Introduction 

Structure and Physical Properties 

     Anthracene (structure see Appendix A-1) is common in the natural environment. This chemical has several nicknames such as anthracin, green oil and paranaphthalene. It has a crystalline structure, and is pale yellow in color. It exhibits a weak aromatic odor. It is a combustible solid, with a flash point of 121.1 C (6). Anthracene is a relatively small PAH. Some larger PAHs can have nine or more rings. Some of the physical and chemical properties of anthracene are listed in Table 1. 

     Phenanthrene is an isomer of anthracene. Instead of three rings in a line, as in the latter, phenanthrene has one of its rings removed from the line (structure see Appendix A-1). As a result, many of the physical properties of the two are very similar (see Table 1). The major differences lie in the melting point and the properties directly related to solubility. Phenanthrene is purified as brown to white monoclinic crystals, and also has the characteristic faint aromatic smell. 
 
 
 

       Benzo[a]pyrene (3,4-benzpyrene) is the largest of the four compounds, with five rings (see Appendix A-1). It also has the faint aromatic odor. Pure benzo[a]pyrene is pale yellow, and is found as monoclinic or orthorhombic crystals (The Merck Index 1989). These can be separated from a mixture of PAHs using various standard separation techniques, and recrystalized from benzene and methanol. Benzo[a]pyrene is the only one of the four compounds that is a known human carcinogen. Some properties of benzo[a]pyrene are found in Table1. 

Commercial Uses 

     Naphthalene is the most abundant distillate of coal tar. Its most common use is as a household fumigant against moths (hence the name mothballs). Naphthalene is an important hydrocarbon raw material used in the manufacture of pthalic anhydride ( used in dye making), of celluloid and hydronapthalenes (used in lubricants), and of motor fuels. At one time, it was used as an insecticide and vermicide. However, this use is decreasing due to the low toxicity of the vapor. Naphthalene is also of some use as an antiseptic and as a soil fumigant (4). Commercial production of this chemical is a result of crystallization from the intermediate fraction of condensed coal and tar from the heavier fraction of cracked petroleum (4). The demand for naphthalene in the United States in 1986 was 250 million pounds, in 1987 255 million pounds, and in 1991 approximately 270 million pounds (20). 

     Anthracene, like many PAHs is used in the production of fast dyes as well as fibers and plastics. It is one of the most important feedstocks for the production of anthraquinone. Anthracene can also be used in insecticides and for a wood preservative (17). This compound is widely abundant. It is found in any type of coal or tar. It is possible to isolate PAHs from roofing material via flash chromatography. Flash chromatography is used for purification of organic as well as inorganic compounds. It is a phase separation between a stationary phase and a rapid organic solvent. The process takes less that 15 minutes to complete once the solvent is determined (16). It is also relatively inexpensive. 

     Like anthracene, phenanthrene is used in the production of dyes. It is also used in the manufacture of explosives, and is an important starting material for phenanthrene based drugs. This leads directly to use in biochemical research for the pharmaceutical industry. In New York State, a mixture of phenanthrene and anthracene tar is used to coat water storage tanks to prevent rust (17). 

     Benzo[a]pyrene, or B[a]p is not produced commercially in the US. B[a]p is not allowed for commercial use in Canada and US (15). Thus large scale manufacturing processes are not available. It is formed when gasoline, garbage or any plant or animal materials are burned. So B[a]p is usually present in soot and smoke. It is also found in the coal tar pitch industry, and is used to join electrical parts together. It is also found in creosote, a chemical used as a preservative for wood. B[a]p is used extensively as a positive control in a variety of laboratory mutagenic and carcinogenic short-term tests. 

Entry into the Environment 

     Naphthalene enters into the atmosphere through emissions and exhaust produced by its presence in fuel oil and gasoline. Another major contributor of naphthalene (in the atmosphere) is the aluminum smelting industry (4). It accounted for about 21% of the total atmospheric emissions measured in Canada in 1991 (4). Entry into the soil and water compartments arises mainly due to discharges and spills during the storage, transportation and disposal of fuel oil and coal tar (20). The Toxic Release Inventory database reports that 39,024,998 pounds of production-related naphthalene wastes were generated for the year 1995, and a total of 2,659,941pounds of naphthalene released in the United States, mostly to the air. 

     Anthracene is released into the environment from a variety of sources. It can be released via non-point sources as well as industrial effluents from the dye and pesticide manufacture. As seen in the Toxic Release Inventory most of the anthracene released was to the air. According to this index in 1995 a total of 5488 pounds of anthracene was released from industry in the United States. Of that amount 5475 pounds was released to the air while the remaining was released to ground and water. The total waste produced from the production of anthracene was 22938 pounds in 1995 (12). Its natural sources are coal and tar, and can be released by the incomplete combustion of organic compounds. Therefore it is a constituent of the exhaust from automobiles, lawn mowers, and charcoal grills. It can also seep into the ground and surface water from coal piles (6). 

     There are three major industries that produce phenanthrene are: 

• Petroleum refining and petroleum related products
• Steel
• Carbon and graphite products

     The largest sources of B[a]p in the air are open burning and home heating with wood and coal. Factories that produce coal tar also contribute small amounts of B[a]p to the air. It is estimated that greater than 100 metric tons per year (18) of the total emissions of B[a]p to the atmosphere are a result of coal fired residential furnaces, coke production, forest fires, and burning coal refuse banks. More than 10 metric tons of B[a]p is released from coal-fired industrial boilers, residential fireplaces, iron and steel sintering, commercial incinerators, open burning of auto components and leaves, trucks and automobiles, and tire wear (18). The toxic release inventory does not list B[a]p probably because it is not produced in the United States. 

Human Health Effects and Toxicity 

     There is little data available for the toxic effects of naphthalene on humans. Short-term low exposure to naphthalene may cause eye and skin irritation (3). At slightly higher levels (above 10 ppm), headaches, fatigue and nausea occur. If naphthalene is ingested it has the potential to cause hemolytic anemia, a condition that involves the breakdown of red blood cells (3). Naphthalene poses more of a threat to African-Americans and people of Mediterranean descent due the higher incidence of problems with the enzymes that produce red blood cells (11). Acute effects can be detected 2 to 4 days after exposure. Long term effects of exposure to levels of naphthalene greater than 10 ppm may lead to chronic effects. Naphthalene is a suspected human carcinogen, and has been proven to cause damage to the kidneys and to the liver. Chronic exposure can lead to reproductive defects including fetal damage and decreasing fertility. Higher incidences of lung and skin tumors have been reported for people who have been occupationally exposed to naphthalene and other PAHs (4). 

     Several studies have been conducted using animal subjects. Rabbits exposed to high levels of naphthalene developed cataracts (3). A case study involving marine invertebrates found that short-term exposure to naphthalene proved to be moderately toxic (3). Acute toxic effects can include low growth or even death in birds, animals, fish and plants. Toxic thresholds for naphthalene include a LC50 of 240ug/L for embryos of largemouth bass (4). 

     Naphthalene is listed by RCRA as a toxic hazardous waste (U165), however it is not currently regulated in drinking water. It is on the list of proposed additions. The Code of Federal Regulations (CFR) lists naphthalene as a constituent of a number of specific and non-specific hazardous wastes. Among these are F025, F034, K087, and K145. The Occupational Safety and Health Administration (OSHA) has issued regulations regarding permissible exposure limits for naphthalene. These limits are: 10 ppm time weighted average and 15 ppm as short term exposure limits (11). Naphthalene is also regulated under the Clean Air Act (CAA), the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), the Federal Insecticide, the Fungicide, and Rodenticide Act, the Clean Water Act, the Emergency Planning and Community Right-to-Know Act of 1986 (EPCRA) and the Resource Conservation and Recovery Act (RCRA) (11). 

     Anthracene exposure can cause skin and eye irritation, which can be aggravated by sunlight. Repeated exposure may cause alteration of skin pigments as well as cancerous growth, although there is no carcinogenic data for anthracene. Inhaled anthracene can cause bronchitis like symptoms (6). There is limited information on human reproductive implications. Anthracene may cause genetic mutations in cells (6). 

     Anthracene is not currently considered a toxic substance, and does not appear on the RCRA hazardous waste lists. It is also not specified as a constituent of specific or non-specific industrial wastes by the CFR. Additionally, there is no drinking water limit for anthracene. 

     Phenanthrene is also a skin and eye irritant, with increasing effects in sunlight due to photosensitization. There is currently no data available for human oral and inhalation exposure. It is however, a suspected carcinogen, although there is no data for humans. There have been several studies of carcinogenic effects in animals. Out of fourteen studies listed by the Integrated Risk Information System (IRIS), only one yielded a positive result. A group of 30 mice were given a single dermal application of 1.8 mg in benzene, followed by twice weekly applications of tetradecanoylphorbol acetate (TPA, a promoter ) for 35 weeks (Scribner, 1973). They found a 40% tumor incidence compared to 0% in the control group. There are some uncertainties in this study. It is unclear whether the TPA dose was 3mg total or 3mg/ application. Also it was unclear whether the control group received benzene. This is particularly noteworthy since benzene is a known human carcinogen. 

     Phenanthrene is not currently listed by RCRA as hazardous, and is not listed in the CFR as a constituent of any specific or non-specific industrial processes. There is also no drinking water standard for phenanthrene, and it can be land disposed. However, much of the recoverable phenanthrene is either used for on-site energy production (incineration) or recycled by simple separation and purification techniques. 

     The general population may be exposed to dust, soil, and other particles that contain B[a]p. Many water supplies throughout the United States contain low levels (.55ppb) of the compound (18). Food grown in contaminated soil or air may contain B[a]p. B[a]p has been found in various types of cereals, vegetables, fruits, and meats. High temperature cooking processes, like charcoal grilling or charring, can increase the amount of B[a]p found in food. It is also a component of tobacco products, and is one of the cancer causing agents in cigarette smoke. 

     The greatest chance of high level exposure to B[a]p is likely to occur in the workplace. People who work in coal tar production plants, coking plants, asphalt production plants, coal gasification sites, and smoke houses receive higher doses than the general population. 

     B[a]p is a known precursor to cancer causing metabolites in laboratory animals (Casarett 1995). Various studies have determined that B[a]p is toxic (8). B[a]p is converted by cytochrome P450 to a variety of oxides that react with DNA, making them highly mutagenic. The U.S. Department of Health and Human Services has determined that B[a]p may reasonably be anticipated to be a carcinogen to humans as well as animals. It is reported that mice fed high levels of B[a]p during pregnancy had trouble reproducing as did their offspring (1). The newborn animals of pregnant mice fed B[a]p had other harmful effects including birth defects and low body weight. It is possible that similar effects could happen to humans exposed to B[a]p. 

     Even though B[a]p is a known carcinogen, OSHA does not ban its use in the workplace. They set rules governing the exposure limits of employees. B[a]p is regulated along with substances known as "coal tar pitch volatiles." Exposure is limited to two tenths of a milligram of coal tar pitch volatiles per cubic millimeter of air (11). The Institute for Occupational Safety and Health (NIOSH) does not have a specific workplace limit for B[a]p, either. 

Environmental Fate  

     A mixture of PAHs released in the water would be expected to sorb to particulate matter and dissolved organic matter. The degree of sorption depends on the mechanism, which is determined by the identity of the sorbate. They would also tend to bioaccumulate in aquatic organisms, due to their relative non-polarity. The bioaccumulation is only a problem for organisms that lack microsomal oxidase (17). This enzyme allows for the breakdown of PAHs to more water-soluble products for excretion. The fraction not sorbed would be subject to photolysis and subsequent breakdown. The reported half-lives for aquatic fate vary due to the nature of the system (17). This includes stratification of the water column, flow of water, quantity of natural organic matter and many other characteristics. 

     Volatilization biodegradation, and adsorption to particulate matter and sediment are all processes, which affect the fate of naphthalene in the water compartment. When a petroleum product enters into the water column, the lighter hydrocarbons spread out along the surface of the water and then evaporate. The volatilization half-life of naphthalene is .4 – 3.2 hours (4). The naphthalene that does not evaporate sorbs to the particulate matter or is transformed into a water in oil emulsion. 

     Again, volatilization plays an important role in the fate of naphthalene in the soil. Degradation through microbial activity also occurs but the extent is dependent upon the temperature, the soil type, and the presence of the proper organisms. The low molecular weight PAHs, like naphthalene, are expected to volatize or biodegrade within 3 to 4 months (4). 

     Anthracene released to the ground would sorb to the soil. It would be a minimal problem for groundwater contamination. Anthracene would be subject to biodegradation in the ground. It would not hydrolyze due to the lack of functional groups. The half-life for biodegradation of terrestrial released anthracene is about 100 to 150 days (17). The strong adsorption to particulate matter makes it less bioavailable. 

     Most of the anthracene released to the atmosphere would volatilize or sorb to particulate matter. It would be subject to photolysis immediately. It has a very short half-life associated with atmospheric fate. It is on the order of hours to days depending on the degree of adsorption. Non-adsorbed compound will have a shorter half-life than adsorbed compound. It is broken down via ozonation and hydroxide radical photolysis (17). 

     If a release of phenanthrene were to occur to the soil, most of the compound would sorb to the soil. It is not expected to volatilize to a great extent. Biodegredation is expected to be the major removal porcess from the soil, given the right conditions. The low solubility of phenanthrene would limit removal by leaching to ground water. 

     An atmospheric release of phenanthrene would also lead mostly to sorption. Photolysis occurs rapidly with a half-life of about 49 hours. Phenanthrene not converted by photolysis would be subject to wet and dry deposition. The small amount of the compound that is not sorbed would be found in the vapor phase, where it can react with photochemically produced hydroxyl radicals with a half-life in days (19). 

     A release of phenanthrene to the water would also be governed by sorption. Most of the compound released should be removed from the water by sorbing to the particulate matter and dissolved organic matter. The degree of sorption depends upon the sorption mechanism, which depends on the type of sorbant. 

     B[a]p released to the atmosphere would likely be associated with particulate matter and may be subject to moderately long distance transport. The degree of transport depends on the particle size distribution and climactic conditions, which determines the rate of wet and dry deposition. B[a]p released to soil would absorb very strongly to the soil and would not leach to the groundwater. It would not hydrolyze, and evaporation from soils and surfaces may not be significant due to its higher molecular weight. B[a]p released to water, also would absorb very strongly to sediments and particular matter. It would not hydrolyze, but would be expected to bio-concentrate in aquatic organisms without metabolism. B[a]p would undergo significant photo-degradation near the surface of waters. Adsorption to sediments and particulates may significantly retard biodegradation, photo-degradation, and evaporation (18). 

     Biodegradation tests of B[a]p in soils have resulted in a wide range of reported half-lives: 2 days (Mackay 1992) to 1.9 year (Mackay 1992). Based on these values and the apparent lack of a significant competing fate process, biodegradation may be an important process in soils. B[a]p also has been shown to be susceptible to metabolic activity in some natural waters lacking carbon or another energy source (18). However, in most waters and in sediments it has been shown to be stable towards biodegradation. 

Analysis of Exams Data 

     The results from Tables 15 and 17 show the distribution of the chemicals at steady state in the pond environment with a stream load of 0.0001 kilograms per hour. Under these conditions the majority ( > 75%) of all four chemicals released into the pond would end up in the benthic sediment. Naphthalene has a relatively high aqueous solubility in comparison to the other compounds; therefore, almost 25% of the quantity released to the pond remained dissolved in the water column. With the increased size of the compound there is a decreased solubility. The trend for the distribution in the sediment can also be seen with the increased log of K_ow for each compound as well as the increasing molecular weight. K_ow is inversely proportional to the aqueous solubility. As the K_ow increases its tendency to adsorb to particulate matter increases. It is a characteristic of PAHs to bind tightly to sediments. These compounds are do not have polar functional groups to allow them to dissolve in water. Larger compounds have more surface area to bind to the particulate matter, so these compounds would end up in the sediment. As seen in both tables the increasing molecular weight gives a higher percentage of compounds being located in the benthic sediment. 

     Most of the PAHs that remain in the water column end up dissolved. In the case of naphthalene, anthracene and phenanthrene, over 95% of the quantity remains dissolved. Benzo(a)pyrene has a lower dissolution percentage due to its reduced aqueous solubility and elevated K_ow. A larger fraction of the B[a]p is sorbed to the sediment or bio-accumulated in the biota. The trend is increasing size decreases the fraction of PAH that will be found in solution. 

     Most of the PAHs that remain in the benthic sediment are sorbed to the sediment. The amount that ends up dissolved or in biota is minute. The trend is increasing size increases the fraction of PAH sorbed to the sediment. Naphthalene is the only one of our compounds that has a significant dissolved fraction. Once again this is due to its small size and higher aqueous solubility. 

     Table 18 gives an analysis of the steady state fate of the organic chemical. The model reports PAHs are subjected to either surface water-borne export or volatilization. The amount, which is volatized, is increased as the size of the compound decreases. Naphthalene, the smallest compound, experiences volatilization of approximately 89% of the load whereas B[a]P, the largest compound, experiences volatilization of roughly only 1% of the load. The bent isomer of anthracene (phenanthrene) experiences slightly more volatilization when compared to the straight chain version. This is possibly due to a lower heat of vaporization in anthracene. The Van der Waals forces are slightly weaker in phenanthrene, and causes an increase in the vapor pressure by about two orders of magnitude. As the vapor pressure increases, the amount volatilized also increases. Conversely, as the amount of the load that undergoes surface water-borne export increases, the size of the compound also increases. When analyzing the results for the isomers, it can been seen that phenanthrene experiences half as much surface water export in comparison to anthracene. 

     Table 20 in the Exams model provides an exposure analysis summary. It summarizes the exposure and fate data found in Tables 15 and 17. It also provides an estimate of the persistence of each compound in the environment and the length of time to achieve greater than 95% cleanup. The clean-up times of the four compounds are: 

• Naphthalene – 8 months
• Phenanthrene – 30 months
• Anthrecene – 53 months
• B[a]P – The model indicated that after 84 years, there was no loss of the initial chemical burden. Therefore, there is little chance to achieve total clean-up

Conclusions 

     Size plays a major role in the fate of PAHs. It affects the most important fate parameters like solubility, vapor pressure, K_H, and Kow. Although none of the PAHs dissolve to a great extent, increasing size of the compound decreases the aqueous solubility. Larger molecules encounter a greater difficulty in being totally associated with water molecules. This is indeed the trend observed in the model. Increasing size increases the sorption. Larger molecules have more surface area to sorb to the sediment or particulate matter. We also observed this trend in PAHs. Finally, increasing size decreases the amount of volatilization. Larger molecules are more difficult to vaporize, due to stronger intermolecular forces between the PAH molecules. 

     We did however noticed an anomaly in the general trend. We analyzed our compounds with the assumption that the trends would follow the following order: naphthalene, anthracene, phenanthrene, and B[a]p. According to the EXAMS model, the fate of phenanthrene was more similar to naphthalene than anthracene. This could be due to differences in the amount of intermolecular interactions in the two isomers. 

Appendix A-1 Structures 
 

NAPHTHALENE	ANTHRACENE

PHENANTHRENE	BENZO[A]PYRENE

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