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Keywords:

  • soil washing;
  • contaminated soil;
  • PAHs;
  • Brij35;
  • Triton X100

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. NOMENCLATURE
  8. LITERATURE CITED

Polycyclic aromatic hydrocarbons (PAHs) are potential carcinogenic and hazardous compounds having raised appreciable environmental concerns around the world in recent decades. This research investigates the effect of soil washing on removal of PAHs in contaminated soil. The study was conducted by collecting soil samples from a petrochemical complex in south of Iran. Testing was carried out at three temperatures of 20°C, 40°C, and 80°C and washing periods of 30 min and 60 min. Moreover, two different concentrations of Triton X-100 and Brij 35 surfactants were used throughout the experiments. The results of this research indicated that the maximum removal efficiency of PAHs was obtained using Brij 35 at concentration of 5 g/L, temperature of 80°C and washing duration of 60 min. The highest removal efficacies for anthracene, naphthalene, fluorene, and benzo(a)pyrene in coarse and fine fractions of the samples occurred at 76.24%, 86.32%, 78.54%, 85.81% and 57.50%, 63.39%, 60.87%, 79.94%, respectively. Moreover, the highest removals of total PAHs are 81.66% and 61.49%, accordingly. Furthermore, using Brij 35, benzo(a)pyrene presented maximum changes in of PAHs in the samples. It is due to the high solubilizing ability of Brij 35 for benzo(a)pyrene, its low adsorption onto soil, and also its lower CMC in comparison with the other surfactant. According to the results, the higher the surfactant concentration, washing time, and temperature are, the more removal efficiency will be. For the parameters tested, the rates of PAHs removals from the samples decreased in the order of surfactant concentration, temperature and washing duration, respectively. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 107–113, 2014


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. NOMENCLATURE
  8. LITERATURE CITED

Soil contamination is an increasing problem worldwide and in cases where a mixture of different contaminants is present, more advanced remediation methods are likely needed, resulting in higher remediation costs [1]. Organic micro-pollutants are widespread in natural environment, often related to human activities. Heavily contaminated soils are frequent around industrial plants using petroleum or coal. Among these compounds, polycyclic aromatic hydrocarbons (PAHs) are a class of special interest. They are highly toxic and carcinogenic substances, often produced by incomplete combustion of carbon compounds [2]. The mineralogical composition of the soil is an important factor for the mobility of metals, and possibly, for their bioavailability to plants [3]. Figure 1 presents a soil contamination scenario due to leaching of an above ground storage tank.

image

Figure 1. Floating of petroleum compounds in a soil media (Madadian, 2012). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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PAHs are a large group of over 200 different compounds containing two or more fused aromatic rings made of carbon and hydrogen atoms [4, 5]. At seriously contaminated sites, the contamination level of PAHs has been reported to be as high as 300 g/kg soil [6]. PAHs, containing two to eight rings, are identified as one of the persistent organic pollutants (POPs) in urban environments, and some of them are classified as priority pollutants by both the USEPA and the European Community. They are introduced into the environment mainly via incomplete combustion of organic matter in nature and anthropogenic processes [7].

From a remediation perspective, the simultaneous removal of POPs and heavy metals (HMs) is difficult due to differences in their physical–chemical properties [8]. Retention of organic contaminants on coarser soil fractions and aquifer material after soil washing or flushing may be influenced by several factors other than particle surface area, including the hydrophobicity of the contaminant, the properties of the washing medium, and the characteristics of the soil particles [9].

The first step in the design of a full-scale washing treatment is a viability analysis, which involves several laboratory and analytical determinations to examine the main characteristics of the soil [10, 11]. In the second step, experiments on a pilot-scale can be performed in similar equipment to full-scale ones [12].

Currently, various soil treatment technologies, including excavation and landfilling, isolation, containment, electrokinetics (EK), biological treatment, soil washing, and soil flushing have been applied to remediate HM contaminated sites. Soil washing is an ex situ process based on the idea of water rinsing to remove contaminants from soil and to transfer them to a concentrated liquid phase. The process for organic and/or inorganic contaminant extraction from soil can be done in one of the two following ways: by transferring them to the washing solution and/or by concentrating them in a smaller volume of solids by particle or gravimetric separation, flotation and/or attrition. Soil washing is a chemical process often associated with mineralogical processes which can be used for soils, sediments, residues, and sludge [13, 14]. A schematic process of soil washing is illustrated in Figure 2.

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Figure 2. Schematic process of soil washing (Madadian, 2012). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Soil washing refers to ex situ techniques that employ physical and/or chemical processes. This method is an innovative treatment technology that uses liquids and a mechanical process to scrub soils, removing hazardous contaminants, and concentrating the contaminants into a smaller volume [15, 16]. Soil washing is to transfer contaminants from the soil solid phase to the aqueous phase by dissolving or suspending them with some chelating agents or acid solutions, or to concentrate them into a small volume of soil via separating them from sand fractions [17, 18]. Surfactant enhanced ex situ soil washing can offer the convenience, efficiency and economy desirable for innovative and alternative soil washing technologies [19].

In this study, to expedite PAHs removal rate and increase the efficiency, two nonionic detergents were used as surfactants at 2.5 g and 5 g concentration in 1 L of water. In colloidal and surface chemistry, the critical micelle concentration (CMC) is defined as the concentration of surfactants above which micelles form and all additional surfactants added to the system go to micelles [20]. The CMC is an important characteristic of a surfactant. Before reaching the CMC, the surface tension changes strongly with the concentration of the surfactant. After reaching the CMC, the surface tension remains relatively constant or changes with a lower slope [21].

One approach that has been considered for enhancing PAH bioremediation in contaminated soils is the application of nonionic surfactants [22]. The theoretical justification for this solution is based upon two hypotheses, first that surfactant micelles may sequester PAHs which are sorbed to the soil matrix, and second that the surfactant micelles may increase the concentration of PAHs in the aqueous phase because the PAHs are more soluble in the micelles. Where the rate of PAH degradation is limited by mass transfer from the solid phase to the aqueous phase, the PAH oxidation rates by microorganisms may then be enhanced [23, 24].

The objective of this research was to investigate the effect of the mentioned washing reagents on treatment of PAHs contaminated soils. Moreover, the effect of temperature of the solution and washing time were investigated to evaluate the efficiency of the soil washing technique in remediation of the PAHs contaminated soil.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. NOMENCLATURE
  8. LITERATURE CITED

Initial Characterizations of the Soil Samples

The research was carried out by collecting soil samples from two different locations of a petrochemical complex in Asalooyeh, south of Iran. The soil was sandy loam comprising of 23.26% clay and silt, 66.25% sand, and 10.49% gravel. The composition of the soil was determined using sieve analysis. These places were suspected contamination due to olfactory and visual signs which were quiet evident. Sampling method was performed using the manner described in chapter four of test methods for evaluating solid waste, physical/chemical methods [25] (Figure 3). The samples were homogenized according to ASTM standard D422-63. The initial characterizations of soil samples indicated the pH of 6.69 [26], and the water content of 5.7% [27]. Initially, the soil was screened using sieve #18 and particles larger than 1 cm were removed from the soil. Following preparation the soil samples, they transferred into the glass jars. The soils inside the jars were then homogenized by shaking the jars for 10 min. The jars were then capped and placed in a refrigerator at 4°C for further tests.

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Figure 3. Sampling procedure from contaminated site in a Petrochemical complex in Asalooyeh. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Surfactant Preparation

In this research, two washing agents called Triton X-100 (Sigma) and Brij 35 (Merck) were used as nonionic surfactant with two different concentrations of 2.5 g/L and 5 g/L for each to wash the samples. To achieve these concentrations, the pertinent amounts of the surfactants were dissolved in 1 L of water. The mixture of the each surfactant and water was placed in a glass container. The container was then capped and agitated for 10 min for homogeneity of the washing reagent.

Washing of Soil Samples

The amount of soil used for each experiment was 100 g. The soil was transferred to a 1000 mL glass container. Next 600 mL of the prepared solution added to the container and placed on a jar test apparatus (Figure 4). The experiments were conducted under three different temperatures of 20°C (laboratory temperature), 40°C, and 80°C each at washing sequences of 30 min and 60 min. In these washing times, the samples were agitated in rotation speed of 250 rpm. In addition, two different concentrations of prepared solutions applied in each run of the tests in order to observe the effect of surfactant concentration on treatment the samples. Each of the samples was tested under different temperature and washing sequences and solution concentrations. Replicate samples were tested in this regard. Following washing procedure, the soil samples were passed through sieve #200 and particles were divided into coarse and fine soil fractions. The samples were then allowed to dry for 24 h in order to be prepared for extraction process.

image

Figure 4. Jar test apparatus. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Extraction Method

The extraction of anthracene, naphthalene, fluorene, and benzo(a)pyrene from soil was conducted according to USEPA SW846 Method 5035. In this regard, specific weight of soil (5 g) was placed in a Soxhlet apparatus using a thimble. The extraction was carried out using 200 mL mixture of Hexane-Dichloromethane (1:1) for 8 h. The extracted liquid was then concentrated using a rotary evaporator in temperature of 30°C and rotation speed of 90 rpm. At the end of the experiments, the samples were analyzed for PAHs according to U.S.EPA method SW846 using a Gas Chromatography (GC) apparatus. A total of 60 samples were analyzed on a GC-14B, SHIMADZU with a flame-ionization detector using a DB-1, fused silica capillary column. It should be noted that through the tests, lots of waste water has been generated which were transferred in treatment plant to be analyses for further uses in soil washing system.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. NOMENCLATURE
  8. LITERATURE CITED

The characteristics of the soil used in this research have been presents in Table 1. In this research two nonionic surfactants were used due to their higher solubilization capacities and due to their lower cost if compared with cationic and anionic ones [28].

Table 1. Physicochemical properties of soil
PropertiesDescription
Soil textureSandy loam
Moisture content5%
pH6.69
Organic content1.024 (µS)

primary analysis of the samples indicated that the initial concentrations of anthracene, naphthalene, fluorene, and benzo(a)pyrene for coarse and fine fractions of the soil are 31.23 mg/L, 63.45 mg/L, 48.23 mg/L, 5.92 mg/L and 52.44 mg/L, 70.12 mg/L, 64.57 mg/L, 6.18 mg/L, respectively. Moreover, the results indicated that the highest removal efficiencies of 76.24%, 86.32%, 78.54%, and 85.81% were achieved for anthracene, naphthalene, fluorene, and benzo(a)pyrene. These optimum amounts were attained using Brij 35 with concentration of 5 g/L and at temperature of 80°C and washing time of 60 min. By applying the considered parameters, secondary concentrations of contaminants were changed which are highlighted as follow.

The Effect of Different Parameters on PAHs Removal

Surfactant concentration

The results indicated that using a Brij 35 with concentration of 5 g/L has the greatest PAHs removal effectiveness. In this regard, the highest removal efficacies of anthracene, naphthalene, fluorene, and benzo(a)pyrene for coarse and fine fractions of the soil are 76.24%, 86.32%, 78.54%, 85.81% and 57.50%, 63.39%, 60.87%, 79.94%, respectively. Moreover, the highest removals of total PAHs are 81.66% and 61.49%, accordingly. It should be noted that these results were obtained in optimum condition of temperature of 80°C and washing time of 60 min along with using Brij 35 (concentration of 5 g/L) as surfactant. On the other hand, the lowest removals occurred when the Triton X-100 was used with concentration of 2.5 g/L. In this case, the temperature and washing time were constant and equal to 20°C and 30 min, respectively. The effects of the surfactant concentration on the removal levels were tested to improve the desorption process [28].

Washing Time

To assess the effect of duration of washing on the treatment of PAHs, two washing times of 30 min and 60 min were tested. The results indicated that the maximum treatment will be occurred in 60 min washing time. In comparison with washing time of 30 min, the 60 min duration results in more removal efficiencies which are the same as what indicated in surfactant concentration section.

Temperature

The experiments were conducted under three temperatures of 20°C, 40°C, and 80°C. The average removal values of PAHs at 20°C and 40°C and 80°C are depicted in Figure 5. It should be noted that by raising the washing solution's temperature from 20°C to 40°C and 80°C, the average PAHs removal efficiencies were increased sharply.

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Figure 5. Removal efficacies of the PAHs using various solutions for (a) coarse fractions, and (b) fine fractions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Following the aforementioned results, it could be concluded that in the optimum conditions of temperature of 80°C and washing time of 60 min, the highest efficiencies were achieved using Brij 35 as surfactant with concentration of 5 g/L (Figures 5a and 5b). Table 2 indicates the secondary concentrations of the pollutants after soil washing process using different agents.

Table 2. Secondary concentrations of PAHs after soil washing process (in optimum condition)
ContaminantPrimary concentration (mg/L)Secondary concentration (mg/L)Description
Distilled waterTriton X-100 (2.5 g/L)Triton X-100 (5 g/L)Brij 35 (2.5 g/L)Brij 35 (5 g/L)
Anthracene31.2315.179.648.708.957.42Coarse fraction
Naphtalene63.4528.9218.7916.3413.108.68
Fluorene48.2325.4815.0813.2412.9010.35
Benzo(a)pyrene5.924.631.230.941.180.84
Total PAHs148.8374.2644.7539.2336.1627.29
Anthracene52.4431.8526.1325.5824.5222.10Fine fraction
Naphtalene70.1239.7231.3629.2329.2025.63
Fluorene64.5743.0833.4232.7030.9225.04
Benzo(a)pyrene6.184.922.861.982.101.24
Total PAHs193.31119.5693.7884.4986.7574.01

On the other hand, the lowest removal effectiveness occurred in temperature of 20°C and washing time of 30 min. Figures 6a and 6b demonstrates the elimination efficiencies of four PAHs in soil. The secondary concentrations of the PAHs are also presents in Table 3.

Table 3. Secondary concentrations of PAHs after soil washing process (in the worst condition)
ContaminantPrimary concentration (mg/L)Secondary concentration (mg/L)Description
Distilled waterTriton X-100 (2.5 g/L)Triton X-100 (5 g/L)Brij 35 (2.5 g/L)Brij 35 (5 g/L)
Anthracene31.2320.0917.0116.8116.4115.55Coarse fraction
Naphtalene63.4538.7829.2426.3225.1722.00
Fluorene48.2332.7524.0423.1324.1122.61
Benzo(a)pyrene5.925.242.832.112.181.87
Total PAHs148.8396.8673.1268.3767.8762.03
Anthracene52.4433.9931.8731.3729.6729.25Fine fraction
Naphtalene70.1248.8944.1441.7041.1541.13
Fluorene64.5746.1243.1440.9939.3637.88
Benzo(a)pyrene6.185.183.202.782.952.04
Total PAHs193.31134.17122.35116.84113.13110.30
image

Figure 6. The lowest removal efficacies of the PAHs using various solutions for (a) Coarse fractions, and (b) Fine fractions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. NOMENCLATURE
  8. LITERATURE CITED

The results of this research show that soil washing is an appropriate technique for coarse grained soils. As soil is a cover to protect groundwater from any possible pollution, this method could help to improve the capability of the soil. In addition, there are lots of animal and plant species which are endangered by presence of contaminants like PAHs. Therefore, soil washing play a significant role in remediation of polluted soil to protect the environment by conserving natural resources of soil. The soil used in the experiments was a sandy loam which was appropriate for soil washing. Also the results indicated that the higher the temperatures of the washing solution, the more PAHs removal efficiencies were accomplished. Increasing the temperature from 20°C to 40°C and 80°C accelerates the molecular movement of the solution and consequently enhances the probability of collision between washing solution and PAHs, hence leading to the highest increase of 81.66% in removal efficiency for the total PAHs. Moreover, analysis of the samples also showed that the optimum PAHs removal in washing tests was accomplished by Brij 35 surfactant at concentration of 5 g/L and under washing time of 60 min and the temperature of 80°C, thus denoting the efficacy of soil washing technique in remediation of PAHs contaminated soil. In optimal conditions, the reduction of benzo(a)pyrene was more than the other PAHs. It could be due to the high solubilizing ability of Brij 35 for benzo(a)pyrene, its low adsorption onto soil, and also its lower CMC in comparison with the other surfactant.

NOMENCLATURE

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. NOMENCLATURE
  8. LITERATURE CITED
CMC

critical micelle concentration

EK

electrokinetics

HM

heavy metal

PAH

polycyclic Aromatic Hydrocarbon

POP

persistent organic pollutant

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. NOMENCLATURE
  8. LITERATURE CITED
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