Petroleum Coal Tar and Related Products Richard W Niemeier PhD


In 1619, Dudley discovered that when certain (bituminous) coals wre heated in the absence of oxygen, referred to as "charking," "coak" was produced, which could be substituted for wood charcoal. By 1800, commercial products included coke-oven gases ("illuminating gas"), a low-boiling distillate known as coal naphthas (light oil of tar, also known as light liquid naphtha or benzine), and a "buttery-solid" condensation product containing carbon, "naphthaline," paranaphthaline or anthracene, paraffin, chrysene, pyrene, phenanthrene, fluorene, and biphenyl, called coal tar. Further fractional distillation of coal tar yields coal tar creosote "oil" (containing phenols, cresols, and xylenols, in contrast to pharmaceutical creosote, which contains guiacol, cresol, phenol, and xylenol from beechwood tar), coal tar pitch, and naphthalene (1). Because coal tar, coal tar pitch, and coal tar pitch volatiles (CTPV) may occur together, they are combined in this review.

The chemical composition of coal tar, coal tar pitch, and related materials is complex and variable. The estimated number of compounds present in these complex mixtures is in the thousands. Because of variation in source materials and manufacturing processes, including different temperatures and times of carbonization, no two coal tars or pitches are chemically identical, and their toxicity may differ with their origin (2). In general, however, approximately 80% of the total carbon present in coal tars exists in aromatic form (3).

Benzo[a]pyrene (B[a]P) is probably the most potent, widespread occupational carcinogen in coal tar, coal tar pitch and its volatiles, coke oven emissions, and creosote, all of which have corresponding work exposure standards (4); however, there is no occupational workplace standard for B(a)P. It may account for more than 75% of the carcinogenic activity of coal tar pitch fume condensate (5). Individuals who work in tarring facilities, roofing operations, power plants, and asphalt and coke manufacturing facilities may be exposed to benzo[a]pyrene and related PAHs. These mixtures may differ qualitatively and quantitatively.

Coal tar is completely or nearly completely soluble in benzene and nitrobenzene and it is partially soluble in acetone, carbon disulfide, chloroform, diethyl ether, ethanol, methanol, petroleum ether, hexane, and sodium hydroxide solution, and slightly soluble in water. It has a characteristic naphthalene-like odor. Coal tar is heavier than water and on ignition it burns with a reddish, luminous, and very sooty flame. Coal tar fumes are highly flammable and are easily ignited by heat, sparks, or flames. Vapors are heavier than air. They may travel to a source of ignition and flash back and may form explosive mixtures with air. Vapors will spread along ground and collect in low or confined areas (sewers, basements, tanks). Vapor explosion is a potential hazard indoors, outdoors, or in sewers. Some may polymerize explosively when heated or involved in a fire. Runoff to a sewer may create a fire or explosion hazard. Containers may explode when heated. Coal tar may be transported hot (3).

The greatest complexity occurs when toxicity is based on the effects of a class of compounds or of a material of a certain physical description. Some polynuclear aromatic hydrocarbons (PNA) and polycyclic aromatic hydrocarbons (PAHs) are carcinogens of varying potency, and they usually exist in mixtures with other PNAs/PAHs and with compounds (activators, promoters, inhibitors) that modify their activity. Analysis of each individual compound is very difficult and when done does not yield a clear answer. Given the complexity of the mixture of biologically active agents and their interactions, a calculated equivalent dose would have little accuracy. In these instances, it is common to measure some quantity related to the active agents and to base the occupational exposure limit on that index. An occupational exposure limit for PNAs has been based on the total weight of benzene-or hexane-soluble airborne material (6). This limit may be appropriate for coal tar pitch volatiles for which it was developed, but it may not work for other PNA/PAH containing materials. Crude oil, asphalt fumes, and cracked petroleum stocks may contain PNA/PAH. The coal dust particles mixed in with coal tar pitch volatiles are not soluble in benzene, but almost all of the petroleum-derived materials admixed with PNAs/PAHs are soluble in benzene. For example, a heavy aromatic naphtha may or may not contain PNAs/PAHs depending on the manufacturing process but is completely soluble in benzene. Thus a measurement of the benzene-soluble fraction of a heavy aromatic naphtha aerosol will reveal nothing about the PNA/PAH content. Alternate indices include the single carcinogen B(a)P, the sum of a subset of six carcinogenic PNAs (benz[a]anthracene, benzo[b] fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthen, benzo[a]pyrene, and benzo[e]pyrene) or 14 or more individual PNAs (7, 8). The NIOSH Manual of Analytical Methods (6) contains numerous methods for coal tar pitch volatiles (#5042), coal tar naphtha (#1550), benzene, cresols, PNAs, and PAHs (6).

Some of the highest measured levels of coal tar pitch volatiles (CTPV) have occurred in the aluminum reduction industry, especially in the Soderberg process potrooms where concentrations as high as 63 mg/m have been reported (9). That same review mentioned that the highest level reported in a roofing operation using coal tar pitch was

review of coke oven plant workers exposed to CTPV stated that one category of worker, the lidman, had the highest

exposure, a range up to 18 mg/m , and an average exposure of 3.2 mg/m . The NIOSH Immediately Dangerous to Life and Health (IDLH) value has been revised for CTPV from 700 mg/m to 80 mg/m based on toxicity data in animals (11-14).

The problem of differentiating the several classes of compounds in a mixed atmosphere such as coal tar pitch volatiles adds complexity to sampling method selection, and it is sometimes necessary to make, and clearly state alongside the results, certain simplifying assumptions. It is commonly assumed when measuring the more toxic soluble form of an element, that the "safe" assumption may be made that all the element present was soluble (8).

In a study of bioremediation effectiveness, the ability of indigenous soil microorganisms to remove these contaminants from aqueous solutions was determined by GC analysis of organic extracts of biotreated groundwater. Changes in potential environmental and human health hazards associated with the biodegradation of this material were determined at intervals by "Microtox" assays and fish toxicity and teratogenicity tests. After 14 days of incubation at 30°C, indigenous microorganisms effectively removed 100, 99, 94, 88, and 87% of measured phenolic and lower molecular weight polycyclic aromatic hydrocarbons and S-heterocyclic, N-heterocyclic, and O-heterocyclic constituents of creosote, respectively. However, only 53% of the higher molecular weight polycyclic aromatic hydrocarbons were degraded. Despite the removal of a majority of the organic contaminants through biotreatment, only a slight decrease in the toxicity and teratogenicity, of biotreated goundwater was observed (3).

Table 22.1. Typical Fractions from Continuous Tar Distillation"


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