# Bioremediation of Hydrocarbon-Polluted Soil: Evaluation of Different Operative Parameters
## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Kinetic Model**} & \multirow{2}{*}{**Parameters**} & \multicolumn{6}{c}{**Treatments**} \\ \cline{3-8}  & & **B1** & **B2** & **B3** & **B4** & **B5** & **B6** \\ \hline \multirow{3}{*}{First-order} & k (d\({}^{-1}\)) & 0.0023 & 0.0019 & 0.0022 & 0.0023 & 0.0036 & 0.0026 \\  & t\({}_{1/2}\) (d) & 301 & 365 & 315 & 301 & 193 & 267 \\  & R\({}^{2}\) & 0.81 & 0.75 & 0.98 & 0.90 & 0.94 & 0.93 \\ \hline \multirow{3}{*}{Second-order} & k (kg\(\cdot\)g\({}^{-1}\cdot\)d\({}^{-1}\)) & 0.0003 & 0.0002 & 0.0003 & 0.0003 & 0.0005 & 0.0003 \\  & t\({}_{1/2}\) (d) & 310 & 468 & 314 & 307 & 185 & 319 \\ \cline{1-1}  & R\({}^{2}\) & 0.84 & 0.81 & 0.97 & 0.92 & 0.97 & 0.97 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Kinetic parameters for the factor Treatments in test B.  
Consequently, kinetics is considered crucial to the scale-up process, and for the final in-situ application. For both tests, this study took into account the reaction rate at the first and second-order models. In general, the R\({}^{2}\) values are high in both models for both tests, confirming that the application is satisfactory. For test M, both kinetic models fit well. For the second-order model, the coefficients of determination are higher, except for M5. In the case of M2, the first and the second-order model have the same R\({}^{2}\). The best fit is for M3 with 93% of the variability explained by the second-order model. Within this model, all the treatments gave similar results for the k value, with the t1/2 values in the range 46-83 days, in agreement with values reported in similar studies [47]. In test B, although both models fit well, it was found that the kinetic models in all treatments, except B3, had higher coefficients of determination, R\({}^{2}\), for the second-order model. The best fit is for B3, B5, and B6, while the B5 has also a higher degradation rate and the shortest half-life (t\({}_{1/2}\) = 185 days) among all the treatments, in line with values achieved by other authors [26]. The half-life time of the other treatments is always over 300 days, being the treatment B2 with the highest value (468 days). The kinetic behaviour might be explained by the complex nature of oil sludge and the occurrence of complex biochemical mechanisms of hydrocarbon bioremediation [33]. The structured method implemented in this study enables to set up and evaluate two different experimental runs. In microcosms polluted with diesel oil, combinations of water content and carbon to nitrogen ratio were assumed as treatments. The treatment M1 (8 WC%-120 C/N) showed the best diesel oil removal, with the shortest half-life time and the highest degradation within the second-order kinetic model. In the experimental runs on biopsies to bioremediate oily sludge supplementing different renewable organic wastes as treatments, the treatment B5 (biopile with beef manure as a bulking agent) showed the best hydrocarbons removal, with the shortest half-life time and the highest degradation within the second-order kinetic model. The results validate the robustness of the framework used which should be applied to the evaluation of bioremediation runs based on categorical factors or labels as variables of interest.