Photo-Fenton oxidation of phenol with Fe(III)-tetra-4- carboxyphenylporphyrin/SiO2 assisted with visible light

Article abstract of DOI:10.1016/j.jphotochem.2014.08.004

In this work we studied the photo-Fenton oxidation of phenol in aqueous solution using tetra(4-carboxyphenyl)porphyrin iron (III) adsorbed on silicon dioxide (TCPPFe/SiO₂) under irradiation of visible light for the generation of hydroxyl radicals. The TCPPFe/SiO₂ was synthesized by the Alder's method and, it was characterized by spectrophotometry UV-vis, FT-IR and electron paramagnetic resonance (EPR). The hydroxyl radicals were detected by chemical trapping through indirect fluorescence of the 2-hydroxyterephthalic acid. The secondary products of the phenol photo-oxidation were analyzed by High Pressure Liquid Chromatography (HPLC) where hydroquinone, catechol and benzoquinone were identified as main products. Finally, we determined and compared the kinetic parameters of the photo-oxidation process using a pseudo-first order model.

Full text of DOI:10.1016/j.jphotochem.2014.08.004

Journal of Photochemistry and Photobiology A: Chemistry 294 (2014) 75–80
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photo-Fenton oxidation of phenol with
Fe(III)-tetra-4-carboxyphenylporphyrin/SiO2 assisted with visible
light
Carlos E. Diaz-Uribe∗, William A. Vallejo L., Jarol Miranda
Grupo de Investigación en Fotoquímica y Fotobiología, Programa de Química, Facultad de Ciencias Básicas, Universidad del Atlántico, Kilómetro 7 Vía
Puerto Colombia, Barranquilla, Colombia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2014
Received in revised form 5 August 2014
Accepted 7 August 2014
Available online 17 August 2014
In this work we studied the photo-Fenton oxidation of phenol in aqueous solution using tetra(4-
carboxyphenyl)porphyrin iron (III) adsorbed on silicon dioxide (TCPPFe/SiO2) under irradiation of visible
light for the generation of hydroxyl radicals. The TCPPFe/SiO2 was synthesized by the Alder’s method
and, it was characterized by spectrophotometry UV–vis, FT-IR and electron paramagnetic resonance
(EPR). The hydroxyl radicals were detected by chemical trapping through indirect fluorescence of the
2-hydroxyterephthalic acid. The secondary products of the phenol photo-oxidation were analyzed by
High Pressure Liquid Chromatography (HPLC) where hydroquinone, catechol and benzoquinone were
identified as main products. Finally, we determined and compared the kinetic parameters of the photo-
oxidation process using a pseudo-first order model.
Keywords:
Porphyrin
Hydroxyl radical
SiO2
© 2014 Elsevier B.V. All rights reserved.
Phenol
1. Introduction
Fenton reaction can be used to destroy a wide variety of organic
compounds: a ferrous ion reacts with hydrogen peroxide (H2O2) to
Phenols are a particular group of dangerous pollutants in
groundwater due to their mutagenic and carcinogenic effects in
humans [1,2]. Phenol is a typical pollutant from petrochemical,
chemical and pharmaceutical industries; it is highly toxic to the
aquatic ecosystem [3]. Biological, physical and chemical methods
can be used to treat wastewater contaminated with phenols [4,5].
Biological processes are generally delayed for practical applica-
tions and physical methods may only change the physical state
of the pollutant where the components are transferred from one
phase to another [6]. The chemical treatment of phenols (such as
chlorination) leads to the formation of chlorinated phenols and
their derivatives have been reported to be even more toxic than
phenols [7]. In recent years, the well known “advanced oxidation
processes” have emerged as an alternative to treat phenol pollut-
ants in water [8,9]. These procedures are not selective and they
can react with almost every chemical pollutant. Among these, the
form hydroxyl radical (HO•) which is a powerful oxidizing agent;
this reaction has been already used to degrade samples of phenol
[10,11]. The rate of this reaction could be increased when exposed
to UV–vis light. This method is known as photo-assisted Fenton
process and constitutes an attractive alternative of advance oxi-
dation process [12,13]. However, direct irradiation of the samples
(containing phenol) with UV light could also generate organic com-
pounds that are more toxic than phenol such as dibenzo-p-dioxins
[14]. Moreover, the Fenton homogeneous catalysis implies several
drawbacks in practical applications like high iron concentration and
a strict final separation process. These drawbacks increase the final
costs of the treatment. Photo-Fenton processes based on hetero-
geneous catalysis systems solve part of these problems providing
an easy separation and recovery of the catalyst from the treated
wastewater, wherein it is noncorrosive, and, besides, it is envi-
ronmentally friendly [15,16]. Being the UV radiation expensive for
practical applications, the usage of photosensitizers containing an
iron center in their chemical structures may serve as a source of
HO• radicals via a Fenton process.
∗
Corresponding author.
E-mail addresses: carlosdiaz@mail.uniatlantico.edu.co, carloslip5@hotmail.com
In this work, we have studied the photo-Fenton oxidation of
phenol in aqueous solution assisted by TCPPFe adsorbed on SiO2.
(C.E. Diaz-Uribe).
http://dx.doi.org/10.1016/j.jphotochem.2014.08.004
1010-6030/© 2014 Elsevier B.V. All rights reserved.

76
C.E. Diaz-Uribe et al. / Journal of Photochemistry and Photobiology A: Chemistry 294 (2014) 75–80
The adsorption TCPPFe was used for assisting the photo-Fenton
reaction under visible light as source reducing the costs of imple-
mentation of this technology in practical application.
2. Materials and methods
2.1. Reagents and equipment
All reagents used in this work were of analytical grade. The
UV–vis spectra were recorded in a Hewlett-Packard 8453 spec-
trophotometer. FT-IR spectra (KBr) were recorded in a Bruker
Tensor 27 spectrometer. EPR study of TCPP and TCPPFe was made
in a JEOL (JES-PE-3X) spectrometer. The samples were measured
in quartz tubes at normal conditions, with 100 kHz magnetic
field modulation, 20 mW microwave power and 5 G modulation
amplitude. Moreover, we used the X-band (9 GHz) in the EPR mea-
surements.
2.2. Preparation and characterization of the catalysts
2.2.1. Synthesis of TCPP
The TCPP was synthesized according to the methods already
described in the literature [17]. Pyrrole (30 mmol) was added to
a mixture of 4-carboxybenzaldehyde (30 mmol), propionic acid
(105 mL) and nitrobenzene (45 mL). The mixture was heated for
1 h at 120 ◦C; afterward, the solvent was removed under vacuum.
Then, the porphyrin was dissolved in 250 mL of 0.100 M NaOH and
was precipitated with a 1.0 M HCl solution. Finally, it was dissolved
in ethanol and recrystallized in a rotary evaporation system.
Fig. 1. Photoreactor (a) immersion well, (b) sampling device, (c) reactor body and
(gas inlet).
TCPPFe/SiO2 and 0.200 mL of H2O2 and a buffer solution to adjust
pH at four (4) different values (1.8, 2.4, 2.8 and 3.2). These reactions
were performed under visible light irradiation under the condi-
tions indicated in Section 2.3, in the batch photo-reactor shown
in Fig. 1. Finally, the identification and quantification of the oxi-
dation products were performed by HPLC (Agilent 1100) with a
diode array detector and Agilent Zorbax C18 column as station-
ary phase. Furthermore, we used an isocratic flow of the mobile
phase (70% acetonitrile and 30% water) rate 1.0 mL min−1. Phenol
and the aromatic intermediates were identified by adjusting the
UV detector at ꢀ = 210 nm (phenol, hydroquinone, catechol) and
245 nm (p-benzoquinone).
2.2.2. Synthesis of TCPPFe and adsorption on SiO2
The TCPPFe was synthesized as follows: the metal-free por-
phyrin (0.33 mmol) was mixed with iron (III) chloride hexahydrate
in 70 mL of N,Nꢀ-dimethylformamide (DMF) for 2 h in reflux system.
The DMF was removed by distillation and the TCPPFe was precipi-
tated in water. The precipitate was dissolved in 0.100 M NaOH and
recrystallized for adding 1.00 M HCl. Finally, the TCPPFe was fil-
tered and dried at room temperature. The TCPP and TCPPFe were
characterized by UV–vis and EPR spectroscopy. The adsorption of
TCPPFe on the SiO2 was performed as follows: TCPPFe was added
to a mixture (20 mL of absolute ethanol, 3 mL of tetraethoxysilane
(TEOS) and 0.5 mL of distilled water) at pH 3 (adjusted with HNO3),
the mixture was refluxed for 24 h, the solvent was evaporated and,
finally, the solid was dried at room temperature [18].
3. Results and discussion
2.3. Detection of hydroxyl radical
3.1. Preparation and characterization of the catalysts
For the generation of hydroxyl radical, 0.01 g of TCPPFe/SiO2
was added to 0.200 mL of H2O2 solution at pH 3; the mixture
was introduced in a batch photo-reactor in nitrogen atmosphere
under visible irradiation (100 W OSRAM halogen immersion lamp).
Fig. 1 shows the general scheme of photo-reactor. The light was fil-
tered through a 1.0 M potassium dichromate solution to remove
wavelengths < 500 nm. The incident photon flow per unit vol-
ume (Io = 5.8 × 10−5 Einstein L−1 s−1) was determined according to
chemical actinometry for using a K[Cr(NH3)2(CNS)]4 0.0100 M (Rei-
necke salt) solution [19]. The formation of hydroxyl radical was
detected monitoring the fluorescence emission spectrum of 2-
hydroxyterephthalic acid (excitation at 315 nm) in a Shimadzu
RF-5301PC spectrofluorometer.
3.1.1. UV–vis analysis
Fig. 2 shows the UV–vis spectrum of the TCPPFe and the TCPP in
ethanol, and it also shows a typical Soret band and four Q bands for
the TCPP. The band at 419 nm was assigned to the Soret band arising
from the transition of a1u(␲)–eg*(␲), and the other four absorp-
tion signals at (514, 548, 588 and 645 nm) were attributed to the
Q bands corresponding to the a2u(␲)–eg*(␲) transitions [20]. The
UV–vis spectra of the TCPPFe exhibited one Soret band and only
one Q band. This decrease in the number of Q bands is typical of
metalloporphyrins. When the metal ion coordinates with nitrogen
atoms the symmetry of the molecule increases; therefore the num-
ber of Q bands decreases. Moreover, the Soret band of TCPPFe was
shifted to blue (6 nm), the delocalized ␲ bonds decrease the aver-
age electron density of the metalloporphyrin allowing an increase
of the energy necessary for electronic transitions. This blue shift of
the Soret band also confirms the synthesis of the TCPPFe [21].
The absorption spectrum of TCPP adsorbed on SiO2 exhibited
a red shift with respect to the ethanol solution. This could be
explained because of the formation of hydrogen bonds between the
2.4. Oxidation of phenol
The oxidation of phenol was performed in nitrogen atmosphere
according to the following procedure: an aqueous solution of phe-
nol (100 mg L−1) was added to a solution containing 0.010 g of

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77
the delocalized ␲ electrons of porphyrin with the magnetic field
then this result is in accordance with that reported here. Fig. 3(b)
shows the EPR spectrum of TCPPFe. This spectrum shows three sig-
nals for the iron (III), high spin, and their spectroscopic factors (g).
The determined g factors in this work agree well with those for high
spin porphyrin-Fe(III) (the signal at g = 6.0 and 2.0 for axially sym-
metric species and g = 4.3 for rhombic symmetry) [24]. These results
confirm the results of UV–vis characterization and the synthesis of
the TCPPFe.
3.2. Identification of hydroxyl radical
We used the chemical trapping reaction of terephthalic acid
plus hydroxyl radical to form 2-hydroxyterephthalic acid. The flu-
orescence emission spectrum of the solution was measured during
illumination. Fig. 4 shows the fluorescence spectrum of the com-
pound generated by the presence of hydroxyl radicals and the
corresponding chemical reaction. This figure shows an increase
of the maximum emission band of the 2-hydroxyterephthalic
acid during the reaction time. Due to the fluorescent product (2-
hydroxyterephthalic acid) formed during irradiation are generated
due to the specific reaction between hydroxyl radicals and tereph-
thalic acid, these results corroborates that hydroxyl radicals were
generated by the TCPPFe under visible light irradiation.
Fig. 2. UV–vis spectra of TCPP (dark line) and TCPPFe (clear line) in ethanol solution.
carboxyl groups of the porphyrin and SiOH on the surface, which
affects the electronic structure of the ␲ system of porphyrin, as
was found in other previous studies [22,23]. The amount of TCPP
adsorbed on SiO2 is 0.21 mmol TCPP/g SiO2.
3.1.2. EPR analysis
3.3. Photo-oxidation process
The sensitizers were also characterized by EPR. Fig. 3(a) shows
the EPR spectra of TCPP; this spectrum exhibits only one signal.
The EPR signals of porphyrins are generated by the interaction of
3.3.1. Kinetic study
The molar fraction of phenol oxidized via a photo-Fenton pro-
cess was calculated using the following equation:
Pho − Ph(t)
Xremoval
=
(1)
Pho
where X is the molar fraction of degraded phenol, Pho is the initial
concentration and Ph(t) is the concentration of phenol depending
on time irradiation. The photo-oxidation of phenol was also studied
at four different pH values (1.8, 2.4, 2.8 and 3.2), and the results are
shown in Fig. 5. In the range of pH studied, the results indicate that
the pH greatly affects the degradation of phenol where the yield of
the process varied in between 5% and 20%. In addition, Fig. 5 reveals
that, the pH that yields the major oxidation of phenol via a photo-
Fenton process is 2.8, under the current experimental conditions
and range of pH studied. In fact, it is known that Fenton processes
are more efficient at pH close to 3 because the oxidation potential
of the hydroxyl radical adopts its highest value (2.80 V) [25]. The
profile of disappearance of phenol shown in Fig. 5 is similar profile
Fig. 4. The fluorescence emission spectra of 2-hydroxyterephthalate recorded at
different reaction times (insight: oxidation by hydroxyl radicals).
Fig. 3. EPR spectra of: (a) TCPP and (b) TCPPFe. The free radicals formed are stable
and were detected directly by EPR at room temperature.

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C.E. Diaz-Uribe et al. / Journal of Photochemistry and Photobiology A: Chemistry 294 (2014) 75–80
Fig. 7. Rate constants (k), percentage of degradation (% degradation) of the photo-
oxidation process at different pH values.
Fig. 5. Photo-Fenton oxidation of phenol at different pH values under experimental
conditions specified in Section 2.3.
hour of the reaction, catechol (CT), hydroquinone (HQ) and ben-
zoquinone (BQ) are the main products. After 1 h of irradiation, we
observed that the concentration of other reaction products (short
chain acids) increased in the reaction medium. This compound was
not separated and identified by HPLC. However, recent studies have
reported that the oxidation of phenols by hydroxyl radicals fin-
ishes with the formation of short chain acids such as cis-muconic,
succinic, oxalic, acetic and formic acids [29]. The breakdown of
hydrogen peroxide to form hydroxyl radicals activates the phe-
nol aromatic ring through a strong resonance electron-donating
effect. It is well known that this effect is felt most strongly at the
ortho and para positions. This fact was confirmed by our results;
hydroquinone catechol appeared in amounts sufficient to quan-
tify, along, low yield of benzoquinone was detected. Our results
indicated that ortho-hydroxylation was preferred over the meta
or para-substitution in phenol photo-oxidation in the presence of
hydrogen peroxide.
to a first-order kinetic decay [26]. This kind of kinetics follows the
equation:
[Ph(t)] = [Ph(o)]e−kt
(2)
where (t) represents the time in minutes and k is the apparent reac-
tion rate constant (min−1). The assumption of a pseudo-first order
model has been used in several studies for characterizing the effect
of different experimental conditions on the rate of degradation of
pollutants [27,28]. The data shown in Fig. 5 were fit to pseudo-first
order using Eq. (2). The rate constant (kꢀ) for each sample was cal-
culated from the slope of a linear fitting of the natural logarithm
of Ph(t) (Ln(Ph(t))) as function of time (see Fig. 6). Furthermore, the
determined rate constants and yield of the photo-oxidation pro-
cess are shown in Fig. 7. Under these experimental conditions, we
obtained both the highest rate constants and the highest yield of
photo Fenton oxidation process at a pH 2.8.
Different routes have been proposed for oxidation of phenol
in aqueous solution [30,31]. It is known that phenol degradation
occurs through a sequential step mechanism that may generate
multiple intermediates. For the photo-Fenton process studied in
this work, we propose a possible reaction mechanism as presented
3.3.2. Mechanism of reaction
The change in concentration of the oxidation products of the
photo Fenton oxidation process is shown in Fig. 8. In the first
Fig. 8. Change of the concentration of products from the photo-oxidation of phenol
under visible light using the TCPPFe(III)/SiO2 system.
Fig. 6. Ln(Ph(t)) vs (t) for test showed in Fig. 5.

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79
References
[1] A. Santos, P. Yustos, A. Quintanilla, F. Garcia-Ochoa, J.A. Casas, J.J. Rodríguez,
Evolution of toxicity upon wet catalytic oxidation of phenol, Environ. Sci. Tech-
nol. 38 (2004) 133–138.
[2] R. Shijin, Phenol mechanism of toxic action classification and prediction: a
decision tree approach, Toxicol. Lett. 144 (2003) 313–323.
[3] A. Adak, A. Pal, Removal of phenol from aquatic environment by SDS-
modified alumina: batch and fixed bed studies, Sep. Purif. Technol. 50 (2006)
256–262.
[4] A. Haddadi, M. Shavandi, Biodegradation of phenol in hypersaline conditions by
Halomonas sp. strain PH2-2 isolated from saline soil, Int. Biodeterior. Biodegrad.
85 (2013) 29–34.
[5] J. Shim, J.M. Lim, P.J. Shea, B.T. Oh, Simultaneous removal of phenol, Cu and
Cd from water with corn cob silica-alginate beads, J. Hazard. Mater. 15 (2014)
129–136.
[6] P.R. Gogate, A.B. Pandit, A review of imperative technologies for wastewater
treatment: I. Oxidation technologies at ambient conditions, Adv. Environ. Res.
8 (2004), 501-501.
[7] H. Dwivedi, C. Pande, Photochemical degradation of halogenated compounds:
a review, Sci. Revs. Chem. Commun. 2 (2012) 41–65.
[8] R. Alnaizy, A. Akgerman, Advanced oxidation of phenolic compounds, Adv.
Environ. Res. 4 (2000) 233–244.
[9] S. Esplugas, J. Giménez, S. Contreras, E. Pascual, M. Rodrıı´guez, Comparison of
different advanced oxidation processes for phenol degradation, Water Res. 36
(2002) 1034–1042.
[10] A. Babuponnusami, K. Muthukumar, Advanced oxidation of phenol:
a
comparison between Fenton, electro-Fenton, sono-electro-Fenton and photo-
electro-Fenton processes, Chem. Eng. J. 183 (2012) 1–9.
[11] S. Karthikeyan, R. Boopathy, V.K. Gupta, G. Sekaran, Preparation, characteriza-
tions and its application of heterogeneous Fenton catalyst for the treatment of
synthetic phenol solution, J. Mol. Liq. 177 (2013) 402–408.
Fig. 9. Mechanism proposed for the photo-oxidation of phenol by hydroxyl radical.
[12] B. Tryba, A.W. Morawski, M. Inagaki, M. Toyoda, The kinetics of phe-
nol decomposition under UV irradiation with and without H2O2 on TiO2,
Fe-TiO2 and Fe-C-TiO2 photocatalysts, Appl. Catal. B: Environ. 63 (2006)
215–221.
[13] H. Kusic´, N. Koprivanac, A.L. Bozic´, I. Selanec, Photo-assisted Fenton type pro-
cesses for the degradation of phenol: a kinetic study, J. Hazard. Mater. 136
(2006) 632–644.
[14] M. Mun˜oz, Z. de Pedro, J. Casas, J. Rodriguez, Assessment of the generation of
chlorinated byproducts upon Fenton-like oxidation of chlorophenols at differ-
ent conditions, J. Hazard. Mater. 190 (2011) 993–1000.
[15] H. Hadjltaief, P. Da Costa, P. Beaunier, M.E. Gálvez, M. Zina, Fe-clay-plate as a
heterogeneous catalyst in photo-Fenton oxidation of phenol as probe molecule
for water treatment, Appl. Clay Sci. 91 (2014) 46–54.
in Fig. 9, which is similar to that reported by Wang et al. [32]: (1)
TCPPFe (oxidation state: Fe3+) absorbs radiation in visible region,
and then an electron is transferred from the porphyrin macrocycle
to Fe3+. (2) The metal is reduced from Fe3+ to Fe2+ and (3) imme-
diately the Fe2+ interacts catalytically with H2O2 which produces
hydroxyl radicals. Afterward, (4) the hydroxyl radicals react with
phenol generating catechol, hydroquinone and benzoquinone. (5)
The chain reaction proceeds until the formation of short chain acids
such as cis-muconic acid, succinic acid, oxalic acid, formic and acetic
acid. Finally, these compounds are transformed into carbon dioxide
and water; the general scheme is shown in Fig. 9.
[16] R. Giovannetti, L. Alibabaei, L. Petetta, Aggregation behaviour of a tetracar-
boxylic porphyrin in aqueous solution, J. Photochem. Photobiol. A: Chem. 211
(2010) 108–114.
[17] A.D. Adler, F.R. Longo, J.D. Finarelli, J. Goldmacher, J. Assour, L. Korsakoff, A
simplified synthesis for meso-tetraphenylporphyrin, J. Org. Chem. 32 (1967)
476–1476.
[18] G. Granados-Oliveros, E. Páez-Mozo, F. Martínez Ortega, M. Piccinato, F. Silva,
C.L. Barbosa, E. Di Mauro, M. da Costa, A. Tsutomu, Visible light production of
superoxide anion with MCarboxyphenylporphyrins (M = H, Fe, Co, Ni, Cu, and
Zn) free and anchored on TiO2: EPR characterization, J. Mol. Catal. A: Chem. 339
(2011) 79–85.
[19] J.F. Cornet, A. Marty, J.B. Gros, Revised technique for the determination of mean
incident light fluxes on photobioreactors, Biotechnol. Prog. 13 (1977) 408–415.
[20] M. Gouterman, G.H. Wagnière, L.C. Snyder, Spectra of porphyrins: Part II. Four
orbital model, J. Mol. Spectrosc. 11 (1963) 108–127.
[21] W. Zheng, N. Shan, L. Yu, X. Wang, UV–vis, fluorescence and EPR properties of
porphyrins and metalloporphyrins, Dyes Pigments 77 (2008) 153–157.
[22] C.E. Diaz-Uribe, M.C. Daza, E.A. Páez-Mozo, F. Martínez, O.C.L.B. Guedes,
E. Di Mauro, Visible light singlet oxygen production with tetra(4-
carboxyphenyl)porphyrin/SiO2, J. Photochem. Photobiol. A: Chem. 259 (2013)
47–52.
[23] M. Trytek, M. Majdan, A. Lipke, J. Fiedurek, Sol–gel immobilization of octaethyl-
porphine and hematoporphyrin for biomimetic photooxidation of ␣-pinene, J.
Catal. 286 (2012) 193–205.
[24] M. Assis, R. John, L. Smith, Hydrocarbon oxidation with iodosylbenzene
catalysed by the sterically hindered iron(III) 5-(pentafluorophenyl)-10,15,20-
tris(2,6-dichlorophenyl)porphyrin in homogeneous solution and covalently
bound to silica, J. Chem. Soc. Perkin Trans. 2 (1998) 2221–2226.
[25] A. Babuponnusami, K. Muthukumar, Degradation of phenol in aqueous solution
by Fenton, Sono-Fenton, Sono-photo-Fenton methods, Clean – Soil Air Water
39 (2) (2011) 142–147.
4. Conclusions
We have used the TCPPFe adsorbed on SiO2 for catalyzing the
oxidation of phenol samples through a photo-Fenton reaction. The
main advantage of this system is that the reaction efficiently occurs
under visible light.
The UV–vis and EPR measurements proved the synthesis of
TCPP and TCPPFe under our experimental conditions. Chemical
trapping through reaction with terephthalic acid corroborated the
presence of hydroxyl radicals during photo-oxidation process. The
main products of the photo-oxidation were identified as cate-
chol, hydroquinone and benzoquinone. In addition, a pseudo-first
order model allowed us to obtain and compare the efficiency
of the photo-oxidation process at different pH (1.8, 2.0, 2.8 and
3.2); results indicated that rate constants were in the range of
1.7 × 10−2–3.6 × 10−2 min−1. In this case, we also found that at
pH 2.8 the photo-oxidation process occurs with the highest rate
constant (3.6 0.2 × 10−2 min−1). Finally, we have proposed a pos-
sible reaction mechanism in order to explain the photo-oxidation
process by the action of TCPPFe.
[26] S. Lakshmi, R. Renganathan, S.J. Fujota, Study on TiO2-mediated photocatalytic
degradation of methylene blue, J. Photochem. Photobiol. A: Chem. 88 (1995)
163–167.
Acknowledgments
[27] Z. Shourong, H. Qingguo,
A study on dye photoremoval in TiO2 sus-
pension solution, J. Photochem. Photobiol. A: Chem. 108 (1997) 235–
238.
The authors are grateful to Laboratório de Fluorescência e
Ressonância Paramagnética Eletrônica (LAFLURPE) for the assis-
tance in the EPR analysis.
[28] T. Zhang, Photooxidative N-demethylation of methylene blue in aqueous TiO2
dispersions under UV irradiation, J. Photochem. Photobiol. A: Chem. 140 (2001)
163–172.

80
C.E. Diaz-Uribe et al. / Journal of Photochemistry and Photobiology A: Chemistry 294 (2014) 75–80
[29] Y. Liu, Y. Zhu, J. Xu, X. Bai, R. Zong, Y. Zhu, Degradation and mineralization
mechanism of phenol by BiPO4 photocatalysis assisted with H2O2, Appl. Catal.
B: Environ. 142 (2013) 561–567.
[31] E. Grabowska, J. Reszczyn´ ska, A. Zaleska, Mechanism of phenol photodegrada-
tion in the presence of pure and modified-TiO2: a review, Water Res. 46 (2012)
5453–5471.
[30] A. Babuponnusami, K. Muthukumar, A review on Fenton and improvements to
the Fenton process for wastewater treatment, J. Environ. Chem. Eng. 2 (2014)
557–572.
[32] H. Wang, W. Liang, Q. Zhang, W. Jiang, Solar-light-assisted Fenton oxidation
of 2,4-dinitrophenol (DNP) using Al2O3-supported Fe(III)-5-sulfosalicylic acid
(ssal) complex as catalyst, Chem. Eng. J. 164 (2010) 115–120.