Benzene is a clear, colourless liquid, that is highly flammable with a low boiling point and high vapour pressure. Soluble in most organic solvents and compounds; benzene is a hexagonal cyclic aromatic hydrocarbon with a ring containing six carbon atoms, with a hydrogen atom attached to each carbon. Benzene is highly refractive, nonpolar and nonreactive with low viscosity and low surface tension (Environment Canada, 1990). The low boiling point and high vapour pressure of benzene causes a rapid evaporation at Standard temperature and pressure. Because of the ring structure benzene is basically stable in the atmosphere. It is relatively photochemically inactive and it contributes little to photochemically induced pollutants (Environment Canada, 1990).
Table 1. Benzene's solubility in water (kg m-3) at various temperatures ( C). As the temperature increases, the solubility also increases (Key Chemicals Data Books Benzene, 1978).
Temperature ( C) Solubility in Water (kg m-3)
9.4 1.79
16.8 1.77
24.0 1.80
31.0 1.83
38.0 1.92
44.7 2.03
51.5 2.14
58.8 2.34
65.4 2.57
Henry's law constant for benzene is 54.3 measured in 10-4 atm-m3/mol.
Table 2. Average vapour pressure of benzene at various temperature ranges (Weast and Liche, 1989).
Temperature Range Vapour Pressure
-58 to -30 sol. 67.01
-30 to + 5 sol. 138.16
0 to 42 liq. -77.03
42 to 100 liq. -16.10
The Kow of benzene is 2.13 (http://www.pegnet.com/MtBE2/mtbe_sl_TOC.htm)
A range of 13 to 51.8 ppm benzene in air may be analyzed by gas chromatography (GC). This is currently the most practical method for identifying and measuring many volatile organic compounds. Detectors using infrared absorption, with carbon monoxide or carbon dioxide lasers as sources, have a detection limit of 3 ppb for benzene (Environment Canada, 1984).
Air is drawn through a glass sampling tube. The sample is desorbed with chloroform and identified using the Friedel-Crafts alkylation reaction. Anhydrous aluminum chloride (about 100mg) is placed in a test tube and heated until it sublimes. When it has cooled, a drop of the extract and two drops of chloroform are added with shaking. the appearance of an orange-red colour indicates the presence of an aromatic ring (Environment Canada, 1984).
Concentrations of benzene greater than 1 ppm can be measure by direct injection of the water sample into the gas chromatograph (GC). This is the method of choice when the identification of a spill is necessary or when low levels of benzene are present (Environment Canada, 1984).
Concentrations of 4 to 40 ppm benzene in water can be measured using 1 cm path length cells with a precision of +/-10%. The advantage of this method over methods that involve heating the sample is that sample loss through volatilization is minimized. This method lacks high sensitivity but is adequate for spills. It is not specific and assumes that the identity of the spill is known (Environment Canada, 1984).
This method is suitable for concentrations of benzene greater than 10 ppm. The precision is +/-6%. This is a simple and inexpensive method which does not require complex instrumentation. It is not highly sensitive or specific but is adequate for spills of a known substance (Environment Canada, 1984).
The water sample is extracted with chloroform and the Friedel-Crafts alkylation reaction is used to identify aromatic hydrocarbons. Anhydrous aluminum chloride (about 100mg) is placed in a test tube and heated until it sublimes. When is has cooled, a drop of the extract and two drops of chloroform are added with shaking. The appearance of an orange-red colour indicates the presence of an aromatic ring (Environment Canada, 1984).
Concentrations of benzene at the ppm level may be detected using a flame ionization detector. The detection limit may be extended to the ppb level by the use of a photoionization detector or an infrared detector. This is the method of choice when the identification of a spill is necessary or when low levels of benzene are present (Environment Canada, 1984).
Concentrations of 4 to 40 ppm in soil may be measured using 1 cm path length cells. This is a simple, inexpensive method. It lacks sensitivity but is adequate for spills of a known composition.
This method is used for the detection of concentrations greater than 10 ppm benzene in soil. It lacks sensitivity and specificity, but is adequate for spills of a known composition.
The Friedel-Crafts alkylation reaction is used to identify aromatic hydrocarbons. A sample of soil is extracted with Freon 113 and the Freon evaporated. The residue is taken up in chloroform. Approximately 100 mg of anhydrous aluminum chloride is placed in a test tube and heated until it sublimes. When it has cooled, several drops of the chloroform indicates the presence of a compound containing an aromatic ring (Environment Canada, 1984).
Benzene may be produced from petroleum, coal or natural gas condensates. It exists as a component of gasoline and is used as a solvent and as an intermediate in the production of numerous chemicals including ethylbenzene, cyclohexane and meleic anhydride (Environment Canada, 1990).
Benzene is recovered from petroleum or coal through one of four following processes:
Catalytic reforming is used to prepare high octane blending stocks for gasoline production. Several different solvents are used to extract or separate benzene from the reformate. Glycols and sulfolane are most commonly used. The extract is then heated and sent through clay towers to remove any olefins present (See Equation 1). Benzene, toluene and xylene are then separated by fractionation. It is estimated that 1% of the total benzene produced from catalytic reforming is emitted into the atmosphere, thus indicates a 99% recovery (Environment Canada, 1990). The unrecovered benzene is not necessarily emitted, but leaves the process in a stream that may be used as a feedstock.
Equation 1 (possible reactions)
C6H14 ------ C6H6 + 4H2 (hexane)
(CH3)2C6H10 ------ C6H6 + 2CH4 + H2 (dimethyl cyclohexane)
CH3C5H9 ------- C6H6 + 3H2 (methyl cyclopentane)
Overall benzene content may be 5-10% before being recovered and purified (Environment Canada, 1984).
Benzene may also be produced from toluene through dealkylation or disproportionation. Dealkylation of toluene can be acomplished through thermal or catalytic processes. Pure toluene (or toluene mixed with aromatics or paffins) is heated, charged to the reactor in the presence of excess hydrogen. Toluene reacts with hydrogen to yield benzene and methane (See Equation 2). About 70-80% conversion of toluene to benzene is accomplished (Environment Canada, 1990). Toluene disporportionation produces benzene and xylenes by catalytic reaction. This process is similar to toluene dealkylation, but can occur under less severe conditions.
Equation 2
CH3C6H5 + H2 -------- C6H6 + CH4 (toluene)
Another method where benzene can be recovered is from ethylene production through pyrolysis of natural gas concentrates or naphthas. Pyrolysis gasoline is a liquid byproduct that is formed as part of the steam cracking process and contains a significant amount of benzene. Due to the high concentration of benzene in pyrolysis gasoline, some plants recover motor gasoline aromatics as well as benzene.
The last method to extract benzene is through distillation of coke oven light oil. Approximately 11 to 18 liters of light oil can be produced from coke ovens (Environment Canada, 1990). Light oil contains 50-85% benzene.
A survey of commercial use patterns indicates that 765 kilotonnes of isolated benzene were produced in Canada in 1990 and 131 kilotonnes were imported, for a total Canadian supply of 896 kilotonnes (Canadian Environmental Protection Act, 1993). Isolated benzene is produced at four industrial plants in Sarnia/Corunna are in Ontario, at two plants in Alberta and at two plants in Montreal, Quebec. Actual numbers of benzene produced in 1981 are avaible in Table 3 (Environment Canada, 1984).
Table 3: Benzene Production levels in Canada during 1981 (Adapted from Environment Canada, 1984).
Company, Plant Location Kilotonnes/Year (1981)
Esso Chemical Canada, Sarnia, Ont. 100 Petro Canada, Montreal, Que. 145 Gulf Canada, Montreal, Que. 119 Petrosar, Corunna, Ont. 165 Polysar, Sarnia, Ont. 67 Shell Canada, Corunna, Ont. 67 Sunchem, Sarnia, Ont. 63 Texaco Canada, Mississauga, Ont. 20 Total 746
Benzene is widely used as an industrial solvent. Due to the toxicity of benzene, it is not known which Canadian industries are still using benzene as a solvent but Table 2 presents industries and manufactured products possibly using benzene.
Possible Industries and Manufactured Products using Benzene as a solvent
rubber tires floor recovering rubber products laboratories adhesives degreasing of metal furniture printing inks pharmaceuticals printing and publishing alcohol production paint removers general organic synthesis synthetic rubber textiles
(Environment Canada, 1990)
Benzene is mainly used in the production of ethylbenzene/styrene, cumene, cyclohexane and maleic anhydride (Environment Canada, 1990).
The following companies were the major buyers of benzene in the year 1982.
Dow Chemical Canada, Inc., Sarnia, Ont. Petro-Canada, Montreal, Que. Gulf Canada Products, Montreal East, Que. Monsanto Canada Inc., LaSalle, Que. Polysar Ltd., Sarnia,Ont. Recochem Inc., Montreal, Que.
(Environment Canada, 1984).
Mechanisms affecting the environmental fate of benzene include photo-oxidation, volatilization, advection, and biodegradation. The atmosphere and surface waters should be the major sinks for benzene because of its relatively high vapour pressure, high water solubility, and low octanol/water partition coefficient. Processes in the atmosphere play a determining role in benzene's ultimate fate in the environment (Government of Canada, 1993).
Photo-oxidation is the major degradation pathway for benzene in air. Benzene is oxidized in reactions with hydroxyl radicals and, to a lesser extent, tropospheric ozone and nitrate radicals (NO3). Under typical urban atmospheric conditions, half-lives attributable to reactions with hydroxyl radicals were calculated to be 9 days, more than 235 days with nitrate radicals, and more than 470 days with ozone. Other estimates for overall half-lives of benzene have ranged from 0.1 to 21 days. Major products of photo-oxidation include: phenol, nitrophenol, nitrobenzene, butanedial, formaldehyde, carbon dioxide, and carbon monoxide. Since the atmospheric half-life of benzene is relatively short, long-range transport of benzene is unlikely (Government of Canada, 1993).
Volatilization and biodegradation are the major processes involved in the removal of benzene from water. The half-life of benzene in water 1 metre deep was estimated to be 4.8 hours as a result of volatilization. Reported half- lives of benzene have ranges from 33 to 384 hours for aerobic biodegradation in surface waters. For anaerobic biodegradation in deeper waters or in groundwater, half-lives ranged from 28 days to 720 days (Government of Canada, 1993).
The primary mechanisms reponsible for loss of benzene from soil are volatilization to the atmosphere and runoff to surface water. Biodegradation also accounts for a small proportion of loss. Benzene released below the soil surface, for example from leaking underground storage tanks, can leach into groundwater. With organic carbon sorption coefficients reported for benzene ranging from 12 to 213, benzene is considered to be moderately to highly mobile in the soil (Goverment of Canada, 1993).
Using the Level III Fugacity Modelling developed for Southern Ontario, the overall residence time in the environment was predicted to be short (3.5 days, considering both degradation and movement of benzene out of the area) The reaction residence time was also short (9.7 days, considering loss through degradation reactions only) (Government of Canada, 1993).
Benzene does not bioconcentrate in aquatic biota to a significant degree. Relatively low bioconcentration factors (BFCs) have been reported for aquatic bacteria, algae, macrophytes, and fish. The highest reported value was for Daphnia pulex, with a BFC of 225. Once the organisms are removed from contaminated water, benzene is rapidly cleared by the organisms. For Daphnia pulex, 85% of accumulated benzene was removed during the 72 hours following withdrawal from contaminated water. The depuration of benzene in fish is also rapid. Half-lives were estimated to be less than 0.5 days in eel, Anguilla japonica, and less than 1 day in striped bass, Morone saxatilis (Government of Canada, 1993)
Benzene enters water and soil through petroleum seepage and weathering of exposed coal-containing strata. It enters groundwater from petroliferous rocks, and air from volcanoes, forest fires, and releases of volatile chemicals from plants. The magnitude of emissions from natural sources is not known but, based on concentrations in rural areas, it is believed to be generally low in comparison with anthropogenic sources (Government of Canada, 1993).
It has been estimated that in 1985, 34 150 tonnes of benzene were released into the atmosphere in Canada. Major sources were combustion of gasoline and combustion of diesel fuels, which together acounted for 76% of total atmospheric releases. Light-duty vehicles accounted for 61% of total releases. Other sources of release to the atmosphere included emissions during benzene production (6.5% of total releases); other chemical production (7.7%); primary iron and steel production (1.0%); solvent uses (1.5%); residential fuel combustion (4.1%; and gasoline marketing (1.9%). Total emissions of benzene into the atmosphere are expected to decline in the future, primarily because of the planned reduction of emissions of volatile organic compounds from light-duty vehicles and the efforts to reduce volatile organic compound emissions from a variety of other sources in order to control ground-level ozone (Government of Canada, 1993).
Benzene can enter soil from oil and gasoline spills, leaking underground storage tanks, and seepage from waste disposal sites. Contamination of surface water may result from spills of chemicals and petroleum products and from discharges of industrial and municipal effluents. Estimates of total environmental loadings from such sources in Canada are not available (Government of Canada, 1993).
It is estimated that every year in Canada, 34 kilotonnes of benzene are released into the atmosphere, 1 kilotonne into water, and 0.2 kilotonnes into soil (Government of Canada, 1993).
In the bacterial degradation of benzene, the aromatic hydrocarbons are oxidized to catechol (orthodihydroxybenzene) or its derivatives by a dioxygenase and a dehydrogenase. After the initial oxidation of the benzene ring, further degradation occurs via enzymatic pathways where the ring is opened between the two hydroxyl substituents (ortho cleavage) or adjacent to them (meta cleavage). The resulting chain compounds are converted into small, potentially useful, metabolites (Schwarzenbach, Gschwend, and Imboden 1993).
Equation 3
C6H6 ---------- C6H4(OH)2 ---------- (META OR ORTHO: SEE BELOW)
ORTHO PATHWAY:
C6H4(OH)2 ---------- succinate + acetate
META PATHWAY:
C6H4(OH)2 ---------- acetaldehyde + pyruvate
The rate of biodegradation is determined by the rate of the microbial population growth. Environmental factors such as temperature, pH, ionic strength, and oxygen concentration affect the composition, growth rate, and enzymatic processes of the microbial community. They not only influence the rate at which biologically mediated transformations will occur, but in fact can dictate whether these processes even occur. In environments where optimal conditions do not exist, such as low oxygen concentrations, the presence of benzene is much more persistent (Schwarzenbach, Gschwend, and Imboden 1993).
Environment Canada. (1990). National Inventory of Sources and Emissions of Benzene (1985). Report EPS 5/AP/1, Minister of Supply and Services Canada. pp. 1-29
Environment Canada. (1984). Enviro Technical Information for Problem Spills--Benzene. Minister of Supply and Services Canada, Toronto. pp. 9-11.
Chao, J. (1978). Key Chemicals Data Books: Benzene. Thermodynamics Research Center, Texas. pp.14.
Government of Canada. (1993). Canadian Environmental Protection Act: Priority Substances List Assessment Report--Benzene. Minister of Supply and Services Canada, Toronto. pp. 4-15.
Morrison, R.T. and R.N. Boyd. (1973).Organic Chemistry. Allyn and Bacon Inc., Toronto. pp. 424, 426.
Schwarzenbach, R.P., P.M. Gschwend, and D.M. Imboden. (1993).Environmental Organic Chemistry. John Wiley & Sons. New York, pp. 485-497.
Weast, R.C. and D.R.Liche. 1989. CRC Handbook of Chemistry and Physics, CRC Press Inc., Florida. pp. D-214-216.
http://www.mcm.uc.edu/geology/crest.orsanco/oqm-cri1.htm, Jan. 29, 1997
http://www.hhmi.org/science/labsafe/lcss/lcss14.htm, Feb. 4, 1997
http://www.pegnet.com/MtBE2/mtbe_sl_TOC.htm, Jan, 29, 1997