## 3 Results and Discussion
### Biodesulfurization of DBT, 4 MDBT and Their Mixture in a Bubble Column bioreactor
(**a**) Single system of DBT, (**b**) single system of 4-MDBT, nd (**c**) mixture DBT and 4-MDBT. Symbols: (\(\bullet\)) DBT, (\(\Circuit\)) 2-HPB, (\(\blacktriangle\)) 4-MDBT. with that of 4-MDBT (Table 2). This difference may be attributed to the lower affinity of the methylated substrate (4-MDBT) for the 4S pathway enzymes, as demonstrated by [PERSON] et al. [33], who reported that the apparent K\({}_{\text{m}}\) value for 4-MDBT is 7.5 times higher than that of DBT. Furthermore, the size of these molecules is different and that could affect the rate of trans-membrane mass transport. DBT is a smaller molecule and is expected to diffuse faster inside the cell compared with 4-MDBT [35]. The specific product formation rate (q\({}_{\text{p}}\)) for DBT desulfurization was equal to 17.8 \(\pm\) 0.8 mmol 2-HBP/kg DCW/h (Table 2) which compares favorably with the product formation rates reported in the literature for the biodegradation of DBT under optimized conditions by resting cells of various bacteria in biphasic systems (model oil) (Table 3). The main difference between the approach of the present study and the majority of the cases reported in the literature is that the cells were used directly from the culture, avoiding the washing or concentration steps. Biodosulfurization by resting cells is affected by various process parameters, for instance the organic fraction phase (OFP), cell mass concentration in the aqueous phase, OSC concentration in the organic phase and oxygen [33]. Differences in the product formation rates could be attributed to the different experimental conditions applied. Furthermore, high 2-HBP production rates are often associated with a low organic fraction phase (OFP) (Table 3). OFP is among the most important technical bottlenecks in an industrial setting, since the higher the organic phase concentration, the lower the costs associated with the subsequent separation of the aqueous phase from the desired product, water handling and disposal [11]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{3}{*}{**Microorganism**} & **DBT (mM)** & **X** & & & & **Specific** & & \\  & **in the** & **(g DCW/L)** & & & **Desulfur-** & **q\({}_{\text{p}}\)** & & \\  & **Organic** & **in Aqueous** & **OFF (\%)** & **Y\({}_{\text{BDS}}\) (\%)** & & **ization Rate** & **(mmoles** & **2-HBP/kg** & **References** \\  & **Phase** & **Phase** & & & & **DBT/kg** & **DCW/h)** & & \\  & & & & & & **DCW/h)** & & \\ \hline \multicolumn{2}{c}{_Gordonia_} & & & & & & & \\ _alkanitorans_ & 0.5 & 8.25 & 35 & N/A & N/A & 6.7 1  & [20] \\ strain 1B & & & & & & & & \\ \hline \multicolumn{2}{c}{_Mycobacterium_} & & & & & & & \\ _godii X/B_ & 4.8 & 12.0 & 10 & 99 & 16.5 & 10.4 & [23] \\ \hline \multicolumn{2}{c}{_Pseudomonas_} & & & & & & & \\ _delafieldi R-8_ & 10 & 50 & 50 & 97 & 11.4 & N/A & [27] \\ \hline \multicolumn{2}{c}{_Rhodococcus_} & & & & & & & \\ \multicolumn{2}{c}{FUM94} & & & & & & & \\ \hline \multicolumn{2}{c}{_Rhodococcus_} & & & & & & & \\ _erythpropils_ & 3 & 10 & 50 & 100 & 30.0 & 14.6 & [37] \\ \multicolumn{2}{c}{SHT87} & & & & & & & \\ \hline \multicolumn{2}{c}{_Splingomonas_} & & & & & & & \\ _subarctica_ & 17b & 28.2 & 20 & 81.5 & 1.63 & 1.44 & [29] \\ \hline \multicolumn{2}{c}{_Rhodococcus_} & & & & & & & \\ _qingshengii_ & 1.4 & 8.4 & 50 & 100 & 21.6 & 17.8 &
\begin{tabular}{c} Present \\ study \\ \end{tabular} \\ \hline \hline \end{tabular}  
* \({}^{1}\) biodosulfurization under continuous process using cells directly from a continuous culture. N/A: not available.  
\end{table} 
Table 3: Biodosulfurization of DBT in model oil by resting cells of various desulfurizing bacteria. The desulfurization efficiencies (Y\({}_{\text{BDS}}\)) of DBT and 4-MDBT in the binary mixture declined to 59.6 and 41.2% compared with the single systems. Both compounds were desulfurized simultaneously by the cells within the initial 5 h of desulfurization without any preference for either one.