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Remediation of soils combining soil vapor extraction and bioremediation: Benzene
Chemosphere 80 (2010) 823–828
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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Remediation of soils combining soil vapor extraction and bioremediation: Benzene António Alves Soares a, José Tomás Albergaria a, Valentina Fernandes Domingues a, Maria da Conceição M. Alvim-Ferraz b, Cristina Delerue-Matos a,* a b
Requimte, Instituto Superior de Engenharia do Porto, Rua S. Tomé, 4200-072 Porto, Portugal LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia da Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal
a r t i c l e
i n f o
Article history: Received 4 February 2010 Received in revised form 8 June 2010 Accepted 9 June 2010
Keywords: Benzene Bioremediation Soil contamination Soil vapor extraction
a b s t r a c t This work reports the study of the combination of soil vapor extraction (SVE) with bioremediation (BR) to remediate soils contaminated with benzene. Soils contaminated with benzene with different water and natural organic matter contents were studied. The main goals were: (i) evaluate the performance of SVE regarding the remediation time and the process efficiency; (ii) study the combination of both technologies in order to identify the best option capable to achieve the legal clean up goals; and (iii) evaluate the influence of soil water content (SWC) and natural organic matter (NOM) on SVE and BR. The remediation experiments performed in soils contaminated with benzene allowed concluding that: (i) SVE presented (a) efficiencies above 92% for sandy soils and above 78% for humic soils; (b) and remediation times from 2 to 45 h, depending on the soil; (ii) BR showed to be an efficient technology to complement SVE; (iii) (a) SWC showed minimum impact on SVE when high airflow rates were used and led to higher remediation times for lower flow rates; (b) NOM as source of microorganisms and nutrients enhanced BR but hindered the SVE due the limitation on the mass transfer of benzene from the soil to the gas phase. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Soil contamination is closely related to industrial activities, disposal of municipal and industrial wastes or environmental accidents. A small amount of an organic compound released in the soil is enough to contaminate large volumes of soil and groundwater, in many cases exceeding the limits of contamination defined by law; the situation is much more serious when the pollutants are toxic or even carcinogenic. One of the most popular soil contaminant is benzene that is mainly used as an intermediate product to produce styrene, phenol and cyclohexane. The Annual Status Report from Treatment Technologies for Site Cleanup (USEPA, 2007) concluded that benzene, as non chlorinated volatile organic compound and integrated in the group of contaminants that include Benzene, Toluene, Ethylbenzene and Xylene (BTEX), was found in 24% of the documented remediation projects. For each specific case of soil contamination, it is essential to select the most appropriate remediation technology as well as to preview the remediation time and efficiency (Albergaria et al., 2006). There are two general approaches in remediating soils: an engineering approach and an ecological approach (Logan, 1992). The engineer* Corresponding author. Address: Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugal. Tel.: +351 22 508 1688; fax: +351 22 508 1449. E-mail address: [email protected] (C. Delerue-Matos). 0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2010.06.036
ing approach relies exclusively on external technologies for soil remediation, while ecological remediation involves the manipulation of inherent soil processes to mobilize, immobilize, transform or degrade contaminants (Haimi, 2000). One of the most widely used in situ technologies is Soil Vapor Extraction (SVE) that is extremely efficient for the remediation of soils contaminated with volatile and/or semi volatile organic compounds located in the unsaturated zone of the soil. This technique consists of the application of vacuum to the soil matrix, producing an airflow in the soil that transports the contaminants to extraction wells (Grasso, 1993; Khan et al., 2004). As the air moves through the soil, mass transfer of contaminant occurs into the mobile gas phase, as a consequence of free pollutant volatilization, desorption from the soil, and dissolution from the aqueous phase. Field studies have demonstrated that SVE can remove more than 90% of contaminants (Poulsen et al., 1999; Albergaria et al., 2006). However the success of a SVE project depends on several parameters including contaminant characteristics such as vapor pressure and solubility, soil properties such as natural porosity, permeability, natural organic matter (NOM) or soil water content (SWC) and operational conditions like the temperature or rate of the airflow. SWC is one of the parameters that more strongly affects the remediation time due to its influence on contaminant availability and on soil permeability, which is one of the most important factors in the volatile organic compound (VOC) migration into the soil (Harper et al., 1998).
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Increasing the SWC the soil porosity decreases, rendering the movement of the air into the soil and influencing negatively the remediation process (Poulsen et al., 1999). An increase of SWC also decreases the mass transfer coefficient between the non-aqueous liquid phase and the gas phase, thereby reducing the interfacial area between those two phases (Harper et al., 1998; Yoon et al., 2002). Albergaria et al. (2006) concluded that in soils contaminated with cyclohexane, the increase of the SWC from 0% to 6% tripled the remediation time and decreased the process efficiency (99– 97%). NOM is also extremely important on the SVE process because of its influence on the mobility and availability of the contaminant in the soil. If organic matter is present in the soil, even in small amounts, it will dominate the sorption process because it is responsible for most of the sorption capacity of VOC in soils (Grasso, 1993). After the release in the soil the contaminant is initially adsorbed to the soil macropores followed by penetration into the smaller pores, which hinders desorption during the remediation process (Sun et al., 2003). Alvim-Ferraz et al. (2006b) performed SVE in soils with different NOM (0% and 7.5%) contaminated with cyclohexane and demonstrated that the NOM is directly proportional to the remediation time (increased from 1.8 to 13 h) and inversely proportional to the efficiency of the remediation process (decreased from 99% to 90%). Concluding, SWC and NOMs of the soil influence negatively the remediation process (higher influence of NOM), turning it less efficient and more time consuming, and consequently more expensive (Alvim-Ferraz et al., 2006a). Bioremediation (BR) is a common remediation technology used, according to ‘‘The Annual Status Report from Treatment Technologies for Site Cleanup”, in 12% of the documented remediation projects. As hydrocarbons are prone to biodegradation by the naturally occurring microorganisms (Leahy and Colwell, 1990), BR has been proposed as a method for the treatment and disposal of oily materials (Salanitro et al., 1997; Head and Swannell, 1999). The biodegradation of hydrocarbons has been intensively studied in controlled conditions (Chaîneau et al., 2003; Gogoi et al., 2003). Thus, soil fauna can take part in the process, increasing overall, and especially microbial, metabolic activity in the soil. On the other hand they can be used as indicators of soil contamination, either to assess the toxicity and risk of the contaminated soil or to evaluate the efficacy of the remediation process, that is, to evaluate the toxicity and risk on the end product (Haimi, 2000). Microbial activity is affected by several physicochemical parameters. The factors that directly influence BR are energy sources, electron acceptors, nutrients, pH, temperature, NOM and SWC. NOM represents a storehouse of carbon and energy (Boopathy, 2000). It is also associated with high microbial numbers and a great diversity of microbial populations. As said before, NOM is responsible for the sorption of organic compounds to the soil matrix and represents a critical factor for the development of biodegradation processes (Scow et al., 1986; Thomas et al., 1986). SWC is a key factor for biodegradation activity. Studies made with total petroleum hydrocarbon showed that low water content soil had no significant biodegradation (Viñas et al., 2005). SVE complemented with BR is an attractive approach of cleaning up petroleum hydrocarbons because it contains the high extraction efficiencies of the early stage of SVE and the extremely low cost of the BR to complete the remediation process. This work reports the study of the combination of SVE with BR to remediate soils contaminated with benzene. Soils contaminated with benzene and with different SWC and NOM were studied. The main goals were: (i) evaluate the performance of SVE regarding the remediation time and the process efficiency; (ii) study the combination of both technologies in order to identify the best option capable to achieve the legal clean up goals; and (iii) evaluate the influence of SWC and NOM on SVE and BR.
2. Experimental 2.1. Reagents Pro-analysis benzene was obtained from Riedel-de-Haën. Mineral medium was obtained from mineral salts medium (MinE) basis as described in Kelly et al. (1994). 2.2. Apparatus and chromatography The SVE process was monitored in an Ai Cambridge GC95 equipped with a flame ionization detector and a Chrompack Hayesep Q 80–100 mesh (3 m 6.35 mm 4 lm) column. The injectors and the detectors were set at 230 °C and the column worked isothermally at 200 °C. Flame gases were air at 270 cm3 min 1 and hydrogen at 30 cm3 min 1. Nitrogen at 30 cm3 min 1 was the carrier gas. Chromatographic data were recorded in Barspec Data System Software (Barspec System, Inc., Israel). The BR process was monitored in a Shimadzu GC-2010 equipped with a flame ionization detector and a TRB 35 NF-2670 (30 m 0.53 mm 3 lm) column. The injectors and the detectors were set at 250 °C and the column worked isothermally at 200 °C. Flame gases were air at 400 cm3 min 1 and hydrogen at 40 cm3 min 1. Helium at 30 cm3 min 1 was the carrier gas. Chromatographic data were recorded in GC Solution Analysis. The benzene quantification was performed by direct calibration method. 2.3. Soil preparation and characterization International standard methodologies were used for the characterization of the prepared and real soils, including the determination of apparent density (ASTM D4531-86), particle density (DIN 18124), pH (USEPA 9045 D), the contents of water (ASTM D 2216) and NOM (Wakley–Black method) (Albergaria et al., 2006). Porosity was calculated through soil apparent and particle densities. Table 1 shows the results of the characterization of the prepared soils. The presence of shell debris in the sandy soils (P0,0, P2,0, P3,0 and P4,0, the first and second subscripts indicating, respectively, the contents of NOM and water) is responsible to the relatively high values of pH observed. 2.4. Equilibrium isotherms The equilibrium isotherms were used to verify if the SVE process achieved the clean up goals. The isotherms related the contaminant concentration in the gas phase with the level of contamination. To obtain the equilibrium isotherms, experiments were performed in a stainless steel column with 37 cm height and 10 cm of internal diameter. The experiments involved four stages: (i) introduction of the soil in the column; (ii) soil contamination; (iii) equilibrium settling; and (iv) determination of the concentration of the contaminant in the gas phase through gas chromatographic analysis. In the first stage, a mass of soil was introduced in the column in fractions of 500 g. After each introduction, the soil was compacted using always the same procedure in order to guarantee similar soil porosity. The second stage consisted of the addiction of different amounts of chilled/frozen contaminant (25–400 mg) on the top of the column allowing the percolation and distribution of the contaminant into the soil matrix. The soil was then left isothermically at 296 K. To evaluate if the equilibrium was reached, the concentration of the contaminant in the gas phase (Cgas) was monitored in four different levels of the column along time. When the concentration was similar at different levels
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A.A. Soares et al. / Chemosphere 80 (2010) 823–828 Table 1 Characteristics of prepared soils (particle size < 2 mm). Soil
Apparent density (g cm
P0,0 P2,0 P3,0 P4,0 P1,4 P2,14 P4,24
1.5 1.3 1.3 1.2 1.1 1.0 0.9
3
)
Particle density (g cm 2.5 2.5 2.5 2.5 2.9 3.1 3.3
3
)
Porosity (%)
pH
Water content (%)
Natural organic matter content (%)
42 49 50 51 62 68 73
8.8 8.8 8.8 8.8 6.5 6.1 5.8
0.0 2.0 3.0 4.0 0.72 2.5 4.3
<0.02 <0.02 <0.02 <0.02 4.0 14 24
(less than 5% deviation), equilibrium was considered to have been reached, what happened within 24 h. 2.5. Soil vapor extraction experiments The SVE experiments were performed both in sandy and in humic soils. The sandy soil was collected at 3 m deep in different places of a beach and the humic soil was collected in a forest area at 1–2 cm depth, both in a region in Porto, Portugal. All samples were stored in adequate vessels. The preparation of the sandy soils involved four steps: (a) washing the sandy soil till clean water was obtained, (b) drying, first at room temperature for 5 d and then at 105 °C for 24 h, (c) sieving in a 2 mm sieve; and (d) adding water in order to induce the desired water contents (2%, 3% and 4%). The preparation of the humic soils involved three steps: (a) drying at 55 °C for 48 h; (b) sieving in a 2 mm sieve; and (c) mixing the correct amounts of sandy and humic soil to induce the desired organic matter contents (4%, 14% and 24%). SVE experiments were performed in the same columns used for the equilibrium isotherms and prepared in the same way except the amount of benzene added (always 1.0 g of benzene). After the establishment of the equilibrium, the column was connected to the laboratorial installation shown in Fig. 1. To start the SVE the vacuum pump (B) was switched on, allowing different flow rates (controlled by the flow meter R) to percolate through the column (C) and then to a sampling system (S) where the contaminated emissions were monitored by gas chromatography. An activated charcoal (A) recipient was placed before the pump to protect the pump and to avoid atmospheric contamination. The remediation process was considered finished when the concentration of the contaminant in the gas phase was below 1.0 g m 3. This time was considered the SVE remediation time and the remaining level of contamination and the process efficiency were calculated.
2.6. Bioremediation experiments The BR experiments were performed in the soils that after SVE presented contamination levels above legal limits. The preparation of these soils to the BR tests consists of the addition of water and substrate to the soil in order to induce the real and potential metabolic activity of indigenous microorganisms. The defined SWC 1 was 20% and the addition of MinE was 10 mL kgsoil . The soils that, after SVE, presented levels of contamination higher than the levels imposed by law (10 mg kg 1) were treated by BR. The BR experiments were performed after the addition of water and substrate to the soil column and consist of the monitoring of the concentration of benzene in the gas phase of the soil until the legal level of contamination was achieved. The time required to reach this level was considered the BR time. The sum of the SVE remediation time with the BR time was defined as global remediation time. 3. Results 3.1. Equilibrium isotherms Fig. 2 shows the equilibrium isotherms that relate the concentration of benzene in the gas phase and the level of contamination in the soil matrix for all the experimented soils. This equilibrium isotherm allowed, through the determination of the concentration of benzene in the gas phase, the evaluation of the level of contamination remaining in the soil after the SVE. For each equilibrium isotherm a trend line was calculated and used to quantify the remaining level of contamination. It is also indicated a line that represents the legal limit, 10 mg kg 1. The isotherms show that the SWC has no significant influence on the concentration of contaminant in the gas phase, what indicates that an increase of the SWC has no influence on the amount
Fig. 1. Laboratorial installation.
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Fig. 2. Equilibrium isotherms at 297 K for all the prepared soils.
of contaminant that is in the gas phase of the soil and that can be easily extracted. The opposite is observed with the organic matter where the increase of its content led to a major reduction of the concentration of benzene in the gas phase of the soil, hindering the SVE process. 3.2. Soil vapor extraction experiments The SVE experiments were monitored by gas chromatography until the concentration of benzene in the gas phase of the soil reached 1.0 g m 3. In the end of each SVE, the remediation time and the level of contamination remaining in soil were calculated. These results are presented in Table 2. The last column of the table indicates the level of achievement of the clean up goal for each SVE performed. If the remaining level of contamination after the SVE
was below the legal limit it was considered that the goal was achieved and the remediation complete. This situation was indicated in the Table with a ‘‘A”. If the level of contamination in the end was within 10 and 20 mg kg 1 the clean up goal was considered to be closely achieved (indicated with ‘‘C”). This means that there is still a small amount of benzene in the soil above the legal limit that can be achieved by the continuation of the SVE or the utilization of the BR. Attending to low quantity of benzene that needs to be removed from the soil, the utilization of SVE can be the best option. In cases where the final level of contamination was above 20 mg kg 1 (identified with an ‘‘X”) the remediation was considered incomplete and should be complemented with BR. The results presented in Table 2 allow concluding that the SWC has minimum impact on the SVE process when higher airflow rates are used; however, for lower airflow rates, the SWC creates a neg-
Table 2 Results obtained in the SVE experiments for all soils. 1
)
Remediation time (h)
Level of contamination in the end (mg kg
1
Soil
Flow rate (L h
Process efficiency (%)
Clean up goal achievement
P0,0
18 9.8 5.7 2.0
2.4 2.4 4.5 6.3
14 13 15 15
)
94 95 94 94
C C C C
P2,0
18 10 5.3 4.0
2.5 4.0 6.4 6.9
19 12 11 11
92 95 96 95
C C C C
P3,0
19 9.9 6.5 2.7
2.3 3.8 8.1 11.3
19 16 11 13
93 94 96 95
C C C C
P4,0
18 9.4 6.7 3.2
2.6 4.0 8.1 12.8
15 13 12 7
94 95 95 97
C C C A
P1,4
19 9.2 5.7 2.2
3.6 5.1 11.2 34.8
14 9 8 7
95 97 97 97
C A A A
P2,14
18 10 4.8 2.2
3.3 7.3 13.2 41.6
114 70 86 70
78 86 83 87
X X X X
P4,24
18 9.6 5.6 2.5
4.2 8.5 22.5 45.4
170 135 118 92
78 85 85 88
X X X X
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A.A. Soares et al. / Chemosphere 80 (2010) 823–828 Table 3 Remediation and bioremediation times for each experiment.
Fig. 3. Monitoring of the bioremediations performed in soils P2,14 (a) and P4,24 (b).
ative impact on the remediation process originating a considerable increase of the remediation time. The SVEs performed in soils with higher NOM had remediation times 75–620% higher and were less efficient. In some of these less efficient cases, BR was needed to achieve the legal limits. This proves the negative impact of NOM on this remediation process. 3.3. Bioremediation experiments The BR tests were performed in the soils that had remaining levels of contamination higher than 20 mg kg 1. Four bioremediation experiments were replicated once and, for these cases, a relative deviation lower than 8% was observed. The BR was considered fin-
Soil
Airflow rate in SVE (L h 1)
Contamination level (mg kg 1)
Bioremediation time (h)
Global remediation time (h)
P2,14
2.2 4.8 10 18
70 86 70 114
117 270 117 728
159 283 124 731
P4,24
2.5 5.6 9.6 18
92 118 135 170
238 250 305 646
283 273 314 650
ished when the concentration of benzene in the gas phase of the soil reached 0.6 g m 3. Fig. 3a shows the monitoring of the BR processes performed in the P2,14 soil with three levels of contamination (70, 86 and 114 mg kg 1). Another test was carried out in a sterilized soil to verify if significant adsorption phenomena occur in the BR process. The results of this test indicated that during the BR process, the decrease of the concentration of benzene in the gas phase is only due to the activity of the microorganisms, as it is shown in Fig. 4. Fig. 4 shows that in the sterile soil, the quantity of contaminant in the gas phase maintains approximately constant indicating that no significant adsorption on the microorganism occurred. In the other test, where the microorganisms were active, the concentration of benzene decrease indicating that it was being degraded by the microorganisms. Fig. 3b presents the monitoring of the BR performed in the P4,24 soil with four levels of contamination (92, 118, 135 and 170 mg kg 1). For this soil the BR was considered finished when Cgas reached 0.3 g m 3. From Fig. 3a and b the BR times were calculated. These results are presented in Table 3. The fluctuations observed in the concentration of benzene in the first 48–72 h are a consequence of the movement of benzene vapors in soil towards the establishment of the equilibrium. The results allow concluding that the BR times are directly proportional to the level of contamination and inversely proportional to the organic matter content. To evaluate the influence of organic matter content on the BR process, it was compared the tests performed in the soils P2,14 and P4,24 with similar levels of contamination (around 90 and 115 mg kg 1). It was observed that the BR times decreased between 10% and 65%, respectively for levels of contamination of 90 and 115 mg kg 1. This can be explained by the fact that soils with higher organic matter contents may be more populated by indigenous microorganisms that increase their capability to degrade contaminants (proven by the higher slope observed in the P4,24 curves in Fig. 3b, compared to those with similar contamination in Fig. 3a). Through this, it is observed that organic matter content has opposite effects on the remediation of soils through the combination of SVE and BR. Higher NOM hinder SVE but enhances BR what explains the inconsistent behavior observed in the global remediation time.
4. Conclusions
Fig. 4. Comparison between sterile and non-sterile tests.
The remediation experiments performed in soils contaminated with benzene allowed concluding that: (a) SWC showed minimum impact on the SVE process performed with high airflow rates; but for low airflow rates an increase on the remediation time was observed; (b) NOM showed an important and negative impact on the SVE process originating an increase of the remediation times between 75% and 620%; (c) the level of contamination remaining in soil after SVE also increased requiring BR to complete the cleaning
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process; (d) only the soils with higher organic matter content (P2,14 and P4,24) required BR; (e) BR times are directly proportional to the level of contamination and inversely proportional to NOM; (f) soils with higher amounts of NOM showed better degradative conditions performing the BR in less time; and (g) SVE combined with BR showed to be an efficient option to the remediation of soils contaminated with benzene. Acknowledgement The authors are grateful to Fundação para a Ciência e Tecnologia (Project PTDC/ECM/68056/2006) for the material support for this work. References Albergaria, J.T., Alvim-Ferraz, M.C.M., Delerue-Matos, C., 2006. Remediation efficiency of vapor extraction of sandy soils contaminated with cyclohexane: influence of air flow rate, water and natural organic matter content. Environ. Pollut. 143, 146–152. Alvim-Ferraz, M.C.M., Albergaria, J.T., Delerue-Matos, C., 2006a. Soil remediation time to achieve clean-up goals: I: influence of soil water content. Chemosphere 62, 853–860. Alvim-Ferraz, M.C.M., Albergaria, J.T., Delerue-Matos, C., 2006b. Soil remediation time to achieve clean-up goals: II: influence of natural organic matter and water contents. Chemosphere 64, 817–825. Boopathy, R., 2000. Factors limiting bioremediation technologies. Bioresour. Technol. 74, 63–67. Chaîneau, C.H., Yéprémian, C., Vidalie, J.F., Ducreux, J., Ballerini, D., 2003. Bioremediation of a crude oil-polluted soil: biodegradation, leaching and toxicity assessments. Water Air Soil Pollut. 144, 419–440. Gogoi, B.K., Dutta, N.N., Goswami, P., Krishna Mohan, T.R., 2003. A case study of bioremediation of petroleum-hydrocarbon contaminated soil at a crude oil spill site. Adv. Environ. Res. 7, 767–782. Grasso, D., 1993. Hazardous Waste Site Remediation, Source Control. Lewis Publisher, Boca Raton.
Haimi, J., 2000. Decomposer animals and bioremediation of soils. Environ. Pollut. 107, 233–238. Harper, B.M., Stiver, W.H., Zytner, R.G., 1998. Influence of water content on SVE in a silt loam soil. J. Environ. Eng. Sci. 124, 1047–1053. Head, I.M., Swannell, R.J.P., 1999. Bioremediation of petroleum hydrocarbon contaminants in marine habitats. Curr. Opin. Biotechnol. 10, 234–239. Kelly, D.P., Baker, S.C., Trickett, J., Davey, M., Murrell, J.C., 1994. Methanesulphonate utilization by a novel methylotrophic bacterium involves an unusual monooxygenase. Microbiology 140, 1419–1426. Khan, F.I., Husain, T., Hejazi, R., 2004. An overview and analysis of site remediation technologies. J. Environ. Manage. 71, 95–122. Leahy, G.J., Colwell, R.R., 1990. Microbial degradation of hydrocarbons in the environment. Microbiol. Mol. Biol. Rev. 54, 305–315. Logan, T., 1992. Reclamation of chemically degraded soils. Adv. Soil Sci. 17, 13–35. Poulsen, T.G., Moldrup, P., Yamaguchi, T., Schjonning, P., Hansen, J.A., 1999. Predicting soil-water and soil-air transport properties and their effects on soil-vapor extraction efficiency. Ground Water Monit. Remediat. 19, 61–70. Salanitro, J.P., Dorn, P.B., Huesemann, M.H., Moore, K.O., Rhodes, I.A., Jackson, L.M.R., Vipond, T.E., Western, M.M., Wisniewsky, H.L., 1997. Crude oil hydrocarbon bioremediation and soil ecotoxicity assessment. Environ. Sci. Technol. 31, 1769–1776. Scow, K.M., Simkins, S., Alexander, M., 1986. Kinetics of mineralization of organiccompounds at low concentrations in soil. Appl. Environ. Microbiol. 51, 1028– 1035. Sun, H., Tateda, M., Ike, M., Fujita, M., 2003. Short and long term sorption/desorption of polycyclic aromatic hydrocarbons onto artificial solids: effects of particle and pore sizes and organic matters. Water Res. 37, 2960–2968. Thomas, R.S., Dakessian, S., Ames, R.N., 1986. Aggregation of a silty clay loam soil by mycorrhizal onion roots. Soil Sci. Soc. Am. J. 50, 1494–1499. USEPA, 2007. Treatment Technologies For Site Cleanup: Annual Status Report, 12th ed.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Remediation of soils combining soil vapor extraction and bioremediation: Benzene António Alves Soares a, José Tomás Albergaria a, Valentina Fernandes Domingues a, Maria da Conceição M. Alvim-Ferraz b, Cristina Delerue-Matos a,* a b
Requimte, Instituto Superior de Engenharia do Porto, Rua S. Tomé, 4200-072 Porto, Portugal LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia da Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal
a r t i c l e
i n f o
Article history: Received 4 February 2010 Received in revised form 8 June 2010 Accepted 9 June 2010
Keywords: Benzene Bioremediation Soil contamination Soil vapor extraction
a b s t r a c t This work reports the study of the combination of soil vapor extraction (SVE) with bioremediation (BR) to remediate soils contaminated with benzene. Soils contaminated with benzene with different water and natural organic matter contents were studied. The main goals were: (i) evaluate the performance of SVE regarding the remediation time and the process efficiency; (ii) study the combination of both technologies in order to identify the best option capable to achieve the legal clean up goals; and (iii) evaluate the influence of soil water content (SWC) and natural organic matter (NOM) on SVE and BR. The remediation experiments performed in soils contaminated with benzene allowed concluding that: (i) SVE presented (a) efficiencies above 92% for sandy soils and above 78% for humic soils; (b) and remediation times from 2 to 45 h, depending on the soil; (ii) BR showed to be an efficient technology to complement SVE; (iii) (a) SWC showed minimum impact on SVE when high airflow rates were used and led to higher remediation times for lower flow rates; (b) NOM as source of microorganisms and nutrients enhanced BR but hindered the SVE due the limitation on the mass transfer of benzene from the soil to the gas phase. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Soil contamination is closely related to industrial activities, disposal of municipal and industrial wastes or environmental accidents. A small amount of an organic compound released in the soil is enough to contaminate large volumes of soil and groundwater, in many cases exceeding the limits of contamination defined by law; the situation is much more serious when the pollutants are toxic or even carcinogenic. One of the most popular soil contaminant is benzene that is mainly used as an intermediate product to produce styrene, phenol and cyclohexane. The Annual Status Report from Treatment Technologies for Site Cleanup (USEPA, 2007) concluded that benzene, as non chlorinated volatile organic compound and integrated in the group of contaminants that include Benzene, Toluene, Ethylbenzene and Xylene (BTEX), was found in 24% of the documented remediation projects. For each specific case of soil contamination, it is essential to select the most appropriate remediation technology as well as to preview the remediation time and efficiency (Albergaria et al., 2006). There are two general approaches in remediating soils: an engineering approach and an ecological approach (Logan, 1992). The engineer* Corresponding author. Address: Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugal. Tel.: +351 22 508 1688; fax: +351 22 508 1449. E-mail address: [email protected] (C. Delerue-Matos). 0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2010.06.036
ing approach relies exclusively on external technologies for soil remediation, while ecological remediation involves the manipulation of inherent soil processes to mobilize, immobilize, transform or degrade contaminants (Haimi, 2000). One of the most widely used in situ technologies is Soil Vapor Extraction (SVE) that is extremely efficient for the remediation of soils contaminated with volatile and/or semi volatile organic compounds located in the unsaturated zone of the soil. This technique consists of the application of vacuum to the soil matrix, producing an airflow in the soil that transports the contaminants to extraction wells (Grasso, 1993; Khan et al., 2004). As the air moves through the soil, mass transfer of contaminant occurs into the mobile gas phase, as a consequence of free pollutant volatilization, desorption from the soil, and dissolution from the aqueous phase. Field studies have demonstrated that SVE can remove more than 90% of contaminants (Poulsen et al., 1999; Albergaria et al., 2006). However the success of a SVE project depends on several parameters including contaminant characteristics such as vapor pressure and solubility, soil properties such as natural porosity, permeability, natural organic matter (NOM) or soil water content (SWC) and operational conditions like the temperature or rate of the airflow. SWC is one of the parameters that more strongly affects the remediation time due to its influence on contaminant availability and on soil permeability, which is one of the most important factors in the volatile organic compound (VOC) migration into the soil (Harper et al., 1998).
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Increasing the SWC the soil porosity decreases, rendering the movement of the air into the soil and influencing negatively the remediation process (Poulsen et al., 1999). An increase of SWC also decreases the mass transfer coefficient between the non-aqueous liquid phase and the gas phase, thereby reducing the interfacial area between those two phases (Harper et al., 1998; Yoon et al., 2002). Albergaria et al. (2006) concluded that in soils contaminated with cyclohexane, the increase of the SWC from 0% to 6% tripled the remediation time and decreased the process efficiency (99– 97%). NOM is also extremely important on the SVE process because of its influence on the mobility and availability of the contaminant in the soil. If organic matter is present in the soil, even in small amounts, it will dominate the sorption process because it is responsible for most of the sorption capacity of VOC in soils (Grasso, 1993). After the release in the soil the contaminant is initially adsorbed to the soil macropores followed by penetration into the smaller pores, which hinders desorption during the remediation process (Sun et al., 2003). Alvim-Ferraz et al. (2006b) performed SVE in soils with different NOM (0% and 7.5%) contaminated with cyclohexane and demonstrated that the NOM is directly proportional to the remediation time (increased from 1.8 to 13 h) and inversely proportional to the efficiency of the remediation process (decreased from 99% to 90%). Concluding, SWC and NOMs of the soil influence negatively the remediation process (higher influence of NOM), turning it less efficient and more time consuming, and consequently more expensive (Alvim-Ferraz et al., 2006a). Bioremediation (BR) is a common remediation technology used, according to ‘‘The Annual Status Report from Treatment Technologies for Site Cleanup”, in 12% of the documented remediation projects. As hydrocarbons are prone to biodegradation by the naturally occurring microorganisms (Leahy and Colwell, 1990), BR has been proposed as a method for the treatment and disposal of oily materials (Salanitro et al., 1997; Head and Swannell, 1999). The biodegradation of hydrocarbons has been intensively studied in controlled conditions (Chaîneau et al., 2003; Gogoi et al., 2003). Thus, soil fauna can take part in the process, increasing overall, and especially microbial, metabolic activity in the soil. On the other hand they can be used as indicators of soil contamination, either to assess the toxicity and risk of the contaminated soil or to evaluate the efficacy of the remediation process, that is, to evaluate the toxicity and risk on the end product (Haimi, 2000). Microbial activity is affected by several physicochemical parameters. The factors that directly influence BR are energy sources, electron acceptors, nutrients, pH, temperature, NOM and SWC. NOM represents a storehouse of carbon and energy (Boopathy, 2000). It is also associated with high microbial numbers and a great diversity of microbial populations. As said before, NOM is responsible for the sorption of organic compounds to the soil matrix and represents a critical factor for the development of biodegradation processes (Scow et al., 1986; Thomas et al., 1986). SWC is a key factor for biodegradation activity. Studies made with total petroleum hydrocarbon showed that low water content soil had no significant biodegradation (Viñas et al., 2005). SVE complemented with BR is an attractive approach of cleaning up petroleum hydrocarbons because it contains the high extraction efficiencies of the early stage of SVE and the extremely low cost of the BR to complete the remediation process. This work reports the study of the combination of SVE with BR to remediate soils contaminated with benzene. Soils contaminated with benzene and with different SWC and NOM were studied. The main goals were: (i) evaluate the performance of SVE regarding the remediation time and the process efficiency; (ii) study the combination of both technologies in order to identify the best option capable to achieve the legal clean up goals; and (iii) evaluate the influence of SWC and NOM on SVE and BR.
2. Experimental 2.1. Reagents Pro-analysis benzene was obtained from Riedel-de-Haën. Mineral medium was obtained from mineral salts medium (MinE) basis as described in Kelly et al. (1994). 2.2. Apparatus and chromatography The SVE process was monitored in an Ai Cambridge GC95 equipped with a flame ionization detector and a Chrompack Hayesep Q 80–100 mesh (3 m 6.35 mm 4 lm) column. The injectors and the detectors were set at 230 °C and the column worked isothermally at 200 °C. Flame gases were air at 270 cm3 min 1 and hydrogen at 30 cm3 min 1. Nitrogen at 30 cm3 min 1 was the carrier gas. Chromatographic data were recorded in Barspec Data System Software (Barspec System, Inc., Israel). The BR process was monitored in a Shimadzu GC-2010 equipped with a flame ionization detector and a TRB 35 NF-2670 (30 m 0.53 mm 3 lm) column. The injectors and the detectors were set at 250 °C and the column worked isothermally at 200 °C. Flame gases were air at 400 cm3 min 1 and hydrogen at 40 cm3 min 1. Helium at 30 cm3 min 1 was the carrier gas. Chromatographic data were recorded in GC Solution Analysis. The benzene quantification was performed by direct calibration method. 2.3. Soil preparation and characterization International standard methodologies were used for the characterization of the prepared and real soils, including the determination of apparent density (ASTM D4531-86), particle density (DIN 18124), pH (USEPA 9045 D), the contents of water (ASTM D 2216) and NOM (Wakley–Black method) (Albergaria et al., 2006). Porosity was calculated through soil apparent and particle densities. Table 1 shows the results of the characterization of the prepared soils. The presence of shell debris in the sandy soils (P0,0, P2,0, P3,0 and P4,0, the first and second subscripts indicating, respectively, the contents of NOM and water) is responsible to the relatively high values of pH observed. 2.4. Equilibrium isotherms The equilibrium isotherms were used to verify if the SVE process achieved the clean up goals. The isotherms related the contaminant concentration in the gas phase with the level of contamination. To obtain the equilibrium isotherms, experiments were performed in a stainless steel column with 37 cm height and 10 cm of internal diameter. The experiments involved four stages: (i) introduction of the soil in the column; (ii) soil contamination; (iii) equilibrium settling; and (iv) determination of the concentration of the contaminant in the gas phase through gas chromatographic analysis. In the first stage, a mass of soil was introduced in the column in fractions of 500 g. After each introduction, the soil was compacted using always the same procedure in order to guarantee similar soil porosity. The second stage consisted of the addiction of different amounts of chilled/frozen contaminant (25–400 mg) on the top of the column allowing the percolation and distribution of the contaminant into the soil matrix. The soil was then left isothermically at 296 K. To evaluate if the equilibrium was reached, the concentration of the contaminant in the gas phase (Cgas) was monitored in four different levels of the column along time. When the concentration was similar at different levels
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Apparent density (g cm
P0,0 P2,0 P3,0 P4,0 P1,4 P2,14 P4,24
1.5 1.3 1.3 1.2 1.1 1.0 0.9
3
)
Particle density (g cm 2.5 2.5 2.5 2.5 2.9 3.1 3.3
3
)
Porosity (%)
pH
Water content (%)
Natural organic matter content (%)
42 49 50 51 62 68 73
8.8 8.8 8.8 8.8 6.5 6.1 5.8
0.0 2.0 3.0 4.0 0.72 2.5 4.3
<0.02 <0.02 <0.02 <0.02 4.0 14 24
(less than 5% deviation), equilibrium was considered to have been reached, what happened within 24 h. 2.5. Soil vapor extraction experiments The SVE experiments were performed both in sandy and in humic soils. The sandy soil was collected at 3 m deep in different places of a beach and the humic soil was collected in a forest area at 1–2 cm depth, both in a region in Porto, Portugal. All samples were stored in adequate vessels. The preparation of the sandy soils involved four steps: (a) washing the sandy soil till clean water was obtained, (b) drying, first at room temperature for 5 d and then at 105 °C for 24 h, (c) sieving in a 2 mm sieve; and (d) adding water in order to induce the desired water contents (2%, 3% and 4%). The preparation of the humic soils involved three steps: (a) drying at 55 °C for 48 h; (b) sieving in a 2 mm sieve; and (c) mixing the correct amounts of sandy and humic soil to induce the desired organic matter contents (4%, 14% and 24%). SVE experiments were performed in the same columns used for the equilibrium isotherms and prepared in the same way except the amount of benzene added (always 1.0 g of benzene). After the establishment of the equilibrium, the column was connected to the laboratorial installation shown in Fig. 1. To start the SVE the vacuum pump (B) was switched on, allowing different flow rates (controlled by the flow meter R) to percolate through the column (C) and then to a sampling system (S) where the contaminated emissions were monitored by gas chromatography. An activated charcoal (A) recipient was placed before the pump to protect the pump and to avoid atmospheric contamination. The remediation process was considered finished when the concentration of the contaminant in the gas phase was below 1.0 g m 3. This time was considered the SVE remediation time and the remaining level of contamination and the process efficiency were calculated.
2.6. Bioremediation experiments The BR experiments were performed in the soils that after SVE presented contamination levels above legal limits. The preparation of these soils to the BR tests consists of the addition of water and substrate to the soil in order to induce the real and potential metabolic activity of indigenous microorganisms. The defined SWC 1 was 20% and the addition of MinE was 10 mL kgsoil . The soils that, after SVE, presented levels of contamination higher than the levels imposed by law (10 mg kg 1) were treated by BR. The BR experiments were performed after the addition of water and substrate to the soil column and consist of the monitoring of the concentration of benzene in the gas phase of the soil until the legal level of contamination was achieved. The time required to reach this level was considered the BR time. The sum of the SVE remediation time with the BR time was defined as global remediation time. 3. Results 3.1. Equilibrium isotherms Fig. 2 shows the equilibrium isotherms that relate the concentration of benzene in the gas phase and the level of contamination in the soil matrix for all the experimented soils. This equilibrium isotherm allowed, through the determination of the concentration of benzene in the gas phase, the evaluation of the level of contamination remaining in the soil after the SVE. For each equilibrium isotherm a trend line was calculated and used to quantify the remaining level of contamination. It is also indicated a line that represents the legal limit, 10 mg kg 1. The isotherms show that the SWC has no significant influence on the concentration of contaminant in the gas phase, what indicates that an increase of the SWC has no influence on the amount
Fig. 1. Laboratorial installation.
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Fig. 2. Equilibrium isotherms at 297 K for all the prepared soils.
of contaminant that is in the gas phase of the soil and that can be easily extracted. The opposite is observed with the organic matter where the increase of its content led to a major reduction of the concentration of benzene in the gas phase of the soil, hindering the SVE process. 3.2. Soil vapor extraction experiments The SVE experiments were monitored by gas chromatography until the concentration of benzene in the gas phase of the soil reached 1.0 g m 3. In the end of each SVE, the remediation time and the level of contamination remaining in soil were calculated. These results are presented in Table 2. The last column of the table indicates the level of achievement of the clean up goal for each SVE performed. If the remaining level of contamination after the SVE
was below the legal limit it was considered that the goal was achieved and the remediation complete. This situation was indicated in the Table with a ‘‘A”. If the level of contamination in the end was within 10 and 20 mg kg 1 the clean up goal was considered to be closely achieved (indicated with ‘‘C”). This means that there is still a small amount of benzene in the soil above the legal limit that can be achieved by the continuation of the SVE or the utilization of the BR. Attending to low quantity of benzene that needs to be removed from the soil, the utilization of SVE can be the best option. In cases where the final level of contamination was above 20 mg kg 1 (identified with an ‘‘X”) the remediation was considered incomplete and should be complemented with BR. The results presented in Table 2 allow concluding that the SWC has minimum impact on the SVE process when higher airflow rates are used; however, for lower airflow rates, the SWC creates a neg-
Table 2 Results obtained in the SVE experiments for all soils. 1
)
Remediation time (h)
Level of contamination in the end (mg kg
1
Soil
Flow rate (L h
Process efficiency (%)
Clean up goal achievement
P0,0
18 9.8 5.7 2.0
2.4 2.4 4.5 6.3
14 13 15 15
)
94 95 94 94
C C C C
P2,0
18 10 5.3 4.0
2.5 4.0 6.4 6.9
19 12 11 11
92 95 96 95
C C C C
P3,0
19 9.9 6.5 2.7
2.3 3.8 8.1 11.3
19 16 11 13
93 94 96 95
C C C C
P4,0
18 9.4 6.7 3.2
2.6 4.0 8.1 12.8
15 13 12 7
94 95 95 97
C C C A
P1,4
19 9.2 5.7 2.2
3.6 5.1 11.2 34.8
14 9 8 7
95 97 97 97
C A A A
P2,14
18 10 4.8 2.2
3.3 7.3 13.2 41.6
114 70 86 70
78 86 83 87
X X X X
P4,24
18 9.6 5.6 2.5
4.2 8.5 22.5 45.4
170 135 118 92
78 85 85 88
X X X X
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A.A. Soares et al. / Chemosphere 80 (2010) 823–828 Table 3 Remediation and bioremediation times for each experiment.
Fig. 3. Monitoring of the bioremediations performed in soils P2,14 (a) and P4,24 (b).
ative impact on the remediation process originating a considerable increase of the remediation time. The SVEs performed in soils with higher NOM had remediation times 75–620% higher and were less efficient. In some of these less efficient cases, BR was needed to achieve the legal limits. This proves the negative impact of NOM on this remediation process. 3.3. Bioremediation experiments The BR tests were performed in the soils that had remaining levels of contamination higher than 20 mg kg 1. Four bioremediation experiments were replicated once and, for these cases, a relative deviation lower than 8% was observed. The BR was considered fin-
Soil
Airflow rate in SVE (L h 1)
Contamination level (mg kg 1)
Bioremediation time (h)
Global remediation time (h)
P2,14
2.2 4.8 10 18
70 86 70 114
117 270 117 728
159 283 124 731
P4,24
2.5 5.6 9.6 18
92 118 135 170
238 250 305 646
283 273 314 650
ished when the concentration of benzene in the gas phase of the soil reached 0.6 g m 3. Fig. 3a shows the monitoring of the BR processes performed in the P2,14 soil with three levels of contamination (70, 86 and 114 mg kg 1). Another test was carried out in a sterilized soil to verify if significant adsorption phenomena occur in the BR process. The results of this test indicated that during the BR process, the decrease of the concentration of benzene in the gas phase is only due to the activity of the microorganisms, as it is shown in Fig. 4. Fig. 4 shows that in the sterile soil, the quantity of contaminant in the gas phase maintains approximately constant indicating that no significant adsorption on the microorganism occurred. In the other test, where the microorganisms were active, the concentration of benzene decrease indicating that it was being degraded by the microorganisms. Fig. 3b presents the monitoring of the BR performed in the P4,24 soil with four levels of contamination (92, 118, 135 and 170 mg kg 1). For this soil the BR was considered finished when Cgas reached 0.3 g m 3. From Fig. 3a and b the BR times were calculated. These results are presented in Table 3. The fluctuations observed in the concentration of benzene in the first 48–72 h are a consequence of the movement of benzene vapors in soil towards the establishment of the equilibrium. The results allow concluding that the BR times are directly proportional to the level of contamination and inversely proportional to the organic matter content. To evaluate the influence of organic matter content on the BR process, it was compared the tests performed in the soils P2,14 and P4,24 with similar levels of contamination (around 90 and 115 mg kg 1). It was observed that the BR times decreased between 10% and 65%, respectively for levels of contamination of 90 and 115 mg kg 1. This can be explained by the fact that soils with higher organic matter contents may be more populated by indigenous microorganisms that increase their capability to degrade contaminants (proven by the higher slope observed in the P4,24 curves in Fig. 3b, compared to those with similar contamination in Fig. 3a). Through this, it is observed that organic matter content has opposite effects on the remediation of soils through the combination of SVE and BR. Higher NOM hinder SVE but enhances BR what explains the inconsistent behavior observed in the global remediation time.
4. Conclusions
Fig. 4. Comparison between sterile and non-sterile tests.
The remediation experiments performed in soils contaminated with benzene allowed concluding that: (a) SWC showed minimum impact on the SVE process performed with high airflow rates; but for low airflow rates an increase on the remediation time was observed; (b) NOM showed an important and negative impact on the SVE process originating an increase of the remediation times between 75% and 620%; (c) the level of contamination remaining in soil after SVE also increased requiring BR to complete the cleaning
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process; (d) only the soils with higher organic matter content (P2,14 and P4,24) required BR; (e) BR times are directly proportional to the level of contamination and inversely proportional to NOM; (f) soils with higher amounts of NOM showed better degradative conditions performing the BR in less time; and (g) SVE combined with BR showed to be an efficient option to the remediation of soils contaminated with benzene. Acknowledgement The authors are grateful to Fundação para a Ciência e Tecnologia (Project PTDC/ECM/68056/2006) for the material support for this work. References Albergaria, J.T., Alvim-Ferraz, M.C.M., Delerue-Matos, C., 2006. Remediation efficiency of vapor extraction of sandy soils contaminated with cyclohexane: influence of air flow rate, water and natural organic matter content. Environ. Pollut. 143, 146–152. Alvim-Ferraz, M.C.M., Albergaria, J.T., Delerue-Matos, C., 2006a. Soil remediation time to achieve clean-up goals: I: influence of soil water content. Chemosphere 62, 853–860. Alvim-Ferraz, M.C.M., Albergaria, J.T., Delerue-Matos, C., 2006b. Soil remediation time to achieve clean-up goals: II: influence of natural organic matter and water contents. Chemosphere 64, 817–825. Boopathy, R., 2000. Factors limiting bioremediation technologies. Bioresour. Technol. 74, 63–67. Chaîneau, C.H., Yéprémian, C., Vidalie, J.F., Ducreux, J., Ballerini, D., 2003. Bioremediation of a crude oil-polluted soil: biodegradation, leaching and toxicity assessments. Water Air Soil Pollut. 144, 419–440. Gogoi, B.K., Dutta, N.N., Goswami, P., Krishna Mohan, T.R., 2003. A case study of bioremediation of petroleum-hydrocarbon contaminated soil at a crude oil spill site. Adv. Environ. Res. 7, 767–782. Grasso, D., 1993. Hazardous Waste Site Remediation, Source Control. Lewis Publisher, Boca Raton.
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