## 3 Results and Discussion
### Solubilisation Test and Soil-Washing Tests with Hradcany Soil  
The capacity of a surfactant to retain hydrophobic hydrocarbon contamination in suspension is an important property for application in extractive soil cleaning or enhanced bioremediation. In the test in which all surfactants were employed in a concentration of 500 mg/L, biosurfactant H performed best, keeping 45 mg/L of kerosene (C10-C40) in emulsion. The synthetic surfactant performed second best, closely followed by biosurfactants A and B (Table 5). The weakest performing biosurfactant, E, kept only 30% in solution. The variability existing among the biosurfactants produced by _Yarrowia lipolytica_ (B-F) emphasized the important influence of cultivation conditions on the quality of the produced sophorolipids. Properties of soil from the contaminated Hradcany site are given in Table 6. There was a clear decreasing trend in porosity with depth (Table 6). A decrease in porosity would make it more difficult for the surfactant to enter soil pores and remove the hydrophobic contaminants. Moreover, the initial concentrations of contaminants were higher at greater depths (Table 6). The predominantly clayey soil favoured increased adsorption and lesser diffusion of the hydrophobic contaminants. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Surfactant** & **A** & **B** & **C** & **F** & **H** & **Triton X-100** & **Tween 80** & **Control** \\ \hline
**\(E_{24}\)(\%) with crude oil** & 17.7 & 15.7 & 15.7 & 13.7 & 13.7 & 5.9 & 13.7 & 15.7 \\
**\(E_{24}\)(\%) with kerosene** & 4.4 & 2.1 & 2.1 & 1.1 & 4.3 & 22 & 6.1 & 4.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Emulsification index \(E_{24}\) of different surfactants (500 mg/L solutions) tested with crude oil and kerosene. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Surfactant** & **A** & **B** & **C** & **F** & **H** & **Tween 80** & **Triton X-100** \\ \hline
**Diameter (cm)** & 6.7 & 4 & 0 & 1.8 & 7.6 & 5 & 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Clear zone diameter for oil spreading test in surfactant solutions (500 mg/L). Soil treatment with biosurfactant H resulted in maximum transfer of contaminants from the soil to the liquid phase (Figure 2a) and, accordingly, the lowest residual contaminant concentration in soil (Figure 2b). Overall, all selected biosurfactants were at least equally effective in contaminant removal as the synthetic surfactants. Washing with H reduced the C10-C40 concentration from 472 mg/kg to 283 mg/kg in sample 7, which corresponds to a reduction by approximately 40%. For soil 11, the reduction was from 1081 mg/kg to 350 mg/kg (approximately 68%). Next most efficient was Triton X-100, with a removal of 36% (from 472 mg/kg to 301 mg/kg) and 38% (from 1081 mg/kg to 673 mg/kg) from the solid phase of soils 7 and 11, respectively. The next best performing biosurfactants were F with 11% and 32% removal and A with 12% and 20% reduction, for soil 7 and soil 11 respectively. Differences in contaminant removal efficiency could be related to the overall cultivation conditions and the producer yeast type. Biosurfactant H (_Candida bombicola_ CB2107) produced in an Erlenmeyer \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Soil 7** & **Soil 10** & **Soil 11** \\ \hline
**Depth (m)** & 2.2–2.5 & 2.8–3 & 3–3.1 \\
**Texture** & sand/clay & clay & clay \\
**Moisture content (\% m/s)** & 10.4 & 14.5 & 16.0 \\
**pH (25 °C)** & 6.14 & 5.52 & 5.97 \\
**Porosity (\% volume of pores)** & 38 & 25 & 25 \\
**Initial c(C10–C40) (mg/kg)** & 472 & 837 & 1081 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Properties of the tested soils samples. Figure 2: Kerosene, measured as c(C10–C40) in mg/L in aqueous phase for soil washing (for soil 7 and 11) with surfactants (**a**) and residual c(C10–C40) in soil (mg/kg) post-washing with surfactants (**b**). Initial c(C10–C40) of soils 7 and 11 were 472 mg/kg and 1080 mg/kg respectively.