# Selective Removal of Diesel Oil Hydrocarbons in Aerobic Bioremediation
## 3 Results  
### Identification of Diesel Oil Hydrocarbons  
Diesel oil is a distillate containing different compounds including _n_-alkanes (from C8 to C24), branched alkanes, cycloalkanes, and aromatic hydrocarbons. Each chromatogram was compared with the chromatograms of standards, which were obtained with GC-FID analysis under the same operative conditions. Considering the retention time, the hydrocarbon species were identified in the order shown in Table 1.  
Diesel oil is composed of 65-70% saturated hydrocarbons and 30-35% aromatic ones. By the standards, it was possible to identify the _n_-alkanes and the PAHs, which represent 35% and 10% of the total compounds, respectively. Table 2 reports the average distribution of the diesel oil components from literature data [25,26] and the value taken as reference for this study. \begin{table}
\begin{tabular}{c c} \hline \hline
**Compound** & **Retention Time** \\  & **(min)** \\ \hline _n_-C8 & 6.3 \\ Acenaphthylene & 6.5 \\ _n_-C9 & 7.6 \\ _n_-C10 & 8.3 \\ _n_-C11 & 10.3 \\ _n_-C12 & 12.3 \\ _n_-C13 & 14.3 \\ _n_-C14 & 16.0 \\ Fluorene & 17.1 \\ _n_-C15 & 17.8 \\ _n_-C16 & 19.4 \\ Phenanthrene & 20.2 \\ _n_-C17 & 20.9 \\ _n_-C18 & 21.0 \\ Anthracene & 22.4 \\ Pyrene & 22.5 \\ _n_-C19 & 23.8 \\ _n_-C20 & 25.1 \\ _n_-C21 & 26.4 \\ _n_-C22 & 27.6 \\ Benzo(x)anthracene & 28.8 \\ _n_-C23 & 29.9 \\ _n_-C24 & 31.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Identified hydrocarbons and their retention time. Referring to Table 2 and integrating our GC-FID analysis, the chemical composition of hydrocarbons was determined considering the number of carbon atoms. The composition of hydrocarbons from C9 to C24 was calculated summing the area of peaks contained between two linear paraffines. For example, for C9 compounds, the integration was done between the \(n\)-C8 peak and the \(n\)-C9 one, and so on for the other compounds. The diesel oil composition by hydrocarbons based on carbon number is shown in Table 3, at the beginning (t = 0 days) and the end (t = 138 days) of the bioremediation process. The data are related to the studied microcosm, with C/N = 180 and u% = 12% by mass. Looking at Table 3, the following points are considered. In commercial diesel oil, the compounds with C14 have the highest concentration, which is kept also at the end of the biodegradation test (t = 138 days). The data show that diesel oil is mainly composed of compounds C10-C16. Considering the composition at the test end (t = 138 days), the compounds with C12-C20 have the highest concentration, and this means that the biodegradation process affects the compounds to a different extent. \begin{table}
\begin{tabular}{c c c} \hline \multirow{2}{*}{**Compound**} & \multicolumn{2}{c}{**Composition**} \\  & **Range** & **Adopted Value** \\ \hline Paraffins & 9.0–49.0 & 16.4 \\ Monocycloparaffins & 9.5–34.0 & 14.6 \\ Discyloparaffins & 10.0–32.5 & 13.8 \\ Tricycloparaffins & 0.7–41.5 & 9.5 \\ Tetracycloparaffins & 0.0–3.0 & 2.9 \\ Alkylbenzenes & 2.8–29.0 & 3.5 \\ Benzocycloparaffins & 2.5–18.0 & 2.6 \\ Benzocycloparaffins & 0.0–14.0 & 3.6 \\ Naphalenes & 0.0–7.5 & 6.1 \\ C\({}_{n}\)H\({}_{(2n-14)}\) & 0.0–5.5 & 3.6 \\ C\({}_{n}\)H\({}_{(2n-16)}\) & 0.0–8.5 & 4.7 \\ Triatronatics & 0.0–8.5 & 2.5 \\ Aromatic sulfur compounds & 0.0–6.0 & 2.3 \\ \hline \end{tabular}
\end{table} 
Table 2: Composition of commercial diesel oil: average values and reference for this study.