**3.1 Anode modification**

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.


Table 1. Physical and textural properties of Guanajuato soil.

Electrode Materials a Key Factor to Improve Soil Electroremediation 225

Otherwise, anode replacement by the RVC-TiO2 option (Anode II, Cathode II), it enhances proton production, so in short time pH goes to acidic values, and after 5 hours it stabilizes around 5. Keeping the cathode as bare RVC makes that the cathodic well pH go to slightly higher alkaline values, than those registered when the anode was bare RVC; but still pH values are lower than the ones obtained with the clean soil. It seems that pollutant inclusion makes a more resistive system by which hydroxyl production is lowered respect of the rates

0 5 10 15 20 25 30

Time, hrs

Fig. 2. Graph of pH evolution at anodic and cathodic wells during the electrokinetic

experiments for clay soil contaminated with 12 mg Kg-1 of phenanthrene, applying a current density of 25 mA cm-2, and using different electrode materials: (I) Bare RVC anode and

For these experiments drained volume was collected, results are presented in Figure 3. As it can be observed keeping a bare RVC cathode, and switching from (I) bare RVC to (II) RVC-TiO2 anodes, exhibit similar water transport during the first 3 hours; but after that water transport is increased for the RVC-TiO2 anode; therefore this modification it allows enhancing the amount of water being displaced from anode to cathode. After 10 hours, water transport reach and steady rate of transport, corresponding regression lines are described as follows: for the RVC (system I) y= 72.31x-54.995, R2=0.995, while the RVC-TiO2

achieved with clean soil.

0

cathode; and (II) RVC-TiO2 anode with RVC cathode.

(system II) is described by y= 88.15x-2.0373, R2= 0.999.

2

4

 Anode I Anode II Cathode I Cathode II

6

pH

8

10

12

14

Based on the above described soil characteristics, it was considered important to determine how this type of soil responses to the action of an electric field. To accomplish this step, clean soil was wetted with deionized water, and later on tested with the bare RVC electrodes. Measurements of pH were done at the electrode interface (anode, cathode) and two middle points (4 and 7 cm). Experimental pH profiles are presented in Figure 1. As it can be observed, natural pH is slightly alkaline (pH=8); also, even though protons are generated at the anode, its penetration is slow, and their amount is not enough to get a high pH depletion at this position; otherwise intermediate points (4 and 7 cm) show an alkalinization since its values are increased by up to 2 units; also, at the cathodic position, pH response satisfies expectations of high alkaline values, since final pH is closer to 13. Considering that alkaline pH favors organic pollutants removal (Murillo-Rivera et al, 2009), then this soil is considered adequate to evaluate hydrocarbon removal.

Fig. 1. Graph of pH evolution at anodic and cathodic wells during the electrokinetic experiments for clean soil, applying a current density of 25 mA cm-2, and using bare RVC anode and cathode electrodes.

Artificial polluted sample was prepared as it is described in the methodology section. Next step is to use this sample with different anode materials, and establish if these are useful on improving electrokinetic process performance for removal of phenanthrene from polluted soil. In Figure 2 it is shown the pH evolution at the anodic and cathodic wells, for a 24 hours soil electroremediation experiment. It can be observed that using bare RVC electrodes (Anode I and Cathode I) makes pH at the anode be slightly depleted during the first hours, but later on occurs an increase of its value, which keeps it around 8 (the initial value) during the rest of the experiment; otherwise, at the cathodic well a fast alkalinization is observed, this remains around 10.5 during the whole experiment, but this value is lower than the one obtained with clean soil.

Based on the above described soil characteristics, it was considered important to determine how this type of soil responses to the action of an electric field. To accomplish this step, clean soil was wetted with deionized water, and later on tested with the bare RVC electrodes. Measurements of pH were done at the electrode interface (anode, cathode) and two middle points (4 and 7 cm). Experimental pH profiles are presented in Figure 1. As it can be observed, natural pH is slightly alkaline (pH=8); also, even though protons are generated at the anode, its penetration is slow, and their amount is not enough to get a high pH depletion at this position; otherwise intermediate points (4 and 7 cm) show an alkalinization since its values are increased by up to 2 units; also, at the cathodic position, pH response satisfies expectations of high alkaline values, since final pH is closer to 13. Considering that alkaline pH favors organic pollutants removal (Murillo-Rivera et al, 2009),

0 5 10 15 20 25 30

 Anode 4 cm 7 cm Cathode

Time, hours

Artificial polluted sample was prepared as it is described in the methodology section. Next step is to use this sample with different anode materials, and establish if these are useful on improving electrokinetic process performance for removal of phenanthrene from polluted soil. In Figure 2 it is shown the pH evolution at the anodic and cathodic wells, for a 24 hours soil electroremediation experiment. It can be observed that using bare RVC electrodes (Anode I and Cathode I) makes pH at the anode be slightly depleted during the first hours, but later on occurs an increase of its value, which keeps it around 8 (the initial value) during the rest of the experiment; otherwise, at the cathodic well a fast alkalinization is observed, this remains around 10.5 during the whole experiment, but this value is lower than the one

Fig. 1. Graph of pH evolution at anodic and cathodic wells during the electrokinetic experiments for clean soil, applying a current density of 25 mA cm-2, and using bare RVC

then this soil is considered adequate to evaluate hydrocarbon removal.

0

anode and cathode electrodes.

obtained with clean soil.

2

4

6

pH

8

10

12

14

Otherwise, anode replacement by the RVC-TiO2 option (Anode II, Cathode II), it enhances proton production, so in short time pH goes to acidic values, and after 5 hours it stabilizes around 5. Keeping the cathode as bare RVC makes that the cathodic well pH go to slightly higher alkaline values, than those registered when the anode was bare RVC; but still pH values are lower than the ones obtained with the clean soil. It seems that pollutant inclusion makes a more resistive system by which hydroxyl production is lowered respect of the rates achieved with clean soil.

Fig. 2. Graph of pH evolution at anodic and cathodic wells during the electrokinetic experiments for clay soil contaminated with 12 mg Kg-1 of phenanthrene, applying a current density of 25 mA cm-2, and using different electrode materials: (I) Bare RVC anode and cathode; and (II) RVC-TiO2 anode with RVC cathode.

For these experiments drained volume was collected, results are presented in Figure 3. As it can be observed keeping a bare RVC cathode, and switching from (I) bare RVC to (II) RVC-TiO2 anodes, exhibit similar water transport during the first 3 hours; but after that water transport is increased for the RVC-TiO2 anode; therefore this modification it allows enhancing the amount of water being displaced from anode to cathode. After 10 hours, water transport reach and steady rate of transport, corresponding regression lines are described as follows: for the RVC (system I) y= 72.31x-54.995, R2=0.995, while the RVC-TiO2 (system II) is described by y= 88.15x-2.0373, R2= 0.999.

Electrode Materials a Key Factor to Improve Soil Electroremediation 227

Collected drained volume values, cell characteristics and soil mass were used to mathematically obtain cumulative electroosmotic flow (mL cm-2 min-1 Kg-1), results are shown in Figure 4. From the plot can be established that effectively catalytic activity of TiO2 allows for getting higher electroosmotic flow. As it can be observed in three hours the RVC-TiO2 anode (II) reached steady response, while the bare RVC anode (I) provides a much lower electroosmotic flow which seems to smoothly reach steady response at similar times, but after 10 hours, a new perturbation takes place and it goes to a transient response, the last

As it was mentioned in the methodology section, electroremediated soil sample was cut in slices and recovered residual phenantrene was injected in an inverse phase Hypersil chromatographic column. Elution time for phenanthrene was 5.8 min, while humic and fulvic acids appear at about 2 min of elution time (Chongsan et al, 2006; Xing & Kang, 2005; Yanzheng et al, 2007). In Figure 5 it is shown a chromatogram for the soil extracted phenanthrene before any electrokinetic experiment, this reference signal is about 1 arbitrary

0 2 4 6 8 10 12

Time, min

Fig. 5. Chromatogram of the Phenanthrene standard, reference signal obtained from the

Obtained chromatograms for each soil section, after the electrokinetic experiment with bare RVC anode and cathode are shown in Figure 6, and those for the RVC-TiO2 anode with bare

As it can be observed in Figure 6, none of the positions amount concentrations higher than 0.25 A. U., also there are several peaks between the humic and fulvic acids (2 min) and the phenanthrene (5.8 min), these peaks are smaller than other signals, and they can be

taking place at an slower rate in respect to the initial one.

0.0

RVC cathode are shown in Figure 7.

artificially polluted soil.

0.2

0.4

0.6

Absorbance, A.U.

0.8

1.0

unit (A.U.).

Fig. 3. Graph of collected cumulative volume at the cathodic well during electrokinetic experiments for clay soil contaminated with 12 mg Kg-1 of phenanthrene, applying a current density of 25 mA cm-2, having a bare RVC cathode and using different anode materials: (I) Bare RVC anode; and (II) RVC-TiO2 anode.

Fig. 4. Comparison of cumulative electroosmotic flow registered at the cathodic well during the electrokinetic experiments for clay soil contaminated with phenanthrene using different anode materials: (I) Bare RVC anode and cathode; and (II) RVC-TiO2 anode with RVC cathode.

 I II

0 5 10 15 20 25 30

Time, hours

0 5 10 15 20 25 30

 I II

Time, hours

Fig. 4. Comparison of cumulative electroosmotic flow registered at the cathodic well during the electrokinetic experiments for clay soil contaminated with phenanthrene using different anode materials: (I) Bare RVC anode and cathode; and (II) RVC-TiO2 anode with RVC

Fig. 3. Graph of collected cumulative volume at the cathodic well during electrokinetic experiments for clay soil contaminated with 12 mg Kg-1 of phenanthrene, applying a current density of 25 mA cm-2, having a bare RVC cathode and using different anode materials: (I)

0

Bare RVC anode; and (II) RVC-TiO2 anode.

0.000

0.005

0.010

0.015

0.020

0.025

Electroosmotic flow, mL cm-2 min-1Kg-1

cathode.

0.030

0.035

0.040

500

1000

1500

Cumulative volumen, mL

2000

2500

Collected drained volume values, cell characteristics and soil mass were used to mathematically obtain cumulative electroosmotic flow (mL cm-2 min-1 Kg-1), results are shown in Figure 4. From the plot can be established that effectively catalytic activity of TiO2 allows for getting higher electroosmotic flow. As it can be observed in three hours the RVC-TiO2 anode (II) reached steady response, while the bare RVC anode (I) provides a much lower electroosmotic flow which seems to smoothly reach steady response at similar times, but after 10 hours, a new perturbation takes place and it goes to a transient response, the last taking place at an slower rate in respect to the initial one.

As it was mentioned in the methodology section, electroremediated soil sample was cut in slices and recovered residual phenantrene was injected in an inverse phase Hypersil chromatographic column. Elution time for phenanthrene was 5.8 min, while humic and fulvic acids appear at about 2 min of elution time (Chongsan et al, 2006; Xing & Kang, 2005; Yanzheng et al, 2007). In Figure 5 it is shown a chromatogram for the soil extracted phenanthrene before any electrokinetic experiment, this reference signal is about 1 arbitrary unit (A.U.).

Fig. 5. Chromatogram of the Phenanthrene standard, reference signal obtained from the artificially polluted soil.

Obtained chromatograms for each soil section, after the electrokinetic experiment with bare RVC anode and cathode are shown in Figure 6, and those for the RVC-TiO2 anode with bare RVC cathode are shown in Figure 7.

As it can be observed in Figure 6, none of the positions amount concentrations higher than 0.25 A. U., also there are several peaks between the humic and fulvic acids (2 min) and the phenanthrene (5.8 min), these peaks are smaller than other signals, and they can be

Electrode Materials a Key Factor to Improve Soil Electroremediation 229

0 2 4 6 8 10 12

 0.1 0.3 0.5 0.7 0.9

> I II I.C

Time, min

0.0 0.2 0.4 0.6 0.8 1.0

Dimensionless distance anode-cathode

Fig. 8. Comparison of residual phenanthrene concentrations in each soil slice for the system of (I) bare RVC electrodes, and the (II) RVC-TiO2 anode, RVC cathode, versus initial

Fig. 7. Chromatograms of residual phenanthrene concentrations in each soil slice for the

0.0

% [C]/[C0

concentration (IC).

]

system of TiO2-RVC anode and bare RVC cathode.

0.2

0.4

0.6

Absorbance, A.U.

0.8

1.0

associated to a phenanthrene decomposition by products from lateral reactions, which take place as electrolyte moves through the soil during the electrokinetic experiment; the higher residual phenanthrene concentration for bare RVC electrodes was about 0.25 A.U. and it occurs at the 0.7 dimensionless position, that is the section before to the one closer to the cathode.

Fig. 6. Chromatograms of residual phenanthrene concentrations in each soil slice for the system of bare RVC electrodes.

Otherwise, when the experiment was run with the RVC-TiO2 anode (Figure 7) the presence of smaller peaks it is practically null; also, an opposite phenomena is observed since in this case the higher residual concentration was about 0.9 A.U., while the lower one is not less than 0.6 A. U., this last takes place at the 0.3 cm position (near the anode), In general, with this option phenanthrene removal was lower than the one attained with the bare RVC electrodes.

In order to make more explicit the above expressed, concentration was calculated from each soil slice chromatogram, this was done by an integration of the area under phenanthrene peak; in this way, its residual concentration was estimated. Results are reported in Figure 8 as percentage of the original concentration in soil (12 mg Kg-1=100%), it can be observed that effectively higher removal was obtained with the bare RVC electrodes, an average of 80%; and even though replacing the anode by the RVC-TiO2 provides higher oxidation conditions and a faster water transport; this fact does not allowed for getting a right residence time for solubilizing and transporting phenanthrene, since removal amounts an average of 20%. It is noticeable that for the bare RVC electrodes, higher residual phenanthrene concentration took place at the same position where it is the lower one when the anode was RVC-TiO2.

associated to a phenanthrene decomposition by products from lateral reactions, which take place as electrolyte moves through the soil during the electrokinetic experiment; the higher residual phenanthrene concentration for bare RVC electrodes was about 0.25 A.U. and it occurs at the 0.7 dimensionless position, that is the section before to the one closer to the

0 2 4 6 8 10 12

 0.1 0.3 0.5 0.7 0.9

Time, min

Fig. 6. Chromatograms of residual phenanthrene concentrations in each soil slice for the

Otherwise, when the experiment was run with the RVC-TiO2 anode (Figure 7) the presence of smaller peaks it is practically null; also, an opposite phenomena is observed since in this case the higher residual concentration was about 0.9 A.U., while the lower one is not less than 0.6 A. U., this last takes place at the 0.3 cm position (near the anode), In general, with this option phenanthrene removal was lower than the one attained with the bare RVC

In order to make more explicit the above expressed, concentration was calculated from each soil slice chromatogram, this was done by an integration of the area under phenanthrene peak; in this way, its residual concentration was estimated. Results are reported in Figure 8 as percentage of the original concentration in soil (12 mg Kg-1=100%), it can be observed that effectively higher removal was obtained with the bare RVC electrodes, an average of 80%; and even though replacing the anode by the RVC-TiO2 provides higher oxidation conditions and a faster water transport; this fact does not allowed for getting a right residence time for solubilizing and transporting phenanthrene, since removal amounts an average of 20%. It is noticeable that for the bare RVC electrodes, higher residual phenanthrene concentration took place at the same position where it is the lower one when the anode was RVC-TiO2.

cathode.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

system of bare RVC electrodes.

electrodes.

Absorbance, A.U.

Fig. 7. Chromatograms of residual phenanthrene concentrations in each soil slice for the system of TiO2-RVC anode and bare RVC cathode.

Fig. 8. Comparison of residual phenanthrene concentrations in each soil slice for the system of (I) bare RVC electrodes, and the (II) RVC-TiO2 anode, RVC cathode, versus initial concentration (IC).

Electrode Materials a Key Factor to Improve Soil Electroremediation 231

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

0 100 200 300 400 500

 anode middle cathode

Time, min

Fig. 9. Graph of pH profiles for array I of the DSA anode and Ti cathode, experimental

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

0

2

Array I

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

4

6

pH

are shown in Figure 11.

8

10

12

14

at pH 12.

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

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 additional physical barriers like a thick filter paper.
