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Enhanced ozonation of simulated dyestuff wastewater by microbubbles
Chemosphere 68 (2007) 1854–1860 www.elsevier.com/locate/chemosphere
Enhanced ozonation of simulated dyestuff wastewater by microbubbles Li-Bing Chu a, Xin-Hui Xing a
a,*
, An-Feng Yu a, Yu-Nan Zhou a, Xu-Lin Sun b, Benjamin Jurcik b
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China b Air Liquide Laboratories, Tsukuba-shi, Ibaraki-Pref. 300-4247, Japan
Received 20 December 2006; received in revised form 9 March 2007; accepted 9 March 2007 Available online 27 April 2007
Abstract The ozonation of synthetic wastewater containing azo dye, CI Reactive Black 5, was investigated using a microbubble generator and a conventional bubble contactor. The microbubble generator produced a milky and high intensity microbubble solution in which the bubbles had a mean diameter of less than 58 lm and a numerical density of more than 2.9 · 104 counts ml1 at a gas flow rate of less than 0.5 l min1. Compared with the bubble contactor, the total mass transfer coefficient was 1.8 times higher and the pseudo-first order rate constant was 3.2–3.6 times higher at the same initial dye concentration of 100 mg l1, 230 mg l1 and 530 mg l1 in the proposed microbubble system. The amount of total organic carbon removed per g of ozone consumed was about 1.3 times higher in the microbubble system than in the bubble contactor. The test using terephthalic acid as the chemical probe implied that more hydroxyl radicals were produced in the microbubble system, which contributed to the degradation of the dye molecules. The results suggested that in addition to the enhancement of mass transfer, microbubbles, which had higher inner pressure, could accelerate the formation of hydroxyl radicals and hence improve the oxidation of dye molecules. 2007 Elsevier Ltd. All rights reserved. Keywords: Microbubbles; Ozonation; Mass transfer; Hydroxyl radical; Dyestuff wastewater
1. Introduction Ozone is a well-known powerful oxidant. It is widely used to improve the taste and color of drinking water as well as to remove organics and inorganic compounds in drinking water and wastewater (Kasprzyk-Hordern et al., 2003). The ozonation process is a typical example of gas absorption with a chemical reaction where the total reaction rate can be affected by both the reaction kinetics and mass transfer (Zhou and Smith, 2000). It has been determined that the rate-limiting step comes from the low gas– liquid mass transfer rate due to the low solubility of ozone (Danckwerts, 1970). The rate of ozone mass transfer depends on the mixing characteristics of the gas–liquid
*
Corresponding author. Tel./fax: +86 10 62794771. E-mail address: [email protected] (X.-H. Xing).
0045-6535/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.03.014
contactor used, the kinetics of ozone decay in the water, and the number and size of the ozone bubbles produced (Shin et al., 1999; Rosal et al., 2006). The effectiveness of ozonation can be increased by a higher surface area of ozone through the generation of smaller bubbles (Mitani et al., 2005). Many factors hinder the use of ozone, including its relatively low solubility and stability in water, the high cost of ozone production, and the selectivity of oxidation. Ozone can slowly react with some organic compounds such as aromatics (Kasprzyk-Hordern et al., 2003). Moreover, in many cases, ozonation cannot completely mineralize the organics to carbon dioxide, leading to partial oxidation products such as aldehydes, organic acids and ketones (Slavinskaya and Selemenev, 2003). In order to enhance ozonation efficiency, much attention has been paid to the development of advanced oxidation processes using O3/ H2O2, UV/O3, or ultrasound/O3, and catalytic ozonation
L.-B. Chu et al. / Chemosphere 68 (2007) 1854–1860
using metal ions, metal oxides or activated carbon (Ruppert et al., 1994; Kasprzyk-Hordern et al., 2003). These processes mainly aim at the generation of hydroxyl radicals, which are more powerful oxidant than molecular ozone. Up to now, these advanced techniques are mainly in the laboratory investigation stage due to the complexity and high cost of these techniques. To make full use of the advantages of ozonation in industry, further study is needed to find efficient techniques to enhance the mass transfer and oxidation of ozone. Microbubbles are defined as bubbles with diameters less than several tens of lm. Compared to conventional bubbles with diameters of several mm, microbubbles have a huge interfacial area and bubble density, low rising velocity in liquid phase and high inner pressure (Serizawa et al., 2003). Microbubbles have shown promise due to their potential to be widely used in such fields as biomedical engineering, environmental engineering, drug reduction in shipbuilding, marine culture, mining and domestic bathes (Burns et al., 1997; Kodama et al., 2000; Soetanto and Chan, 2000; Walker et al., 2001; Serizawa et al., 2003). The application of microbubble technology in ozonation processes provides a new approach for improving ozonation efficiency. In this paper, a widely used azo reactive dye, CI Reactive Black 5 (RB 5) was chosen as a representative model dye to study the performance of ozonation by microbubble technology. Azo dyes are non-biodegradable by the conventional activated sludge process, which causes serious disposal problems (Vandevivere et al., 1998). Ozonation has been reported to be very effective in reducing the color of dyestuff wastewater, but it is not effective in mineralization of organic substances (Hao et al., 2000; Wang et al., 2003). The main purpose of our study was to examine the enhanced mass transfer and the ozone oxidation of dyestuff wastewater by microbubble technology. The mechanism of enhanced degradation of the dye by ozone microbubbles was also analyzed.
2. Materials and methods 2.1. Experimental apparatus The microbubble generator was made by Kyowa Engineering Co., Ltd. (Japan). It consisted of a recycling pump, a gyratory accelerator and an injector (Fig. 1). The principle of microbubble generation is described in the Supplementary Data. The working volume of the Plexiglas reactor was 20 l. Ozone was produced from pure oxygen by an ozone generator (ED-OG-R4, Eco Design, Japan) and fed into the reactor by the microbubble system or a conventional gas diffuser, which was used for comparison. The titanic porous diffuser was cylindrical, with a pore size of 45 lm. A 2% (w/v) potassium iodine solution was used to adsorb the ozone remaining in the off gas at the exit of the reactor.
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Fig. 1. Schematic diagram of the experimental set-up.
2.2. Materials A commercial dye, RB 5, with the chemical formula of C26H21N5O19S6 Æ 4Na (Scheme 1) and molecular weight of 991.8, was purchased from Tianjin Dyestuff Chemical Factory (China) and used without further purification. The synthetic wastewater was prepared by dissolving the RB 5 in deionized water.
2.3. Analysis The concentration of ozone in the gas phase was analyzed by the iodometric method with KI solution (Pires and Carvalho, 1998). The liquid-phase ozone concentration was measured using the indigo colorimetric methods (Bader and Hoigne, 1982). The RB 5 concentration and decolorization capacity during the ozonation process were determined from the absorption intensity at its maximum visible absorption wavelength (595 nm) by a 722s spectrophotometer (Shanghai Precision & Scientific Instrument Co., Ltd., China). The optical spectra of a sample in the UV–vis adsorption wavelength (200–800 nm) were scanned by a UV–vis spectrometer (UV 757, Shanghai Cany Precision Instrument Co., Ltd., China). The total organic carbon (TOC) concentration was measured using a TOC analyzer (TOC-5000A, Shimadzu Corporation, Japan). Anions were analyzed using a high performance liquid chromatograph (10A, Shimadzu Corporation, Japan) OH
M
NH2
N=N
N=N
NaO3S
SO3Na
M:-SO2CH2OSO3Na Scheme 1. Chemical structure of RB 5.
M
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equipped with a shim-pack IC-A3 column and a conductivity detector (CDD-10AVP, Shimadzu Corp., Japan). The mobile phase was 8 mM p-hydroxybenzoic acid, 3.2 mM bis–tris and 50 mM boric acid. The pressure and temperature of the column were 6.86 MPa and 40 C, respectively. The concentration of hydroxyl radicals produced from ozone was tested qualitatively using terephthalic acid (TA) as the chemical probe. This method has been used to detect hydroxyl radicals produced in aqueous phase pulsed electrical discharge reactors and by ultrasound (Fang et al., 1996; Mayank and Locke, 2006). Briefly, TA reacts with the hydroxyl radicals to form 2-hydroxytephthalic acid, which produces bright stable fluorescence when the excitation wavelength is set at 315 nm and the emission is measured at 425 nm. In the present study, ozone was absorbed into deionized water(R = 18.2 MXcm) at a temperature of 7 ± 2 C and pH of 2.0 ± 0.2. 5 ml samples were withdrawn from the reactor to react with 5 ml of 0.5 mM TA. Then the samples were analyzed with a fluorescence spectrophotometer (F-2500, Hitachi, Japan). The size of the microbubbles was measured using a microscope (E600, Nikon, Japan) equipped with a CCD digital camera (SPOT RT SUPER, Diagnostic Instruments, Inc., USA). The recorded images were analyzed to determine the bubble sizes using Image-Pro Plus software (Media Cybernetics Incorporated, USA). After the produced microbubbles reached stability, 1 cm-long cells were filled with samples, and immediately closed. The preparation and observation were conducted immediately to ensure that there was no obvious change in the microbubble size inside the fully filled and closed cells. This method might not accurately reflect the distribution of the original bubble sizes because some large bubbles may have been broken during the sampling, but it did provide a rough idea of the features of the microbubble generator. 2.4. Experimental procedure The experiments were divided into two parts. First, the enhancement of the mass transfer was studied. The temperature of the reactor was kept at 18 ± 2 C. The ozone gas was absorbed into deionized water until the ozone in the reactor reached saturation. Because the self-decomposition constant (kd) of ozone is much lower than the total mass transfer coefficient(kLa), the influence of kd was not taken into account in the derivation of kLa. Consequently, the values of kLa could be determined by the following equation: dC ¼ k L aðC CÞ dt
ð1Þ
where C is the concentration of dissolved ozone in the bulk liquid phase (mg l1) and C* is the concentration of saturated ozone in water (mg l1). The gas holdup (eg) was calculated by measuring the liquid height in the reactor with or without gas dispersion.
Secondly, the ozonation of the simulated dyestuff wastewater was studied. Prior to this experiment, a three-wayvalve was used to bypass the ozone-containing gas into the KI solution, with a gas flow rate of 0.5 l min1, to determine the inlet ozone concentration and to ensure the stability of the gas. The input ozone concentration was approximately 132 mg l1. Then the ozonation process was initiated when the ozone-containing gas was sparged into the reactor, which was operated in semi-batch mode by feeding the ozone continuously. The temperature was kept at 20 ± 2 C. Samples were taken at appropriate time intervals to analyze the color, pH and TOC. In addition, the ozone concentration in the off-gas was detected during the ozonation. The ozonation experiments were performed using both the microbubble generator and a conventional bubble contactor for comparison. All the experiments were repeated to ensure precision and the average value for each parameter was used. The relative standard deviation was less than 6% for most of the measured data. 3. Results and discussion 3.1. Enhanced mass transfer of ozone by microbubbles With the introduction of gas, the water in the reactor gradually became almost milky because many fine bubbles were formed. After gas feeding was stopped, the water remained milky and bubbly for 4–5 min, indicating the very low upward velocity of microbubbles. The eg depended on the gas flow rate (see Supplementary Data). As expected, the eg in the microbubble system was higher than that in the bubble diffused system due to both the high surface area-to-volume ratio and the increased bubble density. It was observed that the lower the gas flow rates, the smaller the bubbles were produced in the microbubble generator. Therefore, the difference of the eg between the two systems became smaller with the increase of the flow rate. A mixture of microbubbles and larger bubbles (approximately a few mm in diameter) was obtained when the gas flow rate was higher than 0.5 l min1. By observing more than 600 bubbles, the bubble size distribution in terms of count as a function of gas flow rates was obtained (see Supplementary Data). The count mean diameter, the total number of gas bubbles per volume and the specific surface area were calculated according to the equations described in Supplementary Data. A mean diameter of less than 58 lm, numerical density of more than 2.9 · 104 counts ml1, and specific surface area of more than 334 m1 were obtained at a gas flow rate of less than 0.5 l min1. With the increase of the gas flow rate, more bubbles with larger sizes were observed. Accordingly, the bubble density and the surface area-to-volume ratio decreased. The obtained high bubble concentration and large specific surface area demonstrated the high intensity of microbubble generation. Fig. 2 shows changes in the concentrations of dissolved ozone and off-gas ozone with time in the two systems at a
L.-B. Chu et al. / Chemosphere 68 (2007) 1854–1860 90
16
80 70
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10 8
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-1
kLa=0.143 min
40
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30 4 20 2
-1
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Output ozone conc. (mg l )
-1
Dissolved ozone conc. (mg l )
-1
kLa=0.256 min
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Time (min) Fig. 2. Time course of dissolved and output ozone concentrations in deionized water at a gas flow rate of 0.5 l min1. Symbols: circles – microbubble generator; triangles – bubble contactor.
gas flow rate of 0.5 l min1. The concentrations of dissolved ozone rose rapidly in the initial period because of the large driving forces for the mass transfer of ozone and then reached saturated. Clearly, the microbubble generator exhibited more efficient mass transfer of ozone and higher concentrations of dissolved ozone compared to the bubble contactor. The total mass transfer coefficient in the former was 1.8 times higher than that in the latter. This is consistent with the expectation that smaller bubbles will result in higher rates of mass transfer of ozone from the bubbles by maximizing the surface area-to-volume ratio. 3.2. Enhanced ozonation of dyestuff wastewater by microbubbles As the ozonation progressed, the color of the RB 5 solution was gradually bleached from black, to brown, to yellow, and then to colorless. Fig. 3 presents the results
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of the ozonation of the RB 5 solution at various initial dye concentrations. The results showed that the lower the initial dye concentration, the higher the decolorization rate. This dependency of decolorization rate on the initial dye concentration may be due to the production of more than one kind of intermediate during the ozonation processes (Shu and Huang, 1995). The reaction kinetics could be treated as a pseudo-first order reaction and the pseudo-first order rate constant k was used to evaluate the decolorization efficiency. The same pseudo-first order behaviors have also been reported elsewhere (Shu and Huang, 1995; Wu and Wang, 2001). The pseudo-first order rate constant in the proposed microbubble system was 3.2–3.6 times faster than that in the bubble contactor at the same initial dye concentration. Considering the fact that the total mass transfer coefficient in the former is 1.8 times higher than that in the latter, we suggested that microbubbles might affect the degree of ozone oxidation in addition to enhancing the mass transfer. The UV absorption spectra at different ozonation time were obtained to study the destruction of the dye molecules. RB 5 exhibited absorption bands at 595 and 311 nm. The disappearance of the visible band was due to the fragmentation of the azo link due to ozone molecule and hydroxyl radical attack, which has been proposed as the first step in the degradation of azo dyes (Ince and Tezcanli, 2001). In addition to this rapid bleaching effect, the decay of the absorbance at 311 nm was considered as evidence of the degradation of aromatic groups in the dye molecules and their intermediates. The abatement rates at 595 nm and 311 nm are shown in Fig. 4. Degradation of aromatic dye moieties proved to be a more recalcitrant process. Ozone could cleave the conjugated double bonds more thoroughly. The benzene rings could also be destroyed to some extent. Clearly, the microbubble system exhibited a faster abatement rate than the bubble contactor.
1.0
1.0
530 mg l -1 230 mg l -1 100 mg l
0.8
C/Co
Normalized absorbency (At /A0 )
-1
0.6 0.4 0.2 0.0
Symbols: hollow-311 nm solid-595 nm
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Time (min) Fig. 3. Degradation of RB 5 solution by ozonation at a gas flow rate of 0.5 l min1. Symbols: circles – microbubble generator; triangles – bubble contactor.
0
20
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Time (min) Fig. 4. Rate of abatement at two absorption bands at an initial concentration of 230 mg l1 during the ozonation of RB 5 solution. Symbols: circles – microbubble generator; triangles – bubble contactor.
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3.3. Analysis of the enhancement mechanism by microbubbles To further examine the mechanism of the enhanced ozonation by microbubbles, the performance of the two systems was compared in terms of decolorization, TOC removal, nitrate and sulfate concentrations, ozone utilization rate and enhancement factor due to chemical reaction E at an initial dye concentration of 230 mg l1 (see Fig. 5 and Supplementary Data). The E value is defined as the ratio of the rate of absorption with chemical reaction to the maximum rate of physical absorption. It was calculated according to the method by Zhou and Smith (1997). As demonstrated in Fig. 5, the color removal initially occurred rapidly, and slowed down, finally reaching 100%. The rate of decolorization was obviously faster in the microbubble system. The time required for 99% removal of color was about 30 min after introduction of ozone into the microbubble generator, compared to 70 min for the bubble contactor. In terms of mineralization of the RB 5 solution, the TOC removal efficiency could be increased markedly by using
Color removal efficiency (%)
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Ratio of consumed to applied ozone (%)
Time (min)
8
Applied ozone dose (g) Fig. 5. Performance of the two systems in color, TOC removal efficiency, ozone utilization rate and E at an initial concentration of 230 mg l1. Symbols: circles – microbubble generator; triangles – bubble contactor.
microbubble technology. The TOC removal efficiency of the microbubble generator reached 80% (at 80 min); it was only 34% (at 130 min) for the bubble contactor. It is interesting to note that the TOC removal lagged behind the color removal during the ozonation. As shown in Fig. 5, when almost 95% of the color had been removed, the TOC of the solution began to decline gradually. This could have been because the ozone first attacked the chromophore, the azo link groups of the RB 5, and produced intermediates (Ince and Tezcanli, 2001). Then, the reaction between the ozone and the intermediates containing organic carbons became dominant, which resulted in TOC reduction. The sulfate and nitrate concentrations were measured during the ozone treatment of RB 5 wastewater by ozone (see Supplementary Data). The formation of nitrate was most likely the result of the oxidation and cleavage of the azo and amino groups (Wang et al., 2003). The cleavage and oxidation of the sulfonic acid resulted in the increase of sulfate. The high initial sulfate concentration of the aqueous RB 5 solution was presumably caused by the presence of sulfate salts within the dye. The concentrations of sulfate released and nitrate formed were higher for the microbubble generator than those for the bubble contactor. These results were consistent with the TOC reduction. The concentration of off-gas ozone of the microbubble system (0.04–2.5 mg l1) was significantly lower than that for the bubble contactor (0.08–74.3 mg l1), which demonstrated the very high ozone utilization of the microbubble system. The ratio of the consumed ozone to the applied ozone was used as a quantitative indicator to represent the ozone utilization rate, as depicted in Fig. 5. The applied ozone MA (mg) and consumed ozone MR (mg) are defined by Eqs. (2) and (3), respectively: M A ¼ QG C A0 t
ð2Þ
MR = Ozone applied – ozone exited – ozone accumulated in holdup gas and liquid Z t ¼ QG ðC A0 C Ae Þdt C Ai V H C Ab V L ð3Þ 0
where QG is the gas flow rate (l min1), CA0 is the input ozone concentration (mg l1), t is the reaction time (min), CAi is the ozone concentration in the holdup gas (mg l1), and is taken as the off-gas ozone concentration CAe in the calculation for simplicity, VH is the volume of holdup gas (L), VL is the working volume of the reactor (L). The dissolved ozone concentration (CAb) was found to be lower than 0.08 mg l1 after the completion of decolorization, so it was not included in this calculation. When the bubble contactor was used, the MR/MA value decreased slowly, then sharply. The decreasing of the MR/ MA value might have been caused by the lower reactivity of the dye solution with ozone. It was thus proposed that the initial decolorization reaction occurred readily, and the rate of both the reaction and ozone utilization was high. This also was confirmed by the significant enhancement
Fluorescence intensity
2000
TA+ micro-ozone-bubble water
1600
TA+normal-ozone-bubble water
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0.20
-1
of the mass transfer, which was primarily due to the fast ozonation reaction between the dissolved ozone and RB 5 solution (Qiu et al., 2001). At the later stage, the ozonation products were hard to oxidize, resulting in a much lower reaction rate. The E value reached unity, indicating the chemical reaction had little influence on the mass transfer rate. Chen et al. (2005) investigated the ozonation of RB 5 by using a rotating packed bed (RPB) and completely stirred tank reactor (CSTR) as ozone contactors. The experimental results indicated that the MR/MA value decreased with ozone doses from 0.52 to 0.14 in the RPB and 0.80 to 0.18 in the CSTR. However, by using the microbubble generator, the MR/ MA value and enhancement factor decreased a little for the applied ozone doses as ozonation proceeded, indicating high reactivity during the whole ozonation process. Considering the high TOC removal efficiency in the microbubble system described above, we propose that the microbubble technology can enhance the ozone oxidation as well as the mass transfer. In general, ozone oxidation pathways include direct ozone attack or free radical attacks. Since the oxidation potential of hydroxyl radicals is much higher than that of ozone molecules, direct oxidation is slower than radical oxidation. Microbubbles with higher internal pressure could induce the decomposition of ozone and contribute to the production of hydroxyl radicals. According to the Young–Laplace equation DP ¼ 2cr (DP is the pressure difference, c is the surface tension, r is the radius of bubble), the smaller the bubble size, the higher the internal pressure at this conditions. This suggestion was proven by hydroxyl radical detection (see Fig. 6). The fluorescence intensity of the samples in the two systems increased with the continuous ozone injection and then reached steady state (see Supplementary Data). The fluorescence intensity of the samples from the microbubble system was much higher than that from the bubble contactor. Ozone molecules could not react with TA. There was almost no fluorescence detected in
Removed TOC/consumed ozone (gg )
L.-B. Chu et al. / Chemosphere 68 (2007) 1854–1860
0.15
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Microbubble generator Bubble contactor
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Applied ozone dose (g) Fig. 7. Ratio of removed TOC value (g) to consumed ozone dose (g) in the two systems at an initial concentration of 230 mg l1.
the presence of the hydroxyl radical scavenger of 90 mM T-butanol in the ozone water sample. These results implied that more hydroxyl radicals were produced in the microbubble system, which contributed to the degradation of the dye molecules. In order to eliminate the effects of the mass transfer, the ratio of the removed TOC (g) to the consumed ozone dose (g) was plotted as a function of the implied ozone dose (see Fig. 7). This ratio is the amount of removed TOC per g of consumed ozone. It was calculated when the TOC of the solution began to decrease steadily, which was from 25 min to 90 min for the microbubble system and from 50 min to 130 min for the bubble contactor. It was obvious that for the same ozone consumption, the TOC removal in the former was 1.3 times higher than that in the latter indicating its higher reactivity. As shown above, the microbubble technology will be a promising process for the enhanced ozonation of dyestuff wastewater. One of the important factors pertaining to the industrial application is the energy cost. Compared to the conventional ozonation process, a liquid pump will be required for the microbubble system, but the consumed ozone doses were greatly decreased because of its higher reaction activity. The total energy cost will be further analyzed by optimizing the scale and operation of the system. 4. Conclusions
1200 800 ozone water+T-butanol+TA
400 TA
ozone water
0 350
400
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650
A microbubble technology was applied to increase the mass transfer rate of ozone as well as to enhance the ozone oxidation of synthetic RB 5 wastewater in this paper. Experiments were performed using both the microbubbles generator and the bubble contactor, which is commonly used in ozonation systems, for the purpose of comparison. Main conclusions obtained are as follows:
Wavelength (nm) Fig. 6. Fluorescence detection of hydroxyl radicals formed in ozone water under different conditions.
(1) The microbubble generator produced a milky and high intensity microbubble solution in which the bub-
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bles had a mean diameter of less than 58 lm, a numerical density of more than 2.9 · 104 counts ml1, and a specific surface area of more than 334 m1 at a gas flow rate of less than 0.5 l min1. (2) Compared with the bubble contactor, the total mass transfer coefficient was 1.8 times higher and the pseudo-first order rate constant was 3.2–3.6 times faster at the same initial dye concentration in the proposed microbubble system at a gas flow rate of 0.5 l min1. (3) The amount of TOC removed per g of ozone consumed was about 1.3 times higher in the microbubble system than in the bubble contactor. The results of the test using TA as the chemical probe implied that more hydroxyl radicals were produced in the microbubble system, which contributed to the degradation of the dye molecules. (4) According to the results of mass transfer, decolorization, mineralization and ozone accumulation for treating synthetic dyestuff wastewater, the microbubble system is a promising process for enhancing both mass transfer and oxidation of ozone. The total energy cost should be analyzed further.
Acknowledgement This research was supported in part by the China Postdoctoral Science Foundation (2005038346). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.chemosphere.2007.03.014. References Bader, H., Hoigne, J., 1982. Determination of ozone in water using the indigo dye method. Ozone Sci. Eng. 4, 169–176. Burns, S.E., Yiacoumi, S., Tsouris, C., 1997. Microbubble generation for environmental and industrial separations. Sep. Purif. Technol. 11, 221– 232. Chen, Y.H., Chang, C.Y., Su, W.L., Chiu, C.Y., Yu, Y.H., Chiang, P.C., Chang, C.F., Shie, J.L., Chiou, C.S., Chiang, S.I., 2005. Ozonation of CI Reactive Black 5 using rotating packed bed and stirred tank reactor. J. Chem. Technol. Biotechnol. 80, 68–75. Danckwerts, P.V., 1970. Gas–Liquid Reactions. McGraw-Hill, New York. Fang, X., Mark, G., Sonntag, C., 1996. OH radical formation by ultrasound in aqueous solutions Part 1: The chemistry underlying the terephthalate dosimeter. Ultrason. Sonochem. 3, 57–63.
Hao, O.J., Kim, H., Chiang, P.C., 2000. Decolorization of wastewater. Crit. Rev. Env. Sci. Tec. 30, 449–505. Ince, N.H., Tezcanli, G., 2001. Reactive dyestuff degradation by combined sonolysis and ozonation. Dyes Pigments 49, 145–153. Kasprzyk-Hordern, B., Ziolek, M., Nawrocki, J., 2003. Catalytic ozonation and methods of enhancing molecular ozone reactions in water treatment. Appl. Catal. B-Environ. 46, 639–669. Kodama, Y., Kakugawa, A., Takahashi, T., Kawashima, H., 2000. Experimental study on microbubbles and their applicability to ships for skin friction reduction. Int. J. Heat Fluid Fl. 21, 582–588. Mayank, S., Locke, B.R., 2006. Quantification of hydroxyl radicals produced in aqueous phase pulsed electrical discharge reactors. Ind. Eng. Chem. Res. 45, 5819–5825. Mitani, M.M., Keller, A.A., Sandall, O.C., Rinker, R.G., 2005. Mass transfer of ozone using a microporous diffuser reactor system. Ozone Sci. Eng. 27, 45–51. Pires, M., Carvalho, L.R.F., 1998. An artifact in air carbonyls sampling using C-18 DNPH-coated cartridge. Anal. Chim. Acta 367, 223–231. Qiu, Y.Q., Kuo, C.H., Zappi, M.E., 2001. Performance and simulation of ozone absorption and reactions in a stirred-tank reactor. Environ. Sci. Technol. 35, 209–215. Rosal, R., Rodriguez, A., Zerhouni, M., 2006. Enhancement of gas–liquid mass transfer during the unsteady-state catalytic decomposition of ozone in water. Appl. Catal. A-General 305, 169–175. Ruppert, G., Bauer, R., Heisler, G., 1994. UV-O3, UV-H2O2, UV-TiO2 and the photo-fenton reaction – comparison of advanced oxidation processes for waste-water treatment. Chemosphere 28, 1447–1454. Serizawa, A., Inui, T., Yahiro, T., Kawara, Z., 2003. Laminarition of micro-bubble containing milky bubbly flow in a pipe. In: Third European–Japanese Two-Phase Flow Group Meeting, Certosa di Pontignano, pp. 21–27. Shin, W.T., Mirmiran, A., Yiacoumi, S., Tsouris, C., 1999. Ozonation using microbubbles formed by electric fields. Sep. Purif. Technol. 15, 271–282. Shu, H.Y., Huang, C.R., 1995. Degradation of commercial azo dyes in water using ozonation and UV enhanced ozonation process. Chemosphere 31, 3813–3825. Slavinskaya, G.V., Selemenev, V.F., 2003. Ozonation of fulvic acids of natural waters in aqueous solutions. Russian J. Appl. Chem. 76, 1472– 1475. Soetanto, K., Chan, M., 2000. Fundamental studies on contrast images from different-sized microbubbles: Analytical and experimental studies. Ultrasound Med. Biol. 26, 81–91. Vandevivere, P.C., Bianchi, R., Verstraete, W., 1998. Treatment and reuse of wastewater from the textile wet-processing industry: Review of emerging technologies. J. Chem. Technol. Biotechnol. 72, 289–302. Walker, A.B., Tsouris, C., DePaoli, D.W., Klasson, K.T., 2001. Ozonation of soluble organics in aqueous solutions using microbubbles. Ozone Sci. Eng. 23, 77–87. Wang, C.X., Yediler, A., Lienert, D., Wang, Z.J., Kettrup, A., 2003. Ozonation of an azo dye CI Remazol Black 5 and toxicological assessment of its oxidation products. Chemosphere 52, 1225–1232. Wu, J.N., Wang, T.W., 2001. Ozonation of aqueous azo dye in a semibatch reactor. Water Res. 35, 1093–1099. Zhou, H.D., Smith, D.W., 1997. Process parameter development for ozonation of kraft pulp mill effluents. Water Sci. Technol. 35, 251–259. Zhou, H.D., Smith, D.W., 2000. Ozone mass transfer in water and wastewater treatment: Experimental observations using a 2D laser particle dynamics analyzer. Water Res. 34, 909–921.
Enhanced ozonation of simulated dyestuff wastewater by microbubbles Li-Bing Chu a, Xin-Hui Xing a
a,*
, An-Feng Yu a, Yu-Nan Zhou a, Xu-Lin Sun b, Benjamin Jurcik b
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China b Air Liquide Laboratories, Tsukuba-shi, Ibaraki-Pref. 300-4247, Japan
Received 20 December 2006; received in revised form 9 March 2007; accepted 9 March 2007 Available online 27 April 2007
Abstract The ozonation of synthetic wastewater containing azo dye, CI Reactive Black 5, was investigated using a microbubble generator and a conventional bubble contactor. The microbubble generator produced a milky and high intensity microbubble solution in which the bubbles had a mean diameter of less than 58 lm and a numerical density of more than 2.9 · 104 counts ml1 at a gas flow rate of less than 0.5 l min1. Compared with the bubble contactor, the total mass transfer coefficient was 1.8 times higher and the pseudo-first order rate constant was 3.2–3.6 times higher at the same initial dye concentration of 100 mg l1, 230 mg l1 and 530 mg l1 in the proposed microbubble system. The amount of total organic carbon removed per g of ozone consumed was about 1.3 times higher in the microbubble system than in the bubble contactor. The test using terephthalic acid as the chemical probe implied that more hydroxyl radicals were produced in the microbubble system, which contributed to the degradation of the dye molecules. The results suggested that in addition to the enhancement of mass transfer, microbubbles, which had higher inner pressure, could accelerate the formation of hydroxyl radicals and hence improve the oxidation of dye molecules. 2007 Elsevier Ltd. All rights reserved. Keywords: Microbubbles; Ozonation; Mass transfer; Hydroxyl radical; Dyestuff wastewater
1. Introduction Ozone is a well-known powerful oxidant. It is widely used to improve the taste and color of drinking water as well as to remove organics and inorganic compounds in drinking water and wastewater (Kasprzyk-Hordern et al., 2003). The ozonation process is a typical example of gas absorption with a chemical reaction where the total reaction rate can be affected by both the reaction kinetics and mass transfer (Zhou and Smith, 2000). It has been determined that the rate-limiting step comes from the low gas– liquid mass transfer rate due to the low solubility of ozone (Danckwerts, 1970). The rate of ozone mass transfer depends on the mixing characteristics of the gas–liquid
*
Corresponding author. Tel./fax: +86 10 62794771. E-mail address: [email protected] (X.-H. Xing).
0045-6535/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.03.014
contactor used, the kinetics of ozone decay in the water, and the number and size of the ozone bubbles produced (Shin et al., 1999; Rosal et al., 2006). The effectiveness of ozonation can be increased by a higher surface area of ozone through the generation of smaller bubbles (Mitani et al., 2005). Many factors hinder the use of ozone, including its relatively low solubility and stability in water, the high cost of ozone production, and the selectivity of oxidation. Ozone can slowly react with some organic compounds such as aromatics (Kasprzyk-Hordern et al., 2003). Moreover, in many cases, ozonation cannot completely mineralize the organics to carbon dioxide, leading to partial oxidation products such as aldehydes, organic acids and ketones (Slavinskaya and Selemenev, 2003). In order to enhance ozonation efficiency, much attention has been paid to the development of advanced oxidation processes using O3/ H2O2, UV/O3, or ultrasound/O3, and catalytic ozonation
L.-B. Chu et al. / Chemosphere 68 (2007) 1854–1860
using metal ions, metal oxides or activated carbon (Ruppert et al., 1994; Kasprzyk-Hordern et al., 2003). These processes mainly aim at the generation of hydroxyl radicals, which are more powerful oxidant than molecular ozone. Up to now, these advanced techniques are mainly in the laboratory investigation stage due to the complexity and high cost of these techniques. To make full use of the advantages of ozonation in industry, further study is needed to find efficient techniques to enhance the mass transfer and oxidation of ozone. Microbubbles are defined as bubbles with diameters less than several tens of lm. Compared to conventional bubbles with diameters of several mm, microbubbles have a huge interfacial area and bubble density, low rising velocity in liquid phase and high inner pressure (Serizawa et al., 2003). Microbubbles have shown promise due to their potential to be widely used in such fields as biomedical engineering, environmental engineering, drug reduction in shipbuilding, marine culture, mining and domestic bathes (Burns et al., 1997; Kodama et al., 2000; Soetanto and Chan, 2000; Walker et al., 2001; Serizawa et al., 2003). The application of microbubble technology in ozonation processes provides a new approach for improving ozonation efficiency. In this paper, a widely used azo reactive dye, CI Reactive Black 5 (RB 5) was chosen as a representative model dye to study the performance of ozonation by microbubble technology. Azo dyes are non-biodegradable by the conventional activated sludge process, which causes serious disposal problems (Vandevivere et al., 1998). Ozonation has been reported to be very effective in reducing the color of dyestuff wastewater, but it is not effective in mineralization of organic substances (Hao et al., 2000; Wang et al., 2003). The main purpose of our study was to examine the enhanced mass transfer and the ozone oxidation of dyestuff wastewater by microbubble technology. The mechanism of enhanced degradation of the dye by ozone microbubbles was also analyzed.
2. Materials and methods 2.1. Experimental apparatus The microbubble generator was made by Kyowa Engineering Co., Ltd. (Japan). It consisted of a recycling pump, a gyratory accelerator and an injector (Fig. 1). The principle of microbubble generation is described in the Supplementary Data. The working volume of the Plexiglas reactor was 20 l. Ozone was produced from pure oxygen by an ozone generator (ED-OG-R4, Eco Design, Japan) and fed into the reactor by the microbubble system or a conventional gas diffuser, which was used for comparison. The titanic porous diffuser was cylindrical, with a pore size of 45 lm. A 2% (w/v) potassium iodine solution was used to adsorb the ozone remaining in the off gas at the exit of the reactor.
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Fig. 1. Schematic diagram of the experimental set-up.
2.2. Materials A commercial dye, RB 5, with the chemical formula of C26H21N5O19S6 Æ 4Na (Scheme 1) and molecular weight of 991.8, was purchased from Tianjin Dyestuff Chemical Factory (China) and used without further purification. The synthetic wastewater was prepared by dissolving the RB 5 in deionized water.
2.3. Analysis The concentration of ozone in the gas phase was analyzed by the iodometric method with KI solution (Pires and Carvalho, 1998). The liquid-phase ozone concentration was measured using the indigo colorimetric methods (Bader and Hoigne, 1982). The RB 5 concentration and decolorization capacity during the ozonation process were determined from the absorption intensity at its maximum visible absorption wavelength (595 nm) by a 722s spectrophotometer (Shanghai Precision & Scientific Instrument Co., Ltd., China). The optical spectra of a sample in the UV–vis adsorption wavelength (200–800 nm) were scanned by a UV–vis spectrometer (UV 757, Shanghai Cany Precision Instrument Co., Ltd., China). The total organic carbon (TOC) concentration was measured using a TOC analyzer (TOC-5000A, Shimadzu Corporation, Japan). Anions were analyzed using a high performance liquid chromatograph (10A, Shimadzu Corporation, Japan) OH
M
NH2
N=N
N=N
NaO3S
SO3Na
M:-SO2CH2OSO3Na Scheme 1. Chemical structure of RB 5.
M
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equipped with a shim-pack IC-A3 column and a conductivity detector (CDD-10AVP, Shimadzu Corp., Japan). The mobile phase was 8 mM p-hydroxybenzoic acid, 3.2 mM bis–tris and 50 mM boric acid. The pressure and temperature of the column were 6.86 MPa and 40 C, respectively. The concentration of hydroxyl radicals produced from ozone was tested qualitatively using terephthalic acid (TA) as the chemical probe. This method has been used to detect hydroxyl radicals produced in aqueous phase pulsed electrical discharge reactors and by ultrasound (Fang et al., 1996; Mayank and Locke, 2006). Briefly, TA reacts with the hydroxyl radicals to form 2-hydroxytephthalic acid, which produces bright stable fluorescence when the excitation wavelength is set at 315 nm and the emission is measured at 425 nm. In the present study, ozone was absorbed into deionized water(R = 18.2 MXcm) at a temperature of 7 ± 2 C and pH of 2.0 ± 0.2. 5 ml samples were withdrawn from the reactor to react with 5 ml of 0.5 mM TA. Then the samples were analyzed with a fluorescence spectrophotometer (F-2500, Hitachi, Japan). The size of the microbubbles was measured using a microscope (E600, Nikon, Japan) equipped with a CCD digital camera (SPOT RT SUPER, Diagnostic Instruments, Inc., USA). The recorded images were analyzed to determine the bubble sizes using Image-Pro Plus software (Media Cybernetics Incorporated, USA). After the produced microbubbles reached stability, 1 cm-long cells were filled with samples, and immediately closed. The preparation and observation were conducted immediately to ensure that there was no obvious change in the microbubble size inside the fully filled and closed cells. This method might not accurately reflect the distribution of the original bubble sizes because some large bubbles may have been broken during the sampling, but it did provide a rough idea of the features of the microbubble generator. 2.4. Experimental procedure The experiments were divided into two parts. First, the enhancement of the mass transfer was studied. The temperature of the reactor was kept at 18 ± 2 C. The ozone gas was absorbed into deionized water until the ozone in the reactor reached saturation. Because the self-decomposition constant (kd) of ozone is much lower than the total mass transfer coefficient(kLa), the influence of kd was not taken into account in the derivation of kLa. Consequently, the values of kLa could be determined by the following equation: dC ¼ k L aðC CÞ dt
ð1Þ
where C is the concentration of dissolved ozone in the bulk liquid phase (mg l1) and C* is the concentration of saturated ozone in water (mg l1). The gas holdup (eg) was calculated by measuring the liquid height in the reactor with or without gas dispersion.
Secondly, the ozonation of the simulated dyestuff wastewater was studied. Prior to this experiment, a three-wayvalve was used to bypass the ozone-containing gas into the KI solution, with a gas flow rate of 0.5 l min1, to determine the inlet ozone concentration and to ensure the stability of the gas. The input ozone concentration was approximately 132 mg l1. Then the ozonation process was initiated when the ozone-containing gas was sparged into the reactor, which was operated in semi-batch mode by feeding the ozone continuously. The temperature was kept at 20 ± 2 C. Samples were taken at appropriate time intervals to analyze the color, pH and TOC. In addition, the ozone concentration in the off-gas was detected during the ozonation. The ozonation experiments were performed using both the microbubble generator and a conventional bubble contactor for comparison. All the experiments were repeated to ensure precision and the average value for each parameter was used. The relative standard deviation was less than 6% for most of the measured data. 3. Results and discussion 3.1. Enhanced mass transfer of ozone by microbubbles With the introduction of gas, the water in the reactor gradually became almost milky because many fine bubbles were formed. After gas feeding was stopped, the water remained milky and bubbly for 4–5 min, indicating the very low upward velocity of microbubbles. The eg depended on the gas flow rate (see Supplementary Data). As expected, the eg in the microbubble system was higher than that in the bubble diffused system due to both the high surface area-to-volume ratio and the increased bubble density. It was observed that the lower the gas flow rates, the smaller the bubbles were produced in the microbubble generator. Therefore, the difference of the eg between the two systems became smaller with the increase of the flow rate. A mixture of microbubbles and larger bubbles (approximately a few mm in diameter) was obtained when the gas flow rate was higher than 0.5 l min1. By observing more than 600 bubbles, the bubble size distribution in terms of count as a function of gas flow rates was obtained (see Supplementary Data). The count mean diameter, the total number of gas bubbles per volume and the specific surface area were calculated according to the equations described in Supplementary Data. A mean diameter of less than 58 lm, numerical density of more than 2.9 · 104 counts ml1, and specific surface area of more than 334 m1 were obtained at a gas flow rate of less than 0.5 l min1. With the increase of the gas flow rate, more bubbles with larger sizes were observed. Accordingly, the bubble density and the surface area-to-volume ratio decreased. The obtained high bubble concentration and large specific surface area demonstrated the high intensity of microbubble generation. Fig. 2 shows changes in the concentrations of dissolved ozone and off-gas ozone with time in the two systems at a
L.-B. Chu et al. / Chemosphere 68 (2007) 1854–1860 90
16
80 70
12
60
10 8
50
-1
kLa=0.143 min
40
6
30 4 20 2
-1
14
Output ozone conc. (mg l )
-1
Dissolved ozone conc. (mg l )
-1
kLa=0.256 min
10
0
0 0
10
20
30
40
50
Time (min) Fig. 2. Time course of dissolved and output ozone concentrations in deionized water at a gas flow rate of 0.5 l min1. Symbols: circles – microbubble generator; triangles – bubble contactor.
gas flow rate of 0.5 l min1. The concentrations of dissolved ozone rose rapidly in the initial period because of the large driving forces for the mass transfer of ozone and then reached saturated. Clearly, the microbubble generator exhibited more efficient mass transfer of ozone and higher concentrations of dissolved ozone compared to the bubble contactor. The total mass transfer coefficient in the former was 1.8 times higher than that in the latter. This is consistent with the expectation that smaller bubbles will result in higher rates of mass transfer of ozone from the bubbles by maximizing the surface area-to-volume ratio. 3.2. Enhanced ozonation of dyestuff wastewater by microbubbles As the ozonation progressed, the color of the RB 5 solution was gradually bleached from black, to brown, to yellow, and then to colorless. Fig. 3 presents the results
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of the ozonation of the RB 5 solution at various initial dye concentrations. The results showed that the lower the initial dye concentration, the higher the decolorization rate. This dependency of decolorization rate on the initial dye concentration may be due to the production of more than one kind of intermediate during the ozonation processes (Shu and Huang, 1995). The reaction kinetics could be treated as a pseudo-first order reaction and the pseudo-first order rate constant k was used to evaluate the decolorization efficiency. The same pseudo-first order behaviors have also been reported elsewhere (Shu and Huang, 1995; Wu and Wang, 2001). The pseudo-first order rate constant in the proposed microbubble system was 3.2–3.6 times faster than that in the bubble contactor at the same initial dye concentration. Considering the fact that the total mass transfer coefficient in the former is 1.8 times higher than that in the latter, we suggested that microbubbles might affect the degree of ozone oxidation in addition to enhancing the mass transfer. The UV absorption spectra at different ozonation time were obtained to study the destruction of the dye molecules. RB 5 exhibited absorption bands at 595 and 311 nm. The disappearance of the visible band was due to the fragmentation of the azo link due to ozone molecule and hydroxyl radical attack, which has been proposed as the first step in the degradation of azo dyes (Ince and Tezcanli, 2001). In addition to this rapid bleaching effect, the decay of the absorbance at 311 nm was considered as evidence of the degradation of aromatic groups in the dye molecules and their intermediates. The abatement rates at 595 nm and 311 nm are shown in Fig. 4. Degradation of aromatic dye moieties proved to be a more recalcitrant process. Ozone could cleave the conjugated double bonds more thoroughly. The benzene rings could also be destroyed to some extent. Clearly, the microbubble system exhibited a faster abatement rate than the bubble contactor.
1.0
1.0
530 mg l -1 230 mg l -1 100 mg l
0.8
C/Co
Normalized absorbency (At /A0 )
-1
0.6 0.4 0.2 0.0
Symbols: hollow-311 nm solid-595 nm
0.8
0.6
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0.0
0
20
40
60
80
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Time (min) Fig. 3. Degradation of RB 5 solution by ozonation at a gas flow rate of 0.5 l min1. Symbols: circles – microbubble generator; triangles – bubble contactor.
0
20
40
60
80
100
120
Time (min) Fig. 4. Rate of abatement at two absorption bands at an initial concentration of 230 mg l1 during the ozonation of RB 5 solution. Symbols: circles – microbubble generator; triangles – bubble contactor.
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3.3. Analysis of the enhancement mechanism by microbubbles To further examine the mechanism of the enhanced ozonation by microbubbles, the performance of the two systems was compared in terms of decolorization, TOC removal, nitrate and sulfate concentrations, ozone utilization rate and enhancement factor due to chemical reaction E at an initial dye concentration of 230 mg l1 (see Fig. 5 and Supplementary Data). The E value is defined as the ratio of the rate of absorption with chemical reaction to the maximum rate of physical absorption. It was calculated according to the method by Zhou and Smith (1997). As demonstrated in Fig. 5, the color removal initially occurred rapidly, and slowed down, finally reaching 100%. The rate of decolorization was obviously faster in the microbubble system. The time required for 99% removal of color was about 30 min after introduction of ozone into the microbubble generator, compared to 70 min for the bubble contactor. In terms of mineralization of the RB 5 solution, the TOC removal efficiency could be increased markedly by using
Color removal efficiency (%)
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60 40
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TOC removal efficiency (%)
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Enhancement factor E
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3 40 2 20
1 0
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2
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6
7
Ratio of consumed to applied ozone (%)
Time (min)
8
Applied ozone dose (g) Fig. 5. Performance of the two systems in color, TOC removal efficiency, ozone utilization rate and E at an initial concentration of 230 mg l1. Symbols: circles – microbubble generator; triangles – bubble contactor.
microbubble technology. The TOC removal efficiency of the microbubble generator reached 80% (at 80 min); it was only 34% (at 130 min) for the bubble contactor. It is interesting to note that the TOC removal lagged behind the color removal during the ozonation. As shown in Fig. 5, when almost 95% of the color had been removed, the TOC of the solution began to decline gradually. This could have been because the ozone first attacked the chromophore, the azo link groups of the RB 5, and produced intermediates (Ince and Tezcanli, 2001). Then, the reaction between the ozone and the intermediates containing organic carbons became dominant, which resulted in TOC reduction. The sulfate and nitrate concentrations were measured during the ozone treatment of RB 5 wastewater by ozone (see Supplementary Data). The formation of nitrate was most likely the result of the oxidation and cleavage of the azo and amino groups (Wang et al., 2003). The cleavage and oxidation of the sulfonic acid resulted in the increase of sulfate. The high initial sulfate concentration of the aqueous RB 5 solution was presumably caused by the presence of sulfate salts within the dye. The concentrations of sulfate released and nitrate formed were higher for the microbubble generator than those for the bubble contactor. These results were consistent with the TOC reduction. The concentration of off-gas ozone of the microbubble system (0.04–2.5 mg l1) was significantly lower than that for the bubble contactor (0.08–74.3 mg l1), which demonstrated the very high ozone utilization of the microbubble system. The ratio of the consumed ozone to the applied ozone was used as a quantitative indicator to represent the ozone utilization rate, as depicted in Fig. 5. The applied ozone MA (mg) and consumed ozone MR (mg) are defined by Eqs. (2) and (3), respectively: M A ¼ QG C A0 t
ð2Þ
MR = Ozone applied – ozone exited – ozone accumulated in holdup gas and liquid Z t ¼ QG ðC A0 C Ae Þdt C Ai V H C Ab V L ð3Þ 0
where QG is the gas flow rate (l min1), CA0 is the input ozone concentration (mg l1), t is the reaction time (min), CAi is the ozone concentration in the holdup gas (mg l1), and is taken as the off-gas ozone concentration CAe in the calculation for simplicity, VH is the volume of holdup gas (L), VL is the working volume of the reactor (L). The dissolved ozone concentration (CAb) was found to be lower than 0.08 mg l1 after the completion of decolorization, so it was not included in this calculation. When the bubble contactor was used, the MR/MA value decreased slowly, then sharply. The decreasing of the MR/ MA value might have been caused by the lower reactivity of the dye solution with ozone. It was thus proposed that the initial decolorization reaction occurred readily, and the rate of both the reaction and ozone utilization was high. This also was confirmed by the significant enhancement
Fluorescence intensity
2000
TA+ micro-ozone-bubble water
1600
TA+normal-ozone-bubble water
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0.20
-1
of the mass transfer, which was primarily due to the fast ozonation reaction between the dissolved ozone and RB 5 solution (Qiu et al., 2001). At the later stage, the ozonation products were hard to oxidize, resulting in a much lower reaction rate. The E value reached unity, indicating the chemical reaction had little influence on the mass transfer rate. Chen et al. (2005) investigated the ozonation of RB 5 by using a rotating packed bed (RPB) and completely stirred tank reactor (CSTR) as ozone contactors. The experimental results indicated that the MR/MA value decreased with ozone doses from 0.52 to 0.14 in the RPB and 0.80 to 0.18 in the CSTR. However, by using the microbubble generator, the MR/ MA value and enhancement factor decreased a little for the applied ozone doses as ozonation proceeded, indicating high reactivity during the whole ozonation process. Considering the high TOC removal efficiency in the microbubble system described above, we propose that the microbubble technology can enhance the ozone oxidation as well as the mass transfer. In general, ozone oxidation pathways include direct ozone attack or free radical attacks. Since the oxidation potential of hydroxyl radicals is much higher than that of ozone molecules, direct oxidation is slower than radical oxidation. Microbubbles with higher internal pressure could induce the decomposition of ozone and contribute to the production of hydroxyl radicals. According to the Young–Laplace equation DP ¼ 2cr (DP is the pressure difference, c is the surface tension, r is the radius of bubble), the smaller the bubble size, the higher the internal pressure at this conditions. This suggestion was proven by hydroxyl radical detection (see Fig. 6). The fluorescence intensity of the samples in the two systems increased with the continuous ozone injection and then reached steady state (see Supplementary Data). The fluorescence intensity of the samples from the microbubble system was much higher than that from the bubble contactor. Ozone molecules could not react with TA. There was almost no fluorescence detected in
Removed TOC/consumed ozone (gg )
L.-B. Chu et al. / Chemosphere 68 (2007) 1854–1860
0.15
0.10
Microbubble generator Bubble contactor
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0.00 1
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8
Applied ozone dose (g) Fig. 7. Ratio of removed TOC value (g) to consumed ozone dose (g) in the two systems at an initial concentration of 230 mg l1.
the presence of the hydroxyl radical scavenger of 90 mM T-butanol in the ozone water sample. These results implied that more hydroxyl radicals were produced in the microbubble system, which contributed to the degradation of the dye molecules. In order to eliminate the effects of the mass transfer, the ratio of the removed TOC (g) to the consumed ozone dose (g) was plotted as a function of the implied ozone dose (see Fig. 7). This ratio is the amount of removed TOC per g of consumed ozone. It was calculated when the TOC of the solution began to decrease steadily, which was from 25 min to 90 min for the microbubble system and from 50 min to 130 min for the bubble contactor. It was obvious that for the same ozone consumption, the TOC removal in the former was 1.3 times higher than that in the latter indicating its higher reactivity. As shown above, the microbubble technology will be a promising process for the enhanced ozonation of dyestuff wastewater. One of the important factors pertaining to the industrial application is the energy cost. Compared to the conventional ozonation process, a liquid pump will be required for the microbubble system, but the consumed ozone doses were greatly decreased because of its higher reaction activity. The total energy cost will be further analyzed by optimizing the scale and operation of the system. 4. Conclusions
1200 800 ozone water+T-butanol+TA
400 TA
ozone water
0 350
400
450
500
550
600
650
A microbubble technology was applied to increase the mass transfer rate of ozone as well as to enhance the ozone oxidation of synthetic RB 5 wastewater in this paper. Experiments were performed using both the microbubbles generator and the bubble contactor, which is commonly used in ozonation systems, for the purpose of comparison. Main conclusions obtained are as follows:
Wavelength (nm) Fig. 6. Fluorescence detection of hydroxyl radicals formed in ozone water under different conditions.
(1) The microbubble generator produced a milky and high intensity microbubble solution in which the bub-
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bles had a mean diameter of less than 58 lm, a numerical density of more than 2.9 · 104 counts ml1, and a specific surface area of more than 334 m1 at a gas flow rate of less than 0.5 l min1. (2) Compared with the bubble contactor, the total mass transfer coefficient was 1.8 times higher and the pseudo-first order rate constant was 3.2–3.6 times faster at the same initial dye concentration in the proposed microbubble system at a gas flow rate of 0.5 l min1. (3) The amount of TOC removed per g of ozone consumed was about 1.3 times higher in the microbubble system than in the bubble contactor. The results of the test using TA as the chemical probe implied that more hydroxyl radicals were produced in the microbubble system, which contributed to the degradation of the dye molecules. (4) According to the results of mass transfer, decolorization, mineralization and ozone accumulation for treating synthetic dyestuff wastewater, the microbubble system is a promising process for enhancing both mass transfer and oxidation of ozone. The total energy cost should be analyzed further.
Acknowledgement This research was supported in part by the China Postdoctoral Science Foundation (2005038346). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.chemosphere.2007.03.014. References Bader, H., Hoigne, J., 1982. Determination of ozone in water using the indigo dye method. Ozone Sci. Eng. 4, 169–176. Burns, S.E., Yiacoumi, S., Tsouris, C., 1997. Microbubble generation for environmental and industrial separations. Sep. Purif. Technol. 11, 221– 232. Chen, Y.H., Chang, C.Y., Su, W.L., Chiu, C.Y., Yu, Y.H., Chiang, P.C., Chang, C.F., Shie, J.L., Chiou, C.S., Chiang, S.I., 2005. Ozonation of CI Reactive Black 5 using rotating packed bed and stirred tank reactor. J. Chem. Technol. Biotechnol. 80, 68–75. Danckwerts, P.V., 1970. Gas–Liquid Reactions. McGraw-Hill, New York. Fang, X., Mark, G., Sonntag, C., 1996. OH radical formation by ultrasound in aqueous solutions Part 1: The chemistry underlying the terephthalate dosimeter. Ultrason. Sonochem. 3, 57–63.
Hao, O.J., Kim, H., Chiang, P.C., 2000. Decolorization of wastewater. Crit. Rev. Env. Sci. Tec. 30, 449–505. Ince, N.H., Tezcanli, G., 2001. Reactive dyestuff degradation by combined sonolysis and ozonation. Dyes Pigments 49, 145–153. Kasprzyk-Hordern, B., Ziolek, M., Nawrocki, J., 2003. Catalytic ozonation and methods of enhancing molecular ozone reactions in water treatment. Appl. Catal. B-Environ. 46, 639–669. Kodama, Y., Kakugawa, A., Takahashi, T., Kawashima, H., 2000. Experimental study on microbubbles and their applicability to ships for skin friction reduction. Int. J. Heat Fluid Fl. 21, 582–588. Mayank, S., Locke, B.R., 2006. Quantification of hydroxyl radicals produced in aqueous phase pulsed electrical discharge reactors. Ind. Eng. Chem. Res. 45, 5819–5825. Mitani, M.M., Keller, A.A., Sandall, O.C., Rinker, R.G., 2005. Mass transfer of ozone using a microporous diffuser reactor system. Ozone Sci. Eng. 27, 45–51. Pires, M., Carvalho, L.R.F., 1998. An artifact in air carbonyls sampling using C-18 DNPH-coated cartridge. Anal. Chim. Acta 367, 223–231. Qiu, Y.Q., Kuo, C.H., Zappi, M.E., 2001. Performance and simulation of ozone absorption and reactions in a stirred-tank reactor. Environ. Sci. Technol. 35, 209–215. Rosal, R., Rodriguez, A., Zerhouni, M., 2006. Enhancement of gas–liquid mass transfer during the unsteady-state catalytic decomposition of ozone in water. Appl. Catal. A-General 305, 169–175. Ruppert, G., Bauer, R., Heisler, G., 1994. UV-O3, UV-H2O2, UV-TiO2 and the photo-fenton reaction – comparison of advanced oxidation processes for waste-water treatment. Chemosphere 28, 1447–1454. Serizawa, A., Inui, T., Yahiro, T., Kawara, Z., 2003. Laminarition of micro-bubble containing milky bubbly flow in a pipe. In: Third European–Japanese Two-Phase Flow Group Meeting, Certosa di Pontignano, pp. 21–27. Shin, W.T., Mirmiran, A., Yiacoumi, S., Tsouris, C., 1999. Ozonation using microbubbles formed by electric fields. Sep. Purif. Technol. 15, 271–282. Shu, H.Y., Huang, C.R., 1995. Degradation of commercial azo dyes in water using ozonation and UV enhanced ozonation process. Chemosphere 31, 3813–3825. Slavinskaya, G.V., Selemenev, V.F., 2003. Ozonation of fulvic acids of natural waters in aqueous solutions. Russian J. Appl. Chem. 76, 1472– 1475. Soetanto, K., Chan, M., 2000. Fundamental studies on contrast images from different-sized microbubbles: Analytical and experimental studies. Ultrasound Med. Biol. 26, 81–91. Vandevivere, P.C., Bianchi, R., Verstraete, W., 1998. Treatment and reuse of wastewater from the textile wet-processing industry: Review of emerging technologies. J. Chem. Technol. Biotechnol. 72, 289–302. Walker, A.B., Tsouris, C., DePaoli, D.W., Klasson, K.T., 2001. Ozonation of soluble organics in aqueous solutions using microbubbles. Ozone Sci. Eng. 23, 77–87. Wang, C.X., Yediler, A., Lienert, D., Wang, Z.J., Kettrup, A., 2003. Ozonation of an azo dye CI Remazol Black 5 and toxicological assessment of its oxidation products. Chemosphere 52, 1225–1232. Wu, J.N., Wang, T.W., 2001. Ozonation of aqueous azo dye in a semibatch reactor. Water Res. 35, 1093–1099. Zhou, H.D., Smith, D.W., 1997. Process parameter development for ozonation of kraft pulp mill effluents. Water Sci. Technol. 35, 251–259. Zhou, H.D., Smith, D.W., 2000. Ozone mass transfer in water and wastewater treatment: Experimental observations using a 2D laser particle dynamics analyzer. Water Res. 34, 909–921.