Environmental Persistence of Benzene

Natural sources of benzene include gas emissions from wild fires and volcanoes, yet benzene is most notable as a constituent of crude oil. Benzene quickly dissipates in the environment, and is generally only found in high concentrations after an industrial accident or chemical spill. Benzene can readily pass from soil or water into the air, and can then reenter the soil or aquatic ecosystems as a residue in snow or rainwater. In the absence of water, benzene volatilization from soil is not rapid. When uniformly distributed at 1 and 10 cm depths, volatilization half-lives of benzene from dry soil were 7 and 38 days, respectively.

The soil organic carbon sorption coefficient (Koc) ranges from 60 to 83, which helps to explain why benzene is so mobile in hydrated soil, as the compound will rapidly leach into groundwater. Conversely, benzene released into water would not be expected to adsorb to sediment or suspended solids to any significant degree. When deposited directly onto soil, the composition of soil can have a direct impact on benzene sorption, as soils with increasing organic matter possess increasing affinity for the compound. The low density of benzene (0.878 7 gcm~2 at 15 °C) would facilitate its migration to the air interface with water, enhancing volatilization at the surface. The half-life of benzene in surface water (25 °C at 1m depth) is estimated to be 4.8 h based on evaporative loss, while half-lives in groundwater can reach one year. When examined in a model river system that is 1 m deep, with a flow rate of 1 ms-1 and a wind velocity of 3 ms-1, the half-life of benzene due to volatilization at 25 °C was reduced to 2.7 h. Other estimates of volatilization half-lives for a model river and model lake are 1 h and 3.5 days, respectively.

Benzene is chemically stable in soil and water, yet oxi-dative degradation of benzene occurs within a few hours to days after volatilization into the air. Atmospheric hydro-xyl radical attack represents the most significant process involved in environmental degradation ofairborne benzene. In addition, oxidation by nitrate radicals and ozone are known to occur at a low level. Since benzene does not absorb light of wavelengths above 290 nm, direct photooxidation of benzene is unlikely. However, photochemically produced hydroxyl radicals will react with benzene, giving a calculated atmospheric half-life of approximately 13 days. Acceleration of benzene degradation can occur in polluted air by interaction with nitrogen oxides and sulfur dioxide (both present in smog), dramatically reducing the half-life of benzene to 4-6 h. Atmospheric degradation of benzene can lead to the formation of phenol, nitrophenol, nitrobenzene, formic acid, and peroxyacetyl nitrate.

Although benzene is quite stable in purified water, a slow process of benzene degradation, known as indirect photolysis, will occur at water-soil interfaces. This process involves energy transfer from activated humic and fulvic acids, which are ubiquitous, primary constituents of soils in aquatic areas. These acids act as photosensitizers by indirectly generated singlet oxygen or hydroxyl radicals which react with benzene. Benzene that remains in soil and is not photochemically oxidized can be available for biodegradation, a process which is maximum at low benzene concentrations (near 1 ppm).

Of particular ecological significance is the microbial degradation of low levels of benzene by several species of aerobic bacteria, through nitrogen- and oxygen-enhanced oxidative processes. Environmental factors which affect biodegradation of benzene include the presence of specific microbial populations and their nutritional sources, temperature, pH, and levels of dissolved oxygen. In addition, the concentration of benzene itself can affect its biodegradation. At concentrations above 2 ppm, biodegradation of benzene is not observed, presumably due to toxicity to the microbes themselves. The highest rates of biodegradation occur at concentrations below 1 ppm, where decay rates of 20-50% per day have been reported. Microbial metabolism of benzene occurs through formation of cis-dihydrodiols, and further transformation to catechols, which are susceptible to ring opening. Soil bacteria which have been reported to degrade benzene under aerobic conditions include Pseudomonas species, Nocardia species, and Nitrosomonas europaea. Benzene is not readily biodegraded under anaerobic conditions, since low oxygen levels make it necessary to use an alternative electron acceptor (which could include nitrate, carbonate, or ferric iron). Sufficient quantities of the alternative acceptor species would be necessary for substantial anaerobic oxidation. Microbial degradation of benzene would also depend on the metabolic capacity of the bacterial community. Those bacteria containing monohydroxylases will cause ring hydroxylation of benzene to phenol, while dioxygenases will catalyze formation of pyrocatechol and hydroquinone. Controlled experiments have demonstrated that aqueous (soil-free) cultures of bacteria can begin to degrade benzene within 12 h, with a half-life of approximately 60 h and almost complete degradation by 90 h.

In most environmental contaminations, several aromatic hydrocarbons are present. Benzene metabolism can be altered by the presence of other aromatic hydrocarbons. For example, low concentrations of some aromatic compounds, such as xylene, may induce the expression of mixed function oxidases in some organisms and increase benzene metabolism. Other aromatic compounds, such as toluene, may compete for metabolic processes and reduce benzene metabolism. Since metabolic activation of benzene is necessary for its toxicity, compounds which compete for oxidative metabolism, such as toluene, may inadvertently reduce the toxicity of benzene by reducing formation of reactive metabolites.

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