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Environmental fate & pathways

Additional information on environmental fate and behaviour

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additional information on environmental fate and behaviour
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In conducting a PBT assessment of hydrocarbon substances, the Hydrocarbon Block Method is used (EC, 2003) together with predictive tools for assessing the primary half-life. The review shows that hydrocarbons are degraded (under aerobic conditions) via mono-oxygenases or di-oxygenases, and are subsequently carboxylated and ultimately hydroxylated. In further assessing the type of metabolites formed, it has been demonstrated that for all the major classes of hydrocarbons, the major metabolites are in most cases less toxic, and always less bioaccumulative than the parent molecule. Consequently, it is concluded that for PBT and risk assessment purposes, the metabolites of hydrocarbons do not require any further assessment. This is further supported by CONCAWE 2019 (An Evaluation of the Persistence, Bioaccumulation and Toxicity of Petroleum Hydrocarbons) attached to IUCLID section 13.
Specific hydrocarbon classes and how they degrade
1. Alkanes
The primary route of degradation for alkanes is to the alcohol. For hydrocarbons with longer chain lengths, it is probable that the occurrence of formation of the secondary alcohol increases, (Kachholz and Rehm , 1977). Fritsche and Hofrichter (2000) make similar observations.
2. Isoalkanes
The major difference between the metabolism under aerobic conditions of alkanes and isoalkanes relates to the impact of the branching on the rate of metabolism (Fritsche and Hofrichter, 2000). However, the initial metabolites are always alcohols.
3. Naphthenics (Cycloalkanes)
Naphthenics (or cycloalkanes) are relatively resistant to microbial attack as the absence of an exposed terminal methyl group hinders the primary attack (Fritsche and Hofrichter, 2000). The route of degradation of cycloalkanes is again generally via the alcohol and as a general rule alkyl side chains on cycloalkanes facilitate degradation.
4. Aromatic compounds
Given that a large number of aromatic compounds are formed by organisms, for example, aromatic amino acids, phenols, or quinines, it is not surprising that many microorganisms have evolved catabolic pathways to degrade them. Hence aromatic hydrocarbon molecules are readily degraded by microorganisms when the respective molecules are similar to other natural compounds. They are converted enzymatically to the natural intermediates of degradation, for example catechol and protecatechuate (Fritsche and Hofrichter, 2000). Although the introduction of aliphatic substituents normally alters the location in the molecule where the initial microbial attack takes place, the eventual degradation products should not be significantly different.
5. Polyaromatic compounds
The biochemical pathway for the aerobic biodegradation of PAHs has been extensively investigated. It is understood that the initial step in the aerobic catabolism of a PAH molecule by bacteria uses a multicomponent enzyme system to oxidise the PAH to a dihydrodiol. These dihydroxylated intermediates may then be processed through either an ortho cleavage type of pathway, in which ring fission occurs between the two hydroxylated carbon atoms, or a meta- cleavage type of pathway, which involves cleavage of the bond adjacent to the hydroxyl groups, leading to central intermediates such as protocatechates and catechols. These compounds are further converted to tricarboxylic acid cycle intermediates (van der Meer et al. 1992). For the lower molecular weight PAHs, the most common route involves the fission into a C3 compound and a hydroxyl aromatic acid compound. The aromatic ring can thereafter either undergo direct fission or can be subjected to decarboxylation, leading to the formation of a dihydroxylated compound. This compound can be dissimilated as described above. When degraded via these pathways, the low molecular weight PAHs can be completely mineralized to CO2 and H2O (Volkering and Breure, 2003).
The following observations were made by Volkering and Breure (2003);
- Many bacterial and fungal species have the enzymatic capacity to oxidise PAHs
- The aerobic transformation of PAHs always involve incorporation of oxygen into the molecule
- The initial step is usually performed via a dioxygenase and forms a dihydrodiol
Their conclusions were that the transformation products of PAHs were in general more polar than the parent and that there were metabolites that were potentially harmful to mammals and could also have important environmental consequences.
Photodegradation via photolysis is a major source of elimination of PAHs from various environmental compartments (Nikolaou et al. 1984), however, this is not the only pathway that occurs. For instance, anthracene is able to form the photoproduct 9,10-anthraquinone (ATQ) (Fox and Olive 1979; Nikolaou et al. 1984; Mallakin et al. 2000). There are a wide variety of other oxyPAHs that are formed via PAH photomodification. Some of these include phenanthrenequinone (McConkey et al. 1997), many hydroxylated anthraquinones (Mallakin et al. 2000; Brack et al. 2003), and a host of other unidentified compounds, resulting from the photomodification of a variety of PAHs (Huang et al. 1997).
There is little specific data available to evaluate the persistence and bioaccumulation of specific PAH photodegradation products. However, accompanying photomodification, which is generally via oxygenation, should lead to an increase in susceptibility to biotransformation in a similar manner to that observed after phase I metabolism by the mixed function oxidases (Prince and Walters, 2006) which will minimise the risk of persistence and bioaccumulation of such compounds.

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