1820
Ind. Eng. Chem. Res. 2008, 47, 1820-1827
Kinetics and Modeling of IPA Oxidation Using Ozone-Based Advanced Oxidation Processes Jerry J. Wu,* Jing-Sheng Yang, and Manickavachagam Muruganandham Department of EnVironmental Engineering and Science, Feng Chia UniVersity, Taichung, Taiwan
In this study, an oxidation model incorporating the mass transfer and kinetics approach based upon the rate constants for the reactions with ozone and ‚OH radicals was developed to predict the degradation of isopropyl alcohol (IPA) and formation of its main byproduct, acetone. The self-decomposition of ozone in buffered water at pH 3 and 7 occurred via a first-order reaction. The ozone mass transfer coefficients are considerably higher at pH 7 than pH 3 due to the participation in the chemical reaction toward the hydroxyl ion. The oxidation model considering side reactions between ozone or ‚OH radicals and byproducts has been established using nonlinear simultaneous differential equations. The reaction coefficients for IPA degradation are 0.008, 0.0236, 1.4931, and 1.4909 s-1 at pH 3 and 7 in O3 and O3/UV processes, respectively. The larger reaction coefficient found in the ozone/UV system indicates several competing mechanisms for hydroxyl radicals. 1. Introduction
2. Experiment
Isopropyl alcohol (IPA) is an important solvent widely used as a cleaning and dehydrating agent in the semiconductor manufacturing process. A characteristic of IPA from semiconductor plants is its low biodegradability, and its degradation intermediate, acetone, causes toxicity by ingestion, inhalation, or absorption. Thus, to establish the kinetics of the reaction of ozone with IPA and to predict its reaction pathway is important. During the ozonation process, ozone gas is usually brought into continuous contact with water under dynamic conditions. Because the mass transfer of ozone is generally the ratedetermining step concerning the mechanism of the ozone reaction, factors including water qualities and operational parameters affecting mass transfer should be examined. The mass transfer rate is influenced by operating variables such as pH, temperature, agitation speed, and gas flow rate.1 The rate of ozone transfer and the subsequent rate of ozone decomposition depend upon the contact system efficiency and the reaction rates of ozone with constituents in the water. The ozone reaction rate depends on the water temperature and on the concentration and type of constituents contained in the water. Rapid reaction with oxidizable inorganic and organic material will maintain a low apparent equilibrium concentration of ozone within the liquid film and increase the rate of ozone transfer compared to water without oxidizable inorganic and organic material. The driving force for ozone transfer is maximized when the ozone absorbed is rapidly consumed by reaction with constituents within water. In fact, when ozone reacts very fast ozone decomposes at the gas surface and no molecular ozone is transferred into the water. The main objective of this study was to determine the mass (ozone) transfer coefficients on ozone decomposition and on ozone solubility. Also, the rate constants for the single reactions of IPA with both oxidants-ozone and OH radicals in ultrapure water were set up. Then, these kinetic rate constants were used to model and predict the kinetics of the oxidation of IPA during ozone and ozone/UV processes. In addition, the influence of raw water-quality parameters (pH, byproducts, UV light, etc.) on the oxidation process and IPA removal efficiency was established.
2.1. Chemicals. IPA and acetone were purchased from SHOWA Chemical Co. Ltd. For all of the experimental works, deionized Milli q-Plus water with a resistance of 18.2 MΩ was used. The pH of aqueous solutions was adjusted and buffered using 0.1 N H2SO4, K2HPO4, and boric acid where needed. In all gas chromatography analyses, compressed air, nitrogen, and hydrogen gas with a purity of more than 99.99% were used for all experiments. 2.2. Reactor Configuration. To better enhance the ability of ozone mass transported from the gaseous phase into aqueous media, new facilities were fabricated using a venturi injector (Mazzi Corp.) and a static in-line mixer. A semibatch reactor (Figure 1) made of stainless steel with a capacity of 5 L was utilized to investigate the mass transfer of ozone. To prevent any interference from impurities, deionized distilled water (18.2 MΩ) was used in all experiments. Ozone was generated from the corona discharge using an ozone generator (Model RXO-5, Ozonair), which produced an ozone concentration of ca. 5-6% (by weight) in the oxygen gas stream. The oxygen stream was produced by an oxygen generator (Model AS-12, Airsep). The flow rate was controlled by a mass flow rate instrument (Model 5850E, Brooks) and liquid flow rate in venturri injector. The use of a static in-line mixer would accelerate the complete mixing between gas bubbles and the liquid stream. In addition, a water jacket around the stainless steel reactor was designed to maintain the desired temperature (25 °C) with water circulated from a thermostatic water bath. The concentrations of gaseous ozone were monitored using an UV spectrophotometer (Genesys 5, Spectronic Inc.) bypassing ozone gas, from both the inlet and outlet, through one 2 mm and another 10 mm flow cells. The concentration of ozone was also determined spectrophotometrically, and absorbance of the ozone was monitored at the wavelength 254 nm. In this case, the liquid was measured through a dissolved ozone detector (MOCA 3600, Orbisphere). A medium-pressure mercury UV lamp (ESCO) with the intensity of 20 mW/cm2 was used for ozone/UV process. Before initiating each experiment, a three-way valve was used to bypass the inlet ozone into a potassium iodine solution (trap) until it was observed that the inlet concentration was constant. When a desired concentration of inlet ozone was obtained, the run was started by rotating this valve and admitting ozone in enriched
* To whom correspondence should be addressed: Tel.: + 886-424517250 ext. 5206. Fax: +886-4-24517686. E-mail:
[email protected].
10.1021/ie070954z CCC: $40.75 © 2008 American Chemical Society Published on Web 02/21/2008
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 1821
d[O3] m ) KLa (RCin g - [O3]) - kd[O3] dt
(1)
When the ozonation system combines with UV emission, the dissolved ozone would be accelerated to decompose into more complex radical routes and produce more ‚OH radicals. The ozone mass-transfer plus the UV absorption model could be modified as eq 2.
d[O3] m ) EKLa (RCin g - [O3]) - kr[O3]ΦUV - kd[O3] dt
Figure 1. Schematic diagram of the reactor used for the advanced oxidation process. Legend: 1, oxygen generator; 2, ozone generator; 3, mass-flow controller; 4, UV spectrophotometer; 5, trap solution (KI); 6, check valve; 7, pressure gauge; 8, flow cell for ozone inlet; 9, flow cell for ozone outlet; 10, Venturi injector; 11, static in-line mixer; 12, UV lamp; 13, circulation pump; 14, dissolved ozone monitor; 15, circulation pump; 16, dissolved ozone sensor; 17, sampling port; 18, semi-batch reactor; 19, thermostatic water bath.
oxygen into the reactor. During the run, the ozone absorbance in the inlet and outlet gas streams and in the liquid stream was monitored continuously at 30 s intervals. Then, the ozone masstransfer parameters were determined from the ozone process alone or combined O3/UV in this system. IPA and its byproducts’ concentrations were measured by withdrawing samples at regular intervals at different pHs. 2.3. Experimental and Analytical Procedures. For a standard reaction run, 5 L of aqueous solution was used. It was prepared by adjusting the initial IPA concentration equal to 500 mg/L. The experiment was performed by ozone and ozone/UV processes, and samples were withdrawn for quantitative analysis during the reaction course in these processes. The reaction was carried out over acidic conditions at pH 3 and a neutral condition at pH 7 to assess their effects on the oxidation rate for IPA degradation. The concentration of solvents, including isopropyl alcohol and acetone, were determined using a gas chromatograph, Shimadzu (GC-17A), equipped with an auto-injector, AOC-20i. The separation column was an EQUITY-1 capillary column made by Supelco Inc. with 30 m in length and 0.32 mm i.d. filled with 1.0 µm film thickness. Samples of 2 µL were injected into the GC for detection, and the flow rate of carrier gas was 1 mL/min. The oven temperature was held at 50 °C for 5 min, and the temperature was then ramped at 100 °C /min to 250 °C. The total run time for each sample was 7 min. 2.4. Ozone Mass-Transfer Model. Before ozone can react with any substance in the liquid phase, it must pass across an interface between gaseous ozone and liquid. As ozone was a sparingly soluble gas, its transfer efficiency was controlled by some physical parameters such as temperature, gaseous flow rate, mixing turbulence, and reactor geometry, and such chemical parameters as pH, ionic strength, and composition of the aqueous phase. Two variables, such as the mass-transfer coefficient and partition coefficients, play an important role on ozone mass transport within aqueous media.2 Thus, they would be the focus of this study to be determined. A mass transfer model could be developed and used to predict the ozone concentration profile during the period of mass-transfer. The ozone mass-transfer model could be expressed as eq 1.
(2)
2.5. OH Radical Kinetic Study. The measurement of the transient concentration of both of the ‚OH radicals and dissolved ozone during the ozonation process has been proposed by Elovitz and von Gunten.3 The approach was based on the measurement of the decrease of an ozone-resistant reference compound that reacted rapidly with hydroxyl radicals. In the present study, the parent compound selected was IPA and its degradation major byproduct, acetone, that fulfills those requirements. The rate constants with ‚OH radicals are 3.1 × 109 L/(mol‚s)4 and 8.3 × 107 L/(mol‚s).5 To evaluate the importance of ozone and ‚OH reactions, a new parameter, the Rct value, has been defined as the ratio of the exposures of ‚OH and O3 (i.e., concentration of oxidant integrated over the reaction time) as shown in eq 3.
Rct )
OH radical exposure ) O3 exposure
∫[OH] dt ∫[O3] dt
(3)
The Rct value could be calculated from the experimentally measured IPA, acetone, and O3 concentrations using the following expressions 4 and 5.
( ) ( ) ln
ln
[IPA]t
[IPA]0
[Acetone]t
[Acetone]0
) -kOH,IPA Rct
∫0t [O3] dt
) -kOH,acetone Rct
∫0t [O3] dt
(4)
(5)
2.6. Integrated Oxidation Model Development. A model combining mass transfer and chemical oxidation kinetics has been developed to successfully predict the accumulation of dissolved ozone concentration and the decrease of mixed malodorous organics.2 In the present research, the parent compound was switched into a single species (IPA) but considering its oxidation byproduct (acetone). Therefore, the integrated model incorporating with mass-transfer and oxidation kinetics could be rewritten. Considering the degradation of IPA and the formation of acetone during lab-scale ozonation, the kinetic parameters regarding with the transport of ozone in pure water determined using an ozone mass transfer model developed in previous section and reaction mechanisms for IPA degradation would be fitted with our experimental observance. Incorporated into this model were the concepts of the enhancement factor, defined as the ratio between the actual and maximum physical absorption rates and chemical kinetics. The following fundamental assumptions were made: 1. The liquid phase is completely mixed. 2. The flow pattern of gaseous ozone through the system is completely mixed. 3. Constant temperature and pressure are maintained during the experiment.
1822
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008
4. Ozone is consumed mainly by IPA and the major byproduct, acetone. 5. The enhancement factor is constant during the ozonation process. Model I describes mathematically the mass transfer and the ozonation reactions by the following eqs 6-8.
where kf is the pseudo first-order reaction rate constant for the reaction of ozone with the byproducts or other compounds (1/ s). The overall reaction coefficient was estimated by fitting our experimental data to find a minimum residual R, which is defined as below in eq 14,
d[O3] ) EKLa (RCin g - [O3]) - ηIPAkIPA[O3][IPA] dt ηacetonekacetone [O3][acetone] - kd [O3]m (6) -
d[IPA] ) ks[IPA] + (kOH-IPA)[O3]Rct[IPA] + dt kIPA[O3][IPA] (7)
d[acetone] ) (kOH-IPA)[O3]Rct[IPA] + kIPA[O3][IPA] dt kp[acetone] - kacetone[O3][acetone] (kOH-IPA)[O3]Rct[acetone] (8) The enhancement factor is defined as the ratio between the actual and maximum physical absorption rates shown in eq 9 and 10,
E)
NO3 KLa[O/3]
N O3 )
mi - mo V
(9)
(10)
R)
x
N
(Xj - Xexperiment)2 ∑ j)1 N
(14)
where Xj is the predicted concentration of the parent compounds in reactor, Xexperiment is the observed concentration during the experiment, and N is the number of total observations. If considering with the ozone/UV process into the expression of integrated model mentioned above, the occurrence of UV light would produce more hydroxyl radicals in the aqueous system. Therefore, the decomposition of dissolved ozone could be moderately modified as the following expression and the reaction terms in the degradation of IPA or acetone would remain the same.
d[O3] ) EKLa(RCin g - [O3]) - ηIPAkIPA[O3][IPA] dt ηacetonekacetone[O3][acetone] - kf[O3] kd[O3]m - kr[O3]ΦUV (15) -
d[IPA] ) ks[IPA]i + (kOH-IPA)[O3]Rct[IPA] + dt kIPA[O3][IPA] (16)
where NO3 (mol/L‚s) was determined from the difference between the molar flux of ozone at the reactor inlet and outlet, mi and mo (mol/s) respectively, KLa is the liquid-phase volumetric mass transfer coefficient (1/s), [O/3] is the equilibrium-dissolved ozone concentration at the water-gas interface (mol/ L), and V is the liquid volume (L) in the reactor. The enhancement factor was determined in the solution containing all of the target compounds. Another important feature to be considered is the occurrence of competition reactions involving ozone and the intermediates formed during ozonation. Ozonation of IPA compounds yields acetone as its primary byproducts. These compounds are still as reactive with ozone as the parent component. These circumstances would make kinetic studies more difficult to carry out. To overcome the difficulty in using the mathematical model, a first-order reaction rate constant, (kf), was used to describe the consumption of ozone by other intermediates. This reaction rate constant (kf) was incorporated into model II, which can be expressed mathematically as the following eqs 11-13,
d[acetone] ) (kOH-IPA)[O3]Rct[IPA] + kIPA[O3][IPA] dt kp[acetone] - kacetone[O3][acetone] (kOH-IPA)[O3]Rct[acetone] (17)
d[O3] ) EKLa (RCin g - [O3]) - ηIPAkIPA [O3][IPA] dt ηacetonekacetone[O3][acetone] - kf[O3] - kd[O3]m (11)
where kd is the first-order rate constant for this ozone decomposition. Parts a and b of Figure 2 show the rate of ozone decomposition in buffered water at pH 3 and 7, respectively; both occurred via a first-order reaction. The self-decomposition of dissolved ozone was calculated using eq 18 and found to be 0.0009 s-1 and 0.0014 s-1 at pH 3 and 7, respectively. The self-decomposition rate constant values at acidic pH are lower than those at neutral pH because too little hydroxyl ion reacts with ozone. 3.2. Mass Transfer in Ozonation and Ozone/UV Processes. In this study, the ozone volumetric mass transfer coefficient KLa (s-1) in liquid film was calculated according to the following eq 19,
d[IPA] ) ks[IPA]i + (kOH-IPA)[O3]Rct[IPA] + dt kIPA[O3][IPA] (12) d[acetone] ) (kOH-IPA)[O3]Rct[IPA] + kIPA[O3][IPA] dt kp[acetone] - kacetone[O3][acetone] (kOH-IPA)[O3]Rct[acetone] (13)
3. Results and Discussion 3.1. Ozone Decomposition in Water. The order and rate of the decomposition of dissolved ozone were determined by observing the reduction in the ozone concentration with buffered water at pH 3 and 7. Investigations of its decomposition in natural water from several European locations had shown that this reaction follows first-order kinetics.5 Accordingly, the integrated ozone decomposition rate could be represented by the following eq 18,
ln
[O3] [O3]0
) -kdt
(18)
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 1823
Figure 2. First-order decomposition of dissolved ozone in a buffered solution (a) at pH 3 and (b) at pH 7.
Figure 3. Determination of the KLa values performed with eq 30 (a) at pH 3 and (b) at pH 7.
Table 1. Comparison of Mass Transfer Coefficients with the Other Studies
Table 2. Fitting Parameters Obtained from the Mass-Transfer Equations
Conditions
Buffered at pH 3
references
KLa (s-1)
VL (L)
ozone inlet gas flowrate Q (mL/min)
Q/VL
this study Gao et al. (2004)12 Wu et al. (2002)2
0.0064 0.008 0.0045
5 10 1.5
400 mL/min 1000 mL/min 300 mL/min
80 100 200
Buffered at pH 7
R
KLa (s-1)
R
KLa (s-1)
0.49
0.0047
0.39
0.0064
where the [O3] is the dissolved ozone concentration, Cin g is the inlet gaseous ozone concentration, R is the ozone partition coefficient (CL/Cg), and kd is the ozone self-decomposition constant. The volumetric mass transfer coefficient KLa is the most important parameter in gas-liquid contactors. The equation can be used to describe this relationship (eqs 20 and 21),
better mass-transfer efficiency in comparison with the other studies using traditional porous aeration. The relations between pH values, KLa of ozone, and saturated ozone concentration were examined. The mass-transfer coefficient, KLa, could be calculated from Figure 3 at pH 3 and 7 via the slope determination by plotting the dissolved ozone accumulation rate and the dissolved ozone concentration as in eq 19. As shown in Table 2, the KLa values seem to increase with the increase of pH. These results might be explained by the following chemical reactions (eqs 22 and 23) in the aqueous phase.
KLa ) KL*a
O3 + OH- f HO2 + O2-
(22)
O3 + HO2 f 2O2 + ‚OH
(23)
d[O3] m ) KLa (RCin g - [O3]) - kd[O3] dt
KL )
D O3 A ,a) δ V
(19)
(20) (21)
where DO3 is the dispersion coefficient of dissolved ozone that related with temperature and pressure, δ is the liquid film thickness that related with chemical reaction in solution, A is the surface area (ozone bubble)/water volume. The average value of KLa calculated from this study, along with comparisons with previous researchers, is presented in Table 1. The conditions for this study were at neutral pH, 5 L of liquid volume, and corresponding to ozone inlet gas flowrate of 400 mL/min. The value of KLa ) 0.0064 s-1 is compared favorably with the other results in Table 1, which shows a large degree of variation due to the variety of parameters such as the volume of liquid and the ozone inlet gas flowrate used in the studies. It is found that the mass-transfer coefficient of ozone in this study is relatively large if considering the ratio of the operated gas flowrate and the liquid volume (V/QL), which indicates that the system using the venturi injector and the static in-line mixer would have a
From the above reactions, it is demonstrated that O3 reacts with OH-. Consequently, the enhancement factor would be involved in the mass-transfer coefficient. The partition coefficient for ozone was reported to be 0.49 and 0.39 at pH 3 and 7, respectively. The R value obtained in this study is relatively large compared with the report by Laity et al.4 It is probably due to the use of the venturi injector, in which many fine gas bubbles are produced under conditions of high speed and low pressure. Thus, the equilibrium-dissolved ozone concentration would be increased significantly. When the pH value was increased from 3 to 7, the R value would also be decreased. This can be explained by the Henry’s constant (in a helium atmosphere) that is calculated by means of the correlation proposed by Sullivan.6 This correlation takes into account the pH effect on ozone solubility by means of hydroxide ion concentration [OH-] expressed in moles. If the solution has more hydroxide ions, the equilibrium concentration of dissolved
1824
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008
Table 3. O3/UV Mass-Transfer for E and krΦUV Buffered at pH 3 krΦUV
E 1.40
Buffered at pH 7 (s-1)
0.006
E
krΦUV (s-1)
1.38
0.0091
ozone will become small. Therefore, the R value at pH 3 is greater than that at pH 7 due to a lower concentration of hydroxide ions. In the O3/UV system, ozone was transferred from the gas into the liquid phase and the rate of this mass transfer was generally enhanced by the UV light. The outlet ozone concentration with or without UV reached a steady value after about 20 min. Furthermore, this value decreased with UV irradiation as expected, and the mass transfer enhancement factor due to UV light could be calculated from the following expression5 by eq 24,
E)
{(O3)inf - (O3)eff}UV {(O3)inf - (O3)eff}non-UV
(24)
where E is the mass transfer enhancement due to UV light, (O3)inf is the influent ozone gas concentration, and (O3)eff is the effluent ozone gas concentration at steady state. In Table 3, the enhancement factor in ozone mass transfer due to UV light for different pHs of 3 and 7 are almost identical. Besides, the kr value is a rate constant for ozone decomposed by UV light. Probably the most important concept in photochemical kinetics is the quantum yield, ΦUV. It is defined as the efficiency of the photochemical reaction, that is, the number of O3 molecules being decomposed per photon absorbed, as shown in eq 25.7
ΦUV )
number of molecules reacted ozone number of photons of light absorbed
(25)
3.3. Kinetic Study. The determination of the ‚OH radical concentrations was a complicated issue because there was no easy method for measuring their concentration in situ. Several indirect methods could be found in the literature that used computer simulations and predicted the ‚OH radical concentrations from the ozone decomposition.8,9 However, because of the complexity of the organic and inorganic matrix of natural waters such as those of the present research, even wellestablished models fail to adequately predict ‚OH radical concentrations. One proposal was the experimental approach proposed by Elovitz and von Gunten,3 who designed a way to measure the transient concentrations of both the ‚OH radical and O3 during an ozonation process. The method was based upon the measurements of the decrease of an ozone-resistant reference compound that reacts rapidly with ‚OH radicals. Thus, in our study, the compound selected was IPA, which fulfills those requirements: its rate constant with OH radicals is 3.1 × 109 L/(mol‚s). To evaluate the importance of ozone and ‚OH reactions, a new parameter, the Rct value, had been defined as the ratio of the exposures of ‚OH and O3 as shown in eq 3 (i.e., concentration of oxidant integrated over the reaction time).3 The Rct value could be calculated from the experimentally measured IPA and O3 concentrations using eq 4 mentioned in section 2.5. Thus, a plot of the first term versus the exposure of ozone should be a straight line, from whose slope the Rct value could be deduced. If Rct remains constant during the ozonation process, it represents the ratio of the ‚OH radical concentration to the ozone concentration. Under these circumstances, the concentration of ‚OH radicals could be easily determined from the experimentally measured ozone concentration or from the ozone concentration calculated using the kinetics equation for ozone
Figure 4. Determination of the Rct values in experiments performed with eq 4 (a) at pH 3 and (b) at pH 7. Table 4. Ozone Kinetic Reaction Buffered at pH 3 Rct
∫[O3] dt
5.84 × 10-8
0.07
Buffered at pH 7
OH radical
Rct
4.08 × 10-9 M 7.49 × 10-8
∫[O3] dt
OH radical
0.049
3.67 × 10-9 M
Table 5. Stoichiometric Factors and Rate Constants for the Direct Reaction of Ozone with IPA and Acetone
IPA acetone a
stoichiometric factor
kO3 (M-1 s-1)
1.05 1.25
1.9a 0.032a
Value was determined by (Hoigne and Bader).10
decomposition (eq 26). Figure 4 shows the plot for the experiments carried out with IPA degradation. The data lie satisfactorily close to straight lines, which confirmed the goodness of the proposed model. A regression analysis gave the slopes that were divided by kOH,IPA 3.1 × 109 L/(mol‚s)4 to obtain the Rct values in Table 4. In this experiment, the phosphate buffer system was used at pH 7.2 This demonstrated that, at this pH, hydrogen phosphate (HPO42-) and dihydrogen phosphate (H2PO4-) are the predominant ‚OH radical scavengers in oxidizing trichlorobenzene by advanced oxidation processes. Thus, the concentration of ‚OH radicals at pH 3 and 7 were almost identical because any free radicals generated from the decomposition of ozone are likely to be scavenged by the phosphate species present in the buffer system that is being used. Moreover, kOH,acetone was 8.3 × 107 L/(mol‚s)5 and the Rct values of acetone were similar to 2.66*Rct-IPA. 3.4. Model Establishment. The model incorporating with mass transfer and chemical oxidation kinetics was developed to predict the accumulation of the dissolved ozone concentration and the decrease in the concentrations of IPA and the formation of the main byproduct, acetone, during a lab-scale ozonation system. The mass-transfer model and partition coefficients for ozone in water were employed. The parameters regarding to the stoichiometric factors and rate constants for the reaction of ozone with IPA and acetone are summarized in Table 5. To measure the stoichiometric factors for IPA and acetone reacting with ozone directly, the experiment was performed individually
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 1825
Figure 5. Comparison of IPA degradation and the formation of its main byproduct, acetone, between experimental data and model prediction in O3 process, (a) at pH 3 and (b) at pH 7.
by placing aqueous ozone and the target organic (IPA or acetone) at pH 2 in the closed glass vessel (300 mL) under the control of the initial molar ratio ([IPA]0/[O3]0 or [acetone]0/ [O3]0) equal to 10, thus allowing ozone to be consumed predominately by its reaction with the target compound. It is found that stoichiometric factors are 1.05 and 1.25 for IPA and acetone, respectively. The stoichiometric factors were determined to range from 1 (for olefinic compounds) to 2.5 (for aromatic hydrocarbons).10 The stoichiometric factors determined in this study are satisfactorily consistent with the reported data.11 The stoichiometric factor approaching 1 means 1 mol of ozone can be proportionally consumed by 1 mol of target compound used in our experiment. To evaluate the effects of stripping and oxygenation on the removal of the IPA and
Figure 6. Comparison of IPA degradation and the formation of its main byproduct, acetone, between experimental data and model prediction in O3/ UVprocess, (a) at pH 3 and (b) at pH 7. Table 6. Reaction Coefficients (kf) Simulated for IPA Degradation under Different pHs (s-1)
O3 O3/UV (s-1)
pH 3
pH 7
0.0080 1.4931
0.0236 1.4909
acetone, oxygen was used to purge the solution. The maximum flowrate, 400 mL/min, was used. Only a slight decrease in IPA and acetone concentration was observed during the reaction period of 120 min, which indicates the stripping effect by introducing gas in the system can be ignored. The enhancement
1826
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008
factor (E) can be determined by eq 26, using the ratio of gaseous ozone consumption under the pH buffered solution containing desired initial concentration of IPA and pH buffered solution without IPA. It is found that the enhancement factors are 4.25 for pH 3 and 3.9 for pH 7, indicating that the occurrence of a chemical reaction would substantially accelerate the physical mass transfer rate of ozone into solutions around 4 fold for IPA solution.
E)
{(O3)inf - (O3)eff}with chemical reaction {(O3)inf - (O3)eff}without chemical reaction
(26)
3.5. Comparison between Experimental Result and Model Prediction. In previous experiments, although the mass-transfer coefficients, partition coefficients, stoichiometric factors, and the enhancement factor had been determined, the reaction factor (kf) was still unknown. Once these parameters are determined, the oxidation model can be used to simulate the degradation progress of the target compound in ozonation and UV processes. IPA and its main byproduct acetone were oxidized under the following conditions: pH 3 and 7, temperature of 25 °C, flowrate of 400 mL/min. In Figure 5, the experimental data and the predicted values determined using models I and II are shown. The values for the minimum residuals used in model II at pH 3 and 7 are 0.0012 and 0.0024 M for acetone, respectively. When the reactions between intermediates and ozone were ignored, the predicted concentration profile of acetone decreased much faster than the experimentally determined ones, such as that the values for the minimum residuals used in model I at pH 3 and 7 are both null. Therefore, for the oxidation of IPA and its main byproduct of acetone, it is very crucial to consider the effect of side reactions with other intermediates during the ozone oxidation process. Thus, the reaction coefficient (kf) should be used to take into account competing reactions. Additionally, while adding UV light to enhance ozone decomposition, models I and II should be modified by considering the parameters, including the krΦUV and E values in the presence of UV light. The comparison of experimental observations and model predictions can be seen in Figure 6. When O3 and O3/ UV processes at pH 3 and 7 are compared, it is found the Rct value in the O3/UV process was 40 times greater than the O3 process alone due to the production of more OH radicals. For this reason, the degradation of IPA in model I was faster in the O3/UV process than in the O3 process. Then, the reaction coefficients were also compared. The reaction coefficients (kf) compared with other byproducts are much larger in the O3/UV process than that found in the O3 process alone, indicating that more unselective radical reactions happened with the minor byproducts in the system. The fitted values for the reaction coefficient are summarized in Table 6. By comparing the similar oxidation model developed by Wu and Masten (2002),2 it is found that the reaction coefficients found in this study are much smaller, where kf are 400 (1/s) and 40 000 (1/s) for pH 2 and 7, respectively, which were used for reacting with mixed aromatic organics such as phenol, p-cresol, p-ethylphenol, indole, and skatole. However, the parent compound, IPA, used in this research has relatively lower reactivity toward ozone and hydroxyl radicals due to its chemical structure and functional groups. 4. Conclusions The combination of a mass transfer and kinetic study has led to the development of a model to successfully predict the degradation of IPA and its main byproduct of acetone formation
in ozone and ozone/UV processes. In this study, several important conclusions have been drawn. 1. The self-decomposition of ozone in buffered water at pH 3 and 7 follows a first-order kinetic model at 25 °C. It is apparent that the rate of ozone decomposition at pH 3 is significantly lower than that at pH 7. 2. The mass-transfer coefficient increases with increasing pH value. The effect of pH on the mass-transfer coefficient can be explained by the chemical reaction in the ozone system including self-decomposition and its reaction with the hydroxyl ion in solution. Therefore, the enhancement factor caused by the ozone reaction with the hydroxyl ion at pH 7 is still significant. 3. The factors, such as the partition coefficient and the UV quantum yield, are found to be related to the pH. If the pH increases, the concentration of ozone will be dramatically consumed by hydroxyl ions. Therefore, the values of the partition coefficient and quantum yield are less at pH 7 than at pH 3. 4. For better predicting the fate of target organics and its major intermediates, an integrated model, combining with mass transfer and oxidation kinetics in O3 and O3/UV processes, should consider the side reactions involving ozone or hydroxyl radicals. Using model I, which ignores the effect of side reactions, the predicted rates of the oxidation of IPA and its main byproducts of acetone are much greater than those observed experimentally. The reaction coefficient for IPA was found to be 0.008, 0.0236, 1.4931, and 1.4909 at pH 3 and 7, in O3 and O3/UV processes, respectively. The larger reaction coefficient found in the ozone/ UV system indicates that more competing mechanisms for hydroxyl radicals happen in such complex reaction system. Acknowledgment The authors wish to express their appreciation to National Science Council in Taiwan for the financial support of this project under the contract number of 95-2221-E-035-060. Nomenclature [O3] ) dissolved ozone concentration (mol/L) E ) enhancement factor due to chemical decomposition by UV light (dimensionless) KLa ) mass-transfer coefficient of ozone in solution in the absence of ozone-reactive compounds (1/s) R ) partition coefficient of ozone (dimensionless) Cin g ) gaseous concentration of inlet ozone (mol/L) kr ) decomposition rate constant by UV (photon/mol‚s) kd ) self-decomposition rate constant of dissolved ozone (mol/ L)1 - m‚s-1 ηi ) stoichiometric factor for IPA or acetone (dimensionless) ki ) reaction rate constant for IPA or acetone with dissolved ozone (L/s‚mol) kf ) reaction coefficient from the byproducts or other compounds, which also consume available ozone (l/s) ks ) coefficient for the removal of IPA by stripping (l/s) kp ) coefficient for the removal of acetone by stripping (l/s) m ) self-decomposition order of dissolved ozone (dimensionless) Φuv ) quantum yield (mol/photon) [IPA] ) concentration of IPA compound (mol/L) [acetone] ) concentration of acetone compound (mol/L) NO3 ) actual ozone absorption rate defined by equation (mol/ L‚s) [O3] ) dissolved ozone concentration, (mol/L) [O3*] ) equilibrium-dissolved ozone concentration at the water-gas interface (mol/L) V ) liquid volume of the reactor (L)
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 1827
Xj ) predicted concentration for the parent compounds in the reactor (mol/L) Xexperiment ) observed concentration during the experiment (mol/L) Literature Cited (1) Sotelo, J. L.; Beltran, F. J.; Benitez, F. J.; Beltran-Heredia, J. Henry’s Law Constants for the Ozone-Water System. Wat. Res. 1989, 23, 1239. (2) Wu, J. J.; Masten, S. J. Oxidation Kinetics of Phenolic and Indolic Compounds by Ozone: Applications to Synthetic and Real Swine Manure Slurry. Wat. Res. 2002, 36, 1513. (3) Elovitz, M. S.; von Gunten, U. Hydroxyl Radical/Ozone Ratios during Ozonation Processes. I. The Rct Concept. Ozone: Sci. Eng. 1999, 21, 239. (4) Laity, J. L.; Burstein, L. G.; Appel, B. R. SolVents Theory and Practice; American Chemical Society: Washington, DC, 1973; Vol. 124, p 95. (5) Garoma, T.; Gurol, D. M. Degradation of tert-Butyl Alcohol in Dilute Aqueous Solution by an O3/UV Process. EnViron. Sci. Technol. 2004, 38, 5246. (6) Sullivan, D. E. Ph.D. Dissertation, Vanderbilt University, Nashville, TN, 1979.
(7) Horspool, W. M.; Armesto, D. Organic Photochemistry: A ComprehensiVe Treatment; Ellis Horwood: New York, 1992. (8) Tomiyasu, H.; Fukutomi, H.; Gordon, G. Kinetics and Mechanism of Ozone Decomposition in Basic Aqueous Solution. Inorg. Chem. 1985, 24, 2962. (9) Westerhoff, P.; Song, R.; Amy, G.; Minear, R. Applications of Ozone Decomposition Models. Ozone: Sci. Eng. 1997, 19, 55. (10) Hoigne, J.; Bader, H. Rate Constants of Reactions of Ozone with Organic and Inorganic Compounds in Water-I. Wat. Res. 1983, 17, 173. (11) Beltran, F. J.; Garcia-Araya, J. F.; Acedo, B. Advanced Oxidation of Atrazine in Water -I. Ozonation. Wat. Res. 1994, 28, 2153. (12) Gao, M. T.; Hirata, M.; Takanashi, H.; Hano, T. Ozone Mass Transfer in a New Gas-Liquid Contactor-Karman Contactor. Sep. Purif. Technol. 2005, 42, 145.
ReceiVed for reView July 13, 2007 ReVised manuscript receiVed November 21, 2007 Accepted December 30, 2007 IE070954Z
