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Decomposition of 2-mercaptothiazoline in aqueous

solution by ozonation

Y.H. Chen

a

, C.Y. Chang

a,*

, C.C. Chen

a

, C.Y. Chiu

b

, Y.H. Yu

a

,

P.C. Chiang

a

, Y. Ku

c

, J.N. Chen

d

, C.F. Chang

a

aGraduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Road, Taipei 106, Taiwan bDepartment of Environmental Engineering, Lan-Yang Institute of Technology, I-Lan 261, Taiwan

cDepartment of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan dGraduate Institute of Environmental Engineering, National Chiao-Tung University, Hsin-Chu 300, Taiwan

Received 11 June 2003; received in revised form 31 October 2003; accepted 5 February 2004

Abstract

This study investigates the ozonation of 2-mercaptothiazoline (2-MT). The 2-MT is one of the important organic additives for the electroplating solution of the printed wiring board industry and has been widely used as a corrosion inhibitor in many industrial processes. It is of concern for the aquatic pollution control especially in the wastewaters. Semibatch ozonation experiments in the completely stirred tank reactor are performed under various concentrations of input ozone. The concentrations of 2-MT, sulfate, and ammonium are analyzed at specified time intervals to elucidate the decomposition of 2-MT during the ozonation. In addition, the time variation of the dissolved ozone concentration

(CALb) is continuously monitored in the course of experiments. Total organic carbon (TOC) is chosen and measured as a

mineralization index of the ozonation of 2-MT. The results indicate that the decomposition of 2-MT is efficient, while the mineralization of TOC is limited via the ozonation only. Simultaneously, the yield of sulfate with the maximum value of about 47% is characterized by the increases of TOC removal and ozone consumption. These results can provide some useful information for assessing the feasibility of the treatment of 2-MT in the aqueous solution by the ozona-tion.

 2004 Elsevier Ltd. All rights reserved.

Keywords: Ozone; 2-Mercaptothiazoline; Sulfate; Total organic carbon; Water treatment

1. Introduction

Organic corrosion inhibitors, such as thiourea, phenylthiourea, and 2-mercaptothiazoline (2-MT), have been used for the protection against corrosion in many industrial processes (Wang et al., 2001). The combina-tion of an exocyclic thione group and a heterocyclic molecule, which may contain nitrogen, carbon, and sulfur atoms, generates a group of compounds with

considerably coordinational potential (Lizarraga et al., 1997). Accordingly, these inhibitors have been fre-quently detected in wastewater effluents as well as river water (Fiehn et al., 1998). Furthermore, most of them are of high mobility within the aquatic system. Among them, 2-MT with a five-member heterocyclic ring has been commonly used as biocorrosion inhibitor, anti-fungal in medical application, and coating agent of metallic surfaces attributed to its adsorption or com-plexation on the surface of metal associated with the electron donor properties of N or S atoms (Mahal and Mukherjee, 1999). It has been also used as a brightening and stabilization agent in the electroplating solution of *

Corresponding author. Tel./fax: +886-2-2363-8994. E-mail address:cychang3@ntu.edu.tw(C.Y. Chang).

0045-6535/$ - see front matter  2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2004.02.001

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printed wiring board industry (Fang, 1996). The 2-MT is harmful according to its Material Safety Data Sheet. Thus, 2-MT is of concern as an aqueous pollutant in water and wastewater treatment.

Ozonation is considered as an effective way to reduce the total organic carbons (TOCs) by oxidizing the stream solutions with ozone (Koch et al., 2002). Ozone may attack on the pollutants via two different reaction pathways: (1) the direct ozonation by the ozone mole-cule, and (2) the radical ozonation by the highly oxi-dative free radicals such as hydroxyl free radicals, which are formed by the decomposition of ozone in the aque-ous solution (Gurol and Singer, 1982; Sotelo et al., 1987). The radical ozonation is non-selective and vig-orous. The ozonation process in the acid condition mainly takes place through the direct oxidation reaction,

which is selective (Beltran and Alvarez, 1996;

Hautani-emi et al., 1998).

However, the information about the ozonation of 2-MT is found scarce and desirable. The objective of this study is to examine the phenomena during the ozonation of 2-MT. Semibatch ozonation experiments in the completely stirred tank reactor are proceeded under various concentrations of input ozone. The concentra-tions of 2-MT, sulfate, and ammonium are analyzed at specified time intervals to study the decomposition of 2-MT. TOC is chosen as a mineralization index of the ozonation of 2-MT. Moreover, the values of the

dis-solved ozone concentration (CALb) and pH are measured

continuously in the course of experiments.

2. Materials and methods 2.1. Chemicals

The initial concentration (CB0) of aqueous solution

consisting of 2-MT as the only organic target is 100

mg l1. The 2-MT with chemical formula of C

3H5NS2,

which was purchased from Aldrich (Milwankee, WI, USA) and used without any further purification, has molecular weight of 119.21, melting point of 105–107 C, and CAS registry number of 96-53-7. The molecular structure of 2-MT is shown in Fig. 1. All solutions used for the experiments were prepared with deionized water without buffers. The initial values of pH and TOC

(CTOC0) of solution were measured as 5.50 and 29.1

mg l1, respectively. The volatility of 2-MT in the

aqueous solution was demonstrated to be negligible from the air-stripping test.

2.2. Instrumentation

The airtight reactor with inside diameter of 17.2 cm was made of Pyrex glass with an effective volume of 5.5 l. It was equipped with water bath jacket to maintain a

constant solution temperature at 25 C in all

experi-ments. The design of reactor was based on the criteria of the shape factors of a standard six-blade turbine (Mc-Cabe et al., 1993). The gas diffuser in cylindrical shape with pore size of 10 lm was located at the bottom of the

reactor. About 3.705 l solution (VL) was used in each

experiment. The total sampling volumes were within 5% of the solution. The stirred speed was kept as high as 800 rpm to ensure the completely mixing of liquid and gas according to the previous tests (Chang et al., 2001). The generation of ozone was controlled by the power input of the ozone generator (model SG-01A, Sumitomo, Tokyo,

Japan) with constant gas pressure (1 kgf cm2). The

ozone generator used in this research employed two steel plate electrodes with ceramic dielectric medium. Ozone-containing gas generated by pure oxygen was introduced into the reactor with a gas flow rate (QG) of 1.78 l min1

at 273 K, 1 atm. 2.3. Analytical method

The input (CAGi0) and outlet (CAGe) gas ozone

con-centrations were measured by an UV photometric ana-lyzer (Seki, model SOZ-6004, Tokyo, Japan), which was calibrated by the KI titration method (Rankness et al., 1996). The Orbisphere’s model 3600 liquid ozone mon-itor with a sensor of membrane-containing cathode, which was calibrated by the indigo method, was used to

analyze the dissolved ozone concentration (CALb) in the

aqueous solution. The pH meter (model 300T, Suntex, Taipei, Taiwan) was used to measure the pH value of the solution. A circulation pump was used to transport the liquid from the reactor to pass through the sensors with

a flow rate of 0.18 l min1, and to re-circulate it back

during the ozonation. All fittings, tubings, and bottles were made of stainless steel, Teflon, or glass.

Samples were drawn out from the reactor at desired time intervals in the course of experiments. The residual dissolved ozone in the sample was removed by nitrogen

gas stripping. The concentrations of 2-MT (CB) were

analyzed using high performance liquid

chromatogra-phy (HPLC) system equipped with 250· 4.6 mm column

(model BDS C18 (5 lm), Thermo Hypersil-keystone, Bellfonte, PA, USA), and UV/Visible detector (model 1706, Bio-Rad, Hercules, CA, USA) at 275 nm. The

HPLC effluent with flow rate of 1.0 ml min1 had the

O3 SO42- CO 2 , H2O , NH4 + , SO 4 2-N S HS fast

Intermediates slow Products

Fig. 1. Molecular structure of 2-mercaptothiazoline (2-MT) and simplified scheme of decomposition of 2-MT via ozonation.

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composition with 73.5 mM [CH3(CH2)3]4N(HSO4):

CH3CN of 74:26. The injection volume of analytic

solution was 20 ll. The detection limit of CB was 0.01

mg l1. The ionic chromatography (IC) employed to

analyze the anionic concentrations of sulfate (CSO2

4 ) and

nitrate (CNO

3) was equipped with 150· 5.5 mm column

(model AN300, MetaChem, Lake forest, CA, USA) and detector (model conductivity series IV, LabAlliance, Lemont, PA, USA). The effluent with flow rate of 2.0

ml min1 had the composition with NaHCO

3/Na2CO3

of 1.7 mM/1.8 mM. The IC employed to analyze

the cationic concentrations of ammonium (CNHþ4) was

equipped with 250· 4 mm column of model IonPac

CS12A and detector of model CSRS-II (both with Dionex, Sunnyvale, CA, USA). The effluent with flow

rate of 1.0 ml min1 had the composition with 22 mM

CH4O3S. The TOC concentration (CTOC) of sample was

analyzed by the TOC analyzer (model 700, O.I. Cor-poration, TX, USA). The instrument employed the UV-persulfate technique to convert the organic carbon into carbon dioxide for the subsequent analysis by an

infra-red CO2 analyzer calibrated with the potassium

hydro-gen phthalate standard.

2.4. Experimental procedures

The semibatch experiments of 2-MT ozonation were

performed to examine the time variations of CALb, CB,

CSO2 4 , CNO



3, CNHþ4, CTOC, and pH. Before starting the ozonation experiments, the ozone-containing gas was bypassed to the UV photometric analyzer to measure the ozone concentration and assure the stability. A part of gas stream at a preset flow rate was directed into the reactor when reaching the preset conditions. The ex-perimental apparatus employed in this work is shown in Fig. 2.

3. Results and discussion

3.1. Time variations of CALb, CB, CSO2 4 , CNH

þ

4, CTOC, and pH in ozonation of 2-MT

The variations of CALb, CB, CSO24 , CNHþ

4, CTOC, and

pH with the ozonation time (t) with CAGi0¼ 40 mg l1

are shown in Fig. 3. The results indicate that the CALb,

CSO2

4 , and CNH

þ

4 increase while the CB, CTOC, and pH

decrease with the ozonation time. The time for the reduction of 2-MT below the detection limit denoted as tf;MTis a characteristic time of great concern during the

ozonation of 2-MT. The comparison of ozonation

re-sults at tf;MT under various experimental conditions is

summarized in Table 1. The YSO2

4 ð¼ CSO24 =ð2CB0ÞÞ and

gTOC ð¼ ðCTOC0 CTOCÞ=CTOC0Þ denote the percentage

of yield of sulfate and removal efficiency of TOC, respectively. The units of CSO2

4 and CB0are in M.

Fig. 2. Experimental apparatus sketch. –––, – –, –– - - ––: ozone gas stream, experimental solution, isothermal water. Components: 1. oxygen cylinder, 2. drying tube, 3. ozone generator, 4. flow meter, 5. three-way valves, 6. stirrer, 7. lamps, 8. reactor, 9. sample port, 10. liquid ozone sensor, 11. pH sensor, 12. oxidation reduction potential (ORP) sensor, 13. circulation pump, 14. thermostat, 15. gas ozone detector, 16. KI solution, 17. vent to hood.

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The results illustrate that the decomposition of 2-MT accompanies with the noteworthy generation of sulfate while with the low diminution of TOC. The values of YSO2

4 at tf;MT are about 24 ± 5% for the three cases

examined. The intermediates generated from the

ozon-ation of 2-MT at tf;MT still contribute over 97% of the

initial TOC (Table 1). Nevertheless, the TOC value gradually decreases with further oxidation of successive products formed. In addition, the pH value of the solution decreases rapidly in the early period (t < tf;MT)

and then approaches a constant value as the ozonation time increases. The increase of acidity (decrease of pH value) follows the generation of sulfate because one sulfate molecule is associated with two protons. Note that the theoretical pH value at tf;MTis calculated as 3.09

for the average value of YSO2

4 of 24%, closing to the

experimental pH values of 3.01–3.09 as indicated in

Table 1. Moreover, the value of CALbremains low in the

early ozonation period and then increases rapidly in a short time. A small quantity of ammonium is detected during the ozonation of 2-MT.

3.2. Decomposition of 2-MT and formation of sulfate and ammonium

The effect of the input ozone dosage on the

decom-position of 2-MT is shown in Fig. 4. The value of tf;MT

with CAGi0¼ 40 mg l1(5 min) is one-third and one-half

of those with CAGi0¼ 10 (tf;MT¼ 15 min) and 20 mg l1

(tf;MT¼ 10 min), respectively. Further, the pseudo-first

order rate constant (kB) for the expression of CB=CB0¼

ekBt can be obtained yielding k

B¼ 0:0167 CAGi0 min1

(with CAGi0in mg l1) from the experimental data. We

may note that the mercapto substituent, which is an electron donor, has a high reactivity with ozone mole-cules toward the electrophilic reaction (Langlais et al., 1991). Referring to the ozonation mechanism of 2-mer-captobenzothiazole reported by Fiehn et al. (1998), we can propose a simplified scheme for the decomposition of 2-MT via the ozonation as shown in Fig. 1. The initial attack of ozone on 2-MT is mainly toward the mercapto

group to generate sulfate. The value of YSO2

4 would be

close to 50% when all the sulfur atoms of the mercapto substitute are oxidized to sulfate. Thus, we can estimate that only nearly a half of the sulfur atoms of the mer-capto substituent has been oxidized to form sulfate at tf;MT, with average value of YSO24 of 24% (Table 1).

As shown in Fig. 5, which illustrates the time varia-tion of YSO2

4 , the generation rate of sulfate is accelerated

by the input ozone dosage. The YSO2

4 approaches

con-stant values of about 45–48% in 30 to 90 min for the

0 5 10 15 0.0 0.2 0.4 0.6 0.8 1.0 CB / C B0 ( -)

Ozonation time (min)

Fig. 4. Time variation of CB=CB0 for ozonation of 2-MT in

semibatch system.



,M, and : CAGi0¼ 10, 20, and 40 mg l1.

CB0¼ CBat t¼ 0. 0 60 120 180 240 2 4 6 pH 0 10 20 30 CTOC (mg /l ) 0 40 80 120 CSO 4 2 -( mg /l ) 0 40 80 120 CB ( mg /l ) 0 1 2 CNH 4 + ( mg /l ) 0 4 8 CAL b (mg/ l)

Ozonation time (min)

Fig. 3. Time variations of concentrations (CALb, CB, CSO2

4 ,

CNHþ

4, CTOC), and pH for ozonation of 2-MT in semibatch

system. Concentration of input ozone gasðCAGi0Þ ¼ 40 mg l1.

Table 1

Comparison of ozonation results at specific times (tf;MT) for

reduction of 2-MT below detection limit under various experi-mental conditions Experimental conditiona t f;MT (min) YSO2 4 (%) gTOC (%) pH Case 1 CAGi0¼ 10 mg l1 15 24 1.6 3.09 Case 2 CAGi0¼ 20 mg l1 10 29 2.5 3.04 Case 3 CAGi0¼ 40 mg l1 5 19 2.7 3.01 a

Initial values of CB0, CTOC, and pH are 100 mg l1, 29.1

mg l1, and 5.50, respectively.

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cases examined. Furthermore, the broken mercapto group may probably form hydrogen sulfide, which is a

weak acid (with pKa1 ¼ 7:0 and pKa2¼ 13:9) and has

high volatility (H2SðaqÞ$ H2SðgÞ with pK of H2SðaqÞ¼

1.01) (Morel and Hering, 1993). Some hydrogen sulfide would then be stripped from the aqueous solution by the gas stream continuously introduced before being

oxi-dized. This cause thus makes the value of YSO2

4 a little smaller than 50%. The results also reveal that the sulfur atom of the heterocyclic ring is hard to be oxidized. In

addition, the generation of sulfate (YSO2

4 ) is found

consistent with the increase of gTOCfor the ozonation of

2-MT as depicted in Fig. 6. The value of YSO2

4 increases

rapidly with gTOC when gTOC<10%. For gTOCP10%,

the formation rate of sulfate becomes slow with YSO2

4 greater than 40%.

In a study on the ozonation of pyridine homologs,

Wibaut (1959) reported that the C@N bond does not

react with ozone directly, but rather undergoes hydro-lytic splitting to form ammonia during the decomposi-tion of ozonide. In the present study, ammonium is also detected in the later period of the ozonation of 2-MT. The concentration of ammonium, although having a low

reactivity with ozone (Hoigne et al., 1985), increases

with the ozonation time. The percentage of conversion

of ammonium (CNHþ4=CB0, MM1) reaches 13% at

t¼ 240 min with CAGi0¼ 40 mg l1, which is close to the

value of gTOCof 16% while smaller than that of YSO2

4 of

48%. This may be due to the cause that the CNHþ

4 and

gTOC depend on the destruction of the heterocyclic

molecule, while the YSO2

4 is mainly generated from the

oxidation of the mercapto group. However, the nitrate

concentration (CNO

3) is detected below the detection

limit of 0.01 mg l1.

3.3. Removal of TOC associated with variation of CALb

For an illustration on the effect of CAGi0 on the

elimination of TOC, Fig. 7 shows the variation of mean

mineralization rate, ðCTOC0 CTOCÞ=t, with gTOC. The

results reveal that the input ozone dosage proportionally enhances the mineralization rate of 2-MT. For example, the average values ofðCTOC0 CTOCÞ=t with CAGi0¼ 10,

20, and 40 mg l1 for g

TOC¼ 0–10% are about 0.037,

0.088, and 0.18 mg l1min1, respectively. However, the

values of ðCTOC0 CTOCÞ=t for all cases decrease

0 5 10 15 ηTOC (%) 0.00 0.05 0.10 0.15 0.20 0.25 (C TOC0 - C TOC ) / t (mg/l-min) C ALb /C AGi0 (-) 0.00 0.05 0.10 0.15 0.20 0.25

Fig. 7. ðCTOC0 CTOCÞ=t and CALb=CAGi0 vs. gTOC for

ozona-tion of 2-MT in semibatch system. Notaozona-tions are the same as specified in Fig. 4. 0 60 120 180 240 0 10 20 30 40 50 YSO 4 2- (%)

Ozonation time (min) Fig. 5. Time variation of YSO2

4 for ozonation of 2-MT in semibatch system. YSO2 4 ¼ CSO 2 4 =ð2CB0Þ (MM 1). Notations are

the same as specified in Fig. 4.

0 5 10 15 ηTOC (%) 0 10 20 30 40 50 YSO 4 2- (%) Fig. 6. YSO2

4 vs. gTOC for ozonation of 2-MT in semibatch

system. gTOC¼ ðCTOC0 CTOCÞ=CTOC0, CTOC0¼ CTOC at t¼ 0.

Notations are the same as specified in Fig. 4. ––: polynomial curve fitting of experimental data with R2¼ 0:939, where R2

(determination coefficient)¼ 1) ½PðCe CpÞ2=PðCe CeÞ2.

Ce, Ce¼ experimental data and corresponding average value.

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remarkably lower than 0.02 mg l1min1with the higher

gTOC. Comparing with the results of the ozonation of

2-naphthalenesulfonate with CAGi0¼ 11 and 44 mg l1

reported by Chen et al. (2002), which reported the values ofðCTOC0 CTOCÞ=t ¼ 0:27–1.64 mg l1min1for

gTOC¼ 0–100% with CTOC0¼ 104 mg l1, we note that

the mineralization rate of 2-MT ozonation is relatively low. The comparison indicates that the organic com-pound with the heterocyclic structure is more resistant to the ozonation than that with the benzene ring.

The variation of CALb=CAGi0with gTOCis also shown

in Fig. 7. It is seen that the value of CALb=CAGi0remains

low for gTOC<5%, but rapidly increases as gTOC

in-creases from 5% to 10%. The CAGi0=CALb (inverse of

CALb=CAGi0) approaches to the steady state value of

4.62 ± 0.32 as gTOC>10%, which is close to the Henry’s

constant of ozone (HA) at 298 K with values between

3.34 and 4.78 reported by Chiu et al. (1997). The low ozone consumption also indicates the inactivation of the ozonation products. Moreover, the masses of ozone

applied (mO3A) and consumed by chemical reactions

(mO3R) are defined by Eqs. (1) and (2), respectively. The mathematic derivation of Eq. (2) is shown in detail in Appendix A.

mO3A¼ QG CAGi0 t ð1Þ

mO3R¼ Z t

0

QGðCAGi0 CAGeÞdt  CALbVL CAGeVF

ð2Þ In Eqs. (1) and (2), VLand VFare the volumes of solution

and free space in the reactor, respectively. Fig. 8 illustrates the variations of YSO2

4 and mO3A= ðCB0VLÞ (mol mol1) with mO3R=ðCB0VLÞ (mol mol

1).

The mO3A=ðCB0VLÞ and mO3R=ðCB0VLÞ denote the mole ratios of ozone applied and consumed to the 2-MT treated, respectively. The results indicate that the value of YSO2

4 is approximately proportional to mO3R=ðCB0VLÞ when mO3R=ðCB0VLÞ < 4. The ratio of YSO2

4 to mO3R= ðCB0VLÞ is about 0.11% in this region. As mO3R=ðCB0VLÞ

increases higher than 5, the value of YSO2

4 reaches a

constant value of about 47%. Furthermore, Fig. 8 de-scribes the association of mO3A with mO3R, where the

value of mO3R=mO3A represents the efficiency of ozone

utilization. The values of mO3R=mO3A are high when

mO3R=ðCB0VLÞ < 4 and similar for various concentrations of input ozone. In the region with mO3R=ðCB0VLÞ < 4, the efficiency of ozone utilization is greater than 51%. However, the value of mO3R=mO3Adecreases significantly when mO3R=ðCB0VLÞ > 4, indicating the low reactivity of subsequent intermediates with ozone. The efficiency of ozone utilization becomes smaller than 10% when mO3R=ðCB0VLÞ > 7.

In summary, the ozonation treatment is effective for the decomposition of 2-MT in the aqueous solution. The

enhancing effect of ozone dosage on the decomposition rate of 2-MT is remarkable. However, the ozonation alone cannot achieve a complete mineralization of 2-MT. The nitrogen and sulfur atoms on the heterocyclic ring of 2-MT are hard to be oxidized to form nitrate and sulfate. The results obtained in this study are useful for the proper design of the ozonation treatment of 2-MT in the aqueous solution.

4. Conclusions

1. Ozonation is employed as an effective way for the decomposition of 2-mercaptothiazoline (2-MT) in the aqueous solution. The decomposition of 2-MT takes place accompanied with the formation of sul-fate and ammonium, the diminution of total organic carbons (TOCs), and the consumption of ozone (mO3R).

2. The rates of decomposition of 2-MT, generation of sulfate, and elimination of TOC increase significantly with the input ozone dosage. When the decomposi-tion of 2-MT is completed, the remarkable yield of sulfate (YSO2

4 ) and the low removal efficiency of

TOC (gTOC) are about 24% and 2.3%, respectively.

3. The values of YSO2

4 during the ozonation of 2-MT

have apparent relationships with gTOC and mO3R=

CB0VL, respectively. The YSO24 and gTOCfor the ozon-ation of 2-MT have the maximum values of about 48% and 16%, respectively. Furthermore, the nitro-gen and sulfur atoms on the heterocyclic structure of 2-MT are found hard to be oxidized to form ni-trate and sulfate.

0 2 10 mO3R / (CB0VL) (mol mol-1) 0 10 20 30 40 50 YSO 4 2- (%) 0 30 60 90 120 150 m O 3A / (C B0 V L ) (m ol mol -1 ) 4 6 8 Fig. 8. YSO2

4 and mO3A=ðCB0VLÞ vs. mO3R=ðCB0VLÞ for ozonation

of 2-MT in semibatch system. Notations are the same as specified in Fig. 4. ––: polynomial curve fitting of experimental data. YSO2

4 and mO3A=ðCB0VLÞ: R

2¼ 0:986 and 0.978.

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4. In the early period of ozonation, say gTOC<10%, the elimination rate of TOC is remarkably enhanced by the input ozone dosage with higher efficiency of ozone utilization. However, in the later period of ozonation, the successive intermediates show the low mineralization rate and efficiency of ozone utili-zation accompanied with the accumulation of the dis-solved ozone. The results reveal that the products generated from the ozonation of 2-MT are evidently resistant toward further oxidation by ozone.

Acknowledgements

This study was supported by the National Science Council of Taiwan under Grant No. NSC 89-2211-E-002-107.

Appendix A. Mathematic derivation of Eq. (2)

The governing equations of mass balance of ozone in a semibatch stirred vessel, which simultaneously con-sider the gas–liquid mass transfer, chemical reactions, and gas flow convection, can be described by Eqs. (A.1)–(A.3) according to the previous studies of Wu and Masten (2001) and Chiu et al. (2003). The proper assumptions of the model are listed as follows (Anselmi et al., 1984, 1985; Chiu et al., 1997, 2003).

1. The homogenous conditions are valid for the bulk li-quid, holdup gas, and free space.

2. Henry’s law applies.

3. Chemical reactions in gas phase are neglected. For the ozone balance in liquid phase:

VLðdCALb=dtÞ ¼ ErAkLA0 aðVLþ VHÞðCAGi=HA CALbÞ

Xconsumption rates of ozone

by reactions ðA:1Þ

For the ozone balance in holdup gas:

VHðdCAGi=dtÞ ¼ QGðCAGi0 CAGiÞ  ErAk0LAaðVLþ VHÞ

 ðCAGi=HA CALbÞ ðA:2Þ

For the ozone balance in free space:

VFðdCAGe=dtÞ ¼ QGðCAGi CAGeÞ ðA:3Þ

In the above equations, CAGi, ErA, kLA0 a, and VH denote

the ozone concentration in holdup gas, enhancement factor of ozone, volumetric mass transfer coefficient, and volume of holdup gas, respectively.

The initial conditions of Eqs. (A.1)–(A.3) are:

t¼ 0; CALb¼ CAGi¼ CAGe¼ 0 ðA:4Þ

The sum of Eqs. (A.1)–(A.3) yields as Eq. (A.5): VLðdCALb=dtÞ þ VHðdCAGi=dtÞ þ VFðdCAGe=dtÞ

¼ QGðCAGi0 CAGeÞ

Xconsumption rates of ozone by reactions

ðA:5Þ Furthermore, integrating Eq. (A.5) with respect to t from 0 to t gives

VLCALbþ VHCAGiþ VFCAGe

¼ Z t

0

QGðCAGi0 CAGeÞdt  mO3R ðA:6Þ

In Eq. (A.6), mO3R¼ Rt

P

consumption rates of ozone by reactionsÞ dt.

Accordingly, Eq. (A.6) can be simplified to Eq. (2) by neglecting the relatively small term of VHCAGi, which had

been employed and justified in the previous study of

Chen et al. (2002). Apparently, mO3R depends only on

the ozone consumption and can be calculated from the measurable values of CALband CAGe. Thus, mO3Rcan be used to assess the performance of ozonation under various experimental conditions.

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數據

Fig. 1. Molecular structure of 2-mercaptothiazoline (2-MT) and simplified scheme of decomposition of 2-MT via ozonation.
Fig. 2. Experimental apparatus sketch. –––, – –, –– - - ––: ozone gas stream, experimental solution, isothermal water
Table 1. Moreover, the value of C ALb remains low in the
Fig. 7. ðC TOC0  C TOC Þ=t and C ALb =C AGi0 vs. g TOC for ozona-

參考文獻

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