Bio-catalytic degradation of dibenzothiophene (DBT) in petroleum distillate (diesel) by Pseudomonas spp

Abstract

Biodesulfurization (BDS) was employed in this study to degrade dibenzothiophene (DBT) which accounts for 70% of the sulfur compounds in diesel using a synthetic and typical South African diesel in the aqueous and biphasic medium. Two Pseudomonas sp. bacteria namely Pseudomonas aeruginosa and Pseudomonas putida were used as biocatalysts. The desulfurization pathways of DBT by the two bacteria were determined by gas chromatography (GC)/mass spectrometry (MS) and High-Performance Liquid Chromatography (HPLC). Both organisms were found to produce 2-hydroxy biphenyl, the desulfurized product of DBT. Results showed BDS performance of 67.53% and 50.02%, by Pseudomonas aeruginosa and Pseudomonas putida, respectively for 500 ppm initial DBT concentration. In order to study the desulfurization of diesel oils obtained from an oil refinery, resting cells studies by Pseudomonas aeruginosa were carried out which showed a decrease of about 30% and 70.54% DBT removal for 5200 ppm in hydrodesulfurization (HDS) feed diesel and 120 ppm in HDS outlet diesel, respectively. Pseudomonas aeruginosa and Pseudomonas putida selectively degraded DBT to form 2-HBP. Application of these bacteria for the desulfurization of diesel showed promising potential for decreasing the sulfur content of South African diesel oil.

Figure 1

Cell growth of Pseudomonas aeruginosa and Pseudomonas putida with DBT degradation and 2-HBP formation. Experimental conditions: Initial DBT concentration 46 ppm, Temperatures 30 °C and 37 °C for Pseudomonas aeruginosa and Pseudomonas putida, respectively, Shaking speed 130 rpm.

Figure 2

Degradation of DBT and formation of 2-HBP formation by resting cells of Pseudomonas aeruginosa and pseudomonas putida. Experimental conditions: Initial DBT concentration 500 ppm, Initial cell concentration 1.2 g DCW/L, Temperature 37 °C (Pseudomonas aeruginosa), 30 °C (Pseudomonas putida), Shaking speed 130 rpm.

Figure 3

Effect of cell concentrations of Pseudomonas putida during degradation of DBT. Experimental conditions: Initial DBT concentrations 500 ppm, Temperature 30 °C, Shaking speed 130 rpm.

Figure 4

Effect of cell concentration of resting cells of Pseudomonas putida and Pseudomonas aeruginosa on the DBT degradation and production of 2-HBP (a) 0.3 g DCW/L (b) 0.6 DCW/L (c) 0.9 g DCW/L (d) 1.2 g DCW/L. Experimental conditions: Initial DBT concentrations 500 ppm, Temperature 37 °C (Pseudomonas aeruginosa) 30 °C (Pseudomonas putida), Shaking speed 130 rpm.

Figure 5

Effect of initial DBT concentrations on the growth of (a) Pseudomonas aeruginosa (b) Pseudomonas putida at 1:4 oil: water ratio for biphasic. Experimental conditions: Temperature 37 °C (Pseudomonas aeruginosa), 30 °C (Pseudomonas putida), Shaking speed 130 rpm.

Figure 6

Effect of initial DBT concentration on DBT degradation by Pseudomonas aeruginosa and Pseudomonas putida (a) 250 ppm, (b) 500 ppm (c) 750 ppm (d) 1000 ppm. Experimental conditions: Cell concentration 1.2 g DCW/L, Temperature 37 °C (Pseudomonas aeruginosa), 30 °C (Pseudomonas putida), Shaking speed 130 rpm.

Figure 7

Effect of biphasic (oil–water ratio) on the growth of (a) Pseudomonas aeruginosa (b) Pseudomonas Putida. Experimental conditions: Initial DBT concentration of 500 ppm, Temperature 37 °C (Pseudomonas aeruginosa) 30 °C (Pseudomonas putida), Shaking speed 130 rpm.

Figure 8

Effect of biphasic media on biodesulfurization of DBT and formation of 2-HBP by (a) Pseudomonas aeruginosa (b) Pseudomonas putida. Experimental conditions: Oil-to-water ratio 1:4. Initial DBT concentration 500 ppm, Temperature 37 °C (Pseudomonas aeruginosa) 30 °C (Pseudomonas putida); shaking speed 130 rpm.

Figure 9

Biodesulfurization of diesel sample obtained after HDS (120 ppm) Experimental conditions: Cell centration 1.2 g DCW/L, Temperatures 30 °C and 37 °C for resting cells of Pseudomonas putida and Pseudomonas aeruginosa, respectively, Shaking speed 130 rpm.

Figure 10

Biodesulfurization of diesel sample before HDS (5200 ppm). Experimental condition: Cell concentration 1.2 g DCW/L, Time 8 h, Temperatures 30 °C and 37 °C for resting cells of Pseudomonas aeruginosa and Pseudomonas putida, respectively, Shaking speed 130 rpm.

Figure 11

Analysis of diesel samples using GC/MS (a) standard DBT in hexane and standard 2-HBP in ethyl acetate, (b) real diesel sample obtained before hydrodesulfurization, (c) biodesulfurized real diesel sample.

Figure 12

GC–MS mass spectra of (a) DBT (b) 2-HBP.