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JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH Part A—Toxic/Hazardous Substances & Environmental Engineering Vol. A39, No. 8, pp. 1989–2010, 2004
A Review of the Occurrence and Fate of Naphthenic Acids in Aquatic Environments John V. Headley1,* and Dena W. McMartin2 1
National Water Research Institute, Environment Canada, Saskatoon, Canada 2 Division of Environmental Engineering, University of Saskatchewan, Saskatoon, Canada
ABSTRACT Naphthenic acids are comprised of a large collection of saturated aliphatic and alicyclic carboxylic acids found in hydrocarbon deposits (petroleum, oil sands bitumen, and crude oils). Naphthenic acids enter surface water systems primarily through effluent discharge, but also through groundwater mixing and erosion of riverbank oil deposits. Of the possible environmental receptors (i.e., air, soil, and water), the most significant is water. Ambient levels of naphthenic acids in northern Alberta rivers in the Athabasca Oil Sands are generally below 1 mg L1. However, tailings pond waters may contain as high as 110 mg L1. The complexity of natural naphthenic acids in petroleum deposits poses an analytical challenge as reflected by the several techniques reported for quantitation of naphthenic acids in the environment. Although naphthenic acids are known to be persistent biomarkers used in identification of oil source maturation, little is established regarding their relative degradation pathways in aquatic environments. Published research related to the potential for microbiological degradation and adsorption to typical Athabasca Oil Sands soils reveal that naphthenic acids are likely to persist in the water column and, with prolonged exposure, accumulate in sediments. However, other than a very general knowledge of environmental
*Correspondence: John V. Headley, Research Scientist, Aquatic Ecosystem Research Protection Branch, National Water Research Institute, Environment Canada, 11 Innovation Boulevard, Saskatoon, Canada; Fax: 306 975 5143; E-mail:
[email protected]. 1989 DOI: 10.1081/ESE-120039370 Copyright & 2004 by Marcel Dekker, Inc.
1093-4529 (Print); 1532-4117 (Online) www.dekker.com
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Headley and McMartin persistence, the occurrence and fate of naphthenic acids has been sparsely studied. This article brings together some of those environmental persistence results, as well as detailed information regarding the origin of naphthenic acids in tailings ponds, chemistry and toxicological considerations, current analytical methods for aquatic sampling, and areas of future remediation research. Key Words: Naphthenic acids; Crude oil; Oil sands; Aquatic environment; Diagenesis; Degradation.
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INTRODUCTION The environmental effects of oil sands material and tailings pond water on the fauna of aquatic systems have been the subjects of much recent research. In Alberta, Canada where the Athabasca Oil Sands are mined, the provincial environmental legislation (Alberta Environmental Protection and Enhancement Act, Section 23, 1993) prohibits the release of potentially toxic waste streams and no oil sands tailings are deliberately released to ground or surface water supplies.[1] Significant environmental and regulatory attention has been focused on the naphthenic acids fraction of oil sands material due to its persistence in the environment and aquatic toxicity at the levels found in the tailings pond water of bitumen extraction facilities. In the Athabasca Oil Sands, naphthenic acids constitute about 2% by weight of total bitumen samples.[2,3] These compounds may enter surface water systems through such mechanisms as groundwater mixing and erosion of riverbank oil deposits in oil producing regions such as the Athabasca Oil Sands (Fig. 1).[2,3] Currently, crude oil production in the Athabasca Oil Sands accounts for more than 25% of Canada’s total production figures and is rapidly expanding.[4] With oil production industry growth of this magnitude, concerns related to naphthenic acid contamination of aquatic resources are magnified. Naphthenic acids are natural constituents of bitumen and, during the oil sands extraction process, are solubilized and concentrated in tailings. Since naphthenic acids exist at low concentration in aquatic environments, have similar structure and behavior as naturally occurring dissolved organic carbon components in surface water, and have significant complexity dependent upon oil source and geological factors, they present an analytical challenge. The absence of adequate separation and identification procedures for naphthenic acid mixtures has hampered studies to determine specific information on structural relationships, environmental reactivity, and degradation pathways in the environment. Consequently, most research conducted to date have focused on the environmental fate, transport and degradation, isolation of specific toxic naphthenic acids, and epidemiology.[2,3,5,6] In general, the occurrence and fate of naphthenic acids in aquatic environments has been sparsely studied. This article provides a comprehensive look at the microbiological degradation and adsorptive properties of naphthenic acids in aquatic environments, as well as detailed information regarding the origin of naphthenic acids in tailings ponds, chemistry and toxicological considerations, current analytical methods for aquatic sampling, and areas of future remediation research.
Figure 1.
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Map of Canada and Alberta showing the Athabasca oil sands region. (View this art in color at www.dekker.com.)
Note: The dark grey formations in the map of Alberta indicate oil sands deposits.
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CHEMISTRY OF NAPHTHENIC ACIDS Naphthenic acids are a family of carboxylic acid surfactants (Fig. 2), primarily consisting of cyclic terpenoids used in source and geochemical characterization of petroleum reserves.[2,3] The compound group is composed predominately of alkylsubstituted cycloaliphatic carboxylic acids with smaller amounts of acyclic aliphatic (paraffinic or fatty) acids. Aromatic olefinic, hydroxy, and dibasic acids are also present as minor components of naphthenic acids. The cycloaliphatic acids include single rings and fused multiple rings. The carboxyl group is usually bonded or attached to a side chain rather than directly to the cycloaliphatic ring (Fig. 2).[3,7,8] The components of naphthenic acids are commonly classified by their structures and the number of carbon atoms in the molecule. The compounds are represented by the general formula CmH2mþzO2 where m indicates the carbon number and z represents the number of hydrogen atoms that are lost as the structures become more compact. This is the most useful classification system and is based on the same definition of characterizing petroleum compounds or group type used for hydrocarbon analyses.[3,9] The z value is equal to 0 for saturated linear or branched hydrocarbon chains, and changes to 2 in monocyclic naphthenic acids, 4 in bicyclic, 6 in tricyclic and so on. The molecular weights change by 14 mass units (CH2) between m series and by 2 mass units (H2) between z series.[2,3,10] Previous studies indicated that the z ¼ 4 series predominates in oil sands tailings pond wastewater.[9] The correlation between z series, y families, and molecular weights is shown in Table 1.[3] The polarity and nonvolatility of naphthenic acids increases with molecular weight, giving individual compounds various physical, chemical, and toxicological properties.[2,3,10–12] In general, however, as a group the naphthenic acids have physical and chemical characteristics that can be used to describe the overall mixture as shown in Table 2.
Figure 2.
General chemical structures of naphthenic acids.
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Table 1. Molecular weights of different z series and m families of naphthenic acids (CmH2mþzO2).
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Number of carbon atoms
M.W. z ¼ 0 (open chain)
M.W. z ¼ 2 (1 ring)
M.W. z ¼ 4 (2 rings)
M.W. z ¼ 6 (3 rings)
172 186 200 214 228 242 256 270 284 298 312
170 184 198 212 226 240 254 268 282 296 310
168 182 196 210 224 238 252 266 280 294 308
166 180 194 208 222 236 250 264 278 292 306
10 11 12 13 14 15 16 17 18 19 20
Table 2.
Physical and chemical properties of naphthenic acids.[2,3,10–12]
Parameter Color Odor State Molecular weight Solubility Density Refractive index pKa Log Kow (octanol water partition coefficient) Boiling point
General characteristic Pale yellow, dark amber, yellowish brown, black Primarily imparted by the presence of phenol and sulphur impurities; musty hydrocarbon odor Viscous liquid Generally between 140 and 450 amu
