Decatungstate anion as an efficient photocatalytic species for the transformation of the pesticide 2-(1-naphthyl)acetamide in aqueous solution

Article abstract of DOI:10.1016/j.jphotochem.2016.10.036

The degradation and mineralization of the plant growth regulator 2-(1-naphthyl)acetamide (NAD) was studied by excitation in the presence of the catalyst polyoxometalate decatungstate anion (W₁₀O₃₂^4?) in aqueous solution under UV (365 nm) or simulated solar light exposure. Our results indicate that the photocatalytic degradation of NAD is dependent on molecular oxygen concentration: in aerated conditions, 95% degradation was achieved after 22 h irradiation, and followed first-order kinetics with a rate constant of 3.2 × 10^?3 min^?1, while under de-aerated conditions almost no degradation was observed (6.0% after 22 h). Upon UV irradiation, the catalyst W₁₀O₃₂^4? enhanced NAD photodegradation by a factor of about 20 compared to its direct degradation. Oxygen appeared to play a key role on the regeneration of the catalyst, promoting the photocatalytic cycle. The primary photoproducts of NAD photocatalytic degradation were assessed by LC-ESI–MS/MS, from which a mechanism of degradation involving electron transfer and hydrogen atom abstraction is proposed. Under these conditions, mono- and di-hydroxylated and oxidized products similar to those obtained under direct photolysis have been identified. In addition, tri-hydroxylation and hydroxyl-naphthoquinone products have been identified exclusively when photolysis was carried out in presence of this catalyst. For prolonged photolysis times, it is expected that the irradiation of the tri-hydroxylated products leads to the opening of the aromatic ring and to mineralization. Furthermore, mineralization was achieved, and led to the formation of the inorganic ions NO₂^? (<6.0 μg L^?1), NO₃^? (2.6 mg L^?1) and NH₄⁺ (<0.5 μg L^?1).

Full text of DOI:10.1016/j.jphotochem.2016.10.036

Accepted Manuscript
Title: Decatungstate anion as an efficient photocatalytic
species for the transformation of the pesticide
2-(1-naphthyl)acetamide in aqueous solution
Author: Eliana Sousa Da Silva Mohamed Sarakha Hugh D.
Burrows Pascal Wong-Wah-Chung
PII:
S1010-6030(16)30534-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2016.10.036
JPC 10420
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
1-7-2016
28-10-2016
29-10-2016
Please cite this article as: Eliana Sousa Da Silva, Mohamed Sarakha, Hugh D.Burrows,
Pascal Wong-Wah-Chung, Decatungstate anion as an efficient photocatalytic
species for the transformation of the pesticide 2-(1-naphthyl)acetamide in
aqueous solution, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2016.10.036
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Decatungstate anion as an efficient photocatalytic species for the transformation of
the pesticide 2-(1-naphthyl)acetamide in aqueous solution
Eliana Sousa Da Silva,a,b,c,1 Mohamed Sarakha, Hugh D. Burrows and Pascal Wong-
a,c
b
Wah-Chunge*
aClermont Université, Université Blaise Pascal, Institut de Chimie de Clermont Ferrand
(ICCF) UMR CNRS 6296, Équipe Photochimie, BP 80026, F-63171 Aubière Cedex,
France
bCentro de Química, Department of Chemistry, University of Coimbra, Rua Larga,
3
004-535, Coimbra, Portugal
cCNRS, UMR 6505, 24 avenue des Landais, F-63173 Aubière, France
dClermont Université, ENSCCF, BP 10448, F-63000 Clermont-Ferrand, France
e
Aix Marseille Univ, CNRS, LCE, Marseille, France
1Present affiliation: Laboratory of Separation and Reaction Engineering – Laboratory of
Catalysis and Materials (LSRE–LCM), Departamento de Engenharia Química,
Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465
Porto, Portugal
*
Corresponding author: E-mail addresses: pascal.wong-wah-chung@univ-amu.fr, (P.
Wong-Wah-Chung), Tel: +033 4 42 90 84 16; fax: +033 4 91 28 84 27,
elianas@qui.uc.pt (E.S. Da Silva), mohamed.sarakha@univ-bpclermont.fr (M.
Sarakha), burrows@ci.uc.pt (H.D. Burrows)

GRAPHICAL ABSTRACT
Highlights
4
-
-
(
-
W O32 is effective on the photodegradation of the pesticide 2-(1-naphthyl)acetamide
10
NAD).
4
-
NAD degradation is enhanced in the presence of the catalyst W10
O32 when compared
to its direct degradation.
4
-
O, NO - , NO -
-
W
10
+.
O
32 efficiently promotes NAD mineralization to CO
2
, H
2
3
2
and
NH
Oxygen is crucial to regenerate the catalyst W10
cycle.
The first step of the photodegradation probably corresponds to an electron transfer
process.
Hydroxylation and oxidation products were identified. No reactivity of NAD amide
side chain was observed.
4
-
O
324-, thus closing the photocatalytic
-
-

Abstract
The degradation and mineralization of the plant growth regulator 2-(1-
naphthyl)acetamide (NAD) was studied by excitation of the catalyst polyoxometalate
decatungstate anion (W10
324-) in aqueous solution under UV (365 nm) and simulated
O
solar light exposure. Our results indicate that the photocatalytic degradation of NAD is
dependent on molecular oxygen concentration: in aerated conditions, 95% degradation
was achieved after 22 h irradiation, and followed first-order kinetics with a rate constant
-3
-1
of 3.2×10 min , while under de-aerated conditions almost no degradation was observed
4
-
(
6.0% after 22 h). Upon UV irradiation, the catalyst W10O32 enhanced NAD
photodegradation by a factor of about 20 compared to its direct degradation. Oxygen
appeared to play a key role on the regeneration of the catalyst, promoting the
photocatalytic cycle. The primary photoproducts of NAD photocatalytic degradation
were assessed by LC-ESI-MS/MS, from which a mechanism of degradation involving
electron transfer and hydrogen atom abstraction is proposed. Under these conditions,
mono- and di-hydroxylated and oxidized products similar to those obtained under direct
photolysis have been identified. In addition, tri-hydroxylation and hydroxyl-
naphthoquinone products have been identified exclusively when photolysis was carried
out in presence of this catalyst. For prolonged photolysis times, it is expected that the
irradiation of the tri-hydroxylated products leads to the opening of the aromatic ring and
to mineralization. Furthermore, mineralization was achieved, with the formation of the
-
-1
-
(2.6 mg L ) and NH
(< 0.5 μg L-1).
-1
+
inorganic ions NO
2
(< 6.0 μg L ), NO
3
4
Keywords: 2-(1-naphthyl)acetamide, pesticides, decatungstate anion, photocatalysis,
polyoxometalates, mineralization

1
. Introduction
The extensive use of pesticides in agriculture and in urban environments has
contributed in recent years to their ubiquitous presence in surface and groundwater [1, 2].
Owing to their nature, most pesticides are persistent in the environment due to their
resistance to chemical and/or photochemical degradation [3, 4]. They may build up in
animal tissues, as well as contaminating food and drinking water, with the consequent
serious threat to human and animal health [5-9]. The pesticide 2-(1-naphthyl)acetamide,
hereafter designated as NAD, (Fig. 1), belongs to the family of plant growth regulators.
It is applied in agriculture as a component in many commercial plant rooting and
horticultural formulations (Amid-Thin W, Amcotone, etc.) with the goal of improving the
growth of fruits (apple, pear, peach, grape, etc.), as well as preventing their drop shortly
before harvest [10-12]. The use of plant growth regulators to decrease pre-harvest
abscission on citrus fruits was first reported in 1941 by Gardner [13]. He discovered that
the application of NAD to pineapple oranges early in the harvest season at a concentration
-1
of 100 mg L reduced fruit drop for twelve weeks or longer [13]. However, NAD is
-1
considered harmful to aquatic organisms, with a LC50 of 44 mg L for fish, and a half-
life of 38 days in water [10]. Its prolonged application in agriculture over the last few
years makes it a potential water pollutant [10, 12].
Recently, we reported the direct photolysis of NAD in aqueous solution under UV
and simulated solar light exposure [14]. Under these conditions, the photochemical
transformation of NAD mainly involves the aromatic moieties, without any participation
of the amide side chain. Mono- and di-hydroxylated NAD derivatives, as well as
oxidation products, such as naphthoquinone, furanone and coumarin derivatives, were
identified as the primary photoproducts. The effect of the irradiation on NAD toxicity
was also assessed, and a direct correlation was drawn between the increase in toxicity and
the formation of these primary photoproducts. These results aroused our interest in
developing a methodology that is able both to transform NAD photochemically through
the use of solar light, and to mineralize it, specifically, by transforming it into
environmentally innocuous compounds such as CO
compounds.
2
, H O, and nitrogen inorganic
2
Advanced oxidation processes (AOPs) [15-20] are among the current methods
described in literature that have enormous potential to reduce the impact of these
pollutants in the environment, since they can lead to total mineralization, or at least to

their transformation into less harmful products. The AOPs typically use UV and UV-near
visible radiation, and/or strong oxidizing agents, which produce highly reactive oxygen

), etc., that are
species, such as hydroxyl radicals (HO ), superoxide anion (O
2
responsible for the transformation of these organic compounds into less harmful products
15, 19, 20]. Fenton, photo-Fenton, H /UV, O , O /UV, O /UV [21-23], and
heterogeneous photocatalysis using semiconductor materials, such as TiO , CdS, ZnO,
WO , etc. [20, 24-28] are some of the methods reported for water remediation. Although
UV/TiO is by far the most widely studied system [29-32], another important branch of
[
2
O
2
3
3
3
2
3
2
AOPs developed in recent decades, which also provides promising results for water
remediation, involves the use of polyoxometalates (POMs) in the presence of light [33-
3
9]. Comparative studies on the action mode of UV/TiO
2
vs UV/POMs suggest that
[40-43], but have, as
POMs present the same photochemical characteristics as TiO
2
advantage, their low cost and relatively low toxicity. POMs are well-defined early
transition metal-oxygen clusters, with unique electronic versatility, and are able to
undergo photoinduced multi-electron transfer, without any change in their structure [44-
4
8]. These characteristics broaden the range of application to fields such as chemistry,
biology, medicine, catalysis, photocatalysis and materials science [41, 49-53]. The most
frequent application of POMs is probably in the field of catalysis, associated with their
attractive redox properties [47, 54, 55]. The decatungstate anion, W10O
324-, is one of the
most photochemically active polyoxometalates, and exhibits attractive properties as a
photocatalyst [56, 57], since it possesses a UV absorption spectrum which partially
overlaps the UV solar emission spectrum, diminishing the energy consumption through
the possible use of solar light [56, 58, 59]. It is established in the literature that the
4
-
absorption of light by W10
O32 leads to the formation of a charge transfer excited state
(W O
10
324-*) that decays in approximately 30 ps to a longer-lived, extremely reactive and
non-emissive transient, designated as wO (lifetime of 35 ns in water) [60-64]. Such a
transient species, which is a relaxed excited state, exhibits oxyradical-like character due
to the presence of an electron deficient oxygen center, and does not react with oxygen.
However, it reacts extensively with organic substrates, producing radicals through
hydrogen atom abstraction or electron transfer mechanisms. In the presence of oxygen,
these radicals may lead to the formation of peroxy- compounds [62, 65-67]. Both
mechanisms lead to the formation of the one-electron reduced form of decatungstate,
5
-
4-
W
W
10
10
O
32 or its protonated form HW10
O
32 [47, 68], which regenerates the starting
4
-
O
32 in the presence of oxygen [69], completing the catalytic cycle. Such a cycle can

lead to the continuous degradation of organic substrates and to their total elimination, as
confirmed by studies concerning the degradation of various types of organic pollutants in
water [35, 37, 58, 70-72].
Consequently, based on these photochemical properties of W10O
324-, we became
interested in assessing the transformation of NAD in the presence of this photocatalyst in
aqueous solution under UV and simulated solar light irradiation, with particular attention
4
-
in evaluating the catalytic activity of W10O32 on its degradation. This was done by
following its degradation and mineralization processes. The disappearance kinetics,
initial rate constants and quantum yields of degradation were determined as a function of
oxygen concentration. In addition, the degree of mineralization was evaluated through
measurement of both the total organic carbon concentration and the inorganic species
produced. The primary organic products formed during the photocatalytic degradation
were identified, and a mechanism of the photodegradation is proposed. Moreover, a
comparison is made regarding the products formed from NAD under normal photolysis
and photocatalysed degradation.
2
. Materials and methods
2
.1. Materials
2
-(1-Naphthyl)acetamide (NAD), methanol (Chromasolv,  99.9% for HPLC)
and formic acid (reagent grade,  95%) were purchased from Sigma Aldrich, and iso-
propanol from Riedel-de Haën. Sodium decatungstate (Na O) was
synthesized according to a literature procedure [73], and fully characterized. Ultrapure
4
W
10
O32.7H
2
-1
water, delivered by a Millipore Milli-Q system (18.2 M cm ; no control of ionic
strength), was used throughout this study. Aqueous solutions of the mixture [NAD
+W
10
O
324-] were freshly prepared at room temperature by directly dissolving the
necessary amount of Na O powder in aqueous NAD solution, prior to starting
4
W
10
O32.7H
2
the irradiation. Solutions were de-aerated and oxygenated by bubbling argon and oxygen,
respectively, for 30 min prior to irradiation and during the whole irradiation period, when
necessary.
2
.2. Irradiation systems
For determination of the quantum yields, homogeneous aqueous solutions of NAD
4
-
in the presence of W10O32 were irradiated at 365 nm with a LOT Oriel grating

monochromatic system equipped with a xenon lamp (1000 W). The beam was parallel
and the reactor was a spectrophotometric quartz cell of 1 cm path length. The photonic
1
5
flux at 365 nm, measured by ferrioxalate actinometry [74] was equal to 7.40×10 photons
−
1
−2
s cm .
Irradiation with Suntest CPS photoreactor (Atlas) was used to simulate the effect
-2
of solar light. This is equipped with a Xe lamp (I = 750 W m ) and filters that prevent
the transmission of wavelengths below 290 nm. The temperature was maintained constant
at 20 ºC by the water flow from a thermostat bath.
For the determination of the initial rate constants, half-lives and evolution of
4
-
photoproducts, irradiations of NAD at 365 nm in absence/presence of W10O32 were
carried out in an elliptical stainless steel cylinder under constant stirring. Three high
pressure mercury lamps (black lamp, Mazda MAW 125 W) were installed symmetrically
in the lower part of the cylinder. Although these emitted mainly at 365 nm (97% of the
emission), two incompletely filtered rays at 313 (2.0%) and 334 nm (1.0%) were also
present. The reactor, a water-jacketed Pyrex tube (ø = 2.8 cm) containing 50 mL solution
and located in the centre of the system, was cooled by the water flow from a thermostat
bath (T = 20 °C) during irradiation experiments. To obtain further information on the
reactive species involved in NAD degradation pathway, an aerated aqueous mixture of
4
-
-4
-1
-1
[
NAD+W10O32 (3.010 mol L )] containing an excess of iso-propanol (1.0 mol L ),
known to be hydrogen atom donor [45, 50], was irradiated at 365 nm using the previously
described system.
2
.3. Analysis
UV/vis absorption spectra were recorded on a Cary 3 double beam
spectrophotometer using a 1 cm path length quartz cell. pH measurements were made
with a JENWAY 3310 pH-meter to ± 0.01 pH unit. Attenuated Total Reflectance (ATR)
Fourier Transform Infrared (FTIR) spectrum was measured in the range 500-4000 cm-1
-1
(
resolution of 2 cm with 15 scans) on a FTIR Nicolet 5700 spectrometer (Thermo
Electron Corporation) equipped with a Smart Orbit accessory.
The disappearance of NAD and the formation of photoproducts were monitored
by high performance liquid chromatography (HPLC), using a system equipped with a
Waters 996 diode array detector (DAD), two Waters 510 pumps and an automatic injector
(Waters 717). The wavelength of detection was set at 280 nm. A reverse phase Nucleodur
column was used (Eclipse XDB-C18, 5 μm, 250 mm×4.6 mm) with 50% water (pH 3.5

with formic acid) and 50% methanol as eluent. The injected volume was 25 μL and the
-1
flow rate 1 mL min . Each sample was analyzed by HPLC in triplicate, with a maximum
error of 3.0%.
The photoproduct identification was carried out at Université Blaise Pascal using
a Waters liquid chromatography (LC) system (Alliance 2695) coupled to a quadrupole
time-of-flight mass spectrometer (Micromass) equipped with a pneumatically assisted
electro-spray ionization source (ESI). The LC system was also coupled with a Waters 996
diode array detector. Data acquisition and processing were performed by MassLynx NT
3
.5 software. Chromatographic separation was achieved by using 50% water (pH 3.5 with
formic acid) and 50% methanol as elution program in a Phenomenex column (Kinetex
-1
MS C18, 3.5 µm, 100 mm2.1 mm). The flow rate was set at 0.2 mL min and the
injected volume was 3.0 µL. The electro-spray source parameters were in positive mode:
capillary voltage 3000 V (2100 V in negative mode), cone voltage 35 V, extraction cone
voltage 2 V, desolvation temperature 250 °C, source temperature 100 °C, ion energy 2 V
and collision energy (CE) between 10 to 30 eV (7 eV in negative mode).
Ionic liquid chromatography was used to measure the evolution in concentration
of nitrate, nitrate and ammonia ions as a function of irradiation time, upon mineralization
of NAD in presence of W10
column was used, while for ammonia a Dionex ICS1500 system with CS16 column was
O
324-. For the anions, a Dionex DX320 system with AS11
-1
employed, with 1.0 mL min flow.
The mineralization of NAD was monitored by measuring the total organic carbon
concentration (TOC) with a Shimadzu Model TOC-5050A equipped with an automatic
-1
sample injector. The calibration curve was obtained within the range 0-49 mg L by using
potassium hydrogen phthalate and sodium hydrogen carbonate for organic and inorganic
carbon, respectively. Three injections were made for each sample with an error of roughly
5
.0%.
3
. Results and discussion
3
.1. Synthesis and characterization of the catalyst sodium decatungstate
The catalyst sodium decatungstate, Na
form as decatungstate anion or W10
324-) was synthesized according to a literature
procedure [73], and characterized by UV-Vis and infrared spectroscopies. Fig. 1 presents
4
W
10
O
32.7H O, (designated in its anionic
2
O
4
-
−4
-1
the UV absorption spectrum of an aqueous solution of W10
natural pH (around 5.0).
O32 (3.0×10 mol L ) at its

4
-
The UV spectrum of W10
O32 presents two absorption bands with maxima at 267
and 323 nm, in agreement with the literature [56, 58, 68]. The first band is attributed to
6
-
4-
H
2
W
12
O
40 and the band at 323 nm corresponds to W10O32 [68]. Since the metal ions in
0
the fully oxidized POMs have a d electronic configuration, the only absorption band that
occurs in the UV range of the electronic spectra is due to the oxygen-to-metal (OM)
charge transfer (LMCT) process [47, 60, 61, 75, 76]. The Fourier Transform Infrared
4
-
(
FTIR) spectrum of W10
O32 (Fig. S1 (support information)) exhibits several vibrational
bands with varying intensities that are in perfect agreement with literature data [73].
4
-
3
.2. Photodegradation of NAD in presence and absence of W10
O32 under simulated solar
light irradiation
−
4
-1
As illustrated in Fig. 1, the UV absorption spectrum of NAD (3.0×10 mol L )
in aqueous solution presents a broad band with vibrational structure extending from 250
-1
-1
to 320 nm, with maximum centred at 280 nm (ε280 nm = 6540 L mol cm , [77]). Despite
the small overlap of this absorption band with the emission spectrum of solar light (Fig.
1
), NAD is susceptible to degrade under environmental conditions. Therefore, simulated
solar light was used as light source (Suntest system) for the irradiation of an aqueous
−
4
-1
NAD solution (3.0×10 mol L ). Its transformation kinetics (Fig. 2) shows that NAD is
effectively degraded under these conditions: 51% of disappearance in 8 h. The main
products formed under these conditions have already been reported in our previous work
4-
[
14] and will be referred ahead in the text. However, as also observed in Fig. 1, W10
O
32
presents an important overlap (much greater than NAD) with the emission spectrum of
solar light within the range 290-400 nm, and this allows it to be used as a photocatalyst
4
-
−4
-1
for water decontamination. Hence, the effect of the catalyst W10
O32 (3.0×10 mol L )
on NAD degradation in aqueous solution was assessed using the Suntest system. The
4
-
results, given in Fig. 2, provide evidence of the photocatalytic activity of W10
O32 in
NAD degradation: 89% disappearance is seen after 8 h of irradiation. These results are in
4
-
agreement with literature studies regarding the effectiveness of W10O32 as a catalyst for
the degradation of organic pollutants under solar irradiation [34, 56, 58, 70].
Nevertheless, in this situation, both direct and induced phototransformation may
4
-
contribute to NAD degradation since NAD and W10
Consequently, and since the main goal of this work was to study the degradation of NAD
promoted by the catalyst W10
324-, further studies were performed using a powerful and
selective irradiation system at 365 nm. Under these conditions, the direct photolysis of
O32 both absorb at  > 290 nm.
O

4
-
NAD is absent, and only W10
O32 will be responsible for NAD disappearance. This
allows assessment of the efficiency of the catalyst on the degradation process.
3
.3. Direct degradation of NAD under irradiation at 365 nm
The direct irradiation of aerated aqueous NAD solutions (3.010 mol L ) was
-4
-1
4
-
first evaluated in the absence of W10O32 with the 365 nm light source. This test will help
4
-
us to assess the catalytic efficiency of W10
conditions (Fig. 3), good stability of the pesticide is observed after 22 h of irradiation
5.0% of degradation) and no formation of products was observed. Since NAD does not
O32 towards NAD degradation. Under these
(
absorb at 365 nm, the small disappearance observed can be explained by the presence of
the incompletely filtered rays at 313 nm present in the 365 nm irradiation system.
4
-
3
3
.4. Photocatalytic degradation of NAD in the presence of W10
.4.1. Effect of oxygen concentration
O32 at 365 nm
-4
-1
The photocatalytic degradation of aqueous NAD solutions (3.010 mol L ) in
4
-
-4
-1
the presence of W10O32 (3.010 mol L ) under illumination at 365 nm was studied as
a function of oxygen concentration: aerated, de-aerated and oxygen saturated conditions.
The kinetic results are depicted in Fig. 3, while the initial rate constants, half-lives and
4
-
quantum yields of NAD transformation in the presence of W10
O32 are reported in Table
1
. The kinetic profile presented in Fig. 3 provides evidence for the significant role that
oxygen plays in the NAD photocatalysed degradation. The degradation rate follows the
order: aerated  oxygenated > de-aerated.
Under aerated conditions, 95% of the NAD disappeared after 22 h of irradiation,
-3
-1
with an estimated first-order rate constant of 3.210 min . This indicates that the
4
-
catalyst W10
direct NAD degradation (k  1.5×10 min ). Furthermore, the addition of oxygen to the
solution did not significantly increase the photocatalytic activity of W10
324-, as can be
O32 enhanced NAD degradation by a factor of 19 when compared with
-4
-1
O
seen in Fig. 3. In fact, the quantum yields, half-lives and rate constants given in Table 1
indicate that degradation is of the same order of magnitude in aerated conditions when
compared with oxygenated conditions. Such a result indicates that the oxygen
concentration existing in solution was sufficient for the regeneration of the initial form
W O
324-, leading to the continuous degradation of NAD through the photocatalytic cycle.
10
5
-
The presence of oxygen is vital to reoxidize the reduced form W10O32 to its initial form

W O
324-, closing the photocatalytic cycle [56, 69], but is not involved in the initial
10
photochemical step. The oxygen consumption in the reoxidation step with reduced
tungstate probably leads to the formation of superoxide anion, which can further
participate in the degradation process leading, on protonation, to hydroperoxyl species
[
56]. However, no further studies were done to detect these species.
Under de-aerated conditions, at the earliest times of irradiation, a slight
degradation of NAD was observed (Fig. 3), after which it was almost completely inhibited
(94% after 22 h). This small decrease may be due to the direct degradation of NAD but
4
-
also to the presence of some residual oxygen left in solution, and W10O32 availability
4
-
present in initial times. The involvement of residual oxygen and initial W10O32 in this
step is clearly demonstrated by the quantum yield determined under these conditions
using monochromatic irradiation at 365 nm (Table 1). After this, a plateau was reached,
owing to the formation of decatungstate excited species, wO [60-64], that decays to form
the one electron reduced species W10
since there is no more oxygen to oxidize it back to the initial W10
O
325-. This accumulates in the course of the reaction
324-, leading to the
O
breaking of the catalytic cycle. Moreover, the formation of a blue color was observed
after the earliest times of irradiation, and persisted till the end of the irradiation, which is
consistent with the formation of the reduced species W10O
325-, that presents an absorption
maximum at 780 nm [76]. When this solution was left exposed to air, the blue color
disappeared, confirming that oxygen plays an important role on regenerating the reduced
O32 to the starting form of the catalyst, W10O
324-.
5
-
species W10
The irradiation of aerated and oxygenated NAD aqueous solutions in presence of
4
-
W
10
O
32 at 365 nm revealed the formation of several photoproducts, as will be discussed
ahead in text. On the other hand, no products were observed in de-aerated conditions or
4
-
even when NAD photolysis was performed at 365 nm in absence of W10O32 (see section
3
.3).
3
.4.2. Effect of iso-propanol
4
-
-4
The kinetic profile of the aqueous aerated mixture [NAD+W10
O
32 (3.010 mol
-1
-1
L )] irradiated at 365 nm in the presence of an excess of iso-propanol (1.0 mol L ) is
depicted in Fig. 3. Under these conditions, 11% of NAD disappeared in the first 60 min
of irradiation, after which, complete inhibition of degradation was observed until 22 h of
irradiation. This initial decrease of NAD concentration was accompanied by the

formation of the blue color, typical of the one electron reduced species W10O
325-, whose
yield is known to increase in presence of iso-propanol [45]. Within the first 60 min, NAD
disappearance may be explained by the competition between iso-propanol and the
4
-
available W10O32 to form the reactive species wO responsible for NAD transformation.
After this time, the inhibition of NAD degradation in the presence of iso-propanol may
be explained by the efficient trapping of the reactive excited species wO by the alcohol.
This suggests the involvement of hydrogen atom abstraction (HA) by the excited state of
W
10
O
324-, wO, from iso-propanol [50], leading to the formation and accumulation of the
reduced form W10
325-, as shown by the blue coloration. The classical reaction of the
O
excited species wO with iso-propanol via HA is proposed in reactions (1) to (4):
h푣
5
−
ꢁ
̇
(1)
wO + (CH )CHOH(CH ) →ꢀ W O + (CH )COH(CH ) + H
3
3
10 32
3
3

̇
(
CH )COH(CH ) + O ꢂ (CH )CO(CH ) + HO
(2)
(3)
(4)
3
3
2
3
3
2
W O + O + H ꢂ W O + HO
5
−
ꢁ
4−
10
32
2
10 32
2

ꢃHO ꢂ H O + O
2
2
2
2
The overall photocatalytic oxidation of iso-propanol in the presence of oxygen
4
-
and W10
O
32 is given by reaction (5), in which the alcohol is transformed into acetone
and hydrogen peroxide:
4
−
(
CH )CHOH(CH ) + O + W O →ꢀ (CH )CO(CH ) + H O
2 2
(5)
3
3
2
10 32
3
3
hv
The results obtained with iso-propanol are in agreement with literature
mechanistic studies, which show that in photosensitized oxidations of organic substrates
4
-
by W10O32 in the presence of oxygen, either HA or electron transfer (ET) or even both
mechanisms may occur [67, 79-81]. We note, however, that some authors suggest that
the reactive species wO also reacts with organic substrates in aqueous solution by indirect
H abstraction via hydroxyl radicals. These radicals may be formed through the reaction
of wO with water molecules (oxidative trapping holes) [65, 71] since the excited state
potential of the decatungstate is more positive than the one-electron oxidation of water

[
71] (reaction 6). The HO radicals thus formed react with organic substrates mainly via
-C H abstraction (reaction 7). The highly reducing hydroxyl alkyl radical formed
(
CH )퐶 OH(CH ) reacts further with the polyoxometalate (reaction 8). In the absence of
̇
3
3
strong oxidant agents (such as oxygen), electrons accumulate on the polyoxometalate,

driving the redox potential to more negative values until an oxidant in solution is able to
act as electron acceptor and close the photocatalytic cycle.
hv
ꢁ
−
5−

ꢁ
(6)
푤O (ꢄ + e ) + H O →ꢀ W O + HO + H
2
10 32
HO + (CH )CHOH(CH ) ꢂ (CH )COH(CH ) + H O
(7)
(8)
̇
3
3
3
3
2
4
−
5−
ꢁ
̇
W O + (CH )COH(CH ) ꢂ W O + (CH )CO(CH ) + H
10
32
3
3
10 32
3
3
However, even though there are some experimental results in the literature that

appear to support the formation of HO radicals in these systems, this possibility is still a
matter of debate and no consensus exists on it.
3
.5. Mineralization process
The mineralization of NAD aqueous aerated solution (3.010 mol L ) in the
-4
-1
4
-
-4
-1
presence of W10O32 (3.010 mol L ) was followed by TOC measurement as a function
of irradiation time using 365 nm light (Fig. 4). Under our experimental conditions,
4
-
W
10
O
32 efficiently promotes NAD mineralization: 73% was achieved after 500 h of
irradiation. However, from Figs. 2 and 4, it can be seen that the degradation of NAD in
4
-
the presence of W10O32 takes place much faster than mineralization. Almost 100% of
NAD degradation is achieved after 22 h of irradiation, while more than 500 h are
necessary for complete mineralization. This is probably related with the formation of huge
amounts of intermediate species formed during the course of NAD photodegradation, that
require longer time to be further oxidised, leading therefore to mineralization on a longer
time scale.
The formation of the inorganic anions (nitrates, nitrites) and ammonium ions
4-
following photolysis in the presence of W10
O32 was also evaluated under the same
experimental conditions. The concentration of NO -
during the course of reaction presents
2
-1
+
-1
-,
values at trace levels, < 6.0 μg L and NH
4
presents concentrations < 0.5 mg L . NO
3
on the other hand, has a more important contribution, and a correlation can be made
between the decrease of TOC and the formation of NO -
, as shown in Fig. 4. After 500 h
of irradiation, 73% of NAD is mineralized and the concentration of NO -
is equal to 2.6
mg L , very close, within the experimental errors, to the expected value calculated from
3
3
-1
-1
the NAD concentration (2.7 mg L ). These results confirm the mineralization of NAD on
photolysis in the presence of W10
O
324-.

4
-
3
.6. Identification of NAD degradation products in the presence of W10
O
32
4
-
Solutions of NAD irradiated at 365 nm in the presence of W10
by HPLC-DAD and by LC-MS. Since many products were formed under these
conditions, aerated and oxygenated solutions of (NAD+W10
324-) irradiated at certain
times of conversion (10, 30 and 50%) were further analysed by LC-MS/MS with electron
O32 were analysed
O
+
-
spray ionisation (ES) in positive (ES ) as well as negative mode (ES ) to identify the main
photoproducts. Although both ionization modes were employed, the best response was
+
obtained with ES , as shown in the chromatogram presented in Fig. 5. Several products
are formed and almost all present shorter retention times than NAD (10.8 min), indicating
the formation of more polar photoproducts. The exception was the product eluted at 11.3
min which corresponds to 1-naphthylacetic acid (1-NAA), in agreement with a previous
study that reports this as NAD degradation product [12]. Based on these results, thirteen
+
-
main products were identified by the molecular ion ([M+H] and [M-H] ) and their mass
fragments ions (Table 2).
3
.6.1. Characterization of the parent compound NAD
The NAD mass spectrum and the fragmentation pattern obtained by LC-MS/MS
+
(
ES mode) is given in Fig. S2 and Scheme S1 (support information), respectively. In
+
addition to the molecular ion of NAD [M+H] with m/z = 186, a main fragment ion was
observed at m/z = 141. This corresponds to the loss of the amide group [M+H-CONH
]+
3
and is in agreement with literature data [82, 83]. Another fragment ion with m/z = 169
was observed in the mass spectrum, although with a low abundance. This ion corresponds
to the loss of ammonia [M+H-NH
]+. Further fragmentation of the molecular ion at 186
3
yields the ion with m/z = 115 with higher collision energy (CE), as depicted in the inset
+
of Fig. S2, which may be explained by the loss of an olefin group [M+H-CONH
3
-C
2
H
2
] .
3
.6.2. Characterization of the product 1-naphthylacetic acid
The only product with retention time greater than NAD (11.3 min) corresponds to
the compound 1-naphthylacetic acid (1-NAA). The amide group of NAD is hydrolysed
to yield the respective acid 1-NAA and NH ) (Scheme 1). Since 1-NAA is
(and NH +
commercially available, its identification was made by comparing the obtained retention
3
4

time and the UV-vis absorption spectrum (λmax  280 nm) of a sample injected in LC-
DAD using the same conditions as NAD.
3
.6.3. Characterization of hydroxylated products
Firstly, the presence of hydroxylated NAD derivatives was assumed regarding
mass differences between by-products and NAD of 16, 32 and 48 mDa. Furthermore,
fragmentation of by-products helps confirming their formation.
In fact, products with retention times of 9.4 and 10.3 min were detected from the
+
early stages of the irradiation. These products, both having m/z ratio 202 ([M+H] ), but
different fragmentation patterns, were attributed to mono-hydroxylated forms of NAD
(Table 2). The hydroxylation of the aromatic ring to give the mono-substituted products
can occur at either of the two aromatic rings of NAD. Taking into account the
fragmentation results, the product at 10.3 min presents as main fragment ions m/z = 185,
1
57 and 143, corresponding to the loss of NH +
3
, acetamide (CO-NH +
groups, respectively, thus confirming the assignment. The other product observed
3
) and CH -CO-
2
+
NH
3
at 9.4 min presents as main fragment ions m/z = 184 and 156 that indicate the loss of a
water molecule and of CO-H
2
O, respectively, is also in agreement with hypothesis.
+
Several photoproducts with molecular ion peak at m/z = 218 ([M+H] ) were
detected with retention times 6.6, 7.4, 7.8, 8.2, 8.6 and 9.0 min. Fig. 6 shows, as example,
2
the MS spectrum of m/z = 218 for the compound eluted at 8.2 min. The diverse fragments
suggest the formation of various isomers of di-hydroxylated products, either involving
one or both aromatic rings (Table 2). Fragmentation of isomers leads to the loss of one or
two water molecules, amino group, amido side-chain and combination that confirm the
structure of di-hydroxylated products.
+
Two products with molecular ion peak m/z = 234 ([M+H] ) and retention times of
6
.0 and 7.1 min have been detected. Main fragments of the products correspond to the
loss of one or two H O molecule, together with the loss of carbonyl function and amido
2
side chain. The presence of these fragments is in good accordance with tri-hydroxylated
derivatives of NAD (Table 2).
Such hydroxylated compounds (mono and dihydroxylated) are among the major
products found in photodegradation studies of naphthalene and naphthalene derivatives
in solution, either under direct or sensitized degradation [84-88] [89]. Although several
authors have proposed the formation of the tri-hydroxy compounds in the mechanism of

naphthalene degradation [89], they have not been clearly identified in these studies, but
results are in accordance with our assignment.
3
.6.4. Characterization of the naphthoquinone and derivative products
A photoproduct with retention time of 8.8 min was assigned to the
+
naphthoquinone formed from NAD. The molecular ion with m/z = 216 ([M+H] ) presents
fragments at 199, 171 and 143. These indicate the loss of NH
3
, CO-NH +
and also CO-
3
CO-NH groups, respectively, which is in agreement with the proposed naphthoquinone
3
compound. Furthermore, these results were also corroborated by the LC-ESI-MS analysis
-
in negative mode (m/z = 214 for [M-H] ). Although two possible isomers may be
attributed to this compound, the most stable structure corresponds to the one having the
carbonyl groups in the 1,4-position of the free aromatic ring (Table 2) since the 1,2-
naphthoquinone will be unstable, and likely to lead to the opening of the ring. Two
+
products with a molecular ion peak [M+H] with m/z = 232 and retention times of 6.8 and
9
2
1
.5 min were detected, and identified as the hydroxy-naphthoquinone derivatives (Table
). The main fragments of the product at 6.8 min have a ratio m/z = 214, 200, 172 and
55. These are attributed to the loss of H
-CO-NH +
. The product eluted at 9.5 min has as fragmentation pattern 216, 200, 172
and 155 (Fig. 7). These correspond to the loss of one (216) and two oxygen atoms (200),
-CO and O , respectively. Naphthoquinones and hydroxyl-naphthoquinone
-CONH +
2
O molecule, H
2
O-CH
2
, H
2
O-CH
2
-CO and H
2
O-
CH
2
3
O
2
2
3
derivatives have been reported in the literature as naphthalene oxidative degradation
products [87, 88, 90].
3
.7. Proposed mechanistic pathways
Based on the above results, a mechanism of the primary steps of NAD
4
-
photocatalysed degradation by W10O32 involving electron transfer and HA abstraction
4
-
is proposed in Scheme 2. The absorption of light by W10
O32 in its ground state leads to
4
-*
the formation of an oxygen-to-metal charge transfer excited state W10
O
32
[61] that
decays in few picoseconds in both aerated and de-aerated solutions to form the very
reactive non-emissive transient species wO [61, 91]. According to the photoproducts
identified by LC-ESI-MS, the first step of the photodegradation should correspond to an
electron transfer process involving the reactive species wO and the aromatic ring of NAD,
with consequent formation of NAD+ radical cation and the reduced decatungstate
5
-
+
species, W10O32 [76, 92]. The reaction of NAD radical cation with water leads to the

hydroxylation of the aromatic ring. Two mono-hydroxylated products were identified as
primary products. Such hydroxylated products have previously been seen in naphthalene
and naphthalene derivative photodegradation [84-88]. Subsequent reaction of the mono-
hydroxylated products with oxygen leads to the formation of the di- and tri-hydroxylated
NAD products. The proposed naphthoquinone derivative may arise from two different
pathways. One involves hydrogen abstraction (HA) of the di-hydroxylated NAD product
(step b). Since the exact positions of the OH groups on the aromatic ring are not known,
the structures drawn correspond to the most stable one in 1,4-position. Another possibility
starts with the deprotonation of the NAD radical formed, followed by an oxygenation
process, leading to the formation of an unstable endoperoxide at positions 1,4-, which
dissociates in the presence of light into the 1,4-naphthoquinone derivative (step a). The
hydroxylation of this naphthoquinone derivative leads to the hydroxyl-1,4-
naphthoquinone. The hydroxyl-1,4-naphthoquinone derivative would be expected to have
greater stability than the parent 1,2-naphthoquinone as the hydroxyl group would stabilize
the quinone due to hydrogen bonding between the hydroxyl group and the oxygen on
carbon 1. For prolonged photolysis times, it is expected that the irradiation of the tri-
hydroxylated products leads to the opening of the aromatic ring, with formation of various
aliphatic derivatives, which, in turn, can undergo oxidation and decarboxylation steps
with the formation of CO
2
as final product. The opening of the ring has been reported in
several studies concerning the degradation of naphthalene and its derivatives, either by
5-
direct or photocatalysed degradation [86, 87, 89, 90]. In the presence of oxygen, W10
O
32
is oxidized to its initial form, W10
O
324-, closing the catalytic cycle (Scheme 2). The
oxygen consumption is accompanied by its reductive activation to form the superoxide
anion, which ultimately leads to the formation of peroxy products.
A comparison between the products of NAD degradation under direct excitation
previously studied [14], and those identified in this work (photocatalysed excitation by
W O
324-) reveals some similarities, but also some differences, suggesting differences in
10
the mechanisms of degradation. The hydroxylated (mono- and di-) as well as the
naphthoquinone products were identified in both situations, whereas the furanone and
coumarin compounds were only found under NAD direct irradiation [14]. In contrast, tri-
hydroxylation and hydroxyl-naphthoquinone products were only identified in the
photocatalysed degradation of NAD in the presence of W10
tri-hydroxylated products may be favoured by the proximity of the catalyst, which
O
324-. The formation of these

ultimately will lead to the ring opening and to the mineralization process, which does not
occur under direct degradation.
4
. Conclusions
The use of the catalyst decatungstate anion (W10
O
324-) in the homogenous
degradation of NAD in water under UV and simulated solar light exposure has proved to
be very efficient. The effect of oxygen concentration on this photocatalysed degradation
indicates a faster degradation of NAD under aerated conditions. This indicates the
important role of oxygen in the regeneration of the catalyst to favor the photocatalytic
cycle. In addition, mineralization studies show that NAD is, indeed, converted into CO
2
,
H
2
O and nitrates as main inorganic products. The identification of NAD primary products
was also assessed, and from this a degradation mechanistic scheme was proposed. The
primary steps of NAD degradation involve electron transfer as well as hydrogen atom
abstraction processes. Electron transfer occurs between the aromatic ring of NAD and the

+
reactive species of excited decatungstate, with consequent formation of NAD radical
cation and the reduced decatungstate species. As a consequence, hydroxylation and
oxidation products were identified and no reactivity of NAD amide side chain was
observed. Similarly, new NAD photoproducts were formed and identified in the presence
of the catalyst when compared with direct photolysis.
Acknowledgments
Eliana S. Da Silva acknowledges the Portuguese Fundação para a Ciência e Tecnologia
(FCT) for her PhD grant (BD/SFRH/BD/43171/2008) under the framework of the
Portuguese POPH/FEDER/QREN program, European Social Fund and National funding
from MCTES. The Coimbra Chemistry Centre thanks FCT (Projects Pest-
C/QUI/UI0313/2011, PEst-OE/QUI/UI0313/2014, UID/QUI/UI0313/2013) and
COMPETE for financial support.

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Figure Captions
4
-
−4
-1
Fig. 1. UV absorption spectra of W10O32 (3.0×10 mol L ) (dotted line) and of NAD
(
−
4
-1
3.0×10 mol L ) (solid line) in aqueous solution. The chemical structure of NAD as
well as the emission spectra of solar light () are also given.
-4
-1
Fig. 2. Kinetics of NAD (3.010 mol L ) degradation in aerated aqueous solution
4
-
-4
irradiated with the Suntest in: () presence and (Δ) absence of W10O32 (3.010 mol
-1
L ) at pH 5.0 obtained by LC-DAD (λdet = 280 nm).
-4
-1
Fig. 3. Evolution of NAD (3.010 mol L ) disappearance in aqueous solution: i) (○) in
4
-
4-
-4
-
aerated conditions and absence of W10
O
32 ; ii) in presence of W10
O32 (3.010 mol L
1)
in: (■) de-aerated, () aerated and () oxygenated conditions; and iii) () in presence
4
-
-4
-1
-1
of W10O32 (3.010 mol L ) and iso-propanol (1.0 mol L ) under aerated conditions.
Irradiation at 365 nm and λdet = 280 nm.
Fig. 4. TOC () and nitrates () evolution profile of aerated aqueous NAD solution
-4
-1
4-
-4
-1
(
3.0×10 mol L ) irradiated at 365 nm in the presence of W10O32 (3.010 mol L ), as
a function of irradiation time.
+
Fig. 5. LC-DAD-ES /MS chromatogram of aerated aqueous solution of the mixture
-4
-1
4-
-4
-1
[
NAD (3.010 mol L ) + W10O32 (3.010 mol L )] (2 h irradiation, det = 250 nm).
2
Fig. 6. MS spectrum for the product with m/z = 218.
2
Fig. 7. MS spectrum for the product with m/z = 232.
Scheme 1. General scheme of NAD hydrolysis into 1-NAA, NH
3
(and NH +
).
4
4
-
Scheme 2. Proposed mechanism for the degradation of NAD photocatalysed by W10
O
32
in aqueous solution.

FIGURE.1
2.0
1.5
1.0
0.5
0.0
1
1
0
.5
.0
.5
NAD
4
-
W O
0 32
1
0.0
450
2
50
300
350
400
Wavelength (nm)

FIGURE.2
1.0
0.8
0.6
0.4
0.2
0.0
NAD
4
-
NAD + W
O
1
0 32
0
2
4
6
8
Irradiation time (h)

FIGURE.3
1.05
0.90
0.75
0.60
0.45
0.30
0.15
0.00
0
5
10
15
20
Irradiation time (h)