Environmental Engineering and Management Journal April 2015, Vol.14, No. 4, 879-886

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

NOVEL ELECTROKINETIC REMEDIATION SYSTEM - BORON DOPED

DIAMOND (EKR-BDD) PROCESS FOR THE DESTRUCTION

OF POLYAROMATIC HYDROCARBONS (PAHs) IN LIQUID PHASE

Alejandro Medel1, Diana Patiño1, Erika Méndez1,2, Yunny Meas1, Luis Godínez1, Juan Manríquez1, Francisco Rodríguez1, Adrián Rodríguez1, Erika Bustos2

1Center for Research and Technological Development in Electrochemistry. Technological Park Querétaro S/N,

Sanfandila, Pedro Escobedo, Querétaro P.C. 76730 2Autonomous University of Puebla, Chemistry Center of the ICUAP, University City. Bldg. 103H. Puebla,

Puebla, Mexico P.C. 72570

Abstract This research evaluated two electrokinetic remediation systems (EKR) for separating phenanthrene from bentonite and its electrochemical destruction by using a Boron Doped Diamond (BDD) electrode. The effect of the electrochemical potential for the oxidation of phenanthrene in liquid phase with Ti/BDD was analyzed by Normal Pulse Voltammetry and Hydroxyl Radical (●OH) analysis using the Spin Trapping Technique. The results showed that 70% of phenanthrene was removed from bentonite through EKR by applying 20 mA for 4 h in alkaline conditions, and that phenanthrene in solution was 100% degraded with Ti/BDD by applying 2.3 V vs Hg|Hg2SO4, for 2.5 h. These results demonstrate the potential application of the electrochemical technology in treating soils contaminated with highly toxic compounds, such as Polyaromatic Hydrocarbons (PAHs) and their final destruction using the EKR-BDD process. Key words: diamond, hydroxyl radical, PAHs, soil Received: January, 2014; Revised final: August, 2014; Accepted: August, 2014

Authors to whom all correspondence should be addressed: e-mail: alejandromedelreyes@gmail.com, ebustos@cideteq.mx; Phone: +52 442 2 11 60 59; Fax: + 52 442 2 11 60 01

1. Introduction Pollution problems affect all natural resources

including soil, air, and water. At the same time, the exploding population and economic development increases the demand for goods, services, and energy. This generates an ever growing volume of industrial and urban waste (Sainchek and Reddy, 2003). Much of the soil is polluted with organic and inorganic compounds like heavy metals, pesticides, pharmaceutical byproducts, dyes, and hydrocarbons, such as Polyaromatic Hydrocarbons (PAHs). The PAHs, like phenanthrene, anthracene and pyrene, are carcinogenic micropollutants that are resistant to

environmental degradation because they are very hydrophobic. These pollutants are also difficult to remove from clay-like soils. With their low aqueous solubility, these pollutants form strong bonds to clay minerals and organic matter present in soil (Luthy et al., 1997). Conventional methods such as extraction, bioremediation, phytoremediation, chemical oxidation, photocatalytic degradation, thermal treatment and integrated remediation are usually not effective for removing these pollutants from soil.

Instead, electrochemical technology is preferred. Electrokinetic remediation (EKR) is a technique based on applying a potential gradient or direct electric current to electrodes inserted in moist

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soil in order to mobilize any pollutants which may be present (Vázquez et al., 2004). This technique is effective in removing pollutant species from even impermeable or clay-like soils (Sainchek and Reddy, 2003) and, depending on their nature, pollutants can be removed by three different electrokinetic transport mechanisms: 1) electro-migration, which allows the transport of charged species which move to the electrodes of contrary charge; 2) electro-osmosis, by transportation of the solvatation water from the cations; and, 3) electro-phoresis, the mechanical movement of colloidal particles or microorganisms present in the soil (Probstein and Hicks, 1993). Besides these transport mechanisms there are other reactions that occur during EKR, such as water oxidation and reduction reactions that occur on the anode and cathode surfaces, respectively (Joseph et al., 1997).

The effluents from an EKR process may contain highly toxic compounds, which cannot be treated by conventional methods. Instead, advanced oxidation processes (AOPs) (Stasinakis, 2008) that generate hydroxyl radicals (●OH) have been shown to be an excellent method for destroying toxic organic compounds. ●OH are oxidizing species whose oxidation potential (2.8 V vs. ENH) (Mota, 2008) degrades pollutants, transforming them into harmless species. AOPs include chemical processes like ozone (O3) in alkaline medium, hydrogen peroxide (H2O2) in combination with ultraviolet light (Rodríguez et al., 2008), as well as Fenton processes (in the presence or absence of light) (Lloyd et al., 1997; Philippopoulos and Poulopoulos, 2003), and photocatalysis (Akira et al., 2000). Although the chemical AOPs are able to degrade the compounds, the destruction of reaction products is more difficult. Recently there has been much interest in using Electrochemical Advanced Oxidation Processes (EAOPs) because these generate ●OH in situ (Panizza and Cerisola, 2009; Brillas et al., 2009). In this category, Electrochemical Oxidation (EO) using Boron Doped Diamond (BDD) as anode has been more accepted than other processes like Electro-Fenton and Photo-Electro-Fenton (Kraft et al., 2003; Medel et al., 2012). This is primarily due to the characteristics of BDD, which is preferred over other materials like graphite, Pt, IrO2, RuO2, SnO2-Sb, and PbO2. BDD has high corrosion resistance, both chemical and electrochemical stability, a wide potential window in aqueous electrolytes and high ●OH production (Panizza and Cerisola, 2005). During the EO process with BDD, organic pollutants can be completely mineralized to CO2 and water, by the ●OH generated on the surface of BDD through the water oxidation process (Eqs. 1) (Marselli et al., 2003) in a parallel reaction to oxygen evolution (Eq. 2).

BDD + H2O → BDD (●OH) + H+ + e- (1) BDD (●OH) → BDD + 1/2O2 + H+ + e- (2)

Considering the advantages that EKR and EO with BDD offer in the removal and destruction of highly toxic pollutants, their coupling (EKR-BDD process) is a totally viable alternative, which eliminates the problem of liquid waste disposal from the EKR process. However, despite the advantages of a coupled EKR-BDD process, very few studies have been published on this application. In this sense, Lee et al. (2009) reported a study on the application of an EKR-BDD process for remediation of kaolin soil contaminated with Acid Blue 25. Their results indicated that 89% of the anionic dye was removed using EKR applying a current density of 30 mA for 7 days, while, the dye solution was completely mineralized in the presence of chloride ions using a BDD electrode. We have not found any studies in the literature that discuss the use of an integrated EKR-BDD process for the removal and destruction of phenanthrene from a soil-water system. However, an EKR-EO process has been reported in the destruction of phenanthrene using graphite anodes (Alcántara et al., 2008).

Given the importance of destroying the pollutants that result from EKR, the purpose of this study is to determine the efficiency of two electrokinetic systems for separating phenanthrene from bentonite and its subsequent electrochemical destruction with a Ti/BDD anode. The effect of the electrochemical potential using Ti/BDD was evaluated by normal pulse voltammetry and ●OH analysis to determine the maximum efficiency for destroying phenanthrene in the liquid phase.

2. Experimental

2.1. Chemicals

Phenanthrene and N,N-dimethyl-p-

nitrosoaniline were obtained from Aldrich. Dichloromethane was obtained from Karal. NaOH and H2SO4 from J. T. Baker, and luminescent marine bacteria Vibrio fischeri (Photobacterium phosphoreum) for toxicity analysis was provided by SDI.

2.2. Instruments

Potentiodynamic measurements (Normal

Pulse Voltammetry and cyclic voltammetry) were carried out with an Autolab PGSTAT 30. The pH and the conductivity were measured with a Corning 450 potentiometer that was equipped with a Pinacle glass electrode. Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) were evaluated using a Hach model DR/2010 digital reactor and Shimadzu Model TOC-VCSN equipment, respectively. The TOC in solid phase was determined using the above equipment, equipped with a SSM–5000a module, while COD was obtained using HACH products. Toxicity analysis was done using a DeltaTox kit, provided by SDI and the analysis of ●OH was performed using Lambda XLS+ equipment.

Novel EKR-BDD process for the destruction of PAHs in liquid phase

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The topographic analysis of the different electrodes evaluated, during the selection process of the best electrocatalitic material, was done by Scanning Electron Microscopy (SEM) with a JEOL JMS-6060LV using an acceleration voltage of 15kv.

2.3. Analytical procedures 2.3.1. Physicochemical characterization of bentonite soil

The synthetic soil used for the EKR experiments was Ca-bentonite. Bentonite was artificially polluted with phenanthrene, previously dissolved in dichloromethane (CH2Cl2), to obtain a target pollutant concentration of 150 mg Kg-1 (mass of phenanthrene/mass of dry soil).

The pollution process was carried out for 10 h, using a thermostatic bath to maintain at a constant temperature (298 K). The time required to saturate the bentonite soil with phenanthrene was verified by sorption isotherms (data not shown). The phenanthrene concentration in the bentonite was verified by soxhlet and gas chromatography. The extract obtained in the first was analyzed by gas chromatography-mass spectrometry (GC-MS) using an Agilent GC 19091J–413 Gas Chromatographer coupled to a 5973N Mass Spectrometer. A HP-5 MS column (dimensions 30m × 0.25mm, 0.25 μm) was used with a stationary phase of 5% phenyl-methyl-siloxane. The carrier gas was helium grade 5 (UAP, Ultra Pure Carrier Grade).

The injection of the sample was performed in 5:1 split mode, with a flow of 0.7 ml min−1. The temperature of the injector was 543 K, with an initial oven temperature of 323 K (maintained for 4 min) and a ramp rate of 282 K per min, until reaching a stable temperature of 573 K for 6 min, for a total time of 37.78 min. Analysis of cation exchange capacity (CEC), organic matter and bulk density were performed according to the Official Mexican Regulation (2000).

2.3.2. Electrokinetic remediation The EKR studies were done using two

systems: (Fig. 1) a batch rectangular cell of 14 x 2 x 4 cm made of acrylic (Fig. 1-A) and a continuous cylindrical system (14 x 3 cm internal diameter, Fig. 1-B) where the flow was 1.5 ml min-1.

In both cases, the separation distance between the electrodes was 6 cm and Ti and Ti/IrO2-Ta2O5 were used as cathode and anode, respectively. A constant electric current of 0.02 A was applied for 4 hours to the cells which contained 35 g of polluted soil and 80 mL of 0.1 M NaOH. At the end of EKR, samples of the solid between the electrodes were divided into 6 segments from the anode (Figure 1-A) to be analyzed by the soxhlet technique and GC-MS.

Also, all the effluents were extracted and characterized (pH, electrical conductivity, COD, TOC and toxicity). To determine toxicity, a lyophilized bacteria, Vibrio fischeri (Photobacterium phosphoreum), was used as a model organism. Vibrio fischeri is a luminescent bacteria, where the decrease of the amount of light emitted is proportional to the degree of toxicity. The removal percentages of TOC and COD were calculated using the following equation (Eq. 3), where C0 was the initial concentration (mg L-1) and Cf was the final concentration (mg L-1) (Abdelwahab et al., 2009; Idris and Saed, 2002).

% removal = [(Co – Cf)/Co] * 100 (3)

2.3.3. Electrochemical treatment of the effluent coming from the EKR process 2.3.3.1. Selection of the anode Cyclic Voltammetry analysis

The analysis was performed in a three electrode cell (60 mL capacity, with a reaction volume of 50 ml). Ti/IrO2-Ta, Ti/SnO2-Sb and Ti/BDD electrodes (2.185 cm2) were used as anodes, a rod of Ti was used as cathode, and a mercury sulfate electrode (Hg|Hg2SO4|K2SO4 (SAT), E° = 0.640 V vs. SHE) was the reference electrode.

A) B) C)a

b e h

c d f g

a

d b h f

j j

i

l

k

m

Fig. 1. A) Rectangular system: a) power supply, b) anodic chamber, c and g) physical barrier, d) Ti/IrO2–Ta2O5 anode, e) Ca-bentonite soil polluted with phenanthrene, f) Ti cathode, h) cathodic chamber. B) Cylindrical EKR system where i) is the cell and

j) is the electrolyte reservoir. C) Experimental system employed to carry out the electrochemical incineration with a Ti/BDD anode, where k) is the electrochemical cell, l) is the power supply, and m) is the heat exchanger

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The temperature was maintained at 298 K,

and the voltammetric profiles for each electrode were obtained applying a scan rate of 100 mVs-1 using 0.5 M H2SO4 as the supporting electrolyte. Before each analysis, the system was deoxygenated using N2 gas. Ti/IrO2-Ta2O5 and Ti/SnO2-Sb electrodes were synthesized by the thermal deposition method using a special formulation. Polycrystalline boron ([B] = 1300 ppm) doped diamond film (Ti/BDD) of 3 μm thickness was synthesized by hot filament chemical vapor deposition (HF-CVD), which was provided by Adamant Technologies. In this analysis the Ti/BDD was previously activated (to eliminate C-sp2 impurities) developing a special methodology (Medel et al., 2013).

2.3.3.2. Oxidation potential selection

Normal Pulse Voltammetry analysis Before performing the degradation

experiments, preliminary analysis using a synthetic solution with a phenanthrene concentration of 150 mgL−1 was carried out in an acid solution (NaOH, adjusted to pH 1with H2SO4) in order to identify the potential to be applied during the electrochemical destruction process. This analysis was performed using Ti/BDD as the anode (2.185 cm2), stainless steel as the cathode, and a mercury sulfate electrode (Hg|Hg2SO4|K2SO4 (SAT), E° = 0.640 V vs. SHE) as the reference electrode. Different electrochemical potential pulses (1.1 to 2.4 V vs. Hg|Hg2SO4|K2SO4 (SAT), E° = 0.640 V vs. SHE) were applied to the Ti/BDD electrode in contact with the phenanthrene solution in individual tests. In all cases, the temperature was maintained at 298 K, and the solution was vigorously stirred with a magnetic bar to achieve an efficient transport of phenanthrene molecules toward the anode. In each test the anodic potential pulses were applied for 180 s.

Between each test the Ti/BDD was reactivated by applying an anodic potential pulse of 2.3 V vs. Hg|Hg2SO4|K2SO4 (SAT), E° = 0.640 V vs. SHE) in 0.5 M H2SO4 for 60 s, using a platinum mesh as cathode. The chronoamperograms which resulted were used to construct a j–E curve from the current density measured at fixed times over a period of 60 ts 120 s. Finally, the oxidation potential for the electrochemical incineration process was selected from this curve.

Hydroxyl radical analysis The production of ●OH with Ti/BDD (2.185

cm2) at different potentials (1.5 to 2.5 V vs. Hg|Hg2SO4|K2SO4 (SAT), E° = 0.640 V vs. SHE) was performed under potentiostatic conditions, at 298 K, using the electrochemical cell described above. The analysis was performed in “qualitative form”, using N,N-dimethyl-p-nitrosoaniline, as a spin trap. This solution (2.5 x10-5 M in 0.5 M H2SO4 (60 mL, under constant agitation)) was transported to a UV-Vis spectrophotometer with the help of a peristaltic

pump through a flow cell at a velocity of 12.5 mL min-1, where the decrease of absorbance ( = 350 nm) marked the generation potential of ●OH under the different electrochemical potentials imposed. The experimental system is shown in Fig. 2.

f

a

b

c

d

e

f

a

b

c

d

e

Fig. 2. Experimental system for in situ and qualitative analysis of ●OH using N,N-dimethyl-p-nitrosoaniline as

spin trap: (a) electrochemical cell, (b) potentiostat-galvanostat, (c) peristaltic pump, (d) UV-Vis spectrophotometer, (e) heat exchanger, and

(f) data acquisition PC 2.3.3.3. Electrolysis

The electrochemical treatment of the aqueous solution coming from the EKR process was performed at room temperature (298 K), under a controlled potentiostatic process, applying a previously selected potential based on the Normal Pulse Voltammetry and ●OH analysis, for 150 min. The electrochemical system was an undivided universal cell of three electrodes (20 mL capacity with a reaction volume of 15 mL) (Fig. 1-C).

The anode was a Ti/BDD electrode (2.185 cm2), stainless steel 304 was used as the cathode, and Hg|Hg2SO4 was the reference electrode. Before the electrolysis experiments, the solution was adjusted to pH 1 with H2SO4. During the experiment, the system was stirred constantly. The mineralization process of phenanthrene was evaluated by TOC decays, as well as by toxicity analysis.

3. Results and discussion

3.1. Physicochemical characterization of bentonite soil

Characterization of the Ca-bentonite without

phenanthrene showed low organic content (0.22%) and a cation exchange capacity (CEC) of 99.6 cmol 100 g-1. This suggested that the predominant mineral was montmorillonite, because the CEC’s of other clay minerals are lower than that of Ca-Bentonite (Barber et al., 1992). This material showed a pH 9.6 with an electrochemical conductivity of 664 S cm-1 and a bulk density of 2.58 mg kg-1.

Novel EKR-BDD process for the destruction of PAHs in liquid phase

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3.2. Electrokinetic remediation TOC analysis in solid phase before and after

the EKR process showed that the phenanthrene removal percentages in the cylindrical and rectangular systems were 61 and 70% respectively, starting from an initial concentration of 2 480 mg kg-

1. In order to verify the efficiency of the two systems, the solution contained in the anodic and cathodic chambers was characterized for electric conductivity, pH, TOC, COD and toxicity.

As shown in Table 1, the electric conductivity was higher in the cathodic chamber than in the anodic chamber of the rectangular system (17.48 and 14.87 mS cm-1, respectively), which was due to the migration of ionic species generated during water electrolysis (Niroumand et al., 2012) (Eqs. 4-5).

At the anode:

2H2O → O2(g) + 4H+ + 4e- (4)

At the cathode:

4H2O + 4e- → 2H2(g) + 4OH- (5)

In the cylindrical EKR system, conductivity was similar to the electrolyte (19.54 mS cm-1) since the solution was thoroughly mixed in both the reactor and in the cathodic effluent. Although it is known that some protons migrate towards the cathode, there was no change in the pH in either reactor (the value was about 12). This effect is important because it has been demonstrated that organic compounds can be removed when pH is basic. On the other hand, the organic pollutant is transported mainly by electro-osmosis, which occurs when solvated water around the cations migrates toward the cathode in an electro-osmotic flow. This phenomenon was also observed for the EKR batch, because the volume of the anodic flow decreased, while in the cathodic chamber it increased between 0.04-0.02 mL min-1. This value was consistent with those reported in literature.

The transport direction was corroborated by TOC and COD measurements. Table 1 shows that for the rectangular EKR system more pollutant was removed in the cathodic chamber than in the anode chamber with TOC > 90 mg L-1 and COD > 80 mg L-

1. For the cylindrical EKR system, TOC values were one third of the value for the rectangular EKR system (36.36 mg L-1). COD was similar in both systems (COD > 80 mg L-1) because of the absence of hydraulic flux.

Toxicity analysis for both reactors showed a higher grade of pollution in both chambers (anodic and cathodic, 100 %, Table 1), as a result of pollutant transport to aqueous solution. To illustrate this and verify the removal of phenanthrene by different electrokinetic phenomena, a normalized distribution of greases and oils extracted after the EKR using a rectangular system is shown in Fig. 3. This shows that Phenanthrene migrated through the soil from anode to cathode with a slight accumulation near the

cathode, particularly for segment 5, which had a1.6 Co/Cf ratio.

Table 1. Physical and chemical properties of the effluent coming from the electrokinetic remediation using 0.1M NaOH as electrolyte, applying 0.02 A for 4 h and using

rectangular and cylindrical EKR systems

Rectangular EKR System

Cylindrical EKR System

Parameters Anodic

Effluent Cathodic Effluent

Anodic Effluent

Cathodic Effluent

Conductivity (mS cm-1)

14.87 17.48 20.07 19.54

pH 12.56 12.63 12.52 12.72 TOC (mg L-1)

32.50 91.20 - 36.36

COD (mg L-1)

24.30 88.40 - 107.70

Toxicity (%) 100.00 100.00 - 100.00

1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

(C

/C0)

Distance from the anode (cm)

Fig. 3. Distribution of greases and oils in EKR-Rectangular system (C/Co) versus distance from the anode (cm)

3.3. Electrochemical oxidation of the effluent coming from the EKR process

3.3.1 Anode selection Cyclic Voltammetry Analysis.

Various materials can be employed as the anode in EO, however, the efficiency of contaminant destruction with electro-generated ●OH depends upon the material used. Therefore, the choice of a specific material is very important because the reaction of organics with the ●OH competes with the side reaction of the anodic discharge of these radicals to oxygen. For this reason, anodes with low electrochemical activity toward oxygen evolution (high O2 overvoltage anodes, O2) are preferred. Therefore, cyclic voltammetry was performed to identify the potential domains of each material. The Fig. 4 shows a comparative analysis of Ti/IrO2-Ta2O5, Ti/SnO2-Sb and Ti/BDD in acid (H2SO4 0.5M) with their respective micrographs obtained by SEM. This graph shows that Ti/BDD with a compact structure has the highest potential window in

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comparison to Ti/IrO2-Ta2O5 and Ti/SnO2-Sb. Although Ti/SnO2-Sb had the second widest potential window, the use of this material is constrained by its instability in acid (Montilla et al., 2005).

-2 -1 0 1 2 3-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

j/m

Acm

-2

E/V vs. Hg/Hg2SO

4

a b

c

Fig. 4. Analysis by cyclic voltammetry of a) Ti/IrO2-Ta2O5, b) Ti/SnO2-Sb and c) Ti/BDD in 0.5 M H2SO4 with their respective micrographies obtained by

SEM applying 15 Kv

In contrast, Ti/IrO2-Ta2O5 is a widely used material, with high electro-chemical stability. However, it has a low O2 and it will not completely destroy toxic compounds (Comninellis, 1994). Both materials (Ti/IrO2-Ta2O5 and Ti/SnO2-Sb) show non-compact morphology with pronounced cracks that can accelerate the deactivation of the anodic surface. Based on this, it was decided that Ti/BDD was the best material to use for the electrochemical destruction of contaminants.

3.3.2. Oxidation potential selection. Normal Pulse Voltammetry analysis

In water oxidation process with BDD at high potentials, the observed current is a measure of the magnitude of the chemical reactions involved, such as the production of ●OH and oxygen evolution. In the presence of a substrate to be oxidized, the main reaction is chemical oxidation by reaction of the ●OH with the organic matter present in the reaction medium. The current related to the latter process can be measured using chronoamperometry, where the current is recorded as a function of time, by applying a potential pulse. If we plot the observed current at a fixed time , (sampled current) as a function of the applied potential, current-potential (i vs. E) curves are obtained (Bard and Faulker, 1980).

The Fig. 5 shows the polarization curve constructed from the chronoamperograms applying different electrochemical potential pulses in the presence of phenanthrene using Ti/BDD as anode. In this figure three zones can be distinguished: zone (I), where the reaction is controlled by the charge transport, i.e., (kinetic control). In this zone (1.1 to 1.4 V vs. Hg|Hg2SO4|K2SO4 (SAT), E° = 0.640 V vs. SHE) the phenanthrene is not yet electroactive, inducing no faradaic current.

1.0 1 .2 1.4 1 .6 1 .8 2 .0 2 .2 2.40 .00

0.01

0.02

0.03

0.04 Zone III

Zone II

2 .3 V

j/m

A c

m-2

E / V V s H g|H g2SO 4

Zone I

Fig. 5. Polarization curve for electrochemical oxidation of phenantrene in acid media using normal pulse voltammetry

Zone (II), mixed control (electron transfer and

mass transport), involves potentials where phenanthrene is oxidized (1.4 to 2.2 V vs. Hg|Hg2SO4|K2SO4 (SAT), E° = 0.640 V vs. SHE) but not so effectively that its surface concentration is zero. Zone (III) is where the process is controlled by the speed at which the species reach the electrode (mass-transfer control). Here, the potentials are applied to induce the same current (2.2 to 2.4 V vs. Hg|Hg2SO4|K2SO4 (SAT), E° = 0.640 V vs. SHE). In this zone of potential the surface concentration of phenanthrene is zero. Hence, phenanthrene arrives as fast as it can be brought by diffusion and the current is limited by this factor. Once the electrode potential becomes so extreme that this condition applies, the potential no longer affects the electrolytic current (Bard and Faulker, 1980). This graph also shows that the current limit (where oxidation occurs at the highest speed for mass transport conditions) occurs at a potential of 2.3 V vs. (Hg|Hg2SO4|K2SO4 (SAT), E° = 0.640 V vs. SHE).

Hydroxyl Radical Analysis

Voltage is an important parameter and its value significantly affects the generation of ●OH. In order to determine the effect of the electrochemical potential on the generation of this reactive specie and to confirm the results from normal pulse voltammetry, an analysis by UV-Vis spectrophotometry (qualitative analysis), using N,N-dimethyl-p-nitrosoaniline (2x10-5 M) as spin trap, was performed. It is important to note that during water oxidation with BDD (Eq. 1) in a medium containing sulfate, the ●OH can induce to the generation of secondary species like H2O2 (Eqs. 6-8), O3 (Eqs. 9-12) and peroxodisulfuric acid, H2S2O8 (Eq. 12) (Michaud et al., 2003).

●OH H2O2 (6)

H2O2 O2 + 2H+ + 2e- (7)

H2O2 + ●OH O2 + 2H2O (8)

●OH ●O + H+ + e- (9)

Novel EKR-BDD process for the destruction of PAHs in liquid phase

885

●O + O2 O3 (10)

2●O O2 (11)

2H2SO4 + 2●OH H2S2O8 + 2H2O (12)

Although N,N-dimethyl-p-nitrosoaniline has been reported to react selectively with ●OH, the presence of oxidants (chlorine, H2O2, O3) or UV light can significantly contribute to its discoloration (Wabner and Grambow, 1985; Muff et al., 2011). Therefore, for this test, two conditions were employed: 1) a high concentration of N,N-dimethyl-p-nitrosoaniline (2x10-5 M); and 2) a short sampling time (10 min). Both conditions were used to prevent the formation of secondary oxidants that could interfere with the accuracy of the results (Medel et al., 2013). The results of this experiment are shown in Fig. 6. As can be seen, maximum ●OH production takes place at 2.3 V vs. (Hg|Hg2SO4|K2SO4 (SAT), E°=0.640 V vs. SHE).

In order to confirm the accuracy of this analysis, parallel measurements of H2O2 were taken, with negative results. The results obtained here are in agreement with the data obtained by normal pulse voltammetry. Based on this, a potential of 2.3 V vs. Hg|Hg2SO4 was selected in order to achieve a high level of efficiency in the electrochemical destruction process of phenanthrene in liquid phase.

1.5 V 1.6 V 1.7 V 1.8 V 1.9 V 2.0 V 2.1 V 2.2 V 2.3 V 2.4 V 2.5 V

0 2 4 6 8 100.4

0.5

0.6

0.7

0.8

0.9

1.0

A/A

o

t (min)

2.3 V

Fig. 6. UV-Vis spectrophotometric analysis of the generation of ●OH (normalized values) applying different

electrochemical potentials. Conditions: 0.5M H2SO4, [N,N-dimethyl-p-nitrosoaniline] = 2 x10-5 M, 298 K

3.3.3. Electrolysis

The effluent obtained from EKR (rectangular system) of both electrolyte chambers was treated by EO using Ti/BDD as anode. A voltage of 2.3 V was applied for 160 min. The Fig. 7 shows the degradation of phenanthrene based on TOC analysis.

The degradation in the anodic effluent was faster than in the cathodic effluent reaching 100% in about 60 min. However, degradation in the cathodic effluent only reached 85% in the same amount of time.

Considering that the initial TOC was higher in the cathodic chamber than in the anodic (Table 1), the incineration process was able to degrade all the organic matter present in the effluent. At the end of the process the toxicity was only 3% in the cathodic effluent, and it was negligible in the anodic effluent. This result shows that the use of electrochemical technology using a BDD electrode can effectively degrade phenanthrene molecules in solution. Thus, the feasibility of the use of EKR-BDD process will be validated in future research using a real soil sample, where the chemical composition could significantly impact the efficiency of the process.

0 20 40 60 80 100 120 140 1600.0

0.2

0.4

0.6

0.8

1.0

TO

C (

C/C

o)

Time (min)

Cathode effluent Anode effluent

Fig. 7. TOC measurement (normalized values) during the electrochemical incineration of anodic and cathodic

effluent using Ti/BDD anode. Conditions: E = 2.3 V vs. Hg|Hg2SO4, 298 K

4. Conclusions

The removal of phenanthrene by EKR from bentonite soil is affected by the type of system used. The best arrangement was the rectangular system. The electrochemical destruction of phenanthrene coming from the EKR was 100% from the anodic effluent and 91% from the cathodic effluent in the rectangular system.

The process showed a low toxicity of only 3% after oxidation. Thus the combination of EKR with electrochemical oxidation (EO) process using BDD as anode, to eliminate organic compounds like phenanthrene, is a viable alternative for the remediation of soils that are highly contaminated with PAHs.

Additional studies must be carried out with real samples to confirm the efficiency of this process.

Acknowledgements The authors would like to thank Mexico’s National Council of Science and Technology (CONACyT) for its financial support of this research under Mixed Fund (FOMIX) - Veracruz–CONACyT, project 9631. The authors also thank John Dye, Peace Corps volunteer at CIDETEQ, and Alejandra Rojo for their review of this manuscript.

Medel et al./Environmental Engineering and Management Journal 14 (2015), 4, 879-886

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