# Photocatalytic Degradation of a Water Soluble Herbicide by Pure and Noble Metal DepositedTiO2 Nanocrystalline Films

Authors: Katerina Pelentridou; Elias Stathatos; Helen Karasali; Dionysios D. Dionysiou; Panagiotis Lianos
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

We present the photocatalytic degradation of a water soluble sulfonylurea herbicide: azimsulfuron in the presence of titania nanocrystalline films. Efficient photodegradation of herbicide was achieved by using low-intensity black light tubes emitting in the Near-UV. The degradation of the herbicide follows first-order kinetics according to the Langmuir-Hinshelwood model. Intermediate products were identified by the LC-MS-MS technique during photocatalytic degradation. In order to increase photodegradation rate of the herbicide, we examined the effect of titania modification by depositing noble metals at various quantities and valence states. The presence of platinum at neutral valence state and optimum concentration induced higher photodegradation rates while silver-modified titania exhibited similar photocatalytic rates with those obtained with pure nanocrystallineTiO2 films. Finally, the effect of initial pH value was also examined. Acidic or alkaline media were unfavorable for azimsulfuron photodegradation.


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## Body

## 1. Introduction

Photodegradation of various organic pollutants by photocatalysis, using
wide bandgap semiconductors, has been extensively studied [1–3]. Among them, TiO2 a relatively inexpensive semiconductor exhibits high photocatalytic activity,
stability in aqueous solution, nontoxicity [4, 5], and so forth.  However, TiO2 usage has a few
drawbacks; for example, it absorbs only in the UVA part of the light spectrum
where solar radiation is only 2-3% of the total reaching the surface of the Earth
[6]. Moreover, the application of TiO2 for photocatalytic oxidation
of organic molecules is limited by high charge carrier recombination rates that
results in low quantum efficiency. In recent years, surface metallization of
TiO2 has received considerable attention as an option to overcome
the high degree of charge carrier recombination [7–10]. Platinum,
and some other noble metals, may be used for this purpose thus providing an
electron sink. In addition, they may extent TiO2 absorbance in the Visible
[11]. The presence of a metal at the surface of TiO2 results in the
formation of a Schottky barrier at the metal-semiconductor
interface, which facilitates the interfacial electron transfer and subsequently
encourages   the charge carrier separation [12].Among the various organic substances, which are known as water
pollutants, herbicides are a major pollution source for both underground and
surface waters. Advanced oxidation processes are used, among others, also for
the degradation of herbicides [13, 14].  Azimsulfuron
(AZS, see Scheme 1 for chemical structure) belongs to the class of sulfonylurea
herbicides, which have a broad spectrum of weed control, low application rate,
and low animal toxicity [15, 16]. Sulfonylurea herbicides,
in addition to playing an important role in modern agriculture, are also
degradable by heterogeneous
photocatalysis, as it has been proven in the past [17, 18]. In the present work, sol-gel prepared TiO2 films, which were
further modified with noble metal ions, were examined for the photodegradation
of AZS in water. The effect of various parameters, such as the amount of metal
deposits and pH value of herbicide aqueous solution, were studied in order to
evaluate the optimum conditions for the photocatalytic oxidation of AZS.Scheme 1
Chemical
structure of AZS.


## 2. Experimental

Azimsulfuron, N-[[(4,6-dimethoxy
pyrimidin-2-yl)-amino]
carbonyl]-1-methyl-4-(2-methyl-2H-tetrazole-5-yl)-1H-pyrazole-5-sulfonamide
(AZS, 99.3% purity, Scheme1), was donated by DuPont de Nemours, Puteaux, France. All the rest of reagents were obtained from Sigma-Aldrich, Mo, USA.
Millipore water was used in all
experiments.


## 2.1. Description of the Photocatalytic Reactor

The
cylindrical reactor schematically shown in Figure1 was used in all experiments
[19]. Air was pumped through the gas inlet using a small pump to ensure continuous
oxygen supply to the reaction solution while simultaneously agitating it.  In cases where experiments were carried out
in the absence of oxygen, the solution was deoxygenated by nitrogen flow and
the openings were sealed.  Four black light
fluorescent tubes of 4 W nominal  power were placed around the
reactor. The whole construction was covered with a cylindrical aluminum
reflector. Cooling was achieved by air flow from below the reactor using a
ventilator. The catalyst was in the form of four-glass rings, covered on both
sides with nanocrystalline TiO2 film. Film deposition is described
below.  The glass rings were of 38 mm of diameter and 15 mm height, stacked and
coaxially placed inside the reactor.
Thus, the total surface of the photocatalyst film was approximately 2×71.6=143cm2.  The intensity of
radiation reaching the surface of the film on the side facing lamps was
measured with an Oriel  radiant power
meter and found equal to  0.79mW/cm2.Figure 1
Schematic representation of the reactor used
in all experiments.


## 2.2.  Nanocrystalline Titania Films  and Metal Deposition

Titania films were
deposited by following the previously reported procedure [20, 21]. Briefly, for
25 mL solution, 3.6 g of the nonionic surfactant Triton X-100
(polyoxyethylene-10-isooctylphenyl ether) was mixed with 20 mL of
ethanol, followed by addition of 1.6 mL of glacial acetic acid and 1.8 mL of titanium
isopropoxide under vigorous stirring. Self organization of the surfactant in this
original sol creates organized assemblies that act as templates defining
nanoparticle size. The surfactant is burned out during calcination. After a few
minutes stirring, the glass rings described above, which were previously
thoroughly washed, sonicated in ethanol and dried in a N2 stream, were
dipped in the above sol and withdrawn slowly by hand. After the film was dried
in air for a few minutes, it was calcined in an oven. The temperature was
increased in a ramp rate of 20C/min⁡° up to 550C° and left
at that temperature for about 10 minutes.
When the titania-covered rings were taken out of the oven, they were
transparent and optically uniform. The above procedure was repeated several
times in order to reach the quantity of catalyst necessary for the purposes of
the present work. The final mass of titania on the four glass rings was 80 mg
(20 mg on each glass ring). Noble metal ions were deposited on titania films by
adsorption from aqueous solutions containing one of the following metal salts:
Na2PtCl4⋅xH2O  or
AgNO3 at various concentrations (from 10−4 to 10−3M for the platinum salt and from 5×10−4 to 10−2M for the silver salt). After the last layer of
TiO2 was deposited and immediately after the film was
taken out from the oven, the rings were submerged in the salt aqueous solution
and were left for half an hour in the dark. Then, the rings were washed, dried,
and subjected to UV radiation for 30 minutes; or they were additionally heated at
500C° for 15 minutes.  UV and heat
treatment were performed to reduce cationic species to neutral metallic particles.


## 2.3. Photodegradation Procedure

The reactor was filled with 100 mL water in which 6 mg of AZS were
dissolved. Standard solutions were employed in all experiments. AZS is not
adsorbed on titania films, however, about an hour of equilibrium was allowed
before illumination. The concentration of AZS in solution was monitored by absorption
spectrophotometry at 240 nm using quartz cuvettes. In order to assure linear
relationship between absorbance and concentration, preliminary measurements
have been made. The linear relationship was established in the range of0→60mgL−1.
Thus, the presently chosen
concentration 60mgL−1 was the maximum concentration of the
established linear concentration versus absorbance range. It was found that
this concentration is also close to the solubility limit of AZS in water. A 3 mL
aliquot was collected for performing absorption spectrophotometry, and it was poured
back into the reactor after measurement. The first concentration measurement
was done in the dark. It was then verified that the herbicide was not adsorbed
on the titania film. Finally, the products of photodegradation have been
monitored by LC-MS-MS analysis.


## 2.4. Apparatus and Methods

Absorption measurements were made with a Cary
1E spectrophotometer. LC-MS-MS analysis
was carried out on a Varian 1200L mass spectrometer equipped with electrospray ionization
interface (ESI). The ion trap was connected to a Varian 1200 LC system
consisting of two individual pumps, a membrane degasser, an autosampler with
temperature control, and a thermostatic column control.  The chromatographic separation was performed
on a reversed phase Varian Polaris C-18 column (150mm×2.1mm, 5μm particle size)
at 30C°. The concentration of AZS at each degradation
stage was calculated by using an external calibration curve from LC-MS-MS
measurements of AZS at five different concentrations. Stock solution was
prepared by diluting the appropriate amount of the analytical standard with
water/methanol (70/30) at concentration 60 ppm. In order to determine the
degradation products formed during AZS photocatalysis, ESI ionization in both
negative- and positive-ion modes was investigated. The results indicated that
the ESI source at positive-ion mode with collision energy at 30 V was most
appropriate for the analysis of the degradation products.


## 3. Results and Discussion

The films made by the sol-gel procedure
described in the Experimental section consisted of anatase nanocrystals of
about 15 nm average size. Size polydispersity is limited and the active surface
area is around110m2/g, as measured by nitrogen adsorption-desorption
isotherms according to the BET analysis [21].


## 3.1. Photodegradation Studies of AZS  with PureTiO2 Films

It has
been proposed that titanium dioxide mediated photodegradation involves the
generation of electron-hole pairs [2], which migrate to the photocatalyst
surface forming surface bound hydroxyl and superoxide radicals according to
following reactions:(1)TiO2+hv→ecb−+hvb+,O2+ecb−→O2−•,H2O++hvb+→OH•+H+. It is commonly known that the hydroxyl and
superoxide radicals are the primary oxidizing species in the photocatalytic
process.  These oxidative reactions result
in the photodegradation of AZS.   The efficiency of this process depends on
oxygen concentration. The absorption spectra of the AZS solution at various
stages of photodegradation are presented in Figure 2, where it is obvious that the
concentration of intact AZS decreased during illumination in the presence of
titania. The characteristic peak at 240 nm completely disappears after 15 hours
of illumination,  using low-power black
light source. Practically, no photolysis of AZS has been observed at the present
illumination level, as seen by curve number 2 of Figure 3. The presence of oxygen
during photocatalytic experiments is absolutely necessary and this is proved by
the data of curve number 3 in Figure 3. Absence of oxygen leads to negligible
photodegradation.  Photodegradation
followed Langmuir-Hinshelwood kinetics:
(2)ln(A0/A)=kappt.  The corresponding first-order rate constant was
0.0020min−1⁡.  By
taking into account  the fact that no AZS
was adsorbed on the titania films, as already said,  it is obvious that photodegradation is carried
out in the liquid phase and it is also obvious that  O2−• species,
which are only formed  in the presence of
oxygen, play a major role.  Furthermore,
the fact that photodegradation is carried out in the liquid phase explains why
it is relatively slow. As a matter of fact, it is known that photodegradation
rates are higher when the target substance is adsorbed on the photocatalyst.Figure 2
Absorption spectrum of AZS in water and variation with time of UV illumination
in the presence ofTiO2 nanocrystalline films.Figure 3
Photodegradation curves of AZS with black
light illumination: in the presence ofTiO2 films (1);  in the absence of photocatalyst (2); and in
the absence of oxygen (3).


## 3.2. Photodegradation of AZS in Presence of
Noble Metals Deposited onTiO2 Films

Silver or platinum metal ions were deposited on theTiO2 surface by submerging titania for several minutes in aqueous
solutions of metal salts. Metal cations could be adsorbed in TiO2 films at substantial quantities because of the relatively high specific surface
area of the films. The platinum complex is in the form of divalent tetrachloride
ion. By accepting two photo-generated electrons, it is converted into zero
valence metal according to the following reaction: (3)PtCl42−+2e−→Pt0+4Cl−. However, XPS
measurements for UV illuminated samples gave peaks with binding energies of
72.85 and 76.13 eV, consistent with published values [22] for Pt(II).
In addition  to UV exposure, thermal
heating (500C°) of the metal-deposited films gave a variation of the
binding energies of platinum,  which were
then located at 70.5 and 73.86 eV, consistent with Pt(0) [22]. It is
obvious that weak UV light intensity from black light irradiation was not
enough for the complete transformation of tetrachloroplatinate ion into zero
valence platinum. However, it is possible that Pt(II) is finally
transformed into Pt(0),  due
to the prolonged UV illumination during photodegradation.  In the case of silver ions, the results were
more predictable as only one electron is enough to reduce silver ions and
create zero valence noble metal on the surface of TiO2,  according to the following reaction: (4)Ag+1+e−→Ag0. This electron is
easy to be generated, either by UV illumination under black light irradiation
or by thermal heating of the films [23]. XPS measurements
gave similar results for both procedures of silver reduction. The
photodegradation of AZS herbicide in the presence of platinum modified titania
films is presented in Figure 4. The films subjected to both UV and thermal
treatment gave better photocatalytic efficiencies as compared with the films, which
were only UV treated. Furthermore, the original salt concentration, which
obviously affects the quantity of deposited metal, affected photodegradation
efficiency.  When the platinum salt
concentration was  5×10−4molL−1,  we obtained the highest
photocatalytic efficiency. At higher metal load, efficiency dropped, possibly
due to screening effects.  The
photocatalytic degradation of AZS in presence of silver loaded TiO2 is presented in Figure 5(a). As in the case of platinum, metal load affected
photocatalysis, 10−2molL−1 silver salt concentration giving
the best results. Silver deposited on TiO2 films gave the plasmon
resonance absorption [23] spectra of Figure 5(b). Absorbance values verified that there is a
direct relation between metal load and salt concentration in the aqueous
solution. Comparative studies of AZS photodegradation with pure or metal-loaded
TiO2 showed that the presence of platinum is effective for the
destruction of the herbicide contrary to silver, which gave similar results as pure
TiO2 films (see Figure 6). The role of platinum in photocatalysis is
not fully understood. The degree of enhancement of the activity of TiO2 by platinisation seems to depend highly on the substance to be photodegraded [24],
the characteristics and the amount of deposited Pt
[25], and the structural and morphological properties
of original TiO2 [26]. In general,
it is accepted, as already said, that in platinized TiO2, a better
separation of charge carriers is observed.Figure 4
Photodegradation curves of
AZS with black light illumination in presence of Pt modifiedTiO2 films at various platinum concentrations and treatments of the films.(a) Photodegradation curves of AZS with black light illumination in the presence of Ag modifiedTiO2 films at various silver concentrations and treatments of the films: (a) -•- from 5×10−4M solution and after UV treatment; (b) -▴- from 10−2M solution and after UV treatment; (c) -▾- from 10−2M solution and after UV and heat treatment.
(a)(b)Figure 6
Comparative
photodegradation curves of AZS with black light illumination in the presence of
pure and modifiedTiO2 films.


## 3.3. AZS Photodegradation Intermediates

Intermediate products of AZS photodegradation
were studied by LC-MS-MS. Two major ions were detected in all photodegradation
stages, one with m/z: 199 and the other with m/z: 244.  These two fragments exactly correspond to the
amino-dimethoxypyrimidine and
1-methyl-4-(2-methyl-2H-tetrazole-5-yl)-1H-pyrazole-5-sulfonamide
moieties, respectively,  which AZS
consists of.  Ion 244 yielded a major
further fragment ion at m/z: 83.3, which could correspond to the sulfonamideSO2NH2−
group.  The amino-dimethoxypyrimidine-containing ion yielded a major
fragment ion at m/z: 74.5, which could correspond to the break down of the
carbon link between the two nitrogen atoms leading to the formation of methylaminocarboxylic acid CH3NHCOOH.  These data are preliminary and further studies on the detailed reaction pathway are being conducted in our laboratories.


## 3.4. pH Effect on the Photodegradation of AZS

The photodegradation of AZS was also studied at various pH values.  The results are presented in Figure7.  pH values were modified by adding either H2SO4 or NaOH, both at a concentration of 0.1molL−1. The fastest
photodegradation was obtained at natural pH (4.7), that is, in a pure solution of
AZS in water. Photodegradation rates in basic pH were low and even lower in
acidic pH.  This behavior could be
explained by the fact that AZS possesses chargeable chemical groups.  It is highly probable that at low pH values, AZS
is positively charged, and at high pH values, it is negatively charged.  However, as it is well known that the surface
of titania is also charged with the same charge at the same pH [27].  Therefore, at low or high pH, there is a
repulsion that separates the catalyst surface from the target molecule and
makes it more difficult for the reactive radical species, which are generated
on the surface of the catalyst, to reach the target molecules; hence the lower
photodegradation rates in these cases. The right axis scale of all graphs of
Figure 7 represents the change of pH in the solution during illumination.  In strongly alkaline and acidic solutions, pH
decreased during photodegradation, but in the case of natural solutions, a pH increase was observed.  The interpretation
of the pH effect on the photocatalytic process is a rather difficult task
because of the multiple roles that various factors could play, such as
electrostatic interactions with the semiconductor surface, chemical structure
of the original substance and of its fragments, and formation of active radicals.Effect of pH variation in the
photodegradation rates with pure nanocrystalline titania films (a) in acidic
pH; (b) in natural pH of AZS in water; (c) in alkaline pH value.
(a)(b)(c)


## 4. Conclusions

The herbicide azimsulfuron can be effectively photodegraded by employing pure or noble metal-modified
titania nanocrystalline films as photocatalysts with black light tubes as low-intensity
UV illumination source.  The catalyst was
deposited by the sol-gel method on glass rings; it could be thus easily
recuperated and repeatedly used in subsequent photodegradation procedures. The
best photocatalytic rates were achieved in the case of platinum modified
nanocrystallineTiO2 films. The pH of AZS aqueous solution affected
photodegradation rates.  The fastest rate
was obtained in the case of natural pH of the solution.



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