum lactate presents good characteristics to be used in large scale application. Other metallic catalysts such as Cu/Fe or Pd/Fe bimetal microscale particles were satisfactorily used for the remedia‐ tion of soils with organochlorines.. Dechlorination of hexachlorobenzene up to 98% was ach‐

Permeable reactive barriers (PRB)are passive remediation systems especially designed for the remediation of contaminated ground water. PRBs consist of digging a trench in the path of flowing groundwater and then filling it with a selected permeable reactive material. As the contaminated groundwater passes through the PRB, contaminants react with the active material in the PRB being absorbed, precipitated or degraded. Clean groundwater exits the PRB. In the design of a PRB several factors have to be taking into account. First, the nature and the chemical properties of the contaminants have to be considered for the selection of the reactive material. For organic contaminants, materials such as active carbon or Fe0 were used. Organic contaminants can be retained in in the porous structure of the active carbon. Native iron has been used for the reductive dechlorination of pesticides and other organo‐ chlorines. The flow rate of groundwater and the reaction rate of the contaminants with the active material in the PRB are used to define the width of the barrier. The resident time of the groundwater in the barrier has to be enough to reach a complete removal or degradation of the contaminants. Finally, the porous structure of the barrier has to confer the barrier it‐ self a permeability value higher than the surrounding soil, to assure that all the groundwa‐ ter pass through the barrier and there will not be bypass. The main advantages of the PRB are the stable operation for long treatment time, even several years, with very low invest‐

nanoparti‐

and the reductive dechlorination

At alkaline pH, H2O2 decomposes in water and oxygen and do not form OH radicals.

applied for the removal of pentachlorophenol from soil. The transportation of Fe0

was found near the cathode due to the combination of Fe0

220 Organic Pollutants - Monitoring, Risk and Treatment

ieved with Cu/Fe [51] and only 60% with Pd/Fe [52].

**4.2. Electrokinetics and permeable reactive barriers**

The combination of electrokinetic remediation with bioremediation has shown some inter‐ esting results that promise this technology a good development in the near future. Basically, the application of an electric field to a polluted site may help in the mobilization of the con‐ taminants. That mobilization makes the contaminants available for the microorganisms. At the same time, soil bacteria are like a colloid with a surface charge. So, they can be moved under the effect of the electric field. The transport of bacteria, even in small distances, may help in the interaction between the bacteria and the contaminants. Finally, the electric field can be used as a transportation mechanism to introduce into the soil the nutrients and other chemicals that may facilitate the bacterial growth and development, as well as the supply of other chemicals that can contribute to the degradation of the contaminants [57, 58].

application of electrical heating soil and groundwater in the source areas, combined with soil vapor extraction and low-yield groundwater pumping, and enhancing biodegradation in the groundwater plume area. Two years of heating and 2.5 years of biodegradation has been resulted in near-complete removal of the contaminants. A full scale implementation of six phase electrical heating technology was used in Sheffield, UK [61]. Terra Vac Ltd. dem‐ onstrated how remediation timescales can be reduced from months/years to weeks, with an electrical heating capable of remediation of soil in difficult geological conditions and in dense populated urban areas. TCE and VC were remediated by electrical heating up to 99.99%. Smith [62] applied the electroheating technology for the remediation of dicholorme‐ thane, ethylene dibromide, triclhoroethane and tetrachloroethane. Electroheating was an ef‐ fective technology for the remediation of such organic contaminants, but during the remediation process, the elevation of temperature increases the solubility of the contami‐ nants in the groundwater, the activity of soil microorganisms is enhanced and some reac‐ tions, such as hydrolysis of the contaminants and the desorption of gases, takes place. Those factors may affect the removal of the contaminants and it influence has to be considered.

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Electrokinetic remediation has been used as a remediation technology in several tests at field scale. In the USA, field projects were carried out or funded by USEPA, DOE, ITRC, US-Ar‐ my Environmental Centre, as well as companies like Electropetroleum Inc. [63], Terran Cor‐ poration, and Monsanto, Dupont, and General Electric which developed the LasagnaTM technology [64, 65]. In Europe, more field projects with electrokinetic remediation have been carried out, specially associated to the commercial activity of the Hak Milieutechniek Com‐ pany [66, 67]. Recently, some field experiences were reported in Japan and Korea [68]. Some of these tests deal about the remediation of polluted sites with organic contaminants such as organochlorides, PAHs and PCBs. Considering the information available in literature, the cost of field application of electrokinetic remediation is about an average value of 200 \$/m3 for both organic and inorganic contaminants, however it must be kept in mind that electro‐ kinetic remediation, like any other remediation technology, is site specific and the costs can

[69].

The scientific knowledge accumulated in the last 20 years conducted to several lessons learned that must be keep in mind in the design of projects for the remediation of contami‐ nated sites. Thus, the remediation of contaminated soils with organic contaminants is site specific. The results obtained in the remediation of a site cannot be assumed for other conta‐ minated sites. This is due to the large influence of the physicochemical properties of the soil and its possible interactions with the organic contaminants in the results of the electrokinetic remediation treatment. Besides, the chemicals used for enhancing the electrokinetic treat‐

**5. Large scale applications**

be vary from less than 100 to more than 400 \$/m3

**6. Future perspectives**

Lageman [59] developed a technology called Electrokinetic Biofence (EBF). The aim of the EBF is to enhance biodegradation of the VOCs in the groundwater at the zone of the fence by electrokinetic dispersion of the dissolved nutrients in the groundwater. EBF which con‐ sists of a row of alternating cathodes and anodes with a mutual distance of 5 m. Upstream of the line of electrodes, a series of infiltration wells were installed, which have been periodi‐ cally filled with nutrients. After running the EBF for nearly 2 years, clear results have been observed. The concentration of nutrients in the zone has increased, the chloride index is de‐ creasing, and VOCs are being dechlorinated by bio-activity. The electrical energy for the EBF is being supplied by solar panels.

#### **4.4. Electroheating**

The removal of volatile and semi-volatile organics from soil can be carried out heating the soil, evaporating the volatile organics and aspirating the vapors, which in turn are trapped in the appropriate absorbent such as active carbon to be finally eliminated by incineration. The heating of soil can be done in several ways. One possibility is the use of an electric cur‐ rent. Soil is not a good electric conductor, so the passing of an electric current generates heat. In electrokientic remediation, it is used a continuous electric current because the objective is to transport the ionic and nonionic contaminants out of the soil. In the case of electroheating, a transportation of the contaminants using the electric field as a driven force is not necessa‐ ry. The electric field is only used as a source of energy that is transformed from electric ener‐ gy into heat. That is why in electroheating the continuous electric field is substituted by an alternate current that supplies the energy but does not induce transportation. Soil is not a good electric conductor. The conductivity of soils is much lower than the typical electric conductor such as metals. The conductivity of soil largely varies with the moisture content and the presence of mobile ions. Anyway, the conductivity of soil is usually low and the heating is easy to achieve with an alternate current. It is recommendable to avoid the use of electroheating in saturated soils. A soil saturated in moisture favors the transportation of current instead of the electric heating.

Electroheating shows several advantages form other technologies designed for the removal of volatile organics from soils. In electroheating, the heating of soil is directly related to the electric field intensity. So, the increase of temperature and the final temperature in the soil can be easily controlled adjusting the intensity of the electric field. Furthermore, the heat is generated into the soil, in the whole volume at the same time, achieving a more uniform temperature in the area to be treated. The uniform temperature permits a uniform removal of the contaminants and a more efficient use of the energy.

Electrical heating was used in the remediation of a contaminated site in Zeist, the Nether‐ lands [60]. The site was severely polluted with chlorinated solvents such as perchloroethy‐ lene (PCE) and trichloroethylene (TCE) and their degradation products are cis-1,2 dichloroethene (C-DCE) and vinyl chloride (VC). Satisfactory results were obtained in the application of electrical heating soil and groundwater in the source areas, combined with soil vapor extraction and low-yield groundwater pumping, and enhancing biodegradation in the groundwater plume area. Two years of heating and 2.5 years of biodegradation has been resulted in near-complete removal of the contaminants. A full scale implementation of six phase electrical heating technology was used in Sheffield, UK [61]. Terra Vac Ltd. dem‐ onstrated how remediation timescales can be reduced from months/years to weeks, with an electrical heating capable of remediation of soil in difficult geological conditions and in dense populated urban areas. TCE and VC were remediated by electrical heating up to 99.99%. Smith [62] applied the electroheating technology for the remediation of dicholorme‐ thane, ethylene dibromide, triclhoroethane and tetrachloroethane. Electroheating was an ef‐ fective technology for the remediation of such organic contaminants, but during the remediation process, the elevation of temperature increases the solubility of the contami‐ nants in the groundwater, the activity of soil microorganisms is enhanced and some reac‐ tions, such as hydrolysis of the contaminants and the desorption of gases, takes place. Those factors may affect the removal of the contaminants and it influence has to be considered.
