**1. Introduction**

28 New Technologies in the Oil and Gas Industry

Janeiro, Brazil.

São Paulo, 2006.

[65] El-Naas M H et al. (2009) Assessment of electrocoagulation for the treatment of

[66] Cerqueira A A (2011) Aplicação da técnica de eletrofloculação utilizando corrente alternada de frequência variável no tratamento de água de produção da indústria do petróleo. 133f. Tese de Doutorado – Universidade do Estado do Rio de Janeiro, Rio de

[67] Ferreira L H (2006) Remoção de sólidos em suspensão de efluente da indústria de papel por eletroflotação. 82f. Dissertação (Mestrado) - Universidade Estadual de Campinas,

[68] Irdemes S et al. (2006) The effect of current density and phosphate concentration on phosphate removal from wastewater by electrocoagulation using aluminum and iron

petroleum refinery wastewater. J. Envir. Management, v. 91, p.180–185.

plate electrode. Sep. Purif. Technology, v. 52, 218–223.

Bioremediation refers to the application of biological agents, typically microbes, to the removal of pollutants from an environment (e.g. through landfarming and biopiles). The effectiveness of bioremediation depends greatly on the presence of suitable microorganisms and nutrients in the subsurface. Therefore, much remains to be done in order that a generally accepted methodology can be developed for a broad range of applications [1]. One approach has been to combine bioremediation with electrokinetics (EK) into a hybrid technology, referred to as electrobioremediation (EKB). EKB uses bioremediation to degrade hydrocarbon contaminants and EK to mobilise them. EK mobilisation of the hydrocarbon products increases their bioavailability, thereby facilitating bioremediation. Whilst commonly used in the remediation of several inorganic contaminants [2,3,4,5], EK has also been successfully applied to the remediation of several soluble organic contaminants, such as phenanthrene, benzene, toluene, and phenol [6,7]. However, the efficiency of this process is severely limited when the compounds have a low solubility or bioavailability. Under these conditions, in situ flushing has the potential to improve the electrokinetically-enhanced soil-solution–hydrocarbon interaction and subsequent contaminant removal by pumping a solution directly into the subsurface of the contaminated site. Nevertheless, in situ flushing is highly dependent on the type of flushing solution employed. If the pollutant is non-ionic, it can be removed by the electroosmotic flux. However, for fine-grained soils, in which the low hydraulic permeability does not allow effective pump and treat techniques, EK remediation may be the only useful process to remove organic pollutants. Indeed, an effective hydraulic velocity of 4x10-7 m/s during EK treatment at 4 V/cm has been achieved in a soil sample with a hydraulic permeability of 5x10-10 [8]., which can be considered essentially impermeable to mechanical pumping [9].

© 2012 Acuña et al., licensee InTech. This is an open access chapter 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. © 2012 Acuña 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.

**Figure 1. The effect of electrokinetic phenomena on porous soil.** The application of an electric current generates hydroxide ions (OH- ) and hydrogen gas (H2) at the cathode and hydrogen ions (H+) and oxygen gas (O2) at the anode. Subsequent diffusion of OH- and H+ introduces a pH gradient throughout the affected subsurface that in turn facilitates electrokinetic migration of soil constituents. Microbes and PAHs (exampled with phenanthrene) migrate to the cathode by electroosmosis (EO). Electronegative microbes also migrate to the anode electrophoretically (EP). Whereas electromigration (EM) dictates the migration of ions (such as sodium and chlorine) and heavy metals (HM).

The underlying mechanism of EK involves the introduction of an electric current into soil. The introduced electric current leads to the migration of contaminants via electroosmosis, electromigration, and electrophoresis. These processes occur as a consequence of the resulting pH gradient that follows the production of hydrogen ions at the anode and hydroxyl ions at the cathode (Fig.1). These phenomena cause changes in a number of soil properties [10]. Electromigration and electrophoresis result in the movement of ions, ion complexes, and charged particles, such as colloidal clay and microorganisms toward the electrode of the opposite charge. Whereas electroosmosis arises from the migration of water towards the cathode, producing an electroosmotic flow which, in turn leads to facilitates the movement of cations, hydrocarbons and microorganisms in the direction of the fluid [11]. Accordingly, changes in the available nitrogen, phosphorus and potassium in soil were observed after EK remediation [12]. The migration of electrolytes causes an increase in the same electroosmotic flux direction, whilst decreasing it at the opposite pole. The loss of moisture from EK-treated soil may also be due to warming by the passage of current, or exothermic reactions that may occur in the soil because the temperature increases between 1- 3 ºC [13]. Consequently, there must be a balance between electroosmotic migration, evaporation by heating or exothermic reactions and the supply of water at the anode.

The pH promotes interactions between metals and other compounds, that are a natural part of the soil, and regulates the availability of pollutants [2,10]. The passage of current directly into soil results in the electrolysis of water, thereby generating hydrogen ions in the anode and hydroxide ions at the cathode. This process occurs according to the following equations:

Cathode (reduction):

30 New Technologies in the Oil and Gas Industry

generates hydroxide ions (OH-

**Figure 1. The effect of electrokinetic phenomena on porous soil.** The application of an electric current

>pH7>

oxygen gas (O2) at the anode. Subsequent diffusion of OH- and H+ introduces a pH gradient throughout the affected subsurface that in turn facilitates electrokinetic migration of soil constituents. Microbes and PAHs (exampled with phenanthrene) migrate to the cathode by electroosmosis (EO). Electronegative microbes also migrate to the anode electrophoretically (EP). Whereas electromigration (EM) dictates the

The underlying mechanism of EK involves the introduction of an electric current into soil. The introduced electric current leads to the migration of contaminants via electroosmosis, electromigration, and electrophoresis. These processes occur as a consequence of the resulting pH gradient that follows the production of hydrogen ions at the anode and hydroxyl ions at the cathode (Fig.1). These phenomena cause changes in a number of soil properties [10]. Electromigration and electrophoresis result in the movement of ions, ion complexes, and charged particles, such as colloidal clay and microorganisms toward the electrode of the opposite charge. Whereas electroosmosis arises from the migration of water towards the cathode, producing an electroosmotic flow which, in turn leads to facilitates the movement of cations, hydrocarbons and microorganisms in the direction of the fluid [11]. Accordingly, changes in the available nitrogen, phosphorus and potassium in soil were observed after EK remediation [12]. The migration of electrolytes causes an

migration of ions (such as sodium and chlorine) and heavy metals (HM).

) and hydrogen gas (H2) at the cathode and hydrogen ions (H+) and

2H2O + 2e- → 2OH- + H2 E° = -0,83V (alkaline)

Anode (oxidation):

H2O → 2H+ + ½ O2 + 2e-E° = +1,23V (acid)

As a result of these reactions, an acid front and a basic front are created at the anode and cathode, respectively [14]. The ideal situation occurs when the contaminant remains are dissolved in the water and not precipitated by changes in pH, when there are no changes by contact with electrodes or interactions between the contaminant and soil particles. This situation is partially fulfilled by heavy metals and some organic compounds, such as phenols or other electrically charged compounds. However, these conditions are not met by hydrocarbons present in oil as they generally have no electric charge or, if they have, it is of very low intensity. Therefore, these hydrocarbons are normally adsorbed on soil particles and are sparingly soluble in water. Under these conditions, electroosmosis is important because it allows the migration of such compounds along the path of the migrating water.

The changes induced by the application of direct current into the soil have direct effects on the microbial activity in situ. Several studies have made efforts to enhance the transport of bacteria or nutrients for effective biodegradation through the application of EKB [15;16; 17; 18, 19].

When the current-intensity is measured with different soil textures, it was found that using only large or small particles was favourable, whereas a sandy clay soil was not favourable to any of the fundamental EK processes [20].

The processes pertaining to EKB are also themselves affected by moisture, pH, chemical nature of the contaminant and zeta potential (ζ) of the soil [2]. The zeta potential is the property that determines the load of a colloid as a function of the charged surface and environment in which it is located. Fully ionisable salts are not colloids, so its ζ is very small; the ζ in most soils is negative. With increased acidity, ζ increases such that it can reach positive values [14]. The increase in ζ impacts on the electroosmotic flow. Soil characteristics as absorbency, ion exchange buffer capacity (pH) and load surface have a marked influence on the EKB. This shows successful results in clay, fine-grained and low permeability soil. Whereas sandy soils should have an impermeable structure at a reasonable depth to allow high humidity or saturation.

The outer surface of bacterial cells possess numerous chemical groups which, at pH 7 or greater, result in an overall negative surface potential [21]. It is therefore possible to speculate about bacterial movement under the influence of an electrical field. Soil pH changes generated allow the bacteria to migrate by electrophoresis into one of the electrodes. As can be seen in Fig.1, the negatively charged membrane causes bacteria to migrate in the direction of anode [15]. Whilst the rate of migration under an electric field is quick (5 cm/h) in aqueous media, it is slow in soil, falling to ~0.8 cm /h. [17]. At low pH, the bacterial membrane charge is positive and the direction of movement is modified to the cathode. This amphoteric property is due to the complexity of surface charges on the bacterial membrane which arise from the combination of acidic (phosphates, carboxylic acids and sulphates) and basic (most notably amines) chemical groups found on the membrane surface. Consequently, it is difficult to predict their performance with biophysical parameters [21]. This difficulty is compounded as some microbes possess the ability to change their surface polarity, thus affording them some flexibility with respect to their relative migration [22]. Nonetheless, bacterial behaviour in the electric field will strongly depend on the field's intensity. The application of the electric field will result in the migration of negatively charged micro-organisms toward the anode and one-dimensional flow of pore fluid from the anode to cathode [22,23]. Importantly, a current of 40 mA at a density of 0.1-0.2 mA/cm2 is preferable in order to achieve a one-dimensional flow of a *Pseudomonas*-loaded pore fluid from the anode to cathode [24]. However, at a current density of 0.1-0.2 mA/cm2, the pH stabilises in the range of 2-3 at the anode destroying the acid-intolerant microbial species and in the range of 8-12 at the cathode killing the baseintolerant species. [22], thereby limiting the efficacy of the EKB process. On the other hand, the application of a recirculating buffer solution and careful regulation of electrolyte concentrations was also shown to afford some control over the pH and thereby improve distribution of a *Pseudomonas* strain which resulted in a 60% degradation of diesel over an 8 day period [24]. Maintenance of soil pH, between 5-7, is therefore necessary in order to achieve the optimum degradation of contaminants by most native soil microbes [22].

Polycyclic aromatic hydrocarbons (PAHs) are a particularly important class of pollutant as they are generated by the incomplete combustion of carbon-based fuels and are ubiquitously found in tar, oil and coal deposits [25]. Consequently, they represent one of the most widespread and abundant class of pollutants. PAHs are of particular concern as members of this class have been identified as being mutagenic, teratogenic and carcinogenic [26]. Their abundance and relative resistance to evaporation [27] makes them ideal candidates as a model contaminant through which the effectiveness of remediation technologies can be assessed.

Our main objectives were to test the effect of EKB on: (i) the removal of PAHs, (ii) to determine the increase in bioaccessibility of PAHs in soil, which would suggest improved bioremediation performance, and (iii) to evaluate the resultant change in the bacterial communities. The measured parameters included the hydrogen ion concentration (pH) values, electrical potential, bacterial count and total petroleum hydrocarbon (TPH) content and these parameters were measured along the length of each soil specimen. The results were analyzed to assess the electrokinetic remedial efficiency.
