**3. Results and discussion**

#### **3.1. Selecting the type of electrodes**

The argument for selecting the electrode material was based on selecting the material with the greatest electro‐active area. **Table 1** shows the electrode materials evaluated with the corre‐ sponding electro‐active areas, having been calculated using the equation Randles‐Sevcik with cyclic voltammetry at different sweep speeds of 20–150 mV/s in the presence of 1 mM Cl3Ru(NH3). It showed a reversible behavior only with the reticulated vitreous carbon (RVC) and quasi‐reversible for all other electrodes in 0.1 M KCl [15].

The electrode showing the highest electro‐active area was the RVC; however, its use was discarded because it has a great capacity to adsorb organic compounds from its surface. Therefore, IrO2‐Ta2O5|Ti anode was used during the different ER treatment, because they have an effective life of 5–10 years [16] and as a cathode of Ti.


**Table 1.** Electro‐active area for different materials.

*In situ*: Soil contamination by hydrocarbon was up to 58,000 mg/kg, a current of 11 A was applied for a period of 7.5 h; in this case hydrate first with water and then 135 L of the support‐

of 9 A was applied for 4 h for each cell in a six‐cell system mounted in series, the soil removed to insert the electrodes was treated *ex situ* and then returned to its place. The volume necessary for moisturizing the soil was 120 L of 0.1M NaOH per cell, and the solution extracted at the

The treatment consisted of applying the electric field for 4 h to the first block of six cells, once it is completed the first block of the treatment is continued with the second block and so on until the end of treatment with a total of 14 blocks for complete 84 cells mount‐ ed on a three‐week period, the *ex situ* process is followed on par with the same operating

DC resistivity measurements were carried out using a Digital Ground Resistance Tester Model 4500 AEMC® INSTRUMENTS applying a current of 2 mA, using four copper electrodes,

Determination of hydrocarbon medium (NMX‐AA‐145‐SCFI‐2008) and heavy (NMX‐AA‐134‐ SCFI‐2006) fractions was performed, as well as polycyclic aromatic hydrocarbons (NMX‐AA‐

The argument for selecting the electrode material was based on selecting the material with the greatest electro‐active area. **Table 1** shows the electrode materials evaluated with the corre‐ sponding electro‐active areas, having been calculated using the equation Randles‐Sevcik with cyclic voltammetry at different sweep speeds of 20–150 mV/s in the presence of 1 mM Cl3Ru(NH3). It showed a reversible behavior only with the reticulated vitreous carbon (RVC)

The electrode showing the highest electro‐active area was the RVC; however, its use was discarded because it has a great capacity to adsorb organic compounds from its surface. Therefore, IrO2‐Ta2O5|Ti anode was used during the different ER treatment, because they have

) contaminated with hydrocarbons was treated, a constant current

ing electrolyte is added (0.1M NaOH) to the cathode hole.

294 Soil Contamination - Current Consequences and Further Solutions

placed at a distance of 1 m, before and after treatment.

146‐SCFI‐2008) before and after electrochemical treatment.

and quasi‐reversible for all other electrodes in 0.1 M KCl [15].

an effective life of 5–10 years [16] and as a cathode of Ti.

**2.6. Application of ER in the field**

**3. Results and discussion**

**3.1. Selecting the type of electrodes**

Antrosol‐type soil (275 m3

conditions.

The removal of fats and oils (F&O) were measured by Soxhlet extraction.

end of the process of ER was treated by an advanced oxidation process.

#### **3.2. Choosing the supporting electrolyte**

Of the solutions prepared from KOH, NaOH, K2HPO4, Na2HPO4, KH2PO4 and Na2HPO4 were chosen for the process of ER KOH and NaOH because they have the highest ionic molar conductivity, in this case for K+ and Na+ 73.5×10‐4 and 50.1×10‐4 sm2 /mol respectively [11].

NaOH was used as electrolyte for the higher removal of HC than KOH, because of its higher molar ionic conductivity.

This behavior can be attributed to the ability of adsorption of K+ in the ground which is higher than that of Na+ (17). Concentration of K+ in solution decreased, causing an increase in electrical resistance in soil, and decreasing the removal efficiency of HC [17–20].

#### **3.3. Choosing the best treatment**

**Table 2** shows the comparison of the three evaluated treatments. It can be observed that the electrochemical treatment shows the best removal rate with 81.9% with a period of 3.5 h [12, 21].


**Table 2.** Comparison of remediation treatments.

According to these tests, the process of ER proved to be the most efficient treatment and with 3.5 h of application time, besides being a technology that can remove both organic and inorganic contaminants in soils with high clay content and low permeability. These character‐ istics make the electrokinetic treatment a viable process to be applied on large scale in HC‐ contaminated soils.

#### **3.4. Choosing the best arrangement of electrodes**

**Table 3** summarizes the three proposed arrangements where the circular one shows the best results in removal of HC (47.81%) in soil and the highest amount of COD in solution (8880

mg/L) associated with the presence of organic pollutants transported into the solution. In the results reported in **Table 3** and **Figure 1**, the lowest and highest amounts of HC re‐ moved from all the sampled points were chosen at each of the arrangements [13, 22, 23].


**Table 3.** Results of F&O in soil and COD in solution of three‐electrode configurations.

**Figure 1.** *Representation of the different configuration of electrodes (1): face to face (A), alternating (B) and circular (C) where the red alligators are the anodes, and black alligators are the cathodes, with their corresponding removal of HC in mg HC/kg of dry soil (2).*

Based on these results, one can be convinced that the circular is the best choice for electrode configuration to be used in fieldwork, because this arrangement allows the concentration of all pollutants to the cathode hollowed by the influence of the electric field where the power lines all converge anode to the cathode.

#### **3.5. ER pilot scale** *ex situ and in situ*

mg/L) associated with the presence of organic pollutants transported into the solution. In the results reported in **Table 3** and **Figure 1**, the lowest and highest amounts of HC re‐ moved from all the sampled points were chosen at each of the arrangements [13, 22, 23].

**Figure 1.** *Representation of the different configuration of electrodes (1): face to face (A), alternating (B) and circular (C) where the red alligators are the anodes, and black alligators are the cathodes, with their corresponding removal of HC in mg HC/kg of dry soil*

*(2).*

**Configuration Removal F&O (%) COD (mg/L) Minimum Maximum**

Face to face 0.51 21.35 3830 Alternating 3.65 29.29 3080 Circular 14.97 47.81 8880

296 Soil Contamination - Current Consequences and Further Solutions

**Table 3.** Results of F&O in soil and COD in solution of three‐electrode configurations.

*Ex situ*: Samples of fats and oils have been collected for analysis as taken from different sections of the soil cell, because the soil heterogeneity represents different behaviors throughout the cell. After three weeks of electrochemical treatment, a decrease of about 84–88% was observed in the concentration of gasoline in the different sampled points (**Figure 2A**) which is due to electro‐migration, electro‐osmosis and electrophoresis, aided by water electrolysis. The contribution of the use of modified anodes IrO2‐Ta2O5|Ti is also considered, provided the chemical conditions are adequate to desorption and/or destruction of hydrocarbons present in the soil particles [14, 24].

**Figure 2.** *ER pilot scale ex situ (A) and in situ (B). Middle fraction HC content in the polluted soil before (A) and after (B) its electrochemical treatment in mg HC/kg of dry soil.*

*In situ*: The amount of F&O was registered in the sampling sections (**Figure 2B**) near the six anodes and cathodes at the beginning (**Figure 2C**) and the end of treatment (**Figure 2D**). **Table 4** shows the removal percentages obtained after a treatment of 7.5 h. In general, a decrease appreciated of pollutant in all sampled points is close to 90%; however, this is not the same in all areas, attributed to soil heterogeneity behavior.

Like in the case of ER *ex situ* treatment efficiency is attributed to transport phenomena occurring during the application of electric field, the use of IrO2‐Ta2O5|Ti anodes, the electrolysis of water, adequate wetting and high clay content in the soil.


**Table 4.** Percentages of HC removal in soil after ER *in situ*.

#### **3.6. Field application of ER**

In **Figure 3**, the blue dots ranging from one to five represent the locations of the sampling points located on the orange lines labeled with B, D and F.

**Figure 3.** *Representation of the experimental setup process ER.*

**Figure 4** shows that for the sampled points, the initial values of the middle fraction HC content (MFHC, **Figure 4A**) determined from the sampling points were higher than 50,000 mg/kg. The electrochemical treatment decreased these values by 74% with average values of 12,000 mg/kg (**Figure 3B**).

The Electrokinetic Treatment of Polluted Soil by Hydrocarbon: From Laboratory to Field http://dx.doi.org/10.5772/64631 299

appreciated of pollutant in all sampled points is close to 90%; however, this is not the same in

Like in the case of ER *ex situ* treatment efficiency is attributed to transport phenomena occurring during the application of electric field, the use of IrO2‐Ta2O5|Ti anodes, the electrolysis of water,

**Anodes Center Center cathode Cathode**

In **Figure 3**, the blue dots ranging from one to five represent the locations of the sampling

**Figure 4** shows that for the sampled points, the initial values of the middle fraction HC content (MFHC, **Figure 4A**) determined from the sampling points were higher than 50,000 mg/kg. The electrochemical treatment decreased these values by 74% with average values of 12,000 mg/kg

1 55.55 94.63 91.75 12.55

all areas, attributed to soil heterogeneity behavior.

298 Soil Contamination - Current Consequences and Further Solutions

adequate wetting and high clay content in the soil.

 21.70 80.16 79.99 52.30 84.48 85.46 44.43 41.03 ‐18.55 27.65 1.66 75.66 30.11 87.84 21.45

**Table 4.** Percentages of HC removal in soil after ER *in situ*.

**Figure 3.** *Representation of the experimental setup process ER.*

(**Figure 3B**).

points located on the orange lines labeled with B, D and F.

**3.6. Field application of ER**

**Position Removal after ER in soil (%)**

**Figure 4.** *Middle fraction HC content in the polluted soil before (A) and after its electrochemical treatment (B) in the sampled points shown in* **Figure 3**.

In the case of heavy fraction HC (HFHC), the results are presented in **Figure 5**. The contami‐ nation content decreased in all points, except 1D and 5D where the slight increase can be possibly due to the sub‐products of the electrokinetic process. The removal rates of the remaining 13 variables are ranging from 11% (1F) to 94% (4F) which demonstrated the feasibility of the field application. It was observed that applying the technique the organic compounds can be removed due to the action of the electric field with the effect of the involved transport processes (electro‐migration, electro‐osmosis, and electrophoresis), to water elec‐ trolysis, the applied electrode configuration and the current.

**Figure 5.** Heavy fraction hydrocarbon content (C28–C40) in different points sampled before and after the process of ER.

The analysis of section B for 16 kinds of poly aromatic hydrocarbons (PAHs) showed that five of them were present in greater abundance. The behavior of these compounds before (blue bars) and after (pink bars) the treatment are presented in **Figure 6**. As it is expected the content of PAHs were various throughout the site; the removal percentages are varying according to the type of compound and the site characteristics: for example, pyrene removal varies 29–90%, the Phenanthrene' removal range is between 18 and 81% and for Benzo (a) pyrene it is 33 and 61%.

**Figure 6.** Behavior of PAH content in section B before (blue bar) and after (pink bar) the electrochemical treatment.

As an additional tool to follow the distribution of the contaminant in the soil DC resistivity measurements were taken with the aim of appreciating a decrease in HC, a diminution in resistivity values reflects a decrease of HC content [24]. The purpose of applying geoelectric measure in the contaminated site is to find a fast, economic, non‐invasive method that could provide a reliable image on the distribution of soil contamination.

The DC resistivity value depends on several geological factors such as the texture class, the minerals present, the moisture content, porosity, these properties change when the soil is exposed to some type of contamination, in this case by organic compounds, which causes an increase in soil resistivity [24–27].

Behavior analysis of apparent resistivity was performed using the Wenner‐Alfa array consist‐ ing of an array of four electrodes and can be used in moderate depths, and is relatively sensitive to vertical changes under the subsurface to the center of the array, but little sensitive to horizontal changes [28, 29].

In **Figure 7**, the distribution of the measured apparent resistivity values can be observed at the test site before (left) and after (right) the treatment. It is remarkable that before the remediation process there were two zones where the apparent resistivity was higher than 20 ohmm (marked with white). After the process of ER, the resistivity values decrease to 2–4 ohmm at the same points, which is associated with a decrease in the amount of HC and increase of salts, as the sub‐products of ER. This can be validated with the results for middle and heavy fraction HC, in the cases of points 2D (removal rates of 61% HFHC and 71% for MFHC), 2F (removal rates HFHC: 35% and 64% for MFHC) 5F (removal rates: 75% for HFHC and 84% of MFHC).

The analysis of section B for 16 kinds of poly aromatic hydrocarbons (PAHs) showed that five of them were present in greater abundance. The behavior of these compounds before (blue bars) and after (pink bars) the treatment are presented in **Figure 6**. As it is expected the content of PAHs were various throughout the site; the removal percentages are varying according to the type of compound and the site characteristics: for example, pyrene removal varies 29–90%, the Phenanthrene' removal range is between 18 and 81% and for Benzo (a) pyrene it is 33 and

300 Soil Contamination - Current Consequences and Further Solutions

**Figure 6.** Behavior of PAH content in section B before (blue bar) and after (pink bar) the electrochemical treatment.

provide a reliable image on the distribution of soil contamination.

increase in soil resistivity [24–27].

horizontal changes [28, 29].

As an additional tool to follow the distribution of the contaminant in the soil DC resistivity measurements were taken with the aim of appreciating a decrease in HC, a diminution in resistivity values reflects a decrease of HC content [24]. The purpose of applying geoelectric measure in the contaminated site is to find a fast, economic, non‐invasive method that could

The DC resistivity value depends on several geological factors such as the texture class, the minerals present, the moisture content, porosity, these properties change when the soil is exposed to some type of contamination, in this case by organic compounds, which causes an

Behavior analysis of apparent resistivity was performed using the Wenner‐Alfa array consist‐ ing of an array of four electrodes and can be used in moderate depths, and is relatively sensitive to vertical changes under the subsurface to the center of the array, but little sensitive to

61%.

**Figure 7.** Apparent resistivity behavior before (A) and after (B) the electrokinetic treatment.

According to the obtained results, DC resistivity survey method can be used as an effective tool for monitoring the process of HC removal in soils. However the readings taken do not represent a value of HC concentration, it is an indirect measure of the reduction of these pollutants in the subsurface with respect to an initial value.
