Dense Nonaqueous Phase Liquids (DNAPLs)

Multi-Component Waste

Creosotes

Creosote is a combustible, yellowish, dark-green to brown or black liquid. It is made by fractional distillation of coal tar. Creosote consists of six major classes of compounds: aromatic hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs) and alkylated PAHs (which can constitute up to 90% of creosote); tar acids/phenolics; tar bases/nitrogen-containing heterocycles; aromatic amines; sulfur-containing heterocycles; and oxygen-containing heterocycles, including dibenzofurans (IPCS 2004). The composition of the mixture varies among lots and across manufacturers.

Many of creosote's chemical and physical properties reflect those of the individual compounds that form the mixture. Some of these compounds are soluble while others are not. Although the main components of creosote are naphthalene and its alkyl derivatives, phenanthrene, fluorene, acenaphthene, alkylphenols, and dibenzofuran, the more soluble components (benzene, toluene, ethylbenzene, xylenes, nitrogen-containing heterocycles, sulfur-containing heterocycles, and phenolics) are generally present at a much higher percentages in leachate and contaminated water (IPCS 2004). As creosote ages, the more volatile and soluble components of the mixture diminish relative to the less volatile and soluble compounds. Aging also affects mobility since the heavier components have higher K_oc values and tend to sorb to soil more than the lighter molecules (the log organic carbon sorption coefficient (K_oc) for PAHs ranges from 2.4 to 7.0 (IPCS 1998)).

Various hazardous materials data sheets show specific gravity values between 1.03 and 1.18, or very slightly heavier than water. However, the reported range can vary more because products sold as creosote may be altered according to the application needed. For instance, blending lighter hydrocarbons, such as diesel, with creosote cuts the cost and improves penetrating ability. Adding heavier coal tar pitch to creosote increases viscosity.

When spilled on the ground, the vapor pressure of the individual compounds affects the fate of the creosote. The vapor pressures of individual components detected in creosote range from 12,700 Pa for benzene to 2.0 x 10^-10 Pa for dibenzo[a,h]anthracene. Generally, low-molecular-weight PAHs (e.g., naphthalene, anthracene, and phenanthrene) are mainly in the gas phase, and high-molecular-weight PAHs are mainly bound to particles. Phenolic compounds, including cresols, as well as the heterocyclic fraction tend to be in the vapor state. However, it is not clear how the specific composition of creosote modifies the distribution behavior of the individual components (IPCS 2004).

Although the transport of creosote compounds from water surfaces to air depends on their individual volatilization rates, volatilization is not considered a dominant process for PAHs and cresols (IPCS 2004).

Generally, the high molecular-weight aromatic organic compounds (more than three rings), with relatively low solubilities and high adsorptive capacities, dominate in sediment, whereas the low molecular-weight aromatic organic compounds (fewer than three aromatic rings) partition selectively into the aqueous phase (IPCS 2004).

The log organic carbon sorption coefficient (K_oc) values for PAHs, which range from 2.4 to 7.0 (IPCS 1998), indicate moderate to minimal mobility in the subsurface. The log K_ow for creosote is 1. Some components of creosote are expected to bioaccumulate.

Creosotes are more likely to degrade under aerobic than anaerobic conditions. The degradation rates are compound-specific with the heavier PAHs being very recalcitrant.

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Adapted from:

International Programme on Chemical Safety
(IPCS). 2004. Concise International Chemical Assessment Document 62: Coal Tar Creosote

This document contains a thorough discussion of the physical and chemical properties of coal tar creosote, its fate and transport, and human health effects.

For Further Information

Environmental Health Criteria 202: Selected Non-Heterocyclic Polycyclic Aromatic Hydrocarbons
International Programme on Chemical Safety (IPCS)
World Health Organization, 1998, 883 pp

IPCS provides a thorough discussion of the physical and chemical properties of selected PAHs, their fate and transport, and human health effects.

Toxicological Profile for Wood Creosote, Coal Tar Creosote, Coal Tar, Coal Tar Pitch, and Coal Tar Pitch Volatiles
Agency for Toxic Substances and Disease Registry (ATSDR)
U.S. Department Of Health And Human Services, 2002, 394 pp

This profile covers coal tar creosote human health effects, chemical and physical properties, manufacturing volume data, potential for human exposure (environmental fate and transport), and analytical methods.