New process for ex situ electrokinetic pollutant removal. I: Process evaluation

Journal of Industrial and Engineering Chemistry 18 (2012) 2162–2176

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New process for ex situ electrokinetic pollutant removal. I: Process evaluation Ahmed Abou-Shady *, Changsheng Peng 1 College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China

A R T I C L E I N F O

Article history: Received 16 January 2012 Accepted 18 June 2012 Available online 26 June 2012 Keywords: Soil electrokinetic New electrokinetic process Pb2+ removal Zn2+ removal Taguchi approach

A B S T R A C T

Soil electrokinetic remediation (SEKR) is a proven technique for treating inorganic, organic, and radionuclide pollutants, particularly in fine-grained soils. The main obstacle to implementing SEKR is the high pH zone adjacent to the cathode surface, which adversely affects electromigration and electroosmosis mechanisms. There have been many attempts to overcome this drawback, but most of the intended solutions are impractical due to excessive cost or the need for chemical additives. We developed a new process for soil electrokinetic treatment in which the cathode is a vertical perforated pipe inserted into the soil. The removals of Pb2+ and Zn2+ from kaolinite were evaluated using the Taguchi approach, in which the effects of five four-level parameters (operation time, electrical potential, cathode gap, concentration, and hydrostatic head) were analyzed. The perforated cathode pipe SEKR system (PCPSS) was designed to investigate vertical soil electrokinetic remediation, which has not been extensively studied. The obtained results showed that increasing the cathode gap and hydrostatic head enhanced electroosmosis role. Satisfactory removal of Zn2+ (93.9%) was achieved, however the maximum removed Pb2+ was 42.7%. The removed Pb2+ and Zn2+ above the cathode gap was much better than the cathode. The effects of cathode gap, concentration, and hydrostatic head did not exhibit any influence on current passing. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Heavy metal pollution in soils arises from natural and anthropogenic sources including excessive use of sewage sludge fertilizers, pesticides, irrigation with wastewater, accidental spills, battery residues, wastewater from industrial processes such as electroplating, smelting, and mining, vehicle exhausts, and landfill leaching [1–3]. Heavy metals are persistent bio-accumulative and toxic chemicals (PBT) that are retained in soil either through adsorption on clay or complexation with co-existing organic matter. The presence of heavy metals negatively affects ecosystems and leads to chronic illnesses in human beings [4,5]. Various techniques have been used to treat polluted soils, including bioremediation, phytoremediation, chemical washing, soil vapor extraction, water flushing, and pump-and-treat methods. However, each of these methods has characteristic drawbacks that limit widespread applicability, such as the need for suitable media for microorganism or plant growth, relatively large reactor sizes, high energy consumption, low heavy metal removal effectiveness, or low soil permeability [6,7].

* Corresponding author. Tel.: +86 152 75288909; fax: +86 532 66782011. E-mail addresses: [email protected], [email protected] (A. Abou-Shady), [email protected] (C. Peng). 1 Tel.: +86 158 53299827; fax: +86 532 66782011.

The basic principles of soil electrokinetic remediation (SEKR) for soil treatment were described by Acar and Alshawabkeh [8] and Probstein and Hicks [9]. In SEKR, pollutants are removed from the soil by connecting an electrical gradient between two electrodes that drives migration of ions toward the oppositely charged electrode. In general, pollutant movement takes place through four mechanisms: electromigration (EG), electroosmosis (EO), diffusion, and electrophoresis. These mechanisms facilitate decontamination by initiating physical, chemical, or hydrological changes such as desorption, adsorption, oxidation, gas generation, dissolution, precipitation, reduction, ion exchange, pH gradient formation, destruction of soil particle active sites, de-complexation, and electrolysis. In general, the effectiveness of SEKR depends on the soil properties (pH, structure, conductivity, porosity, tortuosity, and clay mineral content), pollutant concentration and type, and operating technique (reaction time and electric potential difference). Metal ions are present in various forms in soil including soluble, exchangeable, adsorbed, organic bound, carbonate bound, and residual forms. The removal mechanisms are most effective on loosely bound ions. As the loosely bound ions are removed during processing, the strongly bound ions are converted into loosely bound ions until a new equilibrium is achieved, which is reflected in the dependence on operating time. EG and EO contribute much more to migration than either diffusion or electrophoresis [5,8–17]. SEKR is accompanied by an undesirable change in pH to approximately 12 in the region surrounding the cathode, while the

1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.06.014

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pH at the anode is reduced to 2–3. The elevated cathode pH is a persistent problem requiring an innovative, practical, and costeffective solution to ensure high metal ion solubility, reduce the hindrance of EO, and lower the electrical resistance. In addition, some heavy metals possess amphoteric properties at high pH, and the overall charge of the species changes from positive to negative, e.g., Pb2+ is converted to [PbO2H] and Cr3+ is converted to [Cr(OH)4] [17]. Various modifications have been attempted to overcome this drawback and to improve the performance of SEKR, including:

These proposals are economically hindered by the need for additional reagents and for expensive additional equipment. The aims of our research were to evaluate a new SEKR treatment design employing a vertical cathode consisting of a perforated pipe and to precisely determine the contributions of several operating parameters using the Taguchi approach. The new design was tested in the treatment of kaolinite spiked with Pb2+ and the associated Zn2+ was also investigated. This substrate has been widely described, enabling ready comparison to other available studies.

(1) Combining SEKR and electrodialysis or incorporating cation and anion exchange membranes (CEM and AEM) to hinder advection of the OH surrounding the cathode. This approach has several drawbacks [3,6,17,18]: (a) The CEM is susceptible to precipitation of nonconductive salts which consequently increase the resistance and the required electric potential. (b) The CEM and AEM must be frequently replaced. (c) The catholyte should be siphoned during SEKR operation to improve removal efficiency. (d) Fouling resulting from accumulation of metal hydroxides on surfaces. (e) The need for auxiliary solution additives to reduce membrane clogging. (f) During SEKR operations employing a CEM the imposed current density should not be greater than 0.5 mA cm2 to limit pH increases near the CEM due to water electrolysis. (2) The FIRS technique, in which the ordinary anode is replaced with a sacrificial anode made of iron or manganese. When direct current is applied the sacrificial anode dissociates, producing barrier ions that migrate toward the cathode and precipitate in the acid–base encounter zone [19]. (3) Lasagna processes in which the movement of pollutants is limited to horizontal or vertical treatment layers. The layers are provided with suitable treatment materials including microorganisms, oxidants, sorbents, or buffers [20–22]. (4) Addition of acids such as HNO3, citric acid, acetic acid, HNO3/ HCl, or H2SO4 to reduce the high pH in the cathodic zone and prevent metal ion precipitation [6,21–30]. (5) Addition of chelating agents such as EDTA, NTA, EGTA, DTPA, or DCyTA to complex the ions and veil their charges to protect them from further interactions [24,30,31]. (6) Surfactant additives including anionic (sodium dodecyl sulfate or SDS), cationic (dodecylamine hydrochloride or DAH), and non-ionic (polyethylene oxides such as Brij 30) to enhance desorption and improve solubility of inorganic and organic pollutants [15,32,33]. (7) Placing cation exchange resins saturated with H+ between the cathode and the soil to prevent advance of the OH migration front toward the anode and trap heavy metals accumulating in the cathode zone [4]. (8) Introduce an intervening space between the cathode and soil filled with 0.01 M KNO3. This was found to be suitable for sandy soil [23,34]. (9) Circulating the anolyte solution to the cathode reservoir [35]. (10) Adding depolarizing chemicals such as Ca(OH)2 to the anolyte and HOAc, HCl, or CADEXTM to the catholyte reservoir [17]. (11) Utilizing bipolar electrodes [36]. (12) Providing a flushing system [24]. (13) Incorporating ultrasound to improve porosity and permeability, induce cavitation, and decrease fluid viscosity [7]. (14) Advancing the anode toward the cathode during SEKR operation [13].

2. Experimental 2.1. Overview SEKR processes may be classified into horizontal or vertical designs based on the relative locations of the anode and cathodes. 2.1.1. SEKR horizontal specimen design In horizontal designs the cathode and anode electrodes are inserted in the soil at the level of treatment. The exact configuration is highly variable between researchers. Some examples are: (1) The cathode and anode electrodes are in direct contact with the soil [19,26,27,37–40]. (2) The electrodes are inserted in catholyte and anolyte compartments [4,21,31,38,41–44]. (3) The systems incorporate complete electrodialysis configurations or utilize cation exchange membranes [6,18,31]. (4) Circulating systems are added to regulate the pH adjacent to the cathode and anode using chemical additives [2,7,10,24,27,30,45]. (5) An internal gap filled with KNO3 solution is placed between the soil and the cathode [34]. (6) Rectangular or circular SEKR designs [3,12]. (7) The gap between the cathode and soil is filled with ion exchange resin [4]. (8) The anode reservoir solution is circulated to regulate the cathode electrolyte [35]. (9) The SEKR specimen is provided with a chemical additive reservoir during the treatment [5,17,25,46,47]. (10) Fiberglass, nylon, or filter paper is inserted between the electrodes and the soil [4,35,48]. (11) Multiple anode systems, ceramic castings, and bioremediation [49,50]. (12) Using upward and downward-pointing electrodes [48]. (13) Incorporating bi or monopolar electrodes [36]. (14) Using a lasagna process in which the treatment layers are stratified to prevent migration of the pollutants [20]. (15) Locating the anode between two cathodes [51]. (16) Mounting the cathode vertically in the soil and connecting it to the soil via a perforated tube filled with KNO3 [23]. (17) Incorporating a flushing system [23,52]. (18) Combining SEKR and bioleaching technology [53]. (19) Combining SEKR and ultrasonic techniques [7].

2.1.2. SEKR vertical designs In vertical designs the electrodes are located one above the other. This design has not been as intensively studied. SEKR experiments described in the literature include: (1) Vertical soil electrokinetic approaches in which the cathode is above the anode [28,54].

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(2) Experiments in which the position of the electrodes was changed from horizontal to vertical to study the effect on removal efficiency [55]. (3) Designs in which the anode is above the cathode and a flushing system is employed [52]. (4) Lasagna processes in which the treatment layers are vertical and parallel to the length of the cathode and anode [20–22]. (5) Combinations of vertical SEKR and zeolite adsorption for copper removal [56]. 2.2. Perforated cathode pipe SEKR system (PCPSS) 2.2.1. Description of PCPSS [57,58] The vertical SEKR apparatus depicted in Fig. 1(a) was manufactured from 0.6 cm-thick acrylic in the shape of a

rectangular box 35 cm high  21 cm wide  3 cm deep. The height of the kaolinite sample was 20 cm, and the remaining 15 cm space was used as a water hydrostatic head. The cathode consisted of a stainless steel pipe 5 cm long and 1.6 cm in diameter perforated with 24 holes 3 mm in diameter. The anode was located above the cathode and consisted of a titanium grid coated with iridium and ruthenium oxides and possessing an effective area of 2.8 cm  20.5 cm. The anode was perforated with 8 holes 4 mm in diameter. A power supply (purchased from Shanghai Chemistry Plant, China) was used to provide a constant potential between the electrodes. The current was detected using UT60D and UT60E ammeters coupled to a data acquisition computer. The hydrostatic head was automatically controlled using a sensor (Shenzhen Fast Sensor Co. Ltd.) mounted in the upper portion of the chamber at the desired level. The overall SEKR unit is illustrated in Fig. 1(b).

Fig. 1. Schematic diagrams of perforated cathode pipe SEKR system (PCPSS) (a), and the overall EKR unit (b).

A. Abou-Shady, C. Peng / Journal of Industrial and Engineering Chemistry 18 (2012) 2162–2176

2.2.2. Rationale for application of PCPSS PCPSS was intended to overcome the drawback of high pH in the region surrounding the cathode: (1) The perforated cathode pipe allows one-way fluid flow toward the cathode to reduce the elevated pH. This can overcome problems with reverse EO and the need for chemical additives [38,59,60]. (2) Advection of the base front is hindered. (3) The hydrostatic head enhances the role of EO. 2.2.3. Advantages of PCPSS PCPSS provides many advantages over other techniques: (1) There is no need for chemical additives. (2) Auxiliary equipment such as cathode and anode compartments, chemical reservoirs, pumps, and membranes are unnecessary. (3) It avoids the vertical leakage observed in horizontal designs [49]. (4) PCPSS may be easily implemented in situ, with the cathode pipe directly connected to underground drainage systems.

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compactor was used to remove voids that could increase electrical resistance and energy consumption. 2.3.2. Methodology After each trial the kaolinite was extruded and sliced into 10 equal segments. Samples were removed from each horizontal layer, one located above the cathode pipe and a second above the cathode gap. The samples were air dried at room temperature for 10 days. The following investigations were carried out in duplicate and the results were averaged. Vertical variations in kaolinite pH were measured by mixing 1 g kaolinite in 2.5 mL of 1.0 M KCl and vigorously shaking for a period of 1 h. The pH of the suspension was measured using a pH meter [16]. The concentration of Pb2+ was measured using the Reisenauer method. A 1 g sample of air-dried kaolinite was placed in a covered Teflon beaker. A few drops of distilled water and 0.2 mL of HNO3 were added. The mixture was evaporated to dryness on a hotplate, then sequentially digested using 2 mL HClO4, 5 mL HF, and 1 mL H2SO4. The mixture was heated to 80–90 8C until all kaolinite residues were dissolved [63]. To avoid the adhesion of kaolinite with the Teflon beaker which led to misleading results, about 15 mL of distilled water was added with the digest mixture. 2.4. Experimental scheme

2.3. Materials and methods 2.3.1. Kaolinite preparation Kaolinite was used as a test soil because of its low cation exchange and buffering capacities, low permeability, low swelling rate, and minimal organic matter content [3,25,27,36]. The chemical and physical properties of the commercial kaolinite purchased from Shanghai Chemical Co. are listed in Table 1. Filling the reactor to 20 cm required approximately 1 kg of kaolinite. The material was artificially contaminated by spiking with Pb(NO3)2 solution (purity 99.0%) and aging for one week to ensure homogeneity. Kaolinite contained a relatively high concentration of Zn2+ about 1000–1100 mg kg1. Pb2+ was chosen as the pollutant owing to the large number of studies available for comparison. The kaolinite slurries were well mixed using a plastic rod at saturated moisture content. Approximately 0.4 g of thymol blue indicator per kg of kaolinite was dissolved in 80 mL of 0.01 M NaOH [61] and added to the clay visualize advection patterns in the acid and base fronts adjacent to the electrodes. The indicator appears red-yellow at pH 1.2–2.8 and yellow-blue at pH 8.0–9.6 [62]. After loading the spiked kaolinite in the reactor, a lab-built

Properties

Values

pH Conductivity [1:5] (mS cm1) Cation exchange capacity (CEC) (mequiv. 100 g1) Cations [1:10] (mg L1)

3.95–4.00 625 3.5–4.1

Solubility in acid (%) Zinc (mg kg1) Organic mater (%) Hydraulic conductivity (cm s1) Specific gravity Atterberg limits (a) Plastic limit (%) (b) Liquid limit (%) (d) Plasticity index (%) a

N.D.: not detected.

3. Results and discussion 3.1. pH variation 3.1.1. Variation of anolyte and outlet catholyte pH When a direct current is imposed between two electrodes immersed in water the water molecules begin to dissociate at a potential difference of 1.23 V, known as the reversible potential [67]. The electrolytic reduction of water at the cathode causes H2 Table 2 Orthogonal array (L16OA) illustrates the distribution of EKR influential factors (5 factors at four levels).

Table 1 Kaolinite chemical and physical properties.

Anions [1:10] (mg L1)

The effects of five parameters on PCPSS performance were investigated using the Taguchi approach as outlined in Table 2. The Taguchi approach has been mentioned in details in our previous study [64]. The values of operating time (2, 7, 15, and 20 days), electrical potential (0.5, 1, 2, and 2.5 V cm1), cathode gap (1, 2.5, 4, and 5 cm), concentration (100, 500, 1000, and 2000 mg kg1), and hydrostatic head (1, 5, 10, and 15 cm) were selected based on available electrokinetics studies (Table 3) and other literatures [3,4,5–7,10,12,16–18,21,25–27,29–31,34,36– 38,46,48,49,54,55,65,66].

2+

2+

+

Ca = (22.5), Mg = (3.6), Na = (8.3), and K+ = (3.3) NO3 = (N.D.a), SO42 = (124.6), and Cl = (5.2) <1% 1000–1100 1.96–2 1.1  108 2.23 19.8 29 9.2

Trials

Operating period (day)

Electric gradient (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

2 2 2 2 7 7 7 7 15 15 15 15 20 20 20 20

10 20 40 60 10 20 40 60 10 20 40 60 10 20 40 60

(0.5 V cm1) (1 V cm1) (2 V cm1) (2.5 V cm1) (0.5 V cm1) (1 V cm1) (2 V cm1) (2.5 V cm1) (0.5 V cm1) (1 V cm1) (2 V cm1) (2.5 V cm1) (0.5 V cm1) (1 V cm1) (2 V cm1) (2.5 V cm1)

Cathode gap (cm)

Concentration (mg kg1)

Hydrostatic head (cm)

1 2.5 4 5.5 2.5 1 5.5 4 4 5.5 1 2.5 5.5 4 2.5 1

100 500 1000 2000 1000 2000 100 500 2000 1000 500 100 500 100 2000 1000

5 10 15 1 1 15 10 5 10 5 1 15 15 1 5 10

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Table 3 Summary of researches that focused on the removed Pb2+ by electrokinetic. Soils type

pH variation

Electrical gradient

Elapsed time

Specimen length

Removal (%)

Energy consumption

Ref.

4–11

1000 and 4000 mg kg1

10 and 20 V

137, 233, 240, and 473 h

NA

50–95% 60–70%

20–40 kWh/m3 45–160 kWh/m3

[37]

5.5

7

360–940 mg kg1

0.2 mA cm2

10–90 days

5–15 cm

NA

NA

[16]

4–5 2–4.5 3.8–5.5

11–12 10.5–12.5 4–5.9

790.5 and 863 mg kg1 4432(275) mg g1 100 mg kg1

30 V 19.5 V 1 V cm1

78–119 h 240–480 h 5–20 days

30 cm 12.5 cm 20 cm

83.4–91% 34.19–56.37% 11.2–53.3%

NA NA NA

[34] [18] [38]

4.5–5.5 2–3 NA

4.5–5.5 11–13 NA

455 mg kg1 500–5000 mg kg1 1000 mg kg1

22 V 10, 30, 50, and 100 mA 4–5  103 A

23 days 5, 15, and 30 days 100 days

17 cm 10 cm 15 cm

<20% 13–95% 80–92%

[30] [75] [6]

Different soils Contaminated soil Kaolin Sewage sludge Electroplating sludge

NA 2–3 NA 2–3.5 5.66–8.22

NA 11–13 NA 2.5–10 11.23–11.28

500 mg kg1 500 mg kg1 820 mg kg1 584  41 639.11 mg kg1

8V 50 mA 20 V and 15 mA 1.25 V 32 V

7–15 days 15 days 6 and 30 days 7 days 5 days

18 cm 20 cm 10 and 20 cm 20 cm 30 cm

NA 88–91% 25.4–78.1% 19% 20–34%

Polluted soil Different soils

NA 2.7–4

NA >4 to <7

1000 mg kg1 1000 mg kg1

250 V 20 V

30 h 570 h

20 and 17.5 cm 12 cm

32–48% 0

NA NA 4565.5–11.687.5 A/m2/ (g eg/l) NA NA 15–273 kWh m3 NA 2342.2–4057.4 kWh/t for total pollutants NA NA

Wastewater sludge

6 4.5 2–2.5

12 9.8 2–2.5

10,500 mg kg1

1.25 V cm1

5 days

12 cm

11% 51% 59%

122 kWh t1 140 kWh t1 155 kWh t1

[5]

Illite mixture

>2 2 2

10.5 4.5–6 8

2330–18,000 mg kg1

50–200 mA/cm2

1121–3014 h

10 cm

– – –

93–4171 kWh/m3 49–4412 kWh/m 461–9958 kWh/m

[17]

River mud Kaolinite Kaolinite + HA Clayey sand Sewage sludge

– – – – 2.3

– – – – 6–7

– – – – 890 mg kg1

– – – – 0.84 mA/cm

– – – – 6 days

– – – – 15 cm

54% 69% 66.90% 83% 21%

NA NA NA NA NA

Cathode

Kaolinite Nature soils

3.3–4.5

Calcareous Non-calcareous soils Sand Contaminated soils Silty loam soil artificially contaminated Contaminated soils Contaminated marine clay Silty clay

1

2

[27] [7] [28] [10] [15] [12] [4]

[49]

[46]

Polluted soil Copper ore

NA NA

NA NA

4100 mg kg 0.11% (average)

6 mA/cm 15 mA/cm2

48 days 15 days

NA NA

12.3 (0.5) 45.8 (0.8)

NA NA

[65]

Contaminated soil

<2.5 and >8 < 10

11.5 and >8 < 10

4432 + (275) mg g1

15.5 and 19.5 V

240 h

12.5 cm

13.03–36.28%

NA

[31]

Contaminated soil + artificial spiked Contaminated soil + artificial spiked

NA

NA

12.5 V

10, 20, and 40 days

12.5 cm

34–89%

NA

[66]

NA

NA

0.240 + (0.047) mg g1 + 4.04  102 M 4.432 + (0.275) mg g1 + 2.02  102 M

12.5 V

10, 20, and 40 days

12.5 cm

26–76%

NA

Sewage sludge

<2

>11

139–1110.5 mg kg1

2–6 mA cm2

3–48 h

10 cm (horizontal) 25 cm (vertical)

36–45%

3–37 c-/t

[55]

Kaolinite Kaolinte

<2 3–4

7–12 7–12

5000 mg kg1

0.1 A 0.5 A

100 h

15 cm 12 and 14.5 cm

26–54.46% 14.19–58.44%

417–334 kWh/t 671–1246 kWh/t

[3]

A. Abou-Shady, C. Peng / Journal of Industrial and Engineering Chemistry 18 (2012) 2162–2176

Initial con

Anode

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Fig. 2. The variation of anolyte pH versus time.

evolution and oxidation at the anode causes O2 evolution. An alkaline zone surrounds the cathode and an acidic zone is created at the anode [68]:

NO3  þ 2H2 O þ 3e ! NO þ 4OH

2H2 O þ 2e ! H2ðgasÞ " þ 2OH at the cathode ðE0 Þ ¼ 0:828 (1) 2H3 O þ 2e ! 2H2 O þ H2ðgasÞ " at the cathode ðE0 Þ ¼ 0

(2)

2H2 O  4e ! O2ðgasÞ " þ 4Hþ at the anode ðE0 Þ ¼ 1:229

(3) +

The pH is reduced not only by the high concentration of H , but also in a secondary manner by the release of Al3+ due to kaolinite dissolution [69]. In most SEKR studies the anolyte pH decreased to 2–3 during processing, affecting EO. In our study, the anolyte solution was continually replenished with distilled water of pH 5.99. The pH of trials 3, 4, 7, 10, and 13 increased to 3.5–4.5 with prolonged reaction times due to increased electroosmotic flow and continuous neutralization of the anolyte and to the decreased operating current at extended reaction times, which decreased the rate of electrolysis (Fig. 2). However, in other trials the pH remained between 2 and 2.5. The reduction of water molecules at the cathode leads H2 generation and increasing pH. However, the pH will not increase directly to 12 owing to interaction of OH with residual H+ on the cathode surface [35]: Hþ þ e !

1 H2 2

surface as a consequence of introducing Pb2+ in the form of the nitrate [34]:

(4)

The pH is also limited by formation of metal hydroxides that consume excess OH produced by NO3 reduction at the cathode

(5)

The outlet catholyte pH decreased continuously (Fig. 3) due to the acid front reaching the cathode and neutralizing the high pH. However, the large reduction in pH during trials 3, 4, 7, 10, and 13 was attributed to cavitation-enhanced EO flow. 3.1.2. Vertical pH distribution The soil pH must be lower than 6 to ensure effective removal of Pb2+, and ideally lower than 4 to reduce precipitation of lead salts [37]. However, the soil type also plays an important role in SEKR. Ottosen et al. [16] reported that higher pH values are favorable in calcareous soil (12% CaCO3) due to the increased solubility of cerrusite (PbCO3) at higher pH. In these soils a pH of 4.5–5.5 is preferable, while no removal was observed at pH > 3 in noncalcareous soils. The same trend was reported elsewhere [70]. During the electrokinetic remediation (EKR) operation the acid and alkali fronts migrate toward the opposing electrodes and interact approximately 1/3 of the distance from the cathode to form a zone of abrupt pH change. The movement of these fronts is irregular, and the advection H+ front depends on electromigration, electroosmosis, and diffusion mechanisms. Progress is hindered by high soil buffering capacity as a result of substitution with adsorbed cations. At the cathode zone OH front advection is affected by upstream electromigration and the progress of OH ions is hindered by electroosmosis. The increased soil pH and resulting accelerated void fluid flow create an unsaturated zone that eventually decreases this movement [71].

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Fig. 3. The variation of outlet catholyte pH versus time.

Advancement of the acid front was generally twice as rapid due to the higher solubility of H+ at the anode side (equal to 3.626  107 m2 V1 s1 in the free solution, twice that of OH), the smaller ionic radius of H+, and the formation of hydronium ions [3,8,31,72–74]. The acid front moved toward the cathode with equal horizontal advection and was not affected by differences in cathode gap. The color of the indicator was red for 80% of the anode–cathode separation. The region surrounding the cathode was yellowcolored and the blue indicating high pH did not appear. The visual appearance agrees with the data in Table 4 in which advection of the alkaline front did not exceed 10% of the anode–cathode separation and the pH above the cathode pipe was greater than in the cathode gap in all trials. This indicates that the perforated pipe cathode design has the ability to reduce the pH around the cathode, and the fluid flow through the cathode gap is better than directly above the cathode. The pH was between 2.6 and 3 below the anode. However, the pH ranged between 3.9 and 7.9 directly above the cathode and 3.7 and 5.4 above the cathode gap for all trials except 6, in which it was 6.8. The pH surrounding the cathode was dependent on the cathode gap and the electrical potential gradient. 3.2. Electroosmosis (EO) mechanism 3.2.1. Cumulative outlet catholyte Soil water exists in four forms [68]: 1. Void bulk water that is unconstrained by any forces and may be easily removed using mechanical dewatering processes such as compression or filtration.

2. Interstitial water associated with flocculation phenomena. 3. Surface water adsorbed on the soil surface and containing ions in hydrated forms. The co-ions and counter ions are generally present in their dehydrated forms. 4. Intercellular water associated with chemical bonds. EO plays an important role in pollutant removal during the SEKR process in the form of fluid flow occurring during application of an electric potential difference. In general, EO is governed by the zeta potential of the soil and is inversely dependent on tortuosity and viscosity as described in the following equation [33]: keo ¼

e  e0  z ne  T h

(6)

in which keo is the EO permeability, ne is the effective porosity, and T is the tortuosity. Any factors affecting the zeta potential, the tortuosity, or the viscosity will strongly influence EO. The zeta potential varies inversely with pollutant concentration and is proportional to particle velocity [33]:

z¼

uh e0  e  E

(7)

where z is the potential of the particle suspension, u is the particle velocity, h is the viscosity, e0 is the free space permeability, e is the pore fluid relative permittivity, and E is the field strength. From Eq. (7) it is evident that the zeta potential is affected by the pH and applied electric potential. If the pH is below the point of zero charge (PZC), reverse EO may occur. Increasing the electric potential and adding acetic acid increased EO [10,26,29,38].

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Table 4a The distribution of pH vertically above the cathode. (x/L)

Trail 1

Trail 2

Trail 3

Trail 4

Trail 5

Trail 6

Trail 7

Trail 8

Trail 9

Trail 10

Trail 11

Trail 12

Trail 13

Trail 14

Trail 15

Trail 16

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

2.846 2.920 3.182 3.351 3.642 3.705 3.720 3.706 3.771 6.932

2.940 3.077 3.170 3.340 3.496 3.601 3.643 3.736 3.752 6.184

3.106 3.112 3.170 3.223 3.415 3.431 3.502 3.613 4.024 6.381

3.228 3.195 3.350 3.252 3.503 3.386 3.483 3.511 4.295 7.239

2.707 2.720 3.111 3.250 3.349 3.525 3.504 3.589 3.660 7.986

2.717 2.716 2.688 2.864 3.208 3.370 3.509 3.936 4.762 7.371

3.272 3.211 3.140 3.193 3.170 3.260 3.470 3.518 4.014 7.868

2.941 3.223 2.989 2.950 2.938 3.073 3.182 3.640 4.342 6.644

3.114 3.228 3.286 3.185 3.257 3.394 3.482 3.626 4.070 8.214

3.350 3.166 3.170 3.268 3.263 3.426 3.484 3.429 3.959 7.833

2.700 2.978 2.846 2.721 2.720 2.901 3.020 3.240 3.661 5.750

3.000 3.054 3.104 2.975 2.853 2.963 3.055 3.047 3.486 3.934

3.113 3.131 3.301 3.367 3.206 3.327 3.392 3.523 3.892 6.599

3.178 3.200 3.240 3.226 3.264 3.269 3.350 3.415 4.919 7.510

2.691 2.725 2.728 2.786 2.720 2.994 3.091 3.167 3.756 5.346

2.720 2.704 2.620 2.614 2.717 2.773 2.835 3.811 3.730 5.123

The values highlighted with bold show the brupt increases of pH above the cathode than the cathode gap. Table 4b The distribution of pH vertically above the cathode gap. (x/L)

Trail 1

Trail 2

Trail 3

Trail 4

Trail 5

Trail 6

Trail 7

Trail 8

Trail 9

Trail 10

Trail 11

Trail 12

Trail 13

Trail 14

Trail 15

Trail 16

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

2.942 2.949 3.194 3.352 3.498 3.669 3.704 3.740 3.967 3.728

2.98 2.97 3.148 3.317 3.471 3.610 3.613 3.623 3.656 4.832

3.105 3.102 3.181 3.413 3.427 3.451 3.453 3.536 3.761 3.896

3.254 3.151 3.363 3.485 3.367 3.410 3.518 3.553 3.951 4.704

2.715 2.799 2.831 3.191 3.548 3.572 3.657 3.623 3.678 4.561

2.658 2.216 2.838 2.976 3.107 3.362 3.414 3.843 3.940 6.888

3.160 3.122 3.225 3.175 3.186 3.230 3.240 3.450 3.888 3.920

2.976 2.914 2.866 2.665 2.931 3.063 3.181 3.230 3.810 4.630

3.265 3.162 3.215 3.206 3.308 3.359 3.390 3.510 3.904 4.050

3.232 3.132 3.113 3.225 3.429 3.292 3.497 3.478 3.830 4.384

2.633 2.603 2.675 2.676 2.766 2.841 2.882 3.482 3.584 4.026

2.945 2.945 2.881 3.019 3.013 3.051 3.100 3.999 3.393 3.964

3.218 3.160 3.130 3.161 3.243 3.315 3.415 3.488 3.572 4.493

3.160 3.201 3.230 3.220 3.261 3.273 3.385 3.379 3.721 5.474

2.720 2.697 2.716 2.872 2.780 2.819 3.007 3.328 3.763 5.080

2.660 2.618 2.588 2.780 2.697 2.792 2.877 3.403 3.434 4.771

In general, the electroosmotic flow decreased during the course of the experiment. This behavior corresponded with the drop in pH of the cathode reservoir indicating the arrival of the acid front in the cathode zone, as well as the reduction in current with time [35]. The perforated pipe and application of a hydrostatic head enhanced soil cavitation. Large amounts of catholyte outlet flow were collected during trials 3, 7, 10, and 13 (Fig. 4), in which the hydrostatic head was above 10 cm. In trial 16 the catholyte outlet flow was reduced to 0 due to formation of a barrier of Pb2+ electrodeposits approximately 2 cm thick above the cathodes. This suggests the possibility of utilizing the electrokinetic deposition of Pb2+ as a shield to prevent leakage of soluble waste, by applying high voltages, using a short cathode gap, and employing concentrations up to 1000 mg kg1. The Pb2+ electrodeposits were insoluble in either acidic or basic solutions [58,64].

large Pb2+ electrodeposits above the cathode that retarded the passage of fluid water at higher voltage. Increasing the cathode gap enhanced EO due to cavitation and decreased extent of the OH front between the electrodes. Increasing the Pb2+ concentration decreased EO through a reduction in the zeta potential these results are in harmony with other literatures [37,75]. Increased hydrostatic water head substantially enhanced EO, i.e. cathode gap and water head are proportional to EO, however operating time, voltage, and concentration are inversely with EO. Altin and Degirmenci [37] showed that the high concentration of Pb2+ negatively affected the role of EO. Fig. 5f reveals that the electrical gradient is the most influential factor controlling the EO process, followed by cathode gap, hydrostatic head, operating time, and concentration. This is may be owing to increasing the electrical gradient induces the dragging power of the double layer (10 nm thickness of the charged fluid) towards cathode [23].

3.2.2. Taguchi analysis of EO To precisely evaluate the factors controlling the EO process, the S/N ratios were calculated from the electroosmotic permeability coefficient using [5,21,22]:

3.3. Removal of Pb2+ and Zn2+

qe ¼ ke  ie  A

(8)

in which qe is the electroosmotic flow (cm3 s1), ke is the electroosmotic permeability coefficient (cm2 v1 s1), ie is the electric potential difference (V cm1), and A is the cross-sectional area (cm2). Owing to the negative values of ke, the S/N ratios were calculated using the smaller-the better approach in which lower values of S/N denote to better response, i.e., we sought to maximize the experiential response (SP 2j ). The values of ke and S/N are listed in Table 5. Increasing the values of S/N will effect negatively on EO and vice versa. Fig. 5a–e depicts the inverse effect exerted by increased operating time on EO. The outlet catholyte flow decreased during prolonged operation. Lower potentials (10, 20, or 30 V) were more effective than a potential of 60 V, possibly due to the formation of

3.3.1. Vertical distribution of Pb2+ and Zn2+ According to previous studies investigating EK removal of Pb2+, the fraction removed is proportional to concentration, applied electrical potential, and operating time, although high concentrations negatively affect EO performance through a decrease in zeta potential [36]. At high concentrations most of the Pb2+ will be present in soluble forms, while at low concentrations much of the Pb2+ will be adsorbed at negatively charged sites on the clay surface and must participate in ion exchange to be released into solution. Cations are readily desorbed at the anode through electrostatic forces, then re-adsorbed during their migration toward the cathode [4,8,74]. The percentage of lead removed increased with increasing electric potential, which is not surprising since it is the driving force behind many of the phenomena involved. The EKR process was also time dependent [3,7,17,22,30,47,71,73,76]. The vertical distribution of Pb2+ and Zn2+ in each trial is depicted in Fig. 6. The removal of either Zn2+ or Pb2+ was much better above the cathode gaps than above the cathode (Table 5).

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Fig. 4. Cumulative EO outlet flow versus time.

The relatively high removal vertically above the cathode gap can be ascribed to the following reasons: (a) A reduction in porosity resulted from the precipitation of Pb(OH)2 and Zn(OH)2 in the relatively high pH region over the cathode (Table 4). At elevated pH, Pb2+ is present in various hydroxide forms, including Pb(OH)+ at pH 6.5–10, Pb(OH)2 at pH 10–12, and Pb(OH)3 at pH 8.5–12 [77].

(b) Metal ions existed vertically above the cathode gap are subject to the attraction of double forces compared with that above the cathode, accordingly much better removal is highly expected to occur in this area as is depicted in Fig. 7. (c) The soluble Pb2+ in soil pore fluid is subject to form poor soluble complexes such as PbCl3 or PbCl42 and Pb(SO4)22. This is owing to the relatively high concentrations of sulfate and chloride in kaolinite as it reported in Table 1. If the

Table 5 S/N ratio analogous to electroosmotic coefficient permeability (k), Pb2+ and Zn2+ removals (%), and energy consumption (W h). Trials

Electroosmotic coefficient permeability (k) K (cm2 v1 s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

9.22E05 5.54E05 0.002104 8.51E05 2.15E05 1.85E05 0.00126 1.36E05 3.14E05 6.06E05 1.64E05 9.85E06 0.000645 2.11E05 6.77E06 3.72E06

1

)

S/N

80.70 85.13 53.54 81.40 93.34 94.65 57.98 97.29 90.05 84.35 95.72 100.12 63.81 93.53 103.39 108.58

Pb2+ removal Removal (%)

Zn2+ removal S/N

Energy consumption

Removal (%)

S/N

V1 a

V2 b

V1

V2

V1

V2

V1

V2

19.39 3.42 6.29 43.20 3.41 31.08 15.34 24.55 12.19 14.47 31.80 20.73 8.57 11.52 3.14 8.53

33.41 4.90 5.34 30.22 4.66 30.12 15.88 40.04 3.43 23.99 42.70 19.05 1.81 4.67 27.98 0.29

25.75 10.68 15.97 32.71 10.66 29.85 23.72 27.80 21.72 23.21 30.05 26.33 18.66 21.23 9.94 18.62

30.48 13.80 14.55 29.61 13.37 29.58 24.02 32.05 10.71 27.60 32.61 25.60 5.15 13.39 28.94 10.75

37.56 33.27 67.81 65.92 53.67 81.76 88.65 89.27 87.62 90.57 91.81 90.17 89.45 90.29 89.59 88.64

39.66 44.12 64.68 71.08 53.20 81.06 88.33 93.81 86.00 92.05 93.84 88.74 90.56 88.22 93.90 93.58

31.49 30.44 36.63 36.38 34.59 38.25 38.95 39.01 38.85 39.14 39.26 39.10 39.03 39.11 39.04 38.95

31.97 32.89 36.22 37.04 34.52 38.18 38.92 39.44 38.69 39.28 39.45 38.96 39.14 38.91 39.45 39.42

The values highlighted with bold show the relatively high removal of Pb2+ and Zn2+ above the cathode gap than cathode. a V1: vertical on cathode. b V2: vertical on cathode gap.

Total energy (W h)

S/N

5.02 18.29 51.44 127.88 11.93 37.72 119.60 197.85 23.68 59.31 153.13 479.32 26.87 77.57 188.48 469.80

14.02 25.24 34.22 42.13 21.53 31.53 41.55 45.92 27.49 35.46 43.70 53.61 28.58 37.79 45.50 53.43

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2171

Fig. 5. Effect of EKR influential factors on EO.

concentration of chloride reaches to 0.1 M, the formation of either PbCl3 or PbCl42 will be below 1%. However, at the same concentration of sulfate, 20–30% of the total soluble Pb2+ will form Pb(SO4)22. Take into consideration the high stability constant of Pb(SO4)22 over Pb(Cl)2 [78]. The existing of Pb2+ complexes in anionic forms will induce the migration towards the anode. In general, the relative amount removed of Pb2+ ranged between 0.29 and 42.7% above the cathode and 12.19 and 43.2% above the cathode gap in the eighth through tenth crosssections from the anode. However, the removal of Zn2+ varied among 33.27–91.81% and 39.66–93.90% above the cathode and above the cathode gap in the eighth through tenth cross-sections from the anode. The removal of Zn2+ was relatively high compared with what was observed with Pb2+. This may be owing to the comparatively high adsorption of Pb2+ on clay minerals, and formation of amphoteric complexes such as [PbO2H] that will reverse the migration towards the anode [15,27]. Pb2+ deposits tended to form at a point located 20% of the anode/cathode separation from the cathode, particularly in the area directly above the cathode. This electrodeposit formation adjacent to the cathode during the removal of Pb2+ from natural and artificially polluted soils has not been previously reported, except for a brief mention by Yang and Lin in 1998 [38]. The Pb2+ electrodeposits appeared in the kaolinite samples as a blue-white material that changed to yellow-green after exposure to air. The Pb2+ did not deposit in a thin layer as do other heavy metals such as Cu2+, and Ni2+, but instead formed a network of deposits extending from the cathode surface and in some cases reaching the anode. The Pb2+ electrodeposits could be beneficial from the point of view of reducing plant toxicity and uptake in that the Pb2+ could be removed from soil surface layers and deposited in deeper layers. The electrodeposits were insoluble [58,64]. Concerning the concentration of Pb2+ in the outlet catholyte, The designing of cathode by such shape provides a good circumstance for electrodeposition and precipitation of Pb2+ in hydroxide form inside the cathode pipe which represented a

secure outlet catholyte disposal to be lower than 1 mg L1 [58,64]. It was observed white precipitation in the outlet catholyte which indicates to Pb(OH)2 or Zn(OH)2. The improved Pb2+ and Zn2+ removals without the use of acid additives are due to the design of the PCPSS system, which combines the effects of acid washing, cavitation, electrokinetic processes, and the potentially improved ion migration resulting from a vertical electrode configuration. Fig. 6 summarizes the removal of Pb2+ and Zn2+ under various EKR conditions. The performance was characterized by the following points: (a) The removal above the cathode gap was higher than above the cathode. (b) Pb2+ accumulated near the cathode and formed electrodeposits. Better removal would be expected with other metals such as Cu2+, Ni2+, or Cd2+ that deposit as thin layers on the cathode, i.e., the deposits do not extend as a network such as Pb2+ [58,64]. (c) The removal of Pb2+ as a function of distance tended to oscillate, indicating that desorption of Pb2+ in one cross-section was followed by re-adsorption in the next cross-section. This behavior did not observe with amount removed of Zn2+. (d) Compared to previous studies reported heavy metals removal (Table 3), the PCPSS exhibited satisfactory performance for Zn2+ removal over Pb2+.

3.3.2. Taguchi analysis of Pb2+ and Zn2+ removals To evaluate the effect of the influential factors on Pb2+ and Zn2+ removal, S/N values were calculated using the larger-the better approach in the following equation: RA ¼

R¼

R1 þ R2 þ    þ R8 10

Co  C  100 Co

(9)

(10)

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Fig. 6. The vertical distribution of Pb2+ concentration ratio in kaolinite.

RA is the average removal of Pb2+ or Zn2+ in the eighth through tenth cross-sections from the anode, R is the percentage of Pb2+ or Zn2+ removed in each slice, Co is the initial Pb2+ or Zn2+ concentration (mg kg1), and C is the final Pb2+ or Zn2+ concentration (mg kg1).

Fig. 8(a–e) shows that the S/N ratio of removed Pb2+ tended to oscillate between that above the cathode or cathode gape. The S/N ratio above the cathode gap was greater than above the cathode for Zn2+, indicating better EK performance above the cathode gap (Fig. 9a–e).

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2173

Fig. 7. Schematic diagram illustrates the migration of Pb2+ and Zn2+ vertically above the cathode pipe and cathode gap.

The effect of operating time on Pb2+ removal above the cathode gap was not significant and tended to decline after 15 days (possibly due to the formation of electrodeposits of Pb(OH)2) and the current decreased to ineffective values. Increasing the applied voltage resulted in slight increases in S/N ratio above the cathode until 40 V, however voltage up to 40 V improved the removal. The removal of Pb2+ was proportional to voltage above the cathode gap

for voltage less than 40 V. The unexpected dependence on voltage may due to the formation of electrodeposits. Increasing the cathode gap resulted in decreases in the removal ratio above the cathode, however the removal rate did not change above the cathode gap. Varying the concentration improved the removed Pb2+ above the cathode gap. The effect of hydrostatic head on removal Pb2+ tended to oscillate.

Fig. 8. Effect of EKR influential factors on Pb2+ removal.

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Fig. 9. Effect of EKR influential factors on Zn2+ removal.

The removal of Zn2+ was proportion to operating time, voltage, cathode gape, however different Pb2+ concentration and hydrostatic head did not contribute significantly. Fig. 8f describes the contributions of influential factors above the cathode for Pb2+ removal. Their effects may be ranked above the cathode in the order cathode gap > voltage > hydrostatic head > concentra-

concentration > operating time. Above the cathode gap the order changed to hydrostatic head > concentration > operating time > cathode gap > voltage. On the other hand, the effect of influential factors on Zn2+ removal was found to take the following order operating time > voltage > cathode gap > Pb2+ concentration > hydrostatic head (Fig. 9(f)).

Fig. 10. The variation of current versus time.

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Fig. 11. Effect of EKR influential factors on energy consumption.

3.4. Energy consumption 3.4.1. Temporal variation of current During EKR the current passes along three routes [68]: 1. From the interstitial fluid to the soil particles (predominant). 2. Through particles in contact. This process is more important when the fluid conductivity is relatively low. 3. Through the fluid in pores. This process exerts a significant effect when the fluid conductivity is higher than the soil particles. The temperature occasionally increased during EKR. This effect was reported in several previous papers employing high current densities (up to 5 mA cm2) and was attributed to Joule heating [6,16,39,55,71,79]. In our study high temperatures were observed during the first day of all trials operated at 40 or 60 V, in which the current was quite high. The temperature decreased during the course of the experiments as the current levels were reduced. Generally, the current was elevated at the beginning of treatment due to active pollutant desorption, however as the experiments progressed the current decreased as a result of the following [21,26,35,41,49,51]: 1. Formation of a neutral pH zone at one-third distance from the cathode which diminished the number of charge carriers. 2. Overall decrease in charge carriers due to reduction in ionic strength. 3. Polarization effects (activation, concentration, and resistance). 4. Extended effective inter-cathode distance due to blocking of direct path by salt precipitates. The variation of current versus time is depicted in Fig. 10. During prolonged operation the current decreased to less than 10 mA, except in trials 4, 8, 12, and 16 which were operated at 60 V. In these trials there was a notable increase than others trails.

S/N values. The energy follows: Ee ¼ V  I  dt

consumption

was calculated as

(11)

Ee is the energy consumption (W h), V is the potential (V), I is the current (A), and t is the time (h). Fig. 11(a–e) illustrates the effect of the influential factors on energy consumption. The energy consumption was proportional to operating time and electric potential, but cathode gap, concentration, and hydrostatic head did not exhibit any influence. The influence of the various factors may be ranked in the order of electrical gradient followed by operating time, cathode gap, concentration, and hydrostatic head (Fig. 11f). 4. Conclusions We have developed a new variation of SEKR known as PCPSS. The performance of this technique is characterized by: (1) Improved EO through cavitation. (2) Greater Pb2+ and Zn2+removal above the cathode gap than above the cathode. (3) Satisfactory removal of Zn2+ (93.9%) was achieved. However, the removal of Pb2+ was hindered by deposits formation. (4) Formation of insoluble Pb2+ electrodeposits near the cathode that may be used to form a barrier to prevent waste leakage. The PCPSS process is still undergoing improvements, and further experiments will be carried out to maximize the performance and overcome process limitations as they are identified. Further experiment will be carried out to investigate the suitability of PCPSS in situ. Acknowledgements

3.4.2. Taguchi analysis of energy consumption The effect of various factors on energy consumption was evaluated using the larger-the better approach to calculate

This work was supported by the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education

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