Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Toluene, Ethylbenzene and the Xylenes

4.0 Identity, use and sources in the environment

Toluene, ethylbenzene and xylenes are volatile, flammable and colourless liquids with a sweet, pungent or gasoline-like odour. Toluene may also be referred to as methylbenzene and was formerly known as toluol. Xylenes are also known as dimethylbenzenes. All three compounds are monoaromatic hydrocarbons and thus derivatives of benzene. Toluene and ethylbenzene differ from benzene by the addition of a single methyl or ethyl group, respectively, whereas xylenes have two methyl group substitutions. The position of the methyl groups on the benzene ring determines the isomer of xylene: ortho- (o-), meta- (m-) or para- (p-) xylene, also known as 1,2-, 1,3- and 1,4-dimethylbenzene (U.S. EPA, 1990). Typical mixtures of xylenes contain approximately 40% m-xylene, 24% o-xylene and 19% p-xylene, as well as 17% ethylbenzene (Hancock, 1982).

Toluene, ethylbenzene and xylenes are moderately soluble in water and have relatively low log n-octanol/water partition coefficient (Kow) (Mitra and Roy, 2011). Toluene is the most water soluble of these compounds. All of these compounds have relatively elevated vapour pressures and vapours that are highly flammable and explosive at sufficient concentrations (Vallero, 2004). Specific physicochemical properties for toluene, ethylbenzene, xylenes (which vary in composition and include a variety of impurities, ethylbenzene being the largest) and o-, m- and p-xylene isomers are presented in Table 1.

Table 1. Physical and chemical properties of toluene, ethylbenzene and xylenes
Property Toluene Ethylbenzene Xylenes o-Xylene m-Xylene p-Xylene
CAS, Chemical Abstracts Service; Kow, n-octanol/water partition coefficient
a Budavari et al. (1989); b Lide (1994); c Budavari et al. (1996); d Cannella (2007); e U.S. EPA (1995a); f U.S. EPA (1995b); g U.S. EPA (1995c); h ATSDR (1995); i 1 mmHg = 133.3 Pa; j Verschueren (1983); k HSDB (1999); l Welch et al. (2005).
Molecular formula C6H5CH3 C6H5CH2CH3 C6H4(CH3)2
CAS No. 108-88-3 100-41-4 1330-20-7 95-47-6 108-38-5 106-42-3
Molecular weight (g/mol) 92.13a 106.17b 106.17c
Melting point (°C) -95.0a -95.0d -47.4 -25.0c -47.4c 13–14c
Boiling point (°C) 110.6a 136.2d 137.0–140.0c 144.4c 139.0c 138.4c
Log Kow 2.69e 3.15f 3.12–3.20g 2.77h 3.20h 3.15h
Vapour pressure (mmHg)i 22.0
Density (g/mL) 0.87
0.87 l 0.86 h 0.86 h 0.88 h 0.86 h
Solubility in water (mg/L) 515
Conversion factors for air (ppm to mg/m3) 3.77 4.34 4.34

Alexander et al. (1982) measured aqueous odour and taste thresholds for various chemicals, including toluene, ethylbenzene and xylenes. The odour threshold values were reported as milligrams of compound per litre of odour-free water at 60°C. The authors stated that the odour thresholds measured at 60°C should be applicable to ambient temperature, since the temperature effect appeared to be small. The taste threshold values were reported as milligrams of compound per litre of odour-free water at 40°C. For toluene, two odour threshold measurements of 0.024 mg/L were reported; for ethylbenzene, two odour threshold measurements of 0.0016 and 0.0032 mg/L were reported. Also for toluene, two taste threshold measurements of 0.12 and 0.16 mg/L (average value 0.14 mg/L) were reported; for ethylbenzene, two taste threshold measurements of 0.064 and 0.080 mg/L (average value 0.072 mg/L) were reported. For the xylenes, Middleton et al. (1958) stated that taste and odour could be detected at concentrations ranging from 0.3 to 1.0 mg/L; WHO (2003b) reported from VanGemert and Nettenbrijer (1977) and from Verschueren (1983) that the odour threshold for xylene isomers in water ranges from 0.02 to 1.8 mg/L.

Toluene is primarily manufactured by catalytic reforming of petroleum, ethylbenzene by alkylation of benzene, and xylene by the dehydrocyclodimerization and methylation of toluene and benzene (Camford Information Services Inc., 1996, 2003, 2004). Many industrial and technical applications exist for all three compounds. They are primarily used in the synthesis of specific chemical compounds or as industrial solvents. Toluene is a common solvent used in making paints, paint thinners, fingernail polish, lacquers, adhesives and rubber, as well as in some printing and leather tanning procedures (ATSDR, 2000). It is a common gasoline additive (ATSDR, 2000) and is used in the production of various organic compounds, including benzene and toluene diisocyanate (U.S. EPA, 1995d). It is also employed as a carbon source in the synthesis of multiwall carbon nanotubes (Mi et al., 2005). The uses of toluene are estimated as follows: 46% for benzene production, 37% as a gasoline additive, 8% as a solvent, 7% for toluene diisocyanate production and 2% for other purposes (U.S. EPA, 1995d). Ethylbenzene is used almost exclusively as an intermediate in the production of styrene. In 2003, 789 kilotonnes of styrene were manufactured by two Canadian manufacturers (Ontario and Alberta); 811 kilotonnes of styrene were forecasted to be manufactured in 2006 (Camford Services, 2004). It is also present in gasoline (< 15%) and in mixed xylenes (< 25%) (IARC, 2000). Xylenes are frequently used as solvents and in paint thinners, varnishes and cleaning agents (ATSDR, 2007). Similarly to toluene and ethylbenzene, xylenes are fuel additives that comprise approximately 10% of gasoline. Approximately 82% of xylenes produced is p-xylene, which is used in the production of terephthalic acid and dimethyl terephthalate, both of which are needed to produce polyethylene terephthalate plastic bottles and polyester clothing. o-Xylene and m-xylene isomers are used to a lesser extent in the production of phthalic anhydride and isophthalic acid, respectively (Swedish Chemicals Agency, 2010).

In 2009 Canadians used 42.3 billion litres of gasoline; the Ontario market accounts for 39% of motor gasoline sales in Canada, followed by Québec (21%), Alberta (13%), British Columbia (11%), and Atlantic Canada (7%) (NRCan, 2011). Canadian domestic sales of diesel fuel in 2009 were 26.0 billion litres with approximately 49% of the diesel sales occurring in the western provinces and territories, followed by Ontario (25%), Quebec (18%) and Atlantic Canada (8%). (NRCan, 2011). Toluene, ethylbenzene and xylenes (along with benzene) are part of the aromatic fraction in gasoline and diesel fuel. In 2008, the aromatic content for typical Canadian gasoline ranges from 24.8 to 30.3 % volume (Rahumathulla et al. 2010). The average weight % aromatic content in diesel fuel (regular or low sulphur summer or winter diesel) in Canada ranges from 28.76 to 36.8% (Guthrie et al., 2003). With the widespread use of gasoline and diesel fuel in Canada, unintentional releases to the environment from activities associated with production, transportation and storage, as well as during refuelling of vehicles can contaminate nearby soil and water (Mitra and Roy, 2011). Due to their volatile nature, toluene, ethylbenzene and xylenes will readily vaporize from water and soil upon contact with air (Vallero, 2004). Additional toluene can be released to the atmosphere via several manufacturing processes, including fuel production from crude oil, coke production from coal and as a by-product of styrene production (ATSDR, 2000).

4.1 Environmental fate

Toluene, ethylbenzene and xylenes will tend to partition to air and water due to their relatively high vapour pressure, moderate water solubility and low log Kow. The atmosphere is an important sink for toluene, ethylbenzene and xylenes, as over 98% of the compounds released in the environment will eventually partition to air (ASTER, 1995; IPCS, 1997; OECD, 2002). The primary processes for the removal of toluene, ethylbenzene and xylene from water are volatilization and microbial degradation, the latter being more important with increased water depths (Arthurs et al., 1995). In surface water, toluene, ethylbenzene and xylenes can float near the surface, where they readily volatilize (Mitra and Roy, 2011). Half-lives ranging from 5 hours to 16 days for toluene and from 3.1 hours to 4.1 days for ethylbenzene and xylenes have been reported in conditions ranging from turbulent to static waters (Mackay and Leinonen, 1975; Thomas, 1982; HSDB, 1986, 2010; ATSDR, 2000). Toluene, ethylbenzene and xylenes are readily biodegradable in soil and water under aerobic conditions. Biodegradation half-lives ranging from 2 to 92 days, from 0.1 to 231 days and from 24 to 161 days have been reported for toluene, ethylbenzene and xylenes, respectively (Barker et al., 1986; Wilson et al., 1986; Slooff and Blokzijl, 1988; Mackay et al., 1992; Earl et al., 2003). Biodegradation of these compounds can be slowed significantly when their concentrations are sufficient to cause microbial death or under anaerobic conditions. For example, ethylbenzene degradation can extend to 1155 days under anaerobic conditions (Earl et al., 2003). Slower degradation of toluene, ethylbenzene and xylenes can contribute to transportation of the compounds considerable distances from the original source (Zogorski et al., 2006).Toluene, ethylbenzene and xylenes are not expected to bioconcentrate or biomagnify to a great extent in aquatic organisms (ATSDR, 2010).

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