**1. Introduction**

218 Electrochemical Cells – New Advances in Fundamental Researches and Applications

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Hydrochemistry of groundwater from the Tierra de Pinares region (Douro basin, Spain) affected by high levels of arsenic. *Proceedings of Arsenic 2008, 2nd International Congress on arsenic in the environment: Arsenic from nature to humans.* pp. 83-84, Pollution of top and subsurface soil, as well as underneath groundwater has been one of the consequences of industrial activities; actually environmental professionals are facing a time consuming issue when they look for potential solutions for heavy metals and organic compounds removal. Although many soil remediation technologies are available, electrokinetic processing has been an emerging technology offering advantages for a wide variety of pollutants being either organic or inorganic; as well as its versatility of being applied in soil wetting conditions ranging from unsaturated to saturated; one of the main advantages of this technology is the fact that this process can be applied to low permeability soils, like clays.

Initially electrokinetics was applied for soil consolidation, in this process water flux is forced by an electrical field action, an approach to explain how it works is based on setting up a soil structural change analysis based on modification of soil matrix, plasticity index and crystalline state (Gray, 1970). For clayey soils it has been accepted that they behave like an osmotic membrane, therefore it is important to understand how physicochemical factors affect its response in regulating osmotic pressure into the soil matrix (Fritz, 1986). Another report (Darmawan, 2002) reinforce the necessity of knowing how the solid matrix response to the electric field, since obtained electrical current is function of electrolyte concentration, buffering capacity, and chemical form of involved metals, these can be in either soluble, electrostatically adsorbed, or surface complexed forms; also, metal migration is favored when soil is dominated by clay minerals, otherwise migration is lowered when soil has a high buffering capacity and/or high humic content, the last one acts like an additional resistance to the current transference throughout the soil.

Electrokinetics as a remediation experimental procedure requires having a wetted soil in which electrodes are inserted and terminals are connected to a power source. As soon as an electric field is generated, electrode reactions take place producing protons (H+) at the anode

Electrode Materials a Key Factor to Improve Soil Electroremediation 221

reduce its activity as experimental conditions favor metal deposition and so far electrode passivation. Then, in order to increase active sites number, reduce passivation and increase useful lifetime; some efforts had addressed electrode modification using oxide materials such as CarbonTiO2, TiSnO2-Sb, TiIrO2-Ta2O5, TiRuO2. Electrodes prepared in this way have been named Dimensionally Stable Anodes (DSA) and they have proven be effective in organic degradation (Comminellis, 1994), because in addition to their high capacity to generate hydroxyl radicals, they also are mechanically resistant to abrupt pH changes.

Titanium dioxide exists in three crystalline forms namely anatase, rutile, and brookite, from which the first is the one exhibiting higher catalytic activity; which is highly dependent on the surface specific area of the particulate form, a phenomena which is exemplified by Baiju et al (2009), in their work they used anatase-TiO2 in dye removal; these authors provide evidence on how switching from a particulate form to nanotubes it results in a concurrent change in the mechanism of dye removal from an aqueous solution. Although, other authors (Yigit & Inan, 2009) have provided evidence on how a mixture anatase-rutile provides a

In wastewater treatment use of Dimensionally Stable Anodes (DSA) made of a Ti mesh covered with a film of iridium or tin has probed being an effective tool for organics degradation while keeping its mechanical resistance (León et al, 2009); also, there is a theory proposed by Comminellis (1991) in which it is shown that the iridium DSA posses a highly reactive surface which directly oxidizes the substrate, at this respect an improvement in oxidant activity is obtained when titanium mesh is covered with a film including a mixture of oxides like Ir-Ta (Hu et al, 2002). Use of modified electrodes favors electrocatalytic processes by supporting higher current densities and so far reducing the corresponding oxidation potentials, because inner sphere mechanisms allow for hydroxyl radicals generation at an interfacial level, by which there it is possible to increase organic molecules decomposition reaching total mineralization levels. About the electrode matrix it can be affirmed that DSA usually exhibit higher mechanical resistance because they are prepared onto a rigid matrix, while the ones based on Reticulated Vitreous Carbon (RVC)

Considering that few attention have been dedicated to the role that electrode material plays on soil electroremediation, since a higher electrode activity would allow developing conditions which could enhance pollutant removal, specially when the pollutant is an hydrocarbon product. In this sense, this contribution presents results from two research approaches in which it is tested how inclusion of a catalytic specie, like anatase (TiO2) in a reticulated vitreous carbon (RVC) or a titanium anode covered with IrO2-Ta2O5 film will help to improve occurrence of water electrolysis reactions; also, each experimental approach allowed to elucidate the role played by materials used as anode and cathode, as well as the influence exerted by additional resistance factors like electrode position in respect to the soil interphase. Finally, it is an opportunity to test the theory by which it is expected an enhancement of electrokinetic hydrocarbon removal as consequence of the higher electrode

Experimental work is organized as follows: 1) Electrodes made of Reticulated Vitreous Carbon (RVC), in which anode was modified by inclusion of an anatase deposit (TiO2), and keeping constant a RVC cathode, this provides two combinations bare RVC electrodes and RVC-TiO2 anode with bare RVC cathode; 2) Anode made of a titanium plate covered with

higher efficiency in humic acid mineralization.

have higher reactive surface but a lower mechanical resistance.

activity.

and hydroxide (-OH) at the cathode; concentration of these ions increases exponentially creating and acid front which moves from anode to cathode, and a basic front moving from cathode to anode; during its passage through the soil protons and hydroxides interact with sorbed pollutants releasing them into solution. Soluble ion transport occurs by three mechanisms: 1) Diffusion due to concentration gradients, 2) Convection due to fluid movement and 3) Migration due to the electric field.

A sample of initial published results (Acar et al, 1994; Hamed et al, 1991; Khan & Alam, 1994; Kim et al, 2002; Pamucku et al 1990; Pamucku & White, 1992; Reddy et al, 1999) is enough to claim that this method is highly efficient on restoration actions for clayey soils having very low heavy metal concentrations, for which regular mining procedures would result very expensive; although, for this method one of the minuses is the time required to get metal removals above 90%. Most of these studies report soil characterization providing information about: sand, clay, silt content; organic matter as well as hydraulic permeability. Although, few reports have covered soil electrical resistance, which according to Vázquez et al (2004), it could be used as a method for analyzing soil behavior in presence or absence of an electrical field.

In order to improve the process and get shortening of experimental times, applied efforts have covered a wide set of conditions. Some examples of reported research have addressed for modifying pH and current density (Hamed& Bhadra, 1997), chemical conditioning of electrode wells (Reed et al, 1995; Murillo-Rivera et al, 2009), cation inclusion (Colleta et al, 1997), as well as addition of complexing (Yeung et al, 1996) and lixiviant agents (Cox et al 1996); another approach has been the inclusion of reactive barriers into the soil matrix (Cundy & Hopkinson, 2005; Ruiz et al, 2011).

When treating PAHs (Polyaromatic Hydrocarbons) soil pollution it is important to care about lateral effects like lowering the electroosmotic flow rate (EOFR), which can be consequence of controlling pH at the electrode wells, by doing this an affectation of soil and/or solution chemistry can be induced producing an accumulation at the neutral or alkaline soil regions (Saicheck & Reddy, 2003). Another factor affecting EOFR derives from surfactant inclusion, then it becomes necessary to evaluate if it is worthy lowering EOFR for increasing PAHs removal (Kolosov et al, 2001). Also, pollutant mobilization should be evaluated as a function of pH control and surfactant addition, since flow can occurs in anodic direction (Ribeiro et al, 2005), or be enhanced in the cathodic direction.

Finally, organics and metal removal can be affected by geometry cell and flow direction, in this sense a report about an upward electrokinetic soil remediation (Wang et al, 2007) points out that removal efficiency is increased for organics when electroremediation cell is smaller in diameter, or larger in height; otherwise for metals, removal is improved when the cell is smaller in diameter or shorter in height.

Electrode materials are a key parameter to assure that electron transference takes place at fast rates. Selection of materials should be based on thermodynamic and kinetic response, so interface molecular interactions are fast enough to release the oxidant species. Also, it must be considered aspects like mechanical, thermic and corrosion resistance; as well as the procedures and solutions used in surface cleaning, pretreatment, and surface activation. In this sense, materials which satisfy these requirements are: vitreous carbon, titanium, stainless steel, platinum, gold and silver; but it must not be discarded that all materials can

and hydroxide (-OH) at the cathode; concentration of these ions increases exponentially creating and acid front which moves from anode to cathode, and a basic front moving from cathode to anode; during its passage through the soil protons and hydroxides interact with sorbed pollutants releasing them into solution. Soluble ion transport occurs by three mechanisms: 1) Diffusion due to concentration gradients, 2) Convection due to fluid

A sample of initial published results (Acar et al, 1994; Hamed et al, 1991; Khan & Alam, 1994; Kim et al, 2002; Pamucku et al 1990; Pamucku & White, 1992; Reddy et al, 1999) is enough to claim that this method is highly efficient on restoration actions for clayey soils having very low heavy metal concentrations, for which regular mining procedures would result very expensive; although, for this method one of the minuses is the time required to get metal removals above 90%. Most of these studies report soil characterization providing information about: sand, clay, silt content; organic matter as well as hydraulic permeability. Although, few reports have covered soil electrical resistance, which according to Vázquez et al (2004), it could be used as a method for analyzing soil behavior in presence or absence of

In order to improve the process and get shortening of experimental times, applied efforts have covered a wide set of conditions. Some examples of reported research have addressed for modifying pH and current density (Hamed& Bhadra, 1997), chemical conditioning of electrode wells (Reed et al, 1995; Murillo-Rivera et al, 2009), cation inclusion (Colleta et al, 1997), as well as addition of complexing (Yeung et al, 1996) and lixiviant agents (Cox et al 1996); another approach has been the inclusion of reactive barriers into the soil matrix

When treating PAHs (Polyaromatic Hydrocarbons) soil pollution it is important to care about lateral effects like lowering the electroosmotic flow rate (EOFR), which can be consequence of controlling pH at the electrode wells, by doing this an affectation of soil and/or solution chemistry can be induced producing an accumulation at the neutral or alkaline soil regions (Saicheck & Reddy, 2003). Another factor affecting EOFR derives from surfactant inclusion, then it becomes necessary to evaluate if it is worthy lowering EOFR for increasing PAHs removal (Kolosov et al, 2001). Also, pollutant mobilization should be evaluated as a function of pH control and surfactant addition, since flow can occurs in

Finally, organics and metal removal can be affected by geometry cell and flow direction, in this sense a report about an upward electrokinetic soil remediation (Wang et al, 2007) points out that removal efficiency is increased for organics when electroremediation cell is smaller in diameter, or larger in height; otherwise for metals, removal is improved when the cell is

Electrode materials are a key parameter to assure that electron transference takes place at fast rates. Selection of materials should be based on thermodynamic and kinetic response, so interface molecular interactions are fast enough to release the oxidant species. Also, it must be considered aspects like mechanical, thermic and corrosion resistance; as well as the procedures and solutions used in surface cleaning, pretreatment, and surface activation. In this sense, materials which satisfy these requirements are: vitreous carbon, titanium, stainless steel, platinum, gold and silver; but it must not be discarded that all materials can

anodic direction (Ribeiro et al, 2005), or be enhanced in the cathodic direction.

movement and 3) Migration due to the electric field.

(Cundy & Hopkinson, 2005; Ruiz et al, 2011).

smaller in diameter or shorter in height.

an electrical field.

reduce its activity as experimental conditions favor metal deposition and so far electrode passivation. Then, in order to increase active sites number, reduce passivation and increase useful lifetime; some efforts had addressed electrode modification using oxide materials such as CarbonTiO2, TiSnO2-Sb, TiIrO2-Ta2O5, TiRuO2. Electrodes prepared in this way have been named Dimensionally Stable Anodes (DSA) and they have proven be effective in organic degradation (Comminellis, 1994), because in addition to their high capacity to generate hydroxyl radicals, they also are mechanically resistant to abrupt pH changes.

Titanium dioxide exists in three crystalline forms namely anatase, rutile, and brookite, from which the first is the one exhibiting higher catalytic activity; which is highly dependent on the surface specific area of the particulate form, a phenomena which is exemplified by Baiju et al (2009), in their work they used anatase-TiO2 in dye removal; these authors provide evidence on how switching from a particulate form to nanotubes it results in a concurrent change in the mechanism of dye removal from an aqueous solution. Although, other authors (Yigit & Inan, 2009) have provided evidence on how a mixture anatase-rutile provides a higher efficiency in humic acid mineralization.

In wastewater treatment use of Dimensionally Stable Anodes (DSA) made of a Ti mesh covered with a film of iridium or tin has probed being an effective tool for organics degradation while keeping its mechanical resistance (León et al, 2009); also, there is a theory proposed by Comminellis (1991) in which it is shown that the iridium DSA posses a highly reactive surface which directly oxidizes the substrate, at this respect an improvement in oxidant activity is obtained when titanium mesh is covered with a film including a mixture of oxides like Ir-Ta (Hu et al, 2002). Use of modified electrodes favors electrocatalytic processes by supporting higher current densities and so far reducing the corresponding oxidation potentials, because inner sphere mechanisms allow for hydroxyl radicals generation at an interfacial level, by which there it is possible to increase organic molecules decomposition reaching total mineralization levels. About the electrode matrix it can be affirmed that DSA usually exhibit higher mechanical resistance because they are prepared onto a rigid matrix, while the ones based on Reticulated Vitreous Carbon (RVC) have higher reactive surface but a lower mechanical resistance.

Considering that few attention have been dedicated to the role that electrode material plays on soil electroremediation, since a higher electrode activity would allow developing conditions which could enhance pollutant removal, specially when the pollutant is an hydrocarbon product. In this sense, this contribution presents results from two research approaches in which it is tested how inclusion of a catalytic specie, like anatase (TiO2) in a reticulated vitreous carbon (RVC) or a titanium anode covered with IrO2-Ta2O5 film will help to improve occurrence of water electrolysis reactions; also, each experimental approach allowed to elucidate the role played by materials used as anode and cathode, as well as the influence exerted by additional resistance factors like electrode position in respect to the soil interphase. Finally, it is an opportunity to test the theory by which it is expected an enhancement of electrokinetic hydrocarbon removal as consequence of the higher electrode activity.

Experimental work is organized as follows: 1) Electrodes made of Reticulated Vitreous Carbon (RVC), in which anode was modified by inclusion of an anatase deposit (TiO2), and keeping constant a RVC cathode, this provides two combinations bare RVC electrodes and RVC-TiO2 anode with bare RVC cathode; 2) Anode made of a titanium plate covered with

Electrode Materials a Key Factor to Improve Soil Electroremediation 223

Soil sample corresponds to a hydrocarbon polluted weathered soil, for which a physical characterization is done by using the ASTM D4318 as well as the USCS-P13-B-2 procedure, by which it was established soil type and textural composition. Since this soil has hydrocarbon pollutants, its concentration was determined as oil and grease by the Soxhlet technique. According to published results (Murillo-Rivera et al, 2009), 0.1M NaOH solution is an electrolyte that works fine for hydrocarbon polluted soils, so this one was chosen as the

Experimental cell was a rectangular one (10 cm length, 2 cm width, 4 cm high), a current density of 20 mA cm-2 was imposed with a PDC-GP 4303DU Power Source, in a

Considering that a DSA anode provides oxidant species at higher rates, then for this hydrocarbon polluted soil electroremediation it was chosen an electrode arrangement, considering a modified DSA made of a titanium plate covered with an iridium-tantalum film (TiIrO2-Ta2O5), and two types of cathode: carbon felt (CF) and a titanium plate (Ti). Also, in these set of experiments it was considered two electrode positions: in the first one, a physical barrier of filter paper was included between soil and electrode, while in the second

Registered experimental parameters were: applied electrical current, developed electrical potential, with these it was possible to calculate cell resistance and energy consumption. At the end of each experiment soil was cut in 3 sections (anode, middle, cathode), hydrocarbon removal was estimated from soil residual concentrations, which were extracted by the Soxhlet technique, and later analyzed by UV-Vis (XLS, Perkin-Elmer) gas chromatography

Soil characterization results are reported in Table 1. As it can be observed this soil is classified as a Low plasticity Clay (CL), then it will no exhibit a great volume change during experimentation; also, clay and silt content indicate that this is a low permeability soil.

> plasticity clay (CL)

Plasticity Chart (Helwany,

2007, page 13)

Parameter Value Methodology Liquid Limit (LL) % 36 ASTM D4318-10 Plastic Limit (PL) % 24 ASTM D4318-10 Plasticity Index (PI) % 12 ASTM D4318-10

Sand % 34 USCS-P13-B-2 Silt % 53 USCS-P13-B-2 Clay % 13 USCS-P13-B-2

**2.2 Cathode modification** 

galvanostatic mode during 4 hours.

**3. Results** 

**3.1 Anode modification** 

electrolyte for soil wetting, and wells replenishment.

one the electrode was set in direct contact with soil sample.

coupled to mass spectroscopy (CG-EM, Agilent GC 19091-413).

Classification Low

Table 1. Physical and textural properties of Guanajuato soil.

IrO2-Ta2O5, and two cathode materials carbon felt (CF) and titanium plate (Ti), plus a variation of its position respect to the soil matrix. These experiments are described in the following paragraphs.
