**2. Methodology**

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

The activity was carried out with testing cyclic voltammetry using a potentiostat BAS Epsilon and a glass cell with three electrodes as a reference electrode Ag|AgCl saturated with KCl, wire Ti as auxiliary electrode and plaques of different materials to evaluate as working electrodes. The supporting electrolyte used in these tests was phosphate buffer solution at pH 12 (*i* = 0.1), because it has been reported that hydrocarbons are best removed in alkaline medium [10].

#### **2.2. Choosing the supporting electrolyte**

Solutions of KOH, NaOH, K2HPO4, Na2HPO4, KH2PO4 and Na2HPO4 were all prepared at 0.1M in water, which was used to wet soil for an electrolysis. UV‐Vis spectrophotometry was used to verify the removal of HC after electrochemical treatment in the different solutions used [11].

#### **2.3. Choosing the best treatment**

In the last three decades, there have been several investigations at laboratory and pilot even applying electrokinetic basis to remove a variety of contaminants. The electro‐remediation has been successfully tested in the USA [1–7]. There are even companies offering it as an alternative remediation method within the portfolios of their services a large scale in soils with high clay

The ER method has demonstrated its ability to remove some organic contaminants in studies at laboratory, pilot or field [6], but its main application was on sites contaminated with metals in order to remove elements such as chromium, cadmium, mercury, lead, zinc, etc. [7].

In several studies, the application of the ER process has helped to achieve efficiencies close to 100% removal, particularly if the pollution is caused by a single metal (Pb). In *on‐site* applica‐

One example is the consortium formed by Monsanto, DuPont and General Electric, where the applied technology was called LasagnaTM ER *in situ* to remove trichloroethylene, achieving

Another practical example was developed by Sandia National Laboratories, for electrochem‐ ical *in situ* remediation of soil contaminated with chromium, where electrodes of Iridium/ Titanium were used with applying a power of 1572 kW/h; after 5 months of continuous

Also, the ER was made at the Centro de Investigación y Desarrollo Tecnológico en Electroquí‐ mica, S. C. (CIDETEQ) at laboratory level in order to be able to apply it at pilot and on field level. For that reason, several investigations were developed that led to get familiar with different aspects of field application helping implementation of the technique in a petroleum industrial area. Meanwhile the Geological and Geophysical Institute of Hungary developed an analytical method for investigating the physical and chemical characteristics of soil.

The activity was carried out with testing cyclic voltammetry using a potentiostat BAS Epsilon and a glass cell with three electrodes as a reference electrode Ag|AgCl saturated with KCl, wire Ti as auxiliary electrode and plaques of different materials to evaluate as working electrodes. The supporting electrolyte used in these tests was phosphate buffer solution at pH 12 (*i* = 0.1), because it has been reported that hydrocarbons are best removed in alkaline

Solutions of KOH, NaOH, K2HPO4, Na2HPO4, KH2PO4 and Na2HPO4 were all prepared at 0.1M in water, which was used to wet soil for an electrolysis. UV‐Vis spectrophotometry was used to verify the removal of HC after electrochemical treatment in the different solutions used [11].

tions, the results depended on soil‐type variables and the type of pollutant [3].

content.

removal of 98% [8].

**2. Methodology**

medium [10].

**2.1. Selecting the type of electrodes**

**2.2. Choosing the supporting electrolyte**

treatment 64% efficiency was obtained [9].

292 Soil Contamination - Current Consequences and Further Solutions

The technologies described below were compared in order to find the best treatment for decontaminating soils; in all the three cases the removal of oil by Soxhlet extraction at the end of treatment was evaluated. The initial content of fats and oils of the contaminated soil was 4000 mg HC/kg of dry soil [12].

*Soil washing surfactant Triton X‐114*: Triton X‐114 (4% V/V) was passed at a flow rate of 1.5 mL/ min into a tubular reactor containing 30 g soil, for a period of 5 h.

*Biological treatment with solid culture*: 30 g of soil was added to agro‐industrial waste bagasse and filter cake with a‐residue soil agro‐industrial 100:2:2, together they were placed in glass containers while maintaining a temperature of 28°C, for a period of 15 days with aeration every 3 days for 20 min.

*Electro‐remediation of contaminated soil*: a tubular reactor was used with 30 g soil and 0.1 M NaOH as supporting electrolyte with a flow rate of 1.5 mL/min, by applying a current of 2 mA for a period of 3.5 h; the working electrodes were titanium mesh (cathode) and Ti|IrO2‐Ta2O5 (anode).

#### **2.4. Choosing the best configuration of electrodes**

Three‐electrode configurations were evaluated: (a) face to face consisting of four cathodes and eight anodes (all rectangular) placed opposite the cathodes; (b) the arrangement of alternating electrodes consisted of six cathodes and six anodes alternating rows of three; and (c) the circular configuration resided in a central cathode and six anodes around this one [13]. The sample amount was 1.9 kg for the three cases and hydrated for a period of 18 h with 800 mL of 0.1M NaOH; the current applied was 0.23 A for a period of 6 h. The used working electrodes were made of Titanium plates and IrO2‐Ta2O5|Ti as cathodes and anodes respectively, all at a distance of 6 cm.

The removal process was followed by Soxhlet extraction on the ground and in the solution for determination of chemical oxygen demand (COD), samples for fats and oils were obtained near the anodes and cathodes, as well as in the half‐cell.

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

The arrangement of circular electrodes was used during ER pilot scale *in situ* and *ex situ*. The cathode was used in the center of the electrochemical cell, and the IrO2‐Ta2O5|Ti anodes were used around this one. All the electrodes were used during ER pilot system with dimensions of 60 cm length × 24 cm diameter, which were placed 117 cm between them. In these experi‐ ments the amount of soil type Vertisol pelic treated was 3.3 m3 [13, 14].

*Ex situ*: The soil was contaminated with 1126 mg/kg by gasoline. To ER a constant current of 9 A during 4.5 h by day was applied, adding every day 60 L of 0.7 μM NaOH as supporting electro‐ lyte.

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

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

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

Antrosol‐type soil (275 m3 ) contaminated with hydrocarbons was treated, a constant current 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 end of the process of ER was treated by an advanced oxidation process.

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 conditions.

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

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