Biodesulfurization of Dibenzothiophene and Its Alkylated Derivatives in a Two-Phase Bubble Column Bioreactor by Resting Cells of Rhodococcus erythropolis IGTS8

Abstract

Biodesulfurization (BDS) is considered a complementary technology to the traditional hydrodesulfurization treatment for the removal of recalcitrant sulfur compounds from petroleum products. BDS was investigated in a bubble column bioreactor using two-phase media. The effects of various process parameters, such as biocatalyst age and concentration, organic fraction percentage (OFP), and type of sulfur compound—namely, dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), 4,6-dimethyldibenzothiophene (4,6-DMDBT), and 4,6-diethyldibenzothiophene (4,6-DEDBT)—were evaluated, using resting cells of Rhodococcus erythropolis IGTS8. Cells derived from the beginning of the exponential growth phase of the bacterium exhibited the highest biodesulfurization efficiency and rate. The biocatalyst performed better in an OFP of 50% v/v. The extent of DBT desulfurization was dependent on cell concentration, with the desulfurization rate reaching its maximum at intermediate cell concentrations. A new semi-empirical model for the biphasic BDS was developed, based on the overall Michaelis-Menten kinetics and taking into consideration the deactivation of the biocatalyst over time, as well as the underlying mass transfer phenomena. The model fitted experimental data on DBT consumption and 2-hydroxibyphenyl (2-HBP) accumulation in the organic phase for various initial DBT concentrations and different organosulfur compounds. For constant OFP and biocatalyst concentration, the most important parameter that affects BDS efficiency seems to be biocatalyst deactivation, while the phenomenon is controlled by the affinities of biodesulfurizing enzymes for the different organosulfur compounds. Thus, desulfurization efficiency decreased with increasing initial DBT concentration, and in inverse proportion to increases in the carbon number of alkyl substituent groups.

Streamlining the biodesulfurization process: development of an integrated continuous system prototype using Gordonia alkanivorans strain 1B

“…In addition to increasing agitation speed, or biocatalyst concentration, some authors propose the use of immobilized biocatalysts, different bioreactor designs, or the incorporation of surfactants/emulsifiers into the process. 18,22,42–49 The latter are probably amongst the most promising approaches to efficiently increase mass transfer, since they facilitate/prolong emulsion formation, increasing the area of contact between aqueous and organic phase, and facilitating the diffusion of hydrophobic compounds such as DBT, which consequently increases BDS activity of the biocatalysts.…”

“…In addition to increasing agitation speed, or biocatalyst concentration, some authors propose the use of immobilized biocatalysts, different bioreactor designs, or the incorporation of surfactants/emulsiers into the process. 18,22,[42][43][44][45][46][47][48][49] The latter are probably amongst the most promising approaches to efficiently increase mass transfer, since they facilitate/prolong emulsion formation, increasing the Fig. 2 Response surfaces for 2-HBP production (mM) based on the responses obtained in ED1 at 3 h (A) and 6 h (B) [for the factors X 1the ratio of n-heptane : water, between 1 : 9 (0.1) and 9 : 1 (0.9), and X 2initial DBT concentration in the organic phase, between 0.250 mM and 2 mM], and ED2 at 3 h (C) and 6 h (D) for the factors [X 1 -the ratio of n-heptane : water, between 0.1 and 0.9, and X 2 -cell (biocatalyst) concentration in the water phase, between 5 g L −1 and 20 g L −1 (DCW)].…”

“…Under these conditions, and unlike other designs, such as the bubble column or the traditional sequential continuously stirred tank reactor, there is no need for pH control or active aeration, which reduces emulsion formation and decreases evaporation of volatile fuel compounds, as well as fuel oxidation. 18,46 There is also no active production of gas, which is more likely to occur with growing cells or processes of reductive BDS, since the gas collecting bag did not have to be emptied throughout the assays, despite the system being airtight. Furthermore, the more efficient mixing, with a central propeller, results in increased mass transfer with less intensive agitation, leading to the formation of fewer clogs, especially when compared to the shake-asks or closed tubes.…”

Deciphering the biodesulfurization potential of two novel <i>Rhodococcus</i> isolates from a unique Greek environment

“…Desulfurization activity was determined through a resting cells assay [28] . In brief, the content of 4 to 8 wells from each condition (depending on the growth stage) was removed, mixed in a 1.5 mL Eppendorf tube, centrifuged for 5 min at 1000xg, and cell pellets were resuspended in 50 mM Hepes buffer (pH = 8.0) to a final concentration between 0.5 and 2 g DCW/L.…”