Enhanced Electrokinetic Remediation of Pyrene-Contaminated Soil Through pH Control and Rhamnolipid Addition

Abstract

Electrokinetic (EK) remediation is considered an effective technology for removal of persistent, toxic, and hydrophobic polycyclic aromatic hydrocarbons, such as pyrene, from soils. We propose a method involving use of pH control and rhamnolipid addition for enhancing pyrene removal efficiency of EK remediation. Pyrene removal efficiency was successfully improved by cycling the electrolyte, exchanging electrodes, and implementing an electrolyte pH of 7, with high values of 12.12%, 10.48%, and 14.00% being achieved, respectively, compared to the 0.35% achieved with traditional EK treatment. Effect of rhamnolipid addition on the EK removal of pyrene was also investigated. When rhamnolipid was added to the electrolyte, solubility of pyrene increased, thus resulting in improved removal efficiency. With an increase in rhamnolipid concentration from 0.5 to 2 g/L, pyrene removal efficiency increased from 14.74% to 24.81%, reaching 93.60% in the section closest to the anode. These results indicate that use of pH control and rhamnolipid addition is effective in enhancing the EK remediation of pyrene-contaminated soil.

Introduction

Soil contaminated with polycyclic aromatic hydrocarbons (PAHs) is a worldwide concern due to the potential risks to human and ecological health. PAHs are hydrophobic compounds and are persistent in soil, strongly binding with organic matter, thus making their removal particularly difficult. Their removal is especially difficult from low permeability clay soils using traditional remediation technology (Grasso et al., 2001; Li and Chen, 2002; Sposito, 2008; Pazos et al., 2010). New techniques must therefore be developed for effective and efficient PAH removal (Alcántara et al., 2010).

Electrokinetic (EK) remediation is a promising technology for the removal of contaminants from underground water and soils (Shrestha et al., 2003; Maturi and Reddy, 2006; Cho et al., 2009; Kim et al., 2009; Li et al., 2010) and involves the application of a low-intensity direct current (DC) electric field to the contaminated soils. When a DC is applied between the electrodes inserted into the soil, a series of effects take place, namely electromigration, electroosmosis (Acar and Alshawabkeh, 1993; Reddy and Saichek, 2004; Zhang et al., 2014), and electrophoresis, which ultimately result in the removal of pollutants from the soil (Suer and Lifvergren, 2003; Kimura et al., 2007).

EK has received a great amount of attention in recent years as it is an effective and highly efficient process for the removal of contaminants. In particular, several previous studies have demonstrated that the EK process has been applied successfully in the remediation of heavy metals (Gordon et al., 1998; Giannis et al., 2010; Kim et al., 2014; Zhang et al., 2014) and organic contaminants (Reddy et al., 2006; Alcántara et al., 2012; Cang et al., 2013; Zhou et al., 2013). However, the low water solubility, high hydrophobicity, and slow desorption rates of PAHs make them difficult to remove from soil using traditional EK remediation. One variable that is considered important in the EK remediation process is pH. The standard low pH of soil impedes electroosmosis, thus resulting in a decrease in removal efficiency (Saichek and Reddy, 2003a). We therefore investigated various approaches of ways to control the pH of the soil.

Use of surfactants, cosolvents, or cyclodextrins has been proposed to enhance the efficiency of EK remediation (Reddy et al., 2006). However, the addition of solvents to soil can be considered as the addition of further pollutants, and so further treatment would be required to remove them (Gonzini et al., 2010). Rhamnolipid is a biosurfactant, which exhibits high specificity, high biodegradability, and low toxicity levels, thus making it more attractive than chemical surfactants. It is widely used in a range of fields, such as in agriculture, ecological environments, and in the petroleum industry. Gonzini et al. (2010) studied the effects of rhamnolipid addition on the removal of gas oil from soil using the EK method. Results showed that by increasing the dose of rhamnolipid, the efficiency of gas oil removal increased to 86.7%. In addition, Chang et al. (2009) investigated the remediation performance of EK technology integrated with the nonionic surfactant Triton X-100 and rhamnolipid for the removal of phenanthrene from soils. It was found that Triton X-100 exhibited a total phenanthrene removal efficiency of 10%, while a higher efficiency of 30% was calculated for rhamnolipid addition.

We herein report a series of EK experiments to investigate the feasibility of using pH control and rhamnolipid addition for the removal of PAHs from low permeability soil. Pyrene was selected as our test pollutant. Various rhamnolipid concentrations were examined along with the effect of pH control to assess the impact of both factors on the removal of pyrene.

Materials and Methods

Preparation of soil samples

Clay soil used in this study was sampled from campus surface soil (5–20 cm depth) in Wuxi, Jiangsu Province, China. After collection, the soil sample was air-dried, and all stones, leaves, and large particles were removed. Finally, the soil was sieved through a nylon sieve (2 mm mesh) before further use.

Soils were artificially polluted using the hydrophobic organic pollutant pyrene (Sigma-Aldrich, 98%). Pyrene (1.5 g) was dissolved in hexane (100 mL) and added to the sieved soil (3 kg), to give a final contaminant concentration of 500 mg/kg. The contaminated soil was manually stirred and blended homogeneously. The soil was kept under a ventilation hood for 7 days to allow evaporation of the hexane. During this time, the soil was stirred periodically to ensure uniform distribution of the contaminants. Finally, the soil pH, electrical conductivity (EC), cation exchange capacity (CEC), soil organic carbon (SOC) concentration, and initial pyrene concentration were measured. The physicochemical properties of the soil samples are listed in Table 1.

Table 1.

Physicochemical Properties of Initial Soil Samples

Properties	Value
Soil texture	Clayed
pH (%)	7.72
Moisture content (%)	11.95
CEC (cmol/kg)	11.67
Organic content (%)	10.85
Hydraulic conductivity (cm/s)	1.11 × 10⁻⁸

CEC, cation exchange capacity.

EK setup

A schematic of the EK test setup used in this study and as described in our previous studies (Zhang et al., 2014) is shown in Fig. 1. The setup consisted of an EK cell, two electrode chambers, a DC power supply, and two peristaltic pumps to circulate the solution in the electrode chambers. The EK cell was composed of poly(methyl methacrylate) (PMMA) and measured 20 × 15 × 10 cm (length × width × height). The electrode chambers measured 5 × 15 × 10 cm (length × width × height). Graphite rods were selected as electrodes due to their inertia. Two perforated PMMA plates were installed at either end of the EK cell, and glass filter paper was placed between the EK cell and the electrode chambers to inhibit the migration of soil particles into the electrode chambers. Two gas vents were provided on top of each electrode chamber to allow gas produced during electrolysis reactions to escape.

FIG. 1.

Schematic diagram of experimental setup: 1, soil column; 2, anode chamber; 3, cathode chamber; 4, graphite electrode; 5, glass filter paper; 6, gas valve; 7, peristaltic pump; and 8, direct current power supply.

Remediation experiments

A constant voltage gradient of 1 V/cm was applied for 15 days in all experiments. Solutions of NaOH (0.01 M) and acetic acid (0.01 M) were used to control the pH in the electrode chambers. To enhance pyrene solubility, a range of rhamnolipid concentrations were used. During the experiments, current intensity and the pH in the electrode compartments were read periodically. Tables 2 and 3 give a summary of the experiments carried out.

Table 2.

Summary of Electrokinetic Experiments Enhanced by pH Control

Experiment	Voltage gradient (V/cm)	Duration (h)	Electrolyte	pH control
Test-1	1	360	0.01 M NaCl	Without
Test-2	1	360	0.01 M NaCl	Cycling the electrolyte
Test-3	1	360	0.01 M NaCl	Exchanging electrodes per 12 h
Test-4	1	360	0.01 M NaCl	Controlling electrolyte pH = 7

Table 3.

Summary of Electrokinetic Experiments Enhanced by Rhamnolipid

Experiment	Voltage gradient (V/cm)	Duration (h)	Electrolyte
Test-1	1	360	0.01 M NaCl
Test-5	1	360	0.5 g/L rhamnolipid in 0.01 M NaCl
Test-6	1	360	1 g/L rhamnolipid in 0.01 M NaCl
Test-7	1	360	2 g/L rhamnolipid in 0.01 M NaCl

Analytical methods

All chemicals used were of analytical grade, with the exception of methanol, where high-performance liquid chromatography (HPLC) grade was used. The pH value of the soil was recorded by adding deionized water (25 mL) to the dry soil sample (10 g), and the pH value was measured using a Mettler Toledo EL20 pH meter after 30–60 min of contact time. The EC of the soil was measured using an EC meter and a 1:2.5 soil to water ratio. CEC and SOC measurements were carried out using an ammonium acetate extraction method and dichromate oxidation method, respectively (Cang et al., 2013; Zhang et al., 2014).

Following the experiment, the soil sample was removed from the EK cell and divided into five sections (S1–S5, where S1 was closest to the anode and S5 closest to the cathode). Each sample was analyzed with relation to pH value and pyrene concentration. The pyrene present in the soil samples was extracted using the following modified ultrasonic extraction method (Saichek and Reddy, 2003a). A sample of the dry soil (2 g) was weighed into a centrifuge tube, thoroughly mixed with Na₂SO₄ (2 g), and 10 mL of an extraction solution (dichloromethane/acetone, 1:1, v/v) was added to the mixture. The centrifuge tube was then placed in an ultrasonic bath and extracted for 30 min. After this time, the liquid and solid phases were separated by centrifugation at 5000 rpm for 5 min. The extraction process was repeated thrice. Subsequently, solvent from the extract was removed at reduced pressure using a rotary evaporator, until the volume had reduced to ∼1 mL. Finally, the collected sample was transferred to a 10 mL volumetric flask with methanol, and the sample subjected to HPLC analysis (Dionex UltiMate U3000 HPLC system) to determine the pyrene concentration.

Above HPLC was equipped with a reverse-phase C18 column (250 × 4.6 mm, 5 μm particle size) and a diode array UV detector. Before injection, the samples were filtered through a 0.22 μm Teflon filter. The detector wavelength was set at 254 nm and the injection volume at 20 μL. A mixture of methanol and water (90:10, v/v) was used as the mobile phase at a rate of 1.0 mL/min.

Determination of PAHs by means of HPLC-FLD (fluorescence detection) has been sufficiently verified (USEPA, 1994; APHA, 1995). Ganzler and Salgo (1986) were the first to report the use of microwave energy to irradiate solid matrices such as soils, seeds, foodstuffs, and animal feed in the presence of solvents with high dipole moments with the aim of extracting different organic pollutants. Recently, researchers have presented a microwave-assisted sample treatment technique for the isolation of B(a)P from fatty foods and received higher extraction efficiency, lower solvent consumption, and shorter sample preparation time (García-Falcón et al., 2000).

Results and Discussion

EK treatment with pH control

Figure 2 shows the variation in electric current during EK remediation enhanced by pH control at a range of pH levels. The initial values of the electric current of the four test conditions, namely test-1, test-2, test-3, and test-4, were found to be 15, 12, 19, and 33 mA, respectively. In test-1, the current initially increased, reaching a maximum of 53 mA after 108 h, after which time the current gradually decreased to ∼10 mA. In test-2 and test-3, the electric currents were low, varying between 12 and 50 mA. In the case of test-4, when the anodic and cathodic electrolyte pH values were maintained at 7 using 0.1 M NaOH and 0.1 M acetic acid, it was found that the electric current increased sharply, yielding the highest electric current of all experiments carried out. This effect may be caused by the addition of NaOH, as a number of studies demonstrated that the current passing through the soil is influenced by the ionic concentration in the soil pore fluid (Mitchell, 1993; Saichek and Reddy, 2005; Hamdan and Reddy, 2008; Maturi and Reddy, 2008). Thus, when NaOH was added, Na⁺ and OH⁻ ions were introduced to the anode electrolyte. The OH⁻ ions then neutralized the H⁺ ions produced by the electrolysis reaction at the anode, while the Na⁺ ions electromigrated toward the cathode, resulting in an increase in the current (Saichek and Reddy, 2003a).

FIG. 2.

Changes of electric current during electrokinetic (EK) remediation enhanced by pH control.

Measured pH values in different sections of soil following EK remediation enhanced by pH control are shown in Fig. 3. As previously described, the soil column was divided into five sections following remediation, with each section labeled S1–S5, where S1 was the area closest to the anode and S5 the area closest to the cathode. The initial pH value of soil was found to be 7.72 (Table 1). As observed in Fig. 3, during EK remediation without pH control (test-1), the pH value of the soil gradually increased moving from the anode to the cathode, turning from acidic to alkaline. Generally, the electrolysis of water at the anode and the cathode resulted in variation of the soil pH (Reddy et al., 2006). Upon application of an electrical field, electrolysis took place, with H⁺ and OH⁻ ions being generated at the anode and the cathode, respectively. The generated H⁺ ions migrate through the soil column toward the cathode by electromigration and electroosmotic flow, lowering the pH of the soil in the region near the anode. Meanwhile, OH⁻ ions migrate toward the anode, increasing the soil pH near the cathode, and neutralizing H⁺ ions (Maturi and Reddy, 2008). As a result, an acidic zone was present in the soil near the anode, and an alkaline zone was present near the cathode.

FIG. 3.

Changes in soil pH after EK remediation enhanced by pH control.

As expected, in test-1, where no pH control was used, the pH value of the soil following remediation was 3.53 close to the anode and gradually increased to 8.90 toward the cathode. Following EK remediation with pH control by either cycling the electrolyte (test-2) or exchanging the anode and cathode every 12 h (test-3), the differences in soil pH were lesser than for test-1. In test-4, remediation was carried out maintaining both the anodic and cathodic electrolytes at pH 7. In this case, the difference in soil pH was the lowest of the four treatments and ranged from pH 6–8, which was close to the initial pH value of the soil. This indicates that EK remediation with appropriate pH control can effectively reduce the alteration of soil pH during the process, which may be beneficial for future use of the soil.

Figure 4 shows the residual pyrene concentrations in the different sections (S1–S5) following EK remediation enhanced by pH control. Treatment without pH control resulted in minimal pyrene removal. As shown in Fig. 4, test-1 gave a pyrene removal efficiency of 53.48% in S1, accompanied by pyrene enrichment in S2–S5 after the treatment, giving a total pyrene removal efficiency of 0.35%. This was expected, since pyrene is hydrophobic and exhibits low solubility in water. It should be noted in this study that the principle mechanism of pyrene migration through soil is electroosmotic flow, and it was reported by Saichek and Reddy (2003b) that electroosmotic flow under acidic pH is negligible in low-permeability clay soils. In test-2, the remediation enhanced by electrolyte cycling gave a total removal efficiency of 12.12%, with a maximum efficiency of 65.40% being reached in S1. Thus, the efficiency of test-2 was significantly higher compared with test-1. In the case of test-3, the overall pyrene removal efficiency was 10.48%. Test-4 was enhanced by maintaining the electrolyte at pH 7 and resulted in the highest removal efficiency of all experiments (14.00%). It therefore appears that with the addition of NaOH and acetic acid, the free ions in the soil pore fluid increased, resulting in ion migration and a subsequent increase in current. The increased pyrene migration and removal in the case of test-4 can be explained in terms of electromigration. Under the conditions of test-4, the movement of pyrene was driven by increased electromigration, and thus, the pyrene removal efficiency increased. The results shown in Fig. 4 indicate that pH control during the EK remediation is favorable for pyrene removal.

FIG. 4.

Distribution of pyrene concentration after EK remediation enhanced by pH control (C₀ = initial concentration; C₁ = final concentration).

EK treatment enhanced by rhamnolipid addition

Figure 5 shows the change in electrical current across the soil during EK remediation enhanced by the addition of rhamnolipid. A similar phenomenon was observed as that shown in Fig. 2, where the initial electric currents for the different treatments were relatively small. For example, in test-1, test-5, test-6, and test-7, the initial electric currents were 15, 10, 13, and 16 mA, respectively. As can be seen in Fig. 5, the electric current in test-1 (no rhamnolipid addition) was more unstable than the EK remediation treatments enhanced by rhamnolipid addition (test-5, test-6, and test-7). In test-1, the initial electric current was low, but when an electrical field was applied, H⁺ and OH⁻ ions were generated at the anode and cathode by the electrolysis of water. The transport of these ionic species through the soil resulted in an increase in the electric current in test-1, with a peak value of 53 mA being reached rapidly. Over time, the decrease in soil pore fluid led to increasing resistance in the soil medium, and as a result, the electric current gradually decreased to a stable value of 10–20 mA. This decrease may also be caused by the precipitation of nonconductive solids and by electrode polarization (Acar and Alshawabkeh, 1993; Kornilovich et al., 2005; Colacicco et al., 2010).

FIG. 5.

Changes in electric current during EK remediation enhanced by rhamnolipid.

Similar trends were observed for the electric currents in test-5, test-6, and test-7 as for test-1, although EK remediation enhanced by rhamnolipid addition (test-5, test-6, and test-7) exhibited lower electric currents than test-1. Rhamnolipid is a biosurfactant with a high molecular weight, and so a high rhamnolipid concentration could result in the formation of a viscous liquid, giving increased resistance and hindering the electromigration of free ions in the soil, thus yielding the observed decrease in current (Wang and Mulligan, 2004). Compared to test-5 and test-6, no noteworthy difference in electric current was observed for test-7, indicating that within a certain rhamnolipid concentration range, a change in concentration does not significantly affect the electric current across the soil columns.

In Fig. 6, the change in soil pH in sections S1–S5 following EK remediation enhanced by different rhamnolipid concentrations can be seen. The initial pH value of soil in these experiments was 7.72. As observed in Fig. 6, in test-1, test-5, test-6, and test-7, the soil pH values gradually increased moving from the anode to the cathode. Comparison of test-5, test-6, and test-7 with test-1 shows that with rhamnolipid enhancement, no significant difference in soil pH was observed between these four tests in S1, S3, S4, and S5. However, the soil pH values in section S2 were significantly higher for test-5, test-6, and test-7 than for test-1. This may be attributed to the rhamnolipid-induced reduction in electric current (Fig. 5) and hindered electromigration of H⁺ and OH⁻ ions.

FIG. 6.

Changes in soil pH after EK remediation enhanced by rhamnolipid.

Figure 7 shows the distribution of pyrene concentration in different sections after EK remediation enhanced by rhamnolipid addition. As previously mentioned, pyrene is highly hydrophobic and is poorly soluble in water, thus making its removal from soil problematic. In test-1, traditional EK remediation gave an extremely low total pyrene removal efficiency of 0.35%. However, in test-5, the remediation was enhanced by the addition of rhamnolipid (0.5 g/L) to the electrolyte, with a total pyrene removal efficiency of 14.74% being recorded. These results suggest that rhamnolipid addition enhances pyrene migration in soil and, thus, increases the corresponding removal efficiency. This may be caused due to the rhamnolipid increasing pyrene solubility, thus allowing it to be driven more easily by the pore fluid. Furthermore, rhamnolipid is known to increase bacterial growth, resulting in an increase in the biodegradation rate in soil (Tiehm et al., 1997; Chang et al., 2009). In the case of test-6, 1 g/L rhamnolipid was added to the electrolyte, giving a total pyrene removal efficiency of 23.84%, almost twice the efficiency recorded for test-5. Where 2 g/L rhamnolipid was added (test-7), a maximum removal efficiency of 24.81% was observed, which reached a peak of 93.60% in S1. However, when test-6 and test-7 were compared, little enhancement was observed, indicating that an increase in rhamnolipid concentration effectively improves the pyrene removal efficiency, but beyond a certain point, the enhancement to EK remediation is insignificant. As biosurfactants such as rhamnolipid often have a high cost, it is crucial to comprehensively consider the appropriate concentration of rhamnolipid for the task in hand.

FIG. 7.

Distribution of pyrene concentration after EK remediation enhanced by rhamnolipid (C₀ = initial concentration; C₁ = final concentration).

Conclusions

Effects of pH control and rhamnolipid addition to the EK remediation of pyrene-contaminated soil were investigated. We found that the removal of pyrene from contaminated soil was difficult by means of traditional EK treatment due to the hydrophobic nature of pyrene, and thus, a removal efficiency of only 0.35% was obtained. With the implementation of pH control, the removal efficiency was significantly increased, reaching 14.00% in the test where the electrolyte pH was maintained at pH 7. The addition of the biosurfactant rhamnolipid was also found to effectively enhance the pyrene removal efficiency, with a maximum of 24.81% being achieved over a whole sample (2 g/L rhamnolipid), which increased to 93.60% in the section of the sample closest to the anode. These results indicate that both pH control and addition of a biosurfactant result in enhancement of the EK remediation of pyrene-contaminated soil. Furthermore, as rhamnolipid is a biosurfactant, electro-bioremediation can be applied to soil remediation as an environmentally friendly technology.

Footnotes

Acknowledgments

Financial support for this study was received from a grant by the National Special Project on Water Pollution Control and Management (no. 2012ZX07503-002). The authors are grateful to editors and reviews for the helpful suggestions about their study.

Author Disclosure Statement

No competing financial interests exist.

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