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Fossil Fuels

Electric filed assisted technology for improving the screening and application of thiophene-biodegrading strain Chaozheng Zhang, Lin Huang, Qiaoqiao Tang, Ying Xu, Xiaotong Lan, Jie Li, Wei Liu, and Hua Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03656 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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Electric filed assisted technology for improving the screening and application of thiophene-biodegrading strain Chaozheng Zhang1,*, ‡, Lin Huang1, ‡, Qiaoqiao Tang1, Ying Xu1, Xiaotong Lan1, Jie Li1, Wei Liu2, Hua Zhao1,* 1Key Laboratory of Ministry of Education Industrial Fermentation Microbiology, Tianjin Key Laboratory of Industrial Microbiology, Tianjin Engineering Research Center of Microbial Metabolism and Fermentation Process Control, College of biotechnology, Tianjin University of Science and Technology, Tianjin, 300457, P. R. China 2School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300 130, P. R. China KEYWORDS Thiophene; Electric field assisted screening; Electric field assisted biodegradation; Identification

ABSTRACT For the screening of functional strain and biodegradation, two devices of electric field assisted screening (EFAS) and electric field assisted biodegradation (EFAB) were developed in the current work. A Candida tropicalis CZ12 capable of thiophene degradation was enriched and isolated from oily soil by EFAS device at voltage range of 10-14 v. In EFAB, the

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thiophene degradation velocity of this strain was accelerated by assisted electric field and it could reach 74% at 2h in aqueous thiophene solution. Furthermore, the removal efficiency of the continuous EFAB device for treating thiophene-containing model fuel was 79.76% under the conditions of aqueous-hydrocarbon ratio 5:1, electric field assisted voltage 15v and residence time 120min. It was clear that above advantages of EFAS and EFAB was conducive to facilitating the applications in biorefractory organics.

1. Introduction In recent years, it has become a major challenge to improve the quality of fossil fuels for meeting the requirements of new stringent environmental regulations in the petroleum refining industry. SOx in automobile emission could cause the serious denaturation of metal catalysts in the exhaust gas treatment unit, thereby aggravating the pollutants emission1. In order to reduce air pollution, the requiring concentration of the total sulfur contents in fuel listed in the environmental regulations of most countries should be lower than 10 ug g-1 2. It had been reported that fuel cells could be continuously operated even if the sulfur concentration in fuel was as low as 0.1μg g-1 3. Thus, it was significant to develop a cost-effective desulfurization technology for meeting the increasing consumption requirements. Thiophene and its derivatives are the major sulphur-containing compositions in gasoline. It was very difficult to remove thiophene from gasoline through traditional desulfurization process due to the characteristics of strong aromaticity and low electron density around the S atom4. Obviously, the key in improving the quality of gasoline was to effectively remove of thiophene. It had attracted the more attention of environmentalists and entrepreneurs to deeply optimize the desulfurization technology for removing thiophene from fuel, especially gasoline. Currently, some strategies had been proposed, involving selective adsorption desulfurization5, extraction desulfurization 6, membrane


2

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desulfurization7, oxidation desulfurization8, photochemical catalytic desulfurization4, biodesulfurization

10.

Among them, biodesulfurization became the more desired one

9

and

2c, 11

The

strains which could be used to remove organic sulfur in fuel mainly included Rhodococcus Gordonia

13,

Stachybotry

11c

Microbacterium

14,

Shewanella

10b,

Mycobacterium

15,

Lysinibacillus

12, 16,

and Sphingomonas 17, and they were screened by using dibenzothiophene as the

desired degradation material. In fact, thiophene is the organic sulphide that is most difficult to be degraded in fuel. In the current work, electric field assisted technology would be used to screen a strain from oily soil for effectively removing thiophene from gasoline. The interfacial mass transfer between oil and aqueous, the limited step of biological removal, played a critical role in determining the efficiency of biodesulfurization in fuel. Thus, it was necessary to greatly improve the interfacial mass transfer between oil and aqueous11c. Previous study had found that at a given interphase potential, the transformation ratio of dibenzothiophene in model fuel in one day of Rhodococcus erythropolis PTCC1767 and Bacillus subtilis DSMZ 3256 reached 76% and 71%, respectively 18. However，Alipoor et al.

19

had reported that the desulfurization reactions

in model fuel containing thiophene of 466 ppm was about 88% by using one dynamic electrochemical technique. Immobilization technology by adding surfactants and micro-wave assistance could significantly strengthen the interface mass transfer, resulting in a high efficiency of biodesulfurization 20. In recent years, some novel experimental devices were also developed to improve the efficiency of biodesulfurization by prolonging the working time of biological catalyst or enhancing the cellular density

20b, 20c, 21.

However, the efficiencies of above

technology and devices on biodesulfurization were not satisfactory. Based on the study on various operating equipments, Monticello22 had divided a desulfurization equipment into two parts, one was the enzyme and cell production part, and


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another was bioreactor part. However, in Monticello’s work, only future research direction was asserted, but the specific form of equipment had not been discussed. So far, this was an urgent issue needed to be solved. The objective was to investigate the feasibility of thiophene biodegradation through the electric filed assisted (EFA) technology. A strain with the capacity in thiophene biodegradation was separated by electric field assisted screening (EFAS) and its morphology features were also identified. The efficiency of this strain on thiophene degradation in aqueous phase was evaluated. A novel device coupling bioreactor and electric field assisted biodegradation (EFAB) was constructed for treating the model gasoline containing thiophene. Finally, the desulfurization capacities of thiophene under different operating conditions in the coupling device were also determined. 2. Materials and methods 2.1 Matrix of isolation and cultivation media The matrix of isolation was sampled from oily soil near gas station (Tianjin, China). The screening medium composition for isolate and growth contained (in g L-1) the following: K2HPO4, 2.00; KH2PO4, 2.00; NH4Cl, 0.4; MgSO4, 0.20; FeSO4·7H2O, 0.01; glucose 1.00; and 2 ml L-1 trace element solution (agar, 15 for solid medium). The final pH was adjusted to 7.3. Different amount of thiophene-alcohol solution was added before inoculating. The trace element solution was accordance to the previous work23. The thiophene-alcohol solution was prepared according to the following procedure: 1 mL of thiophene was added into 10 mL of alcohol and then sealed preservation. 2.2 Enrichment of thiophene-degrading strain


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The matrix of isolation (about 1 g) was used as the inoculums to enrich thiophene-degrading bacteria at 30°C in a 250 ml conical flask containing 100 ml of the screening medium, including thiophene at a low concentration (0.2 mg L-1) for the first cultivation and 2.2 mg L-1 thiophene for the last one (0.4 mg L-1 interval), as the only sulfur source. The enrichment culture technique was also used to preliminary screen the strains capable of biodegrading thiophene. The liquid culture was transferred to the next medium after the inoculums were cultured at 30 °C for 24 h, so for six times. 2.3 EFAS device The EFAS device was illustrated in Fig.1. The device consisted of two parts. One part was cathode tank and the other was anode tank. The two tanks were linked by a fluid bridge when the bridge was immersed with plenty of medium. Interchange of substance between two tanks would be conduct through the fluid bridge. Each tank had a platinum sheet (10 mm×10 mm×0.1 mm) as the electrode plate. A direct-current power was connected with the electrode plates which came out of the lid with two holes.


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Fig. 1 The EFAS device (1—lid, 2—cathode tank, 3—anode tank, 4—fluid bridge, 5-1—cathode plate, 5-2—anode plate, 6—direct-current power) 2.4 EFAS of the strains 50 mL of enriched culture were inoculated into the EFAS device containing 200 mL fresh nutrient medium. The EFAS device was placed into a thermostatic incubator at 30 °C. The 3~20 V of voltages from the direct-current power were selected to output between cathode plate and anode plate. Power supply was cut off when the bacterial colony appeared on the cathode plate. Then the cathode plate was taken out and the bacterial colony was eluted into a 250 ml conical flask containing 50 mL fresh nutrient medium. The inoculums were cultured at 30 °C for 12 h. Then the culture was transferred to EFAS device and executed screening process once again. Two screening process later, the culture was streaked out on agar plates composed of the solid medium and incubated for 1 d at 30 °C. The five colonies were picked out from the agar plates. They were all microscopy purity and their morphologies were similar. One of them was selected for further work. It was numbered CZ12 and preserved on tube slants at 4 °C. 2.5 Identification of the strain CZ12 The microbial morphology of CZ12 was observed through a microscope (Olympus BX51, Japan).The carbon sources utilizing characteristic of strain CZ12 preserved on the tube slants was firstly investigated by a Biolog test (Biolog ML3420, USA). It was also subsequently indentified by ITS sequence analysis. The photograph of the isolated strain was taken with a scanning electron microscope (FEI Apreo HIVac, Czech). 2.6 Growth characteristics experiment The screening medium and yeast extract peptone dextrose (YEPD) medium were selected to culture the strain CZ12. The growth characteristics of strain CZ12 were investigated with


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Microbiology reader (OY Growth Curves Bioscreen C, Finland). The strain in test tube was inoculated into the 100 mL of medium in 250 ml of conical flask with three rings. After incubation for 16 hours, the strain solution was transferred a 250 ml of triangular bottle with 5% inoculation containing 100 mL of medium. The initial concentrations of thiophene in both cultures were also 2.5 mg L-1. The value of OD590 was measured interval 1h by Microbiology reader. And the concentrations of thiophene in both fermentation mediums were also detected at the end of experiment. The experiments were carried out three times in parallel. The average values and the relative standard deviations(RSD) of the obtained data were calculated at the same time. 2.7 Biodegradation experiment The inoculums was prepared by inoculating three loops of cells from the plate into a 250 mL conical flask containing 100 ml of the screening medium and incubated in a shaker for 16h at 30 oC

and 180 rpm. Afterwards, this culture was inoculated into a 200 mL screening mediun

(adding 2.5 mg L-1 thiophene) in a 500 mL conical flask with an inoculation of 5%. The operations were carried out three times in parallel. The concentration of thiophene in the culture was detected when it was incubated for 24 h. The detected conditions were accordance to the previous work 24. The degrading efficiency was obtained through calculating the average values and RSD of the obtained data. 2.8 EFAB experiment The inoculums with 10% inoculation were inoculated into screening device. It was placed in an incubator at 30 oC. The thiophene concentration in the culture was detected at an hour interval. The effect of an electric field on the degradation of thiophene was investigated by means of a unelectricfied control experiment and a blank group without bacteria. The


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experiments were executed three times. The degrading efficiency was acquired through calculating the average values and RSD of the obtained data under different conditions. 2.9 Biodegradation experiment for model gasoline under continuous operation The experimental device was constructed with a 10 L of fermentation tank system (bioreactor) with the loading device (BioStat A plus, Sartorius, Germany) and an EFAB channel (15 cm width× 50 cm length × 15 cm height) installed with two Platinum plate electrodes (40cm length × 8 cm width × 1 mm thickness) on the long sides. The bioreactor was used to cultivate the strain and biodegrade the residual thiophene from the EFAB channel. The EFAB channel was the location of matter exchange and biodegradation, in which thiophene was transferred into aqueous phase from organic phase and degraded by the strain CZ12. The fermentation tank and EFAB channel were independently operated. In order to improve the removal efficiency of thiophene in organic phase, they were combined organically. The phase separators were used to separate the aqueous phase from organic phase. The schematic of the experimental device was presented in Fig. 2. The biodegradation experiment of thiophene in two-phase system was carried out in this device. The model gasoline was prepared by adding thiophene into n-octane. The concentration of thiophene was 500 mg L-1. The experimental temperature was controlled at 30 oC. The EFA voltages were changed as 12 v, 15 v and 21 v; the residence times were changed as 30 min, 60 min and 120 min; and the aqueous-organic ratios were changed as 1:1, 2:1 and 5:1. After operating stably for 1 h under each experimental condition, the thiophene concentrations in the sweet gasoline were detected, thereby calculating the removal efficiency of thiophene. All the operations were executed three times, and the average values of the obtained data were used as the results. At the same time, the relative standard deviations(RSD) were calculated.


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Fig. 2 The EFAB device 3. Results and discussion 3.1 Isolation and identification of strain CZ12 According to the previous research, the thiophene-degrading bacteria which was capable to utilize thiophene through gradually adding the concentration of thiophene in medium could be successfully enriched from oily soil10a. In order to avoid the occurrence of growth inhibition and achieve the sufficient utilization of thiophene, a low thiophene concentration was first selected. The thiophene concentration in the medium would be enhanced if that a low thiophene concentration was adopted. The preliminary screening and domestication of bacteria could be realized through the enrichment process. Subsequently, secondary screening would be executed. The enriched culture was inoculated into the EFAS device, in which different voltages were loaded between cathode plate and anode plate. After working for 5h, the response of the two electrode plate was obtained under the different voltage conditions. No response could be


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monitored from the cathode plate if the voltage was lower than 9v. However, massive bubbles were popped out from the cathode plate when the voltage was higher than 15v. In a voltage range from 10v to 14v, the colonies on the cathode plate could be formed only took two hours. The anode plate did not respond under any experimental conditions. After two continuous screening, one colony was obtained on cathode plate. Microscopic observation of the colony showed that the forms of microbial cells were consistent. One strain was obtained and it was numbered CZ12. Scanning electron microscope showed that the strain CZ12 was budding yeast (seen in Fig. 3). The cells of the strain CZ12 were bacilliform in 3.0–4.0 × 5–8 μm, Gram positive. The small and oyster white colonies on the medium plate possessed a diameter of 1 mm to 2 mm. According to the procedure of yeast identification, the strain CZ12 was identified. The results from the Biolog microstation system suggested that the metabolic characteristics of strain CZ12 was same as those of Candida tropicalis. The sequence of the strain CZ12 was determined by ITS rDNA sequencing, and then being blasted into the public database (Gene Bank). The comparison result indicated that the strain was C. tropicalis with an identification of 99%. Thus, in the current work, the strain CZ12 was named C. tropicalis CZ12. Although the feasibility of molds on the removal of sulfur had been reported, there were no any researches on the removal of sulfur (degrade thiophene) by using yeast (Torkamani et al., 2008).


10

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Fig. 3 The photograph of the strain CZ12 under a scanning electron microscope 3.2 Biological characterization of C. tropicalis CZ12

1.0 0.9 0.8 0.7

OD590

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.6 0.5

Screening medium YEPD medium

0.4 0.3 0.2 0.1 0

5

10

15

20

25

30

Time / h

Fig. 4 Growth characterization of C. tropicalis CZ12 in different mediums The growth characterizations of strain CZ12 in YEPD and screening medium were investigated. As shown in Fig. 4, the concentration of cell in YEPD medium was higher than that in the screening medium over the growth period (with OD590 said). Meanwhile, the degradations of thiophene in YEPD medium and screening medium were 27.8 ± 4.3% and 46.2 ± 3.5%, respectively. This result indicated that the nutritious YEPD medium was conducive to the growth of C. tropicalis CZ12. The weak capacity in thiophene degradation of C. tropicalis CZ12 might be caused by the fast growth velocity. 3.3 Biodegradation of thiophene in aqueous phase


11

Energy & Fuels

120

Degradation rate / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 EFAB Biodegradation EFA degradation

60 40 20 0 0

2

4

6

8

10

Time / h

Fig. 5 The effect of EFA on degradation Since EFA method had been used to screened C. tropicalis CZ12, the effect of EFA on thiophene biodegradation would be investigated. The EFA degradation (only EFA), Biodegradation (only inoculation) and EFAB were simultaneously executed with an initial thiophene concentration of 2.2 mg L-1. As shown in Fig. 5, the thiophene degradation efficiency reached 19.5% about 2 h only in the condition of EFA. This result was inconsistent with the research of19. The thiophene degradation efficiencies about 10 h under the conditions of biodegradation and EFAB were 89.0% and 99.6%, respectively. Many studies had well proven that the degradation efficiency of C. tropicalis CZ12 on desulfurization was higher than other microbes 11c, 12-14. Under the EFA condition, thiophene moved towards the cathode plate due to the action of the electric field. This result might be caused by the low electron density around the sulfur atom in thiophene. Then, the electrons would be transmitted from cathode plate to sulfur atom, resulting in thiophene degradation. The low degradation efficiency was caused by the competition between thiophene and other cations in the solution against the electrons from cathode plate.


12

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After 2 h, there was not a significant increase in the thiophene degradation efficiency because the moving velocity of thiophene was slower than that of cations in the electric field. The thiophene degradation mainly caused by microbial biodegradation when C. tropicalis CZ12 was inoculated into the EFAS device. The degradation rates in biodegradation and EFAB at 2h was as high as 72% and 74%, respectively. However, the degradation rate in EFAB within 2h was slightly greater than that in biodegradation. In EFAB, the cells in the culture were clustered on the cathode plate. The formed cluster would lead to the blocking between electrons and other electron acceptor (such as cations). The accumulated cells would obtain large amounts of electrons from the cathode plate, thus enhancing the oxidation rate of thiophene by strengthening electron transfer in the respiratory chain of C. tropicalis CZ12. After 2h, the thiophene degradation efficiencies in both EFAB and biodegradation were constant due to the decrease of thiophene concentration in the broth. Although in EFAB, the thiophene biodegradation rate could be significantly improved within 2 h, it had a negligible effect on the final degradation efficiency. All the reported desulphurization techniques have not met the requirements of industrialization because of its low removal efficiency. The removal ratio of dibenzothiophene in model fuel in 10 days of Rhodococcus rhodochrous (3552), Arthrobacter sulfureus (3332) and Gordonia rubropertincta (289) all reached 99%25. The time consumption was a pain point for biological desulfurization. However, in this current research, the removal efficiency of thiophene in EFAB device reached 74% in 2 hours and 98% in 5 hours. The removal efficiency was obviously higher than other reports2a, 12, 15, 25. 3.4 EFAB under continuous operation


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The thiophene biodegradation in the aqueous phase was promoted by loading voltage in EFA device. The thiophene biodegradation in a two-phase system was investigated by using continuous EFAB experimental device (Fig. 2) and the results were listed in Table 1. With the extension of residence time, the removal efficiencies of thiophene at different aqueous-organic ratios increased. However, the effect of EFA voltage on the removal efficiency of thiophene did not show the same trend. When the EFA voltage was 21V, some bubbles were come out from the cathode plate and the removal efficiency of thiophene at any residence time was lower than that of other voltages. Thus, 15v was the suitable EFA voltage for biodegrading thiophene by using this device. The increase of aqueous-organic ratio was conducive to obtaining high removal efficiency, but leading to the reduction of the system handing capacity for gasoline. The removal efficiency of thiophene reached 79.76% under the conditions of aqueous-organic ratio 5:1, EFA voltage 15v and residence time 120min. Then, the residence time of this current work was far less than that of the previous research 11c, 18, 20b, 20c. Despite the removal efficiency of thiophene in EFAB was similar to that of other researches 11c, 18, it was lower than that in the work of Alipoor 19.

It was clear that the thiophene degradation velocity in EFAB was significantly faster than that

in other degradation technologies, such as ultrasonic assisted degradation 20a.

Table1 The removal efficiency of thiophene under different conditions Aqueous-organic ratio

Voltage(V)

1:1 12

Residence time(min)

Removal efficiency(%, n=3)

30

15.51±1.37

60

23.22±1.82

120

41.75±3.22


14

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15

21

12

2:1

15

21

12

5:1

15

21

30

18.77±1.85

60

31.42±2.66

120

59.36±3.17

30

4.47±1.04

60

7.35±1.18

120

11.46±1.03

30

19.46±1.09

60

34.52±1.38

120

62.35±1.97

30

20.65±1.21

60

36.77±1.93

120

69.76±2.04

30

7.32±0.84

60

13.66±0.86

120

20.12±0.99

30

21.35±1.11

60

39.46±1.16

120

72.33±1.44

30

28.56±1.20

60

56.18±1.53

120

79.76±2.12

30

18.11±0.96

60

32.24±1.05

120

38.36±0.95


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4. Conclusions In this work, EFAS was used to screen the strain capable of degrading thiophene and it was also proposed to achieve the degradation of thiophene. A strain was obtained at an EFAS voltage range of 10-14v, and it was identified as C. tropicalis CZ12. Despite the growth velocity of this strain in the YEPD medium was faster than that in the screening medium; its capacity in thiophene degradation was weak. The thiophene degradation efficiency in aqueous phase was accelerated and it reached 74% at 2h in EFAB. The performance of C. tropicalis CZ12 on the removal of thiophene in the continuous EFAB device was satisfactory. The removal efficiency of thiophene was as high as 79.76%. The advantages of EFAS and EFAB contributed to extending their applications in other biorefractory organics.

AUTHOR INFORMATION Corresponding Author *Corresponding author. Tel.: +86 22 60602067; E-mail address: [email protected]， [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources This work has been supported by the Foundation (No. 2017KF001) of Key Laboratory of Industrial Fermentation Microbiology of Ministry of Education and Tianjin Key Lab of Industrial Microbiology (Tianjin University of Science & Technology) and the Foundation (No.


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ZXKF20180304) of Tianjin Engineering Research Center of Microbial Metabolism and Fermentation Process Control. ACKNOWLEDGMENT This work has been supported by the Foundation (No. 2017KF001) of Key Laboratory of Industrial Fermentation Microbiology of Ministry of Education and Tianjin Key Lab of Industrial Microbiology (Tianjin University of Science & Technology) and the Foundation (No. ZXKF20180304) of Tianjin Engineering Research Center of Microbial Metabolism and Fermentation Process Control. Thanks teacher Qianchan Pang for her support to our researches in SEM. ABBREVIATIONS EFA electric filed assisted; EFAS electric field assisted screening; EFAB, electric field assisted biodegradation; YEPD yeast extract peptone dextrose. REFERENCES 1.

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(a) Bhatia, S.; Sharma, D. K., Biodesulfurization of dibenzothiophene, its alkylated

derivatives and crude oil by a newly isolated strain Pantoea agglomerans D23W3. Biochemical Engineering Journal 2010, 50 (3), 104-109; (b) Maass, D.; Todescato, D.; Moritz, D. E.; Oliveira, J. V.; Oliveira, D.; Aa, U. D. S.; Guelli Souza, S. M., Desulfurization and denitrogenation of heavy gas oil by Rhodococcus erythropolis ATCC 4277. Bioprocess &


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