Open Journal of Applied Sciences, 2012, 2, 86-92
doi:10.4236/ojapps.2012.22011 Published Online June 2012 (
Photocatalytic Tr eatment of Micr ocystin-LR-Containing
Wastewater Using Pt/WO 3 Nanoparticles under Simulated
Solar Light
Chao Zhao, Yingnan Yang, Zhenya Zhang*
Graduate School of Life and Environmental Science, University of Tsukuba, Tsukuba, Japan
Email: *
Received April 7, 2012; revised May 10, 2012; accepted May 20, 2012
This study investigates the photocatalytic degradation of microcystin-LR (MC-LR) under simulated solar light using Pt
modified nano-sized tungsten trioxides (Pt/WO3). Photocatalytic activity was higher during the degradation of MC-LR
with Pt/WO3 than with pure WO3 or TiO2. The catalyst loading greatly affect the degradation performance. The rate of
degradation is influenced by the initial pH of the reaction solution. This study also investigates the photocatalytic inac-
tivation of cyanobacteria. The results show that the algal growth was successfully controlled by the Pt/WO3. This study
suggests Pt/WO3 photocatalytic oxidation with solar light is a promising treatment for water containing MC-LR.
Keywords: Microcystin-LR; Photocatalytic Degradation; Solar Light; Tungsten Trioxide
1. Introduction
An intensification of agricultural and industrial activities
resulting from an increase in population has led to eutro-
phication in superficial freshwater bodies and has there-
fore induced more frequent cyanobacteria blooms world-
wide. The toxins released into freshwater by cyanobacte-
ria are well-documented [1]. The most commonly occur-
ring toxins released by cyanobacteria are called Micro-
Microcystins are a family of cyclic heptapeptides heap-
totoxins containing the unique C20 amino acid,
ienoic acid, which is abbreviated Adda. The most abun-
dant and frequently detected microcystin is microcys-
tin-LR (MC-LR), which has leucine (L) and arginine (R)
in the variable positions. Microcystins are strongly heap-
totoxic because they disrupt protein phosphatases 1 and
2A [2], which may promote primary liver cancer and
cause the death of animals and humans. The World
Health Organization (WHO) has determined a provi-
sional guideline value of 1.0 μg/L for MC-LR in drinking
Various water treatment processes have been evalu-
ated to determine their efficacy in degrading these toxins
because microcystins are considered a threat to human
health. However, MCs are chemically stable across a
range of pH values and temperatures, due to a cyclic
structure; consequently, traditional water treatment proc-
esses are unsuccessful in removing MCs [3-5].
Photocatalytic oxidation is an advanced oxidation tech-
nology that has been deemed an environmentally friendly
water treatment option in recent years [6-10]. When a
photocatalyst is illuminated with a light of an appropriate
wavelength, pairs of electrons (e) and electron holes (h+)
are generated on the surface of the catalyst by photons.
These pairs react with oxygen and water molecules or
hydroxyl groups adsorbed on the surface of the catalyst
and form highly reactive oxygen species, such as hy-
droxyl radicals (•OH), superoxide ions (2) or hydrop-
eroxyl radicals (•OOH) [11]. Reactive oxygen species
can nonselectively oxidize a large number of organic
wastes, including dyes, pesticides, bacteria and herbi-
cides [6-8,12]. Previous research proved that photocata-
lytic oxidation with TiO2 photocatalyst could effectively
destroy MCs, even at extremely high toxin concentra-
tions [13,14]. However, TiO2 has a large absorption band
gap (Eg) of 3.2 eV that restricts its universal use because
it can only absorb UV light [15]. Conversely, with an Eg
between 2.4 eV and 2.8 eV, tungsten oxide (WO3) is a
photocatalyst that absorbs visible light irradiation up to
480 nm [11]. Compared with mixed metal oxides and
doped oxides, WO3 is inexpensive to prepare and stable
in acidic and oxidative conditions, which makes it a
promising material for photocatalytic applications. Pre-
vious research showed that WO3 degradation of organic
species under visible light intensified in the presence of
*Corresponding author.
Copyright © 2012 SciRes. OJAppS
suitable co-catalysts, such as Pt, Pd and CuO [16-18].
However, there is no literature on the photocatalytic de-
gradation of MCs under visible light with WO3-based
The antimicrobial activity of photocatalyic reaction
was first demonstrated by Matsunaga and coworkers [19],
since then, photocatalysis has been shown to be capable
of killing a wide range of organisms including Gram-
negative and Gram-positive bacteria, including endospores,
fungi, algae, protozoa and viruses, and has also been
shown to be capable of inactivating prions [20-22]. More-
over recently works also reported that TiO2-photocata-
lysis has the ability to inhibit the growth of the la-
mentous algae, Oedogonium and Cladophora [23,24].
In our study, the photocatalytic degradation of MC-LR,
a model toxin, was investigated using Pt/WO3 under
simulated solar irradiation. Variations in sample parame-
ters, such as catalyst loading and initial pH, were present
in this study and are discussed later. To investigate the
photocatalytic inactivation of cyanobacteria, Microcystis
aeruginosa (M. aeruginosa) was selected as test species,
for it is the most common blue-green algae and easily
causes eutrophication.
2. Experimental
2.1. Chemicals and Preparation of Photocatalysts
MC-LR standard (95% purity; FW 995.2 g/mol) and
WO3 powder was purchased from Wako (Wako Pure
Chemical Industries, Ltd., Japan). Sigma-Aldrich (Sigma-
Aldrich Co. LLC., USA) supplied hexachloroplatinic
acid (H2PtCl6·6H2O). Ishihara (Ishihara Sangyo Ltd., Ja-
pan) supplied the nanoparticle compound TiO2 (ST-21).
Pt-loaded WO3 sample (Pt/WO3) (0.5% w/w) was pre-
pared by a photo deposition method [18]. An aqueous
suspension containing the particulate WO3 and
H2PtCl6·6H2O was exposed to visible light (λ > 400 nm)
provided by a 300 W Xe lamp (LX-300F, Cermax, CA)
fitted with a cutoff filter (L-42, HOYA, Japan). After 2 h
of irradiation, methanol (10 vol%) was added and the
suspension exposed to further irradiation for 2 h. The
as-prepared sample was collected by centrifugation and
washed twice with Milli-Q water and finally dried at
105˚C for 2 h. The amount of deposited Pt was deter-
mined by analyzing the concentration of unused chloro-
platinic acid remaining in the centrifuged solution after
photo-deposition. The chloroplatinic acid concentration
was analyzed by a inductively coupled plasma mass
spectrometry (ICPS-8100, Shimadzu Co. Ltd., Japan).
The prepared sample was characterized. UV-visible spec-
trum of the sample was recorded on a spectrophotometer
(UV-2550, Shimadzu Co. Ltd., Japan). X-ray powder
diffraction (XRD) measurement was carried out by using
a X-ray diffractometer (Rigaku Smartlab).
2.2. Algal Culture
M. aeruginosa was obtained from National Institute for
Environment Studies (NIES) (Ibaraki, Japan), The com-
position of the MA medium used in algal growth tests
was listed in Tabl e 1. 1 L of MA medium was added to a
3 L conical flask and was autoclaved at 121˚C for 20 min.
The cultivation was carried out in the cultivating box
with illumination for 10 days. The continuous light was
provided by a fluorescence lamp with an automated 12
h/12 h light/dark cycle. The light intensity during the
light phase was 1500 lx. The temperature was controlled
at 25˚C ± 1˚C.
2.3. Photocatalytic Tests
The reactor was a 6-mL vessel equipped with a magnetic
stirrer. A solar lamp (XC-100B, SERIC Ltd., Japan) was
used as the irradiation source, and the light intensity was
measured with a photometer (LI-250A, LI-COR Inc.,
USA). The photoemission spectrum was measured with
an optical fiber spectrometer (USB4000, Ocean Optics
Inc., USA).
An aliquot of stock MC-LR solution (50 mg/L) was
added to the test solution to achieve an initial concentra-
tion of 1 mg/L. A suspension with catalyst particles was
transferred to the reactor containing MC-LR to obtain a
final volume of 5 ml. The catalyst loading was varied
depending on the experiment condition. The initial pH
was adjusted with H2SO4 or NaOH. Before irradiation,
the suspension was stirred for 60 min in the dark to
equilibrate the solution. During irradiation, samples were
taken and centrifuged every 30 minutes for analysis.
Table 1. Composition of culture medium.
Component Concentration (L–1)
Ca(NO3)2·4H2O 50 mg
KNO3 100 mg
NaNO3 50 mg
Na2SO4 40 mg
MgCl2·4H2O 50 mg
β-Na2glycerophosphate 100 mg
Na2EDTA 5 mg
FeCl3·6H2O 0.5 mg
MnCl2·4H2O 5 mg
ZnCl2 0.5 mg
CoCl2·6H2O 5 mg
Na2MoO4·2H2O 0.8 mg
H3BO3 20 mg
Bicine 500 mg
Copyright © 2012 SciRes. OJAppS
Copyright © 2012 SciRes. OJAppS
Photocatalytic inhibition of M. aeruginosa was carried
out in a 200-mL beaker equipped with a magnetic stirrer.
The irradiation source was the same solar lamp with an
automated 12 h/12 h light/dark cycle. All the experiment
equipments were placed in a clean bench to prevent the
interference of dust and microorganism brought by air.
150 ml of algal solution and a suspension with catalyst
particles were added to the beaker for irradiation. Sam-
ples were taken every 2 days for analysis.
counts. Then the samples were dripped to a hemocy-
tometer and covered with a clear cover for counting. The
enumeration was achieved by a microscope.
3. Results and Discussion
3.1. Characterization of Phoctocatalyst Samples
Figure 1 shows the absorption (100-reflectance) spectra
for WO3 and Pt/WO3 powders. The absorption of WO3
increased at approximately 460 nm, which is consistent
with previously reported value [16]. For Pt/WO3 sample,
with the contribution of Pt doping the spectrum shows
stronger broad absorption in the visible light region. The
XRD patterns of WO3 and Pt/WO3 samples are illustrated
in Figure 2. Compared to tungsten oxide JCPD files (No.
43-1035) and those reported by others [23,24], the dif-
fraction patterns of the samples assigned those of WO3
monoclinic structure. As shown, no sign of crystallite Pt
is detected in the patterns with Pt/WO3. It can be related
to the fact that lower Pt concentrations lie below XRD
instrumental detection limit and indicated that the Pt
doping did not influence the crystal structures of WO3.
2.4. Detection of Hydroxyl Radicals (•OH)
Photoluminescence (PL) with terephthalic acid as a probe
molecule was used to detect •OH in the photocatalytic
reaction system. Terephthalic acid reacts with •OH to pro-
duce highly uorescent 2-hydroxyterephthalic acid [25].
In a beaker, a photocatalyst powder was dispersed in
20 mL of 5 × 10–4 M terephthalic acid aqueous solution
and 2 × 10–3 M NaOH. The solar lamp was used as a
light source. Samples were centrifuged every 20 min for
2.5. Analysis
The degradation of microcystin-LR was monitored by
High-performance liquid chromatography (HPLC) (Jasco-
1500, Jasco, Inc., Japan) with a high-resolution diode ar-
ray detector (Jasco UV-1570) set at wavelength of 238
nm. Samples were separated on a C18 column (5 m, 250
mm, 4.6 mm id) using a mobile phase of acetonitrile and
Milli-Q water containing 0.01 mol/L ammonium acetate
(pH 6.8; 32:68 v/v) and a flow rate of 1 mL/min.
PL spectra generated by the 2-hydroxyterephthalic
acid were measured on a Hitachi F-4500 uorescence
spectrophotometer set at a wavelength of 315 nm. The
pH values of the solutions were measured with a pH me-
ter (TES-1380, TES Co., Taiwan).
The growth of M. aeruginosa was evaluated by cells
enumeration. The samples were diluted with Mill-Q wa-
ter to obtain an appropriate cell density for microscopic Figure 1. Absorption spectrum of (A) Pt/WO3 and (B) Pure
WO3, respectiv ely.
Figure 2. XRD patterns of (a) pure WO3 and (b) Pt/WO3, respectively.
3.2. Photocatalytic Degradation of MC-LR with
As shown in Figure 3, the concentration of MC-LR was
virtually unchanged after 3 h of irradiation when there
was no photocatalyst in the solution, thereby indicating
MC-LR was stable under solar irradiation. After 3 h of
irradiation, 19% of MC-LR was removed from a solution
with only TiO2 added, and 24% of MC-LR was removed
from a solution in which only WO3 was added. Compare
to pure TiO2 and WO3, the performance of modified
WO3 was much better, with 3 h of irradiation. Over 81%
of MR-LR was degraded by the Pt/WO3 composite with-
in 90 min. Furthermore, the removal efficacy was 100%
when the contact time was lengthened to 180 min.
Poor MC-LR removal efficiency by a solution con-
taining only TiO2 was attributable to the light source.
The simulated solar lamp used in this experiment emitted
light mainly with wavelengths greater than 400 nm, and
the TiO2 excitation range is less than 390 nm [15]. The
low photocatalytic activity of pure WO3 is because the
conduction band level of WO3 (+0.5V vs. NHE) is more
positive than the potential for the single-electron reduc-
tion of oxygen (O2/2 = 0.56 V vs. NHE; O2/HO2 =
0.13 V vs. NHE) [17]. Without co-catalysts, the high
conduction band of WO3 restricts the compound’s activ-
ity with an organic compound [10].
3.3. Photocatalytic Degradation Mechanism
Pt is a co-catalysts for WO3-induced photocatalytic reac-
tions that can promote O2 reduction in a multi-electron
process [17,26]. In a photocatalyic reaction, the follow-
ing chain reactions have been postulated [11]:
Catalyst hve h
22 ads OO e
 (2)
 (3)
 OO
 (5)
 (6)
  (7)
 (8)
Several highly reactive oxygen species, such as •OH,
HOO• and 2, are generated through the reduction of
O2 to oxidized organic compounds. Accordingly, organic
compounds could be effectively degraded by WO3 in the
presence of a co-catalyst.
Photocatalytic degradation of MC-LR was initiated by
the attack of hydroxyl radical on the conjugated diene
structure of Adda [27], thereby indicating the primary
reactive species in MC-LR degradation was the hydroxyl
radical. Kim et al. proved that the deposition of Pt on
WO3 facilitates the generation of OH radicals under visi-
ble light [28], and our experiments confirmed this phe-
nomenon using photoluminescence (PL). Figure 4 shows
the spectra observed during irradiation of the Pt/WO3
sample. At approximately 425 nm, PL intensity gradually
increased with an increase in irradiation time, thereby sug-
gesting that OH radicals are formed on the photocata-
lyst-water interface via photocatalytic reactions [25,29].
Because the photocatalytic degradation of MC-LR was
initiated by an OH radical [27], Pt/WO3 is particularly
effective in the photocatalytic degradation of MC-LR.
Figure 3. The effect of catalysts on the efciency of photo-
catalytic degradation of MC-LR. Experimental conditions:
MC-LR concentration of 1 mg/L, catalyst concentration of
100 mg/L and simulated solar light intensity of 0.4 mW/cm2.
Experimental conditions: NaOH concentration of 2 × 10–3 M, terephthalic
acid concentration of 5 × 10–4 M, Pt/WO3 concentration of 200 mg/L and
simulated solar light intensity of 0.4 mW/cm2.
Figure 4. PL spectral changes observed during irradiation
of the Pt/WO3 sample.
Copyright © 2012 SciRes. OJAppS
3.4. Effect of Catalyst Loading
Experiments were carried out by varying the catalyst
concentration from 50 to 250 mg/L. The results are
shown in Figure 5. When the catalyst loading was 50
mg/L, the degradation efficiency was low, only 58% of
MC-LR was removed after 3 h irradiation. The increase
in the catalyst loading from 50 to 150 mg/L sharply in-
creased the degradation efficiency. However, when the
catalyst loading further increased to 250 mg/L, the deg-
radation efficiency decreased. The results indicate that
the inactivation efficiency is not proportional to the cata-
lyst loading. The increasing catalyst loading induces an
increase in the availability of active sites on the catalyst
surface and produces a proportional increase in the num-
ber of active radicals by absorbing photons. However, an
increase in the catalyst concentration results in increasing
the light source extinction coefficient, and subsequently
reducing the reaction efficiency [21,30].Based on this
result, 150 mg/L was chosen as the optimum catalyst
3.5. The Effect of Initial pH on the
Photocatalytic Degradation of MC-LR
The pH affects the surface condition of catalysts and
MC-LR and the generation of hydroxyl radical in hy-
droxylation reactions. After 180 min of irradiation, the
removal of MC-LR was 89%, 100% and 77% with pH
values of 3, 6 and 10, respectively (Figure 6).
Although the degradation of MC-LR was initiated by
the attack of hydroxyl radical [27], the number of •OH
ions should be lower at an acidic pH because hydronium
ions favor the presence of an electron hole (Equation (8))
[31]. At low pH, MC-LR degradation would be adversely
affected due to the lack of OH- ions. The initial pH was
Experimental conditions: MC-LR concentration of 1 mg/L, simulated solar
light intensity of 0.4 mW/cm2.
Figure 5. Efciency of photocatalytic degradation of MC-LR
as a function of catalystloading.
Experimental conditions: MC-LR concentration of 1 mg/L, Pt/WO3 concen-
tration of 100 mg/L and simulated solar light intensity of 0.4 mW/cm2.
Figure 6. Efciency of photocatalytic degradation of MC-LR
as a function of initial pH.
adjusted by H2SO4, but Liang et al. reported that 2
ions have an adverse effect on the photocatalytic degra-
dation rate [32]. Given the evidence, we can explain the
lower efciency observed in our experiment at acid pH.
The point of zero zeta-potential (PZZP) for WO3 oc-
curs at approximately pH 2, and WO3 particles are nega-
tively charged when the pH of a solution is greater than 2
[28]. At pH values between 3 and 12, the carboxylic
groups of MC-LR are ionized, and the molecule is nega-
tively charged [33]. In basic conditions, negatively charged
WO3 molecules repel MC-LR and inhibit interactions
between the toxins and the catalysts. Fewer interactions
result in lower photocatalytic activity and a lower degra-
dation rate. Our results are consistent with a study by
Lawton et al. in which the reaction rate of photocatalytic
degradation of MC-LR by TiO2 was lowest at pH 10
3.6. Photocatalytic Inhibition of Algal Growth
under Solar Light
Figure 7 shows the results of photocatalytic inhibition of
algal growth under solar light. After 6 days of irradiation,
M. aeruginosa in the control samples kept growing and
the number of algae cells increased from 1.3 × 106 to 2.2
× 106. However, the growth of M. aeruginosa in the sam-
ples treated by photocatalyst was inhibited and the algae
cells decreased from 1.3 × 106 to 0.1 × 106. These results
show that the photocatalytic treatment had the function
of inactivation of microorganisms.
It is generally believed that the inactivation of micro-
organisms by photocatalylic treatment is mainly due to
oxidative radicals (mainly •OH) produced by photocata-
lyst irradiation [35].
Copyright © 2012 SciRes. OJAppS
Experimental conditions: Catalyst concentration of 150 mg/L and simulated
solar light intensity of 0.8 mW/cm2.
Figure 7. Photocatalytic inhibition of algal growth.
4. Conclusions
Under simulated solar irradiation, WO3 degrades MC-LR
more effectively in the presence of co-catalyst compared
to solutions with pure TiO2 or pure WO3. The highest
rate of MC-LR removal occurred in solutions containing
Pt/WO3. Specifically, 1 mg/L of MC-LR was removed
after 3 h of irradiation by 100 mg/L of Pt/WO3. The op-
timum catalyst loading was 150 mg/L. A neutral pH,
such as a pH of 6, improved the efficacy of toxin re-
moval. The experiment results show that the algal growth
was successfully controlled by the Pt/WO3.
5. Acknowledgements
This work was supported in part by Grant-in-Aid for Re-
search Activity Start-up 22880007 and Scientific Re-
search (A) 22248075 from Japan Society for the Promo-
tion of Science (JSPS).
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