ReviewIn situ bioremediation of monoaromatic pollutants in groundwater: A review
Introduction
Benzene, toluene, ethylbenzene and xylenes isomers (BTEX) are important monoaromatic hydrocarbons that have been found in sites polluted by oil production facilities and industries (Kao et al., 2006). These organic compounds are toxic and contaminate groundwater sources (An, 2004). Groundwater gets polluted by monoaromatic compounds due to release of petrol, gasoline, diesel, petrochemical products from storage tanks and wastes from oil industries (Andreoni and Gianfreda, 2007). These hydrocarbons have higher water solubility than other organic compounds that are present in gasoline such as aliphatics. Generally, solubility of benzene, toluene, ethyl benzene, xylenes and gasoline in water are respectively 18, 25, 3, 20, 50–100 ppm when gasoline is introduced into water (Kermanshahi pour et al., 2005). Percent volume of benzene, toluene, ethylbenzene and xylenes in gasoline, are 1, 1.5, <1–1.5 and 8–10, respectively (An, 2004). Groundwater contaminated by toxic pollutant is a very serious problem because many communities in the world depend upon groundwater as sole or major source of drinking water. Maximum levels for monoaromatic compounds in potable water are 0.05, 1, 0.7 and 10 ppm for benzene, toluene, ethylbenzene and isomers of xylenes, respectively (USEPA, 2006). The detection and determination of light aromatic compounds in limits up to part per billion (ppb) for a water sample can be carried out by various methods including gas chromatography (GC)/flame ionization detector (FID) (Fischer and Werner, 2000, Fraile et al., 2002, Wang et al., 2002, de Nardi et al., 2006), GC/photo ionization detector (PID) (Cunningham et al., 2001, Dórea et al., 2007), GC/mass spectrometer (MS) (USEPA, 1996, Disdier et al., 1999) or GC/solid phase micro extraction (SPME) (Djozan et al., 2004, Karačonji et al., 2006, Lee et al., 2007) through head space or purge and trap (Rosell et al., 2003, USEPA 5030C, 2003, Rosell et al., 2005) depending on sample preparation methods. Other methods such as chemical extraction (benzylsuccinate, trimethylbenzene, catechol 2, 3 dioxygenase), physical methods (depletion of dissolved oxygen, nitrate and sulfate or production of dissolved ferrous iron, sulfide and carbon dioxide), biological (bioassay tools) or numerical, physical and kinetic models can be used for on-line monitoring of monoaromatics degradation during the course of in situ bioremediation (Nadim et al., 2000, Lin et al., 2002, Reusser et al., 2002, Beller, 2002, Johnson et al., 2003, Hua et al., 2003, Schulze and Tiehm, 2004, Kuster et al., 2004, Dobson et al., 2004, Maurer and Rittmann, 2004, Mesarch et al., 2004, Bekins et al., 2005, Gödeke et al., 2006, Hendrickx et al., 2006, Hu et al., 2006, Huang et al., 2006, Kao et al., 2006, Atteia and Guillot, 2007, Biggerstaff et al., 2007, Morasch et al., 2007).
There are different methods for monoaromatic compounds removal from groundwater, such as physical techniques (electro remediation, air sparging, carbon adsorption, filtration, adsorption by zeolites) (Daifullah and Girgis, 2003, Ranck et al., 2005, Yang et al., 2005), chemical methods (chemical oxidation, photo catalysis remediation) (Tiburtius et al., 2005, Mascolo et al., 2007) and biological processes (bioremediation, biodegradation in reactors, phyto-remediation, wetland) (Rozkov et al., 1999, Langwaldt and Puhakka, 2000, Vidali, 2001, Lynch and Moffat, 2005, Wallace and Kadlec, 2005, Farhadian et al., 2006, Martı´nez et al., 2007) methods. These approaches can be applied alone or in combination; the use of several of them is generally encountered for polishing purposes. Some of these complementary methods include sand filtration and the permeable reactive barrier technology, which has been used for treatment of petroleum contaminated groundwater (Guerin, 2002, Khan et al., 2004, Arvin et al., 2005). All above mentioned methods can be divided into in situ and ex situ (pump and treat) remediation technologies. In situ remediation is treatment of the contaminated material in place. Among all remediation technologies for treating xenobiotics or monoaromatic compounds from contaminated groundwater, bioremediation appears to be an efficient and economical process and environmentally sound approach (Vidali, 2001, Dobson et al., 2004, Maliyekkal et al., 2004, Lynch and Moffat, 2005). Ex situ bioremediation is generally costly and difficult due to extraction of contaminated water from subsurface, treatment and recharging the underground. This has led to an interest in using in situ bioremediation for groundwater contaminated by oil products.
In situ bioremediation is known as long term technology since there is less certainty about the uniformity of treatment because of the variability of aquifer and soil characteristics. However, this process has advantages such as relative simplicity, low cost, and potentially remarkable efficiency in contamination removal. In in situ bioremediation, organic pollutants are completely destroyed, therefore no secondary waste stream is produced (Dott et al., 1995).
In situ bioremediation is a biological process where microorganisms metabolize organic contaminants to inorganic material, such as carbon dioxide, methane, water and inorganic salts, either in natural or engineered conditions. When naturally occurring metabolic processes are used to remediate pollutants without any additional alteration of site conditions, the process is called as intrinsic or natural attenuation. Present results indicate that biodegradation is the best method for BTEX removal (Kao and Prosser, 2001, Kao et al., 2006). When working conditions at the site are engineered, i.e. designed to accelerate the bioremediation of contaminants, the process is referred to as engineered or enhanced bioremediation (Scow and Hicks, 2005).
Main factors affecting in situ bioremediation of contaminated groundwater have been widely described in the literature (Kampbell et al., 1996, MacDonald et al., 1999, Boopathy, 2000, Schreiber and Bahr, 2002, McGuire et al., 2005, Farhadian et al., 2006, Andreoni and Gianfreda, 2007). Some of the main points include:
- 1.
Source and concentration of pollutant.
- 2.
Chemistry and toxicity of contamination.
- 3.
Solubility, transport, adsorption, dispersion and volatility of pollutant compounds.
- 4.
Detection, determination and monitoring of pollutants.
- 5.
Chemistry, physics and microbiology of groundwater.
- 6.
Chemistry and mechanics of soil at contaminated site.
- 7.
Hydrogeology and hydrology of contaminated site.
- 8.
Limitations of environmental standards for water and soil.
- 9.
Environment conditions, nutrient sources and presence of electron acceptors.
- 10.
Biodegradability of contaminants, and the presence of a competent biodegrading population of microorganisms.
In in situ bioremediation, anaerobic biodegradation plays a more important role than that of aerobic processes. Aerobic bioremediation process requires expensive oxygen delivery systems and process maintenance is often high due to biofouling in subsurface. But anaerobic processes have advantages such as low biomass production and good electron acceptor availability. Anaerobic processes are sometimes the only possible solution to remove pollutants (Holliger et al., 1997) as it is often difficult to inject oxygen into underground waters.
The microbiology and metabolism of BTEX degradation and interaction between BTEX and other compounds (such as ethanol, MTBE) during their biodegradation is an important factor when in situ bioremediation for monoaromatic removal from groundwater is concerned.
Section snippets
Microbiology and metabolism
Microorganisms such as bacteria, fungi and microalgae play a key role in monoaromatic removal through in situ bioremediation processes (Holliger et al., 1997, Semple et al., 1999, Jindrova et al., 2002, Schulze and Tiehm, 2004, Nikolova and Nenov, 2005). Monoaromatic pollutants act as carbon source for microorganisms. Also, they require macro nutrients (nitrogen and phosphorus), micro nutrients (Ca2+, Mg2+, Na+, K+, S2−, co-factors such as heavy metals), electron acceptor (oxygen is the
In situ bioremediation
In situ bioremediation has been successful for the treatment of groundwater contaminated with mixtures of chlorinated solvents such as carbon tetrachloride (CT), tetrachloroethylene (TCA), trichloroethylene (TCE), or pentachlorophenol (PCP) (Dyer et al., 2003, Klecka et al., 1998, Kao and Prosser, 1999, Schmidt et al., 1999, Ferguson and Pietari, 2000, Goltz et al., 2001, Beeman and Bleckmann, 2002, Antizar-Ladislao and Galil, 2003, Kao et al., 2003, Devlin et al., 2004, Widdowson, 2004). Also,
Conclusion
Monoaromatic pollutants in groundwaters are threatening drinking water resources and therefore have, when present, to be removed. The analysis presented here suggests that in some case, naturally-occurring aerobic biodegradation phenomena can take place at a rate high enough to reach environmental standard limits in a reasonnable time. However, the most common situation is that it is necessary to artificially improve the performances of this process. This approach corresponds to the so-called
Acknowledgements
M.F. strongly acknowledges the Institute for Energy and Hydro Technology, Teheran for a thesis fellowship at Université Blaise Pascal, France. Prof. Mehdi Borghei (Shariff University of Technology, Teheran, Iran) is acknowledged for fruitful discussions and Julien Troquet (Biobasic Environnement, Clermont-Ferrand, France) for providing support to work.
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