**3.2 Cathode modification**

For this set of experiments soil sample was collected at an industrial area located in Nuevo Teapa, Veracruz, México, this is a highly polluted and weathered area. Physical characterization is reported in Table 2.


Table 2. Physical and textural properties of Nuevo Teapa soil

For this set of experiments it was chosen a dimensionally stable anode (DSA) made of a titanium plate with an iridium–tantalum film (TiIrO2-Ta2O5) which was maintained constant; considered cathode materials were: carbon felt (CF) and a titanium plate (Ti). Physical barrier inclusion was evaluated considering two electrode positions: Array I: placing the physical barrier at the soil interphase and the electrode after it; Array II: electrode placed at the soil interphase.

An initial test of soil response was done with the combination DSA-Ti, pH profiles for the option including a physical barrier between electrode and soil (Array I) are presented in Figure 9. As it can be observed soil tends to be acidified, and after 4 hours it is reached a stable pH condition which at the anode is about 2 units lower, and at the cathode is one unit lower, in respect to the initial value. It seems that barrier favors a buffering effect by which the system does not get a drastic pH drop. Although, having a pH variation is evidence of getting a high rate for proton generation and transport throughout soil. It is important to point out that after 3 hours, pH at the middle and cathode sections have similar values, fact which reflects a strong neutralization of transported protons.

Correlating these results with corresponding pH data it can be affirmed that keeping a shorter difference in pH wells, as it happens with bare RVC electrodes, it favors a soil alkaline condition which, even though produces a slower liquid movement, so far this is

Up to here, it was shown that inclusion of one catalytic specie, like anatase, in the anode allowed increasing the oxidant specie production and the electroosmotic flow rate, but obtained phenanthrene removal was lowered. So next step is to analyze what happen if the catalytic activity is maintained at the anode, but cathode is chosen between different materials. Experimental set-up objective was data collection for two different cathode materials, and also to clarify how much the system becomes affected by inclusion of

For this set of experiments soil sample was collected at an industrial area located in Nuevo Teapa, Veracruz, México, this is a highly polluted and weathered area. Physical

> plasticity clay (CI)

For this set of experiments it was chosen a dimensionally stable anode (DSA) made of a titanium plate with an iridium–tantalum film (TiIrO2-Ta2O5) which was maintained constant; considered cathode materials were: carbon felt (CF) and a titanium plate (Ti). Physical barrier inclusion was evaluated considering two electrode positions: Array I: placing the physical barrier at the soil interphase and the electrode after it; Array II:

An initial test of soil response was done with the combination DSA-Ti, pH profiles for the option including a physical barrier between electrode and soil (Array I) are presented in Figure 9. As it can be observed soil tends to be acidified, and after 4 hours it is reached a stable pH condition which at the anode is about 2 units lower, and at the cathode is one unit lower, in respect to the initial value. It seems that barrier favors a buffering effect by which the system does not get a drastic pH drop. Although, having a pH variation is evidence of getting a high rate for proton generation and transport throughout soil. It is important to point out that after 3 hours, pH at the middle and cathode sections have similar values, fact

Plasticity Chart (Helwany,

2007, page 13)

Parameter Value Methodology Liquid Limit (LL) % 42 ASTM D4318-10 Plastic Limit (PL) % 28 ASTM D4318-10 Plasticity Index (PI) % 14 ASTM D4318-10

Sand % 56 USCS-P13-B-2 Silt % 24 USCS-P13-B-2 Clay % 20 USCS-P13-B-2

Classification Medium

Table 2. Physical and textural properties of Nuevo Teapa soil

which reflects a strong neutralization of transported protons.

good enough for phenanthrene removal since it provides a higher residence time.

additional physical barriers like a thick filter paper.

**3.2 Cathode modification** 

characterization is reported in Table 2.

electrode placed at the soil interphase.

Results for Array II are presented in Figure 10, pH profiles for the combination DSA-Ti without barrier clearly show a fast pH drop, and after 1 hour, at the middle section seems to occur an hydroxide accumulation, which could be a factor to accelerate proton penetration, such that at two hours the anode section starts to lowering its pH, and even though it does not reach an acidic condition pH drop is about 5 units at this section, but this pH does not exert a strong impact over the other sections; since it seems that protons penetration displaced hydroxyls to the middle and cathode sections, which suffer a temporary raise in concentration. This behavior corresponds to a pulsed function being displaced from the middle to the cathode section, since when pH starts to decay in the middle, the cathode one start to raise its pH, which it gets a higher value than the initial one. At the final time, the middle section has decreased its pH in 1 unit, while the cathode section has reached stability at pH 12.

Fig. 9. Graph of pH profiles for array I of the DSA anode and Ti cathode, experimental conditions: 0.1 M NaOH wetting electrolyte, current density 20 mA cm-2.

Experimental approach considered a follow up through the global electrical resistance (R, Ohms), which was indirectly calculated from experimental values of electrical potential (E, Volts) and applied current (I, Amperes), parameters related by Ohm's law (R=E I-1). Results are shown in Figure 11.

Electrode Materials a Key Factor to Improve Soil Electroremediation 233

Observing Figure 11 it is evident that allowing electrodes make contact with the soil (array II) provides a less resistive system; and also there is not a clear advantage between using CF or Ti as cathode, since both systems provide similar initial values at 30 min, R=255 ohms for CF, and R=275 ohms for Ti; final resistance values are identical for both systems R=350 ohms. Although, CF exhibit a slightly higher raise (95 ohms) in respect to Ti (75 ohms). Applying a linear regression analysis to CF data, its slope corresponds to 0.462 ohm-min-1, while the Ti slope is 0.339 ohm-min-1 then, even though initial resistance value is higher for

As it can be observed inclusion of the physical barrier makes the experimental system to be more resistive than that where electrodes make contact with the soil, also resistance trends in this system are similar to those of the previous one; since at initial times system with a CF cathode seems to be less resistive at 30 min, R=425 ohms, than that with a Ti cathode at 30 min, R=540 ohms, but at the end of the experiment (240 minutes), the resistance of the system with CF was increased by about 185 ohms, while the one with Ti by only 140 ohms. Applying a regression analysis the slope for CF is 1.087 ohm-min-1, while the one with Ti has a slope of 0.599 ohm-min-1; these values are higher than the ones observed when the electrode is placed at the soil interphase; but again it is confirmed that using Ti as cathode provides a more stable system. The failure of the CF electrode can be attributed to a poisoning effect since there is a possibility that desorbed hydrocarbons get retained at the

At the end of each experiment, residual hydrocarbon content in soil was estimated by a Soxhlet technique at 3 points: 0.25, 0.5, 0.75 anode to cathode dimensionless distance; concentration values are normalized respect to the initial concentration condition an presented as percentage. Results are shown in Figure 12 for carbon felt (CF) cathode, and in

As it can be observed from both Figures (12 and 13) switching the cathode position provides opposite trends in residual hydrocarbon concentrations, since when the physical barrier is between soil and electrode, residual concentration goes from higher to lower in the anode-cathode direction, it seems that transported hydrocarbons are no allowed to accumulate near the cathode; also, CF cathode provides the best conditions for hydrocarbon transport since in the anode-cathode direction hydrocarbon removal goes from 36% to 65% (27% difference), while with the Ti cathode goes from 30 to 42% (12%

In opposite way, when electrodes are at the soil interphase it happens that residual hydrocarbon concentrations increase from anode to cathode, and the higher ones are registered near the cathode; in this case again CF cathode provides the best removal since hydrocarbon removal goes from 60% to 40% (20 % difference), while the Ti cathode removal

Based on the cell resistance results, there is an assumption about carbon felt being passivated due to adsorption of those transported hydrocarbon molecules. In order to assess which type of hydrocarbons migrated, and accumulated at the cathode, CF cathodes were washed with a dichloromethane solution, and eluted samples were used for PAHs

the Ti electrode, this electrode provides a more stable system.

cathode.

difference).

estimation.

Figure 13 for titanium (Ti) cathode.

goes from 40% to 30% (10 % difference).

Fig. 10. Graph of pH profiles for array II of the DSA anode and Ti cathode, experimental conditions: 0.1 M NaOH wetting electrolyte, current density 20 mA cm-2.

Fig. 11. Calculated resistance for soil electroremediation experiments using a modified IrO2- Ta2O5 anode with either Carbon Felt (CF) or Titanium (Ti) cathode. Array I physical barrier inclusion, Array II soil contact experimental conditions: 0.1 M NaOH wetting electrolyte, current density 20 mA cm-2.

0 100 200 300 400 500

Time, min

0 50 100 150 200 250

 CF array I Ti array I CF array II Ti array II

Time, min

Fig. 11. Calculated resistance for soil electroremediation experiments using a modified IrO2- Ta2O5 anode with either Carbon Felt (CF) or Titanium (Ti) cathode. Array I physical barrier inclusion, Array II soil contact experimental conditions: 0.1 M NaOH wetting electrolyte,

Array II

Fig. 10. Graph of pH profiles for array II of the DSA anode and Ti cathode, experimental

 anode middle cathode

conditions: 0.1 M NaOH wetting electrolyte, current density 20 mA cm-2.

0

0

100

200

300

Cell resistance, ohms

current density 20 mA cm-2.

400

500

600

700

2

4

6

pH

8

10

12

14

Observing Figure 11 it is evident that allowing electrodes make contact with the soil (array II) provides a less resistive system; and also there is not a clear advantage between using CF or Ti as cathode, since both systems provide similar initial values at 30 min, R=255 ohms for CF, and R=275 ohms for Ti; final resistance values are identical for both systems R=350 ohms. Although, CF exhibit a slightly higher raise (95 ohms) in respect to Ti (75 ohms). Applying a linear regression analysis to CF data, its slope corresponds to 0.462 ohm-min-1, while the Ti slope is 0.339 ohm-min-1 then, even though initial resistance value is higher for the Ti electrode, this electrode provides a more stable system.

As it can be observed inclusion of the physical barrier makes the experimental system to be more resistive than that where electrodes make contact with the soil, also resistance trends in this system are similar to those of the previous one; since at initial times system with a CF cathode seems to be less resistive at 30 min, R=425 ohms, than that with a Ti cathode at 30 min, R=540 ohms, but at the end of the experiment (240 minutes), the resistance of the system with CF was increased by about 185 ohms, while the one with Ti by only 140 ohms. Applying a regression analysis the slope for CF is 1.087 ohm-min-1, while the one with Ti has a slope of 0.599 ohm-min-1; these values are higher than the ones observed when the electrode is placed at the soil interphase; but again it is confirmed that using Ti as cathode provides a more stable system. The failure of the CF electrode can be attributed to a poisoning effect since there is a possibility that desorbed hydrocarbons get retained at the cathode.

At the end of each experiment, residual hydrocarbon content in soil was estimated by a Soxhlet technique at 3 points: 0.25, 0.5, 0.75 anode to cathode dimensionless distance; concentration values are normalized respect to the initial concentration condition an presented as percentage. Results are shown in Figure 12 for carbon felt (CF) cathode, and in Figure 13 for titanium (Ti) cathode.

As it can be observed from both Figures (12 and 13) switching the cathode position provides opposite trends in residual hydrocarbon concentrations, since when the physical barrier is between soil and electrode, residual concentration goes from higher to lower in the anode-cathode direction, it seems that transported hydrocarbons are no allowed to accumulate near the cathode; also, CF cathode provides the best conditions for hydrocarbon transport since in the anode-cathode direction hydrocarbon removal goes from 36% to 65% (27% difference), while with the Ti cathode goes from 30 to 42% (12% difference).

In opposite way, when electrodes are at the soil interphase it happens that residual hydrocarbon concentrations increase from anode to cathode, and the higher ones are registered near the cathode; in this case again CF cathode provides the best removal since hydrocarbon removal goes from 60% to 40% (20 % difference), while the Ti cathode removal goes from 40% to 30% (10 % difference).

Based on the cell resistance results, there is an assumption about carbon felt being passivated due to adsorption of those transported hydrocarbon molecules. In order to assess which type of hydrocarbons migrated, and accumulated at the cathode, CF cathodes were washed with a dichloromethane solution, and eluted samples were used for PAHs estimation.

Electrode Materials a Key Factor to Improve Soil Electroremediation 235

In order to assess risk exposition levels for oil exploration and production sites, it takes relevance to detect the EPA's 16 priority Polycyclic Aromatic Hydrocarbons (PAHs) (Bojes & Pope, 2007). Analytical techniques that can be applied to PAHs detection consider HPLC coupled with UV-Vis detection, with this technique the 16 priority PAHs can be detected at wavelengths between 227 and 297 nm (Maureen, 2011); another useful technique is gas chromatography (GC) coupled with mass spectroscopy (MS) (Amzad Hossain &

Based on this information, a first approach to PAHs detection was done with UV-Vis, in Figure 14 are shown obtained results for CF cathode in array I (physical barrier included), and array II (soil contact), from these spectra it is obvious that physical barrier presence has enhanced PAHs partition at the interphase soil-water, so keeps hydrocarbon accumulation low in the region nearby; also, the electrode behind the physical barrier has acted like a sink for transported PAHs, so far it is logical to get lower concentrations in the position near the cathode. Otherwise, having the cathode in contact with soil makes PAHs partition to occur

300 400

 C.F. array I C. F. array II

, nm

Fig. 14. UV-Vis spectra of sorbed hydrocarbons onto carbon felt cathode in Array I and Array II. Experimental conditions 0.1 M NaOH, current density 20 mA cm-2, experimental

(phenanthrene) and two with 4 rings ( fluoranthene and pyrene).

Otherwise, observing the spectra, it can be notice that there is not a clear and unique peak; which means there is a possibility of having more than one PAH in the desorbed material. Therefore, a more refined technique should be used for PAHs detection and quantification, requirements widely covered by GC coupled to MS; analytical detection was limited to three of the 16 PAHs in EPA's priority list, the ones chosen were one having three aromatic rings

at slower rate since PAHs face the hydroxide production at the soil boundary.

Salehuddin, 2011).

0.0

time 4 hours.

0.5

1.0

1.5

Absorbance, A. U.

2.0

2.5

3.0

Fig. 12. Graph of normalized residual concentrations for carbon felt (CF) cathode. Array I physical barrier inclusion, Array II soil contact, experimental conditions: 0.1 M NaOH wetting electrolyte, current density 20 mA cm-2.

Fig. 13. Graph of normalized residual concentrations for Titanium (Ti) cathode. Array I physical barrier inclusion, Array II soil contact, experimental conditions: 0.1 M NaOH wetting electrolyte, current density 20 mA cm-2.

 CF array I CF array II

0.0 0.2 0.4 0.6 0.8 1.0

Dimensionless distance anode-cathode

 Ti array I Ti array II

0.0 0.2 0.4 0.6 0.8 1.0

Dimensionless distance anode-cathode

Fig. 13. Graph of normalized residual concentrations for Titanium (Ti) cathode. Array I physical barrier inclusion, Array II soil contact, experimental conditions: 0.1 M NaOH

Fig. 12. Graph of normalized residual concentrations for carbon felt (CF) cathode. Array I physical barrier inclusion, Array II soil contact, experimental conditions: 0.1 M NaOH

wetting electrolyte, current density 20 mA cm-2.

wetting electrolyte, current density 20 mA cm-2.

% [C]/[C0

]

% [C]/[C0

]

In order to assess risk exposition levels for oil exploration and production sites, it takes relevance to detect the EPA's 16 priority Polycyclic Aromatic Hydrocarbons (PAHs) (Bojes & Pope, 2007). Analytical techniques that can be applied to PAHs detection consider HPLC coupled with UV-Vis detection, with this technique the 16 priority PAHs can be detected at wavelengths between 227 and 297 nm (Maureen, 2011); another useful technique is gas chromatography (GC) coupled with mass spectroscopy (MS) (Amzad Hossain & Salehuddin, 2011).

Based on this information, a first approach to PAHs detection was done with UV-Vis, in Figure 14 are shown obtained results for CF cathode in array I (physical barrier included), and array II (soil contact), from these spectra it is obvious that physical barrier presence has enhanced PAHs partition at the interphase soil-water, so keeps hydrocarbon accumulation low in the region nearby; also, the electrode behind the physical barrier has acted like a sink for transported PAHs, so far it is logical to get lower concentrations in the position near the cathode. Otherwise, having the cathode in contact with soil makes PAHs partition to occur at slower rate since PAHs face the hydroxide production at the soil boundary.

Fig. 14. UV-Vis spectra of sorbed hydrocarbons onto carbon felt cathode in Array I and Array II. Experimental conditions 0.1 M NaOH, current density 20 mA cm-2, experimental time 4 hours.

Otherwise, observing the spectra, it can be notice that there is not a clear and unique peak; which means there is a possibility of having more than one PAH in the desorbed material. Therefore, a more refined technique should be used for PAHs detection and quantification, requirements widely covered by GC coupled to MS; analytical detection was limited to three of the 16 PAHs in EPA's priority list, the ones chosen were one having three aromatic rings (phenanthrene) and two with 4 rings ( fluoranthene and pyrene).

Electrode Materials a Key Factor to Improve Soil Electroremediation 237

 IC 0.25 0.5 0.75

Array II

0.1 M NaOH, current density 20 mA cm-2, experimental time 4 hours.

hydrocarbons get adsorbed in the electrode, being difficult its recovery.

Phenanthrene Fluoranthene Pyrene

Fig. 16. Residual concentrations of representative PAHs in Array II, experimental conditions

For anode modification obtained results allows to claim that, effectively inclusion of anatase into the RVC matrix makes electrode reaction being more efficient. Also, by using the modified RVC-TiO2 electrode it is possible to increase the rate at which protons are generated and transported throughout the soil, and so far this influences pH at both electrode wells: anodic and cathodic. Also, it provides a higher electroosmotic flow, but this fast water transport does not allow for an adequate residence time, lowering phenanthrene removal. So far, the bare RVC electrodes provided a lower pH gradient between anodecathode, as well as a lower electroosmotic flow, both parameters are providing a better environment for phenanthrene removal, since with this option it was obtained up to 80%

For cathode modification obtained results have shown that cell resistance is lower when electrodes are in contact with soil sample, and this allowed for higher hydrocarbon mobility, so residual concentration profile exhibits an increasing trend from anode to cathode. Otherwise, physical barrier inclusion increased soil resistance and so far, hydrocarbon mobility is lowered, this fact resulted in a decreasing concentration trend from anode to cathode. From oil and grease extractions it was determined that CF provides higher hydrocarbon removal, although this option is not the best because transported

Even though Ti cathode provided lower hydrocarbon removal as it was estimated from Soxhlet extractions, when extracted samples were tested by GC-MS for quantification of three priority hydrocarbon pollutants, it happens that phenanthrene, fluoranthene and

0

lowering in soil phenanthrene concentration.

20

40

% [C]/[C0

**4. Conclusions** 

]

60

80

100

In order to asses soil electroremediation efficiency in removal of phenanthrene, fluoranthene and pyrene, GC-MS was applied to the soxhlet extracted samples including both original and electroremediated soil. Concentration values were calculated from the area under the curve, and these were converted to percentage taking as reference concentration the one for each PAH registered in the extract from the original weathered soil. Results for the Ti cathode are shown in Figure 15 for the array I (physical barrier inclusion), and Figure 16 for array II (soil contact).

As it can be observed in Figure 15, (array with the physical barrier) there is a higher to lower trend from anode to cathode for the three PAHs which were analyzed, and removals are low; it seems that molecule size exerts an influence on their movement through the soil, since the 3 rings molecule (phenanthrene) has reached removals between 80 and 90%, while those with 4 rings (fluoranthene, pyrene) get similar removals between 60 and 85%. Otherwise, in Figure 16 it can be observed that allowing the electrode to make contact with soil enhances PAHs removal, getting similar residual concentrations for all, in this experiment removals are above 90%.

Correlating these residual concentrations with pH observations (Figures 9 and 10) it seems that the fact of having a physical barrier between soil and electrode, which produces a more resistive system, does not allow for getting a high concentration gradient between electrodes, resulting in lower removals than those obtained when the electrode make contact with the soil; since the last arrangement produces a higher pH gradient between anode and cathode, so far a higher driving force for PAHs transport.

Fig. 15. Residual concentrations of representative PAHs in Array I physical barrier included, experimental conditions 0.1 M NaOH, current density 20 mA cm-2, experimental time 4 hours.

Fig. 16. Residual concentrations of representative PAHs in Array II, experimental conditions 0.1 M NaOH, current density 20 mA cm-2, experimental time 4 hours.
