Effect Of Soil Chemical Properties On The Remediation Of Phenanthrene-contaminated Soil By Electrokinetic-fenton Process

Chemosphere 63 (2006) 1667–1676 www.elsevier.com/locate/chemosphere

Effect of soil chemical properties on the remediation of phenanthrene-contaminated soil by electrokinetic-Fenton process Jung-Hwan Kim

a,* ,

Sang-Jae Han b, Soo-Sam Kim b, Ji-Won Yang

c

a

Environment Energy Engineering (En3), 4th Floor, Dongcheon Building, 267-2 Nonhyun-Dong, Kangnam-Ku, Seoul 135-010, South Korea b Department of Civil and Environmental Engineering, Hanyang University, 1271 sa-1 dong, Aansan, Kyunggi-do 425-791, South Korea c National Research Laboratory for Environmental Remediation, Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (Kaist), 373-1 Geseong-dong, Yuseong-gu, Daejeon 305-701, South Korea Received 14 February 2005; received in revised form 5 October 2005; accepted 5 October 2005 Available online 28 November 2005

Abstract The electrokinetic-Fenton (EK-Fenton) remediation of soil contaminated with phenanthrene was studied. Two different soils were chosen to investigate the effects of chemical properties, such as Fe oxide contents and acid soil buffer capacity. The H2O2 concentrations in pore water, the electrical potential distributions and the electrical currents were monitored to assess the electrochemical effect in relation to the soil properties. Hadong caly had high acid buffer capacity, and thus the amount of electroosmotic flow was lager in the experiment with Hadong clay than with EPK kaolin. The major mechanism of phenanthrene removal was a degradation in the experiment with EPK Kaolin, while it was a simple transport away from the system in experiment with Hadong clay. It was mainly because of the lower acid buffering capacity and better H2O2 stability in case with EPK Kaolin than with Hadong clay.  2005 Published by Elsevier Ltd. Keywords: Electrokinetic-Fenton process; HOCs; Fe oxide; Acid buffer capacity

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are organic contaminants commonly found in subsurface (Warman, 1985). These chemicals not only result in toxicity, but also are carcinogens and endocrine disrupters (Sullivan and Krieger, 1992). Because of their low solu* Corresponding author. Tel.: +82 2 540 3910; fax: +82 2 540 0935. E-mail address: [email protected] (J.-H. Kim).

0045-6535/$ - see front matter  2005 Published by Elsevier Ltd. doi:10.1016/j.chemosphere.2005.10.008

bilities and slow desorption rates, PAHs are difficult to be removed from subsurface environments. Among various technologies employed for the remediation of soils contaminated with organic chemicals, Fenton process is one of the most widely used and studied, in which hydroxyl radicals are produced by the reaction of hydrogen peroxide and ferrous ion. Hydroxyl radicals are non-specific oxidants that react with most organic contaminants at rates close to their theoretical limit, which is near the diffusion-controlled rate in water (109–1010 M1 s1) (Walling, 1975). However, they are

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innovative remediation technique known as the electrokinetically enhanced in situ Fenton oxidation method (Yang and Long, 1999) employs a high H2O2 concentrations as the anode purging solution, which demonstrates a potential to treat contaminants sorbed onto soils possessing low permeability. The H2O2 introduced from the anode chamber is transferred toward the cathode by electroosmotic flow, which is accompanied by the decomposition of contaminants on the mineral surface, where the anions (O2 , HO 2 ) may move towards the anode due to electromigration (Fig. 1). The electrolysis of water produces hydrogen ions at the anode, which in turn generates an acid front, thus affects the contaminant remediation and H2O2 stabilization, which may have direct influences from soil properties. Thus the purpose of the present study is to examine the effect of soil chemical properties, such as buffer capacity and iron contents, on the EK-Fenton process. A bench-scale experiments of the EK-Fenton process were conducted using two soils spiked with phenanthrene as the representative PAH and a low concentration acid (0.01 N H2SO4) as the anode purging solution for pH control. Electrical current, electrical potential distribution and electroosmotic flow, as well as the pH, were measured. The residual H2O2 and phenanthrene concentrations that existed in the soil at the end of each experiment were also detected.

effective only at low pH values, which require pH adjustment during the remediation (Valentine and Wang, 1998). Application of the modified FentonÕs reagent has been effective in degrading contaminants and included the use of high concentrations of H2O2 and iron oxides as catalysts (Tyre et al., 1991). Soil-bound contaminants are more resistant to oxidative attack during chemical treatments than contaminants in solution (Watts et al., 1993; Watts and Dilly, 1996). The effects of sorption on contaminant degradability can be minimized with high H2O2 additions, which results in the generation of non-hydroxyl radical transient oxygen species, such as perhydroxyl radicals (HO2 ), superoxide radical anions (O2 ) and hydroperoxide anion (HO 2 ), which are capable of oxidizing sorbed contaminants (Watts and Stanton, 1999). In the following equations, –Fe3+ and –Fe2+ represent liganded iron or occupying sites at an oxide surface: H O þ –Fe2þ ! OH þ OH þ –Fe3þ ð1Þ 2

2

H2 O2 þ OH ! HO2 þ H2 O HO2 $ O2 þ Hþ ; pK a ¼ 4:8 3þ HO2 þ –Fe2þ ! HO 2 þ –Fe HO þ O ! HO þ O 2

2

2

ð2Þ ð3Þ ð4Þ ð5Þ

2

However, the modified Fenton method has difficulties in treating heterogeneous subsurface environments that contains clayey soil with low permeability. Electrokinetic remediation of soil possessing low permeability represents a technology for both heavy metals and organics removal (Hamed et al., 1991; Acar et al., 1992). Recently, it has been demonstrated that the removal efficiency can be improved by combined application of an electrical field and supplementary reagents for organic removal (Yang and Long, 1999; Reddy and Saichek, 2003). An

Anode

2. Experimental 2.1. Materials The soils selected for the present study were EPK (EdgerÕs Plastic Kaolin from Florida) kaolin and Hadong clay. EPK kaolin was purchased from the

H2O2 & H2O2 stabilizer Supply In-let

Cathode Ground level Soil

Water flow H2 O2 Soil surface Radical species

Organic contaminants

desorption, degradation and removal of contaminants

Anions

Hydrogen ion

-

-

HO2 O2

Hydroxide ion

Fig. 1. Schematic diagram of the electrokinetic-Fenton process catalyzed by minerals in the subsurface.

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1.1 mg l1 and a log Kow of 4.57 at 25 C (Schwarzenbach et al., 1993). The electrolyte was supplied from the anode part and composed of 0.01 N H2SO4 for pH control and 7% H2O2 for oxidation.

Feldspar Corporation and the Hadong clay was an actual soil in Hadong, Korea. Table 1 shows that the mineralogy, physical and chemical properties of these soils differ significantly. As seen in the table, the major mineral of EPK kaolin was kaolinite whereas the Hadong clay soil was largely composed of quartz, kaolinite and halloysite. The two soils had negligible amounts of organic carbon. The concentrations of extractable 2 NO 3 and SO4 in the soil were determined by 0.01 M HCl extraction, and the soluble Cl concentration in the soil by de-ionized water, but the salts in the two soils were only in trace amount. Table 2 shows that the Fe, Mn and Ca concentration, in different forms, were determined by the sequential extraction procedure proposed by the Commission of the European Communities Bureau of (Tokaliog˘lu et al., 2000). The two soils possessed much lesser amounts of Mn, and the Fe concentration in the EPK kaolin was higher than that in Hadong clay, while the concentration of Ca indicated contrary tendency. Phenanthrene was selected to be a representative PAH compound, and has an aqueous solubility of

2.2. Electrokinetic reactor The electrokinetic reactor was set up as shown in Fig. 2. The reactor was divided into four parts: two electrode chambers, an electrolyte reservoirs and a soil cell. The electrokinetic cell used for the tests was 8 cm in diameter and 20 cm long, and made of Plexiglas. Seven passive electrodes were inserted into the soil cell and graphite electrodes of disk shape were placed in contact with the chambers. The electrodes were located at each end of the cell so that the electrolyte solution was only in contact with one face of each electrode. A filter paper was placed at the end of the soil cell. Each electrolyte chamber occupied the space between a porous stone and an electrode. PVC valves were used to control the inflow and outflow of the solution. The inlet at the bottom of the anode chamber was connected to a reservoir containing 2000 ml of

Table 1 Chemical and physical characteristics of soils Parameter

EPK kaolin

Hadong clay

Major mineral component (XRD) Specific gravity of solid particles (ASTM D 854)a

Kaolinite 2.65

Quartz, kaolinite, halloysite 2.62

Composition (%) (ASTM D 422)a Clay Silt Sand Specific surface area (m2 g1) Organic carbon content (%) (Walkley and Black, 1934) Cation exchange capacity (%) (US Soil Conservation Service, 1972)

97 3 0 24.3 <0.1 5.5–6

55 23 22 25.1 <0.1 10.1–10.6

Extractable salts concentration (mg kg1) SO2 4 (extracted with 0.01 M HCl) NO 3 (extracted with 0.01 M HCl) Cl (soluble with de-ionized water) CaO (%) (XRF) Hydraulic conductivity (cm s1) Initial soil pH (500%, water–solid)

42 13 ND 0.18 2 · 108 5.6

13 44 ND 2.16 7 · 108 7

a

ASTM (1996).

Table 2 Distribution of different iron, manganese and calcium fractions in soils Fraction

EPK kaolin (mg kg1) Fe

Exchangeable and bound to carbonates Bound to Fe- and Mn-oxides Bound to organic substances Residual fraction

69 185 44 3866

Total

4163

Mn 2 0.2 0.2 7 10

Hadong clayey soil (mg kg1) Ca

Fe

Mn

Ca

1244 187 64 712

33 89 51 2976

15 11 1 18

1249 222 319 9857

2208

3147

45

11646

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Fig. 2. Schematic diagram of the electrokinetic apparatus.

anolyte solution, which was circulated by a peristaltic pump. A graduated cylinder (2000 ml volume) was used as the catholyte reservoir to measure the water volume transported. Gas vents were provided in the electrode chambers for the escape of any gases resulting from the electrolysis reactions. 2.3. Methods 2.3.1. Preparations of test specimen and reactor For all the electrokinetic tests, phenanthrene dissolved in hexane was mixed with the soil to obtain the required concentration of 200 mg kg1 of dry soil, and then the hexane was allowed to evaporate in a fume hood. A little heterogeneity below 5% through this procedure was generated. The soil was then made up to a moisture content of 75–80% with de-ionized water. The soil was then statically compacted into a cell using an air pressure of 150 kPa for 7 d. The cell was, then, inserted into the main testing chamber. Once the soil was fully packed into the cell, the peripheral equipment was attached to the reactor. At the start of testing, the cathode chamber was filled with de-ionized water and the anode compartment and reservoir were filled with the appropriate electrolyte solution. 2.3.2. Operating conditions The test conditions are summarized in Table 3. A constant electrical potential between electrodes was applied throughout the experiments. The tests with

EPK kaolin and Hadong clay were abbreviated in the prefix by K1,2,3 and H1,2,3, respectively. Seven percent of H2O2 was used as the anode purging solution for all the tests. In K1&2 and H1&2, only H2O2 was used as the anode purging solution, and K2 and H2 were a longterm experiments of K1 and H1, respectively. H2SO4 (10 mN) along with H2O2 were used in the anode purging solution in K3 and H3. Thus test 2&3 in both K&H experiment were performed until the effluent volume was reduced or stabilized. 2.3.3. Chemical analysis At the end of each experiment, the cell filling was sliced into layers of approximately 2 cm, and each sectioned sample was well mixed and was measured for pH and moisture content. The phenanthrene in the liquid samples was extracted by triple liquid–liquid extraction, using a total of about 50 ml dichloromethane. To extract the phenanthrene from the soil, the soil sample (2.5 g) was transferred to a 10 ml borosilicate screw-top tube. 7.5 ml of a 1:1 dichloromethane:methanol mixture was added to the soil, and the soil–solvent suspension was shaken (200 rpm) for 72 h at 30 C. The tube was then centrifuged for 15 min, and the solvent mixture was transferred to a 10 ml test tube. One gram of anhydrous sodium sulfate was mixed with the phenanthrene containing solvent to completely remove residual water. The phenanthrene concentration was determined using a HPLC (Gibson

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Table 3 Summary of test conditions

Applied electrical potential (V) Area (cm2) Length of sample (cm) Initial concentration of spiked phenanthrene (mg kg1) Fluid at the cathode chamber Permeating fluid at the anode chamber Duration (h) Initial water content (%)

K1

K2

K3

H1

H2

H3

H2O2 (7%) 528 39.2

H2O2 (7%) H2SO4 (10 mN) 240 40.2

30 50.24 20 200

H2O2 (7%) 240 42.1

H2O2 (7%) 486 42.3

305 system) equipped with UV and Fluorescence detectors. A mixture of water and methanol, 20:80, was used as the mobile phase, with a constant flow rate of 1.0 ml min1. The detector wavelength was set at 254 nm. Using this procedure, 75–80% of the phenanthrene spiked into EPK kaolin and Hadong clay was recovered. This result indicates that some irreversible sorption/partitioning of the phenanthrene into clay might have occurred, due to EPK kaolin and Hadong clay possessing soil properties of a small particle size and high soil surface area. After the phenanthrene concentration of the 10-sectional samples had been analyzed by HPLC, the average value of the adjacent parts was used as the representative phenanthrene concentration. After the soil had been diluted and centrifuged, the residual H2O2 concentration in the supernatant was analyzed by an iodometric titration method (Kolthoff et al., 1969).

De-ionized water H2O2 (7%) H2O2 (7%) H2SO4 (10 mN) 312 240 42.5 38.5

8 7

Blank EPK Hadong

6 5

pH

Parameter

4 3 2 1 0

0

100 200 300 400

500 600 700

800

900 1000

+

Acid added (mmol H /kg soil)

Fig. 3. Titration curves of soils.

3. Results and discussion

higher than that in EPK Kaolin. Therefore, the resistance to pH change in the Hadong clay might be due to the neutralization of H+ by the calcium carbonates in the soil.

3.1. Soil buffer capacity of two soils

3.2. Electrical current and electrical potential distribution

The method of Yong and Warkentin (1990) was adapted in the present study to construct a titration curve of the soil buffer capacity. The acid solutions were prepared at the concentration of 0, 0.001, 0.002, to 0.01 M. The acid solutions were then added to the soil at a ratio of 1:10 for soil:acid solution, using 4 g of dry soil and 40 ml of acid solution. Blank test without soil was conducted. The titration curves of pH vs acid inputs for the EPK kaolin, Hadong clay and blank are shown in Fig. 3. The titration curves show that the Hadong clay was resistant to pH change, while the EPK kaolin seemed to have no significant resistance to pH changes. Gee et al. (2001) proposed that soil buffer capacity increased as the amount of Ca in the soil increased. In Tables 1 and 2, Ca content in Hadong clay was significantly

Fig. 4 shows the current increased rapidly and reached a peak within 1–24 h, and then decreased and remained nearly constant when H2SO4 was used for pH control. The peak at the very beginning was because the quantity of ions was the greatest due to the dissolution of salts associated with the dry soil particles (Mitchell, 1993). Wada and Umegaki (2001) proposed, when NaCl was introduced from the anode chamber, that the anions in the pore water were Cl ions, and the major cations balancing the anionic charge were H+, Na+ and Al3+. Namely, based on the principle of electrical neutrality, the ionic concentrations was proportional to the amount of salts released from soil surface and introduced from the anode. The increase of these salts concentration resulted in an increase of the electrical conductivity and current.

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H2 SO4 þ CaCO3 þ H2 O

20 K1 (7% Hydrogen peroxide)

18 16

Current (mA)

14

H1 (7% Hydrogen peroxide)

2þ þ CO2 $ 2Hþ þ SO2 4 þ Ca 3 þ H2 O

K3 (7% Hydrogen peroxide,10mN Sulfuric acid)

$ CaSO4  2H2 O ðGypsumÞ þ CO2

ð6Þ

H3 (7% Hydrogen peroxide,10mN Sulfuric acid)

12 10 8 6 4 2 0 0

50

100

150

200

250

300

350

Elapsed time (h)

Fig. 4. Comparison of current in EPK kaolin and Hadong clayey soil.

  Table 1 shows that salts, such as SO2 4 , NO3 and Cl , released from the soils were at very trace levels. Hadong clay, however, contains a significant amount of soluble carbonate (Tables 1 and 2) that reacts with sulfuric acid. And as time elapsed, ionic concentration decreased with the solids produced according to the following equation (Lindsay, 1979):

Therefore, when H2SO4 was used in addition to H2O2 as the purging solution, major salts during the early stage of tests with Hadong clayey soil were HCO and 2  and HO in tests3 with CO2 along with SO , O 3 4 2 2 EPK kaolin. Fig. 5 shows the normalized electrical potential (V/Vmax) distribution. In K1 and H1 that used H2O2 as the only purging solution, the electrical potential difference between the 2 cm and anode end of soil specimen was higher than in the other regions during the 240 h. These results signify that the difference of the electrical conductivity in this region was higher than that in the other regions, and was maintained as time elapsed. Therefore, these phenomenon might have resulted from the concentration gradient of ionic species, O2 and HO 2 generated during the H2O2 decomposition as well 2 as HCO 3 and CO3 released from the soil. On the contrary, in K3 and H3 using H2O2 and H2SO4 as the anode purging solution, the potential gradient in the regions near the anode was much lower than in the other tests, and a very low potential gradient

1

Normalized electrical potential (V/Vmax)

Normalized electrical potential (V/Vmax)

1 0.5h 24h 72h 240h

0.8

0.6

0.4

0.2

0.8

0.5h 24h 72h

0.6

240h

0.4 0.2

0

0

0

2

4 6 8 10 12 14 16 18 20 Distance from anode (cm)

0

(a) K1 (7% H2O2)

4

6 8 10 12 14 16 18 20 Distance from anode (cm)

(b) K3 (7% H2O2, 10mN H2SO4) 1 Normalized electrical potential (V/V max)

1 Normalized electrical potential (V/Vmax)

2

0.5h

0.8

24h 72h

0.6

240h 0.4 0.2 0

0.5h 24h 72h 240h

0.8

0.6 0.4

0.2 0

0

2

4

6 8 10 12 14 16 Distance from anode (cm)

(c) H1 (7% H2O2)

18

20

0

2

4

6 8 10 12 14 16 Distance from anode (cm)

18

20

(d) H3 (7% H2O2, 10mN H2SO4)

Fig. 5. Development of electrical potential profile (Vmax indicate that electrical potential was measured by the passive electrode, inserted at both ends of the soil specimen).

J.-H. Kim et al. / Chemosphere 63 (2006) 1667–1676

dient. When only H2O2 was used, the electrical potential drop near anode resulted in non-linear profile, and thus electroosmotic flow was retarded. In addition, the cumulative electroosmotic flow with Hadong clay was higher than that with EPK kaolin. Generally, according to the Helmholtz–Smoluchowski theory, the electroosmotic flow rate increases with the increase of negative zeta potential and electric-field strength, and then the amount of the negative zeta potential in the soil surface is proportional to the pH increase in the soil. Based on this theory, carbonate minerals in the Hadong clay buffered the acid generated at the anode and maintained the pH high (Fig. 7). Thus the zeta potential is relatively unchanged, which keeps the cumulative electroosmotic flow in Hadong clay higher than in EPK kaolin with a low buffering capacity. Consequently, electroosmotic flows were dependent on linearity of electrical potential distribution and zeta potential according to soil chemical property.

region gradually developed from the anode end toward the cathode as time elapsed. This phenomenon has been confirmed by Wada and Umegaki (2001) that Cl ions were gradually speeded toward the cathode, due to the electroosmotic flow, when NaCl was injected from the anode chamber. Based on this result, the main cause of the potential flattening was the transportation of SO2 4 ions towards the cathode due to the electroosmotic flow. 3.3. Electroosmotic flow and electrical potential distribution

3.4. pH and residual concentration of H2O2 after tests Fig. 7 shows the distribution of the residual H2O2 concentration and the pH of the soil specimen after tests. The pH values were ranged from 3.4 to 7.2 in K1 and from 3.8 to 10.5 in H1. These results indicate that the initial pH and acid buffer capacity of the soil affected the transportation of the acid and base front. Namely, since the EPK kaolin possessed a low initial pH and low acid buffer capacity, the pH in the soil was lower than that in the Hadong clay. The residual H2O2 concentrations in the Z = 0.05–0.15 regions were higher in H1 than in K1, while that in the other regions showed the contrary tendency (Z = normalized distance from the anode end in the soil column; i.e., Z = z/L, where z = distance from anode end in the soil column and L = soil column length). Generally, the H2O2 decomposition rate is proportional to the increase in the pH and amount of Fe oxide

1800 K2 (7% Hydrogen peroxide) H2 (7% Hydrogen peroxide) K3 (7% Hydrogen peroxide, 0.01 N Sulfuric acid)

1400

H3 (7% Hydrogen peroxide, 0.01 N Sulfuric acid)

1200 1000 800 600 400 200 0 0

100

200

300

400

500

600

Elapsed time (h) Fig. 6. Cumulative volume of effluent plotted against elapsed time.

12

3.5

K1 (Hydrogen peroxide) H1 (Hydrogen peroxide) K1 (pH) H1 (pH)

10 8

3 2.5

6 2 4

1.5 1

2 0.5

12

4.5

H2O2 concentration (%)

4

pH

H 2O2 concentration (%)

4.5

4 3.5

K2 H2 K2 H2

(Hydrogen peroxide) (Hydrogen peroxide) (pH) (pH)

3

10 8

2.5 6 2 1.5

4

1 2 0.5

0

0

0

0

0.05 0.15 0.25 0.35 0.45 0.55 0.65 0.75 0.85 0.95

0.05 0.15 0.25 0.35 0.45 0.55 0.65 0.75 0.85 0.95

Normalized distance from anode

Normalized distance from anode

(a) K1 and H1

(b) K2 and H2

Fig. 7. Residual H2O2 and pH distribution in the soil specimens after the tests.

pH

Cumulative volume of effluent (ml)

Fig. 6 shows the cumulative electroosmotic flow. The results indicate that the differences in the anode reagent and soil properties had a significant effect on the cumulative electroosmotic flow. When H2SO4 was additionally used with H2O2 as an anode purging solution, the electroosmotic flow rate was higher than that in the tests using only H2O2. The cumulative electroosmotic flow is closely related to the linearity of electrical potential gra-

1600

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than in K2, it is evident that the treatment efficiencies in tests using EPK kaolin were higher than in Hadong clay. Since the Fe content and soil buffer capacity in EPK kaolin were, respectively, higher and lower than those in Hadong clay, a higher removal efficiency and improvement of H2O2 stabilization were found in tests with EPK kaolin compared with those in Hadong clay. Particularly, in K3 with EPK kaolin as well as H2O2 and H2SO4 as the anode purging solution, the efficiency of phenanthrene increased significantly compared to other tests. This result indicates that the low buffer capacity of EPK kaolin and addition of 0.01 N H2SO4 resulted in increase of advancement rate of acid front, and then the oxidation of phenanthrene with Fenton-like reaction increased. The mechanism of phenanthrene treatment was influenced by the soil properties (Fig. 8). In the tests with Hadong clay, the residual concentration of the phenanthrene in the regions near cathode increased compared to the initial phenanthrene concentration. These results indicate that desorption of the phenanthrene in the

in the soil (Watts et al., 1999a). A higher residual H2O2 concentration in Z = 0.05–0.25 regions and a lower residual H2O2 concentration in the other regions were found for H1 compared with K1. Since the amount of Fe oxide in Hadong clayey soil was lower than that in EPK kaolin, the residual H2O2 concentration in Z = 0.05–0.25 regions were higher in EPK kaolin than in Hadong clay. On the other hand, in H1, due to the high acid buffer capacity of Hadong clay, the decomposition rate of H2O2 increased rapidly as the distance from the anode increased. In addition, although the test period were longer and cumulative electroosmotic flow were higher in H2 than in K2, the residual concentration of H2O2 in the entire soil specimen was lower in H2 than in K2. The lower stability of H2O2 in H1 and H2 also resulted from the higher acid buffer capacity. 3.5. Residual concentration of H2O2 and phenanthrene Although the test period and amount of electroosmotic flow in H2 were longer and larger, respectively,

2.5

0.6

2 0.4

1.5 1

0.2

-1

0.5 0 2

6

10

14

4 3.5

0.8

3 2.5

0.6

2 0.4

1.5 1

0.2

0.5 0

0

18

2

Distance from the anode (cm)

6

14

18

1.5 1

0.2

0.5 0 6

2.5

0.6

2 0.4

1.5 1

0.2 0.5 14

Distance from the anode (cm)

18

0

14

18

5 1.2

4.5

4.5

-1

3

10

Distance from the anode (cm)

4

1

3.5 0.8

3 2.5

0.6 2 0.4

1.5 1

0.2

0.5 0

0 2

6

10

14

18

Distance from the anode (cm)

(e) H2 -1

Relative phenanthrene concentration (C C0 )

4

1

3.5 0.8

3 2.5

0.6 2 0.4

1.5 1

0.2 0.5 0

0 2

6

10

14

Distance from the anode (cm)

(f) H3

Hydrogen peroxide concentration (%)

Fig. 8. The distribution of residual phenanthrene and hydrogen peroxide in the soil specimens after the tests.

18

H2O2 concentration (%)

0.8

H2O2 concentration (%)

3.5

(d) H1

2 0.4

2

H2O2 concentration (%)

Phenanthrene concentration (C C0 )

-1 0

4

1

10

2.5

(c) K3

1.2

4.5

6

3 0.6

5

5

2

3.5

(b) K2

1.2

0

4

0.8

Distance from the anode (cm)

(a) K1

Phenanthrene concentration (C C )

10

4.5 1

0

Phenanthrene concentration (C C0-1)

0

1

5

H2O2 concentration (%)

3

1.2

4.5

H2O2 concentration (%)

3.5

0.8

Phenanthrene concentration (C C0 )

-1

0

4

H2O2 concentration (%)

Phenanthrene concentration (C C )

4.5 1

5

1.2

5

1.2

Phenanthrene concentration (C C0-1)

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regions near anode occurred and then most of the desorbed phenanthrene was transported toward the cathode due to the electroosmotic flow. Namely, since the transportation rate toward the cathode was faster than the degradation rate, the major mechanism of the phenanthrene treatment were desorption and transportation. However, in the tests with EPK kaolin, the increase of the relative phenanthrene concentration in the regions near cathode did not occur, except in K2. These results indicate that the degradation rate of phenanthrene was as fast as the desorption rate at the soil surface. Watts et al. (1999b) suggested that the reductants, such as O2 and HO2 , were responsible for the desorption of the sorbed contaminants during the vigorous Fenton-like reactions. Kawahara et al. (1995) also proposed that, in the Fenton reaction with high H2O2 concentration, a significant increase in the extractability of the PAHs occurred after 1 h of treatment. It was because of the release mechanism of tightly held PAHs due to electron exchange by the structural iron in the clay minerals and the swelling of the layers. The present result also supports the mechanism: the degradation and removal with transportation of desorbed phenanthrene after the desorption of phenanthrene sorbed in the soil during the EK-Fenton process.

4. Summary and conclusions The objective of this investigation was to examine the electrochemical phenomenon, H2O2 stabilization and treatment mechanism in relation to the soil properties during the EK-Fenton process for phenanthrene removal. The experiments employed two clay soils, EPK kaolin and Hadong clay soil, to compare the effects caused by different soil types. The parameters measured during the test were the electrical current, electrical potential gradient distribution, and cumulative electroosmotic flow. At the end of each test, the pH and residual phenanthrene concentrations in the soils were also measured. The following conclusions could be drawn from this study: As was stated by the principle of electrical neutrality, the electrochemical variation (current and potential gradient) during the EK-Fenton process depended on the amount and distribution of salts in the soil specimen. 3 The major salts were CO2 released from 3 and HCO   the soil, O2 and HO2 generated with H2O2 decomposition, and SO2 4 introduced from the anode. The electroosmotic flow was closely related to the profile of electrical potential gradient during the experiment. The more linear profile it resulted, the larger cumulative electroosmotic flow was obtained. In addition, the departure from the linearity indicates a higher electrical resistance due to a shortage of ionic species, which resulted in an undesirably bigger electrical poten-

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tial difference within a short distance. Furthermore, the amount of cumulative electroosmotic flow was lager in the test with Hadong clay than with EPK kaolinite due to a high acid soil buffer capacity of Hadong clay. The major mechanism of phenanthrene treatment was degradation due to a suitable condition for the Fenton-like reaction in the test with EPK kaolinite possessing a low acid buffer capacity and high iron content. The high carbonate content in Hadong clay, however, resulted in a large acid buffer capacity, and thus resulted in the decrease of H+ concentration. Carbonates appear to reduce the H2O2 stabilization and treatment efficiency of phenanthrene by the Fenton-like reaction.

References Acar, Y.B., Li, H., Gale, R.J., 1992. Phenol removal from kaolinite by electrokinetics. J. Geotech. Engrg., ASCE 118, 1837–1851. American Society for Testing and Materials, 1996. Special procedure for testing soil and rock for engineering purposes. In: 1996 Annual Book of ASTM Standards, Philadelphia, PA. Gee, C., Ramsey, M.H., Thornton, I., 2001. Buffering from secondary minerals as a migration limiting factor in lead polluted soils at historical smelting sites. Appl. Geochem. 16, 1193–1199. Hamed, J., Acar, Y.B., Gale, R.J., 1991. Pb(II) removal from kaolinite by electrokinetics. J. Geotech. Eng. 117, 241–271. Kawahara, F.K., Davila, B., Al-Albed, S.R., Vesper, S.J., Ireland, J.C., Rock, S., 1995. Polynuclear aromatic hydrocarbon (PAH) release from soil during treatment with FentonÕs reagent. Chemosphere 31, 4131–4142. Kolthoff, I.M., Sandell, E.B., Meehan, E.J., Buchkenstein, S., 1969. Quantitative Chemical Analysis, fourth ed. Macmillan, New York, pp. 842–860. Lindsay, W.L., 1979. Chemical Equilibria in Soils. John Wiley and Sons, Inc., New York. Mitchell, J.K., 1993. Fundamentals of Soil Behavior. John Wiley and Sons, Inc., New York. Reddy, K.R., Saichek, R.E., 2003. Effect of soil type on electrokinetic removal of phenanthrene using surfactants and cosolvents. J. Environ. Eng. 129, 336–346. Schwarzenbach, R.P., Gschwend, P.M., Imboden, D.M., 1993. Environmental Organic Chemistry. John Wiley and Sons, Inc., New York. Sullivan Jr., J.B., Krieger, G.R., 1992. Hazardous Materials Toxicology: Clinical Principals of Environmental Health. Willians and Wilkins, Baltimore, MD. Tokaliog˘lu, S ß ., Kartal, S ß ., Elc¸i, L., 2000. Determination of heavy metals and their speciation in lake sediments by flame atomic absorption spectrometry after a four-stage sequential extraction procedure. Anal. Chim. Acta 413, 33–40. Tyre, B.W., Watts, B.J., Miller, G.C., 1991. Treatment of four biorefractory contaminants in soils using catalyzed hydrogen peroxide. J. Environ. Qual. 20, 832–838. US Soil Conservation Service, 1972. Soil survey laboratory methods and procedures for collecting soil samples. Soil

1676

J.-H. Kim et al. / Chemosphere 63 (2006) 1667–1676

Survey Investigation: Report 1, US Gov. Print. Office, Washington, DC. Valentine, R.L., Wang, H., 1998. Iron oxide catalyzed oxidation of quinoline by hydrogen peroxide. J. Eviron. Eng. 124, 31–38. Wada, S.I., Umegaki, Y., 2001. Major ion and electrical potential distribution in soil under electrokinetic remediation. Eviron. Sci. Technol. 35, 2151–2155. Walkley, A., Black, I.A., 1934. An examination of the digestion method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Sci. 37, 29–38. Walling, C., 1975. FentonÕs reagent revisited. Accounts Chem. Res. 8, 125–131. Warman, K., 1985. PAH emissions from coal-fired plants. In: Handbook of Polycyclic Aromatic HydrocarbonsEmission Source and Recent Progress in Analytical Chemistry, vol. 2. Marcel Dekker, New York, pp. 21–59. Watts, R.J., Dilly, S.E., 1996. Evaluation of iron catalysts for the Fenton-like remediation of diesel-contaminated soils. J. Hazard. Mater. 51, 209–224.

Watts, R.J., Stanton, P.C., 1999. Mineralization of sorbed and NAPL-phase hexadecane by catalyzed hydrogen peroxide. Water Res. 33, 1405–1414. Watts, R.J., Udell, M.D., Monsen, R.M., 1993. Use of iron minerals in optimizing the peroxide treatment of contaminated soils. Water Environ. Res. 65, 839–844. Watts, R.J., Foget, M.K., Kong, S.H., Teel, A.L., 1999a. Hydrogen peroxide decomposition in model subsurface systems. J. Hazard. Mater. B69, 229–243. Watts, R.J., Bottenberg, B., Hess, T.F., Jensen, M.D., Teel, A.L., 1999b. Role of reductants in the enhanced desorption and transformation of chloroaliphatic compounds by modified FentonÕs reaction. Environ. Sci. Technol. 33, 3432– 3437. Yang, G.C.C., Long, Y.W., 1999. Removal and degradation of phenol in a saturated flow by in-situ electrokinetic remediation and Fenton-like process. J. Hazard. Mater. B69, 259– 271. Yong, R., Warkentin, B.P., 1990. Buffer capacity and lead retention in some clay materials. Water Air Soil Pollut. 53, 53–67.