Photolysis and TiO2 Photocatalytic Treatment of Naproxen: Degradation,
Mineralization, Intermediates and Toxicity
Fabiola Méndez-Arriaga, Jaime Gimenez, and Santiago Esplugas*
Chemical Engineering Departament, University of Barcelona, Marti i Franques 1, 08028, Barcelona, Spain
Abstract: The heterogeneous photocatalytic degradation of Naproxen using TiO2 as photocatalyst was investigated.
Effects of TiO2 loading, temperature, volumetric flow and dissolved oxygen concentration, as operational parameters,
were studied. The experiments were carried out in a 0.078 L Duran reactor equipped with a 1kW Xe-lamp. After 3
hours of photolysis, a Naproxen aqueous solution (0.8 mmol//L) results in a 90% removal with only a 5% of
mineralization whereas the TiO2 photocatalysis leads lower removal (40%) with better mineralization (20%).
Identification of byproducts has shown that demethylation and decarboxylation are the principal initial processes in
the degradation of NPX. The toxicity of the treated solution was evaluated using the Microtox test based on the
bioluminescent bacteria Vibro fisheri, in order to compare the acute toxicity of Naproxen and its photoproducts.
Photocatalysis did not show an improvement in the biodegradability under the operational conditions tested. However,
mineralization data are promising for future studies.
Introduction
Nowadays the excessive use of pharmaceutical
compounds had leaved an unexpected consequence on
the aquatic environment. Drugs and/or their metabolites
go into the sewage treatment plants (STPs) after
human or animal consume, from several sources like
hospital, industry or municipal wastewaters (1). The
incomplete elimination of pharmaceutical compounds
in STPs is already reported in several countries (2, 3,
4). These kinds of pollutants are able to across the
STP without further degradation and finally are discharged to surface waters as rivers or lakes. Naproxen
(NPX) is a Non-Steroidal Anti-Inflammatory (NSAI)
drug and it is one of the most frequently found as
recalcitrant in surface waters (5, 6). It is widely prescribed for the skeleton-muscle pain or inflammatory
rheumatic disorders due to its analgesic and antipyretic
effects. NPX is an aromatic compound characterized
for its 2-arylpropionic acid group, typical of the
NSAI drugs. NPX has been already detected in STP
influents into the range of 250 ng/L to 1.5 g/L, and
its removal has been estimated around 71%. Its
presence has already shown harmful toxicological
consequences (7) and adverse ecological impact on
the microbiological aquatic systems (8).
Since STPs are not sufficiently effective to eliminate pharmaceutical compounds like NPX, new alternative techniques, as the so-called Advanced Oxidation Processes (AOPs), have been tested to diminish
its concentration in water. AOPs make use of different
reacting systems mainly characterized for the promoKeywords: NSAID, Naproxen, photocatalysis, photolysis
*Corresponding author; E-mail address: santi.esplugas@ub.edu
ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
tion of hydroxyl radicals (.OH), and due to its nonselective and powerful oxidative characteristics, several
AOPs have reached good removal rates for recalcitrant
pharmaceutical pollutants in wastewater. For instance
amoxilline, ibuprofen or bisphenol-A using O3, photocatalysis and sonolysis respectively (9, 10, 11) are fine
example of AOP environmental aquatic remediation.
In the case of NSAID several conventional,
advanced or combination of oxidative treatments have
been studied. For example NPX has been degraded
by chlorination, biofilm (12), photolysis (13, 14, 18)
and combined heterogeneous photocatalysis with
separation by nanofiltration (15). Some results of the
treatments above mentioned have shown higher toxicity
of the effluent due to the remained byproducts. NPXbyproducts produced from the chlorination and
photolysis treatment have shown low biodegradability
and high ecotoxicity on algae, rotifers and microcrustaceans organisms. Although some degradation
processes of NPX have been developed, a study
aiming the biodegradability of byproducts from photocatalysis irradiated treatment is still lacking. In the
last basis, the objective of this investigation is to
undertake a study of the heterogeneous photocatalytic
degradation of NPX in aqueous suspension with TiO2
in order i) to identify the main byproducts remained
after photocatalytic treatment and ii) to evaluate the
biodegradability and toxicity of the treated dissolution.
Material and Methods
NPX-Na ((S)-6 methoxy -methyl 2-naphthaleneacetic sodium salt) was purchased from SigmaAldrich and it was used without pre-treatment. Table
1 shows some chemical and physical properties of
J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
436
Figure 1. Experimental laboratory equipment.
Table 1. Physical and chemical properties of NPX.
(C14H14O3)
Structure
Aqueous solubility
15.9 (mg/L) (25 ºC)
Melting Point
152-154 ºC
Log KOW
3.18
pKa
4.15
NPX [SRC Phys. Prop. Database, 2007]. Titanium
dioxide (TiO2) Degussa P-25 (commercial catalyst
~70% anatase, ~30% rutile, surface area 55 m2/g (19))
was used as received. Fresh daily solutions of NPX
were prepared in milliQ water. Irradiation experiments
were carried out with a solar simulator (Solarbox, 1500
Co.fo.me.gra, 220V, 50Hz) equipped with a 1 kWXenon lamp (Phillips) and a tubular-horizontal
photoreactor
(0.078
L
illuminated
volume)
concentrically located between the reflected wallmirrors inside of the box. The photon flux inside of the
photoreactor was evaluated by actinometrical uraniloxalic procedure and it was calculated of 6.3
Einstein/s (290 nm < 
One and a
half L of NPX-TiO2 aqueous mixture were fed in an
external vessel and pumped to the Solarbox. Pure
oxygen (Air Liquide ®) purge was employed and O2
concentration was measured on line by Crison Oxi 330i
WTW Oxi Cal-SL sensor. Temperature of the stirred
vessel was kept constant through an ultra-thermostat
bath (Selecta; Frigiterm –10). Figure 1 depicts a scheme
of the experimental equipment employed.
Samples were withdrawn at several intervals of
time and immediately filtered with a Durapore PVDF
Millipore filter (0.22 m). NPX concentration was
followed by a HPLC Waters using a C18-RP Trace
ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
Extrasil OD52 5 Micromet 25 x 0.46 Teknockroma
column. The mobile phase was an acetonitrile (Panreac
99.8%) and 0.05 M phosphate dehydrogen ammonium
(Aldrich 98% ACS) solution (0.8/0.2) employed in
isocratic mode (1 mL/min). Detection of NPX was
carried out at 254 nm. Dissolved organic carbon (DOC)
and absorption spectrum were obtained by a Shimadzu
TOC-V CNS and a Perkin Elmer UV/Vis Lambda 20
spectrophotometer respectively. Biochemical oxygen
demand (BOD5) and chemical oxygen demand (COD)
were carried out according to Standard Methods (5120)
by respirometric single measuring system OxiTop and
Norm. France NFT 90-101 respectively. Toxicity
measurements were carried out using the Microtox test
measuring the inhibition of bioluminescence of Vibro
fisheri at 15 min of exposure time. Identification of
intermediate products was carried out by electro spray
ionization/ mass spectrometry (ESI-MS) using a Jasco
AS-2050 plus IS mass spectrometer.
Results and Discussion
Preliminary Experiments: Thermo-Degradation
and Adsorption Experiments
As preliminary control experiments and in order to
evaluate possible thermo-degradation or losses by
volatility, 1 L of NPX solution (0.8 mmol/L) without
catalyst and under no pH or ionic-strength control, was
placed in the stirred tank and heated at 20, 40, 60 and
80 ºC. Once reached the step-temperature, samples were
withdrawn and NPX concentration and DOC were
measured. pH and temperature were online measured.
Figure 2 depicts the NPX, DOC and pH evolution for
each 20 ºC-step. It can be noticed that NPX
concentration and DOC remained unchanged during
every step-change of temperature. Thus, NPX do not
suffer any thermo-decomposition or loss by
J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
437
Figure 2. Evolution of NPX, DOC and pH for each increase of
increase.
(a)
volatility in the temperature range tested. On the
other hand, since in heterogeneous systems the
adsorption plays an important role in the evolution of
the photodegradation, adsorption experiments at
constant temperature were carried out. Several ratios
of NPX/TiO2 were mixed in 30 mL vials at free pH
conditions (pH 6.150.15). In the first experimental
series, 1 g/L of TiO2 and several initial concentration
of NPX (ranging from 2 to 1000 mg/L) were assorted.
In the second series, 500 mg/L of NPX were mixed
with TiO2 ranging from 0.03 to 1 g/L. Constant temperature (30 ºC), stirring and close-dark surroundings
were always controlled during 24 h. Samples were
carefully withdrawn from the supernatant and then
filtered with a PVDF 0.22m filter. The equilibrium
amount of NPX was determined by difference between
the initial concentration and the measured supernatant
concentration for each NPX/TiO2 ratio.
Figure 3a depicts the amount of NPX in equilibrium and the amount adsorbed on 1 g/L of TiO2
varying the NPX concentration and Figure 3b shows
the adsorbed amount of NPX and the NPX equilibrium concentration for several quantities of catalyst
for 500 mg/L of NPX. As it is possible to observe
from both figures, the amount remained in the
solution is between 92 and 95% independently of the
initial concentration of NPX and TiO2. The adsorbed
amount is ca. 5 and 8% in the wide range of TiO2
tested. Under free pH conditions, close to the point
zero charge of TiO2 (6.5), NPX is fully soluble in
water. A low adsorption was observed due to no
electrostatic attraction between the surface charge of
TiO2 and NPX in anion form is accomplished. A
highest chemisorption between NPX and TiO2 would
be observed of pH adjusted between 4.1 and 5.3,
which corresponds to the pKa of NPX and TiO2 (20)
respectively. In this range the positive charges of the
surface of the catalyst attracts the anionic form of the
compound and the photocatalytic degradation would
be expected on the surface of the catalyst. However,
due to the typical wastewater streams have pH values
ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
(b)
Figure 3. (a) Adsorption of NPX on 1g/L TiO2 varying the initial
concentration of NPX, without adjust of pH (6) and 30 ºC. (b)
Adsorption of 500 mg/L of NPX varying the concentration of
TiO2, 30 ºC and without adjust of pH (6).
between 6 and 9 (21) and in order to avoid extra pH
adjustment before biodegradability test, no control of
pH was selected for the photocatalytic degradation
assuming the low adsorption percentage between NXP
and TiO2 and that the most possible way of degradation could be reached by migration of ·OH radicals to
the bulk of the suspension.
Degradation of NPX by Photolysis Versus
Photocatalytic Process
To evaluate the degradation of NPX by photolysis,
a 1.5 L NPX solution (0.8 mmol/L) was exposed to
the solar irradiation with the Xe-lamp simulator source.
On the other hand, the degradation of NPX by photocatalytic means was carried out using 0.1 g/L of TiO2
under the same irradiation conditions before described.
In Figure 4 it is depicted the degradation of NPX in
both cases.
As it is possible to observe, NPX concentrations
decrease until 75% when it is continually exposed at
Xe lamp during 2 hours in absence of catalyst with an
initial rate of degradation of 6 mol/min. On the
contrary, an important difference is observed in the
J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
438
0.012
7
0.01
6
0.008
5
4
0.006
3
kr (min-1 )
Initial degradation rate (mol/min)
8
0.004
2
0.002
1
0
0
Photolysis
0.1
0.5
1
TiO2 (g/L)
Figure 4. Degradation of NPX by photolysis and photocatalysis
using 0.1 g/L of TiO2.
case of degradation of NPX when catalyst is present
leading almost 40% removal in the same interval of
time. The chemical transformation of organic compounds by photolysis is carried out through the
fundamental deactivation of the excited states undergoing in a certain number of primary photochemical
processes: rearrangement, formation of radicals, isomerization, ionization etc. (22). Thus the degradation
of organic compounds by photolysis starts when the
target compound absorbs specific energy and reaches
an activation process promoting the electronically
excited molecular states. NPX degradation by photolysis is reached due to its absorption spectrum overlap
with the Xe-lamp emission spectrum in the range of
290 and 350 nm. Thus several pathways are followed
until almost full degradation of the compound in 3 h
is observed. Our experiments have concordance with
previous investigations founded in references (13, 14,
17, 18).
On the other hand, the degradation of NPX by
means of photocatalytic process follows another well
differentiated and recognized process. In suspension,
the photocatalytic mechanism is based on the energy
available to be absorbed for the catalyst (TiO2)
normally accepted between 300 and 400 nm. Under
exited condition, the valence band-electrons are transferred to the conduction band forming the hole

electron pair ( hvb  ecb ) (Eq. 1). The superoxide
radical anions (Eq. 2) and hydroxyl radicals together
with H+ (Eq. 3) are formed by the reduction of the
oxygen and by cleavage of adsorbed molecules of
water.
TiO2 + hv → TiO2* (h+vb /e-cb)
Eq. 1
e- + O2→ ·O2-
Eq. 2
h + H2O →·OH + H
+
+
Eq. 3
When the electron migrates to the surface of
catalyst, most of the above reactions are reached on
ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
Figure 5. Influence of TiO2 loading on the initial degradation
rate and reaction constant.
the irradiated surface of TiO2. As well recognized the
·OH radical is one of the most powerful oxidants
with a redox potential of 2.80 E0V (20). If organic
compounds are adsorbed on the surface of the
catalyst, the ·OH non-selective attack promotes the
cleavage of compound bounds. If not important
adsorption is observed, diffusion of ·OH and/or ·O2radicals to the bulk of the dissolution will be the
dominant process in the degradation. In our case it
was possible to observe that 40% of the initial NPX
was degraded under photocatalytic process.
The low adsorption of NPX on the catalytic surface and the short time-live of the ·OH are some of
the reasons to observe the strong difference between
the degradation by photolysis and photocatalysis -2
folds higher-. Part of the ·OH generated are
recombined, loosed or deactivated before they react
with NPX present in its full soluble state in the bulk
of the dissolution.
Photocatalytic Mineralization
In order to evaluate the effect of the TiO2 loading
on the DOC removal, concentrations of catalyst from
0.1 to 1 g/L were tested for 0.8 mmol/L NPX solution.
In Figure 5 it is depicted the degradation rate of NPX
and the pseudo-first order constant for the initial time
of reaction including the observed by photolysis.
Results showed that the optimal amount of TiO2 to
reach the maximum initial degradation of NPX was
the highest tested. An important remark is that the
initial degradation for photolysis and photocatalytic
experiments is quite similar (between 3 and 7 mol/
L·min). However, after 180 min, DOC removal and
NPX degradation are maxim for a TiO2 concentration
of 0.1 g/L and decrease when TiO2 concentration
increases to 0.5 g/L. From this value, an increase in
the TiO2 concentration does not seem to influence in
a high manner, on the DOC removal and NPX
degradation (see Table 2). The last can be explained
due to at the beginning of the reaction, the most
J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
439
Table 2. Effect of TiO2 load on NPX degradation and DOC
removal at 180 min.
TiO2
g/L
Photolysis
0.1
0.5
1
NPX degradation
mmol/min
0.0400
0.0177
0.0088
0.0084
DOC removal
%
5
20
11
9
important influence in the degradation is the ·OH
attack and it is governed for the catalyst loading.
However throughout the process, slower degradation
rate of NPX can be attributed to the apparition of
probably hydrophobic byproducts which present more
affinity to the surface of TiO2. Preferential degradation
of these hydrophobic compounds reduces the probability to follow the degradation of NPX.
On the other hand operating with the minimum
amount of TiO2, there was a slow degradation of
NPX at the initial time of reaction. This fact may be
explained by the scarcely diffusion and contact of the
·OH able to reach the compound. However, after 180
min, the global NPX degradation and DOC removal
is the highest observed due to two important degradation pathways could be occurring at the same time:
photolysis and ·OH attack of NPX and byproducts.
Photolysis showed a DOC removal of 5% and almost
double mineralization was observed for photocatalysis
using 1 g/L of catalyst -the maximum employed-.
However the higher DOC removal is observed using
the lowest amount of catalyst (0.1 g/L). Only under
low amount of TiO2 both process (photolysis and
photocatalysis) contribute to improve the DOC removal.
Another experimental series varying the volumetric rate (0.1 to 0.4 L/min) and temperature (20 to
40 oC), with 0.1 g/L of TiO2, were carried out following the evolution of NPX and DOC concentration. In
Fig 6a it is depicted the DOC change for several
conditions including the photolysis case. It is noticed
that DOC removal is not dependent of the volumetric
flow or of temperature into 20 to 30 ºC. However if
photocatalytic degradation is maintained at 40 ºC, an
evident increase of the DOC removal is observed
(26%) (see Figure 6b). It is generally accepted that
the photocatalytic reaction rate is not considerable
modified with the variation of temperature (20).
Nevertheless, if volatile compounds are generated,
slight increase in the DOC removal could be attributed
to the formation of a quiet higher thermolabile dissolution, as shown in the photocatalysis degradation
behavior of NPX.
In order to evaluate the effect of the presence of
oxygen during the photocatalytic degradation, O2 was
ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
(a)
(b)
Figure 6. (a) Temperature and recirculation rate effect on DOC
removal with 0.1 g/L TiO2. (º) photolysis; () 0.1 L/min; (■) 0.2
L/min; (×)20 ºC; (+) 30ºC (ж)40 ºC. (b) NPX conversion
(continuous bar line) and DOC removal (doted bar line) at 180 and
240 min of treatment for 20, 30 and 40 ºC using 0.1 g/L of TiO 2.
purged into the stirred-tank and maintained its concentration around 402 mg/L. Figure 7 depicts the
DOC removal and NPX degradation under oxygen
saturated conditions. In presence of oxygen the
complete degradation of NPX was observed in 120
min with an initial degradation rate of 4.4 mol/min see Figure 5-. In photocatalysis it is well known that
the oxygen enhances the photocatalytic activity (22,
23, 24, 25). Due to NPX was complete removed it is
possible to assure that the presence of oxygen promotes
less recombination and more efficiency on the generation and reactivity of ·OH and O2.-. Several authors
have reported the important influence of the oxygenated
surroundings in the photolysis of NPX (18). However
oxygen in saturating conditions did not any enhance
in the DOC removal (see Figure 7). Photocatalytic
reaction under O2 saturated conditions could promotes
parallel reactions involving the organic radicals
generated among the ·OH, H+, ·O2·-, etc with NPX,
however no higher mineralization is achieved due to
possible reactions also with the byproducts produced.
J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
440
Table 3. Intermediate compounds identified by LC/ESI-TOF-MS: NPXo 0.8 mmol/L; pH free; 40 mg/L O2; 40 ºC; 0.1 g/L TiO2, solar
simulated irradiation during 3 hours.
ID
Compound
Ret. Time
min
Calculate
mass
230
Exp.
mass
m/z
229.0813
1
Naproxen
12
3
2-(6-Hydroxy
naphtalen-2-yl)
propanoic acid
9
Dimmer C26H26O4
229[M]-,185[M-COO]-,170[M-C2H3O2]-
21
216
215.0713
215[M]-; 171[M-COO]-;157[171-CH3]-
25
402
401.8813
401[M]-;357[M-C2H4O]-;313[357-C2H4O]-;
245[M-C11H9O]-;187[313-C10H7]-
Figure 7. Photocatalytic degradation of NPX (fill dots) and DOC
conversion (empty dots) under saturated conditions of dissolved
O2 (40 mg/L) using 1 g/L TiO2 at 40 ºC.
Byproducts Generated by Photocatalysis
Intermediates generated after 180 min of treatment (samples for aerated and oxygenated conditions
with 0.1 g/L TiO2) were identified with a Liquid
Chromatography-Electrospray Ionization-Time of
Flight mass (LC/ESI-TOF-MS), using a Jasco AS2050 plus IS mass spectrometer into the m/z range of
65 to 1000 in negative ionization mode. Table 3 lists
the intermediates detected and Figure 8 depicts the
possible reaction mechanism with the corresponding
intermediates. The spectrum showed the presence of
two principal molecular peaks at m/z 215 and 401 at
retention times of 21 and 25 min after the molecular
mass of NPX (m/z= 229) observed at 12 min. The
remained DOC after treatment of NPX 1 consisted
mostly of 2-(6-Hydroxynaphtalen-2-yl) propanoic acid
3 and a dimmer 9 formed from consecutives transformations of the 1-(6-Methoxynaphtalen-2-yl) oxyradical 7. Under O2 saturated conditions it is possible
to assure that the photoreactivity of NPX was much
higher, in agreement with previous results. However
for sample under aerated conditions, the same byproducts were observed only varying the signal
intensity. The evolution of the degradation of NPX
follows several multi-step and interconnected routes.
In general, two well differentiated and simultaneous
ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
Main fragments
m/z
pathways could be assured: demethylation by ·OH
attack and decarboxylation by photolysis. As explained
above the degradation of NPX was directly influenced
for the TiO2 loading, decreasing as higher TiO2 used.
At high TiO2 concentration the low decrease of NPX
can be due to scattering effect and degradation by
photoinitiation was not forceful enough.
The initial degradation of Naproxen 1 leads
demethylated 2 and decarboxilated 5 radical derivatives. The possible hydrogen addition, from the
cleavage of water through the h+ in the surface of
catalyst (Eq 3), to 2 radical generates 2-(6 hydroxynaphtalen-2-yl) propionic acid 3. Although the initial
·OH attack could be possible in  or methyl position, only evidence of demethylation in methyl
position is observed in the molecular ion at m/z 215
in the electronic impact mass spectrum with principal
fragments of 171 and 143. By elemental analysis and
in agreement with the molecular formula of fragments,
(6-methoxynaphthalen-2-yl)-propionic acid or -demethylnaproxen was not noticed to be present. Isidori
et al. (2005) have reported that NSAID as NPX are
rapidly photodemethylated in the environment. However no-proof of 2-(6 hydroxynaphtalen-2-yl) propionic
acid (-demethylnaproxen) as intermediate of photolysis has been reported. We could assure that the ·OH
radical attack can be the promoting step for its
formation in photocatalytic experiences. The above is
also related to the highest degradation rate of NPX at
low TiO2 concentrations due to the photolytic via
showed a major contribution.
The methyl radical generated follows several
reaction pathways –recombination, hydrogen subtraction, ·OH attack- (see Figure 8, left) driving to the
formation of low-weight soluble and/or volatile compounds like methane, ethane, its concomitant alcohols,
aliphatic acids, or ketone. Compounds like ethanol,
highly soluble in water, remains in dissolution. However it is possible that some other of these compounds
leave the dissolution by volatility or evaporation.
J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
441
1
Demethilation
CH3
O
Decarboxylation
-
H3C
O Na+
O
CH
O
+
CH3
O
+
2
-
H3C
5*
O
+ CH3
+ O2
CH3
CH3
O
H2C
C
CH3
H3C
6
-
O Na+
HO
+ ·OH
O
4
O
3
-
-
O
HO
+ h+
O
O
O
O
H3C
CO2
CH3
CH3
O
CH3
O
CH3
H3C
+ ·OH
+ h+
7*
O
OH
HO
CH2
CH2
H3C
H3C
+ ·CH3
+ h+
OH
C
-C2H3OH
O
O
CH3
+ h+
Dimmerization
HO
CH2
+8
H3C
H3C
8
O
O
OH
O
CH2
CH2
+h+
OH
9
H3C
O
O
CH3
Figure 8. Intermediates observed at 180 min of photocatalytic treatment and propose mechanism of reaction.
This is in agreement with the slight increase in the
DOC removal observed in experiments reached at
40 ºC, for instance, for ketone which presents a boiling
point of 56 ºC. It was also observed –as latter
described- that these low-weigh compounds, through
its reactive radicals transformations, are involucrate
in the post-sequential formation of more different
byproducts.
The other main degradation via was the decarboxylation of NPX. In agreement with other
authors its generation way is through photoinitiation
absorption of light (14, 17, 18). Isidori et al. (17) and
Jimenez et al. (18) have reported the formation of 2ethyl-6-methoxynaphtalene (ethylnerolin) by photolysis, however this byproduct could drive to produce
CO2 or bicarbonate 4 by ·OH attack in the central C
atom in presence of TiO2. Interesting, the last can be
corroborated for the evolution of the pH during the
reaction for all condition tested (see Figure 9). pH
increases in an unusual behavior of oxidation photoISSN 1203-8407 © 2008 Science & Technology Network, Inc.
catalytic processes. Thus bicarbonate remains in
solution increasing the pH. This parallel process
competes on the CO2 formation and mineralization
shows an inefficient increase.
In presence of O2, the photodecarboxylated radical
5 is the precursor of two of the most important and
higher yield-amount photolytic byproducts: 1-(6methoxynaphtalen-2-yl)ethanone (acetylnerolin) and
1-(6-methoxynaphtalen-2-yl)ethanol with 34% and 7%
respectively (18). Under O2 conditions the alcohol and
ketone intermediates are formed by oxygen trapping
of the benzilic radical following the break-down of
the unstable hydroperoxide. By consecutive steps,
closely related to peroxide chemistry and constant
hydrogen source, 1-(6-methoxynaphtalen-2-yl)oxyethyl
was also formed and reported as photolytic byproduct.
Jimenez et al. (18) assure that the source of hydrogen
is essential to obtain the oxyethyl moiety. In photocatalytic-enhance conditions this requirement is enough
reached due to the cleavage of O-H bond of water.
J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
442
pH
7.50
6.50
5.50
0
30
60
90
120
150
180
Time (min)
Figure 9. pH evolution for several operational conditions. (♦)
Photolysis, 2 L/min, 30 ºC; (■) 1g/L TiO2, 2 L/min; (▲) 0.5 g/L
TiO2, 2 L/min, 30 ºC; (◊) 0.1 g/L TiO2, 2 L/min, 30 ºC; (○) 0.1
g/L TiO2, 1 L/min, 20 ºC; (●) 0.1 g/L TiO2, 4 L/min; (□) 0.1 g/L
TiO2, 4 L/min, 20 ºC; (∆) 0.1 g/L TiO2, 4 L/min, 40 ºC.
On the other hand DellaGreca et al. (14) assured that
oxyethyl compound and other dimmers reported are
only generated under presence of inorganic salts in
dissolution in coincidence with our conditions.
In our molecular ion detection chromatogram,
any of three above described byphotoproducts ((1-(6methoxynaphtalen-2-yl)ethanone,
1-(6-methoxynaphtalen-2-yl)ethanol or 1-(6-methoxynaphtalen-2yl)oxyethyl)) were observed. However due to the
constant O-H bond is cleaved through the photocatalytic action, the hydroxyolefin radical is formed
leading the benzyl cation with an unpaired electron.
In our experience certain sticky and greasy (oily)
appearance was observed in the dissolution, due to
probably parent olefin radical compound formed.
Actually DellaGreca et al. (14) report the olefin side
chain also as byproduct of photolysis. Although the
olefin-byproduct was not a photocatalytic intermediate,
its short-life time could cleavage the C-C bond of the
naphtalen ring followed by the coupling with the
analogue non-dehydroxyolefined to afford the dimmer
8. Dimmer 9 could be formed from hydrogen subtraction. This step, which introduces the methyl
function on the oxymethyl moiety, could be followed
by a second step consisting in the reaction of the
above radical intermediate with the CH3OHCH2·
radical, formed from the initial demethylation photocatalyzed process and its post-radical activity. Thus
under photocatalytic O2-enhanced process, the continuous source of ·H and ·OH, makes possible the
formation of 9 by union of CH3OHCH2· radical.
Compound 9 showed the molecular ion at m/z 401 in
the electronic impact mass spectrum with the follow
m/z fragments: 313, 357, 245 and 187. Dimmer 9 was
the other byproduct remained in the dissolution; however DellaGreca et al. (14) reported the formation of
ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
two dimmers from the photolysis via no equivalent to
here founded. Summarizing, byproduct 9 is strong
dependent on the ethoxy-group due to the O2 saturated
conditions and the action of inorganic salts present is
water. Although we did not observe any photolysisbyproduct, it is possible to assure that the transformation of NPX is strong associated with the
involvement of different short-lived intermediates, as
above described.
It is possible to observe that the presence of the
TiO2 promotes different and pseudo-complementary
degradation pathways of NPX with additional important
increase of DOC removal. The important contribution
of the photocatalytic degradation is the further reacting
of the methyl radical. The difference also can be
strongly attributed to the presence of O2 in dissolution
which leads the formation of byproducts and the
reactivity continues with the ·OH and with the
promotion of the h+ from the catalyst. This can
indicate that by photocatalysis the pathway goes
further in the oxidative reaction.
Biodegradability and Toxicity
Biodegradability and toxicity assays were carried
out at different time intervals of the photocatalytic
treatment and also for pure NPX solution without
previous treatment. BOD5/COD for the initial NPX
solution was 0.02 and for 30, 60, 120 and 240 min of
photocatalytic treatment, the BOD5/COD values
never were higher than 0.05. This can suggest that
no-biodegradable photoproducts were formed during
the photocatalytic treatment. In Figure 10, changes in
toxicity during the photocatalytic treatment are
presented. As it is possible to observe, in presence of
TiO2 an important increase in the toxicity is observed
after 4 h irradiation. Under saturated dissolved oxygen
it is able to achieve complete degradation of NPX,
however the increase of the toxicity is observed due
to the partial decomposition of NPX that can lead to
the formation of final products more toxic than the
original NPX compound. In our experience, also the
by-products identified did not show as biodegradables
and no improvement was reached in the EC50 values,
always lower than 8%.
Our results are in agreement with the reported by
several authors regarding the toxicity of byproducts
from photolysis degradation (14, 17). Photoproducts
by solar simulator irradiation of NPX were significantly
more toxic than the parent compounds for several
organisms tested (B. calyciflorus, T. platyurus and C.
Dubia, Daphnia magna, Vibrio fisheri) for toxicity
test, acute bioassays, chronic bioassays and genotoxicity/mutagenesis testing respectively. The photo
J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
443
Figure 10. Changes of the EC50 (bars) and % inhibition (points
and line) of bioluminescence of Vibro fisheri as function of
irradiation time in the presence of 0.1 g/L TiO2 and 40 mg/L O2.
decarboxylation products with ethyl, 1-hydroxyethyl
and acetyl side chains have shown a high cytotoxicity.
Moreover, complex ciclodextrin-NPX was also proved
to improve drug phototoxicity without successful (18).
The diminished phototoxicity of the complexed NPX
is associated with the involvement of different shortlived intermediates, probably similar as intermediates
founded by photocatalysis. Similar quiet results are
reported for chlorinated process or by biofilm (12).
Due these compounds exhibit a low final BOD5/
COD, a photocatalytic treatment for degradation of
NPX can be suggested for a post-biological processes,
only if the oxidation state is more advanced. It is
possible that the byproducts of NPX are quite difficult
to improve its biodegradability, however due to by
photocatalysis the mineralization increase, it is possible
to propose it as good DOC removal treatment. With
photocatalytic process not only the complete NPX
concentration is able to disappear but also mineralization is achieved. The focus could be the optimization and solar alternative process to improve the
efficiency of the photocatalytic treatment for NPX
degradation.
shown a nature of photoinitiation production. Photolysis plays an important role on the degradation of
NPX affecting the reaction rate and the main subproducts formed. The byproducts of degradation of
NPX can be summarized as product of several
influences: by photoinitiation, ·OH attack and high h+
reactivity under O2 saturated conditions, inorganic
ions. As possible to conclude, the main subproducts
of photocatalytic treatment involve also the photolysis
byproducts in a further oxidation. In oxidant medium
(O2 saturated), an improvement in the transformation
of NPX is reached, due to lesser hole-electron recombination, but no enhancement in mineralization is
observed. Intermediate products can represent worst
biodegradability conditions for the treated effluent.
The toxicity of the treated solution is not reduced
during photocatalytic process and no coupling for
detoxification with a biological system can be assured
before optimization.
Acknowledgements
Generous support by the Spanish Ministry of
Education and Science (CICYT projects CTQ200500446/PPQ and CTQ2004-02311/PPQ) and University
of Barcelona (predoctoral fellowship) is gratefully
acknowledged.
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Received for review March 26, 2008. Revised manuscript
received June 16, 2008. Accepted June 18, 2008.
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