Kinetics of Ozonation of 2-Mercaptothiazoline in an Electroplating

A bibliometric analysis of industrial wastewater research: current trends and future prospects. Tianlong Zheng , Juan Wang , Qunhui Wang , Chunhong Ni...
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Ind. Eng. Chem. Res. 2006, 45, 4936-4943

Kinetics of Ozonation of 2-Mercaptothiazoline in an Electroplating Solution Combined with UV Radiation Y. H. Chen,*,† C. Y. Chang,‡ C. C. Chen,‡ and C. Y. Chiu§ Department of Chemical and Material Engineering, National Kaohsiung UniVersity of Applied Sciences, Kaohsiung 807, Taiwan, Graduate Institute of EnVironmental Engineering, National Taiwan UniVersity, Taipei 106, Taiwan, and Department of Safety, Health, and EnVironmental Engineering, Lan-Yang Institute of Technology, I-Lan 261, Taiwan

The enhanced ozonation of 2-mercaptothiazoline (2-MT) with ultraviolet (UV) radiation in the acid-based electroplating solution of the printed wiring board industry was studied. The substrate prescription of the electroplating solution is with high concentrations of copper sulfate, sulfuric acid, and chloride ions. Note that the contribution of the present work includes investigation of the effect of UV radiation and construction of the refined model for the O3/UV treatment of 2-MT. The catalysis of UV radiation is found to be significant for the enhancement of the mineralization rate in the ozonation of 2-MT. Furthermore, the multistep reaction kinetics has been proposed to describe the concentration variations of 2-MT, total organic carbons, dissolved ozone, and off-gas ozone in the course of ozonation. This comprehensive kinetics for the ozonation of 2-MT combined with UV radiation is developed in association with the previous study of the sole ozonation of 2-MT in the electroplating solution. The enhancement factor of the gas-liquid mass transfer of ozone is simultaneously considered by the ozonation model. The obtained results can provide useful information for modeling the removal of additives such as 2-MT in the electroplating solution of the printed wiring board industry by the ozonation with 254-nm UV light radiation. Introduction The discarded aged electroplating solution is one of the major wastewater sources in the printed wiring board (PWB) industry. The substrates (the major chemical species) of the recipe solution are inorganics, such as sulfuric acid, copper sulfate, hydrochloric acid, etc., while the minor additives are organics, such as 2-mercaptothiazoline (2-MT), which is used as a brightening and stabilization agent.1 Consequently, the characteristics of a wasted electroplating solution are with high acidity (pH ) 0.180.42) and ionic strength. All of the above features make the solution difficult to treat by conventional treatment processes.1,2 The current major method used to treat the waste electroplating solution of PWB is chemical coagulation, which produces hazardous chemical sludge because of its high heavy-metal content such as copper. In Taiwan, the yield of the waste electroplating solution of PWB is approximately 106 000 CMD, resulting in about 21 000 metric tons of waste sludge per year with a moisture content of 78 wt %.3 Furthermore, in view of the resource recycling, the aged electroplating solution of PWB has great reclamation and recycling potentials with high copper concentration and electric conductivity. Note that the qualities of the organics in the electroplating solution become low and unreliable to the process after electroplating and electrophoresis. For this reason, removing the spent organic additives in order to add new additives is one of the key steps for the reutilization process. 2-MT has also been used as a biocorrosion inhibitor, an antifungal reagent, and a brightening and stabilization agent in many industrial processes.4 Ozonation is an effective way to remove organics and reduce the total organic carbons (TOCs). The compounds * To whom correspondence should be addressed. Tel.: +886-7-3814526 ext. 5115. Fax: +886-7-383-0674. E-mail: [email protected]. † National Kaohsiung University of Applied Sciences. ‡ National Taiwan University. § Lan-Yang Institute of Technology.

would be attacked via two different reaction mechanisms: (1) the direct ozonation by the ozone molecule and (2) the radical oxidation by the highly oxidative free radicals such as hydroxyl free radicals (OH•), which are formed by the decomposition of ozone in the solution. The radical oxidation is nonselective and vigorous. The studies on the ozonation of additives in the electroplating solution are scarce. Recently, Chen et al.5 investigated the decomposition of 2-MT in the electroplating solution by sole ozonation. Their results indicated that the products of 2-MT ozonation have a high resistance toward ozone molecules for further oxidation. Thus, enhanced ozonation with ultraviolet (UV) light radiation is employed to eliminate 2-MT in the electroplating solution in the present study. The purpose of introducing UV radiation in the ozonation processes is to yield more free radicals for the higher ozonation rate.6 Of further note is that the O3/H2O2 process may enhance the removal of 2-MT easier than the O3/UV one in practice. Nevertheless, the application of O3/UV without changing the substrate recipe of the electroplating solution is preferred for reutilization purposes. In addition, the proper ozonation model is essential for rational design and optimal operation. Note that available ozonation kinetics for the description of ozonation with pollutant was developed to consider only one or two reaction steps.7-15 However, the mechanism of pollutant decomposition via ozonation is commonly composed of three to six series of reaction steps.16-22 Accordingly, the available ozonation models are applicable for depicting the decomposition of the original compound and the initial period of ozonation. Recently, Beltra´n et al.23 and Chen et al.5 attempted to propose the multistep reaction kinetics (MSRK) with five reaction steps to describe the sole ozonation of simazine and 2-MT, respectively. The validity of MSRK was demonstrated by the agreement of the prediction with the ozonation results in their studies. Nevertheless, the multistep reaction kinetic model for enhanced ozonation combined with UV radiation is still desired.

10.1021/ie060065w CCC: $33.50 © 2006 American Chemical Society Published on Web 05/28/2006

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For ozone:

The objective of this study is to investigate the ozonation of 2-MT with UV radiation in the electroplating solution of the PWB industry. Ozonation experiments of 2-MT with various ozone dosages and UV intensities are performed. Ozonation combined with UV radiation is found to be an effective procedure for the removal of 2-MT in the electroplating solution. The related MSRK is proposed to simulate the concentration variations of 2-MT, TOC, dissolved ozone, and off-gas ozone simultaneously.

In eq 2, the left-hand-side term represents the variation of the bulk liquid ozone concentration, while the right-hand-side terms stand for ozone gas-liquid mass transfer, ozone self-decomposition, and ozonation reactions, respectively.

Theoretical Analysis

For pollutant:

The model with MSRK is proposed to describe the ozonation processes in the electroplating or aqueous solution. There are three major factors considered in the ozonation kinetic model: (1) the gas-liquid mass transfer, (2) the ozonation reactions, and (3) the enhancement effect of chemical reactions on the gas-liquid mass transfer of ozone. The mass transfer of ozone from the gas to liquid phase can be described by the two-film model. With ozone consumption by chemical reactions, the mass-transfer rate of ozone may be enhanced.24,25 The ratio of the mass-transfer rate of ozone with chemical reactions occurring in the liquid phase to that without is designated by the enhancement factor of ozone (ErA). The ErA value would be greater than the unit when the rate of chemical reactions is fast. Multistep Reaction Kinetic Model. As ozone (denoted as A) is dissolved in water, it may be consumed via selfdecomposition and oxidation with the pollutant (denoted as B) and intermediates (denoted as Ij). In general, the pseudo-firstorder and -second-order reaction rate expressions, which were successfully employed in other ozonation systems of different pollutants, are adopted for the spontaneous ozone decomposition and ozonation reactions, respectively.7,9,26 The second-order reaction rate expression is first-order with respect to ozone and pollutant, respectively. The enhancement of UV radiation for yielding hydroxyl radicals by ozone photolysis at 254 nm is presented in the items of the reaction rate constants (kIB, kIj, and kIP) and light intensity of the UV lamp ([I]). The assumptions of the MSRK model in the semicontinuous ozonation condition are as follows. 1. The homogeneous conditions with complete mixing of liquid and gas flows are valid in the reactor. 2. The series ozonation mechanism is applicable. 3. Second-order chemical reactions are bimolecular. 4. Henry’s law applies. 5. Reactions in the gas phase are neglected. The governing equation for gaseous ozone considers gas convection and gas-liquid mass transfer. The governing equation of holdup gas ozone (CAGi) can be expressed by eq 1.

dCAGi/dt ) QG(CAGi0 - CAGi)/VH - ErAk0LAa(CAGi/HA CALb)/G (1) In eq 1, the left-hand-side term represents the variation of the holdup gas ozone concentration, while the right-hand-side terms stand for gas convection and gas-liquid ozone mass transfer, respectively. For the liquid-phase governing equations of ozone (CALb), pollutant (CBLb), intermediates (CjLb), and product (CPLb), the chemical reaction terms should be considered based on the series ozonation mechanism as in eqs 2-6.5,23 In addition, the variation of TOCs in the solution is described by the lumped contribution of CBLb, CjLb, and CPLb as shown in eq 9. The number of the reaction steps (N + 1) can be adjusted for the different approaches of the reaction kinetics.

dCALb/dt ) ErAk0LAa(CAGi/HA - CALb)/L - kdCALb kBCALbCBLb -

∑kjCALbCjLb - kIP[I]CALbCPLb

dCBLb/dt ) -kBCALbCBLb - kIBm[I]CBLb

(2)

(3)

For the first intermediate (j ) 1): dC1Lb/dt ) kBCALbCBLb - k1CALbC1Lb + kIBm[I]CBLb

(4)

For the following intermediates (N g j g 2): dCjLb/dt ) kj-1CALbC(j-1)Lb - kjCALbCjLb

(5)

with kd ) kdm + kId[I], kB ) kBm + kIB[I], and kj ) kjm + kIj[I]. The reaction rate constants (kd, kB, and kj) in eqs 2-5 are composed of the direct (kdm, kBm, and kjm) and radical (kId, kIB, and kIj) ozonation reaction rate constants. The kIBm is the reaction rate constant of direct photolysis of pollutant, while the reaction rate of direct photolysis of pollutant by UV radiation is considered obviously slower than the O3 and O3/UV treatments. Therefore, it is appropriate to neglect the direct photolysis of intermediates and product in the ozonation model. The concentration of ozonation product (CPLb), which has low reactivity toward ozone and reacts with free radicals, is calculated by eq 6.

dCPLb/dt ) kNCALbCNLb - kIP[I]CALbCPLb

(6)

Note that the effect of the OH• concentration on the radical ozonation reactions is contained and expressed by the proposed reaction rate equations, including the reaction of OH• generation (denoted as kId[I]CALb) and the reactions of OH• with pollutant (denoted as kIB[I]CALbCPLb), intermediate j (denoted as kIj[I]CALbCjLb), and product (denoted as kIP[I]CALbCPLb). The governing equation of off-gas ozone (CAGe) in the free space is

dCAGe/dt ) QG(CAGi - CAGe)/VF

(7)

The initial conditions of eqs 1-7 are

t ) 0, CAGi ) CAGe ) CALb ) CjLb ) CPLb ) 0, CBLb ) CBLb0 (8) The variation of the TOC can be estimated by eq 9,

CTOC/CTOC0 ) (CBLb +

∑RjCjLb + RpCPLb)/CBLb0

(9)

where Rj and Rp are the ratios of TOC contribution per mole of intermediate j and product P to that per mole of pollutant B, respectively. It may be noted that the actual reactions involved may be more complex than the sequential multistep reactions with these simplifying assumptions proposed above. However, the present reaction kinetics model can provide the refined explanation of the experimental data for practical application. Film Model and Enhancement Factor. According to the film model, the value of ErA for ozone absorption can be

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calculated according to eqs 10-16. Note that the resistance of the ozone gas-liquid mass transfer is mainly contributed by the liquid phase. The concentration of ozone in the liquid film (CALF) at the gas-liquid interface (x ) 0) is in equilibrium with that in the gas phase. Furthermore, the volatilities of pollutant, intermediates, and product are usually neglected. Thus, the concentration gradients of pollutant (dCBLF/dx), intermediates (dCjLF/dx), and product (dCPLF/dx) in the liquid film at x ) 0 are taken as zero. At the boundary of the liquid film near the bulk liquid (x ) xM), the concentrations of ozone, pollutant, intermediates, and product in the liquid film are equal to those in the bulk liquid, respectively.

For ozone: DA

d2CALF dx2

) kdCALF + kBCALFCBLF +

∑kjCALFCjLF +

For pollutant: d2CBLF dx2

) kBCALFCBLF + kIBm[I]CBLF

(11)

For the first intermediate (j ) 1): D1

d2C1LF dx2

) -kBCALFCBLF + k1CALFC1LF

(12)

For the following intermediates (N g j g 2): Dj

d2CjLF dx2

) -kj-1CALFC(j-1)LF + kjCALFCjLF

(13)

) -kNCALFCNLF + kIP[I]CALFCPLF

(14)

For product: DP

d2CPLF dx2

The boundary conditions: x ) 0, CALF ) CAGi/HA, dCBLF/dx ) dCjLF/dx ) dCPLF/dx ) 0 (15) x ) xM ) DA/k0LA, CALF ) CALb, CBLF ) CBLb, CjLF ) CjLb, CPLF ) CPLb (16) The calculation of ErA: ErA ) -(dCALF/dx)|x)0/[(CAGi/HA - CALb)/(DA/k0LA)]

of the time step (∆t) adopted in the computer program are 71 points and 0.008 s, respectively. The confidential error range of the mass balance for checking the numerical scheme used is less than 10-6. The values of the parameters of the MSRK model are determined by the best fitting of the prediction compared with the experimental data with respect to the determination coefficient (R2). Experimental Section

kIP[I]CALFCPLF (10)

DB

Figure 1. Molecular structure of 2-MT.

(17)

Computation Algorithm for Solving the MSRK Model. Equations 1-17 represent the governing equations of the MSRK model for predicting the dynamic process of pollutant ozonation with UV radiation. The numerical method with the Turbo C program is employed for solving the equations in this study. Equations 10-16 are first solved using the iterative method to obtain the values of CALF, CBLF, CjLF, and CPLF in the film, yielding the value of ErA from eq 17 at time t. Equations 1-9 are then solved using the fourth-order Runge-Kutta method to compute the values of the variables in the next time step of t + ∆t from the available values at t. This is followed by the computation of ErA at t + ∆t. The computation is conducted up to the set duration. The grids along x ) 0 to xM and the size

Chemicals. The substrate recipe of the electroplating solution was [CuSO4‚5H2O] ) 200 g L-1, [H2SO4] ) 60 g L-1, and [Cl-] ) 0.03 g L-1. The initial concentration of 2-MT (CBLb0) in the solution was 100 mg L-1. 2-MT with the chemical formula of C3H5NS2 purchased from Aldrich (Milwankee, WI) has a molecular weight of 119.21, a melting point of 105-107 °C, and a CAS registry number of 96-53-7. The molecular structure of 2-MT is composed of an exocyclic mercapto group and a heterocyclic molecule containing sulfur, nitrogen, and carbon atoms, as shown in Figure 1. All experimental solutions were prepared with deionized water. The initial concentration of TOCs (CTOC0) was measured as about 29.1 mg L-1. Properties of the Electroplating Solution. The pH value of the electroplating solution was about 0.25. The variation of the pH during the experiments was found to be very slight because of the high acid concentration. The diffusion coefficient (DA), dimensionless Henry’s law constant (HA), mass-transfer coefficient (k0LA) of ozone, and specific area of the gas-liquid interface (a) in the electroplating solution were determined as 1.3 × 10-9 m2 s-1, 4.18, 8.80 × 10-5 m s-1, and 269 m-1, respectively.5 In addition, the reaction rate constants of ozone self-decomposition (kdm) and photolysis (kId) in the electroplating solution were measured as 0.0036 s-1 and 3.626 × 10-6 W-1 m3 s-1, respectively.11,12 The diffusion coefficients of 2-MT (DB), intermediates (Dj), and product (DP) were taken as 7.77 × 10-10 m2 s-1 for the modeling.5,27 Apparatus. The airtight reactor of 17.2-cm inside diameter was made of Pyrex glass with an effective volume of 5.5 L and equipped with a water jacket to maintain a constant solution temperature. The design of the reactor was based on the criteria of the shape factors of a standard six-blade turbine.28 The gas diffuser in cylindrical shape with a pore size of 10 µm was located at the bottom of the reactor. The low-pressure mercury lamps, which emitted principally at 254 nm, provided the UV radiation. The modeling light intensity [I] with units of W m-3 is defined as the average applied power of UV radiation per unit volume of the well-mixed system, which would be proportional to the number of photons absorbed by the solution per unit volume and unit time. The [I] value can be calculated from the product of [IUV](Aq/VL)Fs, where [IUV] is the light intensity measured on the outer surface of the quartz tubes housing the UV lamps with units of W m-2, Aq is the outer area of the quartz tubes submerged in the solution, and Fs is the fraction of the emitting UV light absorbed by the solution.29 Two quartz tubes of 3.8-cm outside diameter symmetrically installed inside the reactor were used to house the UV lamps with a Aq value of 0.0411 m2. The intensity of the UV radiation ([IUV]) was measured by a digital radiometer (Ultra-Violet

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Figure 2. Experimental apparatus sketch: s, ozone gas stream; - -, experimental solution; -‚‚-, isothermal water. Components: (1) oxygen cylinder, (2) drying tube, (3) ozone generator, (4) flowmeter, (5) three-way valves, (6) stirrer, (7) UV lamps, (8) reactor, (9) sample port, (10) liquid ozone sensor, (11) pH sensor, (12) circulation pump, (13) thermostat, (14) gas ozone detector, (15) KI solution, (16) vent to hood.

Products, Upland, CA) with a model UVP-25 radiation sensor. The value of Fs is detected close to the unit in all of the experiments by measuring the transmittance of the UV radiation through the solution. The fed (CAGi0) and discharged (CAGe) concentrations of gaseous ozone were measured by the Seki model SOZ-6004 UV photometric analyzer (Tokyo, Japan), which was calibrated by the KI titration method. The Orbisphere model 3600 liquid ozone monitor with a sensor of a membrane-containing cathode was used to measure the dissolved ozone concentration (CALb) in the solution. The concentration of 2-MT (CBLb) was analyzed using a high-performance liquid chromatography (HPLC) system, with a 250 × 4.6 mm column [model BDS C18 (5 µm); Thermo Hypersil-Keystone, Bellfonte, PA] and a UV/visible detector (model 1706; Bio-Rad, Hercules, CA) at 275 nm. The HPLC effluent with a flow rate of 1.0 mL min-1 had a composition of 73.5 mM [CH3(CH2)3]4N(HSO4)/CH3CN of 74: 26. The injection volume of the solution for the analysis was 20 µL. In addition, the concentration of TOCs (CTOC) was analyzed by the TOC analyzer (model 700; OI Corp., College Station, TX). Procedures. The experimental solution of 2-MT ozonation with 3.705 L (VL) was carried out at 25 °C for 240 min. The stirring speed was 800 rpm to ensure the complete mixing of the gas and liquid phases according to the previous studies.5,12 The volatilities of 2-MT and its intermediates during ozonation are tested to be negligible according to the previous studies.5 A circulation pump was used to transport the liquid from the reactor to the sensor and reflow it back with a flow rate of 0.18 L min-1 during ozonation. Samples were drawn out from the reactor at desired time intervals in the course of ozonation for the analysis of CBLb and CTOC, while the total sampling volume was within 5% of the solution. The residual dissolved ozone in the samples was removed immediately by stripping with nitrogen. An experimental apparatus sketch is shown in Figure 2. The generation of ozone from pure oxygen was controlled by the ozone generator (model SG-01A; Sumitomo, Tokyo, Japan) with a gas flow rate (QG) of 0.0324 L s-1. There were three CAGi0 values of 10, 20, and 40 mg L-1 and two [I] values

of 384 and 769 W m-3 employed to test the effects of the fed ozone dosage and light intensity on ozonation of 2-MT. Before the ozonation processes were started, the ozone-containing gas was bypassed to the photometric analyzer to ensure stability. A part of the gas stream at the preset flow rate was directed into the reactor when the ozonation system was ready to start. The variations of CALb and CAGe during ozonation were continuously monitored. Furthermore, the volatility of 2-MT in the electroplating solution was found to be negligible from the blank tests of oxygen aeration. Results and Discussion The dynamic variations of CBLb, CTOC, CALb, and CAGe in the course of 2-MT ozonation with UV radiation are simultaneously monitored for the investigation. The normalized concentrations in the dimensionless forms are θBLb ()CBLb/CBLb0), θALb [)CALb/(CAGi0/HA)], and θAGe ()CAGe/CAGi0). Furthermore, the MSRK model adopting schemes with various reaction steps is employed to simulate the concentration variations. Variations of θBLb, ηTOC, θALb, and θAGe. As shown in Figure 3, the effect of direct UV photolysis on the removal of 2-MT is found to be relatively weak with kIBm ) 1.34 × 10-7 W-1 m3 s-1. The times required for the complete decomposition of 2-MT with CAGi0 ) 40 mg L-1 are about one-third and half of those with CAGi0 ) 10 and 20 mg L-1, respectively. The first attack of ozone on 2-MT is mainly toward the mercapto group, which has an activating effect by denoting the electron density.30,31 However, the results reveal that the intensity of the UV radiation only slightly affects the destruction rate of 2-MT. It is due to the fact that the presence of the UV radiation would decrease the dissolved ozone concentration and accompany the generation of OH• at the same time. As a result, the overall decomposition rate of 2-MT is only slightly accelerated by the UV radiation. Figure 4 shows the variations of the removal efficiency of TOCs [ηTOC ) (CTOC0 - CTOC)/CTOC0] under different experimental conditions. The ozone dosage would also improve the mineralization rate. Apparently, the mineralization rate decreases with the ozonation time because the sequent intermediates have

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Figure 3. Time variations of θBLb for 2-MT ozonation in the electroplating solution of a semibatch system. θBLb ) CBLb/CBLb0. Symbols: experiments. Lines: prediction based on five-step reaction kinetics. O and -‚-: CAGi0 ) 10 mg L-1 with [I] ) 769 W m-3. 4 and - -: CAGi0 ) 20 mg L-1 with [I] ) 769 W m-3. 0 and -‚‚-: CAGi0 ) 40 mg L-1 with [I] ) 384 W m-3. 3: UV alone with [I] ) 769 W m-3. b, 2, and 9: CAGi0 ) 10, 20, and 40 mg L-1 without UV radiation.5

Figure 4. Time variations of ηTOC for 2-MT ozonation in the electroplating solution of a semibatch system. ηTOC ) (CTOC0 - CTOC)/CTOC0. Symbols: experiments. Lines: prediction based on five-step reaction kinetics. Notations are the same as those specified in Figure 3.

Figure 5. (CTOC0 - CTOC)/t vs ηTOC for 2-MT ozonation in the electroplating solution of a semibatch system. Symbols: experiments. Notations are the same as those specified in Figure 3.

lower reactivities toward oxidation. Note that introduction of UV radiation in 2-MT ozonation makes a significant contribution to mineralization. The case of O3/UV has a higher ηTOC value of about 2 times that of the case of sole ozonation with the same CAGi0. For a further illustration, the intrinsic effect of UV radiation on the elimination of TOC (Figure 5) shows the variations of the mean mineralization rate, (CTOC0 - CTOC)/t, with ηTOC. It is seen that the effect of UV radiation is not significant on the enhancement of the mean mineralization rate when ηTOC < 7%. However, in the later ozonation stage (higher

Figure 6. Time variations of θALb for 2-MT ozonation with UV radiation in the electroplating solution of a semibatch system. θALb ) CALb/(CAGi0/ HA). Symbols: experiments. Lines: prediction based on five-step reaction kinetics. Notations are the same as those specified in Figure 3.

ηTOC), the values of (CTOC0 - CTOC)/t via O3/UV are still high, while those via sole ozonation become comparatively low. This is because of the fact that the intermediates in the later stage of ozonation, such as organic acids, usually have low reactivity toward ozone molecules.31,32 According to the previous studies on the oxidation of protocatechuic acid and 4-chlorobenzaldehyde,33,34 the hydroxyl free radical has a better performance for the degradation of organic acids than ozone molecules. Thus, the oxidation reaction via hydroxyl free radicals becomes predominant to proceed in the regime with higher ηTOC. As shown in Figure 6, the variations of θALb can be divided into three stages. In the first stage of ηTOC < 9%, θALb remains nearly undetectable. In this regime, the rate of ozonation reaction is very fast so that the ozone transferred from the gas phase is consumed immediately in the solution. Then θALb starts to increase rapidly with the ozonation time in the transient regime of 9% e ηTOC < 20%. The accumulation of dissolved ozone is attributed to the fact that the consumption rate of dissolved ozone is smaller than its gas-liquid mass-transfer rate because of the lower reactivities of the refractory intermediates in the reacted solution. Finally, θALb gradually approaches to the constant value of 0.67-0.73, which would be dependent on and independent of [I] and CAGi0, respectively. Meanwhile, the consumption of ozone mostly comes from the self-decomposition and photolysis reactions. This ozone-rich regime of ηTOC g 20% representing the steady state of the dissolved ozone concentration has theoretical values of 1/[1 + kdVL/(QGHA) + kdL/(k0LAa)] of 0.68 and 0.74 for [I] ) 769 and 384 W m-3, respectively. Note that the mineralization rate of 2-MT in the sole ozonation system becomes very slow in the ozone-rich regime because OH• is difficult to generate in such acidic conditions. On the other hand, OH• can be induced by the participation of UV radiation to accelerate the higher mineralization rate. Figure 7 shows that θAGe consistently increases from the beginning to reach the steady state. The ratio of ozone transferred from the gas to liquid phases to that in the feed gas [)ErAk0LAa(θAGi - θALb)(VL + VH)/(HAQG)] would decrease with the ozonation time because of the smaller values of ErA (as illustrated in Figure 8) and θAGi - θALb in the later time. Evidently, θAGe increasing faster with higher CAGi0 values has a steady-state value of 0.84-0.89 close to theoretical values of 1 - 1/[1 + QGHA/(kdVL) + QGHAL/(k0LAaVL)] of 0.87 and 0.89 for [I] ) 769 and 384 W m-3, respectively. Simulation of Ozonation Based on MSRK. On the basis of the MSRK model, the kinetic parameters in the O3/UV system can be assessed by achieving the best fitting for the experimental data in the simulation. As a result, the corresponding values of

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Figure 7. Time variations of θAGe for 2-MT ozonation with UV radiation in the electroplating solution of a semibatch system. θAGe ) CAGe/CAGi0. Symbols: experiments. Lines: prediction based on five-step reaction kinetics. Notations are the same as those specified in Figure 3.

Figure 8. ErA vs ηTOC for 2-MT ozonation with UV radiation in the electroplating solution of a semibatch system. Lines: prediction based on five-step reaction kinetics. Notations are the same as those specified in Figure 3. s: CAGi0 ) 40 mg L-1 without UV radiation.5

the parameters and R2 are summarized in Table 1. The reaction rate constants for pollutants in the ozonation kinetics of 2-MT with UV radiation are greater than those without because of the additional terms related to UV radiation. They account for

the higher mineralization rate in the O3/UV system of 2-MT. The enhanced effect depends on the intensity of the UV radiation. Furthermore, the refractory derivatives from the destruction of 2-MT toward the oxidation are associated with the small reaction rate constants. The TOC contribution of the intermediate Ij is reflected by the factor Rj, while a smaller value of Rj indicates a higher mineralization degree. Note that the values of the kinetic parameters presented in Table 1 are applicable for ozonation of 2-MT combined with UV radiation in the environment of the prescription electroplating solution and the reaction temperature of 25 °C. As for the MSRK model of different reaction steps, it shows good agreement for θBLb, ηTOC, θALb, and θAGe based on the MSRK model of five reaction steps, as depicted in Figures 3, 4, 6, and 7, respectively. However, the fitting of ηTOC, θALb, and θAGe would deteriorate with the MSRK model of less reaction steps significantly. This indicates that the ozonation kinetics of insufficient reaction steps is not adequate to predict the related ozonation results. In the present work, the MSRK model of five reaction steps can simulate the experimental data satisfactorily. For further investigation of the ozonation mechanism, the identification of intermediates during 2-MT ozonation in the electroplating solution would be helpful as a future work. The variations of ErA are simultaneously predicted based on the MSRK model, as shown in Figure 8. ErA has the initial values of 5.05 and 4.89 for [I] ) 769 and 384 W m-3, which are close to the initial values of the Hatta number (xkBDACBLb0/k0LA) of 5.14 and 4.98, respectively. The distinct relationship between ErA and ηTOC reveals that ErA decreases with the mineralization degree continually to approach the constant of the unit. It is obvious that the effect of ErA on the gas-liquid mass transfer of ozone is not negligible in the cases. The inaccurate determination of ErA would also lead to an incorrect estimation of the reaction kinetics. The gas-liquid mass-transfer rate of ozone would be overestimated or underestimated by taking ErA with the initial value or unit, respectively. The case of higher CAGi0 would have the greater value of ErA at the same ηTOC. In addition, the value of ErA for the O3/UV system is greater than that for the sole ozonation system owing to the faster reaction rate. As a result, the modeling

Table 1. Ozonation Kinetics of 2-MT Combined with UV Radiation Adopting Schemes with Various Reaction Steps reaction step 2 3

4

5

a

kinetic parameters kIB ) 29.23 W-1 m3 M-1 s-1, kI1 ) 0.0195 W-1 m3 M-1 s-1, kIP ) 0.000 429 W-1 m3 M-1 s-1 (kBm ) 165 000 M-1 s-1, k1m ) 1.5 M-1 s-1)a kIB ) 29.23 W-1 m3 M-1 s-1, kI1 ) 0.146 W-1 m3 M-1 s-1, kI2 ) 0.0195 W-1 m3 M-1 s-1, kIP ) 0.000 58 W-1 m3 M-1 s-1 (kBm ) 165 000 M-1 s-1, k1m ) 24 000 M-1 s-1, k2m ) 1.55 M-1 s-1)a kIB ) 29.23 W-1 m3 M-1 s-1, kI1 ) 0.146 W-1 m3 M-1 s-1, kI2 ) 0.136 W-1 m3 M-1 s-1, kI3 ) 0.0195 W-1 m3 M-1 s-1, kIP ) 0.000 58 W-1 m3 M-1 s-1 (kBm ) 165 000 M-1 s-1, k1m ) 24 000 M-1 s-1, k2m ) 3500 M-1 s-1, k3m ) 1.55 M-1 s-1)a kIB ) 29.23 W-1 m3 M-1 s-1, kI1 ) 0.146 W-1 m3 M-1 s-1, kI2 ) 0.136 W-1 m3 M-1 s-1, kI3 ) 0.0974 W-1 m3 M-1 s-1, kI4 ) 0.0507 W-1 m3 M-1 s-1, kIP ) 0.000 58 W-1 m3 M-1 s-1 (kBm ) 165 000 M-1 s-1, k1m ) 24 000 M-1 s-1, k2m ) 3500 M-1 s-1, k3m ) 10 M-1 s-1, k4m ) 1.55 M-1 s-1)a

Kinetic parameters were obtained from the sole ozonation of 2-MT.5

factors of TOC contributiona

θBLb

R2 values for predicted results ηTOC θALb θAGe

R1 ) 0.92, RP ) 0.7

0.988

0.863

0.105

0.400

R1 ) 0.96, R2 ) 0.935, RP ) 0.74

0.998

0.979

0.570

0.721

R1 ) 0.96, R2 ) 0.935, R2 ) 0.885, RP ) 0.74

0.997

0.994

0.933

0.888

R1 ) 0.96, R2 ) 0.935, R3 ) 0.905, R4 ) 0.87, RP ) 0.75

0.998

0.998

0.946

0.893

4942

Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006

methodology proposed in the study is useful for the prediction of pollutant ozonation with UV radiation in the solution. Conclusions 1. The results recommend the employment of the ozonation process combined with UV radiation for the removal of 2-MT in the electroplating solution. The concentration variations of 2-MT, TOCs, dissolved ozone (CALb), and off-gas ozone can be properly predicted based on the proposed multistep reaction kinetics. 2. The decomposition rate of 2-MT increases with the fed ozone concentration, while is not significantly enhanced by the presence of UV radiation. On the other hand, the increase of both the fed ozone concentration and the UV radiation intensity can effectively improve the mineralization rate of the sequential intermediates. 3. The progress of ozonation of 2-MT with UV radiation in the electroplating solution can be divided into three stages. First, in the early stage as the removal efficiency of TOCs (ηTOC) < 9%, CALb remains almost undetectable and the enhancement factor of the ozone mass transfer (ErA) is remarkably greater than the unit. Second, in the transient regime, when 9% e ηTOC < 20%, the dissolved ozone starts to accumulate while the ozonation reactions become relatively slow and the ErA value is close to the unit. Third, in the ozone-rich regime, when ηTOC g 20%, CALb reaches the steady-state value, while the ozone consumption mostly comes from the self-decomposition and photolysis reactions of ozone. Acknowledgment This study was supported by the National Science Council of Taiwan under Grant NSC 94-2218-E-151-014. Note Added after ASAP Publication. This article was released ASAP on May 28, 2006 with a minor error in equation 17. The correct version was posted on May 31, 2006. Nomenclature a ) specific gas-liquid interfacial area based on the volume of liquid and gas (m-1) A ) ozone Aq ) outer area of the quartz tubes submerged in solution (m2) B ) pollutant CAGi, CAGi0 ) gas concentrations of ozone of holdup and inlet gases (mg L-1 or M) CAGe ) gas concentration of ozone in free volume (M) CALb ) dissolved ozone concentrations in a bulk liquid (M) CALF, CBLF ) dissolved concentrations of ozone and pollutant in a liquid film (M) CALS ) dissolved ozone concentration of liquid at the gasliquid interface (M) CBLb ) concentration of pollutant in a bulk liquid (M) CBLb0 ) initial concentration of pollutant in a bulk liquid (mg L-1 or M) Ce, Ce ) experimental data and the corresponding average values CjLb ) concentration of intermediate j in a bulk liquid (M) CjLF ) concentration of intermediate j in a liquid film (M) Cp ) predicted values CPLb ) concentration of product in a bulk liquid (M) CPLF ) concentration of product in a liquid film (M) CTOC ) concentration of total organic carbons (mg L-1) CTOC0 ) initial concentration of total organic carbons (mg L-1)

DA, DB ) molecular liquid diffusion coefficients of ozone and pollutant (m2 s-1) Dj, DP ) molecular liquid diffusion coefficients of intermediate j and product (m2 s-1) ErA ) enhancement factor of ozone mass transfer defined by eq 17 FS ) fraction of emitting light absorbed by the solution HA ) dimensionless Henry’s law constant of ozone, CAGi/CALS (M M-1) Ij ) intermediate j [I] ) average applied intensity of UV radiation per unit volume (W m-3) [IUV] ) light intensity measured on the outer surface of the quartz tubes housing UV lamps (W m-2) kB ) reaction rate constant of pollutant B (M-1 s-1) kBm ) reaction rate constant of pollutant B reacted with ozone (M-1 s-1) kd ) decomposition rate constant of ozone (s-1) kdm ) self-decomposition rate constant of ozone (s-1) kj ) reaction rate constant of intermediate j (M-1 s-1) kjm ) reaction rate constant of intermediate j reacted with ozone (M-1 s-1) kIB ) reaction rate constant of pollutant B reacted with OH• induced by UV (W-1 m3 M-1 s-1) kIBm ) photolysis rate constant of pollutant B (W-1 m3 s-1) kId ) photolysis rate constant of ozone (W-1 m3 s-1) kIj ) reaction rate constant of intermediate j reacted with OH• induced by UV (W-1 m3 M-1 s-1) kIP ) reaction rate constant of product P reacted with OH• induced by UV (W-1 m3 M-1 s-1) 0 kLA ) physical liquid-phase mass-transfer coefficient of ozone (m s-1) N ) number of intermediates in the ozonation kinetic model MSRK ) multistep reaction kinetics 2-MT ) 2-mercaptothiazoline P ) product PWB ) printed wiring board R2 ) determination coefficient, 1 - [∑(Ce - Cp)2/∑(Ce Ce)2] QG ) gas flow rate (L s-1) t ) time (s) TOCs ) total organic carbons VF ) volume of free space (L) VH ) volume of holdup gas (L) VL ) volume of bulk liquid (L) x ) distance from the gas-liquid interface of a liquid film (m) xM ) thickness of a liquid film, DA/k0LA (m) Rj, RP ) ratios of TOC contribution per mole of intermediate j and product P to that per mole of pollutant B G, L ) relative gas and liquid holdups, G + L ) 1 ηTOC ) removal efficiency of TOCs, (CTOC0 - CTOC)/CTOC0 (%) θAGe ) dimensionless gas concentration of ozone in free volume, CAGe/CAGi0 θALb ) dimensionless concentration of ozone in a bulk liquid, CALb/(CAGi0/HA) θALF ) dimensionless concentration of ozone in a liquid film, CALF/(CAGi0/HA) θBLb ) dimensionless concentration of pollutant in a bulk liquid, CBLb/CBLb0 Subscripts j ) intermediate j, Arabic numbers

Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006 4943

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ReceiVed for reView January 15, 2006 ReVised manuscript receiVed April 7, 2006 Accepted April 19, 2006 IE060065W