Polycyclic aromatic hydrocarbons, abbreviated PAH, is a common term for high molecular, aromatic hydrocarbons, which means that they contain more than two unsubstituted benzene rings. Sometimes naphthalene is included in this group, although it only contains two rings. Some examples of PAHs together with naphthalene are given in Figure 1.

 

Naphtalene	Phenanthrene	Flouranthrene
Pyrene	Benzo[a]anthracene	Benzo[a]pyrene

 

Figure 1. Structure formula for selected PAHs.

 

 

PAHs are formed as a result of burning of organic material and naturally as a result of thermal geological reaction. They have for many years attracted much attention, because some of them are strong carcinogens, i.e. benz(a)pyrene and benz(a)anthracene. These compounds are constituents of heavy fuel, tar and creosote, for which reason they are often identified at gas works and coal gasification sites, and in connection with oil spills. Point sources of anthropogenic origin are indicated in Table 1.

 

Table 1. Point sources giving rise to PAH contamination¹⁾

 

Source

Coal gasification

Heat and power generation

Coke production

Catalytic cracking

Carbon-black production and use

Asphalt production and use

Coal tar production and use

Refining/destillation of crude oil

Wood treatment and preservation

Fuel operations

Incineration

Landfills/waste disposal

¹⁾ Wilson and Jones (1993)

It should, however, be noted, that they are found everywhere in the environment – and not only at contaminated sites - because of incomplete combustion of fossil fuels, although normally in very low concentrations. It has been shown that the background level in normal agricultural land has increased for many years (Jones et al. 1989), and it is expected to further increase in the future. This is because many of these compounds are rather persistent in the environment even under aerobic conditions. But they are also not very mobile, which can be seen from the high K_ow values shown in Table 1.

 

Table 1. Physio-chemical properties of selected PAHs

 

Compound	Mole weight g mole-1	Boiling point °C	Water solubility mg l-1	Log Kow
Naphthalene	128	218	31.7	3.5
Phenanthrene	178	339	1.29	4.45
Anthracene	178	340	0.075	4.46
Fluoranthene	202	375	0.26	4.90
Pyrene	202	393	0.135	4.90
Benz(a)anthracene	228	435	0.014	5.61
Benz(a)pyrene	252	496	0.004	6.50

 

 

Fate in the soil and groundwater environment

 

Because of the very low water solubility and high K_ow values, they will tend to be sorbed to the organic matter in the soil in stead of being solubilized in the infiltrating water and through this be transported downwards to the groundwater reservoirs. The sorption process is therefore counteractive to efficient biodegradation since it will decrease bioavailability, since the compounds due to sorption will be located in microporous areas of the soil inaccessible to the bacteria, and the biodegradation will thus be controlled by the slow desorptive and diffusive mass transfer into the biologically active areas (Zhang et al. 1998). It has been claimed that a slow sorption following the initial rapid and reversible sorption lead to a chemical fraction that is very resistant to desorption (Hatzinger and Alexander, 1995). This phenomenon is called aging, and the existence of such a desorption-resistant residues may increase the time as the compound stay in the soil dramatically. PAHs have also been shown to be partitioned or incorporated more or less reversibly into the humic substances of the soil after partial degradation and thereby be even more immobilised in the soil (Kästner et al, 1999; Ressler et al. 1999).

 

At the same time they show very low aerobic degradability depending on the environmental conditions and the available concentration. Only two- and three-ringed compounds have been shown to be degraded under anaerobic conditions with nitrate or sulfate as the terminal electron acceptor (Mihelic and Luthy, 1988, Coates et al. 1996). Very low concentrations have a strong influence on the biodegradation of such hydrophobic compounds, and some studies have indicated that the process stops below a certain threshold concentration (Alexander, 1985). The low mobility and high persistence means that they can stay in the soil for decades, and even at sites with contaminations dating at least 50 years back, 4- or 5-ringed PAHs are found near the soil surface.

 

 

Toxic effects

 

PAHs are known for their carcinogenic effects, while their ecotoxicological effects are less emphasised, since the are not considered to have acute toxicity of any significance. This is normally linked to the very low water solubility and bioavailability of these compounds, but this also means that it is extremely difficult to design and carry out toxicity tests (Kelsey and Alexander, 1997). In Table 3, the genotoxicity and ecotoxicity are shown for selected PAHs. 

 

Table 2. Toxicity of selected PAHs

 

Compound	Carcinogen	Ames test response	Sister chromatid replacement	LC50 1)
Naphthalene	-	-	-	3.8
Phenanthrene	-	-	-	0.6
Anthracene	-	-	-	4.46
Fluoranthene	(+)	-	-	0.5
Pyrene	-	+	+	4.90
Benz(a)anthracene	+	+	+	> 1
Benz(a)pyrene	+	+	+	> 1

 

¹⁾ Neanthes arenaceodentata expressed as LC_{50 96h} I mg l^-1 (Cerniglia, 1992; Rossi and Neff, 1978)

 

Management and remediation

 

PAH-contaminated sites are in many cases located in the centre of the cities, since gas works, asphalt factories, etc. often were placed near industrial areas with a need of energy and diverse tar products. These sites are always high on the clean-up priority list, because of the human health effects of some of the high molecular PAHs.

 

In situ remediation of PAHs has not been applied on a wider scale, although some technologies have been suggested, i.e. solvent flushing (Denie et al. 1994). Because of their low mobility, these compounds will in most cases stay in the upper layers of the site, except when the contamination take place directly in the subsurface, which is the case in some coal gasification sites. Therefore, excavation of the contaminated site can be applied with success provided a good knowledge about the distribution of the contamination exists.

 

On site or ex situ bioremediation of PAH-contaminated soil has been reported in a number of cases with or without the use of specific inocula specially adapted for bioaugmentation (Wang et al. 1990; Mueller et al. 1991). Such composting or similar techniques are advantageous because of the price of the operation. However, because of the problems with low bioavailability after incorporation or aging of the soil, it can be very difficult to reach acceptable clean up criteria, and even if such criteria are reached, an issue of extraction of all PAH from the soil for analysis might weaken the interpretation of the results. A number of methods for enhancing bioavailability by addition of surfactant or other agents have been applied with varying success (See Wilson & Jones, 1993; Aronstein & Alexander, 1993).

 

For these reasons, PAH-contaminated soil is often disposed off by incineration or chemical extraction in specifically adapted plants. Such technologies are always preferable, when dealing with heavily PAH-contaminated soil, but it must be noted that reuse of the soil after such treatment will be hampered because the soil has become biologically inactive by the treatment.

 

Publications

 

Alexander, M. (1985) Biodegradation of organic chemicals. Environ. Sci. Technol. 18, 106-111.

 

Aronstein, BN, Alexander, M (1993) Effect of a non-ionic surfactant added to the soil surface on the biodegradation of aromatic hydrocarbons within the soil. Appl. Microbiol. Biotechnol. 39, 386-390.

 

Bossert, I, Bartha, R (1984) The fate of petroleum in soil ecosystems. In Atlas, RM (Ed.) Petroleum Microbiology. MacMillan, NY.

 

Cerniglia CE (1992) Biodegradation of polycyclic aromatic hydrocarbons. Biodegradation, 3, 351-368.

 

Coates, JD, Anderson, RT, Lovley, DR (1996) Oxidation of polycyclic aromatic hydrocarbons under sulfate-reducing conditions. Appl. Environ. Microbiol. 62, 1099-1101.

 

Augustijn, DCM, Jessup, RE, Rao, PSC, Wood, AL (1994) Remediation of contaminated soils by solvent flushing. J. Environ. Eng. 120, 42-57.

 

Hatzinger, PB, Alexander, M (1995) Effect of aging of chemicals in soil on their biodegradability and extractability. Environ. Sci. Technol. 29, 537-45.

 

Jones, KC, Stratford, JA, Waterhouse, KS, Furlong, ET, Giger, W, Hites, RA, Schaffner, C, Johnston, AE (1989) Increases in the polynuclear aromatic hydrocarbon content of an agricultural soil over the last century. Environ. Sci. Technol. 23, 95-101.

 

Kästner, M, Streibich, S, Beyrer, M, Richnow, HH, Fritsche, W (1999) Formation of bound residues during microbial degradation of [¹⁴C]Anthracene in soil. Appl. Environ. Microbiol. 65(5), 1834-1842.

 

Kelsey, JW, Alexander, M (1997) Declining bioavailability and inappropriate estimation of risk of persistent compounds. Environ. Toxicol. Chem. 16, 582-585.

 

Mihelic, JR, Luthy, RG (1988) Microbial degradation of acenaphthene and naphthalene under dinitrification conditions in soil-water systems. Appl. Environ. Microbiol. 54, 1188-98.

 

Mueller, JG, Lantz, SE, Blattman, BO, Chapman, PJ (1991) Bench-scale evaluation of alternative biological treatment processes for the remediation of pentachlorophenol and creosote-contaminated material: slurry phase bioremediation. Environ. Sci. Technol. 25, 1055-1061.

 

Neff, JM (1977) Polycyclic Aromatic Hydrocarbons in the Aquatic Environment. Sources, fates, and Biological Effects. Applied Science Publ. Ldn.

 

Ressler, BP, Kneifel, H., Winter, J. (1999) Bioavailability of polycyclic aromatic hydrocarbons and formation of humic acid-like residues during bacterial PAH degradation. Appl. Microbiol. Biotechnol. 53(1), 85-91.

 

Rossi, SS, Neff, JM (1978) Toxicity of polynuclear aromatic hydrocarbons to the marine polychaete Neanthes arenaceodentata. Marine Pollution Bulletin, 9, 220-223.

 

Wang, X, Yu, X, Bartha, R (1990) Effect of bioremediation on polycyclic aromatic hydrocarbon residues in soil. Environ. Sci. Technol. 24, 1086-1089.

 

Wilson, SC, Jones, KC (1993) Bioremediation of soil contaminated with polynuclear aromatic hydrocarbons (PAHs): A review. Environ. Pollut. 81, 229-49.

 

Zhang, W, Bouwer, EJ, Ball, WP (1998) Bioavailability of hydrophobic organic contaminants: Effects and implications of sorption-related mass transfer on bioremediation. GWMR, winter 1998, 126-138.