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Further description:-  Benzene, toluene, ethylbenzene, and xylene 

Glossary Entry
BTEX is an acronym for benzene, toluene, ethylbenzene, and xylene. This group of volatile organic 
compounds (VOCs) is found in petroleum hydrocarbons, such as gasoline, and other common environmental
contaminants.
Further description: Contaminants

The monoaromatic hydrocarbons, abbreviated BTEX, which stands for benzene, toluene, ethylbenzene, trimethylbenzenes and the three xylene isomers, are aromatic hydrocarbons containing one unsubstituted or methyl-substituted benzene ring, See Figure 1.

 

Benzene

Toluene

o-xylene

 

 

 

 

 

 

1,2,3-Trimethylbenzene

Ethylbenzene

 

 

Figure 1. Structure formula for the BTEX.

 

 

BTEX have in recent years attracted much attention, since they constitute one of the most common and serious threats to groundwater reservoirs and indoor climate deriving from contaminated sites. This is mainly due to the potential effects of benzene, which is considered a strong carcinogen, and which is highly mobile in the soil and groundwater environment, which is also the case for the other BTEX.

 

They are found in numerous sites, including areas used for fuel operations, refineries, gasoline stations, and gasification sites. 

 

Fate in the soil and groundwater environment

 

Because of the relatively high water solubility and low Kow values (See Table 1), these compounds will tend to be dissolved in the water phase or evaporated into the air spaces of the soil. Because of their relative hydrophilic nature, they are not attenuated very much by the soil particles or constituents and can be transported rather long distances if the right conditions are there. In some sites, some BTEX are found several kilometres downstream the source.

 

However, this would not occur, if it were not for the persistency of these compounds under certain redox conditions. As can be seen from Table 2, all of the BTEX are highly biodegradable under aerobic conditions. However, in soil and groundwater oxygen is often depleted, and especially in groundwater, due to the low water solubility of oxygen, the flux of oxygen will in many cases not be enough to support aerobic degradation. Under anaerobic conditions, the biodegradation pattern for these compounds is rather complex, While most of them, including benzene, are shown to be degraded under strict anaerobic condition and sulfate reducing conditions (Edwards et al. 1992; Wilson et al. 1986), the biodegradation under denitrifying conditions are less favourable. Benzene cannot be degraded with nitrogen as terminal electron acceptor (Schreiber and Bahr, 2002), and o-xylene often depend on the existence of primary substrates, either toluene or phenol to be degraded (Flyvbjerg et al. 1993).  In addition to this cometabolic behaviour, also prolonged lag periods for degradation of xylenes, ethylbenzene and 1,2,4-trimethylbenzene are often observed (Hutchins et al. 1991).

 

Table 1. Physio-chemical properties of BTEX

 

Compound

Mole weight g mole-1

Density

g ml-1

Boiling point °C

Water solubility

mg l-1

Vapor pressure mmHg

Log Kow

Benzene

78

0.88

80.1

1780

76

2.13

Toluene

92

0.87

110.8

535

22

2.69

o-Xylene

106

0.88

144.4

175

5

2.77

m-Xylene

106

0.86

139

135

6

3.20

p-Xylene

106

0.86

138.4

198

6.5

3.15

Ethylbenzene

106

0.87

136.2

152

7

3.15

 

 

 

 

 

 

 

 

 

Table 2. Biodegradation of BTEX under different redox conditions

 

Compound

Aerobic conditions

Denitrifying conditions

Sulfate-reducing conditions

Iron-reducing conditions

Methano-genic conditions

Benzene

++

-

+

-

+

Toluene

++

++

+

+

+

m-Xylene

++

++

+

+

+

p-Xylene

++

+

+

 

+

o-Xylene

++

+/-1)

-

-

+/-

Ethylbenzene

++

+/-

 

-

+/-

1,2,4-trimethyl-benzene

++

 

 

 

+/-

 

 

Toxic effects

 

There are not many data for the ecotoxicity of these compounds in the terrestrial environment, since they in most cases will not stay in this environment for very long time and thus are not considered toxic. For the same reason it is extremely difficult to perform toxicity tests in soil. This was also reported by Salanitro et al. (1997), who observed losses of 40-95 % of BTEX applied to soil samples for preparation of toxicity testing. However, a number of studies have been performed estimating the toxicity towards marine or freshwater environments, since they might be exposed for longer periods of time in cases with large bulk spills of light fuel etc. Normally, there will be no additional problems with toxic metabolites, since the aerobic and anaerobic degradation of these compounds will lead to complete mineralization.

 

In Table 2, the carcinogenity and the ecotoxicity towards different marine organisms are shown. 

 

Table 2. Geno- and ecotoxicity (LC50 mg l-1)1) of BTEX

 

Compound

Carcinogen

LC50 Cancer magister

LC50 Paleo-monetes pugio

LC50 Marone saxitilis

Benzene

+

108

27

5.8

Toluene

-

28

9.5

7.3

o-Xylene

-

12

3.7

9.2

m-Xylene

-

6

1.3

11

p-Xylene

-

2

0.86

2

Ethylbenzene

-

13

0.49

4.3

1,2,4-Trimethylbenzene

-

5

5.4

 

 1) After Neff (1977)

 

 Management and remediation

 

In most European countries, contaminated sites containing BTEX are on the clean up priority list, in particular because of the health threats posed by benzene. Since these compounds will be found both in the source zone and the plume area, they cannot be remediated solely be excavation of the contamination soil. To contain the plume, pump-and-treat measures or barrier technology has to be applied. To remove the contamination, either natural attenuation can be relied on (MacDonald, 2000) or in situ remediation technologies such as bioremediation in the form of biostimulation with addition of nutrients and or electron acceptors, soil vapor extraction/bioventing, or reactive barriers can be applied. A number of field trials have shown successful removal of these compounds by in situ remediation (See i.e. Cunnigham et al. 2001), but to achieve complete remediation, the geology and the biogeochemistry of the site must be accurately described, and a thorough monitoring scheme must be prepared in order to get exact information as to when the clean up criteria have been reached. 

 

Publications

 

Cunningham, JA, Rahme, H., Hopkins, GD, Lebron, C., Reinhard, M. (2001) Enhanced in situ bioremediation of BTEX-contaminated groundwater by combined injection of nitrate and sulfate. Environ. Sci. Technol. 35, 1663-1670.

 

Edwards, EA, Wills, LE, Reinhard, M, Grbic-Galic, D (1992) Anaerobic degradation of toluene and xylene by aquifer microorganisms under sulfate-reducing conditions. Appl. Environ. Microbiol. 58(3), 794-800.

 

Flyvbjerg, J, Jørgensen, C, Arvin, E, Jensen, BK, Olsen, SK (1993) Biodegradation of ortho-cresol by a mixed culture of nitrate-reducing bacteria growing on toluene. Appl. Environ. Microbiol. 59, 2286-2292.

 

Hutchins, SR, Sewell GW, Kovacs, DA, Smith, GA (1991) Biodegradation of aromatic hydrocarbons by aquifer microorganisms under denitrifying conditions. Environ. Sci. Technol. 25, 68-76.

 

MacDonald, JA (2000) Evaluating natural attenuation for groundwater cleanup. Environ. Sci. Technol. 34, 346A-353A.

 

Neff, JM (1977) Polycyclic Aromatic Hydrocarbons in the Aquatic Environment. Sources, Fates, and Biological Effects. Applied Science Publ. Ldn.

 

Salanitro, JP, Dorn, PB, Huesemann, MH, Moore, KO, Rhodes, IA, Rice, JLM, Vipond, TE, Western, MM, Wisniewski, HL (1997) Crude oil hydrocarbon and soil ecotoxicity assessment. Environ. Sci. Technol. 31, 1769-1776.

 

Schreiber, ME, Bahr, JM (2002) Nitrate-enhanced bioremediation of BTEX-contaminated groundwater: parameter estimation from natural-gradient tracer experiments. J. Contam. Hydrol. 55, 29-56.

 

Wilson, BH, Smith, GB, Rees, JF (1986) Biotransformation of selected alkylbenzenes and halogenated aliphatic hydrocarbons in methanogenic aquifer material: A microcosm study. Environ. Sci. Technol. 20, 997-1002.

 

 

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