Automated on-line monitoring of the TiO2-based photocatalytic degradation of dimethyl phthalate and diethyl phthalate

Article abstract of DOI:10.1039/c8pp00307f

A fully automated on-line system for monitoring the TiO₂-based photocatalytic degradation of dimethyl phthalate (DMP) and diethyl phthalate (DEP) using sequential injection analysis (SIA) coupled to liquid chromatography (LC) with UV detection was proposed. The effects of the type of catalyst (sol-gel, Degussa P25 and Hombikat), the amount of catalyst (0.5, 1.0 and 1.5 g L^-1), and the solution pH (4, 7 and 10) were evaluated through a three-level fractional factorial design (FFD) to verify the influence of the factors on the response variable (degradation efficiency, %). As a result of FFD evaluation, the main factor that influences the process is the type of catalyst. Degradation percentages close to 100% under UV-vis radiation were reached using the two commercial TiO₂ materials, which present mixed phases (anatase/rutile), Degussa P25 (82%/18%) and Hombikat (76%/24%). 60% degradation was obtained using the laboratory-made pure anatase crystalline TiO₂ phase. The pH and amount of catalyst showed minimum significant effect on the degradation efficiencies of DMP and DEP. Greater degradation efficiency was achieved using Degussa P25 at pH 10 with 1.5 g L^-1 catalyst dosage. Under these conditions, complete degradation and 92% mineralization were achieved after 300 min of reaction. Additionally, a drastic decrease in the concentration of BOD₅ and COD was observed, which results in significant enhancement of their biodegradability obtaining a BOD₅/COD index of 0.66 after the photocatalytic treatment. The main intermediate products found were dimethyl 4-hydroxyphthalate, 4-hydroxy-diethyl phthalate, phthalic acid and phthalic anhydride indicating that the photocatalytic degradation pathway involved the hydrolysis reaction of the aliphatic chain and hydroxylation of the aromatic ring, obtaining products with lower toxicity than the initial molecules.

Full text of DOI:10.1039/c8pp00307f

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Automated on-line monitoring of the TiO2-based
photocatalytic degradation of dimethyl phthalate
and diethyl phthalate†
Cite this: DOI: 10.1039/c8pp00307f
b
Daniel Salazar-Beltrán,a,b Laura Hinojosa-Reyes, a Fernando Maya-Alejandro,
b
Gemma Turnes-Palomino, b Carlos Palomino-Cabello,
Aracely Hernández-Ramírez a and Jorge Luis Guzmán-Mar
*
a
A fully automated on-line system for monitoring the TiO2-based photocatalytic degradation of dimethyl
phthalate (DMP) and diethyl phthalate (DEP) using sequential injection analysis (SIA) coupled to liquid
chromatography (LC) with UV detection was proposed. The effects of the type of catalyst (sol–gel,
Degussa P25 and Hombikat), the amount of catalyst (0.5, 1.0 and 1.5 g L−1), and the solution pH (4, 7 and
10) were evaluated through a three-level fractional factorial design (FFD) to verify the influence of the
factors on the response variable (degradation efficiency, %). As a result of FFD evaluation, the main factor
that influences the process is the type of catalyst. Degradation percentages close to 100% under UV-vis
radiation were reached using the two commercial TiO2 materials, which present mixed phases (anatase/
rutile), Degussa P25 (82%/18%) and Hombikat (76%/24%). 60% degradation was obtained using the labora-
tory-made pure anatase crystalline TiO2 phase. The pH and amount of catalyst showed minimum signifi-
cant effect on the degradation efficiencies of DMP and DEP. Greater degradation efficiency was achieved
using Degussa P25 at pH 10 with 1.5 g L−1 catalyst dosage. Under these conditions, complete degradation
and 92% mineralization were achieved after 300 min of reaction. Additionally, a drastic decrease in the
concentration of BOD5 and COD was observed, which results in significant enhancement of their biode-
gradability obtaining a BOD5/COD index of 0.66 after the photocatalytic treatment. The main intermediate
products found were dimethyl 4-hydroxyphthalate, 4-hydroxy-diethyl phthalate, phthalic acid and phtha-
lic anhydride indicating that the photocatalytic degradation pathway involved the hydrolysis reaction of
the aliphatic chain and hydroxylation of the aromatic ring, obtaining products with lower toxicity than the
initial molecules.
Received 13th July 2018,
Accepted 10th September 2018
DOI: 10.1039/c8pp00307f
rsc.li/pps
exposure is their effect on humans and wildlife reproduc-
1. Introduction
tion,8,9 causing damage to the endocrine system and carcino-
Phthalic acid esters (PAEs) have been classified as priority genic effects.10–12 Due to the risk to human health and the
hazardous substances due to their risk to human health. PAEs environment, certain PAEs including dimethyl phthalate
have been widely used as additives in plastic or polymeric (DMP) and diethyl phthalate (DEP) have been identified as pri-
materials to improve their flexibility, transparency, and ority hazardous substances by the European Union (EU).13,14
durability.1–3 Today, millions of tons of PAEs are produced all
Most commonly, these short chained esters are detected in
over the world annually.4–6 These poorly biodegradable PAEs environmental samples,5,15 including rivers, lakes and ground-
are not chemically bound to polymers and can easily reach the water. Different physical and chemical processes like adsorp-
soil and aquatic systems.7 The main concern related to PAEs tion with activated carbon, chitosan, or activated sludge have
been described for the removal of PAEs from water.5,16,17
However, the pollutants are only concentrated onto adsorbent
aUniversidad Autónoma de Nuevo León, Facultad de Ciencias Químicas, San Nicolás
de los Garzas, Nuevo León, C.P. 66455, Mexico.
materials. Thus, it is necessary to identify appropriate treat-
ment technologies such as advanced oxidation processes
(AOPs), which involve the generation and use of transient
species of high oxidizing power, mainly the hydroxyl radical
(•OH). Within these processes, heterogeneous photocatalysis
has been proposed in recent years as an interesting alternative
E-mail: jorge.guzmamr@uanl.edu.mx
bUniversity of the Balearic Islands, Department of Chemistry, Cra. de Valldemossa
Km 7.5, Palma de Mallorca, E-07122, Spain
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8pp00307f
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for PAEs oxidation; nevertheless, much work has been done on used to identify the degradation intermediates and by-products.
the degradation of single pollutants.18–20 Titanium dioxide Based on the detected intermediates, degradation pathways of
(TiO2) is known to be an excellent photocatalyst because it is DMP and DEP were proposed in the TiO2 photocatalytic
photostable, reusable, inexpensive, nontoxic and easily system. To the best of the authors’ knowledge, the photo-
available.21–23 Anatase and rutile are the two principal catalytic catalytic oxidation of a mixture of PAEs has not been studied
phases of TiO2. Anatase is generally regarded as the most yet under the above-mentioned conditions.
photochemically active phase of TiO2, presumably due to the
combined effect of lower rates of recombination and higher
surface adsorptive capacity.24 Compared with pure phase TiO2
materials, mixed-phase TiO2 can exhibit higher photocatalytic
2. Experimental
2.1. Implementation of an automated SIA photocatalytic
activity, which could be attributed to the effective interparticle
reactor
charge transfer that promotes charge separation and enhances
photoactivity and photoefficiency.25,26
The SIA system (Fig. 1) for the on-line monitoring of the photo-
Degradation studies are primarily based on batch treat- catalytic degradation of DMP and DEP was composed of a
ments, and proper analytical control is often time-consuming syringe pump (SIA) and an 8-port multi-position selection
due to the collection of a considerable number of samples at valve. The SIA system was coupled to the photocatalytic reactor
fixed times during the evolution of the photocatalytic reaction. (a 250 mL Pyrex glass reactor and a UV-visible metal halide
In this sense, sequential injection analysis (SIA) has proved to Philips lamp 25 W, polychromatic radiation from 300 to
be a suitable system for on-line analysis because of its high 700 nm, 563 W m−2) and to a filtration unit through the multi-
sampling frequency, simplicity of automation, minimal position selection valve. The lamp was positioned at a distance
sample and reagent consumption, and little sample of 5 cm above the reactor. Aqueous DMP and DEP mixture
manipulation.27–29
solution with an initial concentration of 5 mg L−1 for each
In this work, the application of heterogeneous photocataly- compound was prepared at different pH values (4, 7 and 10)
sis for the degradation of a binary mixture of DMP and DEP adjusted using diluted aqueous solutions of HCl and NaOH.
under UV-Vis radiation was investigated. The on-line and real- The dosage of the catalyst was between 0.5 and 1.5 g L−1
.
time monitoring of pollutant degradation was carried out Catalyst dispersions were equilibrated in the dark for 1 h prior
using an SIA system equipped with a photoreactor, which is to irradiation. At regular time intervals (30 min), 2 mL of the
coupled to a high performance liquid chromatography instru- sample were automatically loaded from the reactor and filtered
ment. The effect of catalyst type, catalyst dosage and solution through a 0.45 µm Nylon filter to remove the photocatalyst.
pH was studied comparing the photocatalytic activity of three The concentrations of DMP and DEP were analyzed using a
different TiO2-based materials. Under the selected degradation Jasco high performance liquid chromatography (HPLC) system
conditions, complementary tests were performed, following with a C18 Column (Phenomenex Kinetex, 5 mm × 4.6 mm
the mineralization and the biodegradability index. Finally, gas i.d.) and a UV-Vis diode array detector (MD-4017) at 230 nm.
chromatography coupled to mass spectrometry (GC-MS) was Isocratic elution with acetonitrile/water (55 : 45, v/v) mobile
Fig. 1 A schematic depiction of the SIA system used for the on-line monitoring of DMP and DEP during the photocatalytic reaction.
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phase at a flow rate of 1 mL min−1 and an injection volume of biodegradable samples, between 0.2 and 0.4 for samples with
20 µL was used. The calibration curves were elaborated from low biodegradability, and less than 0.2 for non-biodegradable
seven concentration standards from 0.1 and 10 mg L−1, and samples.32
the limits of detection were 0.01 and 0.02 mg L−1 for DMP and
DEP, respectively.
2.5. Identification of degradation intermediates and by-
products
2.2. Characterization of TiO2-based photocatalysts
To determine the intermediate products from the DMP and
Catalyst surface area and porosity were determined from the
DEP photocatalytic reaction, solid-phase extraction (SPE) with
nitrogen adsorption measurements at 77 K. The crystalline
C18 25 mm 3 M Empore extraction disks was used.33 The
phase composition and the average crystallite size were
studied using X-ray diffraction (XRD). The morphology of the
catalysts was analyzed using scanning electron microscopy
(SEM). Diffuse reflectance UV-Vis spectroscopy was used to
extraction disk was conditioned with 10 mL of a dichloro-
methane and ethyl acetate mixture (1 : 1), 10 mL of methanol,
and 10 mL of ultrapure water. Then, the sample was percolated
through the disk with a flow rate of 5 mL min−1 under
obtain the band gap energy. The isoelectric point was deter-
vacuum. The compounds trapped in the disk were collected
mined using a Zeta-Meter apparatus. Detailed information is
using 5 × 4 mL of dichloromethane/ethyl acetate (1 : 1). The
described in the ESI.†
collected fractions were evaporated under a gentle stream of
2.3. Statistical analysis
nitrogen to dryness, reconstituted in 100 μL of the solvent
mixture, and 1 μL was injected into the GC-MS instrument in a
splitless mode. A GC-MS system (5979 inert XL mass selective,
Agilent Technologies) equipped with an Agilent HP-5 ms capil-
lary column (30 m × 0.25 mm i.d.) was used with the following
chromatographic conditions: injector temperature 220 °C,
column temperature program 40 °C, 40–200 °C (5 °C min−1),
200–210 °C (1 °C min−1), 210–280 °C (20 °C min−1) and 280 °C
A three-level fractional factorial design (FFD) with three repli-
cates at the central point was used to investigate the effect of
the process variables as well as to identify the best conditions
for the photocatalytic process. Three independent variables
were type of catalyst (TC), dosage of catalyst (CC), and solution
pH (pH). The degradation percentages of DMP and DEP at
150 min reaction time were the output response variables.
The significance of the effects of each variable was evalu-
ated with analysis of variance (ANOVA) using the statistical
software STATISTICA 10 (StatSoft, Inc. 2011). A p-value of less
than 0.05 was considered significant. To evaluate the statistical
relationships among operating variables of the photocatalytic
process and the responses (degradation percentages of DMP
and DEP) was used a response surface methodology approach
and desirability parameters.
(3 min). Helium was used as the carrier gas at 1.5 mL min−1
The interface was kept at 280 °C.33
.
The by-products of the degradation reaction were analyzed
using ion exclusion chromatography (IEC). At regular time
intervals (60 min), 1 mL of the sample was taken from the
reactor and filtered through 0.45 μm Nylon syringe filters to
remove TiO2 particles. The degradation by-products were ana-
lyzed on a YL9100 HPLC system with an ion exclusion column
(Aminex, 300 mm × 7.8 mm i.d.) equipped with a photodiode
array detector (YL9160) at 200 nm. The mobile phase was
4 mM H2SO4 at a flow rate of 0.8 mL min−1 with an injection
volume of 20 µL.
2.4. Determination of total organic carbon (TOC) and the
biodegradability index
A volume of 8 mL of the sample was taken from the reactor
every 60 min. The samples were filtered through a 0.45 μm
Nylon syringe filter to remove TiO2 particles and acidified to
pH 2 with 1 M HCl prior to analysis. The total organic carbon
(TOC) concentration was measured using a Multi N/C 2100s
analyzer (Analytik Jena AG Corporation).
3. Results and discussion
3.1. Characterization of TiO2-based photocatalysts
For the analysis of the biological oxygen demand (BOD5) The characterization results of TiO2-based materials are sum-
and the chemical oxygen demand (COD), volumes of 25 mL of marized in Table 1. The crystallinity and growth orientation of
the sample were taken every hour during the photocatalytic the TiO2 materials were examined using XRD (Fig. S1†). The
degradation and prepared for their analysis on the same day. composition of the crystalline phase was determined with eqn
COD was determined using the closed reflux method (Method (S1)† showing that Degussa P25 has 82% anatase and 18%
5220 D) and the consumed oxygen was measured at 600 nm rutile, while Hombikat has 76% anatase and 24% rutile
using a Hewlett-Packard 8452A UV-Vis spectrophotometer.30 (Table 1). The contents of crystalline phases were similar to
For the determination of BOD5, a sensor system was used those published by Karunakaran et al.34
(VELP SCIENTIFICA).30
The average crystallite sizes of the anatase phase were deter-
The BOD5 and COD measurements were used to calculate mined using eqn (S2),† obtaining values of 19.5, 20.7 and
the biodegradability index. There is a correlation between the 20.7 nm for sol–gel, Degussa P25 and Hombikat, respectively,
BOD5 and COD values under certain conditions.31
showing slightly higher values for the commercial TiO2-based
The BOD5/COD ratio is usually greater than 0.5 for highly materials compared to that of the material prepared by the
biodegradable samples, between 0.4 and 0.5 for moderately sol–gel method.
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Table 1 Characteristics of TiO2 based catalysts
Crystalline phase (%)
(A: anatase, R: rutile)
Catalyst
Surface area (m2 g−1
)
Pore size (nm)
Eg (eV)
Isoelectric point
Sol–gel
Degussa P25
Hombikat
100 (A)
82 (A)/18 (R)
76 (A)/24 (R)
49.2
51.4
56.9
8.0
22.7
26.0
3.0
3.1
3.1
6.1
6.5
6.9
The surface areas and pore sizes were calculated by the BET 3.3. Significance of process variables and empirical model
method and BJH method, respectively, from the N2 adsorption development for PAE photocatalytic degradation
isotherms (Fig. S2†). Although the surface areas of the three
materials were very similar (Table 1), the fact that commercial
materials show a 3-fold larger pore size than the synthesized
material by the sol–gel method can be attributed to a pore
The significance of the process variables in terms of linear and
quadratic effects was assessed using the Pareto chart at a 95%
confidence level (Fig. S6†). The type of catalyst (TC) consider-
ably affected the degradation percentage of both PAEs followed
blocking by a process of percolation in the sol–gel material
by the quadratic term of TC. The variable dosage of catalyst
(hysteresis loop type H2). A larger pore size can make these
(CC) and the pH of the solution by themselves had little posi-
pores more accessible to PAE molecules, increasing the
tive influence on the degradation percentage of both
adsorption and degradation of the pollutant.35,36
compounds.
From the diffuse reflectance UV-Vis absorption spectrum
In the ANOVA performed for the regression model, R2
(Fig. S3†) and applying the Kubelka–Munk equation,37 the
values of 99.29 and 99.71 were obtained for DMP and DEP,
respectively. These results indicated that the predicted and the
experimental values were similar with an error less than 5%.
band gap energy (Eg) of each catalyst was calculated. The Eg
values of the materials (Table 1) were similar (3.0–3.1 eV),
which means that these materials absorb energy that lies at
Response surface graphs were constructed in terms of desir-
the boundary between the UV and the visible region. The cata-
ability to define the best conditions for degradation of DMP
lysts presented an isoelectric point between pH 6 and 7
and DEP (Fig. 2). Regarding the type of catalyst (Fig. 2A and B),
(Fig. S4†), having the lowest value (6.1) for the TiO2 syn-
it was observed that the degradation efficiency was higher
using Degussa P25, reaching percentages greater than 60% for
both compounds after 150 min of the reaction, followed by the
Hombikat reaching percentages close to 55%, and finally the
thesized by the sol–gel method, followed by Degussa P25 (6.5)
and finally the Hombikat (6.9).
SEM images of the catalysts are shown in Fig. S5.† The
three materials had a similar morphology based on the
TiO2 synthesized by the sol–gel method showed a lower degra-
agglomeration of spherical particles with sizes close to 50 nm
dation percentage (near to 25%). The photocatalytic activity of
for the sol–gel TiO2, lower than 50 nm for Degussa P25, and
TiO2 depends on the surface and structural properties that
include the crystalline composition, surface area, particle size
close to 100 nm for Hombikat. Particle size is an important
parameter influencing catalytic activity; a high photocatalytic
distribution, porosity and band gap energy. The greater photo-
activity is associated with a smaller particle and crystallite size,
catalytic efficiency in the degradation of DMP and DEP using
Degussa P25 and Hombikat catalysts may be due to the fact
due to the fact that a large number of active surface sites are
available for adsorption, which increases the rate of interfacial
that these materials present a mixture of anatase and rutile
charge transfer.36,38
crystalline phases and have a larger pore size, enhancing the
accessibility of PAE molecules, in comparison with the one
synthesized by the sol–gel method with only the anatase phase
and considerably smaller pore size. Studies related to the
3.2. Implementation of the automated SIA photocatalytic
reactor
The SIA system allowed all solutions to be handled during the photocatalytic activity of mixed phases of TiO2 propose that
monitoring of the PAE degradation in a fully automated mode. the transfer of electrons from the anatase phase to an electron
Ten aliquots were collected during the photocatalytic reaction trap of the rutile phase of lower energy serves to reduce the
and analyzed in triplicate. Aliquots of 2 mL of the PAEs/TiO2 recombination rate of the electron–hole pairs in the anatase
dispersion were collected every 30 min and analyzed after the phase and improve the photoactivity and photoefficiency of
removal of TiO2 by on-line filtration. It was not necessary to TiO2-based catalysts. Moreover, the rutile phase helps by chan-
replace the filter during the photocatalytic reaction. Any ging the photoresponse of the anatase phase to the visible
change in the peak shape and resolution, and backpressure region in mixed phase materials.24,26 Among the commercial
problems in the system were observed using the same filter. catalysts, the higher degradation percentages of DMP and DEP
On-line monitoring was compared with manual analysis were obtained when using Degussa P25, which may be due to
showing a relative standard deviation of the peak area values its phase composition (anatase–rutile ratio), while Hombikat
of less than 3.9% in the case of the manual analysis and less presented a higher percentage of the rutile phase that may
than 1.6% in the case of the automated system, demonstrating favor the recombination of electron–hole pairs.24 Muneer et al.
the precision of the proposed automated procedure.
compared the performance of the catalysts Degussa P25 and
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was due to the increase in the total surface area of the catalyst
(active sites) available for the photocatalytic reaction. This
result agrees with that reported by Xu et al., who evaluated
Degussa P25 to degrade butylbenzyl phthalate (BBP) by varying
the catalyst concentration between 0.5 and 5 g L−1, concluding
that the efficiency in the degradation of BBP increased with
larger amounts of TiO2 up to 2.0 g L−1 43
.
Finally, the effect of the pH on the degradation of DMP and
DEP was evaluated (Fig. 2A and C). In heterogeneous photocata-
lysis systems, pH is one of the operating parameters that affects
the surface charge of the catalyst.40 The pH of the solution
modifies the surface charge of the catalyst, while DMP and
DEP are in the molecular form over the entire pH range, so,
they do not exhibit electrostatic interactions with the catalyst
surface.44 The percentage of degradation increased with the
increase in pH values for the three catalysts, reaching the
highest degradation percentage at pH 10. In general, changes
in pH may have a non-significant effect on the adsorption of
PAEs on the TiO2 surface, as well as on the photodegradative
reactions that occur on the surface of the particles.33 However,
all three catalysts presented their highest degradation
efficiency at pH 10; this could be due to the fact that at high
initial pH, more hydroxide ions (OH−) are present in the solu-
tion, promoting the formation of hydroxyl radicals (•OH),
which are produced by the photooxidation of OH− ions
through the holes formed in the surface of TiO2. Since the •OH
radicals are the main oxidizing species in the photocatalytic
process, the degradation of DMP and DEP can be accelerated
in an alkaline medium.45
The estimated parameters by the desirability profiles
(Fig. S7†) indicated that maximum degradation percentage
values of DMP and DEP were 82.7 and 81.3%, respectively, at
150 min of the reaction, obtained with 1.5 g L−1 of the catalyst
Degussa P25 at pH 10.
The photocatalytic reaction was carried out under these
experimental conditions and degradation percentages of 81.0
and 82.2% for DMP and DEP were obtained with an error less
than 2.1%. Control tests such as adsorption and photolysis
were performed under operating conditions described by the
experimental design and compared with the obtained results.
Fig. 3A–B show the complete degradation for DMP and DEP at
250 min reaction time through the photocatalytic process, and
adsorption did not contribute (less than 2% for both com-
pounds) to the degradation. The photodegradation of PAEs
under UV-Vis radiation was almost negligible (approximately
5%), confirming the effectiveness of the photocatalytic process
during the degradation of the phthalate mixture.
Fig. 2 Response surface of the desirability function for the degradation
of DMP and DEP showing the mutual effect of the factors: A) pH and
TC; B) CC and TC; and C) pH and CC.
Hombikat for DEP degradation, concluding that both
materials exhibited a similar behavior in photonic efficiency;
nevertheless, a higher degradation percentage was obtained
when using Degussa P25.39
The effect of the catalyst dosage was investigated (Fig. 2B
and C). Different catalyst loads were evaluated (from 0.5 to
1.5 g L−1). The amount of catalyst in batch photoreactors is
commonly described between 0.2 and 2.5 g L−1. This limit
depends on the geometry and working conditions of the
photoreactor and corresponds to the maximum amount of
TiO2 in which all the exposed surface is fully irradiated.
Exceeding this maximum concentration of the catalyst, a
masking effect of the photosensitive surface occurs due to the
excess of particles.40–42 The degradation efficiency was
3.4. Determination of total organic carbon (TOC) and the
biodegradability index
Likewise, under the same conditions, the mineralization of the
contaminants was evaluated during the photocatalytic process
and the results are presented in Fig. 3C. The efficiency of the
TiO2 P25-based photocatalysis was clearly observed, since 92%
mineralization was reached after 300 min of reaction.
enhanced by increasing the catalyst amount up to 1.5 g L−1
.
The increase in the degradation efficiency of DMP and DEP
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A high mineralization percentage (≥70%) is related to
smaller molecules such as carboxylic acids like oxalic acid,
formic acid and acetic acid, as degradation products, less toxic
than the initial PAEs.22,33,46
The biodegradability of organic matter is an important
factor to know the capability to remove the organic matter
under microbiological processes.32 The biodegradability index
(BOD5/COD ratio) was calculated throughout the photocatalytic
treatment (Fig. 3D). The results indicated that the sample
before being treated had a biodegradability index of 0.16,
which indicates that the effluent is not biodegradable. As the
reaction time elapsed, a decrease in the biodegradability index
was observed, reaching a value of 0.08 at 180 min, which indi-
cates the formation of less biodegradable intermediates than
DMP and DEP. However, the biodegradability index increased
reaching a value of 0.66 after 300 min of reaction, which indi-
cates that the sample is biodegradable. The results obtained
from the analysis of BOD5, COD and TOC, during the photo-
catalytic process demonstrated the efficiency of heterogeneous
photocatalysis for the treatment of these types of emerging
pollutants (DMP and DEP) producing a more biodegradable
and less toxic effluent.
3.5. Identification of degradation intermediates and by-products
The intermediate products from DMP and DEP photocatalytic
degradation were identified by GC-MS technique. A comparison
with available standard compounds, instrumental library searches,
and mass fragmentation patterns was carried out to identify
degradation intermediates. The (a) DMP and (b) DEP photo-
catalytic degradation occurred through the oxidation initiated
by •OH radical attack resulting in the hydrolysis of the ali-
phatic chain and the hydroxylation of the aromatic ring
(Fig. 4). Based on the identified transformation products, OH
attack during photocatalytic treatment occurs mainly through
the hydroxylation of the aromatic ring producing (c) dimethyl
4-hydroxyphthalate, (d) diethyl 4-hydroxyphthalate, (e)
5-hydroxy-2-(methoxycarbonyl) benzoic acid, (g) 2-(ethoxycar-
bonyl)-5-hydroxybenzoic acid, (i) 4-hydroxyphthalic acid, and
(k) 2,4-dihydroxybenzoic acid intermediates. From the hydroxy-
lation reaction, the main intermediates generated were (c)
dimethyl 4-hydroxyphthalate and (d) diethyl 4-hydroxyphtha-
late, which are in concordance with the results reported by
other authors.46,47 The hydrolysis reaction of the aliphatic
chain was also found to occur during the photocatalytic
process. The intermediates formed were (f) 2-(methoxycarbo-
nyl) benzoic acid, (h) 2-(ethoxycarbonyl) benzoic acid, ( j)
phthalic acid, and (l) phthalic anhydride. Obtaining ( j)
phthalic acid and (l) phthalic anhydride via hydrolysis was
also reported by other authors.22,39,46,47 Finally, the aromatic
ring is opened, producing some carboxylic acids, such as (m)
glutaric acid, (n) oxalic acid, and (o) acetic acid.
Fig. 3 Degradation of: (A) DMP and (B) DEP by heterogeneous photo-
catalysis; (C) content of organic carbon and generated oxalic acid; and
(D) biodegradability index during degradation of DMP and DEP (initial
concentration of DMP and DEP of 5 mg L−1 each, catalyst Degussa P25,
dosage 1.5 g L−1 and dissolution pH = 10; n = 3 replicates for graphs (A),
(B) and (C)).
The presence of carboxylic acids was confirmed by IEC.
A mixed standard stock solution containing carboxylic acids
(acetic acid, formic acid, succinic acid, and oxalic acid) com-
monly described after the degradation of PAEs were injected
into the IEC system to identify and quantify the short chain
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tion, not only did the hydrolysis reaction of the aliphatic chain
occur, but also the hydroxylation of the aromatic ring, obtain-
ing intermediates such as dimethyl 4-hydroxyphthalate,
4-hydroxy-diethyl phthalate, phthalic acid, and phthalic anhy-
dride. However, after 300 min of reaction, the main by-pro-
ducts identified were carboxylic acids such as glutaric acid,
oxalic acid and acetic acid.
Therefore, the efficiency of heterogeneous photocatalysis
using TiO2-based materials for the elimination of these highly
toxic and recalcitrant emerging compounds was demonstrated.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful for the financial support from the
Facultad de Ciencias Químicas of the Universidad Autónoma
de Nuevo León, the National Council of Science and
Technology of México (CONACyT), the Spanish Ministerio de
Economía y Competitividad (MINECO) and the European
Funds for Regional Development (FEDER). D. Salazar-Beltrán
acknowledges the University of Balearic Islands and CONACyT
for scholarship support.
Fig. 4 Proposed degradation pathways of DMP and DEP with TiO2
Degussa P25 (initial concentration of DMP and DEP of 5 mg L−1 each,
catalyst dosage 1.5 g L−1 and dissolution pH = 10).
References
1 K. Khosravi and G. W. Price, Microchem. J., 2015, 121, 205–212.
2 M. J. Silva, A. R. Slakman, J. A. Reidy, J. L. Preau,
A. R. Herbert, E. Samandar, L. L. Needham and
A. M. Calafat, J. Chromatogr. B, 2004, 805, 161–167.
3 W. J. Peijnenburg and J. Struijs, Ecotoxicol. Environ. Saf.,
2006, 63, 204–215.
organic acids. However, due to the high detection limits
of this technique and the low abundance of by-products gen-
erated, only oxalic acid was detected at a concentration of
4.6 mg L−1 after 300 min of reaction (Fig. 3C).
4 J. Yang, Y. Li, Y. Wang, J. Ruan, J. Zhang and C. Sun, Trac,
Trends Anal. Chem., 2015, 72, 10–26.
5 M. Julinova and R. Slavik, J. Environ. Manage., 2012, 94, 13–24.
6 G. C. C. Yang, Y. H. Chiu and C. Wang, Electrochim. Acta,
2015, 181, 217–227.
7 D. Salazar-Beltrán, L. Hinojosa-Reyes, E. Ruiz-Ruiz,
A. Hernández-Ramírez and J. L. Guzmán-Mar, Food Anal.
Methods, 2017, 11, 48–61.
8 S. Net, A. Delmont, R. Sempere, A. Paluselli and
B. Ouddane, Sci. Total Environ., 2015, 515–516, 162–180.
9 H. M. Zhao, H. Du, L. Xiang, Y. L. Chen, L. A. Lu, Y. W. Li,
H. Li, Q. Y. Cai and C. H. Mo, Environ. Pollut., 2015, 206,
95–103.
4. Conclusions
On-line monitoring of the photocatalytic degradation of DMP
and DEP was automated by coupling the photoreactor to an
HPLC system through an SIA system. This flow-based method-
ology enabled a real-time response of the degradation process
with RSD values less than 1.6%. In accordance with the FFD,
Degussa P25 showed the highest activity at a pH value and
catalyst amount of 10 and 1.5 g L−1, respectively. Under these
experimental conditions, complete degradation and 92% min-
eralization were achieved after 300 min of reaction.
Additionally, a drastic decrease in COD (from 49.1 O2 L−1
initial to 3.8 mg O2 L−1 after 300 min of reaction) allowed 92% 10 P. Otero, S. K. Saha, S. Moane, J. Barron, G. Clancy and
removal. In the case of BOD5, a decrease of 67% was observed, P. Murray, J. Chromatogr. B, 2015, 997, 229–235.
with an initial and final BOD5 concentration of 7.7 mg O2 L−1 11 U. Heudorf, V. Mersch-Sundermann and J. Angerer,
and 2.5 mg O2 L−1, respectively. Hence, the photocatalytic
Int. J. Hyg. Environ. Health, 2007, 210, 623–634.
treatment appears to be useful when there are easily bio- 12 T. Fierens, K. Servaes, H. M. Van, L. Geerts, H. S. De,
degradable residues presenting a biodegradability index of
0.66 at the end of the reaction. During the photocatalytic reac-
I. Sioen and G. Vanermen, Food Chem. Toxicol., 2012, 50,
2575–2583.
This journal is © The Royal Society of Chemistry and Owner Societies 2018
Photochem. Photobiol. Sci.

View Article Online
Paper
Photochemical & Photobiological Sciences
13 D. Gao, Z. Li, Z. Wen and N. Ren, Chemosphere, 2014, 95, 30 APHA, AWWA and WPCF, in Métodos normalizados para el
24–32.
análisis de aguas potables y residuales, ed. D. d. Santos,
Madrid, España, 1992.
31 A. Asha, A. Muthukrishnaraj and N. Balasubramanian,
Int. J. Ind. Chem., 2014, 5, 1–6.
32 K. Z. Abdalla and G. Hammam, Int. J. Sci. Bas. Appl. Res.,
2014, 13, 42–48.
14 C. Pérez-Feás, M. C. Barciela-Alonso and P. Bermejo-
Barrera, Microchem. J., 2011, 99, 108–113.
15 B. L. Yuan, X. Z. Li and N. Graham, Water Res., 2008, 42,
1413–1420.
16 D. Walter, L. Hsu-Chuen, C. Shu-Fei, T. K. Kuan and
H. Chihpin, J. Environ. Eng. Landsc., 2006, 16, 275– 33 S. Kaneco, H. Katsumata, T. Suzuki and K. Ohta, Chem.
282. Eng. J., 2006, 125, 59–66.
17 N. Adhoum and L. Monser, Sep. Purif. Technol., 2004, 38, 34 C. Karunakaran, D. Vasumathi and P. Vinayagamoorthy,
233–239. Indian J. Chem., 2015, 54A, 1076–1084.
18 F. Ma, S. Zhang, X. Yang, W. Guo, Y. Guo and M. Huo, 35 J. Yu, G. Wang, B. Cheng and M. Zhou, Appl. Catal., B,
Catal. Commun., 2012, 24, 75–79. 2007, 69, 171–180.
19 S. Anandan, N. Pugazhenthiran, T. Lana-Villarreal, G. Lee 36 D. S. Kim, S. J. Han and S. Y. Kwak, J. Colloid Interface Sci.,
and J. J. Wu, Chem. Eng. J., 2013, 231, 182–189. 2007, 316, 85–91.
20 Y. Jing, L. Li, Q. Zhang, P. Lu, P. Liu and X. Lu, J. Hazard. 37 R. López and R. Gómez, J. Sol–Gel Sci. Technol., 2011, 61,
Mater., 2011, 189, 40–47. 1–7.
21 A. Haarstrick, O. M. Kut and E. Heinzle, Environ. Sci. 38 Z. Zhang, C. Wang, R. Zakaria and J. Y. Ying, J. Phys. Chem.
Technol., 1996, 30, 817–824. B, 1998, 102, 10871–10878.
22 Y. H. Chen, L. L. Chen and N. C. Shang, J. Hazard. Mater., 39 M. Muneer, J. Theurich and D. Bahnemann, J. Photochem.
2009, 172, 20–29. Photobiol., A, 2001, 143, 213–219.
23 M. Borges, D. García, T. Hernández, J. Ruiz-Morales and 40 M. N. Chong, B. Jin, C. W. Chow and C. Saint, Water Res.,
P. Esparza, Catalysts, 2015, 5, 77–87. 2010, 44, 2997–3027.
24 D. C. Hurum, A. G. Agrios, K. A. Gray, T. Rajh and 41 D. Friedmann, C. Mendive and D. Bahnemann, Appl.
M. C. Thurnauer, J. Phys. Chem. B, 2003, 107, 4545–
Catal., B, 2010, 99, 398–406.
4549.
42 J. M. Herrmann, Top. Catal., 2005, 34, 49–65.
25 H. Hou, M. Shang, L. Wang, W. Li, B. Tang and W. Yang, 43 X. R. Xu, S. X. Li, X. Y. Li, J. D. Gu, F. Chen, X. Z. Li and
Sci. Rep., 2015, 5, 1–9. H. B. Li, J. Hazard. Mater., 2009, 164, 527–532.
26 G. Li, L. Chen, M. E. Graham and K. A. Gray, J. Mol. Catal. 44 B. Cagnon, S. Chatelain, T. Ferreira de Oliveira,
A: Chem., 2007, 275, 30–35.
27 A. Economou, TrAC, Trends Anal. Chem., 2005, 24, 416–425.
F. Versaveau, S. Delpeux and O. Chedeville, Water, Air, Soil
Pollut., 2017, 228, 1–10.
28 C. Chávez-Moreno, L. Ferrer, L. Hinojosa-Reyes, 45 S. Ahmed, M. G. Rasul, R. Brown and M. A. Hashib,
A. Hernández-Ramírez, V. Cerda and J. Guzmán-Mar,
J. Environ. Manage., 2013, 129, 377–383.
29 E. Guevara-Almaraz, L. Hinojosa-Reyes, A. Caballero-
J. Environ. Manage., 2011, 92, 311–330.
46 B. L. Yuan, X. Z. Li and N. Graham, Chemosphere, 2008, 72,
197–204.
Quintero, E. Ruiz-Ruiz, A. Hernández-Ramírez and 47 Y. J. Jung, B. S. Oh, K. S. Kim, M. Koga, R. Shinohara and
J. L. Guzmán-Mar, Chemosphere, 2015, 121, 68–75.
J. W. Kang, J. Water Health, 2010, 8, 290–298.
Photochem. Photobiol. Sci.
This journal is © The Royal Society of Chemistry and Owner Societies 2018