Surfactant-Enhanced Electrokinetic Remediation of Chromium and Phenanthrene Cross-Polluted Soils

Abstract

Electrokinetic (EK) remediation has potential to simultaneously remove heavy metals (HMs) and organic compounds from soil, but removal efficiency of these pollutants is very low in general if no enhancing treatment is applied. In this work, a new enhanced EK remediation technology to decontaminate a HM–organic cross-contaminated soil by applying different surfactants and pH control were studied. Laboratory-scale EK experiments were performed using different surfactants (Triton X-100, Tween 80, and Dowfax 8390) and controlling catholyte electrolyte pH at 4. Diphenylcarbazide (DPC), which is a complexing agent used in the spectrophotometric determination of chromate, was used as a solubilizer for chromium (Cr). After treatment with 1.0 V/cm of voltage gradient for 15 days, soil pH, electroosmotic flow (EOF), electrical current, and the concentrations and chemical fractionations of soil phenanthrene (PHE) and Cr were analyzed. Results indicated that EOF rate in the Dowfax 8390 system was higher than other surfactants. Using EK technology for removal of Cr and PHE in the presence of Dowfax 8390 was more efficient than in the presence of other surfactants. A surfactant system containing both Dowfax 8390 and solubilized DPC is the most effective surfactant system to remediate Cr-contaminated soil in EK treatment. However, to a certain extent, the removal efficiency of PHE is reduced. In addition, the influence of pH in this system was evaluated. Under the optimal operating conditions, more than 70%, 86%, and 56% of Cr(T), Cr⁶⁺, and PHE were removed from the soil, respectively. Overall, it can be concluded that the enhanced EK remediation system in the simulated setup can be effective for the removal of both HMs and polycyclic aromatic hydrocarbons from clayey soils and will help develop the remediation technology for HM–organic cross-contaminated soil.

Introduction

With the development of industrialization, mixtures of organic and inorganic pollutants are frequently found in the environment. According to the reports of USEPA, more than 67% of contaminated sites are found to contain HMs and organic pollutants simultaneously (USEPA, 2004). In the United States, about 11% of the sites on the National Priority List for the treatment of soil and 14% of the contaminated sites in Japan that exceed environmental quality standards are contaminated by Cr⁶⁺ (Suzuki et al., 2014). Polycyclic aromatic hydrocarbons (PAHs) consist of two or more fused benzene rings such as phenanthrene (PHE), fluoranthene (FLT), and benzo[a]pyrene (BaP), which are potential carcinogenic and mutagenic hydrophobic pollutants found in contaminated soil (Shi et al., 2015). Although several techniques have been studied for the remediation of polluted sites, only a few of these technologies can be applied for cross-contaminated sites (Sharma and Reddy, 2004). This is because of the extraordinary property between HMs and PAHs. Therefore, the remediation of HM and organic pollutant cross-contaminated soil is a major difficulty and challenge.

In the previous researches, electrokinetic remediation (EKR) has shown great potential to remediate soils contaminated with HMs (Jia et al., 2012; Kim et al., 2014; Zhang et al., 2014) or organic contaminants (Alcántara et al., 2012; Zhou et al., 2013). However, only limited work has been done on the application of this remediation in the compound-contaminated sites (Alcántara et al., 2012; Long et al., 2013). In this technology, an electric field is applied to promote the movement of contaminants toward the electrode chambers. Usually, HMs are water/acid soluble and can be easily migrated and integrated with soil Fe–Mn oxides and organic matter. For organic pollutants, electroosmosis is the main transport mechanism. Because of the low solubility of organic pollutant, it is difficult to remove it from the soil environment. Therefore, the removal of organic pollutants is particularly important for the remediation of cross-contaminated sites in the EKR system.

The use of surfactant to enhance removal efficiency of organic compounds from soils during EKR has been investigated by several researchers. This is because surfactants are chemical compounds that, as a result of their structure (hydrophilic and hydrophobic groups), can increase the solubility of organic compounds in water. Usually, surfactants can be classified as anionic, cationic, nonionic, or amphoteric. Cationic surfactants were rarely used for environmental application in soils because they are generally toxic and tend to interact with the soil particles (often electronegative) reducing their effectivity (Bourbonais et al., 1995). For the same reason, amphoteric surfactants were also rarely used. Nonionic surfactants are favorable in EKR because they can be transported into the soil by electroosmosis and, in general, their toxicity is low. Anionic surfactants are usually chosen for surfactant-based remediation procedures because of their lower degree of adsorption on soil than that by cationic and nonionic surfactants (Rouse et al., 2015). For instance, Yang et al. (2005) reported PHE removal as high as 98% with APG (alkyl polyglycoside), whereas Calfax16 L-35 removed lesser than 25%. Park et al. (2007) found that 60% of PHE was removed when SDS (sodium dodecyl sulfate) was used as the surfactant.

Surfactants had been extensively researched in the removal of PAHs and hydrocarbons from contaminated soil, but the role of surfactants in remediation of soil metal contamination is less well understood. In the soil, HMs mainly exist in two forms: adsorption on the surface of soil in the form of ions and precipitation of metal compounds (Evans, 1989). Different from the organic pollutants, HMs are mainly removed from the soil via surfactant-associated complexation (Ochoa-Loza et al., 2001) and ionic exchange (Swarnkar et al., 2012). Mulligan et al. (1999) found that anion surfactants can remove zinc and copper from a hydrocarbon-contaminated soil due to the anionic character of these surfactants. Moreover, the addition of complexing agents can convert soil-bound heavy metal (HM) ions into soluble metal complexes, or they may be used to modify the zeta-potential that increases the electroosmotic flow (EOF) and promote the migration of organic pollutants indirectly (Popov et al., 1997; Nogueira et al., 2007).

In this study, the main objective was to achieve the simultaneous removal of Cr and PHE from soil by applying different surfactants in electrolyte and controlling soil pH. We want to drive Cr and PHE to electrolyte during their migration process. Through this study, a new technology to decontaminate the cross-contaminated soil was developed. It will help us to develop the remediation technology for HM–organic cross-contaminated soil.

Materials and Methods

Soil

The contaminated soil used in this study was sampled from Jiangnan University, Jiangsu Province, China. Weeds, leaves, and rocks were removed before sample collection. The samples were sieved and the fraction that contained particles of 0.83 mm or smaller was selected. The soil characteristics are reported in Table 1.

Table 1.

Characteristics of Soil Used for Experiments

Characteristics	Value
Soil texture	Clayey
pH	7.28
Moisture content (%)	11.95
CEC (cmol/kg)	11.67
Organic content (%)	10.85
Hydraulic conductivity (cm/s)	1.11 × 10⁻⁸
Cr(T) (mg/Kg)	9.85 ± 0.45
Cr⁶⁺ (mg/Kg)	7.53 ± 0.37
PHE (mg/Kg)	4.81 ± 0.28

Sample preparation

The PHE-Cr cross-contaminated soil represents typical contaminates found at some contaminated sites. It was prepared by spiking K₂Cr₂O₇ and PHE (each at about 500 mg/kg as Cr and PHE) according to the following method. First, the soil (300 g, 10% total quantity of soil to be spiked) was spiked with a highly pure PHE (1.500 g, Sigma 98%) in acetone. The spiked soil was transferred in a wide rectangular pan and then left under the fume hood for 1 d to evaporate any traces of acetone. Second, the spiked soil with PHE was further spiked with 30 mL K₂Cr₂O₇ solution (containing 8.487 g Cr) and put in the fume hood for 1 day. Third, the spiked soil (300 g) was thoroughly mixed with uncontaminated soil (2700 g), and then, the total contaminated soil was passed through a 0.84-mm nylon sieve again to ensure homogeneity of treatment. The final concentrations for PHE and Cr in the treated soil were analyzed, and their values were 489 and 512 mg/kg, respectively.

EK reactor

The schematic diagram of laboratory-scale EK reactor is shown in Fig. 1. The EK reactor consists of five compartments: the anolyte reservoir (2 L), anode chamber (4 × 4 × 4 cm), soil cell (20 × 4 × 4 cm), cathode chamber (4 × 4 × 4 cm), and catholyte reservoir (2L).The electrodes were made of graphite, which connected to a direct current power supply at constant voltage (20 V). The graphite electrode with a 30 cm² size covered the whole soil cross-section to provide uniform electric current. Graphite electrodes were used for both the anode and the cathode. Electrode chambers were connected to the soil cell with screws. Filter paper was placed between the soil and electrode compartments to prevent soil particles from penetrating into the electrolyte solution reservoirs. The characteristics of surfactants are shown in Table 2.

FIG. 1.

Schematic of EK treatment system for removal of chromium and PHE from soils. PHE, phenanthrene; EK, electrokinetic.

Table 2.

Physicochemical Properties of Complexing Agents and Surfactants Used in Study

Trade name	Dowfax 8390 (DOSL)	SDBS	Triton X-100	DPC
Chemical name	Diphenyl oxide disulfonate	Sodium dodecyl benzene sulfonate	Polyethylene glycol tert-octylphenyl ether	Diphenylcarbazide
Molecular weight	642	348	625	242
HLB	N/A	10.6	13.4	N/A
CMC (mM/L)	3.0	1.2	0.28	N/A
Molecular formula	C₁₆H₃₃C₁₂H₇O(SO₃Na)₂	C₁₈H₂₉NaO₃S	C₃₂H₅₈O₁₀	C₁₃H₁₄N₄O
Type Company	Anionic	Anionic	Nonionic	N/A
	Dow Chemical, Midland, MI	Sigma Aldrich	Sigma Aldrich	Sigma Aldrich

SDBS, sodium dodecyl benzene sulfonate; DPC, Diphenylcarbazide; HLB, hydrophile–lipophile balance; N/A, not available; CMC, critical micelle concentration.

EK experiment design

The soil sample was manually added in the EK cell and is compacted with a plastic piston. All the experiments were completed at room temperature (22°C ± 3°C). The liquid in the electrode chambers was recirculated with a pump to avoid the development of concentration gradients and the EOF is measured in the expansion vessels. The EOF was measured as the excess of liquid that overflows from the electrode chamber.

A constant DC voltage gradient of 20 V (1 V/cm) was applied in all experiments for a treatment time of 15 days. Readings of current intensity and pH in the electrode compartments were taken periodically. All experiments were performed in duplicate. As shown in Table 3, there were six treatments in this trial. Different surfactants (Triton X-100, SDBS, DOSL) and pH control were used. Acetic acid (0.01 M) was applied to control the pH in the catholyte. During the experiments, electric currents and EOF were monitored. After the treatments, each soil column was divided into five equal sections, labeled as S1–S5 from the anode to the cathode. The soil pH, and the contents and chemical fractionations of Cr(T) in soil subsamples were determined. All analytical determinations were done in triplicate with an experimental error below 10%. In addition, the overall treatment efficiency was calculated as follows: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \omega { \rm { = } } { \frac { { C_0 } - { C_1 } } { { C_0 } } } \end{align*} \end{document}

where C₀ is the initial amount of pollutant concentrations in the soils (milligram) and C₁ is the amount of pollutant concentrations that remains in the soils after treatment.

Table 3.

Summary of Electrokinetic Experiments

Experiment	Voltage gradient(V/cm)	Duration (h)	Anolyte (0.01 M NaCl)	Catholyte (0.01 M NaCl)
T1	1	360	None	None
T2	1	360	10 times CMC SDBS	10 times CMC SDBS
T3	1	360	10 times CMC Triton-X100	10 times CMC Triton-X100
T4	1	360	10 times CMC DOSL	10 times CMC DOSL
T5	1	360	10 times CMC DOSL +DPC(0.5 M)	10 times CMC DOSL +DPC(0.5 M)
T6	1	360	10 times CMC DOSL +DPC(0.5 M)	10 times CMC DOSL +DPC(0.5 M), pH = 4

Chemical analysis

Determination of HMs: Samples were air-dried, passed through a 100-mesh screen (0.149 mm), and digested with a Microwave Digestion System (Multiwave PRO). A flame atomic absorption spectrometry (FAAS) (SHIMADZU AA7000) was used to determine the concentrations of Cr(T) according to USEPA methods 7190 (USEPA 1996a). Cr⁶⁺ was extracted according to the EPA method 3060 A (USEPA 1996b). Concentration of Cr³⁺ was determined by the difference between Cr(T) and Cr⁶⁺. To determine the binding forms of HMs with soil samples before and after EKR, the selective sequential extraction (SSE) approach was adapted from the Tessier180 method (Tessier et al., 1979). The metal species in the soil are separated into the following fractions: F1:exchangeable (metals leached from soaking soil in 1 M of MgCl₂, pH = 7); F2: carbonate-bound (1 M of NaOAc, pH = 5); F3: Fe-Mn oxide-bound (0.04 M of NH₂OH·HCl in 25% (v/v) HOAc, pH = 2 and heated at 96°C); F4: organic matter-bound (0.02 M HNO₃ in 30% H₂O₂, pH = 2); and F5: the residual (digested with a HCl-HNO₃-HF-HClO₄ mixture). After applying SSE, the Cr concentrations in these five fractions were determined by atomic adsorption spectrometry (SHIMADZU AA7000).

For PHE analysis of soil, a dry representative sample weighing 10 g was thoroughly mixed with about 10 g of Na₂SO₄, and the mixture was placed into a Whatman cellulose extraction thimble. The PHE was then extracted using a Soxhlet apparatus according to the procedure outlined in the USEPA test method 3540C (USEPA 1996c). The extraction solution was composed of acetone/hexane (1:1, v/v). After four cycles (5 min, 110°C, and 104 kPa), the extraction was completed, and the collected sample was used to determine the PHE concentration. The PHE concentration was determined by an HPLC (Agilent 1100) equipped with an Alltech Econosphere reverse-phase C18 column (250 × 4:6 mm, 5 μm particle size) and a diode array UV detector was used. Before injection, the samples were filtered through a 0.45-μm Teflon filter. The injection volume was set at 10 μL. A mixture of water and methanol, 10:90, was used as the mobile phase at a constant flow rate of 1.0 mL/min. The detector wavelength was set at 254 nm, and the column temperature was maintained at 30°C. Triplicate standard samples were commonly injected to certify a uniform response and to ensure that the calibration graph and the baseline remained stable, and the detection limit was 0.01 mg/L.

Results and Discussion

Electric current

Electric current is an indication of the amount of ion electromigration (Demir, 2013). As shown in Fig. 2a, the current is different and fluctuated in the different tests. The overall trend of the current is increased first and then decreased, finally remaining stable. The increase of electric current in the first stage could be attributed to desorption of ions from soil particles, and electrolyte can release ions into the soil, which contributed to an increased electric conductivity and enhancement in the current (Colacicco et al., 2010; Li et al., 2012). Meanwhile, mobile ions in soils will be gradually reduced with time, resulting in lower current intensity and precipitation of metal hydroxides near the cathodes (Acar et al., 1996). It can be found from Fig. 2a that the addition of diphenylcarbazide (DPC) increased the current flow. In T5, the peak value of 99 mA was displayed after 2 days, and the current was kept beyond 30 mA afterward. The increase of electric current in the soil column was partly because of using DPC as the solubilizate could exit the micelles and form a Cr⁶⁺-DPC complex with the solid-associated chromate; moreover, the extraction of Cr⁶⁺ [Cr⁶⁺-DPC complex] into the micellar core of the surfactants occurs. Comparing the results of the tests performed with and without pH control, it can be observed that the tests using the acetic acid solution commonly had slightly higher current values, and this is most likely a result of the additional ions introduced by the acetic acid electrolyte. When the H⁺ and CH₃COO⁻ ions were introduced, the H⁺ ions neutralized some of the OH⁻ generated at the cathode by the electrolysis reaction, while CH₃COO⁻ electromigrated toward the anode, consequently increasing the current. This result was in agreement with Zhang et al. (2014) who reported that acidic solution could enhance the desorption of ions from the surface of the soil into pore water, and higher concentrations of ions in pore water increased the electric current under a constant voltage condition.

FIG. 2.

Time course of (a) electrical current and (b) accumulated electroosmotic flow (direction: anode to cathode).

Electroosmotic flow

Figure 2b shows the changes of cumulative EOF with repair time. The EOF toward the cathode was observed without pH control in the cathode. The EOF in the tests increased slowly after a week of EK processing. In addition, the EOF rate in the T3 and T4 system is higher than that in T2. This is ascribed to Triton X-100, a nonionic surfactant, but SDBS and DOSL are anionic ones. In this case, SDBS and DOSL can form a complex with cations, which makes the electrolyte concentration in the anionic surfactant system lower than that in the Triton X-100 system (Miller, 1995). The EOF in T6 stopped and then a reverse EOF toward the anode was detected. The reverse EOF was due to the acidification of the soil below the point of zero charge, the change a sign of the zeta potential from negative to positive (Cameselle, 2015).

pH profiles

Figure 3 shows the distribution of soil pH after the EK experiments. It can be found that the soil pH showed obvious changes before and after the treatments. Due to water electrolysis at electrodes, the soil pH close to the cathode was higher than that near the anode. In the higher pH, metal hydroxides will precipitate in soil close to the cathode, blocking the soil pore, while a low pH value decreases the soil zeta potential, leading to decreased electroosmotic permeability and water flow rate, and thus limiting the migration of contaminants. It is important to keep the pH value suitable and stable in soil in removal of HMs and organics (Jian et al., 2010).

FIG. 3.

Change of soil pH in different sections after treatment.

From T2 to T5, the soil pH decreased to a lesser extent compared to that in T1, suggesting a successful pH control of the electrolytes with adding surfactant. And the pH of the soil samples with both surfactant and acetic acid (T6) after the application of EK ranged from 2.07 to 6.38. Acid soil has high buffering capacity for OH⁻, which can prevent it migrating to the soil column from the electrode chamber.

Removal of Cr and PHE from soil

The Cr and PHE concentrations in the soil sections after treatments are shown in Table 4. After enhanced treatments, the total PHE concentrations were lower than the initial PHE concentration (489 mg/kg). Figure 5 shows the normalized PHE concentration in the soil after the treatment. The removal percent of PHE in T1 was lower than those in other treatments, which indicated that surfactants were favorable to the removal of PHE. In T4, the removal percent of PHE was 44% and the PHE concentrations in soil sections increased from the anode to the cathode by adding DOSL in the electrolyte, which was attributed to the fact that DOSL can increase the molar solubilization ratio (MSR) of PHE, which can greatly increase the solubility of PHE (Deshpande et al., 2000). In all the experiments, the highest removal percent of soil PHE was 56% in T6, especially reached 81% and 74% in S1 and S2, respectively, and was higher than those in other soil sections. The reason for this result can be explained as follows. (1) The acid condition of the soil, Ammami et al.(2015) who reported that the more acidic pH of the treated sediment was favorable to the formation of the protonated neutral form of the used surfactant. In that case, the surfactant can be more prone to transport PAHs, leading to a better removal. (2) Oxidative degradation, Alcántara et al. (2009) found that the anodic oxidation of PAH mixtures occurs heterogeneously on each PAH.

FIG. 5.

Distribution of PHE concentration after electrokinetic remediation.

Table 4.

Cr and Phenanthrene Removals (%) After Electrokinetic Treatment

			Soil section(from anode to cathode)
Pollutants	Tests	Initial (mg/kg)	S1	S2	S3	S4	S5	Removal percent(%)
Cr(T)	T1	512	748	410	222	167	140	31
	T2	512	689	384	171	120	102	40
	T3	512	717	400	194	148	125	35
	T4	512	542	332	174	137	92	48
	T5	512	379	291	143	82	61	61
	T6	512	317	167	128	62	50	70
Cr⁶⁺	T1	482	355	273	198	156	122	54
	T2	482	265	209	167	126	80	65
	T3	482	296	235	187	128	100	61
PHE	T4	482	219	186	150	67	72	71
	T5	482	145	80	61	59	45	84
	T6	482	136	69	48	45	30	87
	T1	489	305	342	426	477	592	12
	T2	489	294	309	407	462	587	16
	T3	489	242	287	368	396	499	27
	T4	489	143	188	280	324	428	44
	T5	489	163	205	231	403	492	39
	T6	489	95	127	178	178	294	56

For the removal of soil Cr, there were different removal efficiencies among the different treatments. Figure 4 shows the distribution of Cr⁶⁺ and Cr(T) in the soil sections after the EK remediation. Due to Cr₂O₇²⁻ migration to the anode from the contaminated soil at the force of EK flux due to electromigration and electroosmosis and the oxidation of Cr³⁺ to Cr⁶⁺, concentration of Cr⁶⁺ in the soil near the anode region (sections I and II) was relatively high, while near the cathode it was low. A similar result that Cr⁶⁺ accumulated at the anode was reported previously (Reddy et al., 1997; Shen et al., 2007). In T1, the removal percent of Cr(T) was only 31%. The removal percent of Cr(T) and Cr⁶⁺ in T2 (40%, 65%), T3 (35%, 61%), and T4 (48%, 71%) was higher than those in T1 (31%, 54%), which was attributed to surfactant-associated complexation (Ochoa-Loza et al., 2001) and ionic exchange (Swarnkar et al., 2012). The removal percents of soil Cr(T) and Cr⁶⁺ in T5 were 61% and 84%, respectively. The removal efficiency was higher than the other treatments; this is because DPC has a CO-NH group, which binds to the chromate anions, and then Cr⁶⁺ exists as soluble anions in the pore water and thus easily removed by EKR. According to the above results, the control of catholyte pH and addition of surfactant DOSL with DPC are the best treatment for the simultaneous removal of PHE and Cr.

FIG. 4.

Distribution of chromium concentration after electrokinetic remediation.

From Table 4, we can see that a lot of Cr³⁺ was precipitated and accumulated in S1 and S2. The results were different from Al-Hamdan et al. (2008) who reported that Cr³⁺ migrated toward the cathode and accumulated either as precipitates or adsorbates at the sections close to the cathode where the high pH conditions exist. This phenomenon may be caused by the following two reasons: (1) Hamada et al. (2003) who found that Cr³⁺ can also form Cr(citrate)⁰ or Cr(citrate)₂³⁻, and then migrated to the anode and (2) the reduction reaction between Cr⁶⁺and PHE.

Fractionation changes

As shown in Fig. 6, a five-step sequential extraction was conducted to analyze residue Cr speciation in soil samples. Metal fractionations in the soil provide important information related to the EK phenomena. The percents of initial F1:exchangeable (EXCH); F2: carbonate-bound (CAR); F3: Fe-Mn oxide-bound (FeMnOX); F4: organic matter-bound (OMB); and F5: the residual (RES) of Cr were 40.7%, 16.6%, 23.4%, 10.1%, and 9.2%, respectively. After the EK process, the soil Cr moved from cathode to anode and its concentration increased from cathode to anode. Soil Cr in T1, 2, 3, and 4 moved to the anode and accumulated in S1. In all the tests, the “EXCH” Cr fraction was decreased from S1 to S5, mainly removed with a maximum removal more than 90% in S5. Other fractions, such as FeMnOX, OMB, and RES, were hardly removed. However, the removal of each fractionation was remarkable in T5 and T6. This is due to most of the Cr being moved out of the soil. In all tests, the FeMnOX, OMB, and RES had no obvious changes, whereas an apparent reduction in EXCH that was likely converted to CAR. Two possible sources of carbonate may come from (1) bicarbonate may be transformed to carbonate (HCO₃⁻+ OH⁻→CO₃²⁻) at the cathode and (2) the oxidation of surfactants by Cr⁶⁺.

FIG. 6.

Fractionation change of Cr after EK experiments (F1:exchangeable; F2: Fe-Mn oxide-bound; F3:carbonate-bound; F4: the residual; and F5: organic matter-bound).

This is because Cr³⁺ in the soil can exist as Cr(citrate)⁰ or Cr(citrate)₂³⁻ and migrate to the anode (Hamada et al., 2003). The percents of different Cr(T) fractionations in S1 were all higher than those in initial soil and S1–S3. In T5, the percents of different Cr(T) fractionations were all lower than those in initial soil. This is because most of the Cr migrate out of the soil column chamber. The soil pH values in different soil sections were lower in T6, so the percent of different Cr(T) fractionations decreased significantly. From Fig. 6 we can see that the residual fractionation (F5) of Cr(T) was increased. This is because during the EK process, a small amount of Cr translated into residual fraction by precipitation, and at the same time, the mass of Cr(T) was decreased resulting in the proportion of residual fractionation (F5) increase.

Removal mechanism of Cr and PHE during EK remediation

To our knowledge, this is the first study in discussing the effect of different oxidants and pH control on the removal percent of HMs and organic pollutants. By adding appropriate surfactants and controlling electrolyte pH together, the HM–organic pollutant cross-contaminated soil was successfully decontaminated, and the highest removal percent of soil Cr⁶⁺ and PHE was 87% and 54%, respectively (from Table 4). The removal mechanisms were complicated and relevant with multiple factors, such as soil pH, the solubility of surfactants, the migration of pollutants, and the interference between Cr and PHE.

First, during EK remediation, soil pH is the key factor in affecting the migration of HMs. In the higher pH, metal hydroxides will precipitate in soil close to the cathode, blocking the soil pore, while a low pH value decreases the soil zeta potential, leading to decreased electroosmotic permeability and water flow rate, and thus limiting the migration of contaminants. It is important to keep pH value suitable and stable in soil in removal of HMs and organic pollutants. Li et al. (2012) achieved 36% removal of Cr(T) and 92.5% removal of Cr⁶⁺ by conditioning the pH of catholyte during EK remediation of a Cr-contaminated soil. In our study, the soil pH values ranged from 2.6 to 8.9 in T5 and as conditioning catholyte pH was kept at 4, the soil pH values ranged from 2.1 to 6.4 in T6. After EK remediation, the Cr(T) and Cr⁶⁺ removal percents were 61%, 84% and 70%, 87% in T5 and T6, respectively. On the contrary, soil pH affected the removal of organic pollutants. This can be attributed to acidic pH conditions that can enhance the solubility of PHE from soil to pore water, which can significantly accelerate the migration of PHE. Therefore, the acid environment forces the pollutants to remain in the interstitial solution, and then, they are transported toward the electrode chamber fostered by the action of the electric field.

Second, the migration of pollutants is very important for the removal of pollutants from soil. Most of HMs are usually positive/negative charged and easily moved to cathode/anode by electromigration under DC electric field. However, the migration of organic pollutants was affected by many factors, such as the kind of cosolvents, EOF, and the kind of surfactants during EK remediation (Probstein and Hicks, 1993; Maturi and Reddy, 2008). From Fig. 2b, the amount of EOF in T3, 4, 5, and 6 was higher than that in T1 and T2, which was in favor of the migration of PHE in soil.

Third, the solubility of surfactants is one of the important factors for the removal of organic pollutants. Among the three surfactants in this study, the anionic surfactant system performed the greater removal efficiency of PHE than the nonionic surfactant system. One removal mechanism is that the anionic surfactant, with a relatively high EOF rate (shown in Fig. 2b), can easily remove PHE. The other can be attributed to the relatively high PAH solubilization ability of anionic surfactant in the EK system (Chang et al., 2009).

The last factor affecting the removal efficiency of combined pollutants could be the interaction among the pollutants such as the oxidation/reduction reaction, which was generally neglected by previous studies about EK remediation of HM–organic cross-contaminated soil. Lu et al. (2012) reported that the high Cr concentration at anode region might relate to the adsorption of Cr³⁺ to Cr⁶⁺ and the potential oxidation reaction for Cr³⁺ is shown in Equation (1). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} 2{\rm Cr}^{\rm 3 +} + 3{\rm O}_2 + 6{\rm H}_2 {\rm O} + 6{\rm e}^{-} \rightarrow 2{\rm Cr}^{6 +} + 12{\rm OH}^{-} \tag{1} \end{align*} \end{document}

Previous studies had shown that under the action of microorganisms in the soil, PAHs can be used as a donor to catalyze the reduction of HMs, so as to achieve the co-detoxification of HMs and organic pollutants (Shen. et al., 1996). Song et al. (2009) found the feasibility of simultaneous Cr⁶⁺ reduction and phenol degradation using pure cultures of bacteria—Pseudomonas aeruginosa. The data in Table 4 also show that the amount of Cr⁶⁺ and PHE reduced, while the concentration of Cr³⁺ increased in the anode region, and the potential oxidation reaction for Cr³⁺ is shown in Equation (2). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} {\rm Cr}_2 {\rm O}_7{}^{2 -} + {\rm H}^{+} + {\rm C}_{14}{\rm H}_{10} \rightarrow {\rm Cr}^{3 +} + {\rm HCO}_3{}^{-} + {\rm H}_2{\rm O} \tag{2} \end{align*} \end{document}

This result is in agreement with that of an earlier study by Nkhalambayausi-Chirwa and Wang (2000). It is assumed that phenol was completely degraded to HCO₃⁻ and H₂O. Thus, the interaction among the pollutants was considered during the enhanced EK remediation of HM–organic compound contaminated soil.

Conclusions

Based on the experimental results of Cr and PHE removal from contaminated soil by the six EK experiments reported in this article, the main conclusions are as follows:

(1) Results showed that the electroremediation in this work was effective in simultaneous removal of PHE and Cr from polluted soil, but removal percents were higher when enhancing agents were used. The removal percents of soil PHE, Cr(T), and Cr⁶⁺ were 12–56%, 31–70%, and 54–87%, respectively.

(2) Results of these laboratory studies suggest that a surfactant system containing DOSL with solubilized DPC is the most effective surfactant system to remediate chromium-contaminated soil in EK treatment. However, to a certain extent, the removal efficiency of PHE is reduced.

(3) The greatest recovery of PHE (56%) and Cr⁶⁺ (87%) in batch tests was obtained in T-6. These results show that acidic environment can prevent the formation of HM precipitation and also promote the PHE desorption from the soil to the pore water.

This study developed a new EK remediation technology to decontaminate the HM–organic pollutant cross-contaminated soil by addition of different surfactants and controlling electrolyte pH. We have demonstrated that the enhanced EK remediation system in the simulated setup can significantly improve the removal efficiency of the pollutants. At the same time, while this research has focused on one specific class of soil compound pollution in the EK system with surfactants, the approach described can be applied to other systems as well such as ionic pollutants (phosphate and arsenate) combined with organic pollutants (PAHs and PCB).

Footnotes

Acknowledgments

This study was supported by grants from the National Special Project on Water Pollution Control and Management (no. 2012ZX07503-002). We are grateful to editors and reviewers for their helpful suggestions about this study.

Author Disclosure Statement

No competing financial interests exist.

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