Lanthanum ion doped nano TiO2 encapsulated in zeozyme and impregnated in a polystyrene film as a photocatalyst for the degradation of diuron in an aquatic ecosystem

Article abstract of DOI:10.1039/c6ra21258a

The occurrence of chlorinated herbicide diuron in water bodies is considered serious pollution and a major health hazard to flora, fauna and mankind. In the present investigation, we studied the photocatalytic degradation of diuron in an aquatic ecosystem using lanthanum ion doped nano TiO₂ (Lnp) encapsulated in NaY zeolite pores (1 : 10) and impregnated in polystyrene film (ZLT). The hydrophobic nature of the polystyrene support resulted in an efficient and highly recoverable heterogeneous system. Catalyst characterization was carried out by FT-IR, XRD, DRS-UV, fluorescence, BET, SEM-EDAX and XPS. BET results revealed the successful loading of lanthanum ion doped TiO₂ (Lnp) inside the NaY zeolite pores via a decrease in surface area for the zeolite encapsulated Lnp (ZLnp) as compared to NaY zeolite alone. DRS UV supported the impregnation of ZLnp in the polystyrene films; the bathochromic shift (Δλ) was 4 nm and the hypochromic shift decreased in intensity 10 fold. The photocatalytic reaction was carried out at a concentration of 20 mg L^-1 of diuron, with 0.01 M H₂O₂ and a catalytic amount of 500 mg L^-1 ZLT under unstirred conditions. Degradation of diuron by ZLT reached 40% after 2 hours. Noteworthy features are the good results under optimized conditions and that the same film models were used successfully in the presence of zebra fish (Danio rerio). The present investigation also demonstrated successful re-use of the photocatalytic film six times without any appreciable loss in catalytic activity. From the abovementioned results, it was proven that ZLT is an efficient and ecofriendly catalyst.

Full text of DOI:10.1039/c6ra21258a

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Lanthanum ions doped nano TiO encapsulated in Zeozyme and
2
impregnated in a polystyrene film - As photocatalyst for
degradation of diuron in aquatic ecosystem
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
DOI: 10.1039/x0xx00000x
B. R. Saranya , V. Sathiyanarayanan and S. T. Maheswari*
www.rsc.org/
The occurrence of chlorinated herbicide, diuron in water bodies is considered as a serious pollutant and a major health
hazard to flora, fauna and mankind. In the present investigation we studied the photocatalytic degradation of diuron in
aquatic ecosystem which was achieved by Lanthanum ions doped nano TiO
1:10) and then impregnated in Polystyrene film (ZLT). The hydrophobic nature of the polystyrene support was utilized for
efficiency and highly recoverable heterogeneous system. Catalyst characterization was done by, FT-IR, XRD, DRS-UV,
Fluorescence, BET and SEM-EDAX. BET results reveal the successful loading of Lanthanum ions doped TiO (Lnp) inside NaY
2
(Lnp) and encapsulated in NaY Zeolite pores
(
2
Zeolite pores, decrease in surface area for encapsulated Lnp inside Zeolite pores (ZLnp) compared NaY Zeolite. DRS UV
supports the impregnation of ZLnp, Bathochromicshift Δ λ = 4 nm and hypochromicshift 10 fold decrease intensity. The
2 2
photocatalytic conditions were carried out at a concentration of 20 mg/L of diuron, 0.01M H O and catalytic amount of
5
00 mg/L under unstirred condition. Based on the results, ZLT showed 40 % degradation of the diuron after 2 hours. The
optimized ratio yielded good results and the same film models were used in presence of Zebra fish (Danio rerio) are the
noteworthy features. Upon observation the present investigation also revealed the reusability of the prepared
photocatalytic film successfully by six times without any appreciable loss in catalytic activity. From the above results it was
proved that ZLT is an efficient ecofriendly catalyst.
3
been reported. However, the environmental exposure to
Introduction
diuron has been scarcely investigated. Thus far, the identified
main input sources of diuron into the aquatic environment
Among the xenobiotic pollutants Diuron [3-(3, 4-
dichlorophenyl)-1, 1-dimethylurea] is one, of the major
concern of current era since this compound is adversely
affecting human and ecosystem health reported and listed in
the Contaminant Candidate List 3 of the US EPA (US
Environmental Protection Agency, 2009) which identifies
potential drinking water contaminants for regulation in the
were runoff and drainage from fields sprinkled with herbicide,
diuron via sewer systems. Especially, the production and
Emission is from infected agricultural plots, as well as their
occurrence in surface waters has hardly been investigated
systematically. It is difficult for the treatment by conventional
techniques. Nevertheless, it has been reported that diuron can
be degraded via photocatalytic reaction on oxide
1
future. It belongs to halogenophenylureas representing an
4
semiconductors (Sayeh, et al., 2007). There are many
important class of contact herbicides that have been used
worldwide for more than 40 years. Due to low solubility and
chemical stability of halogenophenylureas, they penetrate
slowly through soil and contaminate underground sources of
drinking water. Their photo reactivity has been already studied
available oxidation technologies suitable for wastewater
5
treatment such as wet air oxidation, supercritical water
6
7
oxidation, incineration and advanced oxidation processes
8
–14
15
Since first time Frank and Bard examined the
(AOPs).
2
possibilities for the decomposition of cyanide in water, with
several times. It is also commonly found in crops grown and
the aid of TiO was reported thereby increased interest in
2
stored for human or animal consumption, as well as in
processed food, hence their occurrence in food and feed has
usage of TiO in environmental applications. Photocatalysis of
2
TiO can be explained as a “catalytic reaction involving the
2
1
6
production of a catalyst by absorption of light”. The TiO2
semiconductor photocatalyst remains the outstanding among
the other catalyst due to its excellent oxidative property –
a.
Department of Analytical Chemistry, International Institute of Biotechnology and
Toxicology (IIBAT), Padappai, Chennai, Tamil Nadu - 601 301, India. E-mail:
maheswariraja@yahoo.com
1
7-19
b.
Department of Analytical Chemistry, Palamur biosciences Pvt. Ltd, Mahabubnagar Honda - Fujishima effect and physical stability.
Telangana State - 509 002, India.
The
application rendered by TiO is limited by the mechanism of
2
†
Electronic Supplementary Information (ESI) available: [details of any
Footnotes relaꢀng to the title and/or authors should appear here.
fast recombination of electron-hole pairs and their wide band
2
0
supplementary information available should be included here]. See gap, which corresponds with UV light. In the paper of
DOI: 10.1039/x0xx00000x
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Jirkovský et al., a direct photolysis of diuron was studied METHODS
1
under various conditions. In the present investigation, La3+
Preparation of nano particle TiO
2
(np)
2
has been doped onto the TiO and encapsulated into NaY
2
8
TiO powder was prepared by a sol-gel process, in which sol was
2
Zeolite, on the hypothesis that the presence of dopant can
retard the recombination between e−-h+ pair generated from
prepared by mixing titanium tetraisopropoxide (TTIP), 2-propanol,
3
and ammonia at room temperature. 10 cm (0.036) mole of
the irradiated TiO
consequently improve the catalytic activity. Metal doping
with the TiO reduces the mechanism of electron-hole
recombination and sensitization towards visible light.
2
nanoparticles encapsulated into zeolite to
3
2
2
titanium tetraisopropoxide was dissolved in 80 cm (0.82 mole) of
2
-propanol added to the round bottom flask and homogenized by
2
2
3–26
stirring for 10 minutes. To the above mixture 10 ml of ammonia
The
3
solution 1 cm /min was gradually added. Clear solution to white
co incorporation of both La and Ti into the framework of
zeolite will give the chance to occupy most of the internal and
external surface areas of zeolite with active metals thus
enhancing the catalytic activity. Zeolites are good adsorbents
and eco-friendly materials possessing high surface area and
high thermal stability. The behavior of the catalyst and its
practical applications will be influenced by the properties of
the zeolites such as particle size, surface area, pore diameter,
mechanical strength, microbial resistance, thermal stability,
chemical durability, hydrophobic/hydrophilic character, ease
precipitate was obtained. After 1 hour stirring 2-propanol was
evaporated by buchi rotavapour. At the terminal stage, the sample
was dried by convection at 105 °C for 18 h. The sample was then
calcined at 600 °C for 2 h.
Preparation of lanthanum ions doped TiO
2
nano particle (Lnp)
1
g (np) was taken in a round bottomed flask containing 20 ml of 2-
propanol was kept over the magnetic stirrer for stirring for 10
minutes. To the above mixture 200 mg of Lanthanum (III) nitrate
was added and stirred for five hours. The 2-propanol was
evaporated by buchi rotavapour. At the terminal stage, the sample
was dried by convection at 105 °C for 18 h. The sample was then
calcined at 600 °C for 2 h.
2
7
of regeneration, loading capacity, and cost. Herewith we,
report the excellent assessment of Photocatalytic degradation
2
of Diuron by lanthanum ions doped TiO encapsulated in NaY
Zeolite and impregnated in polystyrene film via realistic and
eco friendly channel on continues reproducibility and
reusability. Further, the research was extended to assess the
fish (Danio rerio) acute toxicity of catalyst.directly on here.
Preparation of lanthanum ions doped TiO
encapsulation in NaY Zeolite (ZLnp)
2
nano particle
5
00 mg of zeolite was taken in 100 ml round bottom flask
Experimental Section
containing 30 ml methanol and sonicated for 10 minutes. To the
above mixture 50 mg of lanthanum doped nano particle (Lnp) was
added, allowed it on sonication for 5 hours, methanol was
evaporated using buchi rotavapour. The residual methanol was
dried in the oven at 105 °C for 3 hours.
Materials. Titanium tetraisopropoxide (97%) – Aldrich, Ammonia
solution (25%) – Merck specialities, Mumbai, 2-propanol 99.5% –
Merck specialities, Mumbai, Lanthanum(III) nitrate (98%) – Alfa
aesar, Methanol – Hi media, Dicloromethane – Merck specialities,
Mumbai, Tetrahydrofuron
Polystyrene – Sigma Aldrich, USA.
Physical measurements. IR spectrum was recorded in Jasco FT-IR-
–
Merck specialities, Mumbai,
Preparation of ZLnp impregnated polystyrene film (ZLT)
Thin film of ZLnp impregnated in polystyrene was prepared by
adding 200 mg ZLnp and 5 % polystyrene in Tetrahydrofuron
solution. The above mixture was sonicated for 20 minutes. The
aliquot of above mixture was spread on a uniform smooth surface
and the solvent was evaporated in the oven at 50°C for 2hours. The
same procedure was employed without the addition of ZLnp for the
preparation of bare polystyrene film.
6
600.Powder XRD diffraction data were collected on a Scintag XDS
000 X-ray diffractometer using CuKR radiation. DRSUV Shimadzu
2
UV-VIS-NIR SPECTROPHOTOMETER UV-3600 PLUS. Fluorescence
spectra were measured in ethanol using Perkin Elmer LS45
Fluorescence Spectrophotometer. Nitrogen adsorption isotherms
were measured at -196 °C using an ASAP 2020 POROSIMETER. The
Scanning Electron Micrographs (SEM) of sample was recorded using
a F E I Quanta FEG 200 High Resolution Scanning Electron
Microscope. ICP-OES analyses were performed on a Perkin-Elmer
Optima 2000 DV model. X-ray photoelectron spectra of the
catalysts were recorded in an ESCA-3 Mark II spectrometer(VG
Scientific LT., England) using Al Kα(1486.6eV) radiation as the
source . Spectra referenced with Binding ENERGY OF C1S(284Ev).
The quantification of residues of diuron was done by Shimadzu
prominence High Performance Liquid Chromatograph equipped
with two pumps (model LC-20AT), oven (CTO-20A), Ultra Violet
detector (SPD-20A), and a C18 reverse phase column (25 cm length
x 4.6 mm i.d x 5 μ particle size, Phenomenex). Eluent was a mixture
of acetonitrile and water (80:20 v/v) with 1.0 mL/min flow rate,
oven temperature 40°C, detection was at 235 nm with an injection
volume of 20 μL. The peak of diuron was eluted at 4.6 minutes and
degradant (3, 4-Dichloroaniline) at 4.1 minutes.
Design of Degradation experimental procedure
The selective photocatalytic degradation activities were
investigated by employing the catalyst. A total of 4 aquariums tanks
sized (60 x 30 x 45cm, lbh) filled with 10 L of water were used for
the studies. The aquarium tanks were aerated and nine Zebra fishes
(
Danio rerio) were taken in each aquarium. The first aquarium
contained 20 ppm of diuron solution, ZLT 0.1g and 20 mM of H
added dropwise. The second aquarium contained 20 ppm of diuron
solution and 20 mM of H added dropwise. The third aquarium
was used as a control, without ZLT, diuron, and of H 2. The fourth
aquarium contained 20 ppm of diuron solution, ZLT 0.1g and 20 mM
of H added dropwise in the absence of fish. All fishes were fed
2 2
O
2
O
2
2
O
2
O
2
with Pinar Yem at a concentration of 1 % of their body mass per
day. All the four aquariums were kept under direct sunlight without
unstirred conditions. The course of the degradation was monitored
2
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at the regular intervals and the pattern of degradation was analyzed accommodate these Lnp. The diffraction pattern for the metal
using HPLC method.
could not be detected in the encapsulated Zeolite. This could
possibly be due to low loading of metal present in the zeolite.
RESULTS AND DISCUSSION
IR Spectra. Infrared spectra of the ZLnp impregnated in the
polystyrene film, bare polystyrene film, encapsulated NaY
In the X-ray studies of the samples np, Lnp and ZLT (Fig. S1 in
the ESI†) it is observed that a mixed structure of anatase(A)
and rutile(R) crystalline phases coexisted at La-doped TiO with
2
Zeolite with loading of Lnp and NaY Zeolite, are shown in Fig. 1
the weight percent of anatase phase 86–89% according to the
intensity of highest peak of the two phases. No rutile phase
(
a, b, c and d). A new absorption band at a wavelength of 996
−1
cm was due to Ti–O–Si bond. This result was in agreement
2
9
−1 appear in undoped TiO
2
. The high-angle XRD results for the
as-synthesized samples reveal the coexistence of TiO -anatase
JCPDS, No. 21-1272). The arrows headed downwards
with a reported work by Wang et al,. The peak at 996 cm
was detected in zeolite catalysts loaded with Lnp and it was
ascribed to the replacement of tetrahedral Si with Ti during
2
(
2
9
−1
represent the existence of TiO anatase position in ZLT. The
2
the ion-exchange method. The peak at 996 cm was
detected in presence Zeolite i.e., ZLT and ZLnp and it was
designated to the replacement of tetrahedral Si with Ti during
average grain size calculated from the broadending of the
(
101) peak of anatase using Scherrer’s equation was 6–19 nm.
29
the ion-exchange method . For NaY Zeolite, the peak at 996
30
−1
DRS-UV and Fluorescence. The UV-Vis diffuse reflectance
spectra for NaY Zeolite, Lnp, ZLnp and np (Fig. S2 in the ESI†).
Blue shifts towards shorter wave lengths about 370 nm were
observed for ZLnp while the np had a wave length of about
cm is attributed to Si–OH bond Figure 1(d), and from Figure
(b), it is clear that there is no such peak in bare polystyrene
1
film. Thus, this peak was an indication of successful
impregnation of ZLnp in to the polystyrene film.
3
90 nm.The result obtained was in good agreement with the
3
1
findings of Easwaramoorthi and Natarajan. The origin of such
blue shift towards shorter wave length was due to the
quantum size effect for semiconductors as the particle size
2
decreased after the modification of TiO into the pores of the
zeolite. The rapid decrease in reflectance at certain particular
wave lengths indicated the presence of optical band gap after
3
2
the ion-exchange of Ti species into zeolite. The reduction of
energy gap is facilitated by the doping of semiconductors by
the use of metals and causes the red shift in the reflection
spectra towards higher wavelength, which leads to the
excitation of electron from the valance band to the conduction
band. The doping of Lanthanum ions into the np slightly
Fig. 1 Infrared spectra of (a) ZLnp impregnated in the
polystyrene film (ZLT); (b) Bare Polystyrene film; (c)
Encapsulated NaY Zeolite with loading of Lnp (ZLnp); (d) NaY
Zeolite
XRD studies.
Fig. 3 Fluorescence spectra of ZLT and Bare polystyrene film
enhances the shift towards higher wave length in comparative
to that of np (Fig. S2 in the ESI†). The absorption spectra for
bare polystyrene film and ZLnp impregnated polystyrene thin
film (ZLT) (Fig. S3 in the ESI†). The hyprochromic shift with
tenfold decrease in intensity and additional bathochromic shift
with Δλ = 4 nm has occurred which reveals the quenching of
the polystyrene by the impregnated ZLT.
Fig. 2 Powder XRD spectra of (a) ZLnp; (b) NaY Zeolite
The X-ray powder diffraction (XRD) patterns of encapsulated
Lnp in NaY Zeolite and NaY Zeolite are presented in Fig. 2 (a &
b). XRD patterns of the zeolite host before and after
encapsulation of Lnp indicates no remarkable differences,
although a slight change in intensity of these typical lines 220,
The fluorescence emission occurs due to the recombination
311 and 331 in the encapsulated Lnp were noticed. This of electron hole pair in the semiconductor. Emission spectra of
confirms that NaY zeolite and encapsulation of Lnp have little np and the Lnp (Fig. S4 in the ESI†) reveal that emission
influence on the crystallinity and that the zeolite host can intensity of the Lnp was decreased comparatively with np. This
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is because of suppression of the suppression of shows the presence of different elements on the surface of
recombination of photo generated electron-hole pairs by catalyst such as C, Si, O, Ti, and La. Hence, it was believed that
33
lanthanum loaded on TiO2.
the existence of signals attributed to the elements in these
The fluorescence spectra of bare polystyrene film and ZLT samples indicated the success of metals loading during the
were shown in Fig. 3. Comparing the emission spectrum preparation method. Carbon is the element of Polystyrene
obtained there was a significant drop in the intensity at 611nm film, Silicon and Oxygen are the main elements of zeolite
which justifies that fluorescence quenching has occurred due frame work. The titanium and Lanthanum was successfully
to the impregnation of ZLnp. This is in accordance with the encapsulated inside the supercages of the zeolite (Fig. S5 in
results obtained from UV spectrum.
the ESI†). This conclusion was consistent with results of XRD
and UV-Vis reflectance spectra.
Surface textural studies – BET (Brunauer–Emmett–Teller). The
surface textural properties for the zeolite encapsulated Lnp XPS Analysis. The Catalyst was analysed using X-ray photoelectron
3
5
(Table S1 in the ESI†). Clearly, inclusion of Lnp in the zeolite spectroscopy (XPS),
a
surface sensitive technique
for
cavities considerably reduced the pore volume and the surface determination of elemental composition and oxidation states.
area of the zeolite host. This confirms the existence of Lnp in Fig. 6 and 7 represents the selected area scan for ZLT (C1s, Ti, La,
3
4
the zeolite cavities rather than on the external surface. The O1s, Al and Si). C1s peak was observed in the region of 284.6 eV is
decreasing values in the surface area pore volume and due to C-C neutral bond with greater intensity signifies the ZLnp
adsorption capacity depends on the amount of incorporated was loaded in the polystyrene film in Fig. 6 (a). The Ti core level
Lnp as well as their molecular size and geo-metrical spectrum in Fig. 6 (b) contains two peaks at 458.4 and 464.5 which
3
/2
1/2
conformation inside the zeolite host. Fig. 4 shows the N
2
corresponds to the Ti 2p and Ti 2p binding energy regions.
adsorption/desorption isotherms and pore size distribution for These value match up well with the previously reported data for the
4
ZLnp which are typical type I according to the IUPAC Ti ions in TiO2. Fig. 6 (c) represents the binging energy of La 3d5/2
+
36
37
classification and are characteristics of the microporous nature and La 3d3/2 are 834 .8 eV and 851.2 eV, respectively.
of the materials. This supports the observation that the Lnp
are present within the zeolite cages and not on the external
surface since the zeolite crystallinity was retained.
Fig. 6 (a) C1s peak image (b) Ti peak image (c) La peak image
Fig. 4 A linear plot of BET surface area
SEM Analysis
encapsulation of ZLnp in the bare polystyrene film shown in Al 2p 74.2 eV. An intense peak was observed at 102.5 eV is due to
In Fig. 7 (a) one intense peak was located at the 531.2 corresponds
3
8, 39
.
The SEM images of the ZLT indicated the to O1s of the Na-Yzeolite.
Fig. 7 (b) shows the binding energy of
Fig. 5. This confirmed the Lnp was preserved inside the cages
Silica 2p binding energy in Fig. 7 (c).
Fig. 5 (a) FE-SEM analysis image at 1µm of ZLT (b) FE-SEM
analysis image at 10 µm of ZLT
of the zeolite pores without the leaching of Lnp and protected Fig. 7 (a) O1s peak image (b) Al 2p peak image (c) Si 2p peak
by the polystyrene film. The absence of morphology change image
was also observed by IR spectrum. The EDAX spectrum of Lnp,
4
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Catalytic Activity. The photocatalytic activity of the newly yielded very rapid degradation of diuron and the completion of
fabricated ZLT was conducted in aquatic system for the the diuron below 5% was achieved within five hours The major
degradation of Diuron in the presence of Zebra fish. There was degradant identified was 3, 4 dichloroaniline. Further, any
a constant decrease in the diuron concentration during every abnormalities and mortality to fish was assessed and was
interval of the sample collection.
observed during and even after the completion of the
experiment and 100% of fishes were alive and active. Upon
observation of fishes even after a month of completion of
experiment the same result was noticed.
CONCLUSION
The novelty of the present investigation concludes that the
2
lanthanum ions doped TiO encapsulated in NaY Zeolite and
impregnated in polystyrene film has greater impact on the serious
pollutant such as pesticide residue, like Diuron on the nearby
running water bodies and on its aquatic food chain. The same was
also confirmed by analytical Characterization which exhibited fairly
clear evidence in the spectral (FTIR, Drs-UV, Fluorescence, XPS) and
physico-chemical studies (XRD, BET, SEM) for the well-defined
2
inclusion and distribution of lanthanum ions doped TiO in the
nanopores of the zeolite matrix. It was also found that the ZLT acted
as a heterogeneous catalyst, which is an efficient catalyst for the
Fig. 8 HPLC analysis chromatogram of Diuron
Photo degradation of Diuron under direct sunlight for aquatic
The HPLC analysis chromatogram Fig.
8
shows the system with unstirred condition. The observed DT50 value was 2.01
photocatalytic degradation pattern. The influence of catalyst hours in presence of catalyst. We also made an attempt to
was studied from the experiment without ZLT in the tank the demonstrate the fish acute toxicity of Catalyst as per the OECD
same identical condition adopted and there was a sluggish Guideline No. 203. Based on the results of our study, it could be
degradation profile observed for the sample intervals and also concluded that that ZLT is non toxic to aquatic species. Excellent
the diuron concentration exist more than 90 % even after two reproducible results were obtained when tested for the catalyst
days. Catalyst reproducibility was estimated by conducting the reusability.
experiment with presence of catalyst for six replicates.
Concordance was achieved for the reproducibility. Excellent
Acknowledgements
reproducible results were obtained when tested for the
catalyst reusability. The catalyst was reusable and easily The authors are thankful to the management of IIBAT and
recoverable in all conditions and even after more than three Director Dr. A. Ramesh for providing necessary facilities during
cycles it was reused without any regeneration (Fig. S6 in the the present investigation. The authors are also thankful to IIT–
ESI†). The catalyst selectivity was tested for the diuron degradation Chennai, SRM institute of Nanotechnology Potheri, and B.S.
for all the catalyst (Fig. S7 (a, b, c & d) and Table S3 in the ESI†). Abdur Rahman University, Chennai. We also extend our
The ZLT shows highest selectivity with regarding the DT50 value, sincere thanks to Y. Gowthami, Research Scholar, IIBAT for her
reaction completion time and low catalytic amount. The reusability valuable inputs in the preparation of the manuscript.
of ZLT is high due to excellent recovery when compared with other
catalyst. The ZLT satisfies the key elements of required to be a
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Catalyst was conducted with high amount of catalyst loaded in
the tank in presence of Zebra fishes for a week and found no
visible abnormalities such as loss of equilibrium, swimming
behavior, respiratory function, and pigmentations as per OECD
1
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3
2
4
0
Guideline No. 203.
Additionally the leaching of the
impregnated zeolite was tested by soaking the ZLT in the 0.1M
Hydrochloric acid for 2 hours at 40°C. The collected samples
analyzed in ICP-OES where showed less the 0.01 ppm of silica
and 0.0003 ppm of Titanium was observed. Therefore the
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showed a fruitful result of DT50 value of 2.01 hours (Fig. S8 and
Table S3 in the ESI†). The influence of hydrogen peroxide
showed a no significant decrease of DT50 value but in presence
of catalyst with a dropwise addition of the hydrogen peroxide
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