Biodegradation of waste streams containing benzene, toluene, ethylbenzene and xylene (BTEX): Practical implications and brief perspectives

Benzene (B), toluene (T), ethylbenzene (E) and xylene (X), collectively named as BTEX are mono-aromatic ring compounds with a 6-carbon benzene ring. Due to the presence of the aromatic ring, these compounds, especially benzene, are generally considered to be non-reactive species [1]. However, they are known to undergo hydrogenation and certain substitution reaction depending on the prevailing environmental conditions. Their solubility in water is usually low (i.e. relatively hydrophobic in nature) due to the low octanol-water partition coef icient (Kow) values which favour hydrophilic dissolution [2]. BTEX compounds are relatively insoluble in water and in some situations, their levels have been recorded at up to 1000 mg/L, which is much higher than the allowed maximum contaminant level of 0.005 mg/L for B, 1 mg/L for T, 0.7 mg/L for E, and a total of 10 mg/L for all three forms of X [3]. They are released into the natural environment (air, water, soil and sediments) due to land ill leaching, underground storage tanks, accidents during oil transportation and pipeline leakage from petroleum and chemical industries [4]. BTEX compounds are transported through several metres at oil-spill sites causing them to be persistent pollutants in both soil and water environments [5]. The persistence presence of BTEX in air and the transportation to water bodies has been reported as a result of rainfall [6]. BTEX compounds exists together in the environment suggesting that their toxicity is ampli ied through their interactions with one another. The composition of BTEX in gasoline is: m-xylene (31%), toluene (26%), o-xylene (12%), ethylbenzene (11%), benzene (11%), p-xylene (9%). BTEX are harmful to human and aquatic life due to its inherent toxic and carcinogenic properties. Acute exposure can cause tiredness, dizziness, headache, loss of coordination, skin, sensory irritation, chronic kidney disease and also affect the liver and blood system [7]. BTEX compounds can be released from a variety of sources such as petrochemical industry waste streams, household wastes, municipal land ills as well as groundwater plumes, especially when they are located at a considerable distance from an oil spill site [8,9]. BTEX are also few of the compounds responsible for the formation of ground level ozone and are considered as priority environmental pollutants by the US-EPA [10].