Transformation products and reaction kinetics in simulated solar light photocatalytic degradation of

26 Pages·2013·0.44 MB·English

COVER PAGE

Transformation products

and reaction kinetics in

simulated solar light

photocatalytic degradation

of propanolol using Ce-

doped TiO2
Applied Catal. B. Environ., 129, 13-29, 2013

This version is made available in accordance with publisher policies. Please, cite
only the publisher version using the citation below:

Javier Santiago-Morales, Ana Agüera, María del Mar
Gómez, Amadeo R. Fernández-Alba, Jaime Giménez,
Santiago Esplugas, Roberto Rosal, Transformation
products and reaction kinetics in simulated solar light
photocatalytic degradation of propranolol using Ce-
doped TiO2, Applied Catalysis B: Environmental, Volume
129, 17 January 2013, Pages 13-29, ISSN 0926-3373,
10.1016/j.apcatb.2012.09.023.

Link to official URL:
http://www.sciencedirect.com/science/article/pii/S0926337312004249
Transformation products and reaction kinetics in simulated solar light photocatalytic
degradation of propanolol using Ce-doped TiO2

a b b b,c
Javier Santiago-Morales , Ana Agüera , María del Mar Gómez , Amadeo R. Fernández-Alba , Jaime
d d a,c,
Gimeno , Santiago Esplugas , Roberto Rosal *

a
Department of Chemical Engineering, University of Alcalá, E-28771 Alcalá de Henares, Spain.
b
Department of Analytical Chemistry, University of Almería, E-04010 Almería, Spain.
c
Advanced Study Institute of Madrid, IMDEA-Agua, Parque Científico Tecnológico, E-28805, Alcalá de
Henares, Madrid, Spain
d
Department of Chemical Engineering, University of Barcelona, E-08028, Barcelona, Spain

* corresponding author

Abstract
The visible light photocatalytic degradation of propanolol has been studied using cerium doped titanium dioxide, a
catalyst that showed an increased light absorption in the visible region. The experiments were carried out in ultrapure
water and in a biologically treated wastewater from the secondary effluent of a treatment plant spiked with propanolol.
The best results were obtained for a cerium loading of 0.5 % wt. with a bulk catalyst concentration of 0.14 g/L, for
which propanolol became essentially depleted after 1.5 of irradiation. The extent of mineralization reached 17.4%
after six hours on stream for the same conditions. Both the oxidation with catalyst holes and the reaction with
hydroxyl radicals contribute to propanolol s depletion, but for the reaction conditions tested in this work, the first
mechanism was prevailing and accounted for 60% of the propanolol rate constant. The runs performed in wastewater
matrix leaded to a very low photocatalytic rate compared with pure water, which was attributed to the presence of
radical scavengers and competing substances. Over thirty reaction intermediates were detected by means of exact
mass measurements performed by liquid chromatography coupled to quadrupole-time-of-flight mass spectrometry
(LC-ESI-QTOF-MS/MS) based on the characteristic fragmentation of oxidation by-products.Their relative abundance
was also assessed in catalytic and non-catalytic runs. The most abundant transformation products could be attributed
to the cleavage of the ether bond of propanolol. Other compounds detected derived from the addition of hydroxyl
groups to the aromatic nuclei or to the ring-opening attack of hydroxyl radicals to the naphthol moiety. Finally, the
toxicity of oxidized mixtures has been determined using the green algae Pseudokirchneriella subcapitata and
bioluminescent marine bacterium Vibrio fischeri. Although the toxicity of treated mixtures tends to decrease as
propanolol was depleted, mixtures treated in pure water may lead to the accumulation of toxic transformation
products.
Keywords: propanolol; solar photocatalysis; transformation products; liquid chromatography; mass spectrometry.

1. Introduction propranolol. Maurer et al. reported removal efficiencies
in the 28-35% range for propranolol in two WWTP
The incomplete removal of pharmaceuticals and other
located in the vicinity of Zürich [8]. Rosal et al.
emerging pollutants in conventional wastewater
monitored the primary and final effluents of a WWTP
treatment plants (WWTP) has been identified as the main
located in Alcalá de Henares, Madrid, performing
route by which anthropogenic pollutants reach aqueous
monthly analyses on time-composite samples over a one-
environments [1]. As a consequence, the presence of
year sampling period [9]. They obtained average
pharmaceuticals has become ubiquitous in natural waters,
concentrations of atenolol, metoprolol and propranolol of
even to the extent of entering drinking water facilities
1025 ng/L, 19 ng/L, and 36 ng/L, respectively, with very
[2]. Some of the adverse effects of these pollutants on
low removal efficiencies which did not exceed 15% for
ecosystems have been reported but the risk associated
atenolol and were even lower for the other two.
with chronic exposure remains essentially unknown [3].
The presence of anthropogenic pollutants in treated Several studies have indicated that advanced oxidation
wastewaters also endangers reuse, which is becoming a processes (AOP) are a suitable choice for the removal of
major issue in view of the growing water scarcity [4]. β-blockers from aqueous solution [10, 11].
The occurrence of β-blockers has been repeatedly Heterogeneous photocatalysis belongs to the category of
reported in recent years in the effluents of many WWTP AOP due to the formation of surface reactive oxygen
• • − •
around the world [5-7]. It has been clearly established species (ROS) such as the radicals HO , O2 or HO2 . In
that conventional wastewater treatments using activated this process, the irradiation of certain semiconductors in
sludge are not effective in completely removing the presence of oxygen creates a redox environment

Applied Catal. B. Environ., 129, 13-29, 2013
suitable for the oxidation of diluted organic pollutants (WWTP) located in Gavà-Viladecans (Barcelona), which
[12]. The most common photocatalyst is titanium receives a mixture of domestic and industrial wastewater
dioxide, the performance of which is very well from facilities located around the area. This WWTP has a
documented [13]. Titanium dioxide, however, only capacity of 300000 equivalent inhabitants and was
absorbs the near-UV part of the solar spectrum designed to treat a maximum volume of wastewater of
3 −1
(wavelength shorter than 390 nm) corresponding to the 64000 m day . The main characteristics of the treated
band gap of TiO2, which is about 3.2 eV depending on wastewater are shown in Table 1.
the anatase/rutile ratio, or even higher for nanoparticles.
2.2. Catalyst preparation and characterization
This fact represents a major drawback for the
development of solar photocatalysis, an environmentally The photocatalyst was a cerium doped titanium oxide
friendly process which could reduce the impact of energy with different percentages of cerium (0%, 0.5% and 1%)
consumption related to the use of UV lamps [14]. The prepared by the sol-gel method. Briefly 10 mL of
photocatalytic efficiency of titanium dioxide can be titanium isopropoxide was dissolved in 50 mL of
enhanced by red shifting its absorption to the visible absolute ethanol (Solution A). In parallel, 92 mg of
region (400-800 nm), which can be achieved by doping (NH4)2Ce(NO3)6 was dissolved in 10 mL distilled water
the anatase matrix with certain cationic or anionic and then added to 40 mL of ethanol and 10 mL of
impurities. Rare earth oxides, in particular, enhance the anhydrous acetic acid (Solution B). Solution A was then
photocatalytic activity of TiO2 due to the transitions of 4f added dropwise to a beaker containing Solution B. The
electrons, which can increase the separation rate of precipitate was dried at 100 ºC and calcined at 670 ºC for
photogenerated charges [15]. Cerium has also been one hour. The amounts mentioned produced
identified as one of the most interesting rare earth approximately 2 g of 0.5% Ce-TiO2 catalyst.
4+
dopants due to its ability to shift between Ce (CeO2)
3+ The Brunauer-Emmet-Teller (BET) surface areas were
and Ce (Ce2O3) under oxidizing or reducing conditions 2
1.4, 34.1, 56.2 m /g for 0%, 0.5% and 1% Ce-TiO2
and to the facile formation of labile oxygen vacancies as
catalysts respectively. BET surface area showed a high
a result of the relatively high mobility of bulk oxygen
correlation with cerium loading. This was attributed to
species. Electronically, the effect of cerium on the TiO2
the decreasing particle size of crystals as discussed
bandgap has been interpreted as being due to an n-type
below. All samples showed a well-defined IUPAC type
impurity band at the interface between the titanium and
IV isotherm and average pore diameters calculated using
cerium oxides [16].
the Barrett-Joyner-Halenda (BJH) analysis of the
In this study, the use of cerium-doped titanium dioxide desorption branch showed an increase in pore diameter
was examined as a strategy to increase the degradation with cerium loadings from 8.6 nm for 0.5% Ce to 12.0
rate of the β-blocker propranolol under irradiation in the nm for 1% Ce compared with 3.7 nm for non-doped
visible region. The experiments were carried out in TiO2. This behaviour may be explained by the decrease
ultrapure water and in a biologically treated wastewater in crystallite size due to the incorporation of cerium, as
from the secondary effluent of a WWTP spiked with discussed below.
propranolol. Besides obtaining photocatalytic kinetics, a
The absorbance of the photocatalyst was measured using
number of reaction intermediates were detected by means
a UV/VIS/NIR Perkin Elmer Lambda 900 spectrometer
of exact mass measurements performed by liquid
equipped with an integrating sphere. The spectra are
chromatography coupled to quadrupole-time-of-flight
shown in Fig. 1 together with the emission spectrum of
mass spectrometry (LC-ESI-QTOF-MS/MS). Their
the Xenon lamp corrected with Duran tube transmission
relative abundance was assessed either in catalytic and
in order to display an estimate of the radiation spectrum
non-catalytic runs. Finally, the toxicity of partially
at the inner wall of the photoreactor. The incorporation of
oxidized mixtures was determined using the green algae
a small amount of cerium into TiO2 significantly
Pseudokirchneriella subcapitata and the bioluminescent
increased the absorption of visible light in the 400-600
marine bacterium Vibrio fischeri.
nm range, which corresponds to the maximum in the
2. Materials and methods lamp emission spectrum. A further increase of cerium
above 0.5% did not result in enhanced radiation
2.1. Materials
absorption. The band gap energies estimated by Tauc’s
Propranolol (PRO) and p-chlorobenzoic acid (pCBA) plot were 3.06 ± 0.15 eV, 2.67 ± 0.11 eV and 2.61 ± 0.05
were purchased from Sigma Aldrich (+99% purity). eV for 0%, 0.5% and 1% cerium loadings.
Titanium isopropoxide, ethanol, diammonium cerium
The catalyst was characterized by X-ray
(IV) nitrate, [(NH4)2Ce(NO3)6], sodium phosphate
diffraction (XRD) using a Seifert 3000P
monobasic dihydrate, sodium hydrogen phosphate,
diffractometer (Cu Kα, λ = 1.5406 Å). The unit cell
sodium hydroxide and hydrochloric acid were analytical
parameters of Ce-TiO2 were calculated considering that
grade reagents used as received. Pure water was obtained
TiO2 anatase belongs to the tetragonal system, whose unit
from a Millipore Milli-Q System with a resistivity of at 2
cell parameters comply with the equation 1/dhkl =
least 18 MΩ cm at 25ºC. Wastewater was collected from 2 2 2 2 2
(h +k )/a + l /c . The diffraction peaks from (1 0 1) and
the secondary clarifier of a wastewater treatment plant
(0 0 4) planes were used, obtaining values of a = b = 3.78

Applied Catal. B. Environ., 129, 13-29, 2013
Å and c = 9.49 Å, essentially coincident with those for The analyses of propranolol and pCBA were performed
pure anatase. We used XRD data and the Scherrer by HPLC using a Waters Corporation apparatus equipped
Equation to estimate the size of crystallites in the with a SEA18 5µm 15 x 0.46 Teknokroma column and a
polycrystalline samples with the Scherrer constant, KS, Waters 996 photodiode array detector. The mobile phase
rounded up to 1: was a 60:40 mixture of water acidified with phosphoric
acid at pH 3.0 and acetonitrile. UV detection was carried
B (2θ ) = KS λ [1] out at 214 nm and 237 nm for propranolol and pCBA
L cosθ respectively. The flow rate was 0.7 mL/min and the
injection volume was 10 µL.
The peak width, B, was determined from the full width at
half maximum. The well-defined peak of (2 0 0) plane at A liquid chromatography-electrospray ionization-
2θ = 48.1º was used to obtain crystallite sizes. With the quadrupole time-of-flight–mass spectrometry (LC-ESI-
increase of cerium XRD peaks broadened and their QTOF–MS/MS) system in positive mode was used to
relative intensity decreased. This was attributed to a identify the transformation products from propranolol.
reduction in the average crystallite sizes, which dropped Samples collected at different irradiation times during the
from 38 nm (0.5 % wt. Ce) to 26 nm (1.0 % wt. Ce). The experiments were directly analyzed without previous pre-
doping with cerium was shown to induce a crystal lattice concentration. The analytes were separated using a
expansion that could suppress the growth of anatase HPLC system (vacuum degasser, autosampler and a
crystals and consequently, the average crystallite size binary pump Agilent Series 1200, Agilent Technologies)
decreased [17]. equipped with a reversed-phase SB-C18 analytical
column of 3.0 mm × 250mm, 5 µm particle size (Agilent
The size distribution of water suspended particles was
Technologies). 0.1% formic acid and 5% MilliQ water in
obtained using dynamic light scattering (DLS, Malvern
acetonitrile were used as mobile phase A and 0.1%
Zetasizer Nano ZS). Zeta potential was determined by
formic acid in water (pH 3.5) as mobile phase B. The
electrophoretic light scattering in the same Nano ZS
elution gradient went from 10% A (3min) to 100% A in
instrument. All measurements were conducted at 25 ºC
22 min, to be kept thereafter at 100% A for 3min. The
using 2 mM phosphate buffer (pH 7.5) as dispersing
flow rate was 0.5 mL/min and the injection volume
medium. It was found that the size of catalyst aggregates
20µL. The HPLC system was connected to a quadrupole-
was not significantly affected by the presence of cerium
time-of-flight mass spectrometer (Agilent 6530 Q-TOF
in the structure, with sizes in the 400-600 nm range
MS, Agilent Technologies, Santa Clara, CA). The
irrespective of the amount of cerium. ζ-potential,
instrument was operated in the 4GHz High Resolution
however, dropped slightly from -65 ± 10 mV (TiO2) to -
Mode. Ions were generated using an electrospray ion
48 ± 6 mV (Ce-TiO2) in all doped samples, which
source with Agilent Jet Stream Technology. The
displayed similar values irrespective of cerium loading.
operation conditions were: superheated nitrogen sheath
2.2. Analytical methods gas temperature (400 ºC) at flow rate 12 L/min; nozzle
voltage, 0 V; capillary, 4000 V; nebulizer, 60 psi; drying
Anions were determined using a Metrohm 861 Advance
gas, 5 L/min; gas temperature, 250 ºC; skimmer voltage,
Compact IC with suppressed conductivity detector, a
65 V; octapole RF Peak, 750 V; and fragmentor (in
Metrosep A Supp 4-250 analytical column and a
source CID fragmentation), 90 V. The mass axis was
Metrosep A Supp 4/5 precolumn with 1.8 mM Na2CO3
calibrated using the mixture provided by the
and 1.7 mM Na HCO3 as eluents with a flow of 1
manufacturer throughout the m/z 40–3200 range. A
mL/min. Cations were quantified by means of a Metrosep
second sprayer with a reference solution was used for
C4 Guard/4.0 precolumn and a Metrosep C4-150/4.0
continuous calibration in positive ion mode using the
column using 1.7 mM HNO3 and 0.7 mM dipicolinic acid
following reference masses: 121.0509 and 922.0098 m/z
as eluent with a flow of 0.9 mL/min. Dissolved metals
(resolution: 21,700 ± 500 at 922.0098 m/z). MS/MS
were determined by ICP/MS using a quadrupole mass
spectra were acquired throughout the m/z 40–950 range
spectrometer Agilent 7700X operating at 3 MHz in
at a scan rate of 0.5 s/spectrum. The collision energy was
helium cell gas mode. The injection volume was 20 µL
optimized to obtain the highest number of fragments. The
for both methods. Spectrophotometric measurements to
full mass spectra data recorded were processed with
obtain UV absorption at 254 nm were carried out in a
Agilent Mass Hunter Workstation Software (version
Perkin-Elmer UV/VIS Lambda 20 (220–700 nm range)
B.03.01).
spectrophotometer. The determination of chemical
oxygen demand (COD), total suspended solids and 2.3. Toxicity tests
alkalinity was performed according to the Standard Multigenerational toxicity was evaluated using an algal
Methods for the examination of Water and Wastewater growth inhibition test according to the Technical
[18]. The dissolved organic carbon, DOC, determined as Guideline OECD TG 201[19]. To this end, we cultivated
total content of organic carbon (TOC) and non-purgeable the green alga P. subcapitata in 96-well microplates
organic carbon (NPOC) was measured using a Shimadzu using a total volume of 200 µL. Each well contained 100
TOC-VCSH analyzer. NPOC was used for wastewater µL of sample and 100 µL of growth medium which was
samples, which had a high content of inorganic carbon. prepared using the required amount of concentrated

Applied Catal. B. Environ., 129, 13-29, 2013
OECD medium (pH adjusted to 8.0 ± 0.1) in order to suspended catalyst was used as bulk concentration for
ensure the same concentration of salts in all samples and calculations. Propranolol solution from a concentrate was
controls. The microplates were placed in an algal growth added to the aqueous suspension of the catalyst and
chamber under continuous fluorescent illumination stirred for one more hour to reach adsorption-desorption
-2 -1
(approximately 100 µE m s ), and incubated at 22 ± equilibrium. Then the run started. The experiments were
1°C. Algal growth was assessed by chlorophyll carried out in ultrapure water and in spiked wastewater
fluorescence (excitation 444 nm – emission 680 nm) using the secondary effluent of the WWTP described
using a Fluoroskan Ascent FL plate luminometer. Algae before as a matrix. Samples were withdrawn for analysis
beads and culture media were purchased from at prescribed intervals.
Microbiotest Inc.
3. Results and discussion
Acute toxicity was assessed by measuring the decrease in
3.1. Kinetics
the constitutive bioluminescence of the marine bacterium
V. fischeri following the procedure described in ISO The degradation of propranolol takes place through two
11348-3 [20]. The measurements were performed using a parallel processes: photolysis and photocatalysis. The
Fluoroskan Ascent FL plate luminometer. The incubation photolysis of propranolol, quantified in runs without
period used in this study was 30 min in all cases. Tests catalyst, is due to the absorption of radiation of a
were performed at 17.9 C±0.3ºC and the decrease of light wavelength lower than 330 nm [22]. Fig. 2 shows the
was monitored using the previously mentioned extent of propranolol depletion with and without catalyst.
microplate luminometer. We used the commercially When using 0.14 g/L of 0.5 % wt. Ce-TiO2, propranolol
available Biofix Lumi test (Macherey–Nagel, Germany). completely disappeared within 2 h of irradiation, whereas
The bacterial reagent is supplied freeze-dried and is for a similar run in the absence of catalyst, about 50% of
reconstituted and incubated at 3ºC for 5 min before use. the initial propranolol was still in solution. DOC removal
The analysis medium was 0.34 M NaCl (2% w/v). The also increased from 3% to 17% for runs with the same
toxicity was measured as the percentage of inhibition amount of catalyst with respect to non-catalytic
with respect to the light emitted in the absence of any irradiation. This difference was attributed to the photons
+
toxic influence after 30 min exposure. absorbed by the photocatalyst, which generate holes (h )
and intermediate oxygen containing oxidants such as
2.4. Experimental setup and procedure
hydroxyl radicals (·OH) which oxidized organic matter.
The photocatalytic reactions were carried out in a Duran The rate of propranolol removal can be expressed as
tubular photoreactor placed in a Solarbox (Co.fo.me.gra follows, where rd and ri represent the homogeneous rate
220V 50Hz) and irradiated by a Xe-OP lamp (Philips 1 of photolysis and photocatalysis respectively:
-6 -
kW) with a photon flux of (6.19 ± 0,20) x 10 Einstein s
1
(290-400 nm) determined by 2-nitrobenzaldhide − dcPRO = r
d + ri [2]
actinometry [21]. The runs were performed at 25ºC. The dt
aqueous suspension containing 25 mg/L of propranolol,
Assuming that the heterogeneous process follows a
was continuously driven to the photoreactor from a
- Langmuir-Hinshelwood expression [23, 24], the reaction
feeding tank and recirculated to it, at a rate of 0.65 L min
1 kinetics for the rate of photocatalysis can be written as
by means of a pump (Ecoline VC-280 II, Ismatec). All
follows:
connections and pipes employed were made of Teflon
and/or glass material to avoid losses by adsorption. The
m KPRO cPRO
temperature was controlled using a Huber thermostatic ri = kT LVRPA [3]
bath. pH was measured with a Crison GLP 22 instrument 1 + ∑Kici
i
and kept constant at 7.5 ± 0.1 by using a 2 mM phosphate
buffer. Dissolved oxygen concentration, with an initial where KPRO is the equilibrium constant for propranolol
value of 8.0 ± 0.5 mg O2/L, was measured by a Crison adsorption and kT is a true kinetic constant independent
Oxi 330i WTW Oxi Cal-SL sensor. By allowing contact of photon absorption that includes the primary quantum
with air in the recirculation chamber, an almost constant yield for electron-hole generation within the
concentration of dissolved oxygen was ensured photocatalyst. The kinetic constant takes into account all
throughout the runs. The catalyst concentration varied in other factors that may affect the overall quantum yield,
the 0-0.25 mg/L range. Prior to the runs, the catalyst was with the exception of the substrate concentration and the
dispersed in 400 mL of pure water using an ultrasonic local volumetric rate of photon absorption (LVRPA). The
homogenizer (Bandelin Sonoplus HO2070) operating at exponent m of the LVRPA depends on the efficiency of
50 W for 10 min (70% amplitude). The catalyst dispersed electron–hole formation and recombination at the catalyst
in the aqueous phase was allowed to circulate through the surface. The data available indicate that at weak radiation
photocatalytic system for one hour. The larger particles intensities, the rate of photocatalytic oxidation is first-
were removed to avoid deposition within the reaction order in radiation intensity and, therefore, m = 1 [25].
loop. The concentration of catalyst remaining in The sum in the denominator extends to all adsorbable
suspension was also determined after being dried at compounds in the mixture including unreacted
105ºC and weighed. The effective concentration of propranolol. Gora et al. [26] and Li Puma et al.[27]

Applied Catal. B. Environ., 129, 13-29, 2013
determined that the dark equilibrium constants for the absorption and scattering coefficients and quantum yield,
TiO2 adsorption of several compounds were of the same have to be averaged over the useful spectrum of the
order of magnitude as those observed under irradiation. incident radiation [29].
They also suggested that the transformation products
Photocatalytic runs were performed with a Ce-TiO2 using
should have equilibrium constants similar to that of the
different cerium loadings: 0, 0.5 and 1.0 % wt., the
parent compounds, all of them being essentially
effective bulk catalyst concentration varying in the 0-
independent of radiation absorption. The denominator of
0.27 g/L range. Fig. 3 shows the increase in the first-
Eq. 5 takes the following simplified form:
order rate constant of propranolol degradation using
1 +∑Ki ci =1 + ∑Ki ci,o = 1 + KPRO cPRO,o [4] cerium-doped catalysts with respect to non-doped TiO2.
i i As indicated in Fig. 1, the presence of cerium shifted the
absorption towards higher wavelengths, which is
Eq. 3 is valid for any point inside the reactor. The
probably the reason of their improved performance. The
effective radiant power absorbed within the reaction
best results were obtained for photocatalysts with a
space can be obtained by integrating the LVRPA:
cerium loading of 0.5 % wt. and rate constants roughly
twice those of 1.0 % wt. cerium loading. This behaviour
∫ LVRPAdV = 2 π ∫∫ r (LVRPA) dr dz [5] 4+
was probably due to the role played by Ce in
V r,z
suppressing electron-hole recombination, which turns

In a reactor with recirculation with a total volume V and into a combination centre at higher dopant concentrations
a reactor volume Vr, the rate law combined with the leading to a decrease in photocatalytic activity. In fact, it
material balance in the batch recirculation reaction has been suggested that the decrease in crystallite size
system yields the following first-order rate expression: results in the creation of oxygen vacancies which could
reduce recombination, this being consistent with our
 
observations concerning crystallite size in doped TiO2
kT 
 ∫ LVRPAdVr  [17]. The inset in Fig. 3 shows the fitting to a first-order
ri = KPRO Vr  cPRO [6] rate equation of catalytic and non-catalytic runs. The rate
V 1 + KPRO cPRO,o constant increased with catalyst load in all cases, as
indicated in Fig. 3, in which the error bars represent 95%
Finally, the LVRPA can be calculated from the two-flux-
confidence intervals. The relationship was essentially
absorption-scattering model proposed by Li Puma and
linear, as expected from the dependency of rate constants
Brucato [28]:
on bulk catalyst concentration, cs, expressed in Eq. 5.
LVRPA = κ cs Io e−(κ +σ )cs δ 1−ω2 [7] The degree of mineralization, measured through
dissolved organic carbon (DOC), was 8.5% and 17.4%
The presence of adsorbing compounds does not alter the for catalysts with 1.0 and 0.5 of cerium % wt.
scattering coefficient, σ, which is equal to that of the respectively after six hours of irradiation using a catalyst
photocatalyst particles. In Eq. 7, κ is the specific concentration of 0.14 g/L. The behaviour was similar in
absorption coefficient of the catalyst, ω the scattering other conditions, with the catalyst doped with 0.5 % wt.
albedo, and Io the fluence rate at reactor wall. On the cerium being the most effective in all cases. Although
other hand, the absorption coefficient of the medium is a higher catalyst concentrations (up to 0.27 g/L) led to
sum of the absorption coefficients of all dissolved higher rate constants (Fig. 3), we chose the concentration
compounds and the catalyst: κ cs + Σκi ci. of 0.14 g/L for most runs because of the lower scattering,
this being the most probable reason for the inflection
The rate of the photochemical process, rd, depends on the
observed in Fig. 3 at the highest catalyst loadings.
overall quantum yield and on the LVRPA, in which κ cs
The contribution of hydroxyl radicals to propranolol
of Eq. 7 must be substituted by Σκi ci or κPRO cPRO.
degradation was assessed employing a competitive
−
σ cs +∑κi ci  δ 1−ω2 kinetic approach with pCBA as hydroxyl radical probe.
rd = ΦPRO κPRO cPRO Io e  i  [8] pCBA has been reported for this purpose in UV stud
9ies -1
due to its rapid reaction with •OH (k•OH,pCBA = 5 x 10 M
-1
Assuming again that Σκi ci is essentially coincident with s ) and its low quantum yield for direct photolysis (0.026
κPRO cPRO due to the contribution of transformation using wavelengths in the 250–350 nm range) and low
products from the parent compound, both ri and rd radiation absorption above 290 nm (Duran cut-off),
become first-order in the concentration of propranolol, which implies a low rate of photolysis compared to the
with the following global rate expression: •OH pathway [30]. Besides, no significant adsorption of
pCBA has been observed in dark contact with the
− dcPRO = (kd + ki ) cPRO = k cPRO [9] photocatalyst and, therefore, the direct oxidation of
dt pCBA by holes was considered negligible, the compound
being only depleted by bulk hydroxyl radicals. The
It is interesting to note that, when applied to a
exposure to •OH could be obtained from kinetic data as
polychromatic radiation, all relevant quantities, namely
follows:

Applied Catal. B. Environ., 129, 13-29, 2013
cpCBA,o Photocatalytic degradation runs were also performed
ln = kOH , pCBA
∫ c•OH dt [10] using a wastewater matrix with the Ce-TiO2 (0.5% wt.
cpCBA
Ce) and a catalyst bulk concentration of 0.14 g/L. The
results are shown in Fig. 5. In photolytic runs without
Coming back to Eq. 7, the apparent first-order constant
catalyst, it was observed that the first-order rate constant,
for photocatalysis, ki, is the consequence of two
contributions: the oxidation of compounds adsorbed in kd, was higher than that of the runs performed in pure
water (85% higher). This was probably due to the
catalyst holes, ki,h+, and the bulk reaction intermediated
consequence of the photolysis of nitrate and nitrite,
by hydroxyl radicals ki,•OH:
which has been shown to induce the formation of
dcPRO hydroxyl radicals by absorbing light at 360 nm and 310
− = (kd + ki,h+ + ki,•OH c•OH ) cPRO [11]
nm respectively [32]. First-order catalytic rate constants,
dt
k, were higher than those of pure photolytic runs (15%
Integrating Eq. 11, and combining it with Eq. 10, it is higher), but the difference was much lower than that
possible to relate the logarithmic concentration decay encountered in experiments performed in pure water, in
rates of propranolol and pCBA: which the catalyst increased the degradation rate by up to
four times. This dramatic decrease in the efficiency of the
cPRO,o ki,•OH cpCBA,o
ln
cPRO = (kd + ki,h+ ) t + ki,•OH ∫ c•OH dt = (kd + ki,h+ ) t + kOH , pCBA ln cpCBA photocata-1lyst, reflected in ki v-1alues dropping from 1.71 ±
0.15 min to 0.06 ± 0.01 min , was probably in part a
[12]
consequence of the presence of radical scavengers in
The second order rate constant for the reaction of wastewater, such as bicarbonates, phosphates or organic
propranolol and hydroxyl radicals, ki,•OH, was measured matter. The important role played by catalytic holes in
10
by Benner et al. who reported a value of 1.0 ± 0.2 x 10 reactions performed in pure water suggests that other
-1 -1
M s [31]. By plotting the time-independent terms of compounds in wastewater compete with propranolol for
Eq. 12 as a function of time (Fig. 4), the slope yields the surface adsorption, which would explain this reduction in
experimental value of kd + ki,h+, the sum of the first-order efficiency. Dimitroula et al. also found a decrease in the
rate constants for direct photolysis and photocatalytic oxidation rate of several micropollutants of one order of
oxidation of propranolol in catalyst holes, ki,h+. The result magnitude when comparing the reaction in pure water
was 2.50 x 10-2 ± 4 x 10-4 min-1, and by subtracting the with a wastewater matrix [33]. Ioannou et al. showed that
experimental value of kd (8.2 x 10-3 ± 6 x 10-4 min-1), the mineralization practically did not take place at all when
calculated rate constant for the reaction with holes was irradiating β-blockers with solar light in the presence of
1.6 x 10-2 ± 1 x 10-3 min-1. It was assumed that a low TiO2 [34]. They assumed that photo-generated reactive
fraction of radiation is absorbed by the catalyst. On the species were consumed by attacking organic compounds
other hand, the rate constant, k, for the photocatalytic without converting them into carbon dioxide and in the
degradation of propranolol under the same conditions reaction with chlorides and other scavengers. As a
was 3.53 x 10-2 ± 3 x 10-4 min-1 (Fig. 3). Combining the consequence, the conversion of propranolol approached
-2 -4 - zero (from 20-30%) when dissolving it in a secondary
former results, ki = ki •OH c•OH = 1.03 x 10 ± 7 x 10 min
1 treated wastewater effluent. Similar results have been
and, therefore, it can be estimated that 60% of the
reported elsewhere. Dialynas et al. reported that the
photocatalytic reaction took place through surface holes
organic matter contained in treated wastewater is
(h+) for a catalyst load of 0.14 g/L, the rest being
considerably refractory towards photocatalytic oxidation,
mediated by hydroxyl radicals. Yang et al. calculated that
with a very low removal rate of dissolved organic carbon
77.5% of the UV-TiO2 degradation of propranolol was
[35]. Our COD results point in the same direction and
due to the reaction with hydroxyl radicals, with 19.2%
showed a 13% decrease (28% in pure water), which was
being due to the reaction with oxidizing catalyst holes
essentially the same value obtained without catalyst.
[23]. The difference was probably due to the different
However, concerning DOC, we obtained a 14 % decrease
catalyst load (one order of magnitude greater) which
could leave a high catalyst surface area free to produce after 6 h (0.14 g/L, 0.5 % wt. Ce-TiO2), only somewhat
lower than that in pure water (17%, Fig. 2) and
hydroxyl radicals from water, and to the different light
considerably higher than that of pure irradiation runs (<
source (UV instead of visible light). Adsorption
5%).
experiments performed in the dark enabled us to
calculate that under the aforementioned conditions, about 3.2. Identification of reaction intermediates and reaction
20% of the propranolol in solution became adsorbed on pathway
the catalyst surface (pH 7.5, buffered). This high
The identification of propranolol transformation products
adsorption, shared by other β-blockers as noted by Yang
was performed on samples with 25 mg/L of propranolol.
et al. suggest that degradation is prone to occur mainly on
In catalytic runs, 0.14 g/L of 0.5% wt. Ce-TiO2 was used.
the surface of TiO2 by oxidation with holes [23]. These
The assignment was based on accurate mass
ideas are supported by the fact that experimental runs
measurements recorded by the LC-QTOF-MS/MS
with Ce-TiO2 resulted in a lower rate constant when
instrument described above, operating in positive mode
performing runs at pH 6 in comparison with pH 7.5 (23%
(ESI+). MS/MS spectra were acquired at optimized
lower).

Applied Catal. B. Environ., 129, 13-29, 2013
collision energies to increase fragmentation and thus and P14 were measured in relatively high amounts in the
improve structural information. These measurements photoproducts of solar irradiation (Fig. 7 c and d). The
allowed elemental compositions to be proposed for the main products of photocatalysis retaining the structure of
+
protonated [M+H] molecular ions and their propranolol were P26 (m/z 294.1700, C16H24NO4), P27
characteristic product ions, thus providing a high degree (m/z 308.1492, C16H22NO5), and P29 (m/z 310.1649,
of confidence in structure assignation. Table 2 displays C16H24NO5) all of which are the consequence of the ring-
the ion formula and calculated mass of the product ions, opening attack of hydroxyl radicals on the naphthol
as well as relative mass error and DBE (double bond and moiety.
ring equivalents). The second column of the table
Some other fragments from the ether bond cleavage were
indicates “c” and “s” for transformation products
detected in photocatalytic runs, namely P2 (m/z
identified in photocatalytic runs and solar irradiation
118.0863, C5H12NO2), P3 (m/z 132.1019, C6H14NO2), P5
respectively. “p” stands for the parent compound,
(m/z 150.1125, C6H16NO3) and P6 (m/z 164.0917,
propranolol, for which the accurate mass measurements
C6H14NO4), some of which were absent in non-catalytic
recorded (m/z 260.1645 for C16H22NO2) offer an
runs. Non-catalytic solar irradiation yielded a
excellent agreement of less than 1 ppm error, with
considerably higher amount of the more complex
calculated m/z value. Propranolol product ions yielded
transformation products. These included, the
characteristic signals at m/z 218.1176, 183.0804 and
hydroxylated transformation products P9 and P10 (m/z
157.0648 that corresponded to cleavages in the aliphatic
274.1438, C16H20NO3), with a hydroxyl group in the
chain of isopropyl, aminoisopropyl plus water, and C2H2
aromatic moiety but in which the amine had been
groups, respectively (Table 2). These fragments still
oxidized to a hydroxylamine. P25 (m/z 292.1543,
retained the naphthalene structure. Additionally, product
C16H22NO4) is a naphthol ring-opening product which
ions at m/z 116.1070, 98.0964, 74.0600, 72.0808 and
was detected in much higher amounts in solar runs than
56.0495 correspond to transformations in the side chain
in photocatalytic oxidation. In general, solar irradiation
after cleavage of the ether bond of propranolol. As with
provided a higher amount of hydroxylated and
propranolol, the characteristic fragmentation of oxidation
polyhydroxylated intermediates (such as P23, m/z
by-products provided enough information for the
292.1543, C16H22NO4) in spite of a lower formation of
identification of over thirty transformation products. The
ether cleavage products. Moreover, compounds such as
appearance of characteristic fragments in the set of
P4, P9 and P28, among others, tended to accumulate in
product ions spectra indicates the prevalence of a certain
the solar irradiated mixture, while exhibiting a maximum
fraction of the molecule and suggests the place it should
in photocatalytic runs followed by a decrease after a
occupy in the transformation pathway, a proposal which
period of 1-3 h on stream. This is a consequence of the
is depicted in Fig. 6. Compounds P2, P3, P5 and P21
enhanced oxidation capacity of the photocatalytic system
were only encountered in samples from photocatalytic
in comparison with pure solar irradiation.
runs, the rest having been detected in all samples.
Liu and William [36] and Liu et al. [37] studied the
The relative abundance of the main identified compounds
photodegradation of propranolol by radiation in the 295-
is given in Fig. 7. The most abundant transformation
800 nm range and determined that the main
product obtained in photocatalytic runs was P4 (m/z
transformation product was an isomer of m/z 292
134.1176, C6H16NO2), which is attributed to the cleavage
(protonated), for which they proposed a ring-opening
of the ether bond of propranolol. Yang et al. (2010)
structure. In this study, we also found a series of ring-
studied the TiO2 photocatalytic degradation of
opening products, namely P21, P22, P24 or P25 as well
propranolol (and other β-blockers) and found that the
as a hydroxylation derivative, P23, all of them with m/z
main degradation products corresponded to hydroxylated
292.1543, P25 being similar to Liu’s proposal except for
forms or the parent molecule. They detected several
a different ring-opening position. Romero et al. studied
monohydroxylated and polyhydroxylated isomers as well
the TiO2 photocatalytic degradation of propranolol using
as the product of the ether cleavage. The latter resulted in
visible light radiation [38]. They identified four reaction
the formation of naphthol and an intermediate whose
intermediates, of which m/z 266, 282 and 292
protonated form was detected at m/z 134 assumed to be
corresponded to our compounds P7, P18 and P25. For the
an aminodiol with ion formula (C6H16NO2), which
others, at m/z 308, we proposed a different structure
corresponds with P4. The addition of hydroxyl groups to
(P27-P28, C16H22NO5) with a carboxylic acid instead of a
the aromatic nuclei was probably the first step in the
hydroxylamine. This is justified by the presence of the
degradation pathway. The first photoproducts identified
characteristic fragments C6H12N and C10H7O3 that could
in this study were the isomers P11-P16 (C16H22NO3, m/z
not be explained otherwise. It is interesting to note that
276.1594), which represent the addition of 16 mass units
similar compounds can be obtained as a consequence of
to the parent compound, imputable to monohydroxylated
biological degradation. Marco Urrea et al. studied the
intermediates. P11-P16, however, represented only a very
biological advanced oxidation of propranolol using the
low amount of the whole set of transformation products
extracellular oxidizing species produced by the fungus
detected in photocatalytic runs, none of them being
Trametes versicolor [39]. The main degradation products
among the fourteen displaying higher chromatographic
were identified as monohydroxylated derivatives from
areas and included in Fig 7 a and b. In contrast, P11, P13

Applied Catal. B. Environ., 129, 13-29, 2013
the hydroxyl radical attack on the naphthalene moiety compound [37]. On the other hand, the formation of toxic
and would correspond to our P11-P16 set. or non-degradable compounds during the early stages of
propranolol irradiation or photocatalytic oxidation has
3.3. Toxicity of partially oxidized mixtures
been suggested elsewhere. Romero et al. showed that the
Fig. 8 shows the growth inhibition of biodegradability of visible light irradiated samples
Pseudokirchneriella subcapitata when exposed to increased with time, but only after a period of several
propranolol and partially irradiated mixtures. Growth was hours [38]. During the photocatalytic treatment of
completely inhibited both in pure water and spiked propranolol using TiO2. Ioannou et al. found that the
wastewater for untreated propranolol solutions, as their toxicity of treated mixtures to Daphnia magna increased
concentration was considerably higher than the EC50 during the first part of the reaction to progressively
value, which is 0.77 mg/L [37]. The irradiation of decrease thereafter [34]. This implies the formation of
propranolol decreased the toxicity towards the growth of toxic transformation compounds that could be eliminated
P. subcapitata leading to essentially non-toxic mixtures upon prolonged irradiation.
after 2 h in the presence of 0.14 g/L of Ce-TiO2 (0.5 Ce
4. Conclusions
wt. %). For non-catalytic irradiation, the detoxification
led to growth inhibition of < 5% after 4 h (Fig. 8a). The The visible light photocatalytic degradation of
effect of the presence of the catalyst was much more propranolol can be carried out using cerium doped
marked when using spiked wastewater as matrix. In this titanium dioxide. The results showed that, for a cerium
case, irradiation led to growth inhibition of still 16% after loading of 0.5 % wt. with a bulk catalyst concentration of
6h, whereas the photocatalytic process removed most of 0.14 g/L, propranolol became essentially depleted after
the toxicity within the first two hours (Fig. 8b). In Fig. 1.5 hours of irradiation. In contrast, 8% of initial
8b, we also represented the toxicity of irradiated propranolol remained in solution after 6 h of photolytic
wastewater that inhibited algal growth by 8%, which (non-catalytic) treatment. The effect was related to an
increased up to some 25% for prolonged exposures. It is increased light absorption in the visible region of cerium
interesting to note that during the first hour of treatment doped TiO2.
of propranolol spiked wastewater, there was no
The contribution of hydroxyl radicals to propranolol
noticeable effect on mixture toxicity, which is due to the
degradation was assessed using pCBA as hydroxyl
lower rate of oxidation or photodegradation in the
radical probe. A kinetic competition approach allowed
presence of the compounds dissolved in wastewater.
determination of the rate constant for the photocatalytic
Fig. 9 shows the results obtained for the marine bacteria oxidation of propranolol in catalyst holes. Thus, it was
Vibrio fischeri. Escher et al. determined a median effect estimated that 60% of the photocatalytic reaction took
value of 81 mg/L for 30 min contact time of propranolol place through surface holes (h+), the rest being mediated
with V. fischeri [40]. Propranolol toxicity is much lower by hydroxyl radicals.
than for the green alga and represented a 15% inhibition
Experiments carried out in biologically treated
for the untreated solution in pure water. The toxic effect
wastewater spiked with propranolol showed a very low
of propranolol also increased the background toxicity of
rate for the photocatalytic process compared with pure
wastewater, but only to a modest 10% till about 55%
water. This could be attributed to the presence of radical
bioluminescence inhibition. The non-catalytic irradiation
scavengers but also to substances competing for surface
of propranolol, either in pure water or in wastewater,
adsorption.
resulted in a moderate toxicity reduction. The
photocatalytic degradation of propranolol in pure water, Over thirty reaction intermediates were detected by
however, led to a sharp bioluminescence decrease, which means of exact mass measurements performed by LC-
reached almost 80%. The fact that a similar treatment in ESI-QTOF-MS/MS. The main transformation products
wastewater matrix did not cause a similar toxicity peak could be attributed to the cleavage of the ether bond of
was probably due to the accumulation of transformation propranolol, while other compounds derived from the
products such as P17 or P28. The results with P. addition of hydroxyl groups to the aromatic nuclei or to
subcapitata (Fig 8a) also show that photocatalytic the ring-opening attack of hydroxyl radicals on the
treatments induce a significant increase in toxicity after naphthol moiety.
about 3 h of treatment. The higher toxicity of partially
The toxicity of oxidized mixtures determined using the
oxidized mixtures was probably the consequence of the
green algae P. subcapitata and bioluminescent marine
formation of ring-opening products, as these are the kinds
bacterium V. fischeri showed the formation of toxic
of compound that have been associated in the past with
transformation products, which accumulated in runs
increased toxicity in partially oxidized mixtures [41, 42].
performed in pure water, for which the photocatalytic
Liu et al. used the alga Pseudokirchneriella subcapitata reaction rate was much higher.
and the rotifer Brachionus calyciflorus to conclude that
Nomenclature
propranolol yields phototransformation products that
-1
would generally be less toxic because of their higher ci, cj concentration of a given organic compound, mol L
-1
polarity and hydrophilicity compared to the parent cs bulk concentration of solids in the liquid phase, kg L
-1
c•OH concentration of hydroxyl radicals, mol L

Applied Catal. B. Environ., 129, 13-29, 2013
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-1 -1
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