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 processesin semiconductor manufacturing industry. Traditional biological treatment is known to be difficult for thetreatment of DMSO-containing wastewater because of odor problems. Ozonation of DMSO combined witha biological process is suggested to be a cost-efficient treatment solution, whereas the application of ozoneto wastewater treatment has been limited by its low utilization efficiency and high cost. In this study, weapplied an ozone microbubble generator to increase ozone transfer efficiency in the aqueous solution. Theoxidation of DMSO by ozone microbubbles was investigated in a bubble column reactor with an inner diameterof 20 cm. We studied the dependence of DMSO degradation on the gas and liquid flow rates. Experimentalresults indicate that the ozonation of DMSO is a first-order mass-transfer-controlled reaction and the reactionrate constant increases with increasing gas velocity. Ozone transfer ratio increases with decrease in gas flowrate.

1. Introduction

Microbubbles are often defined as bubbles with diametersof several tens of micrometers and have many characteristicsdifferent from common millibubbles with diameters of theorder of millimeters. A typical property of microbubbles isa high internal pressure, which results from surface tensionat the gas-liquid interface. The Young-Laplace equationstates that

where ∆P is the pressure difference between the inside andoutside of a bubble, σ is the surface tension of the liquid,and db is the bubble diameter. Therefore, the interior gaspressure increases as the bubble becomes smaller. Moreover,Henry’s law indicates that the mass of gas transferred fromthe gas phase to the liquid phase increases with rising gaspressure, which results in shrinkage of the microbubbles.

In recent years, microbubbles have attracted much attentionin the fields of wastewater treatment, chemical process, andmedicine.1 We developed a bubble column using mi-crobubbles and demonstrated that the mass-transfer efficiencyof ozone in the aqueous solution is greatly improved.2 Themass-transfer efficiency of ozone from the gas phase to theliquid phase is always evaluated by the liquid-phase volu-metric mass-transfer coefficient, kLa, because the gas-phasemass-transfer resistance can be ignored because of therelatively low solubility of ozone in water.3 The mass-transfercoefficient, kL, depends on the mixing characteristics of thegas-liquid contactor used and the kinetics of ozone reactionsproduced, while the specific interfacial area, a, is determinedby the number and size of ozone-containing bubbles pro-duced. The large specific interfacial area of microbubblesand their tendency to decrease in size and subsequently to

disappear under water lead to their efficient gaseous solubil-ity. Figure 1 shows a comparison of the volumetric mass-transfer coefficient of oxygen in different bubble columnsusing microbubbles2,4 and millibubbles.5 The mass transferof oxygen from the gas phase to the liquid phase wasimproved when microbubbles with diameter less than 100µm were used.

Dimethyl sulfoxide (DMSO, (CH3)2SO) is widely used asa detergent or a photoresist stripping solvent in the manu-facture of semiconductors and liquid crystal displays, so thata large quantity of DMSO wastewater is discharged fromthe washing and rinsing processes. Traditional biologicalmethods are known to be difficult for the treatment of DMSO-containing wastewater because of odor problems caused byintermediate products such as dimethyl sulfide (DMS), methylmercaptan, and hydrogen sulfide.6 Recently, some researcherssuggested using a combination of advanced oxidation pro-cesses (AOPs) and biological processes to provide a cost-efficient treatment solution for the treatment of DMSO-containing wastewater.7,8 The combination process alwaysconsists of two steps: First, oxidation of DMSO by AOPs todimethyl sulfone (DMSO2) or methane sulfonic acid (MSA),and second, further biodegradation of DMSO2 and MSA tosulfuric acid by activated sludge without producing anyreduced and harmful sulfur-containing byproduct. In theseAOPs, hydroxyl radical (•OH) is assumed to be the mainoxidant responsible for the decomposition of DMSO.

In the first part of this study, we tried to search for anonchemical method to decompose DMSO with air mi-crobubbles. Takahashi et al. reported that free radicals aregenerated from collapsing air microbubbles.9 If free radicalsare generated during collapsing air microbubbles, the produced•OH can degrade DMSO to MSA. This hypothesis was checkedby air microbubble generation under various pH’s andconductivities.

It has been demonstrated that ozone mass transfer can beimproved by the application of microbubbles in our previouswork.2 Consequently, the reaction between ozone and organicsubstance is supposed to be accelerated by using mi-crobubbles. In the second part of this study, we studied theoxidation of DMSO by ozone microbubbles under different

* To whom correspondence should be addressed. Tel.: 81-29-861-8751. Fax: 81-29-861-8496. E-mail: pan-li@aist.go.jp.

† Graduate School of Science and Technology.§ Present address: Research Institute for Environmental Management

Technology, National Institute of Advanced Industrial Science andTechnology, AIST Tsukuba East, 16-1, Onogawa, Tsukuba, Ibaraki305-8569, Japan.

‡ Faculty of Science and Technology.

∆P ) 4σ/db (1)

Ind. Eng. Chem. Res. 2009, 48, 8048–80538048

10.1021/ie801565v CCC: $40.75 2009 American Chemical SocietyPublished on Web 05/12/2009

gas and liquid flow rates. Ozone transfer ratio, the mass ratioof 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 DMSOreagent with distilled water, and its concentration was kept about10 mg/L in all experiments. The pH of DMSO solution wasvaried in the range of 4.3-9.5 by adding 0.1 M hydrochloricacid or 0.05 M sodium hydroxide solutions. Ionic strength ofDMSO solution was varied by adding sodium chloride from 0to 1 wt %.

2.1. Experimental Apparatus. A schematic diagram of theexperimental apparatus is illustrated in Figure 2. The bubblecolumn made of acrylic resin has an inner diameter of 0.20 m

and a height of 1.20 m. Microbubble generation systemconsists 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 thebottom of the bubble column. To increase the gas/liquid flowrate ratio, G/L, a centrifugal pump and microbubble generatorwere combined as mentioned in our previous article.2 Airwas aspirated by the centrifugal pump, and the liquid andair were simultaneously mixed by the pump. The pressurizedmixture of air and liquid was then decompressed throughthe microbubble generator with a high rotating velocity, andmicrobubbles were generated. We observed that the con-densed microbubbles gave the water a milky appearance. Themajority of the microbubbles ranged in size from 20 to 100µm, and the average diameter was about 50 µm, as measuredby a laser diffraction particle size analyzer (LS230, BechmanCoulter, Inc.) in our previous work.10

2.2. Experimental Method. All the experiments werecarried out in semibatch mode. To examine the formation ofthe free radical, air microbubbles were generated at air andliquid flow rates of 0.5 and 15.0 L/min, respectively. Sampleswere taken from the sampling tap in the middle of the reactorevery 5 min, and DMSO and DMSO2 concentrations inaqueous solutions were detected by GC-FID (GC-2010,Shimadzu Co.).

For the experiments on ozone oxidation, ozone wasgenerated from oxygen by a corona-discharge ozone generator(SO-03UN03, Tokyu Car Co.). Ozone concentration insolutions was online monitored by a polarographic ozonemeter (ELP-100, Ebara Jitsugyo Co., Ltd.) during the wholereaction process. A vane pump was used to continuouslysample water to the ozone meter. Ozone concentration in thegas phase was also online monitored using a UV-absorptionozone meter (EG-320, Ebara Jitsugyo Co., Ltd.), and thenthe outlet gas was fed into an ozone destructor. Ozonemicrobubbles were generated at various gas and liquid flowrates. The ozone gas flow rate was measured by a mass flowmeter (model 3340, Kofloc Co.), while the water flow ratewas measured with a flow meter (SP-562, Tecflow Interna-tional IR-Flow Co.). A transistor inverter (VF-nC12004P,Toshiba Co., Ltd.) connected with the centrifugal pump was

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.

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used to control the water flow rate. Unbuffered distilled waterwas used in all the ozonation experiments.

Total organic carbon (TOC) concentration of aqueous sampleswas measured with a TOC meter (TOC-5000, Shimadzu Co.).A pH meter (HM-40S, DKK-TOA Co.) was used to measurepH of the samples.

3. Theoretical Section

In a gas-liquid system where gas dissolution is followed bya chemical reaction, two steps control the overall reaction rate:the mass transfer from gas phase to liquid phase and thechemical reaction in the liquid phase.11,12 The ozone oxidationcan be considered to be a mass-transfer-controlled reactionowing to low solubility of ozone.13 The rate of mass transferof ozone from gas phase into liquid phase is limited by liquidfilm diffusion.12,14-16

It is assumed that the ozonation of DMSO can be presentedby eq 2 with a stoichiometric ratio, b.

The reaction rate can be defined by eq 3 by assuming a first-order reaction.13,17

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 besimplified to

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.

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 isassumed and the slope represents kD.

4. Results and Discussion

4.1. Air Microbubbles. No change in DMSO and TOCconcentrations was observed during the reaction period of20 min by blowing air microbubbles at different pH (4.3-9.5)and NaCl concentrations (0-1 wt %). MSA was not detectedeither, which suggests that no hydroxyl free radicals weregenerated from collapsing air microbubble under the presentconditions. Takahashi et al.9 confirmed hydroxyl free radicalgeneration from the collapse of microbubbles using electronspin resonance spectroscopy. However, the experiments wereconducted under strongly acidic conditions (0.18 M H2SO4,pH ) 0.44), while the lowest pH value in our experimentswas 4.3.

4.2. Ozone Microbubbles. 4.2.1. Effect of Ozone GasFlow Rate on DMSO Oxidation. Gas flow rate of input ozonewas varied from 0.25 to 1.50 L/min, while water flow rateand ozone concentration was kept at 15.0 L/min and 28.2g/Nm3, respectively. Figure 3 shows the changes of CD/CD,0

and DMSO2 concentrations with reaction time. DMSOconcentrations 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.

DMSO + bO3 + H2O f DMSO2 + other products (2)

-dCD

dt) kL,O3

a·(DL,DCD

DL,O3

+PO3

bHO3) (3)

-dCD

dt)

(kL,O3a)DL,D

DL,O3

·CD ) kD·CD (4)

lnCD,0

CD)

(kL,O3a)DL,D

DL,O3

t ) kDt (5)

8050 Ind. Eng. Chem. Res., Vol. 48, No. 17, 2009

crease with time proceeding. The oxidation rate, that is, thedecrease rate of DMSO concentration or the increase rate ofDMSO2 concentration, also increases with increasing ozonegas flow rate. When the input gas flow rate increases, theconcentration of ozone dissolved in liquid also increases asshown in Figure 4a, which resulted in the increase ofoxidation rate.

By assuming that the inlet and outlet ozone gases havethe same flow rate (Qin ) Qout), we define the ozone transferratio, η (%), in the semibatch ozone contactor as follows:12

where Win and Wout are the total input and output amounts ofozone (mg) measured in the gas phase, respectively. Cin andCout are the ozone concentrations in inlet gas at the bottomof the bubble column and off-gas from its top (mg/L).

The change of ozone concentration in off-gas with timeas shown in Figure 4b was used to calculate the ozone transferratio, η (%), by eq 6. The calculated results of η are illustratedin Figure 5. The ozone transfer ratio at the reaction time of10 min decreases from 95 to 65% when the ozone gas flowrate was increased from 0.25 to 1.5 L/min.

The linear relationship between -ln(CD/CD,0) and reactiontime as shown in Figure 2 indicates that the ozonation of DMSOis a first-order mass-transfer-controlled reaction. The reactionrate 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 to0.20 mm/s.

4.2.2. Effect of Water Flow Rate on DMSO Oxidation.The flow rate of water circulated by the centrifugal pumpwas changed from 15.0 to 19.5 L/min, while gas flow rate

Figure 4. Effect of gas flow rate on ozone concentration in liquid and off-gas.

η(t) )Win - Wout

Win× 100

)QinCint - ∫0

tQoutCout dt

QinCint× 100

) (1 - 1Cint

∫0

tCout dt) × 100 (6)

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and ozone concentration were kept at 1.0 L/min and 28.2g/Nm3, respectively. The effect of water flow rate on theoxidation rate is very small as compared with that of the gasflow rate.

5. Conclusions

No •OH free radicals were detected during air microbubblestreatment when the pH and the salt concentrations were changedduring the ranges of 4.3-9.5 and 0-1 wt %, respectively. Theoxidation of DMSO by ozone microbubbles was studied undervarious gas and liquid flow rates. Experimental results indicatethat the ozonation of DMSO is a first-order mass-transfer-controlled reaction. The reaction constant kD increases withincreasing gas superficial velocity. Ozone transfer ratio, namely,

the mass ratio of the ozone transferred from the phase into theliquid phase to the ozone input, increases with decreasing gasflow rate.

Acknowledgment

We are deeply indebted for the support of Organo Co. forthe 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)

Figure 5. Effect of gas flow rate on ozone transfer ratio.

Figure 6. Relationship between reaction constant and gas superficial velocity.

8052 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,O3

a ) 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)

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(3) Johnson, P. N.; Davis, R. A. Diffusivity of ozone in water. J. Chem.Eng. Data 1996, 41, 1485.

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(6) Park, S. J.; Yoon, T. I.; Bae, J. H.; Seo, H. J.; Park, H. J. Biologicaltreatment of wastewater containing dimethyl sulphoxide from the semi-conductor industry. Proc. Biochem. 2001, 36, 579.

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(9) Takahashi, M.; Chiba, K.; Li, P. Free-radical generation fromcollapsing 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 usingmicrobubbles. J. Chem. Eng. Jpn. 2006, 39, 896.

(11) Rice, R. G.; Browning, M. E. Ozone Treatment of IndustrialWastewater; Noyes Data Corp.: Park Ridge, NJ, 1981.

(12) Gould, J. P.; Ulirsch, G. V. Kinetics of the heterogeneous ozonationof nitrated phenols. Water Sci. Technol. 1992, 26, 169.

(13) Hsu, Y. C.; Huang, C. J. Characteristics of a new gas-inducedreactor. AIChE J. 1996, 42, 3146.

(14) Kuo, C. H.; Li, K. Y.; Wen, C. P. Absorption and decompositionof ozone in aqueous solutions. AIChE Symp. Ser. 1977, 73, 230.

(15) Sotelo, J. L.; Beltran, F. J.; Benitez, F. J. Henry’s law constant forthe ozone-water system. Water Res. 1989, 23, 1239.

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ReceiVed for reView October 17, 2008ReVised manuscript receiVed February 19, 2009

Accepted February 19, 2009

IE801565V

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