Ind. Eng. Chem. Res. 2006, 45, 553-557
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Evaluation of Electrical Energy Per Order (EEO) with Kinetic Modeling on Photooxidative Degradation of C. I. Acid Orange 7 in a Tubular Continuous-Flow Photoreactor Mohammad A. Behnajady* and Nasser Modirshahla Research Laboratory, Department of Applied Chemistry, Islamic Azad UniVersity, P.O. Box 1655, Tabriz Branch, Tabriz, I.R. Iran
The photooxidative decolorization of C. I. Acid Orange 7 (AO7), a commercial monoazo textile dye, in the UV/H2O2 process has been investigated in a tubular continuous-flow photoreactor. The decolorization rate follows pseudo-first-order kinetic with respect to the AO7 concentration. The rate constant of the interaction of ‚OH radicals with the AO7 molecules has been estimated through the adoption of a simplified kinetic model (6.1 × 108 M-1s-1). This model allows for predicting the concentration of AO7 in different photoreactor lengths for different flow rates and initial concentrations of H2O2 and AO7. For electrical energy cost evaluation, the figure-of-merit electrical energy per order (EEO) was estimated from experimental and calculated data. The results show EEO is very sensitive to the initial concentrations of H2O2 and AO7. 1. Introduction Dyes are widely used in textile, leather, pharmaceutical, plastic, paint, and food industries. Azo dyes, such as Acid Orange 7 (AO7), are in the wastewater of textile mills and discharged into rivers and lakes, causing environmental problems. Azo dyes constitute over 50% of all textile dyes used in the industry.1 Due to the large degree of aromatics present in these dyes, conventional biological treatment methods are ineffective for decolorization and degradation. For removal of such pollutants, physical techniques such as adsorption on activated carbon, ultrafiltration, reverse osmosis, coagulation, and electrocoagulation2 can be used efficiently. Nevertheless, they are nondestructive and merely transport contaminants from water to sludge.2 Advanced oxidation processes (AOPs) are attractive alternatives to nondestructive physical water treatment processes, because they are able to mineralize organic water contaminants.3 Although AOPs make use of different reacting systems, they are almost characterized by the efficient production of hydroxyl radicals. The hydroxyl radical is a powerful oxidant and a short-lived, highly reactive, and nonselective reagent which is easy to produce. Among the AOPs, a combination of UV radiation and H2O2 has been applied successfully to treat different pollutants in water.4-6 Various types of kinetic models have been postulated to describe the kinetics of the UV/H2O2 process. An overview of these models is given in Table 1. The first type of these models is an empirical rate expression, in which rate constants and reaction orders are determined by dealing with the experimental results with linear regression. Ku and Ho,7 Sundstrom et al.,8 and Elkanzi and Kheng9 tested the power law model for many organic compounds and obtained the reaction order for various operational parameters. A second type of kinetic model for the UV/H2O2 process is based on the reaction mechanism and known chemical and photochemical reactions. Most of the kinetic models of this second type utilize the pseudo-steadystate approximation for the free radical species in the system.5,10 These kinetic models were developed for a constant concentra* To whom correspondence should be addressed. Tel.: +98-4113318681-4 (305). Fax: +98-411-3318687. E-mail: behnajady@ iaut.ac.ir.
Figure 1. Schematic diagram of tubular continuous-flow photoreactor. For details, refer to the text.
tion of H2O2 in the decolorization course. El-Dein et al.10 demonstrated that H2O2 concentration considerably decreases during the decolorization course. Much of the past efforts on the treatment of dye wastewater by the UV/H2O2 process were studied in a batch or recirculated photoreactor. Therefore, studies on the development of the kinetic modeling of the dye removal by the UV/H2O2 process in a continuous photoreactor is scarce. The objective of the present study is to develop a kinetic model for the decolorization of AO7 using the UV/H2O2 process in a tubular continuous-flow photoreactor. This model can be used in order to predict the concentration of AO7 at different lengths of photoreactor and to determine the required electrical energy at different conditions. 2. Experimental Section 2.1. Materials. AO7, a monoazo anionic dye from the acid class, was obtained from Fluka. Hydrogen peroxide solution (30%) was purchased from Merck (Germany). 2.2. Photoreactor. All experiments were carried out in a tubular continuous-flow photoreactor. A schematic diagram has been shown in Figure 1. The photoreactor comprises four quartz tubes (24.4 mm i.d., 26 mm o.d., 81 cm length), which were serially connected by means of transparent rubber tubes from the top to the bottom. The radiation source consists of four
10.1021/ie050111c CCC: $33.50 © 2006 American Chemical Society Published on Web 12/14/2005
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Table 1. Review of Some Kinetic Rate Expressions for Removal of Various Organic Compounds with the UV/H2O2 Process type
reactor type
organic compound
first
second
rate equationa -dCs/dt ) + khCqs CH2O2rIs -dCs/dt ) -dCs/dt ) -dCs/dt ) ((2ØfH2O2khI)/(kbCSO))Cs fH2O2 ) H2O2CH2O2/(sCs + H2O2CH2O2) -dCs/dt ) [kbICH2O2/((kb1I + kb2)CSO + CH2O2)]Cs kH2O2Cms CH2O2n + kuvC0s Ip kuvCms + khCH2O2nC0s kuvCms In + khC0s CH2O2pIq
chlorophenols benzene, toluene, phenol, dimethylphthalate isoprene Acid Orange 7
batch batch batch batch
Reactive Black 5
recirculated
ref 7 8 9 5 10
a
m, n, o, p, q, r, and s are reaction orders; kb, kb1, and kb2 are constants used for the kinetic modeling; kh is the reaction rate constant for the UV/H2O2 system (min-1); kUV is the reaction rate constant for UV alone (min-1); Ø is the quantum yield; and is the molar absorption coefficient (M-1 cm-1).
mercury UV lamps (30 W, UV-C, manufactured by Philips, Holland) in vertical arrays, which were placed in front of the quartz tubes at a distance of 5 cm. 2.3. Procedures and Analysis. For photooxidative degradation of AO7, a solution containing known concentrations of dye and H2O2 was prepared and then 2 L of the prepared solution was transferred into a Pyrex beaker and agitated with a magnetic stirrer during the experiment. The solution was pumped with a peristaltic pump through the irradiated quartz tubes, and AO7 concentration at the inlet and outlet was analyzed with a UVVis spectrophotometer (Ultrospec 2000, Biotech Pharmacia, England) at 485 nm. The light intensity in the center of the photoreactor was measured by a Lux-UV-IR meter (Leybold Co.).
Considering the steady-state approximations for ‚OH and HO2‚, the concentration of ‚OH can be calculated as follows:
[‚OH] )
k1
H2O2 + hυ 982(‚OH)
[AO7]0 ≈ [AO7] + [Int.]
k2
AO7 + ‚OH98Int.
[‚OH] )
Int. + ‚OH98P
k4
H2O2 + ‚OH98HO2‚ + H2O k4 ) 2.7 × 107 M-1s-1 (ref 11) (4)
k1[H2O2] k4[H2O2] + k2[AO7]0 + (k3 - k2)[Int.]
k4[H2O2] + k2[AO7]0 . (k3 - k2)[Int.]
[‚OH] )
k5 ) 8.3 × 105 M-1s-1 (ref 12) (5)
-
d[HO2‚]/dt ) k4[H2O2][‚OH] - 2k5[HO2‚]
2
(7)
k1[H2O2] k4[H2O2] + k2[AO7]0
(12)
d[AO7] ) kap[AO7] dt
(13)
In the above equation, [AO7] is the AO7 concentration, and kap and t are the pseudo-first-order rate constant and the irradiation time, respectively. Considering eq 2 and also a lower reactivity of other radical species such as HO2‚ in comparison with ‚OH, the following equation can be written as
kap ) k2[‚OH]
(14)
The following relationship is obtained by substituting eqs 12 and 14 into eq 13:
k1k2[H2O2] -d[AO7] [AO7] ) dt k4[H2O2] + k2[AO7]0
The kinetic model for photooxidative degradation of AO7 was obtained using eqs 1-5. The corresponding kinetic equations for ‚OH and HO2‚ are
d[‚OH]/dt ) k1[H2O2] - k4[H2O2][‚OH] k2[AO7][‚OH] - k3[Int.][‚OH] (6)
(11)
As we described in our previous work,5 the kinetics of the degradation of AO7 in the UV/H2O2 process is pseudo-first order.
k5
HO2‚ + HO2‚98H2O2 + O2
(10)
Finally,
(3)
where Int. is the formed intermediates and P is the final products. Other reactions involved are given below:
(9)
Since the concentration of intermediates ([Int.]) in comparison to excess of hydrogen peroxide and initial concentration of AO7 is very low and there is not a considerable difference between k2 and k3,13 it can be written as
(2)
k3
(8)
From eqs 8 and 9, the concentration of ‚OH can be written as follows:
(1)
In the above equation, k1 is a function of the quantum yield of the photochemical dissociation of H2O2 and incident UV-light intensity, since in this work UV-light intensity was constant; therefore, k1 may be considered to have a constant value. The ‚OH radicals formed may undergo radical-chain reactions with the substrate, and intermediates are formed. These intermediates react with ‚OH radicals to complete mineralization.
k4[H2O2] + k2[AO7] + k3[Int.]
In the early stages of the process, we can write the following:
3. Results and Discussion 3.1. Kinetic Modeling. The primary and principal step for the UV/H2O2 degradation has been postulated as the initial attack of photon to hydrogen peroxide molecule and, consequently, the formation of hydroxyl free radicals.
k1[H2O2]
(15)
where
kap )
k[H2O2] [H2O2] + k′[AO7]0
(16)
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 555
and
k ) k1k2/k4, k′ ) k2/k4 The corresponding kinetic equation for H2O2 is
d[H2O2]/dt ) -k1[H2O2] - k4[H2O2][‚OH] + k5[HO2‚]2 (17) The following relationship is obtained by substituting eqs 7 and 12 into eq 17:
-d[H2O2] 1.5k1[H2O2]2 + k1k′[H2O2][AO7]0 ) (18) dt [H2O2] + k′[AO7]0 For a plug flow reactor (PFR), the design equation can be written as follows:14
-d[AO7] - rAO7 ) dV ν0
(19)
π V ) di2l 4
(20)
with
and
-rAO7 )
-d[AO7] dt
(21)
In the above equations V, ν0, di, and l are the photoreactor volume, volumetric flow rate, inner diameter of quartz tubular photoreactor, and photoreactor length, respectively. With substitution of eqs 20 and 21 into eq 19, we obtain
π 2 d -d[AO7] 4 i - d[AO7] ) dl ν0 dt
(
)
(22)
Finally, with substitution of eqs 15, 16, and 18 into eq 22, we obtain
(
)
π 2 d k′[H2O2] -d[AO7] 4 i ) [AO7] dl ν0 [H2O2] + k[AO7]0
(23)
and
(
)
π 2 -d[H2O2] 4 di 1.5k1[H2O2]2 + k1k′[H2O2][AO7]0 ) dl ν0 [H2O2] + k′[AO7]0
(24)
On the basis of eq 23, the semilogarithmic graphs of the concentration of AO7 in the presence of different concentrations of H2O2 versus photoreactor length are straight lines. In all cases, R2 (correlation coefficient) values are >0.99, which confirms the proposed first-order kinetics for the decolorization of AO7 in this process. Equation 16 can be tested using experimental data for obtaining model parameters after transforming it to a straight-line equation as follows:
1 1 k′ [AO7]0 ) + kap k k [H2O2]
(25)
A plot of 1/kap against [AO7]0/[H2O2] gives the values of the
Figure 2. Plot of 1/kap versus the initial concentration ratio of AO7 to H2O2.
slope (k′/k) and intercept (1/k) (see Figure 2). From the intercept and slope of this curve, k and k′ values for AO7 are 1.008 min-1 and 22.6, respectively. The reaction rate constant of the interaction of ‚OH radicals with the AO7 molecules (k2) has been estimated from the k′ value to be 6.1 × 108 M-1s-1. This value is in the range of 107-1010 M-1s-1, which has been reported as the rate constant of the reaction of ‚OH radicals with most of the organic compounds.11 After substituting the values of k, k′, and k1 into the kinetic model equations (eqs 23 and 24), the concentrations of AO7 in different photoreactor lengths were obtained for varying initial concentrations of H2O2 and AO7 and also for different flow rates by solving the mentioned equations simultaneously, by an ordinary differential equations (ODE) solver. A comparison between experimental and calculated data for the decolorization of AO7 in different photoreactor lengths was shown in Figure 3 for different initial concentrations of H2O2 and AO7 and different flow rates. As can be seen from these figures, the model allows for predicting the AO7 in different photoreactor lengths at different conditions. The good agreement between the calculated and experimental data in Figure 3 confirms the proposed mechanism and supposed deactivation occurrence of hydroxyl radicals with excess hydrogen peroxide and AO7 concentration. 3.2. Electrical Energy Determination. The evaluation of the treatment costs is, at the moment, one of the aspects which needs more attention. There are a number of important factors in selecting a waste-treatment technology, including economics, economy of scale, regulations, effluent quality goals, operation (maintenance, control, safety), and robustness (flexibility to change/upsets). Although all these factors are important, economics is often paramount. Since the UV/H2O2 process is electric-energy intensive, and electric energy can represent a major fraction of the operating costs, simple figures-of-merit based on electric energy consumption can be very useful and informative. The connection between electric energy (Eel) and the overall reaction kinetics of AOPs is obvious: the electric energy needed to eliminate a specific amount of substrate is directly proportional to the electric power (Pel) of the lamp used and inversely proportional to the treated volume of water and the observed overall rate constant (koverall) of the process:15
Eel ≈
Pel Vkoverall
(26)
Therefore, Bolton et al.15 defined the figures-of-merit “electric energy per mass” (EEM) for use in the zero-order kinetic regime and “electric energy per order” (EEO) for use in the first-order kinetic regime of AOPs. This concept was accepted by the IUPAC as a technical report.15 In the case of low pollutant
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Figure 3. Comparison between experimental and calculated data for the decolorization of AO7 in the UV/H2O2 process: (a) at different initial concentrations of H2O2, [AO7]o ) 40 mg L-1, flow rate ) 140 mL min-1; (b) at different initial concentrations of AO7, [H2O2]o ) 500 mg L-1, flow rate ) 140 mL min-1; (c) at different volumetric flow rates, [AO7]o ) 40 mg L-1, [H2O2]o ) 500 mg L-1. (Io ) 42 W m-2 for all runs.)
concentrations (which applies here), the appropriate figure-ofmerit is the electrical energy per order (EEO), defined as the number of kilowatt hours of electrical energy required to reduce the concentration of a pollutant by 1 order of magnitude in a unit volume of contaminated water. The EEO (kWh/m3/order) can be calculated from the following equation for a batch-type reactor,
EEO )
Pelt × 1000 V × 60 log (C0/C)
(27)
where Pel is the input power (kW) to the AOP system, t is the irradiation time (min), V is the volume of water (L) in the reactor, and C0 and C are the initial and final pollutant concentrations, respectively. The above equation for a plugflow reactor changes to
EEO )
Pel ν0 log (C0/C)
(28)
where ν0 is the volumetric flow rate (m3/h) in the flow-through system.15 Figure 4 shows the predicted EEO values estimated from the kinetic modeling (eqs 23, 24, and 28) and the experimental data as a function of H2O2 concentration. It can be seen that the model correctly predicts the trend of EEO. As can be seen from Figure 4, EEO decreases with increasing H2O2 concentration from 100 to 300 mg L-1. This is a result of the enhancement of the decolorization rate of AO7 in this course, which is due to an
Figure 4. Electrical energy per order versus the initial concentration of H2O2: [AO7]o ) 40 mg L-1, flow rate ) 140 mL min-1, Io ) 42 W m-2.
increase in the hydroxyl radical concentration. The enhancement of the decolorization rate is not very high in the range of 300 to 500 mg L-1 from H2O2 concentration. This is reasonable, because ‚OH radicals efficiently react with H2O2, so that the photooxidative degradation promoter itself contributes to the ‚OH-scavenging capacity and reduces the decolorization rate of AO7.6 On the other hand, the presence of the concentration of H2O2 in eq 23 shows that the reaction rate is proportional and independent to H2O2 concentration at low and high concentrations, respectively. The EEO is inversely proportional to the observed overall rate constant (kap) of the UV/H2O2 process; therefore, with increasing kap, EEO values significantly decrease.
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 557 Table 2. Experimental and Calculated Values of EEO for Decolorization of AO7 with the UV/H2O2 Process in a Continuous-flow Photoreactora EEO (kWh/m3/order)
a
[AO7]0 (mg L-1)
experimental
calculated
19.55 29.40 39.25 48.34
2.51 4.56 5.90 12.56
3.57 5.58 8.17 10.93
[H2O2] ) 500 mg L-1 and Io ) 42 W m-2 for all runs.
As is evident from Table 2, EEO increases with increasing initial concentration of AO7. This is due to the decrease in the decolorization rate of AO7 with increasing initial concentration of the dye. This can be explained by considering that the molar extinction coefficient of the dye at λ < 260 nm is very high, so that a rise in its concentration induces an inner filter effect. Consequently, the solution becomes more and more impermeable to UV radiation. Also, at higher concentrations of AO7, higher intermediates are formed. These intermediates are also highly reactive toward hydroxyl radicals. Thus, AO7 and its intermediates compete effectively for hydroxyl radicals, reducing the removal efficiency.16 The presence of the initial concentration of AO7 in eq 23 indicates that the kap could decrease with increasing initial concentration of AO7, because there is a linear relation between kap and the reverse of the initial concentration of AO7. The decrease of kap is responsible for increasing EEO. 4. Conclusions It can be concluded that the UV/H2O2 process using a continuous-flow photoreactor provides good performance in the decolorization of AO7 as a model compound from textile industry. The results indicated that AO7 concentration at different lengths of photoreactor could be predicted with kinetic modeling at different conditions. The rate constant of the interaction of .OH radicals with AO7 molecules has been estimated from this model (6.1 × 108 M-1s-1). For cost evaluation, electrical energy per order (EEO) based on experimental and calculated data was obtained for different conditions. The results of this study show that EEO is a function of the initial concentrations of H2O2 and AO7. It indicates that applying an optimum H2O2 concentration and reducing the initial concentration of AO7 could reduce the EEO value. Acknowledgment The authors thank the Islamic Azad University of Tabriz branch for financial and other support.
Literature Cited (1) Kirk-Othmer Encyclopedia of chemical technology, third ed.; Wiley: New York, 1978. (2) Daneshvar, N.; Ashassi-Sorkhabi, H.; Tizpar, A. Decolorization of orange II by electrocoagulation method. Sep. Purif. Technol. 2003, 31, 153. (3) Daneshvar, N.; Rabbani, M.; Modirshahla, N.; Behnajady, M. A. Photooxidative degradation of Acid Red 27 in a tubular continuous-flow photoreactor: Influence of operational parameters and mineralization products. J. Hazard. Mater. 2005, 118, 155. (4) Legrini, O.; Oliveros, E.; Braun, A. M. Photochemical processes for water treatment. Chem. ReV. 1993, 93, 671. (5) Behnajady, M. A.; Modirshahla, N.; Shokri, M. Photodestruction of Acid Orange 7 (AO7) in aqueous solutions by UV/H2O2: Influence of operational parameters. Chemosphere 2004, 55, 129. (6) Daneshvar, N.; Rabbani, M.; Modirshahla, N.; Behnajady, M. A. Critical effect of hydrogen peroxide concentration in photochemical oxidative degradation of C. I. Acid Red 27 (AR27). Chemosphere 2004, 56, 895. (7) Ku, Y.; Ho, S. C. The effect of oxidants on UV destruction of chlorophenols. EnViron. Prog. 1990, 9, 218. (8) Sundstrom, D. W.; Weir, B. A.; Klei, H. E. Destruction of aromatic pollutants by UV light catalyzed oxidation with hydrogen peroxide. EnViron. Prog. 1989, 8, 6. (9) Elkanzi, E. M.; Kheng, G. B. H2O2/UV degradation kinetics of isoprene in aqueous solution. J. Hazard. Mater. B 2000, 73, 55. (10) Mohey El-Dein, A.; Libra, J. A.; Wiesmann, U. Mechanism and kinetic model for the decolorization the azo dye Reactive Black 5 by hydrogen peroxide and UV radiation. Chemosphere 2003, 52, 1069. (11) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (AOH/AO-) in aqueous solution. J. Phys. Chem. Ref. Data 1988, 17, 513. (12) Bielski, H. J.; Benon, H. J.; Cabelli, D. E.; Ravindra, L. A.; Alberta, A. B. Reactivity of perhydroxyl/superoxide radicals in aqueous solution. J. Phys. Chem. Ref. Data 1985, 14, 1041. (13) Colonna, G. M.; Caronna, T.; Marcandalli, B. Oxidative degradation of dyes by ultraviolet radiation in the presence of hydrogen peroxide. Dyes Pigm. 1999, 41, 211-220. (14) Levenspiel, O. Chemical reaction engineering; Wiley: New York, 1972. (15) Bolton, J. R.; Bircger, K. G.; Tumas, W.; Tolman, C. A. Figure of merit for the technical development and application of advanced oxidation technologies for both electric and solar-derived systems. Pure Appl. Chem. 2001, 73, 627. (16) Cater, S.; Stefan, M. I.; Bolton, J. R.; Safarzadeh-Amiri, A. UV/ H2O2 treatment of methyl tert-butyl ether in contaminated waters. EnViron. Sci. Technol. 2000, 34, 659.
ReceiVed for reView January 27, 2005 ReVised manuscript receiVed October 27, 2005 Accepted November 3, 2005 IE050111C
