Article pubs.acs.org/EF
Research on the Effect of Surfactants on the Biodesulfurization of Coal Mengjun Zhang,† Tingting Hu,† Gaimei Ren,† Zhenyu Zhu,† and Yu Yang*,†,‡ †
School of Minerals Processing and Bioengineering, Central South University, Changsha, Hunan, China, 410083 Key Laboratory of Biometallurgy, Ministry of Education, 932 South Lushan Road, Changsha, Hunan, China, 410083
‡
ABSTRACT: The hydrophobicity of coal limits the adsorption of microorganisms and the efficiency of biological desulfurization was influenced, while the surfactant can enhance the interaction of the four phases, air, water, bacteria, and coal grain. In this experiment, three types of surfactant, anionic (SDS), cationic (DTAB), and nonionic (Tween 20), were investigated at 30 °C, and the cell concentration, pH value, leaching time, and coal biodesulfurization rate of the coal desulfurization system were detected. It is shown that the total desulfurization rate approached 29.7% in 16 days when 1100 mg/L Tween 20 was added in the shaking test, which represented the optimum efficiency. Further column leaching desulfurizating experiments showed that the desulfurization rate of the experimental group with Tween 20 (12.75%) was notably higher than that of the control group (8.32%). Along with the processing of desulfurization, the cell concentration decreased at first, and then rapidly increased, and finally stayed constant. The pH value of coal water slurry increased. The leaching time increased with the process of the desulfurization system. We concluded that the nonionic surfactant has a significant effect on coal biodesulfurization.
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practices.9 Also, the floatation desulfurization experiments of high-sulfur coal whose surface pretreatment was made by utilizing Rhodopseudomonas spheroides and Thiobacillus ferroxidant by Chinese researchers10,11 have gained satisfactory results. Depending on its dissociation properties of the polar group, surfactants could be classified into anionic, cationic, and nonionic surfactants. The effect of surfactants on the viscosity and emulsification property was found to be generally discussed. The contact of bacteria and coal grain can be enhanced by adding appropriate surfactant in the biodesulfurization process,12,13 because the efficiency of the biodesulfurization will be improved.14 This experiment took the coal from the Liupanshui mine as the experimental object. The mixed culture of Acidithiobacillus caldus (A. caldus for short) and Acidithiobacillus thiooxidans (A. thiooxidans for short) were employed to desulfurize the coal, and three types surfactant, anionic (SDS), cationic (DTAB), and nonionic (Tween 20), respectively, were added. In this context, the effect of the three surfactants on the desulfurization system was demonstrated. The optimal surfactant and its concentration were identified by taking the shaking test, and the column leaching was carried out for the further verification.
INTRODUCTION Coal is still one of the irreplaceable energy resource in terms of its occupancy of the recoverable fossil fuel resources is 66.8%.1 Till now, coal has been used in over 100 power plants, over 500 industrial furnaces, and several thousand of various kilns in China.2 However, a large amount of toxic or harmful gas from the high sulfur coal combustion was released into the air, especially SO2, causing serious pollution and leading to severe environmental problems. However, great quantities of coal are utilized insufficiently and the situation is expected to get worse with the development of the coal chemical industry. Therefore, it is urging research on new applications for coal pitch. The best method to limit the amount of sulfur oxides emitted into the atmosphere is to reduce the amount of sulfur in coal before combustion.3,4 Thus, studies in this area have drawn increased attention from researchers in recent years. Among the existing coal desulfurization technologies, biodesulfurization technology has been one of the most potential ways. He et al. used the acidophilic and thermophilic strain Acidithiobacillus caldus to desulfurize coal and bioleaching coal pyrite.5 Finally, coal pyritic desulfurization with A. caldus was about 47% and the total desulfurization was 19%. Cardona et al. used a consortium of native microorganisms to desulfurize two coals from the southwest of Colombia.6 The results showed that 85−95% reduction of pyritic sulfur and 31−51% of total sulfur were removed, in a period of 30 days. Since the 1990s, the Central Research Institute of Electric Power Industry in Japan started to make use of microbials to improve the efficiency of desulfurization, since it could be adsorbed in the surface of the mineral, and change the mineral’s physicochemical property.7 Fazaelipoor et al. found that a bacterial species (Pseudomonas aeruginosa) could produce a biosurfactant, and examined in coal flotation as a frother.8 Subsequently, Khoshdast et al. found that rhamnolipid could be used as a promising frother or co-frother in mineral processing © XXXX American Chemical Society
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MATERIALS AND METHODS
Coal Sample. The coal sample used in the experiment was collected from Liupanshui, Guizhou Province, China. X-ray diffraction (XRD) was used to analyze the component of the coal sample.15 The result shown in Table 1 indicated that the total sulfur of the coal sample was 4.55%, which was composed of 2.44% of pyritic sulfur, 1.99% of organic sulfur, and 0.12% of SO42−. The sample was ground Received: April 30, 2017 Revised: July 18, 2017 Published: July 20, 2017 A
DOI: 10.1021/acs.energyfuels.7b01116 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Sulfur Content of Different Forms in Coal sulfur forms
content/(%)
total sulfur pyritic sulfur sulfate sulfur organic sulfur
4.55 2.44 0.12 1.99
in a ball mill and sized to less than 0.2 mm for the shaking test and 5 mm for the column leaching test.16 Medium. The 9K medium was used for cultivation of A. caldus and A. thiooxidans, which contained 3.0 g of (NH4)2SO4, 0.1 g of KCl, 0.5 g of K2HPO4, 0.5 g of MgSO4, 0.01 g of Ca(NO3)2, and 1 L of deionized water.17 Sulfur sources, elemental sulfur, were added to the medium at the concentration of 10 g/L. The final pH value was 2.0. Surfactants. We selected three surfactants, sodium dodecyl sulfonate (SDS), dodecyl trimethylammonium bromide (DTAB), and Tween 20, which represent anionic, cationic, and nonionic, respectively. The experimental design of the surfactants on the coal desulfurization are shown in Table 2. Microorganisms and Cultivation. The strain of A. caldus and A. thiooxidans were originally isolated from acid mine drainage (AMD) in the coal mines. The strain was inoculated into 100 mL of sterilized medium and cultured at 30 °C with constant agitation (170 rpm) for 48 h. Shaking Test. The row coal used in the experiment was sterilized and the biodesulfurization process of different surfaces was carried out in flasks with a volume capacity of 100 mL in 250 mL of 9K medium, 15% w/v pulp density, the initial cell concentration of 1.0 × 106 cells· mL−1, and a processing time of 16 days. The desulfurization system consisted of A. caldus and A. thiooxidans (concentration ratio was 1:1). Three surfactants were selected in this study: anionic (SDS), cationic (DTAB), and nonionic (Tween 20), which were added to explore the influence to the coal desulfurization rate and to find the optimal surfactant and its concentration. The control group was without surfactant. After the shaking test, the sulfur content was detected to calculate the efficiency of biodesulfurization. Column Leaching. Glass columns (XK50, Pharmacia Biotech), 20 cm long with a 5 cm internal diameter, were packed with 400 g of airdried coals. The experimental system is shown in Figure 1. The surfactant and its concentration used in the column leaching test were determined by the results of the shaking test. To reduce the energy consumption of the machine, column leaching of desulfurization system of microbial desulfurization processing phase intermittent leaching method.18,19 At the beginning of leaching, 1 mL of the leaching liquor was taken to calculate the mixed bacteria cells concentration and pH value of the desulfurization system. Determination period of the cycle is 12 days. After the end of the column leaching desulfurization cycle, the coal sample was drenched with the dilute hydrochloric acid with 10% first, and then rinsed with distilled water several times, and dried at 60 °C at last. The ratio of total sulfur content before and after treatment was the desulfurization rate. The total sulfur content was detected by the Eschka method to calculate the efficiency of biodesulfurization. According to the GB/T214-2007, the various forms of sulfur content were detected.
Figure 1. Schematic of column desulfurization test.
surfactant, Tween 20, SDS, and DTAB, that have an influence on biodesulfurization rate are shown in Table 3. The three types of surfactants all had an influence on the coal biodesulfurization rate at different levels. Among these, 1100 mg/L Tween 20 reached the highest total sulfur desulfurization rate 29.7%, and the minimum desulfurization rate was 9.9%, shown in SDS concentration with 150 and 250 mg/L at the same time. According to previous studies, cationic surfactants are highly toxic, and the anionic neutral, nonionic surfactants are generally less toxic.20 Cationic surfactants are often used as disinfectants and have a strong inhibitory effect on various bacteria, molds, and fungi.21 Sodium dodecyl sulfate has a great toxicity as a common anionic surfactant, while Tween 20 is a low toxicity nonionic surfactant.22,23 Therefore, the low concentrations of cationic and anionic surfactants and the high concentrations of nonionic surfactants were used in the experiments. Thus, along with the concentration of SDS and DTAB, the growth and the desulfurization activity of the bioleaching microbe were limited, and the efficiency of biological desulfurization was decreased. On the other hand, due to the different molecular structures of surfactants, they had different influence on the desulfurization system.24 The Tween 20 added group has the highest desulfurization rate probably because of the nonionic surfactant’s surface modification functions; meanwhile, nonionic surfactants having no extra electric charge, which would have weak influence to the whole desulfurization system, also resulted in the highest rate. Contrarily, SDS and DTAB have certain effects on the desulfurization system due to their carrying an electric charge.14 The Influence of Tween 20 on Leaching Desulfurization System. In this experiment, the concentration of Tween 20 was 1100 mg/L. The change of cell concentration and pH value of the two groups of the column leaching system are shown in Figure 2. In the first 8 days, the cell concentration of the control group was significantly increased; since then, the bacteria enter the decline period. For the experimental group, the amount of
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RESULTS AND DISCUSSION The Influence of Three Types Surfactant on Coal Biodesulfurization Rate. The results of three types of
Table 2. Experimental Design of the Surfactants on the Coal Desulfurization surfactants SDS/(mg/L) gradients abbreviation
100 S1
150 S2
DTAB/(mg/L) 250 S3
10 D1
70 D2 B
Tween 20/(mg/L) 140 D3
550 T1
880 T2
1100 T3
DOI: 10.1021/acs.energyfuels.7b01116 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 3. Results of Coal Desulfurization Rate with Different Surfactants abbreviation total sulfur content/(%) desulfurization rate/(%)
S1
S2
S3
D1
D2
D3
T1
T2
T3
blank group
3.7 18.7
4.1 9.9
4.1 9.9
3.5 23.1
3.55 22
3.65 19.7
4 12.1
3.8 16.5
3.2 29.7
3.8 16.5
Figure 2. Cell concentration (left) and pH value (right) in the leachates of the column desulfurization test.
system would decrease. The pH value of Tween 20 added group increased in the first 2 days, followed by a sharp decrease. The final pH value was 1.42; meanwhile, the control group was 1.67. The adsorption of bacteria caused the etching of coal particles, which resulted in less mineral acid solution, which made the pH value of the system in the experimental group rise. However, the bacteria adsorbed on the coal particle surface could constantly metabolize pyrite sulfur effectively, and a large number of H+ were generated, which caused a sustained drop in the pH of the desulfurization system.26 The Change of Leaching Time. Figure 3 shows the change of leaching time (LT) of the column desulfurization test. The LT of the desulfurization system rapidly increased on the 2nd day of the Tween 20 added group, and its value was already raised from 5 to 42 min at the 8th day. However, the control group increased slightly; the maximum value was only up to 11 min. The internal of column leaching biodesulfurization system may be changed by adding Tween 20, making the bacterial liquid difficult to drain. The moisture of the control group can drain more fluently because of coal’s hydrophobicity. In the experimental group, the Tween 20 changed the character of the coal particle surface, which made it easier for bacterial to adsorb on the surface of coal particles. Once the microbials adsorbed on the mineral surface, they will produce large amounts of extracellular polymeric substance,27,28 forming a dense biological membrane structure which affected the micro pipelines of water runoff in the column leaching desulfurization system and made it more difficult to drain, which resulted in the significant extension of leaching time of the experimental group. The Influence of Tween 20 on the Coal Biodesulfurization Rate. The desulfurization rates of the test are shown in Figure 4. The Tween 20 added group degraded 12.75% total sulfur from the coal sample, which is higher than the control group 8.32%, and the inorganic sulfur desulfurization rate was 21.78% and 13.04%, respectively. Moreover, the organic sulfur content remained substantially unchanged. Tween 20 promoted the adsorption of the bacteria, which increased the
surface adsorption bacteria content decreased and the bound bacteria content increased after the dispersants were added. The cell concentration of the experimental group declined in the second day; it may be because of a large number of bacterial cells adsorbed to the surface of coal particles, leading to low free cell concentration in the leachate. On the 10th day, the cell concentration of the experimental group was much higher than that of the control group, which was because that with the death of the bacteria, extracellular polymeric substance of the dead bacteria cannot be released into the solution in time, forming an extracellular polymer passivation layer which hindered the readsorption of the bacteria.25 The change of pH value of the two groups of the column leaching system is shown in Figure 3 (right). The pH value of the two groups all trend to decreased. The coal sample itself contained a certain amount of humic acid, and once the coal sample dissolved in the culture medium, the pH value of the
Figure 3. Change of leaching time of the column desulfurization test. C
DOI: 10.1021/acs.energyfuels.7b01116 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
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Figure 4. Desulfurization rate of the column desulfurization test.
secretion of extracellular polymeric substance and built an abundant biological membrane structure. However, the little difference of the desulfurization rate between the two groups could be caused by the extracellular polymer passivation layer which formed at the later stage of biodesulfurization. Therefore, Tween 20 can promote the desulfurization process at some degree, although it mainly playd a “double-edged sword” role for the desulfurization system.
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CONCLUSION Biodesulfurization still has not yet received large-scale industrial applications; however, it has shown some enormous potential in the development of energy industry and environment protection. In these experiments, the bacteria Acidithiobacillus caldus and Acidithiobacillus thiooxidans were employed to desulfurize the coal, and three types of surfactant, anionic (SDS), cationic (DTAB), and nonionic (Tween 20), respectively, were added. The latter surfactant is preferred as it affords a framework of higher quality and stability, as judged by the desulfurization rate. All the signs indicated the necessity to study the microbiological performance as well as the changes of the desulfurization system after surfactant is added. Therefore, the future studies could concentrate on how the biosurfactant influenced the desulfurization system.
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AMD=acid mine drainage LT=leaching time
AUTHOR INFORMATION
Corresponding Author
*Tel: +86-731-88877216. Fax: +86-731-88710804. E-mail:
[email protected]. ORCID
Mengjun Zhang: 0000-0002-4408-0677 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Program of Hunan Science and Technology Plan (2012FJ4322), the Program of New Century Excellent Talents in Ministry of Education of China (NCET-07-0869), and the Program of Central South University Independent Innovation (2017zzts383).
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ABBREVIATIONS XRD=X-ray diffraction SDS=sodium dodecyl sulfonate DTAB=dodecyl trimethylammonium bromide D
DOI: 10.1021/acs.energyfuels.7b01116 Energy Fuels XXXX, XXX, XXX−XXX
