Environmental Life Cycle Costing - Chapter 11 pptx

Petroleum hydrocarbon components are divided into three major classes related to their chemical structure: the alkanes, the alkenes, and the aromatics.2 These compounds enter the environ

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Volatile Organic Compounds

11.1 INTRODUCTION

Volatile organic compounds (VOCs) are those organic compounds with a boiling-point range from 50–100 to 240–260°C (WHO).1 They are composed of a large number of major air pollutants emitted from both industrial and nonindustrial facil-ities Chemically, VOCs include both aliphatic and aromatic hydrocarbons, haloge-nated hydrocarbons, some alcohols, esters, and aldehydes Table 11.1 shows several examples of this group of compounds The importance of VOCs as a class of air pollutants is clear The U.S Environmental Protection Agency has designated them

as one of the six “Criteria Air Pollutants.” In this chapter, we will discuss the sources, characteristics, and health effects of some VOCs

11.2 SOURCES

Both natural and anthropogenic sources contribute to VOC emissions Natural sources of VOCs include petroleum, forest fires, and the transformation of biogenic precursors Main anthropogenic sources include high-temperature combustion of fuels, emissions from crude and refined oil, municipal incineration, burning of crops before or after harvesting as an agricultural practice, emissions from power boats, and others A recent survey by the EPA showed indoor pollution as an important source of VOCs More than 500 VOCs were detected, most only once, in a survey involving 10 buildings The study showed that a variety of VOCs originated from common items such as building materials, cleaning solvents, furnishings, and pes-ticides Since indoor concentrations for all compounds except benzene exceeded outdoor levels, it was concluded that indoor sources contributed to the observed results The importance of indoor VOC pollution in health has been recognized in World Health Organization publications.1

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140 ENVIRONMENTAL TOXICOLOGY

11.3 PETROLEUM HYDROCARBONS

Petroleum is a complex mixture of hydrocarbons, with a characteristic chemical composition and specific physical properties, depending on the geological and geo-graphical origin of the crude oil and the nature of the cracking process used during refining Petroleum hydrocarbon components are divided into three major classes related to their chemical structure: the alkanes, the alkenes, and the aromatics.2 These compounds enter the environment as a result of stationary and mobile sources, and comprise a significant portion of the contaminant mixture found in ground and surface waters, coastal areas, and in the global atmosphere.3,4 The drilling, removal, processing, transportation, storage, and use of petroleum hydrocarbons involve sev-eral operations, during which losses of material, chemical conversions, and dis-charges of wastes can occur Total global emissions and disdis-charges of petroleum have been estimated at about 90 million tons.5

The specific chemical structure and mixtures of these three classes of petroleum hydrocarbons determine their chemical properties, such as solubility and volatility, persistence and resistance to rates of photochemical oxidation, microbial degrada-tion, and their biological toxicities in the environment (Table 11.2)

11.3.1.1 Properties and Use

The alkanes are chains of carbon atoms with attached hydrogen atoms They may

be simple, straight chains (n-normal), branched (iso-, sec-, tert-), or have a simple ring configuration (cyclo-) (Figure 11.1) Low-molecular-weight alkanes have low boiling points and are highly volatile They are slightly soluble in water but extremely

Table 11.1 Examples of Volatile Organic Compounds

Aliphatics Pentane, hexane, heptane, cyclohexane, octane, nonane,

eicosane, dodecane, 2,4-dimethylhexane Aromatic hydrocarbons Benzene, diethylbenzene, trimethylbenzenes,

dimethyl-ethylbenzene, toluene, xylenes, naphthalene, styrene Halogenated hydrocarbons Chloroform, dichloromethane, trichloroethylene,

tetrachloro-ethylene, dichlorobenzenes Alcohols 2-Butylalcohol, 1-dodecanol

Esters Ethyl acetate, 1-hexyl butanoate

Table 11.2 Comparison Between Aliphatics and PAHs Characteristics Aliphatics PAHs

Rate of degradation faster slower Persistence in tissue shorter longer Toxicity less toxic more toxic (some are

carcinogenic)

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VOLATILE ORGANIC COMPOUNDS 141

soluble in fats and oils The lipophilicity enhances their rapid penetration through membranes and into tissues High-molecular-weight alkanes are not soluble in water and are exclusively lipophilic Low-molecular-weight alkanes are used as solvents, degreasers, and as thinners and diluents of paints, enamels, varnishes and lacquers They are also used as extractants of organic compounds from plant and animal tissues, soils, and sediments, and in the production of aviation fuels and gasoline

11.3.1.2 Health Effects

Alkanes act primarily by solubilizing or emulsifying fats, mucous membranes, and cholesterols At low concentrations, alkanes are simple irritants As such, they can cause inflammation, redness, itching, and swelling of the skin, mucous mem-branes, nose, trachea, and bronchioles They also produce anesthesia and narcosis

in the central nervous system (CNS) At high concentrations, acute eczema of the skin and pulmonary edema may develop, as well as unconsciousness or death through asphyxiation by paralysis of the portion of the brain responsible for respiration Alkanes have also been found to penetrate rapidly into the fatty cells of the myelin sheath that surrounds the nerve fibers, where they dissolve the cells and cause degeneration of the axon, interrupting the transference of nerve impulses.6

Figure 11.1 Examples of some of the common components of crude petroleum.

Hydrocarbons

Aromatics

Poly- Naphthaeno- Di- Mono-Cycloalkanes

n-Alkanes

Isoalkanes (e.g.) Aliphatics

Decalin

Benzene

Naphthalene

Indane

Benzo[a]pyrene

CH 3 _ (CH 2 )n _ CH 3

CH 3 _ CH 2 _ CH 2 _ CH

CH

CH 3

3

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Alkanes of higher molecular weight are considered to be virtually nontoxic, though they may affect chemical communication and interfere with metabolic pro-cesses Many of the same high-molecular-weight alkanes are produced biogenically and have been found to occur naturally in marine organisms.2

Alkanes can be excreted, unaltered, by the lungs and can also be metabolized

by the oxidation of the terminal methyl group by molecular oxygen via the MFO system to produce an alcohol Repeated oxidation of the terminal carbon produces

an aldehyde and finally a carboxylic acid, which is broken down by β-oxidation to give rise to acetyl coenzyme A as the final product.6,7

In the atmosphere, low-molecular-weight alkanes react with the hydroxyl radical,

in a process in which a hydrogen atom is abstracted from the alkane to form an alkyl radical This radical adds molecular oxygen and in the presence of high concentrations of NO, forms atmospherically reactive NO2.8

11.3.2.1 Properties and Use

The alkenes are also chains of carbon atoms with attached hydrogen atoms, but the chains contain carbon–carbon double bonds and are considered unsaturated in relation to the total possible number of attached hydrogen atoms, compared to an alkane of similar carbon chain length The double bonds convey to the alkene a planar configuration that allows the formation of geometrical isomers (cis- and

trans-) Alkenes are generally more reactive than alkanes, but less reactive than aromatics They are not found in crude petroleum but are present in some refined products, specifically gasoline and aviation fuels Alkenes undergo addition reac-tions, forming potentially more toxic metabolites They can undergo polymerization

to create long polyethylene chains; oxidation reactions to form oxides that on hydrolysis can form glycols; and halogenation to form extremely toxic chlorinated and brominated hydrocarbon pesticides

11.3.2.2 Health Effects

In experimental animals the cis- isomers have been found to cause weakness, nausea, and vomiting from their adverse effects on the gastrointestinal tract, and tremor and cramps due to their effects on the CNS.6

The aromatic hydrocarbons have a basic six-carbon ring configuration with six hydrogen atoms and three double bonds, and are unsaturated in terms of attached hydrogen atoms The aromatic ring may occur in a single, unattached configuration

as in benzene; as two attached rings to form naphthalene; or as many attached rings, i.e., polycyclic aromatic hydrocarbons (PAHs) (Figure 11.1) The aromatic ring structures may also have substituted methyl and more complex alkyl side chains as

in the case of toluene, the xylenes, cumene, 2-methylnaphthalene, etc The

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tution of the hydrogen atoms with other compounds yields several distinct chemical species with varying degrees of polarity, lipophilicity, persistence, and toxicity.9,10 But it is the fundamental unit of ring structure, with the carbon–carbon resonance stabilized bonds of equal length and energy, that confers increased stability to these compounds, making them not only persistent, but some of the most acutely toxic and carcinogenic compounds in the environment.6,11

Benzene, toluene and the three isomers of xylene are among the most common monocyclic aromatic chemicals found in petroleum.12 They have low molecular weight, low water solubility, high volatility, and flammability compared to other alkanes, alkenes, and polyaromatic hydrocarbons, and have the same toxicological modes of action — narcosis.10,13 Their structures, stability, and ability to be both slightly hydrophilic and lipophilic, enhance their accessibility to more niches, spe-cies, biochemical pathways and sites of action That, in part, accounts for these compounds to be designated as priority pollutants by the EPA.10,12

11.3.3.1 Benzene

Benzene (bp 80.1°C) is chemically the most significant aromatic hydrocarbon because it is the starting material for the manufacture of numerous industrial and agricultural products.6,10,12 It has been in commercial use for over a century, and its toxic effects have been suspected for almost as long.10 Benzene is used as an intermediate in the synthesis of pharmaceuticals and other chemicals such as styrene, detergents, pesticides, and cyclohexane; as a degreasing and cleaning agent; as an antiknock fuel additive;14 as a solvent for extracting pesticides from tissues, soils, and sediments in research and industrial applications; as a thinner and diluent of paints, inks, and lacquers; and as a solvent in the rubber industry.12

In the atmosphere, benzene and more than 70 of its derivatives are present as a result of fossil fuel combustion and emissions from a variety of industrial processes The principal chain reactor is the OH· radical, which can either add to the aromatic ring or abstract a hydrogen atom from the side group A variety of aromatic alde-hydes, alcohols, and nitrates are produced, as well as products of ring cleavage These products have moderately high molecular weight and moderate solubility in water and can therefore be readily deposited on aerosol particle surfaces.8

The toxicological mode of action of benzene is narcosis, affecting the central nervous system At high concentrations, inhalation of air containing approximately

64 g/m3 of benzene can be fatal within a few minutes, and one tenth of that level can cause acute poisoning within an hour.6 Exposure causes skin irritation, fluid accumulation in the lungs (edema), excitation, depression, and may eventually lead

to respiratory failure and death At lower concentrations, benzene can cause blood abnormalities, lower white cell counts and bone marrow damage.6,15 These toxico-logical effects have been attributed specifically to the trans-benzene-1,2-oxide inter-mediate formed during eukaryotic oxidative degradation of benzene6 (Figure 11.2) During oxidation, the oxygen atom is incorporated directly into the ring, forming

an epoxide intermediate The epoxide, which is not immediately degraded, resides

in the cell structures and actively reacts with cell nucleophiles, damaging blood, lymph, and bone marrow cells, as well as affecting liver and kidney function.10 The

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epoxide is eventually converted to phenol by a slower, nonenzymatic, rearrangement process (Figure 11.2) that is finally eliminated from the body as its sulfate.6,9 Benzene

is of most concern, because of its known association with the development of leukemic cancer in humans.16

11.3.3.2 Toluene

Toluene (bp 110.6°C) is produced primarily as a precursor for the synthesis of other chemicals For example, 70% of the product is used for the synthesis of benzene, 15% for the manufacture of other chemicals, and 10% is used as a solvent for paints and as a gasoline additive.12,13 Toluene is used as one of the major substitutes for benzene due to the extreme hazards associated with benzene exposure.6

The toxicological mode of action for toluene is narcosis, producing skin irrita-tions at low concentrairrita-tions and, at higher levels, affecting blood cells, the liver, kidneys, and the CNS, through which it causes headaches, nausea, and impaired coordination.6 Compared with benzene, toluene is less water soluble and more lipophilic, causing greater concentrations of it to be more rapidly transported to the site of action, which increases its potential for toxic effects.17 However, while the methyl group increases toluene concentration and depressant effects at the site of action, the rapid enzymatic degradation of toluene immediately reduces the site concentration, mediating the potential toxicological effects and resulting in a lowered observed toxicity.3,17,18 The mechanism involved in moderating the effects is the rapid oxidation of the aliphatic methyl side-chain instead of the ring structure Benzyl alcohol and benzoate intermediates are formed that are conjugated to hippuric acid (about 70% of the dose is affected) and rapidly eliminated, with the remainder being expired from the lungs unchanged.6,9

11.3.3.3 The Xylenes

The xylenes: ortho, meta, and para (boiling points: 144.4°C, 139.1°C, and 138.3°C, successively) have also been used as replacements for benzene and toluene

in the production of resins, synthetic fabrics, plastics, and as gasoline additives, cleaners, solvents, and lacquers.6,11,13 As in the case of toluene and benzene, the xylenes act as narcotics on the CNS, causing headaches, impaired coordination, edema, and nausea at higher concentrations; and skin irritations, anemia, blood cell damage, and a decrease in blood platelets at lower, chronic exposure levels.6 In oxidative degradation, m-xylene and p-xylene are metabolized to m-toluates and p -toluates that are further oxidized by the meta pathway.6,19-21o-Xylene oxidation does occur, but by a modified co-metabolic pathway with toluene.19 Elimination of xylenes

Figure 11.2 Biotransformation of benzene.

%HQ]HQH

2

+ + (Q]\PDWLF

2+

3KHQRO 5HDUUDQJHPHQW

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is primarily through the excretion of metabolites in the form of methyl hippuric acid (95% of the absorbed dose), 1 to 2% as xylenol, and by exhalation of 3 to 5% as the unchanged solvent

The double methylation of the xylenes makes them virtually insoluble in water They are very lipophilic, with the potential for rapid transport to the site of action

As with toluene, the toxicity of the xylenes is mediated by the reduction in their water-soluble fraction concentrations and their rapid biodegradation The presence of the second methyl group on the benzene ring determines the number of enzymatic steps

in the xylene degradation process and the specific pathway, rate of degradation, and potential for bioaccumulation by its location at the ortho, meta, or para position.3,17-21

11.4 POLYCYCLIC AROMATIC HYDROCARBONS

Polycyclic or polynuclear aromatic hydrocarbons (PAHs) are a group of com-pounds composed of two or more fused aromatic rings in linear, angular, or cluster arrangements By definition, they consist solely of carbon and hydrogen.22 The EPA has focused on the 16 PAHs (Table 11.3) that are included on the list of 126 priority pollutants These were selected on the basis of toxicity, potential for human exposure, and frequency of occurrence at hazardous waste sites

The concern over PAHs is that many have been shown to be carcinogenic to animals, and substantial data exist incriminating them as carcinogenic to humans.23 Eight of the PAHs in Table 11.3 are classified as Group B2 compounds, or probable

Table 11.3 PAHs Identified as Priority Pollutants by

the EPA

Name Abbreviation

Carcinogenic Classification

a D: insufficient data are available to assess their carcino-genic potential; B2: probable human carcinogens.

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human carcinogens The remaining eight are classified as Group D compounds, which means that insufficient data are available to assess their carcinogenic potential No individual PAH has been classified as belonging to Group A, known human carcino-gens However, several complex mixtures from which PAHs have been identified are known human carcinogens, such as cigarette smoke, coal tar, and coke oven emissions

The predominant source of PAHs is the incomplete combustion of organic mate-rial The anthropogenic sources of PAHs can be divided into stationary and mobile categories of emission.22 Vehicular engines are the major contributors to the mobile emissions The stationary fraction encompasses a wide variety of combustion pro-cesses, including residential heating, aluminum production, coke manufacture, incin-eration,24 and power generation The amounts and types of PAHs produced by each

of these vary widely due to differences in fuel type and combustion conditions.22 Not all PAHs are the result of human activity Volcanic eruptions and forest and prairie fires are among the major sources of naturally produced PAHs In addition, there is some evidence that PAHs may also be formed by direct biosynthesis by microbes and plants.25

PAHs are ubiquitous and can occur in the air attached to dust particles, or in soil and sediments as solids PAHs have also been detected in food and water.23 As pure chemicals, PAHs generally exist as colorless, white, or pale yellow-green solids However, the physical and chemical characteristics of PAHs vary with their molec-ular weight.25 Resistance to oxidation and reduction tends to decrease with increasing molecular weight Vapor pressure and aqueous solubility decrease almost logarith-mically with increasing molecular weight As a consequence of these differences, they tend to be environmentally more stable because they are less amenable to microbial degradation.26

In general, the transport of PAHs can be outlined as follows: PAHs released to the atmosphere are subjected to short- and long-range transit and are removed via wet and dry deposition In surface water, PAHs can volatilize, photodegrade, oxidize, biodegrade, bind to particulates or accumulate in organisms In sediments, PAHs can biodegrade or accumulate in aquatic organisms PAHs in soils can biodegrade

or accumulate in plants; PAHs can enter the ground water and be transported within

an aquifer Figure 11.3 illustrates major routes involved in the transport

PAHs are widely distributed in the environment and have been detected in air, water, sediment, soil, food, and other consumer products, such as cosmetics and

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cigarettes As a result, humans are exposed to these chemicals as part of everyday living As stated previously, most of the direct releases of PAHs into the environment are to the atmosphere Important sources of PAHs in surface waters include depo-sition of airborne PAHs, municipal wastewater discharge, urban storm water runoff, and industrial discharges.26 Most of the PAHs in surface waters and soils are believed

to result from atmospheric deposition

Food groups that tend to have the highest levels of PAHs include charcoal-broiled

or smoked meats, leafy vegetables, grains, and vegetable fats and oils.27 The presence

of PAHs on leafy vegetables and grains is believed to be caused by atmospheric deposition and reflects local conditions in the growing area The average American

is estimated to consume between 1 and 5 µg/day of carcinogenic PAHs, with unprocessed grains and cooked meats as the greatest sources of these substances (carcinogenic being defined by Menzie et al.27 as Group B2 compounds) A person who consumes a heavy meat diet has the highest estimated potential dose, on the order of 6 to 12 µg/day A vegetarian diet can offer an elevated PAH intake of 3 to

9 µg/day compared with the average diet if it comprises leafy vegetables, such as lettuce and spinach, and unrefined grains

Using the EPA assumption of a respiration rate of 20 m3/day, the estimated potential dose ranges between 0.02 and 3 µg/day for inhalation by nonsmokers, with

a median value of 0.16 µg/day.27 Tobacco smoke can be a major source of airborne carcinogenic PAHs Mainstream smoke from unfiltered cigarettes may contain 0.1 to

Figure 11.3 Transport and fate of PAHs.

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0.25 µg/cigarette An individual who smokes a pack of unfiltered cigarettes a day

is estimated to inhale an additional 2 to 5 µg/day Indoor air levels associated with

tobacco smoke have been reported in the range of 3 to 29 ng/m3.27 The consequence

is that exposure to secondary cigarette smoke, as well as mainstream smoke, may

be implicated in adverse health effects

The potential dose of carcinogenic PAHs from drinking water (assuming an

average drinking water consumption of 2 L/day) ranges between 0.0002 and 0.12

µg/day, with a median value of 0.006 µg/day.26 Drinking water concentrations were

reported to range between 0.1 and 61.6 ng/L Most drinking water values fell between

1 and 10 ng/L

Carcinogenic PAHs are found in all surface soils.27 As anticipated, urban areas

have higher concentrations than do agricultural and forest soils Typical

concentra-tions of carcinogenic PAHs are in the range of 5 to 100 µg/kg; agricultural soils 10

to 100 µg/kg; and urban soils 600 to 3000 µg/kg Assuming an incidental ingestion

rate of 50 mg soil per day, the potential intake of carcinogenic PAHs for urban

populations ranges from 0.003 to 0.4 µg/day, with the median value being 0.06

µg/day Excluding occupational exposure routes, food may be the major source of

carcinogenic PAHs for nonsmokers Smokers of nonfiltered cigarettes may be

exposed to twice the concentration of carcinogenic PAHs

PAHs enter the human body quickly by all routes of exposure: inhalation,

ingestion, and dermal contact.23 The rate of absorption is increased when they are

present in oily mixtures PAHs are conveyed to all the tissues of the body containing

fat and tend to be stored mostly in the kidney and liver, with smaller amounts in the

spleen, adrenal glands, and ovaries Results from animal studies show that PAHs do

not tend to be stored in the body for prolonged periods and are usually excreted

within a matter of days

The lipophilicity of PAHs enables them to readily penetrate cellular membranes

Subsequent metabolism renders them more water soluble and thus more readily

removed from the body On the other hand, PAHs can also be converted to more

toxic or carcinogenic metabolites One factor that may influence the delicate balance

of toxification/detoxification is the site at which the chemically reactive metabolite

is formed Metabolism of PAHs occurs in all tissues.25 The extent of induction of

enzyme systems following exposure to xenobiotics is known to vary with tissues

For example, the liver is generally more inducible than the lung or skin

In mammals, the cytochrome P450 MFO system is responsible for initiating the

metabolism of xenobiotics As discussed in Chapter 6, the primary function of this

system appears to be to render lipophilic compounds more water soluble Although

this system effectively detoxifies certain xenobiotics, others, such as PAHs, are

transformed into intermediates that are highly toxic, mutagenic, or carcinogenic to

the host For example, oxidative metabolism of benzo[a]pyrene by the MFO system

converts it into a dihydroxy epoxide believed to be a carcinogen that can interact

with DNA (see Chapter 15)

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VOLATILE... para position.3,1 7-2 1

11. 4 POLYCYCLIC AROMATIC HYDROCARBONS

Polycyclic or polynuclear aromatic hydrocarbons (PAHs) are a group of com-pounds composed of two... are available to assess their carcino-genic potential; B2: probable human carcinogens.

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