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Oxidation of Dimethyl Sulfoxide in Aqueous Solution Using Microbubbles Pan Li,*,†,§ Hideki Tsuge,‡ and Keiko Itoh‡ Graduate School of Science and Technology and Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
A large quantity of dimethyl sulfoxide (DMSO) wastewater is discharged from washing and rinsing processes in semiconductor manufacturing industry. Traditional biological treatment is known to be difficult for the treatment of DMSO-containing wastewater because of odor problems. Ozonation of DMSO combined with a biological process is suggested to be a cost-efficient treatment solution, whereas the application of ozone to wastewater treatment has been limited by its low utilization efficiency and high cost. In this study, we applied an ozone microbubble generator to increase ozone transfer efficiency in the aqueous solution. The oxidation of DMSO by ozone microbubbles was investigated in a bubble column reactor with an inner diameter of 20 cm. We studied the dependence of DMSO degradation on the gas and liquid flow rates. Experimental results indicate that the ozonation of DMSO is a first-order mass-transfer-controlled reaction and the reaction rate constant increases with increasing gas velocity. Ozone transfer ratio increases with decrease in gas flow rate. 1. Introduction Microbubbles are often defined as bubbles with diameters of several tens of micrometers and have many characteristics different from common millibubbles with diameters of the order of millimeters. A typical property of microbubbles is a high internal pressure, which results from surface tension at the gas-liquid interface. The Young-Laplace equation states that ∆P ) 4σ/db
(1)
where ∆P is the pressure difference between the inside and outside of a bubble, σ is the surface tension of the liquid, and db is the bubble diameter. Therefore, the interior gas pressure increases as the bubble becomes smaller. Moreover, Henry’s law indicates that the mass of gas transferred from the gas phase to the liquid phase increases with rising gas pressure, which results in shrinkage of the microbubbles. In recent years, microbubbles have attracted much attention in the fields of wastewater treatment, chemical process, and medicine.1 We developed a bubble column using microbubbles and demonstrated that the mass-transfer efficiency of ozone in the aqueous solution is greatly improved.2 The mass-transfer efficiency of ozone from the gas phase to the liquid phase is always evaluated by the liquid-phase volumetric mass-transfer coefficient, kLa, because the gas-phase mass-transfer resistance can be ignored because of the relatively low solubility of ozone in water.3 The mass-transfer coefficient, kL, depends on the mixing characteristics of the gas-liquid contactor used and the kinetics of ozone reactions produced, while the specific interfacial area, a, is determined by the number and size of ozone-containing bubbles produced. The large specific interfacial area of microbubbles and their tendency to decrease in size and subsequently to * To whom correspondence should be addressed. Tel.: 81-29-8618751. Fax: 81-29-861-8496. E-mail:
[email protected]. † Graduate School of Science and Technology. § Present address: Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba East, 16-1, Onogawa, Tsukuba, Ibaraki 305-8569, Japan. ‡ Faculty of Science and Technology.
disappear under water lead to their efficient gaseous solubility. Figure 1 shows a comparison of the volumetric masstransfer coefficient of oxygen in different bubble columns using microbubbles2,4 and millibubbles.5 The mass transfer of oxygen from the gas phase to the liquid phase was improved when microbubbles with diameter less than 100 µm were used. Dimethyl sulfoxide (DMSO, (CH3)2SO) is widely used as a detergent or a photoresist stripping solvent in the manufacture of semiconductors and liquid crystal displays, so that a large quantity of DMSO wastewater is discharged from the washing and rinsing processes. Traditional biological methods are known to be difficult for the treatment of DMSOcontaining wastewater because of odor problems caused by intermediate products such as dimethyl sulfide (DMS), methyl mercaptan, and hydrogen sulfide.6 Recently, some researchers suggested using a combination of advanced oxidation processes (AOPs) and biological processes to provide a costefficient treatment solution for the treatment of DMSOcontaining wastewater.7,8 The combination process always consists of two steps: First, oxidation of DMSO by AOPs to dimethyl sulfone (DMSO2) or methane sulfonic acid (MSA), and second, further biodegradation of DMSO2 and MSA to sulfuric acid by activated sludge without producing any reduced and harmful sulfur-containing byproduct. In these AOPs, hydroxyl radical (•OH) is assumed to be the main oxidant responsible for the decomposition of DMSO. In the first part of this study, we tried to search for a nonchemical method to decompose DMSO with air microbubbles. Takahashi et al. reported that free radicals are generated from collapsing air microbubbles.9 If free radicals are generated during collapsing air microbubbles, the produced • OH can degrade DMSO to MSA. This hypothesis was checked by air microbubble generation under various pH’s and conductivities. It has been demonstrated that ozone mass transfer can be improved by the application of microbubbles in our previous work.2 Consequently, the reaction between ozone and organic substance is supposed to be accelerated by using microbubbles. In the second part of this study, we studied the oxidation of DMSO by ozone microbubbles under different
10.1021/ie801565v CCC: $40.75 2009 American Chemical Society Published on Web 05/12/2009
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Figure 1. Comparison of volumetric mass-transfer coefficient of oxygen in bubble columns using microbubbles (92) and millibubbles (b).
Figure 2. Schematic diagram of the experimental apparatus.
gas and liquid flow rates. Ozone transfer ratio, the mass ratio of ozone transferred into liquid to input ozone, was obtained. The ozonation mechanism of DMSO was discussed. 2. Experimental Section DMSO aqueous solution was prepared by diluting DMSO reagent with distilled water, and its concentration was kept about 10 mg/L in all experiments. The pH of DMSO solution was varied in the range of 4.3-9.5 by adding 0.1 M hydrochloric acid or 0.05 M sodium hydroxide solutions. Ionic strength of DMSO solution was varied by adding sodium chloride from 0 to 1 wt %. 2.1. Experimental Apparatus. A schematic diagram of the experimental apparatus is illustrated in Figure 2. The bubble column made of acrylic resin has an inner diameter of 0.20 m
and a height of 1.20 m. Microbubble generation system consists of a centrifugal pump (20KED04S, Nikuni Co., Ltd.) and a rotating-flow microbubble generator (M2-LM/PVC, Nanoplanet Research Institute Co.), which was set near the bottom of the bubble column. To increase the gas/liquid flow rate ratio, G/L, a centrifugal pump and microbubble generator were combined as mentioned in our previous article.2 Air was aspirated by the centrifugal pump, and the liquid and air were simultaneously mixed by the pump. The pressurized mixture of air and liquid was then decompressed through the microbubble generator with a high rotating velocity, and microbubbles were generated. We observed that the condensed microbubbles gave the water a milky appearance. The majority of the microbubbles ranged in size from 20 to 100 µm, and the average diameter was about 50 µm, as measured by a laser diffraction particle size analyzer (LS230, Bechman Coulter, Inc.) in our previous work.10 2.2. Experimental Method. All the experiments were carried out in semibatch mode. To examine the formation of the free radical, air microbubbles were generated at air and liquid flow rates of 0.5 and 15.0 L/min, respectively. Samples were taken from the sampling tap in the middle of the reactor every 5 min, and DMSO and DMSO2 concentrations in aqueous solutions were detected by GC-FID (GC-2010, Shimadzu Co.). For the experiments on ozone oxidation, ozone was generated from oxygen by a corona-discharge ozone generator (SO-03UN03, Tokyu Car Co.). Ozone concentration in solutions was online monitored by a polarographic ozone meter (ELP-100, Ebara Jitsugyo Co., Ltd.) during the whole reaction process. A vane pump was used to continuously sample water to the ozone meter. Ozone concentration in the gas phase was also online monitored using a UV-absorption ozone meter (EG-320, Ebara Jitsugyo Co., Ltd.), and then the outlet gas was fed into an ozone destructor. Ozone microbubbles were generated at various gas and liquid flow rates. The ozone gas flow rate was measured by a mass flow meter (model 3340, Kofloc Co.), while the water flow rate was measured with a flow meter (SP-562, Tecflow International IR-Flow Co.). A transistor inverter (VF-nC12004P, Toshiba Co., Ltd.) connected with the centrifugal pump was
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used to control the water flow rate. Unbuffered distilled water was used in all the ozonation experiments. Total organic carbon (TOC) concentration of aqueous samples was measured with a TOC meter (TOC-5000, Shimadzu Co.). A pH meter (HM-40S, DKK-TOA Co.) was used to measure pH of the samples. 3. Theoretical Section In a gas-liquid system where gas dissolution is followed by a chemical reaction, two steps control the overall reaction rate: the mass transfer from gas phase to liquid phase and the chemical reaction in the liquid phase.11,12 The ozone oxidation can be considered to be a mass-transfer-controlled reaction owing to low solubility of ozone.13 The rate of mass transfer of ozone from gas phase into liquid phase is limited by liquid film diffusion.12,14-16 It is assumed that the ozonation of DMSO can be presented by eq 2 with a stoichiometric ratio, b. DMSO + bO3 + H2O f DMSO2 + other products (2) The reaction rate can be defined by eq 3 by assuming a firstorder reaction.13,17 -
(
PO3 DL,DCD dCD ) kL,O3a· + dt DL,O3 bHO3
)
(3)
where CD (mM) is the concentration of DMSO, kL,O3a (s-1) is the volumetric mass-transfer coefficient of ozone, DL,D and DL,O3 (m2/s) are the molecular diffusivity of DMSO and ozone, PO3 (kPa) is the partial pressure of ozone gas in gas input, and HO3 (kPa/mol frac) is Henry’s constant of ozone. Saunders et al. also showed that the second term (PO3/bHO3) is negligible in comparison to the first term.17 Therefore, eq 3 could be simplified to
-
(kL,O3a)DL,D dCD ) ·CD ) kD·CD dt DL,O3
(4)
where kD (s-1) is the reaction constant. Assuming that the diffusivities of ozone and DMSO in water and kL,O3a remain constant during the reaction, eq 5 can be obtained by integrating eq 4. ln
(kL,O3a)DL,D CD,0 ) t ) kDt CD DL,O3
(5)
where CD,0 is the initial concentration of DMSO. If there is a linear relationship between ln(CD,0/CD) and the reaction time, the first-order ozonation kinetics of DMSO is assumed and the slope represents kD. 4. Results and Discussion 4.1. Air Microbubbles. No change in DMSO and TOC concentrations was observed during the reaction period of 20 min by blowing air microbubbles at different pH (4.3-9.5) and NaCl concentrations (0-1 wt %). MSA was not detected either, which suggests that no hydroxyl free radicals were generated from collapsing air microbubble under the present conditions. Takahashi et al.9 confirmed hydroxyl free radical generation from the collapse of microbubbles using electron spin resonance spectroscopy. However, the experiments were conducted under strongly acidic conditions (0.18 M H2SO4, pH ) 0.44), while the lowest pH value in our experiments was 4.3. 4.2. Ozone Microbubbles. 4.2.1. Effect of Ozone Gas Flow Rate on DMSO Oxidation. Gas flow rate of input ozone was varied from 0.25 to 1.50 L/min, while water flow rate and ozone concentration was kept at 15.0 L/min and 28.2 g/Nm3, respectively. Figure 3 shows the changes of CD/CD,0 and DMSO2 concentrations with reaction time. DMSO concentrations decrease, while DMSO2 concentrations in-
Figure 3. Effect of gas flow rate on ozonation of DMSO. Closed symbols represent CD/CD,0; open symbols represent the concentration of DMSO2.
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Figure 4. Effect of gas flow rate on ozone concentration in liquid and off-gas.
crease with time proceeding. The oxidation rate, that is, the decrease rate of DMSO concentration or the increase rate of DMSO2 concentration, also increases with increasing ozone gas flow rate. When the input gas flow rate increases, the concentration of ozone dissolved in liquid also increases as shown in Figure 4a, which resulted in the increase of oxidation rate. By assuming that the inlet and outlet ozone gases have the same flow rate (Qin ) Qout), we define the ozone transfer ratio, η (%), in the semibatch ozone contactor as follows:12 η(t) )
)
Win - Wout × 100 Win QinCint -
) (1 -
∫Q t
outCout
0
QinCint 1 Cint
∫C t
0
out
dt
× 100
dt) × 100
(6)
where Win and Wout are the total input and output amounts of ozone (mg) measured in the gas phase, respectively. Cin and Cout are the ozone concentrations in inlet gas at the bottom of the bubble column and off-gas from its top (mg/L). The change of ozone concentration in off-gas with time as shown in Figure 4b was used to calculate the ozone transfer ratio, η (%), by eq 6. The calculated results of η are illustrated in Figure 5. The ozone transfer ratio at the reaction time of 10 min decreases from 95 to 65% when the ozone gas flow rate was increased from 0.25 to 1.5 L/min. The linear relationship between -ln(CD/CD,0) and reaction time as shown in Figure 2 indicates that the ozonation of DMSO is a first-order mass-transfer-controlled reaction. The reaction rate constants obtained from the slopes are plotted in Figure 6. The reaction constant kD increases from 7.0 × 10-4 to 1.9 × 10-3 s-1 with increasing gas superficial velocity from 0.03 to 0.20 mm/s. 4.2.2. Effect of Water Flow Rate on DMSO Oxidation. The flow rate of water circulated by the centrifugal pump was changed from 15.0 to 19.5 L/min, while gas flow rate
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Figure 5. Effect of gas flow rate on ozone transfer ratio.
Figure 6. Relationship between reaction constant and gas superficial velocity.
and ozone concentration were kept at 1.0 L/min and 28.2 g/Nm3, respectively. The effect of water flow rate on the oxidation rate is very small as compared with that of the gas flow rate.
the mass ratio of the ozone transferred from the phase into the liquid phase to the ozone input, increases with decreasing gas flow rate. Acknowledgment
5. Conclusions No •OH free radicals were detected during air microbubbles treatment when the pH and the salt concentrations were changed during the ranges of 4.3-9.5 and 0-1 wt %, respectively. The oxidation of DMSO by ozone microbubbles was studied under various gas and liquid flow rates. Experimental results indicate that the ozonation of DMSO is a first-order mass-transfercontrolled reaction. The reaction constant kD increases with increasing gas superficial velocity. Ozone transfer ratio, namely,
We are deeply indebted for the support of Organo Co. for the present study. Nomenclature db ) bubble diameter (m) CD ) concentration of DMSO (mM) Cin, Cout ) ozone concentrations in inlet gas at the bottom of the bubble column and off-gas from its top (mg/L) DL,D, DL,O3 ) molecular diffusivity of DMSO and ozone (m2/s)
Ind. Eng. Chem. Res., Vol. 48, No. 17, 2009 HO3 ) Henry’s constant of ozone (kPa/mol frac) PO3 ) partial pressure of ozone gas in gas input (kPa) kD ) reaction constant (s-1) kL,O3a ) volumetric mass-transfer coefficient of ozone (s-1) Qin, Qout ) flow rate of inlet and outlet ozone gases (L/min) t ) time (min) uG ) gas superficial velocity (mm/s) Win, Wout ) total input and output amounts of ozone measured in the gas phase (mg) Greek Letters η ) ozone transfer ratio (%) ∆P ) pressure difference between the inside and outside of a bubble (Pa) σ ) surface tension of the liquid (N/m)
Literature Cited (1) The Latest Technology on Microbubbles and Nanobubbles; Tsuge, H., Ed.; CMC Publishing Co.: Tokyo, 2007; p 109. (2) Li, P.; Tsuge, H. Ozone transfer in a new gas-induced contactor with microbubbles. J. Chem. Eng. Jpn. 2006, 39, 1213. (3) Johnson, P. N.; Davis, R. A. Diffusivity of ozone in water. J. Chem. Eng. Data 1996, 41, 1485. (4) Bando, Y.; Takahashi, Y.; Luo, W.; Wang, Y.; Yasuda, K.; Nakamura, M.; Funato, Y.; Oshima, M. Flow characteristics in concurrent upflow bubble column dispersed with micro-bubbles. J. Chem. Eng. Jpn. 2008, 41, 562. (5) Waghmare, Y. G.; Knopf, F. C.; Rice, R. G. The Bjerknes effect: Explaining pulsed-flow behavior in bubble columns. AIChE J. 2007, 53, 1678.
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(6) Park, S. J.; Yoon, T. I.; Bae, J. H.; Seo, H. J.; Park, H. J. Biological treatment of wastewater containing dimethyl sulphoxide from the semiconductor industry. Proc. Biochem. 2001, 36, 579. (7) Shigeta, K. Method and apparatus for treatment wastewater containing dimethyl sulfoxide. Japanese Patent P2000-263069, 1999. (8) Lee, Y. H.; Lee, C. H.; Yoon, J. Y. Kinetics and mechanisms of DMSO (dimethylsulfoxide) degradation by UV/H2O2 process. Water Res. 2004, 38, 2579. (9) Takahashi, M.; Chiba, K.; Li, P. Free-radical generation from collapsing microbubbles in the absence of a dynamic stimulus. J. Phys. Chem. B 2007, 111, 1343. (10) Li, P.; Tsuge, H. Water treatment by induced air flotation using microbubbles. J. Chem. Eng. Jpn. 2006, 39, 896. (11) Rice, R. G.; Browning, M. E. Ozone Treatment of Industrial Wastewater; Noyes Data Corp.: Park Ridge, NJ, 1981. (12) Gould, J. P.; Ulirsch, G. V. Kinetics of the heterogeneous ozonation of nitrated phenols. Water Sci. Technol. 1992, 26, 169. (13) Hsu, Y. C.; Huang, C. J. Characteristics of a new gas-induced reactor. AIChE J. 1996, 42, 3146. (14) Kuo, C. H.; Li, K. Y.; Wen, C. P. Absorption and decomposition of ozone in aqueous solutions. AIChE Symp. Ser. 1977, 73, 230. (15) Sotelo, J. L.; Beltran, F. J.; Benitez, F. J. Henry’s law constant for the ozone-water system. Water Res. 1989, 23, 1239. (16) Munter, R.; Preis, S.; Kamenev, S. Methodology of ozone introduction into water and wastewater treatment. Ozone: Sci. Eng. 1993, 15, 149. (17) Saunders, F. M.; Gould, J. P.; Southerland, C. R. The effect of solute competition on ozonolysis of industrial dyes. Water Res. 1983, 17, 1407.
ReceiVed for reView October 17, 2008 ReVised manuscript receiVed February 19, 2009 Accepted February 19, 2009 IE801565V
