미생물 고정화를 이용한 DMS의 제거

김종연, 김병우

성균관대학교 화학공학과

Removal of DMS(dimethylsulfide) by Immobilization of Biocatalyst

Jong-Yeon Kim, Byoung-Woo Kim

Department of Chemical Engineering, Sung Kyun Kwan University

INTRODUCTION

Dimethyl sulfide is malodorous compound which exceeds the odor threshold at low concentration. It is among the major volatile organic sulfur compounds produced in natural environments(1,2). It can arise from the breakdown of sulphur-cotaining amino acids and lignin, and can be produced by algae, and in soils, freshwater, anaerobic lagoons and oceans(2-4). Methanethiol, DMS and dimethyl disulphide also present a major odor problem in the wood-pulping industry(4). DMS is believed to play an important role in the global sulfur cycle. DMS emissions represent more than 90% of the biogenic sulfur emissiions from oceans and approximately half of the total biogenic sulfur emissions to the atmosphere(5,6). Oxidation products of DMS contribute to the acidity of atmospheric particles and rain, and it was also postulated that DMS plays an important role in marine cloud formation and climate regulation(7).

THEORY

The physical and chemical processes that have been used to remove malodorous gases from waste gas and wastewater include air stripping, activated carbon adsorption, wet-scrubber, and incineration. But, the corresponding costs for such technologies and their disposals are economically incompetitive, and secondary-pollutant issues may arise. Recently, the focus has been shifted to investigation of the microbial alternatives to conventional methods. Biofiltration has been proven to be an effective and inexpensive method, especially when applied to dilute, easily biodegradable waste gases under appropriate sets of conditions. However, the biofilter may cause environmental risks because bioaerosols(fungi and bacteria) can be released from the bioflter into the ambient air. Recently, these disadvantages have been overcome by using an immobilized-cell method.

The immobilized-cell technology has been applied for wastewater treatment because of its many advantages, such as prevention of cell losses, high microbe contents, high tolerance to environmental impact, and high stability during the operation period(8). However, little information is available for DMS(g) removal from air gas by immobilized-cell technology.

In addition to selection of appropriate packing materials, it is also important to use an effective species for optimizing DMS treatment. Autotrophic bacteria, such as sulfur-oxidizers, have been characterized phylogenically Kanagawa et al(9).

Cells immobilized in calcium alginate beads for H2S(g) removal has been regarded as a potential method because of their high removal efficiency, removal capacity, and the economic consideration. In this study, we developed an innovative DMS gas treatment method by an immobilized Thiobacillus thioparus TK-m biofilter and also conducted a quantitative investigation into the principles and operation of a biofilter system for DMS removal.

The DMS removal rate in the immobilized-cell biofilter was calculated using the following equation derived from the Michaelis-Menten equation(10).

[eqn: { 1} over { R}= {Ks } over { Vm} { 1} over { Cln} + { 1} over {Vm }]

where R(g of S/day·(kg of beads)) = removal rate, Cln(ppm) = logarithmic mean concentration of DMS at the inlet and outlet of the biofilter, Vm (g of S/day·(kg of beads)) = maximum removal rate, and Ks (ppm) = saturation constant. Using the linear relationship between 1/Cln and 1/R, Vm and Ks values were calculated from the slope and intercept, respectively.

EXPERIMENTAL

In this experiment, Thiobacillus thioparus TK-m(DSM5368)was used. This  bacteria is the colorless sulfur-oxidizing bacteria. No intracellular sulfur is visible under the light microscope, but these may contain finely dispersed sulfur, visible under the electron microscope after staining with silver salts. Thiobacillus thioparus TK-m deposits sulfrur in the colonies after prolonged incubation in the presence of sulfide, making the colonies(yellowish) white and opque. T. thioparus TK-m is obligate chemolithoautotrophs, being dependent on CO2 and reduced sulfur compounds for biosynthetic carbon and energy. This microorganisms are mesophilic, no growth above 42℃. T. thioparus TK-m oxidizes thiocyanate; final pH in liquid thiosulfate medium 3.5-4.5.

Table. 1. Basic characteristics of the obligately autotrophic colorless sulfur bacteria T. thioparus TK-m.

Spcies	% G+C	Motility	Carboxysomes	pH	S2O32- oxidation to: SO42-	Opt. temp.(℃)	Ubiquinone
Thiobacillus thioparus	61-66	+	+	6-8	+	30	Q-8

Basal medium P contained(g/L):  KH2PO4(40), K2HPO4(40), Na2CO3(8). Medium  Mg contained(g/L): MgCl2·6H2O(20), 1mL trace metal solution and 5mL vitamin mixture. The trace metal solution contained(g/L): Disodium EDTA(50), ZnSO4·7H2O(11), CaCl2(5), MnCl2·4H2O(2.5), FeSO4·7H2O(5), CuSO4·5H2O(0.2). Vitamin mixtures contained(mg/L): tiamine·HCl(10), riboflavin(20), pyridoxine·HCl(10), nicotinic acid(10), p-aminobenzoic acid(10), calcium pantotheate(20), biotin(1), cyanocobalamin(1). And 5% Na2S2O3·H2O(50mL) was used.

Culture(50mL) was grown in 125-mL erlenmyer flask sealed with rubber stopper and shaken at 30℃. After 3 days, the culture were moved to larger volume of medium for enrichment.

T. thioparus TK-m, grown in 100mL medium, was harvested by centrifugation(7500g, 10min) and then washed three times with sterile demineralized water. The organisms were mixed with a sterile 4% sodium algiante solution, and this cell containing solution was then mixed with a sterile 4% CaCl2 solution. Upon mixing, 3 mm diameter immobilized beads were formed. These beads were then activated by being flushed with sterile buffer solution for 5 h.

Fig. 1. Schematic diagram of the deodorizing apparatus used in this study.

A schematic diagram of the deodorizing apparatus used in this study is shown(Fig. 1). The acrylic tube(50 mm by 100mm) was filled with immobilized-cell beads. DMS was diluted with air and supplied to the bottom of the tube through fritted glass difuser. The flow rate of the gas and the air was controlled by mass flow controllers(UNIT co.). The bacterial cultures in the tube were maintained at 25℃ and at pH 6.8 ± 0.4 by titration with 1 M K2CO3. The DMS gas was measured by gas chromatography. A 5g(wet-weight) amount of cell-laden beads was dissolved in 95mL of 0.1 M sodium citrate solution, and the sulfur compounds in the solution were determined. Sulfate concentration in the solution was determined by the turbidometric method, and sulfur was determined by spectrometer at 290nm.

RESULTS and DISSCUSSION

Kanagawa et al studied DMS gas removal in suspension condition(9). In their study, DMS gas (100□L/L) was supplied at 250 Ncm3 per min to the aeration tube. The initial cell concentration was an A660 value of 0.008. The outlet gas contained 25□L of DMS per liter after 2h, but DMS was not detected after 24h(A660, 0.032) or subsequently in the outlet gas by either gas chromatography or the human volunteers. And they measured the maximum degradation rate of DMS by the cells. That showed that the maximum degradation rate was 4.0 g/g per day.

In this immobilized-cells method, It was scheduled to improve the removal capacity of DMS gas. The maximum degradation rate and capacity of the apparatus to remove DMS were measured.

It is shown in Fig. 2. that the growth of T. thioparus TK-m on thiosulfate(A660) was continued for 43 hours. It shows that the concentration of sulfate(A290) is increased stoichiometrically with cell growth.

Fig. 2. The growth of T. thioparus TK-m on thiosulfate.

This is the first study in which it is demonstrated that dimethylsulfide is removed from contaminated air by an identified microorganism, T. thioparus TK-m by immobilization. Our results may demonstrate that the immobilized T. thiobacillus TK-m biofilter can remove DMS from the gas phase with high efficiency. The main product of DMS oxidation is sulfate.

REFERENCES

1. Krouse, J.R., and McCready, R.G.L., Bilgeochemical Cycling of Mineral-forming Ellements, Elsevier, Amsterdam, 401(1979)

2. Rasmussen, R.A., Tellus, 26, 254(1974)

3. Lovelock, J.E., Maggs, R., and Rasmussen, R.A., Nature 237, 452(1972)

4. Sivela, S., and Sundman, V., Arch. Microbilo., 103, 303(1975)

5. Andreae, M.O., and Raemdonck, H., 221, 744(1983)

6. Dacey, J. W.H., King, G.M., Wakeham, S.M., 330, 643(1987)

7. Charson, R.J., Lonelock, J.E., Andreae, M.D., and Warren, S.G., Nature, 326, 655(1987)

8. Ginkel, G. C. V., Tramper, J. K., Luyben, C. A. M., and Klapwijk, A., Enzyme Microb. Technol., 5, 297(1983)

9. Kanagawa, T., and D.P. Kelly, FEMS Microbiol. Lett., 34, 13(1986)

10. Hirai, M., Ohtake, M., and Shoda, M., J. Ferment. Bioeng., 70, 334(1990)