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Ind. Eng. Chem. Res. 2002, 41, 1436-1444
Decomposition of Formic Acid in a Water Solution Employing the Photo-Fenton Reaction Germa´ n H. Rossetti,†,‡ Enrique D. Albizzati,† and Orlando M. Alfano*,‡ Facultad de Ingenierı´a Quı´mica, Universidad Nacional del Litoral (UNL), Santiago del Estero 2654, 3000 Santa Fe, Argentina, and Instituto de Desarrollo Tecnolo´ gico para la Industria Quı´mica (INTEC), Consejo Nacional de Investigaciones Cientı´ficas y Te´ cnicas (CONICET), and Universidad Nacional del Litoral (UNL), Gu¨ emes 3450, 3000 Santa Fe, Argentina
The photodegradation of a specific organic pollutant in aqueous solution using the photo-Fenton system has been modeled and experimentally verified. Kinetic and reactor models accounting for the decomposition rates of formic acid (the model compound) and hydrogen peroxide were developed. The experimental work was performed in a flat-plate, well-stirred reactor placed inside a batch recycling system having a high-flow-rate recirculating pump and a storage tank. The reactor was irradiated from both sides with two tubular lamps placed at the focal axis of two cylindrical reflectors of a parabolic cross section. When predictions of the theoretical model are compared with experimental results, a good representation of the formic acid and hydrogen peroxide concentration evolution in a rather wide range of their initial molar concentrations is obtained; within the range of explored variables, deviations between model predictions and experimental data were always smaller than 9%. Furthermore, model predictions and experimental results of the organic compound degradation show that UV irradiation improves the effectiveness of the Fenton system significantly. Under the adopted operating conditions, a pollutant conversion 63% greater than that obtained with the dark system was observed. 1. Introduction The Fenton reaction is a chemical system involving hydrogen peroxide and ferrous salts that generates highly reactive hydroxyl radicals at low temperature; these radicals react rapidly and nonselectively with most toxic organic compounds.1 Several studies have proposed different reaction mechanisms and kinetic models for the decomposition of hydrogen peroxide by ferric ion in acidic aqueous solutions.2,3 A modern application of this reaction is the degradation of pollutants that may be present in water and wastewater streams. The oxidation ability of the Fenton mixture can be greatly enhanced using UV or UV/visible radiation (the light-enhanced Fenton or the photo-Fenton reaction) in order to increase the production rate of hydroxyl radicals. Using this reaction, the mineralization of various aqueous organic compounds was studied: 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T),4,5 metolachlor and methyl parathion,6 carbon tetrachloride and hexachloroethane,7 2,4,6-trinitrotoluene (TNT) and other nitroaromatic compounds,8 and 2-propanol.9 The feasibility of a large-scale application of the photo-Fenton reaction for the degradation of organic pollutants in an aqueous solution has been investigated by Bauer,10 Bolton et al.,11 and Oliveros et al.,12 among others. Very recently, a combined photochemical and thermal solar radiation process to enhance the degrada* To whom correspondence should be addressed. Fax: +54 342 4559185. Tel.: +54 342 4559175. E-mail: alfano@ intec.unl.edu.ar. † Facultad de Ingenierı ´a Quı´mica, Universidad Nacional del Litoral. ‡ INTEC, CONICET, and UNL.
tion rate of the Fenton reaction13 and a coupled photoFenton-aerobic biological treatment14 to remove organic pollutants have also been proposed. These contributions were mainly concerned with proposing reaction paths and/or mechanisms for the decomposition of the toxic organic compounds or demonstrating the feasibility of the light-enhanced Fenton process to reach the complete mineralization of pollutants. In this work, the photodegradation of a specific organic pollutant in an aqueous solution, using the photo-Fenton system, has been studied. With this purpose, a flat-plate, well-stirred reactor placed inside a batch recycling system has been modeled and experimentally verified. The reactor was irradiated from both sides with two tubular lamps placed at the focal axis of two cylindrical reflectors of a parabolic cross section. Formic acid was chosen as the model substrate. It must be noticed that two parallel reactions must be taken into account in the reactor model: (i) the Fenton (thermal) reaction and (ii) the photo-Fenton (light-activated) reaction. This is particularly important when the total reaction volume is not equal to the photoreactor volume. This point will be specially investigated to assess the oxidation ability of the Fenton system when UV irradiation is used. With this purpose, a kinetic model that can account for the thermal and light-activated reaction rates in a single mathematical expression has been developed. Then, to take into account the irradiated and nonirradiated liquid volumes, a rigorous mass balance for each one of the reactant species has been stated. 2. Kinetic Model A kinetic model was developed to describe the decomposition of formic acid (the model compound) in a water solution. The reaction scheme used in this paper,
10.1021/ie010696k CCC: $22.00 © 2002 American Chemical Society Published on Web 02/13/2002
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1437 Table 1. Reaction Mechanism values of the kinetic constants (M-1 s-1)a
reaction step initiation
hν
+ H2O f Fe2+ + HO• + H+ k1 3+ Fe + H2O2 f Fe2+ + H+ + HO2• k2 Fe2+ + H2O2 f Fe3+ + HO- + HO• k3 H2O2 + HO• f HO2• + H2O k4 H2O2 + HO2• f HO• + H2O + O2 k5 2HO• f H2O2 k6 2HO2• f H2O2 + O2 k7 HO2• + HO• f H2O + O2 k8 Fe3+ + HO2• f Fe2+ + H+ + O2 k9 Fe2+ + HO2• + H+ f Fe3+ + H2O2 k10 HCOOH + HO• f CO2•- + H2O + H+ k11 CO2•- + O2 + H+ f CO2 + HO2• Fe3+
propagation termination
decomposition
k1 ) 2.00 × 10-3 k2 ) 5.30 × 101 k3 ) 2.70 × 107 k4 ) 3.70 × 100 k5 ) 4.00 × 109 k6 ) 8.30 × 105 k7 ) 3.72 × 1010 k8 ) 2.88 × 104 k9 ) 1.20 × 106 k10 ) 1.40 × 108 k11 ) 1.00 × 109
Data taken from Walling and Goosen2 and Buxton et al.21
a
involving initiation, propagation, and termination reactions, comprises 12 reaction steps (Table 1). The reaction mechanism of the photo-Fenton system has been described in detail elsewhere.3,4 The following assumptions have been considered: (i) the steady-state approximation (SSA) may be applied for highly reactive radicals, such as OH• and HO2•, (ii) radical-radical termination reactions are neglected as compared with the propagation reactions, (iii) the ferrous ion concentration remains constant during the reaction time,3 and (iv) the oxygen concentration is always in excess. With these assumptions, it can be deduced that the formic acid reaction rate is given by
( ) Φ h
RF(x,t) ) -
∑λ eaλ(x,t)
1+
(
1+
Φ h
k1CFe3+CP
)( ) 1/2
1/2
k1k2k8 k9
1+
k4CP
k8CFe3+ CFe3+CP (1) k3CP 1+ k10CF
where Φ h is the wavelength-averaged primary quantum yield, eaλ (x,t) is the spectral local volumetric rate of photon absorption (LVRPA), ki are kinetic constants defined in the kinetic scheme, and CP, CF, and CFe3+ are the hydrogen peroxide, formic acid, and ferric ion concentrations, respectively. In eq 1 it is immediately seen that, when ∑λeaλ (x,t) ) 0, the formic acid decomposition rate is not null. In other words, a thermal reaction rate (for the Fenton system) can be readily identified. This term may be formally represented by the following expression:15
( )
k1k2k8 RTF(t) ) k9
1/2
(
Φ h
RF(x,t) ) -
()
∑λ eaλ(x,t)
1+
k3CP k10CF
Φ h
+ 1+
)
∑λ eaλ(x,t)
k1CFe3+CP
1/2
RTF(t)
(3)
Based on a similar procedure, the hydrogen peroxide reaction rate obtained from the proposed kinetic model is
k10CF
∑λ
When eq 2 is substituted into eq 1, the following expression for the formic acid reaction rate is obtained:
Note that eq 3 includes both the thermal and photochemical formic acid degradation rates.
-
k3CP
eaλ (x,t)
steps from the reaction scheme presented in Table 1; i.e., the first reaction step has been excluded.
k4CP k8CFe3+ C C k3CP Fe3+ P 1+ k10CF
1+
(2)
Note that, to obtain eq 2, we have considered 11 reaction
(
Φ h
RP(x,t) )
()
∑λ eaλ(x,t) k3 CP
Φ h
- 1+
{[ ] 1+
1+
k1CFe3+CP
k10 CF k4 CP 1+ k8 CFe3+ k1k2k8 1+
k10 CF
( ) k9
)
∑λ eaλ(x,t)
1/2
×
1/2
CPCFe3+ +
k3 CP
( )
k4
k1 k2
k8 k9
1/2
CP2
}
(4)
Again, when Σλeaλ (x,t) ) 0, the hydrogen peroxide reaction rate does not vanish, and in eq 4, a thermal reaction rate may be identified. Defining a thermal hydrogen peroxide decomposition15
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[
]
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002
k4 CP k8 CFe3+ k1k2k8 RTP(t) ) - 1 + k10 CF k9 1+ k3 C P
( )
1+
1/2
CPCFe3+ -
( )
k4
k1 k2 k8 k9
1/2
CP2 (5)
and substituting eq 5 into eq 4, we finally obtain
Φ h RP(x,t) )
∑λ eaλ(x,t)
1+
k3 CP
(
Φ h
+ 1+
)
∑λ eaλ(x,t)
k1CFe3+CP
1/2
RTP(t) (6)
k10 CF
3. Reactor Model Mass Balances. The adopted experimental device is an isothermal, well-stirred batch recycling reactor. The operating conditions in the system may be summarized as follows: (i) a reactor volume smaller than the tank volume (VR , VT), (ii) perfect mixing in the whole recycling system, (iii) high recirculating flow rate, and (iv) isothermal operation. As is shown in the appendix, a rigorous mass balance for this reactor gives
V - VR T dCi(t) VR ) 〈Ri(x,t)〉VR + Ri (t) dt V V
(7)
Note that the first term on the right-hand side of eq 7 represents the organic pollutant degradation produced by both the light-activated and thermal reactions (photoFenton) taking place inside the irradiated liquid volume, and the second one represents the thermal reaction (Fenton) occurring in the nonirradiated volume. The required reaction rates Ri(x,t) and RTi (t) for the mass balances are obtained from the kinetic model given by eqs 2, 3, 5, and 6. Substituting the reaction rates for the formic acid (i ) F) and the hydrogen peroxide (i ) P), one obtains
dCF
)
dt
VR 〈RF(Φ h V
∑λ eaλ(x,t),CF,CP,CFe
(
)
V - VR
dCP dt
)
VR 〈RP(Φ h V
,ki)〉VR +
3+
V
RTF(CF,CP,CFe3+,ki) (8)
∑λ eaλ(x,t),CF,CP,CFe
,ki)〉VR +
3+
(
)
V - VR V
RTP(CF,CP,CFe3+,ki) (9)
t)0
CF ) C0F
(10)
t)0
CF ) C0P
(11)
It should be noted that the average value on the first term of the right-hand side of eqs 8 and 9 must be retained in order to account for spatial variations of the photo-Fenton reaction rate as a consequence of variations in the LVRPA with position x inside the reactor.
Figure 1. Schematic representation of the flat-plate photoreactor: (a) top view; (b) side view. Key: 1, photoreactor; 2, reactor plate of radiation entrance; 3, UV lamp; 4, parabolic reflector; 5, liquid in; 6, liquid out.
However, as a result of the integration over the irradiated liquid volume (VR), this term is only a function of time. This set of two nonlinear, first-order, ordinary differential equations must be numerically solved with the initial conditions given by eqs 10 and 11. Radiation Field. It has been proposed that a threedimensional model should be used to compute the spectral LVRPA inside a photoreactor similar to our photochemical reactor.16-18 The authors proposed and experimentally validated a rigorous model to evaluate the spectral LVRPA for a similar emitting system (lamp-reflector-reactor geometry) and found that for a restricted set of geometrical and optical parameters the LVRPA variations were not very significant when the radial and angular coordinates were varied. In light of these results, a one-dimensional model has been used in this work to calculate the monochromatic LVRPA.19 Thus,
eaλ (x,t) ) κλ(t) qW,λ exp[-κT,λ(t) x]
(12)
In eq 12 qW,λ is the spectral radiative flux at the reactor wall, κλ the reactant species absorption coefficient, and κT,λ the total absorption coefficient. In the case of a flat-plate reactor of a circular cross section irradiated from both sides with two tubular lamps placed at the focal axis of two parabolic reflectors, similar expressions may be used for each emitting system. Then, from Figure 1, we have
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1439
From side 1:
eaλ (x,t) ) κλ(t) qW1,λ exp[-κT,λ(t) x] (13)
From side 2: eaλ (x,t) ) κλ(t) qW2,λ exp[-κT,λ(t) (L - x)] (14) It will be seen below that qW1,λ ) qW2,λ ) qW,λ; then, the monochromatic LVRPA may be expressed as
eaλ (x,t) ) κλ(t) qW,λ{exp[-κT,λ(t) x] + exp[-κT,λ(t) (L - x)]} (15) where the spectral volumetric absorption coefficient of the reacting species is given by
κλ(t) ) RFe(III),λCFe(III)(t)
(16)
It should be noted that Fe(OH)2+ is the dominant monomeric Fe(III)-hydroxy complex between pH ) 2.5 and 520 and that radiation absorption of Fe2+ and H2O2 is negligible for a wavelength λ > 300 nm. Consequently, the total spectral absorption coefficient of the reacting system is
κT,λ(t) )
∑i
Ri,λCi(t) = RFe(III),λCFe(III)(t)
value
units
69.94 4.40 4.60 3000 1000a
cm3 cm cm cm3 cm3
20 2.39 3.7 60
W W cm cm
2.75 6.0
cm cm
50
cm
Reactor irradiated volume diameter length total liquid volume Lamp Philips TL 20W/08 nominal power output power: 310 nm e λ e 420 nm diameter nominal arc length Reflector parabola characteristic constant distance vertex of parabolic reflectorreactor plate length a
Value used in section 6 of this work.
Table 3. Values of the Molar Absorptivity of Fe(OH)2+ wavelength (nm)
value (m2 mol-1)a
wavelength (nm)
value (m2 mol-1)a
300 310 320 330 340 350 360
457.4 425.9 353.4 270.5 192.3 128.9 81.7
370 380 390 400 410 420
48.4 27.6 15.0 8.1 4.6 2.3
(17)
The spectral radiative flux at the reactor wall (qW,λ) can be obtained from the radiative flux at the wall (qW) and the normalized spectral distribution of the lamp output power provided by the lamp manufacturer (fλ):
qW,λ ) qWfλ
Table 2. Reactor, Lamp, and Reflector Characteristics and Dimensions
a
Values taken from Faust and Hoigne´.20
(18)
When eqs 16-18 are substituted into eq 15, the following final expression may be written:
eaλ (x,t) ) RFe(III),λCFe(III)(t) fλqW × {exp[-xRFe(III),λCFe(III)(t)] + exp[-(L - x)RFe(III),λCFe(III)(t)]} (19) Numerical Solution. The system of two nonlinear, first-order, ordinary differential equations (eqs 8-11 and eq 19 to compute the spectral LVRPA) was numerically solved using a fourth-order, Runge-Kutta method. Integration of these equations provides the formic acid and hydrogen peroxide molar concentrations as a function of time. Values of the kinetic constants were obtained from Walling and Goosen2 and Buxton et al.21 They are indicated in Table 1. Just one parameter was adjusted from experimental data: the wavelength-averaged primary quantum yield. By using the formic acid and hydrogen peroxide experimental concentrations versus time data and a nonlinear regression algorithm, the primary quantum yield was found to be Φ h ) 0.83. A summary of the principal reactor, reflector, and lamp characteristics and dimensions is presented in Table 2. The radiative flux at the reactor walls was evaluated with actinometric measurements employing potassium ferrioxalate in an aqueous solution. The same procedure was used to confirm that the radiative flux at the radiation entrance takes on the same value from both sides. From these experiments, the radiative flux at the reactor wall was determined to be qW ) 3.02 × 10-9 einstein cm-2 s-1. The molar absorptivity and overall quantum yield of the potassium ferrioxalate as
Figure 2. Flow sheet of the experimental device. Key: 1, photoreactor; 2, UV lamp; 3, parabolic reflector; 4, heat exchanger; 5, thermostatic bath; 6, pump; 7, storage tank; 8, stirrer; 9, thermometer; 10, liquid sampling.
a function of wavelength were taken from the specific literature.22 The spectral data for the molar absorptivity of the absorbing species [Fe(OH)2+] were obtained from Faust and Hoigne´20 (see Table 3). 4. Experiments Setup. The employed apparatus was a well-stirred reactor placed inside a batch recycling system (Figure 2). The flat-plate reactor of a circular cross section was irradiated from both sides with two tubular lamps placed at the focal axis of two cylindrical reflectors of a parabolic cross section. Radiant energy was supplied by two black light, mercury arc lamps having an input power of 20 W (Philips TL 20W/08).23 The parabolic
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Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002
reflector was made with a specularly finished aluminum sheet having Alzac treatment.24 The tank was made of Pyrex glass and was equipped with a thermometer, a liquid sampling valve, and a variable-speed stirrer. Also, the experimental setup had an all-glass heat exchanger connected to a thermostatic bath and a centrifugal pump to achieve a high recirculating flow rate of the aqueous solution. The heat exchanger and the recirculating system facilitated the temperature control of the reacting mixture. Further details and characteristics of the experimental setup are indicated in Table 2. Note that the photoreactor volume is 2.33 and 7% of the total volume, depending on the two different operating conditions. Procedure. Experimental runs were conducted at constant pH (3.0) and temperature (298 K). The experimental procedure began when concentrated sulfuric acid was added to the mixture of ferric sulfate (Carlo Erba, RPE) and hydrogen peroxide (Carlo Erba, ACS, 30% P) in water to adjust the pH. Two lamp shutters interposed between the illuminating systems and the reactor windows allowed one to reach the specified operating conditions. Then, formic acid was added to the storage tank, and the lamp shutters were removed to start the reaction. Experiments lasted 2 h, with sampling of the liquid taken at equal time intervals (30 min). Prior to analysis, samples were quenched by addition of sodium sulfite (Anedra, 99%). Formic acid was analyzed with total organic carbon measurements (Shimadzu TOC-5000A), and hydrogen peroxide was determined by using a modified iodimetric technique.25 Analysis of ferrous ion was performed with standard spectrophotometric techniques (absorbance measurements of the Fe(II)-phenanthroline complex at 510 nm with a UV-vis Cary 17D spectrophotometer). 5. Comparison between Predicted and Experimental Results Model predictions and experimental results of formic acid and hydrogen peroxide concentrations as a function of time were compared for various hydrogen peroxide initial concentrations (C0P) and formic acid initial concentrations (C0F). To study the effects produced on the organic compound degradation rates by addition of hydrogen peroxide, a set of experimental runs at constant values of formic acid (2.0 × 10-3 M) and ferric iron (1.0 × 10-3 M) initial concentrations were performed. Figure 3a shows model predictions and experimental data of the time evolution of formic acid relative concentration (CF/C0F) for three different hydrogen peroxide initial concentrations: 6.70 × 10-3, 9.59 × 10-3, and 1.44 × 10-2 M. As might be expected, increasing the hydrogen peroxide initial concentration increases the formic acid conversion. For example, organic pollutant conversions after 2 h of reaction time are 66, 76, and 80% for C0P ) 6.70 × 10-3, 9.59 × 10-3, and 1.44 × 10-2 M, respectively. When predictions of the theoretical model are compared with experimental results, a good representation of the formic acid and hydrogen peroxide concentration evolution in a rather wide range of their initial molar concentrations is obtained; within the range of explored variables, deviations between model predictions (just one adjusted parameter) and experimental data were always smaller than 9%.
Figure 3. Model and experimental concentrations vs time for C0F ) 2.0 × 10-3 M and CFe3+ ) 1.0 × 10-3 M: (a) relative formic acid concentration; (b) hydrogen peroxide concentration. Key: (‚‚‚4) C0P ) 6.70 × 10-3 M, (s]) C0P ) 9.59 × 10-3 M, (- - -O) C0P ) 1.44 × 10-2 M.
Hydrogen peroxide concentration vs time results, corresponding to the three experimental runs previously described, are given in Figure 3b. Again, a good agreement between model predictions and experimental points is observed (the maximum deviation is not larger than 6%). Additional experiments were carried out to investigate the effects of the formic acid initial concentration on the formic acid degradation rates. To do this, constant values of both the hydrogen peroxide initial concentration (C0P ) 1.0 × 10-2 M) and the ferric ion concentration (CFe3+ ) 1.0 × 10-3 M) were employed in the experimental runs. Figure 4a is a plot of model predictions and experimental data of formic acid concentration as a function of time. The formic acid initial concentrations (C0F) were 1.73 × 10-3, 2.01 × 10-3, and 2.56 × 10-3 M. When the profiles are looked at, it is noted that formic acid conversion is slightly larger at lower C0F; for example, formic acid conversions after 2 h of reaction time are 71.2, 76.4, and 78.1% for C0F equal to 2.56 × 10-3, 2.01 × 10-3, and 1.73 × 10-3 M, respectively. In these runs, it was found that deviations between model predictions and experimental data were never larger than 7%. Figure 4b shows the hydrogen peroxide relative concentration (CP/C0P) as a function of time, for the same operating conditions as those used in Figure 4a. A maximum percentage error of 8% was obtained. In the above studies, it has been experimentally verified that the ferrous ion concentration remains almost constant during the reaction time. Moreover, for a ferric ion concentration of 1.0 × 10-3 M, the ferrous ion concentration was determined to be approximately 1.0 × 10-5 M. Thus, CFe3+/CFe2+ = 100.
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1441
Figure 4. Model and experimental concentrations vs time for C0P ) 1.0 × 10-2 M and CFe3+ ) 1.0 × 10-3 M: (a) formic acid concentration; (b) relative hydrogen peroxide concentration. Key: (‚‚‚0) C0F ) 1.73 × 10-3 M, (s]) C0F ) 2.01 × 10-3 M, (- - -3) C0F ) 2.56 × 10-3 M.
Figure 5. Computer simulations of formic acid conversion vs C0P/C0F ratio for (a) CFe3+ ) 1.0 × 10-3 M and (b) C0F ) 2.2 × 10-3 M. Key: (s) Fenton reaction, (- - -) photo-Fenton reaction. Table 4. Comparison between Fenton and Photo-Fenton Conversions
6. Comparison between Fenton and Photo-Fenton Pollutant Conversions To determine the effects of the UV radiation on the organic compound degradation, the reacting system was studied under the following operating conditions: (i) dark solution (Fenton reaction) and (ii) UV-irradiated solution (photo-Fenton reaction). Experimental pollutant conversions for a formic acid initial concentration (C0F) equal to 2.0 × 10-3 M and for two different hydrogen peroxide to formic acid initial concentration ratios (C0P/C0F ) 3.1 and 6.7) were obtained. A smaller total liquid volume (V ) 1000 cm3) and a reaction time of 60 min were also used. It should be noted that the ratio of the photoreactor volume (photo-Fenton) to the total liquid volume (Fenton) is just 0.07. The kinetic and reactor models presented in sections 2 and 3 were used to predict pollutant conversions at the same specified operating conditions. Model predictions and experimental data are illustrated in Table 4. Two important conclusions may be obtained from these results:26 (i) under the adopted operating conditions, the photo-Fenton system produces an organic pollutant conversion up to 63.3% greater than that obtained with the dark system and (ii) a higher increase in pollutant conversion is reached when the hydrogen peroxide to formic acid initial concentration ratio is low. It should also be noted that theoretical and experimental results show good agreement, with the maximum error being 2.6% for the photo-Fenton reaction and the highest hydrogen peroxide to formic acid concentration ratio (C0P/C0F ) 6.7). In the case of the Fenton system, the maximum deviation is 2.2% for the lowest hydrogen peroxide to formic acid concentration ratio (C0P/C0F ) 3.1).
pollutant conversion (%) conversion C0P/C0F error photo- error enhancement ratio Fenton (%) Fenton (%) (%) experimental data model predictions experimental data model predictions
3.1 3.1 6.7 6.7
31.2 30.5 43.1 43.0
2.2 0.2
49.2 49.8 65.2 63.5
1.2 2.6
57.8 63.3 51.3 47.7
To further proceed with the analysis of the conversion enhancement when UV irradiation is used, a parametric study was carried out to compare Fenton and photoFenton pollutant conversions under different operating conditions. In our computer simulations, we have investigated the effect of three significant parameters of the system on the formic acid conversion (XF): the hydrogen peroxide to formic acid initial concentration ratio (C0P/C0F), the formic acid initial concentration (C0F), and the ferric ion concentration (CFe3+). To do this, the following two examples have been considered: (i) a constant ferric ion concentration for three values of the formic acid initial concentration (Figure 5a) and (ii) a constant formic acid initial concentration for three ferric ion concentrations (Figure 5b). Figure 5a shows XF (after a reaction time t ) 1 h) as a function of C0P/C0F, for CFe3+ ) 1.0 × 10-3 M and for C0F ) 1.1, 2.2, and 3.3 × 10-3 M. As a reference, the Fenton pollutant conversion for C0F ) 2.2 × 10-3 M is also plotted (solid line). Note that only one curve is plotted for the Fenton system because, within the range of investigated conditions, no important changes of Fenton conversions were observed when C0F was modified between 1.1 and 3.3 × 10-3 M. For both reaction types, one can observe that increasing the ratio (C0P/C0F) increases the pollutant conversion and that at very high values of (C0P/C0F) the conversion reaches a sort of
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Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002
Table 5. Computed Results of Fenton Conversion, Photo-Fenton Conversion, and Conversion Enhancement (VR/V ) 0.07)
C0P/C0F 5 20 5 20
photoFenton Fenton conversion C0F CFe3+ conversion conversion enhancement (×103+ M) (×103+ M) (%) (%) (%) 1.1 2.2 3.3 1.1 2.2 3.3 2.2 2.2 2.2 2.2 2.2 2.2
1.0 1.0 1.0 1.0 1.0 1.0 0.5 1.0 2.0 0.5 1.0 2.0
38.2 38.2 38.2 54.3 54.3 54.3 20.9 38.2 63.1 31.9 54.3 80.1
64.6 55.5 51.5 66.2 62.1 60.3 36.3 55.2 77.8 40.4 62.1 84.7
69.1 45.3 34.8 21.9 14.4 11.1 73.7 44.5 23.3 26.7 14.3 5.7
plateau. Note that values of C0P/C0F higher than approximately 10 give a nearly constant formic acid conversion for each C0F. On the other hand, comparing the Fenton and photo-Fenton pollutant conversions, one may notice that (i) for a constant value of the ratio (C0P/C0F), the pollutant conversion for the photo-Fenton reaction is always higher than that obtained with the Fenton system and (ii) the conversion enhancement increases as C0F and C0P/C0F are decreased. Table 5 illustrates these results. Figure 5b reports computed values of the XF (after a reaction time t ) 1 h) vs the ratio C0P/C0F, for C0F ) 2.2 × 10-3 M and for CFe3+ ) 0.5 × 10-3, 1.0 × 10-3, and 2.0 × 10-3 M. Once more, the computed results corresponding to the Fenton formic acid conversion are included as reference values (solid lines). After a ratio (C0P/C0F) of about 10, XF reaches some form of saturation, mainly for the photo-Fenton process. It is also observed that the degradation ability of the Fenton reaction may be significantly enhanced using UV radiation. Comparing the Fenton and photo-Fenton processes in this plot, one may conclude that the conversion enhancement increases as the concentration ratio (C0P/C0F) and the ferric ion concentration (CFe3+) are decreased. However, it is interesting to notice that low pollutant conversions are obtained if low values of CFe3+ and C0P/C0F are used. Table 5 provides computed results of Fenton and photoFenton conversions and of conversion enhancement for different values of C0P/C0F and CFe3+. It seems that, to achieve a significant formic acid conversion enhancement and, simultaneously, a rather high pollutant conversion, the following operating conditions should be used for this particular reactor arrangement: 5 e C0P/C0F e 10 and CFe3+ ≈ 1.0 × 10-3 M. 7. Conclusions To summarize, the main conclusions are as follows: (1) Kinetic and reactor models have been developed to describe the degradation of formic acid in an acidic aqueous solution (pH ) 3), using the photo-Fenton reaction. The complete kinetic expression includes both thermal (dark) and photochemical formic acid degradation rates. The kinetic representation permits one to properly describe the organic pollutant transformation in both dark (Fenton) and UV-irradiated (photo-Fenton) regions inside the reacting system. (2) Under the adopted operating conditions, an organic pollutant conversion up to 80% has been achieved after 2 h of operation.
(3) Model predictions and experimental results show that UV irradiation significantly improves the effectiveness of the Fenton reaction. Within the range of investigated variables and for a photoreactor to total liquid volume ratio of 0.07, an organic pollutant conversion 63% greater than that obtained with the dark system was obtained. Furthermore, a higher increase in formic acid conversion was observed when the hydrogen peroxide/organic compound concentration ratio was low. (4) When predictions of the kinetic model are compared with experimental data, a good representation of the formic acid and hydrogen peroxide concentrations versus time data, for a rather wide range of their initial concentrations, was obtained. Deviations between model predictions and experimental results were always smaller than 9%. (5) A parametric study employed to compare Fenton and photo-Fenton processes under different operating conditions has shown that the conversion enhancement increases when (i) the formic acid initial concentration is decreased and (ii) the hydrogen peroxide to formic acid concentration ratio and the ferric ion concentration are decreased. Acknowledgment The authors are grateful to Universidad Nacional del Litoral (CAI+D Nο. 116/17), Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (PIP Nο. 200), and Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (PICT-99 Nο. 06955) for their support to produce this work. They also thank Eng. Claudia M. Romani for technical assistance. Appendix: Mass Balance for the Well-Stirred Reactor Placed inside a Batch Recycling System Starting from the local mass balance for the i component in a well-mixed, batch reactor, we can integrate over the whole liquid volume of the system (V) and apply the divergence theorem to the molar flux term:
∫V
∂Ci(x,t) dV + ∂t
∫ANi(x,t)‚m dA ) ∫VRi(x,t) dV
(A.1)
Considering that molar diffusive fluxes are null and that the batch recycling reactor is closed (no inlet or outlet streams), the second term on the left-hand side of eq A.1 becomes zero. The integrals on the first term of the left-hand side and on the right-hand side of eq A.1 may be divided into two terms: (i) for the irradiated liquid volume (VR) and (ii) for the nonirradiated liquid volume (V - VR). Thus
∫V
∂Ci(x,t) dV + R ∂t
∂C (x,t)
∫V-V i∂t dV ) ∫V Ri(x,t) dV + ∫V-V RTi (x,t) dV
(A.2)
∫V Ci(x,t) dV + dtd ∫V-V Ci(x,t) dV ) ∫V Ri(x,t) dV + ∫V-V RTi (x,t) dV
(A.3)
R
R
d dt
R
R
R
R
R
Using the mean value theorem, we can define the following averaged functions:
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1443
〈Ci(x,t)〉VR ) 〈Ci(x,t)〉V-VR )
∫V Ci(x,t) dV
1 ) V - VR
1 VR
(A.4)
R
1 V - VR
〈Ri(x,t)〉VR ) 〈RTi (x,t)〉V-VR
1 VR
∫V-V Ci(x,t) dV R
∫V Ri(x,t) dV
(A.5) (A.6)
R
∫
dV )
RTi (t)
(A.7)
[
]
V - VR d VR 〈Ci(x,t)〉VR + 〈Ci(x,t)〉V-VR ) dt V V VR V - VR T 〈Ri(x,t)〉VR + Ri (t) (A.8) V V Consider the second term of the left-hand side of eq A.8; in a well-stirred tank, Ci is uniform and can be taken out of the averaged value. Furthermore, because VR , V and the conversion per pass is very small, one can write27,28
| ]
V - VR d VR 〈C (x,t)〉VR + Ci(t) V-VR = dt V i V dCi(t) d V - VR Ci(t) = (A.9) dt V dt
[
Superscripts 0 ) initial condition T ) thermal rate Special Symbol
RT(t) V-VR i
In eq A.7 we have considered that the thermal reaction rate (RTi ) is not a function of position. Then, substituting eqs A.4-A.7 into eq A.3 and dividing by the total liquid volume of the system (V), we obtain
[
R ) a reactor property T ) a tank property W ) a reactor wall property λ ) indicates a dependence on wavelength
]
Substituting eq A.9 into eq A.8, we finally obtain
V - VR T dCi(t) VR ) 〈Ri(x,t)〉VR + Ri (t) dt V V
(A.10)
Nomenclature C ) molar concentration, mol cm-3 ea ) local volumetric rate of photons absorption (LVRPA), einstein cm-3 s-1 fλ ) normalized spectral distribution of the lamp output power k ) kinetic constant, M-1 s-1 L ) reactor depth, cm Ni ) molar flux of component i, mol cm-2 s-1 q ) radiative flux, einstein cm-2 s-1 R ) reaction rate, mol cm-3 s-1 t ) time, s V ) volume, cm3 x ) spatial coordinate, cm X ) conversion Greek Letters R ) molar absorptivity, m2 mol-1 κ ) volumetric absorption coefficient, cm-1 λ ) wavelength, nm Φ ) primary quantum yield, mol einstein-1 Subscripts F ) relative to formic acid Fe2+ ) relative to ferrous ion Fe3+ ) relative to ferric ion i ) relative to species i P ) relative to hydrogen peroxide
〈 〉 ) average value
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Received for review August 21, 2001 Revised manuscript received November 20, 2001 Accepted November 26, 2001 IE010696K
