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3.0 Persistence Assessment of Petroleum Hydrocarbon Blocks
Hydrocarbons have been present in the environment for billions of years. As a highly
reduced form of carbon, these substances provide a valuable source of energy for
consumption by microorganisms. Therefore, mechanisms have evolved to degrade
hydrocarbons, and nearly all hydrocarbons can be degraded under appropriate conditions
(Prince and Walters, 2007).
In order to assess the persistence of the different HCBs, aquatic half-life predictions for
representative constituents were made using the BioHCwin module of the EPISuite v4.0
model. Unacclimated, marine and freshwater experimental biodegradation half-lives from
three different sources (Prince et al. 2007, 2008; EMBSI, 2009a, b and c) were compiled
and compared to model predictions. According to previous work (Prince et al. 2007) halflives
for different classes of hydrocarbons do not differ significantly between marine and
freshwater. Thus, half-life data from both media have been used collectively in the
persistence assessment.
Results of standardized biodegradation experiments may vary significantly depending on
the microbial inoculum that is used. Further, in experiments with hydrocarbon mixtures,
observed biodegradation rates can be influenced by a lag phase that is due to the presence
of other hydrocarbons that are preferentially biodegraded. Consequently, if more than
one reliable half-life value was available for the structure the lowest half-life for the
structure was selected as proof the structure has the potential to biodegrade under unacclimated
test conditions. Persistence of the HCBs was assessed comparing predicted
and experimental half-lives for the corresponding representative structures to the 60-day
criterion for marine water persistence specified in Annex XIII of the REACH regulation.
In the following sections, the results of the persistence assessment will be discussed.
3.1 Normal Paraffins
The BioHCwin model predicts that n-paraffins with chain lengths above twenty-two
carbons will have half-lives exceeding the 60-day persistence criterion (Figure 1).
However, marine and freshwater experimental half-lives for n-paraffins with carbon
chain lengths up to 19 carbons (Table 1) range from 1.9 to 15 days. For the carbon
numbers for which experimental persistence data are available, predicted and
experimental data are below the persistence criterion (Figure 2). In several cases, there
are data for multiple hydrocarbons with the same number of carbons. In fact,
experimental values are far below the persistence criterion, meaning that model
predictions are conservative. Moreover, n-paraffins larger than hexane are known to be
the preferred substrate in oil degradation processes (Prince and Walters, 2007). This is
reflected by the higher experimental half-lives of the shorter chain paraffins (Table 1). It
can be concluded that, based on the BioHCwin predictions, n-paraffins at or above C22
may have half-lives above the persistence criterion.
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