# Selective Removal of Diesel Oil Hydrocarbons in Aerobic Bioremediation
## 4 Discussion  
We paid attention to the identification of hydrocarbons and determined the removal efficiency of diesel oil (\(\eta\)) and single hydrocarbon compounds (\(\eta_{\rm H}\)).  
The results are significant: comparing our data with those obtained by [PERSON] et al. [6], who used GC-MS, the percentage of removal of the same hydrocarbon clusters was very similar, even if their test duration was longer (t = 210 days). [PERSON] et al. [27] confirmed that for the identification of these compounds, the GC-FID analysis generates results in line with those obtained with GC-MS. They achieved the same hydrocarbon removal efficiency as in our study, using similar test durations.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**u\% (by Mass)**} & \multicolumn{3}{c}{**C/N = 120**} & \multicolumn{3}{c}{**C/N = 180**} \\  & **8\%** & **12\%** & **15\%** & **8\%** & **12\%** & **15\%** \\ \hline \(\eta\) \% & 72\% & 63\% & 68\% & 52\% & 65\% & 59\% \\ \hline \end{tabular}
\end{table}
Table 6: Diesel oil removal efficiency \(\eta\) by microcosms with C/N = 120 and 180 at t = 138 days.  
\begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Compounds**} & **k** & **R2** & **t1/2** \\  & **(kg g-1 d-1)** & **(d)** \\ \hline C8–C11 & 0.0018 & 0.94 & 32 \\ C12–C15 & 0.0011 & 0.97 & 40 \\ C16–C19 & 0.0013 & 0.84 & 64 \\ C20–C24 & 0.0041 & 0.99 & 35 \\ Aromatics & 0.0018 & 0.47 & 92 \\ \hline \end{tabular}
\end{table}
Table 5: Second-order reaction rate constants and half-life times.  
In our tested microcosms, the removal efficiency was higher for the alkanes with a medium-length chain than for those with a long-length chain. From our findings (Table 7), the degradation of the C8-C11 cluster was in the range between 72% and 88%, namely the highest. For the other clusters, the efficiency was always lower than 70%.  
In terms of overall removal efficiency, the microcosm with C/N = 120 and u% = 8% by mass gave the highest value, equal to 72%. At the same time, this microcosm also gave the highest values of degradation (\(\eta_{\rm H}\)) for each cluster.  
It was noted that the removal trend for aromatic hydrocarbons (Figure 4) was slower than for the other compounds. Considering the retention times shown in Table 1 and the composition of diesel oil based on carbon atoms (Table 3), it is possible to observe that, except for acenaphthylene, the PAHs were removed after the \(n\)-C16 compound. Thus, the removal trend for the aromatic hydrocarbons below the heaviest hydrocarbon cluster (C20-C24) reflects the molecular complexity and their difficulty to be degraded.  
The low molecular mass compounds are preferentially degraded; thereby, the concentrations of fractions with C10-C12 and C12-C16 are removed [28]. This is due to the lower hydrophobicity of the short and medium-chain alkanes; thus, they are more easily degraded, as other authors confirmed [9]. The percentage of removed hydrocarbons was 63% on average for alkanes and 50% for aromatics in 138 days, which is in line with the values obtained in other studies [5; 29].  
Knowledge of the residual diesel oil composition permits evaluating whether the compound concentrations are lower than those stated in the current legislation for remediated soils. For example, Table 8 presents the maximum concentrations of some hydrocarbons allowed by the current Italian legislation [30], showing that the maximum concentration depends on the specific use of the site. Table 8 also reports the concentrations of the pollutants present in the studied soil (microcosm with C/N = 120 and u% = 8% by mass) at the beginning and the end of the process.  
Looking at the data in Table 8, it is evident that the biological process was not efficient enough to make the site suitable for reuse, despite the high values of removal efficiency; therefore, additional treatment for soil clean-up is compulsory.