Photochemical mineralization of di-n-butyl phthalate with H
2
O
2
/Fe
3+
Chyow-San Chiou
a,∗, Yi-Hung Chen
b, Chang-Tang Chang
a,
Ching-Yuan Chang
c, Je-Lueng Shie
a, Yuan-Shan Li
aaDepartment of Environmental Engineering, National I-Lan University, 1, Section 1, Shen-Lung Road, I-Lan 260, Taiwan bDepartment of Chemical and Material Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan
cGraduate Institute of Environmental Engineering, National Taiwan University, Taipei, Taiwan
Received 21 July 2005; received in revised form 15 November 2005; accepted 28 November 2005 Available online 6 January 2006
Abstract
This study evaluated the performance of photo-Fenton reaction initiated by the UV irradiation with H2O2/Fe3+, denoted as UV/H2O2/Fe3+, to
decompose di-n-butyl phthalate (DBP) in the aqueous solution. The concentration of total organic carbon (TOC) was chosen as a mineralization index of the decomposition of DBP by the UV/H2O2/Fe3+process. A second-order kinetic model with respect to TOC was adequately adopted
to represent the mineralization of DBP by the UV/H2O2/Fe3+process. The experimental results of this study suggested that the dosages with
4.74× 10−5mol min−1L−1H2O2and initial Fe3+loading concentration of 4.50× 10−4mol L−1in the solution at pH 3.0 with 120W cm−2UV
(312 nm) provided the optimal operation conditions for the mineralization of DBP (5 mg L−1) yielding a 92.4% mineralization efficiency at 90 min reaction time.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Photo-Fenton; Mineralization; UV/H2O2/Fe3+; Di-n-butyl phthalate
1. Introduction
Phthalic acid esters (PAEs) are widely used as plasticiz-ers in different resins, especially PVC resin[1], and important additives in special paints and adhensives. The United States Environmental Protection Agency (USEPA) [2] and some of its international counterparts have classified the most common PAEs as priority pollutants and as endocrine-disrupting com-pounds.
PAEs, especially di-n-butyl phthalate (DBP) [3], have become widespread in the environment as they have been found in sediments, waters and soils [4,5], as a result of their low water solubility and high octanol/water partition coefficients, PAEs tend to accumulate in the soil or sediment and in the biota living in the phthalate containing waters. DBP also is a rather stable compound in the natural environment, and the toxic prop-erties of which are even more important with the consideration of its high bioaccumulation rate in different organisms[4]. Huang et al. [6] found that some of PAEs, such as DBP and DEHP
∗Corresponding author. Tel.: +886 3 9357400x747; fax: +886 3 9359674.
E-mail address: chiou33@ms7.hinet.net (C.-S. Chiou).
(di-2-ethylhexyl phthalate), were the main refractory organic compounds in municipal wastewater. In conventional activated sludge plants, a large number of these organic pollutants are diffi-cult to be degraded when passed through the treatment facilities. The application of Fenton reaction to produce hydroxyl radi-cals for decomposing organic pollutants has attracted extensive attention[7]. It has been successfully applied to degrade the aro-matic compounds[8–10]. Through the function of the agent of Fenton reaction, which is a mixture of H2O2with iron salts, can
generate the strong oxidative hydroxyl radicals,•OH according to the following reactions[11]:
Fe2++ H2O2→ Fe3++•OH + OH−, k1= 76 M−1s−1
(1) Fe3++ H2O2→ Fe2++ HO2• + H+, k2= 0.02 M−1s−1
(2) The main short comes of Fenton reaction are that Fe2+ and H2O2must be added continuously to keep the reaction
proceed-ing and produced the large volume of wasted iron hydroxide sludge. In order to overcome the sludge problem and enhance the Fenton reactions, a photo-Fenton process[12,13]was
devel-0304-3894/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2005.11.072
oped by introducing ultraviolet (UV) light to a Fenton process. By applying UV light, Fe2+can be regenerated via the following photo-reactions[14]:
Fe(OH)2++ hν → Fe2++•OH (3) Fe(OH)63++ hν → Fe2++•OH + H+ (4)
Reduction of Fe3+to Fe2+is advantageous to proceed the Fen-ton reaction repeatedly without continuously adding Fe2+, and reduce the amount of iron hydroxide sludge. Furthermore, the reaction mechanism of Eqs.(3)and(4)also generate the strong oxidant,•OH to mineralize the target organic compounds. More-over, a hydrogen peroxide can produce two hydroxyl radicals initiated by UV light, according to Eq.(5):
H2O2+ hν → 2•OH (5)
Among these routes, Eqs. (3)–(5) facilitate the formation of hydroxyl radicals and promote the decomposition rates of organic compounds. Many previous studies had shown that the decomposition of various refractory organic compounds[15–20]
via the photo-Fenton process had been proven to be very effec-tive.
The present study assessed the function of UV light on the enhancing H2O2/Fe3+, denoted as UV/H2O2/Fe3+process,
to mineralize DBP. The concentration of total organic carbon (TOC) was chosen as a mineralization index of the decomposi-tion of DBP by this UV/H2O2/Fe3+process. The effects of pH
value, UV light intensity, H2O2dosage rate and Fe3+
concentra-tion on the mineralizaconcentra-tion of DBP were examined. The related kinetic equations were also established based on the observed experimental results.
2. Materials and methods
Di-n-butyl phthalate, DBP (C16H22O4, MW = 278), with
high quality was purchased from Acros Organics (Belgium) and used without any further purification. Hydrogen peroxide (35%) and ferric nitrate were supplied by Shimakyu Co. (Japan) and Merck, respectively. The TOC of sample was analyzed by a TOC analyzer (Tekmar Dohrmann Phoenix 8000). This instru-ment utilizes the UV-persulfate technique to convert the organic carbon into carbon dioxide (CO2) analyzed by an infrared CO2
analyzer and calibrated with the potassium hydrogen phthalate. The relative standard deviation of TOC analyses of DBP samples with triple measurements are below 5%.
Batch experiments of the oxidation process were conducted in a 2 L well-mixed round-bottle glass flask with water jacket (Fig. 1). The UV irradiation source was two 8 W lamps encased in a quartz tube with wavelengths of 312 nm. UVX Radiometer (UVP Inc., USA) was employed for the determination of UV light intensity. The UV intensity of one 8 W UV lamp at 312 nm is 60W cm−2. A variable speed motor connected to a stainless steel shaft provided the mixing medium. A pH controller was used to control pH value of solution by adding 0.1 M HNO3or
NaOH solution into the reactor. Hydrogen peroxide was supplied by a syringe pump with a constant feed rate.
Fig. 1. The experimental apparatus sketch. Components: (1) syringe pump; (2) reaction vessel; (3) UV lamp; (4) stirrer; (5) pH meter; (6) thermostate.
The effect of pH value of solution on the system perfor-mance was studied at various pH values of 2.0, 2.5, 3.0, 3.5 and 4.0 with the constant dosage rate of H2O2(dCH2O2/dt) at 3.15× 10−5mol min−1L−1 and the initial loading con-centrations of Fe3+ (CFe3+,0) of 3.60× 10−4mol L−1 and DBP of 5 mg L−1, respectively. The influence of the UV irradiation on the system performance was evaluated with and without UV at pH value of 3.0. The study of the effect of CFe3+,0 was evaluated at various initial concentra-tions of 9.01× 10−5, 1.80× 10−4, 2.70× 10−4, 3.60× 10−4, 4.50× 10−4and 5.41× 10−4mol L−1with 120W cm−2UV intensity (312 nm), 3.15× 10−5mol min−1L−1 hydrogen per-oxide and 5 mg L−1 DBP at pH value of 3.0. The effect of hydrogen peroxide was examined by dosing hydrogen peroxide at various dosage rates (3.95× 10−6, 7.90× 10−6, 1.58× 10−5, 3.15× 10−5, 4.74× 10−5and 5.54× 10−5mol min−1L−1) into the reactor. The temperature for the all experiments was main-tained at 298 K.
3. Results and discussion 3.1. Effects of pH value
The pH value was reported as a parameter that affects the performance of the Fenton reaction [21], and the species concentration of Fe3+–hydroxyl complex in aqueous solu-tion. Therefore, the effect of pH value on the destruction of DBP via the photo-Fenton reaction was investigated. As shown in Fig. 2, the mineralization efficiency of TOCDBP, ηTOC,DBP= (TOC0− TOC)/TOC0, increases with the decrease
of pH to pH value of 3.0, but would decrease with the further decreasing of the pH values. Previous studies on the Fenton pro-cess[22,23]reported that the optimal pH value condition for a Fenton reaction is at pH 3, suggesting that H+in the solution at low pH values would consume•OH via Eq.(6)and form H2O
[14]:
•OH+ H++ e−→ H
2O, k = 7 × 10−9M−1s−1 (6)
Hence, the excess H+under low pH condition, pH < 3, decreases
com-Fig. 2. Dependence of mineralization of DBP on time at various pH values adjusted by HNO3. Experimental conditions: initial
concentra-tions of DBP (CDBP0) = 5 mg L−1, and initial concentrations of Fe3+
(CFe3+,0)= 3.60 × 10−4mol L−1, dosing rate of H2O2(dCH2O2/dt) = 3.15 ×
10−5mol min−1L−1, UV intensity (λ312) = 120W cm−2and T = 298 K.
pound. Moreover, Fe(OH)2+is the dominant species among the Fe3+–hydroxyl complex in the aqueous solution from pH 2.5 to 5, and the highest quantity of Fe(OH)2+ is obtained at pH 4
[14]. The concentration of Fe(OH)2+decreases with pH values beyond or below 4, and the lower quantity of Fe(OH)2+results lower concentration of Fe2+ and•OH via Eq.(3). As pH < 3, the concentration of Fe(OH)2+ decreased with decreasing pH value of aqueous solution which subsequently contributed poor
ηTOC,DBP.
Furthermore, the lowerηTOC,DBPvalue for solution at pH > 3 might be due to that the oxidation potential of •OH was decreased with increasing pH [24], and the formation of the precipitation of iron hydroxide (pH > 4). Thus, the optimal pH value for the effective mineralization of DBP was found to be 3.0, and the pH value of solution was maintained at 3.0 for the following experiments.
3.2. Mineralization efficiency of DBP under various conditions
The wavelength of radiation is also an important parame-ter for the production of Fe2+ and hydroxyl radical from the Fe3+–hydroxyl complex in aqueous solution via the photolytic reaction[14,25,26]. As Fe(OH)2+is the dominant species in the aqueous solution from pH 2.5 to 5[14], and the wavelength of UV radiation from 290 to 400 nm will photolysis Fe(OH)2+to produce Fe2+and hydroxyl radical based on the charge transfer band of Fe(OH)2+as indicated in Eq.(3). Besides, according to the study of Pignatello[27], degradation of organic compound by Fe3+/H2O2was imposingly accelerated by irradiation with
light wavelength above 300 nm. Therefore, the wavelength of UV radiation utilized in this study must be during the range from 300 to 400 nm. In general, the most easily available UV lamps with the maximum emitting wavelength during the range from 300 to 400 nm are 312 and 360 nm. Furthermore, Pig-natello[27]also indicated that radiation wavelength at 312 nm
Fig. 3. Dependence of mineralization of DBP on time at various conditions. Experimental conditions: (case a) UV (λ312) = 120W cm−2, dCH2O2/dt =
3.15 × 10−5mol min−1L−1; (case b) UV (λ312) = 120W cm−2, CFe3+,0= 3.60 × 10−4mol L−1; (case c) dCH2O2/dt = 3.15 × 10−5mol min−1L−1,
CFe3+,0= 3.60 × 10−4mol L−1; (case d) UV (λ312) = 60W cm−2,
dCH2O2/dt = 3.15 × 10−5mol min−1L−1, CFe3+,0= 3.60 × 10−4mol L−1;
(case e) UV (λ312) = 120W cm−2, dCH2O2/dt = 3.15 × 10−5mol min−1L−1,
CFe3+,0= 3.60 × 10−4mol L−1. The other conditions are CDBP0= 5 mg L−1,
T = 298 K and pH 3.0.
obtained higher quantum yield than 360 nm. Moreover, the direct photolysis of H2O2 generates•OH according to Eq.(5)
which requires UV radiation wavelength < 360 nm[28]. There-fore, UV radiation wavelength at 312 nm was investigated in this study.
Fig. 3presents ηTOC,DBP as a function of time depending on the various conditions. TheηTOC,DBPof the cases (a and b) inFig. 3were 22.3% and 26.1%, respectively, in 60 min reac-tion time (t) only with sole H2O2or Fe3+under UV irradiation
(denoted as UV/H2O2and UV/Fe3+). Though the hydroxyl
rad-ical could be formed from Eqs.(5), and(3)by UV irradiation, respectively, but the ineffectiveηTOC,DBPmight be attributed to the low amount of•OH production from these equations, and the extremely chemical stability of DBP.
The case (c) inFig. 3presented the mineralization of DBP with adding Fe3+and H
2O2but without UV irradiation, denoted
as H2O2/Fe3+, revealing ηTOC,DBP of 46% at t = 60 min. It is
apparent that the mineralization of DBP was achieved by HO• and HO2• produced from the interaction between Fe3+ and
H2O2. From Eq. (2), HO2• and Fe2+ could be generated and
the generated Fe2+could be utilized to proceed Eq.(1)and pro-duced HO•resulting a better mineralization efficiency than the cases (a and b) inFig. 3.
Finally, the cases (d and e) inFig. 3illustrated the mineraliza-tion of DBP by photo-Fenton reacmineraliza-tion, UV/H2O2/Fe3+, under
different light intensities with the values ofηTOC,DBPas 69.6% and 76.5% at t = 60 min, respectively. The experimental results showed that the introduction of UV radiation results in the more effective effect on the enhancement of ηTOC,DBP compared with the cases (a–c) inFig. 3. Furthermore, the stronger light intensity causes higher mineralization efficiency. The improve-ment of the mineralization efficiency in the UV/H2O2/Fe3+
process is significant by the photo-reduction of Fe3+ to Fe2+, which could react with H2O2establishing a cycle mechanism of
generating hydroxyl radicals. As a result, the mineralization effi-ciency of DBP under various conditions followed the sequence: UV(120W cm−2)/H2O2/Fe3+> UV(60W cm−2)/H2O2/Fe3+
> H2O2/Fe3+> UV(120W cm−2)/Fe3+> UV(120W cm−2)/
H2O2 with the constant dosage rate of H2O2 at 3.15×
10−5mol min−1L−1 and the initial loading concentration of Fe3+of 3.60× 10−4mol L−1.
3.3. Effects of H2O2and Fe3+
The previous works[29,30]reported that there should have an appropriate ratio of H2O2to Fe2+to achieve the optimal
treat-ment efficiency on the organic compounds. However, the optimal ratio depends on the characteristics of the organics. The photo-Fenton process in this study, Fe2+was supplied by the photolysis of Fe3+–hydroxyl complex with a constant rate. Hence, a prop-erly continuous addition of H2O2 is required to maintain an
adequate ratio of H2O2/Fe2+ for processing Eq.(1) with the
supplement of Fe2+from the photolysis of Fe3+–hydroxyl com-plex.
The effect of initial loading concentrations of Fe3+(CFe3+,0) on the mineralization of DBP was shown inFig. 4. The results revealed that a largerCFe3+,0 up to 4.50× 10−4mol L−1 gen-erally gave a higherηTOC,DBP due to the higher aqueous Fe2+ concentration from the photolysis of Fe(OH)2+ which primar-ily increases with increasing the initial concentration of Fe3+. A higherCFe3+,0 can produce more Fe2+ via Eq.(3)and then bring more hydroxyl radicals for the mineralization of DBP via Eq.(1). However, when theCFe3+,0 increased higher than 4.50× 10−4mol L−1, theηTOC,DBP would become lower. It is because that the•OH and•O2H in the solution would be
con-sumed by the excess Fe3+and Fe2+in the solution as indicated in Eqs.(7)and(8) [22], leading the lower photo-Fenton oxidation
Fig. 4. Time variation of mineralization of DBP at variousCFe3+,0. Experi-mental conditions: UV (λ312) = 120W cm−2, CDBP0= 5 mg L−1, dosing rate
of H2O2(dCH2O2/dt) = 3.15 × 10−5mol min−1L−1, T = 298 K and pH 3.0.
Fig. 5. Dependence of mineralization of DBP on time at various H2O2
dos-ing rates (dCH2O2/dt). Experimental conditions: UV (λ312) = 120W cm−2,
CDBP0= 5 mg L−1,CFe3+,0= 4.50 × 10−4mol L−1, T = 298 K and pH 3.0. efficiency: Fe2++•OH → Fe3++ OH−, k3= 4.3 × 108M−1s−1 (7) Fe3++ HO2• → Fe2++ O2+ H+, k5= 1.0 × 104M−1s−1 (8) The effect of various dosing rates of H2O2(dCH2O2/dt) on
ηTOC,DBP was shown in Fig. 5. At higher H2O2
concentra-tions, the production of •OH would increase being beneficial to the mineralization of DBP.ηTOC,DBPincreased with increas-ing dCH2O2/dt from 3.95 × 10−6to 4.74× 10−5mol min−1L−1 but then decreased when the value of dCH2O2/dt was greater than 4.74× 10−5mol min−1L−1. Note that the reaction rate constant between H2O2 and•OH as Eq.(9) [31]is as high as
2.7× 107M−1s−1.
H2O2+•OH → H2O + HO2•, k5= 2.7 × 107M−1s−1
(9) Thus, the reaction via Eq. (9)becomes the predominant reac-tion in comparison with the reacreac-tion between •OH and DBP in the condition of excess H2O2. Hence, the excess
concen-tration of H2O2 being as the scavenger for the •OH would
reduce the mineralization efficiency. As indicated in this study, the optimal operation conditions for the mineralization of DBP are dCH2O2/dt of 4.74 × 10−5mol min−1L−1 andCFe3+,0 of 4.50× 10−4mol L−1 in the solution at pH 3.0 based on the experimental results.
3.4. Kinetic studies of DBP mineralization
The reaction kinetics of a photo-Fenton reaction on the min-eralization of DBP via UV irradiation with H2O2/Fe3+can be
described as: dC
dt = −kC
where C, m, t, and k represent the TOC concentration of the DBP solution, order of the reaction, time, and reaction rate constant, respectively. For a second-order reaction, Eq.(10)becomes:
1
C =
1
C0+ kt (11)
where C0is the initial TOC concentration of the DBP solution.
Consider theηTOC,DBPincreasing withCFe3+,0and dCH2O2/ dt with the range of 9.01 × 10−5 to 4.50× 10−4mol L−1 and 3.95× 10−6 to 4.74× 10−5mol min−1L−1 as shown in
Figs. 4 and 5, respectively. Fig. 6(a) shows the plots at var-ious CFe3+,0 with dCH2O2/dt of 3.15 × 10−5mol min−1L−1.
Fig. 6(b) presents the plots at various dCH2O2/dt for the solution atCFe3+,0= 4.50 × 10−4mol L−1. The results revealed reason-ably good linear fits based on the second-order kinetic model as Eq.(11).
The slope associated with the k value varying with dCH2O2/dt andCFe3+,0 inFig. 6 can be further illustrated inFig. 7. The linear relationships of ln k with lnCFe3+,0and (ln(dCH2O2/dt))
Fig. 6. Analysis of mineralization kinetics of DBP simulated by second-order reaction. (a) Cases with various BOF slag loadings (CFe3+,0) at a constant H2O2
dosing rate of 3.15× 10−5mol min−1L−1. The values of r2of regression lines
of, , , , ♦: 0.978, 0.952, 0.942, 0.966, 0.963. (b) Cases with various H2O2dosing rates at a constantCFe3+,0= 4.50 × 10−4mol L−1. The values of r2of regression lines of, , , , ♦: 0.982, 0.984, 0.906, 0.940, 0.984.
Fig. 7. The relationship between k withCFe3+,0and dCH2O2/dt. Experimental
conditions: UV (λ312) = 120W cm−2, T = 298 K, pH 3.0. (a) ln k vs. lnCFe3+,0 at a constant dCH2O2/dt = 3.15 × 10−5mol min−1L−1, r
2= 0.959. (b) ln k vs.
ln dCH2O2/dt at a constant CFe3+,0= 4.50 × 10−4mol L−1, r2= 0.947. Units of k, dCH2O2/dt, CFe3+,0are in (mg-C L−1)−1min−1, mol min−1L−1, mol L−1,
respectively.
then gave the following correlation:
k = k0(CFe3+,0)x dCH2O2 dt y (12) In Eq.(12),CFe3+,0, dCH2O2/dt, x, and y are the initial loading concentration of Fe3+ (mol L−1), molar dosing rate of H2O2
(mol min−1L−1), orders of concentration dependence ofCFe3+,0 and dCH2O2/dt, respectively. The values of x and y obtained from the slopes ofFig. 7(a and b) were 1.29 and 0.76, respectively. As a result, the k0in Eq.(12)can be obtained by plotting the
obtained k against (CFe3+,0)1.29d(CH2O2/dt)
0.76
, and then gives
k0 of 2.01× 106 ((mg-C L−1)−1 min−1) (mol-Fe3+ L−1)−1.29 (mol-H2O2min−1L−1)−0.76at 298 K.
4. Conclusions
The major results of applying UV/H2O2/Fe3+process to
1. The rank of treatment conditions based on the min-eralization efficiency of DBP has the sequence: UV(120W cm−2)/H2O2/Fe3+> UV(60W cm−2)/H2O2/
Fe3+> H2O2/Fe3+> UV (120W cm−2)/Fe3+> UV (120
W cm−2)/H2O2.
2. The experimental results in this study suggest that the condi-tion with the UV irradiacondi-tion 120W cm−2 UV (312 nm), H2O2 dosages of 4.74× 10−5mol min−1L−1, and initial
loading concentration of Fe3+of 4.50× 10−4mol L−1in the solution at pH 3.0 provides the optimal operation perfor-mance for the mineralization of DBP (5 mg L−1) of 92.4% at 90 min reaction time.
3. The observed experimental data showed a reasonably good expression of the second-order kinetic model with respect to DBP in terms of TOC: dC/dt =−kC2. The reaction rate of mineralization of DBP in terms of TOC (k) can be further correlated to the molar dosing rate of H2O2 and initial loading concentration of Fe3+
by power expressions, yielding k ((mg-C L−1)−1min−1)= 2.01 × 106(CFe3+,0)1.29(dCH2O2/dt)
0.76 where the units of CFe3+,0and dCH2O2/dt are in mol L−1and mol min−1L−1, respectively.
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