Synthesis of Mn-doped TiO2 by novel route and photocatalytic mineralization/intermediate studies of organic pollutants

Article abstract of DOI:10.1007/s11164-019-03771-x

The doping of TiO₂ particles with various manganese (Mn) concentrations (0.25–1.0%) were synthesized using simple sol–gel and modified sol–gel technique. The characteristics of synthesized particles were found employing standard analytical techniques such as X-ray diffraction, scanning electron microscopy and UV–Vis spectroscopy. The photocatalytic activity of the synthesized particles was compared by investigating the mineralization of two selected organic pollutants like ketoprofen and chlorothalonil. The photocatalysts which were produced by improved sol–gel technique show the lower value of band gap energy and small size crystallite and, hence, exhibit better photocatalytic activity. The outcomes also designate that the concentration of dopant Mn 0.75% indicated the highest photocatalytic activity than other concentrations of dopant in the mineralization of both the compounds. The mineralization kinetics of both compounds was studied under various situations like reaction pH and catalyst dosage. The mineralization rates were highly affected by all the above parameters. An effort has also been performed to highlight the intermediates produced during the photooxidation of both the compounds using the GC–MS analysis method. Both compounds show the production of several intermediates. A possible pathway for the production of different products has been suggested.

Full text of DOI:10.1007/s11164-019-03771-x

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03771-x
Synthesis of Mn‑doped TiO2 by novel route
and photocatalytic mineralization/intermediate studies
of organic pollutants
Khalid Umar1 · Mohamad Nasir Mohamad Ibrahim1 · Akil Ahmad2 ·
Mohd Rafatullah2
Received: 27 November 2018 / Accepted: 5 February 2019
© Springer Nature B.V. 2019
Abstract
The doping of TiO2 particles with various manganese (Mn) concentrations (0.25–
1.0%) were synthesized using simple sol–gel and modifed sol–gel technique. The
characteristics of synthesized particles were found employing standard analytical
techniques such as X-ray difraction, scanning electron microscopy and UV–Vis
spectroscopy. The photocatalytic activity of the synthesized particles was compared
by investigating the mineralization of two selected organic pollutants like ketoprofen
and chlorothalonil. The photocatalysts which were produced by improved sol–gel
technique show the lower value of band gap energy and small size crystallite and,
hence, exhibit better photocatalytic activity. The outcomes also designate that the
concentration of dopant Mn 0.75% indicated the highest photocatalytic activity than
other concentrations of dopant in the mineralization of both the compounds. The
mineralization kinetics of both compounds was studied under various situations like
reaction pH and catalyst dosage. The mineralization rates were highly afected by all
the above parameters. An efort has also been performed to highlight the intermedi-
ates produced during the photooxidation of both the compounds using the GC–MS
analysis method. Both compounds show the production of several intermediates. A
possible pathway for the production of diferent products has been suggested.
Keywords Ketoprofen · Chlorothalonil · Doped-titanium dioxide · Semiconductor ·
Intermediate products
*
*
Khalid Umar
khalidumar@usm.my
Mohamad Nasir Mohamad Ibrahim
mnm@usm.my
1
2
School of Chemical Sciences, Universiti Sains Malaysia, 11800 Gelugor, Pulau Pinang,
Malaysia
School of Industrial Technology, Universiti Sains Malaysia, 11800 Gelugor, Pulau Pinang,
Malaysia
Vol.:(0123456789)
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K. Umar et al.
Introduction
The formulation of new pharmaceutical compounds and pesticides plays a vital
role and revolutionizes the life of human beings day by day. The pharmaceuti-
cal compounds are ultimately responsible for the treatment of various danger-
ous diseases. On the other hand, the use of pesticides protects the crops from
pests and fnally increases the productivity. Nowadays, global consumption of
these compounds in human and veterinary medicine are now at very high levels
which creates a lot of problems for the environment [1]. A drug derivative such
as ketoprofen, (non-steroidal anti-infammatory drug) is extensively employed to
prevent rheumatoid arthritis, these molecules are highly soluble in the groundwa-
ter but insoluble and non-degraded in the subsoil, biotic and abiotic due to polar
structure. Therefore, lengthy contact of these chemicals is probable and infu-
ences health [2–6]. Moreover, chlorothalonil is a non-systemic foliar fungicide
employed in many agricultural crops, particularly fruits and vegetables [7, 8]. In
addition, it has been used as an alternative to TBT additive agent in antifouling
paints for the last few years [9]. To prevent the growth of algae, bacteria and
other invertebrates, this type of paint is applied to the hulls of ships [10].
Key sources of pharmaceutical compounds polluting ground and surface water
are the waste of manufacturing pharma-industries, human excretion [11] while
for pesticides, urban areas runof, agricultural felds return fows and leaching are
accountable for water pollution [12]. Even a very small amount of these pollut-
ants is very harmful to our living systems. For example, Chlorpyrifos (concen-
tration, 1–10 mg L−1) is enough to slow down the development in blue green
algae Anabaena sp. and decrease ciliated protozoa in a naturally based microbial
community [13, 14]. Therefore, degradation of these compounds has acquired
increasing attention. In recent years, doping of metal ions with semiconductor
has been made with the aim that they could do work in the visible region and also
recognised as changing both photochemical reactivity and photophysical behav-
iour [15–17]. Although the sol–gel technique is an eye-catching technique and a
simple method for synthesis of TiO2 at low temperature, it is difcult to fnd uni-
form particles employing this technique. Due to some advantages, hydrothermal
technique proved to be a potential technique such that (1) Crystalline product can
be achieved directly (2) It also favours a decline in particles agglomeration. (3)
Due to a recrystallization process, the purity of the obtained product is very high
[15]. In literature, synthesis of nanocrystalline TiO2 with doped metal ions/metal
oxide using the sol–gel method has been proposed [18–20].
As far as the mechanism of photocatalysis is concerned, this involves the exci-
tation of electrons from the valance band to the conduction band. As a result,
the formation of a hole in the valance band and availability of electrons in the
conduction band takes place. After that, if this charge separation continues, these
electrons and holes may travel to the surface of catalyst where they react with
water and oxygen to form radical ions [21–23]. In the case of doping metal ions
with photocatalyts, it establishes a new energy level which reduces the band gap
1 3

Synthesis of Mn-doped TiO2 by novel route and photocatalytic…
and inhibits electron–hole recombination. The doping also improves the absorp-
tion of visible light [16, 24].
The current paper investigates the production of Mn-doped TiO2 with various
methods followed by characterization employing an analytical standard. The com-
parison of photocatalytic activity for both photocatalysts synthesized by two dif-
ferent methods was examined by investigating the two organic pollutants mineral-
ization (drug and pesticides) like ketoprofen and chlorothalonil under a source of
visible light. A study on the photomineralization of ketoprofen and chlorothalonil
sensitized by Mn-doped TiO2 in aqueous solution was carried out by considering
diferent parameters of reaction, like pH reaction and catalyst loading. To obtain the
optimal condition of mineralization, the photocatalytic oxidation method is neces-
sary for diferent feld applications. An efort has also been made to recognize the
intermediate products produced during the photooxidation practise of both the com-
pounds using the GC–MS analysis method.
Experimental
Materials and methods
Ketoprofen, chlorothalonil and titanium isopropoxide, were procured from Sigma-
Aldrich. Mangnese (II) sulphate monohydrate and 2-propanol were obtained from
Merck, Himedia Laboratories Pvt. Ltd. India, Central Drug House, India. Double
distilled water was used in all experiments. TiO2 particles were synthesized by
the simple sol–gel method reported in our earlier paper [19] while to synthesize
TiO2 particles with modifed sol–gel technique, the following procedure was used;
a homogeneous solution I (10 mL of 2-propanol and 10 mL of water) was mixed
slowly with solution II (titanium isopropoxide and 2-propanol) under constant stir-
ring and maintaining temperature at 90 °C. After sometime, the turbidity in the
solution occurred and later the formation of precipitation took place. The precipi-
tation state may be due to hydrolysis of titanium isopropoxide. A solution III was
prepared which consisted of known concentrations (0.25–1.0%) of manganese sul-
phate monohydrate, 2-propanol and water. Doping of TiO2 with Mn was performed
by addition of solution III slowly with solution II. For hydrothermal treatment, the
gel obtained was shifted into a 100-mL, autoclave Tefon-lined vessel at 350 °C for
5 h. Then the suspension was centrifuged, washed with absolute ethanol and dried at
room temperature.
Catalysts characterization
The techniques like standard analytical XRD, SEM and UV–Vis spectroscopy were
used to characterize the synthesized catalysts. XRD was carried out in the 2θ range
of 20–80° (Rigaku Minifex II) with Cu Kα radiations (λ=1.5418 Å) at 30 kV and
15 mA to discover the structural characteristics of TiO2 and doped TiO2 particles. A
Shimadzu UV–Vis spectrophotometer (Model 1601) was used to fnd the band gap
1 3

K. Umar et al.
energy of synthesized particles in the range of 300–800 nm at room temperature.
The surface morphology of synthesized material was recorded using SEM (LEO,
435 VF) at 1 K–100 K magnifcation, WD 15 mm at 20 kV voltage.
Irradiation procedure
Solutions of desired concentrations of each organic pollutant (ketoprofen and chlo-
rothalonil) were synthesized in double distilled water and irradiated in the existence
of Mn-doped TiO2 (2 g L−1) with regular supply of atmospheric oxygen and stirring.
Pyrex glass was used to make an immersion cell photochemical reactor equipped
with a water circulating jacket, magnetic stirring bar, and an opening for air supply.
Aqueous solution of 250 mL of preferred organic pollutants was transferred into the
reactor, and the desired quantity of photocatalyst was added. For system equilibra-
tion, the solution was stirred for 15 min in the dark. The blank solution was kept in
the dark to obtain a zero time reading. A visible light halogen lamp (500 W, 9500
Lumens) was used to irradiate the solution. Samples were collected at regular time
intervals during the experiment and examined after centrifugation.
For the intermediate products characterization, an aqueous solution of the organic
pollutants ketoprofen (0.30 mM, 250 mL) was placed in a photochemical reactor.
After that, Mn-doped TiO2 (0.75%, 3 g L−1) was added to the reactor. The solution
was irradiated through visible light at diferent time intervals with continuous bub-
bling and stirring of atmospheric O2. The photocatalyst was removed by fltration,
and the fltrate was transferred into a separating funnel. After adding the chloroform
to the separating funnel, two layers were formed, i.e., one organic layer and a second
aqueous layer. The vigorous shaking of the separating funnel may be responsible
for the displacement of intermediates compounds from aqueous to an organic layer,
which was consequently dried over anhydrous sodium sulphate and investigated by
GC/MS. The same procedure was used for another organic pollutant, i.e., chlorotha-
lonil (0.20 mM, 250 mL).
Analysis
The mineralization of ketoprofen and chlorothalonil was checked by determin-
ing the total organic carbon (TOC) content using a Shimadzu TOC-VCH analyser.
For GC–MS investigation, a Perkin Elmer (GC-MASS CLARUS -680) which was
equipped with a capillary column PE-5MS (length 30 m, i.d. (250 µm), operating
temperature 50 °C, which was elevated to 250 °C at 10 °C min−1), in a split less
approach of injection volume (1.0 µL) with carrier gas as helium, was employed.
1 3

Synthesis of Mn-doped TiO2 by novel route and photocatalytic…
Results and discussion
Characterization of catalysts
XRD patterns of prepared doped and undoped TiO2 with diferent concentrations
of Mn calcinated at 350 °C for 5 h were measured. Figure 1 indicates the XRD
arrangement of Mn-doped TiO2 calcined at 350 °C as a representative example. The
undoped and doped-TiO2 XRD patterns do not show any noteworthy change. It is
pertinent to mention here that XRD patterns do not show any peak corresponding to
dopant, possibly the sample encloses the low concentration of dopant as compared
to TiO2. Moreover, ionic radii of Mn2+ (0.80 Å) is higher than Ti4+ (0.68 Å). There-
fore, these dopants rarely to act as interstitial contamination, so they probably exist
on the surface of TiO2 [25–27]. For the calculation of the average crystallite size of
synthesized particles (pure and doped TiO2), the Debye Scherer formula was used
which is expressed by the following Eq. 1:
Kꢀ
D =
.
(1)
ꢁ cos ꢂ
This is in which D=size of crystallite, K=factor of shape, β=full width at half
maximum, λ=wavelength, and θ=difraction angle. The crystallites average size of
pure and doped samples, synthesized by both the methods are shown in Table 1.
The size of crystallite was establish to be reduced with the rise in concentration
of dopant. This decrease in size may be due to possible production of some interac-
tion of dopants with titanium, which is responsible for the inhibition of the size of
crystal grains [28]. It is important to mention here that the particles yielded by the
TiO2(H)
Mn-TiO2(H) (0.25)
Mn-TiO2(H) (0.50)
Mn-TiO2(H) (0.75)
Mn-TiO2(H) (1.00)
20
30
40
50
60
70
2 Theta
Fig. 1 XRD patterns of undoped and Mn-doped TiO2 (0.25%, 0.50%, 0.75% and 1.0%). (H=post hydro-
thermally modifed sol–gel method)
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K. Umar et al.
Table 1 Crystallite size of undoped and Mn-doped TiO2 with diferent concentrations of Mn synthesized
by two diferent routes
S. no.
Dopant concent-
ration (%)
Dopant/crystallite size (nm) (sim- Dopant/crystallite size
ple sol–gel method)
(nm) (modifed sol–gel
method)
1
2
3
4
5
0
None/15.9
Mn/15.4
Mn/14.5
Mn/13.8
Mn/13.2
None/13.6
Mn/11.7
Mn/9.9
Mn/8.1
Mn/7.7
0.25
0.50
0.75
1.00
Calcination temp: 350 °C calcination time: 5 h
modifed sol–gel method show the smaller average crystallite size than the simple
one.
UV–Vis light excitation creates photogenerated holes and electrons in the struc-
ture of band (or levels of molecular energy) of the material. So, the UV–Vis absorp-
tion spectra of doped TiO2 and undoped TiO2 synthesized by two diferent routes
with diferent dopant concentrations of Mn were investigated and as a symbolic
model, UV–Vis absorption spectra of Mn-doped TiO2 are revealed in Fig. 2. The
wavelength attained by pure and doped TiO2 particles from UV–Vis spectra has
been used to calculate the band gap energies using the following (Eq. 2) and are
enumerated in Table 2.
2.0
TiO2(H)
Mn-TiO2 (H)(0.25)
Mn-TiO2(H) (0.50)
Mn-TiO2(H) (0.75)
1.5
Mn-TiO2(H) (1.00)
1.0
0.5
0.0
A
B
C
D
E
320
340
360
380
400
420
440
460
480
Wavelength (nm)
Fig. 2 Absorption spectra of undoped and Mn-doped TiO2. (a) Undoped TiO2, (b) 0.25% Mn-doped
TiO2, (c) 0.50% Mn-doped TiO2, (d) 0.75% Mn-doped TiO2 and (e) 1.00% Mn- doped TiO2, (H=post
hydrothermally modifed sol–gel method)
1 3

Synthesis of Mn-doped TiO2 by novel route and photocatalytic…
Table 2 Band gap energy of undoped and Mn-doped TiO2 with diferent concentrations of Mn synthe-
sized by two diferent routes
S. no.
Dopant concentration
(%)
Dopant/band gap (eV) (simple
sol–gel method)
Dopant/band gap (eV)
(modifed sol–gel
method)
1
2
3
4
5
0.00
0.25
0.50
0.75
1.00
None/3.16
Mn/3.07
Mn/2.97
Mn/2.79
Mn/2.88
None/3.16
Mn/2.99
Mn/2.89
Mn/2.67
Mn/2.79
Calcination temp: 350 °C calcination time: 5 h
Band energy(eV) = 1240∕wavelength(nm)
(2)
The UV–Vis absorption spectra of Mn-doped TiO2 display a red shift in the
band-gap alteration. This phenomenon may be understood so that the doping of met-
als introduces the new levels of energy in the TiO2 particle, which reduces the band
gap energy [27].
The morphological properties of doped and undoped TiO2 were determined
through SEM. Figure 3 shows the structural properties of undoped and Mn-doped
TiO2 (0.75%) synthesized by the modifed sol–gel method while Fig. 4 shows images
of undoped and Mn-doped TiO2 (0.75%) produced by the simple sol–gel method.
The SEM images of doped TiO2 particles produced by the modifed sol–gel tech-
nique show a complex and porous nature with a high irregular surface that would
be the outcome in a high area of the surface. On the other hand, doped TiO2 synthe-
sized by the simple sol–gel technique presents an amorphous nature [24].
Fig. 3 SEM images of a undoped TiO2, b Mn-doped TiO2 (0.75%) calcination temp: 350 °C, modifed
sol–gel method, calcination time: 5 h
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K. Umar et al.
Fig. 4 SEM images of a undoped TiO2, b Mn-doped TiO2 (0.75%) calcination temp: 350 °C, simple sol–
gel method, calcination time: 5 h
Photocatalytic activity
Photocatalytic action of Mn‑doped‑TiO2 for the mineralization of organic pollutants
The Mn-doped TiO2 based photocatalytic activity is synthesized by two difer-
ent methods with various concentrations of metal ions ranging from 0.25 to 1.0%
that was verifed by determination of the mineralization of two various organic
derivatives like ketoprofen (drug) and chlorothalonil (pesticide) under a source
of visible light in aqueous suspension with the existence of O2. Figures 5 and 6
illustrate the mineralization rate of both contaminants in the presence of doped
TiO2 and undoped (pure TiO2). Figures show the mineralization rate of pollut-
ants rises with escalation in dopant concentration from 0.20 to 0.75% and more
growth in the dopant concentration leads to reduction in the mineralization rate.
The rise in the photocatalytic action by growing the concentration of dopant
could be ascribed to the fact that the TiO2 doping with dopant can introduce
fresh sites of trapping. These innovative produced sites of trapping also afect
the recombination process of the photo-generated moieties which increases the
photo-oxidation profciency [16]. The shortening of band gap may be another
cause for the rise in the photocatalytic action by increasing the concentration of
dopant thereby efciently absorbing the longer wavelength light. Moreover, at
upper concentrations of dopant there is incidence of many charge carriers trap-
ping and hereafter the chance of electron–hole recombination rises [29, 30]
which decreases the mineralization rate. It is also appropriate to discuss here that
the TiO2 particles which were prepared by the modifed method show the higher
activity as compared to the simple sol–gel method. The photocatalytic perfor-
mance of particles depends on several factors like the morphology, distribution of
particle size, electronic properties, surface area, conficting surfaces hydroxyla-
tion, quantity of adsorbed reactant species, etc. [31].
In this case, the better activity of the particles which were prepared by the modi-
fed method can be elucidated on the basis of the fact that they have lower band
1 3

Synthesis of Mn-doped TiO2 by novel route and photocatalytic…
0.0008
Ketoprofen/doped TiO2 (ssg)
Ketoprofen/doped TiO2 (msg)
0.0006
0.0004
0.0002
0.0000
1.00
0.0
0.75
0.25
0.50
Mn-Dopant concentration (%)
Fig. 5 Photocatalytic activity of undoped and Mn-doped TiO2 [synthesized by the simple sol–gel method
(ssg) and the modifed sol–gel method (msg)] for the mineralization of ketoprofen and chlorothalonil.
Experimental condition: reaction vessel: immersion well photochemical reactor made of Pyrex glass,
ketoprofen (0.12 mM), volume (250 mL), dopant: Mn, dopant concentration (0, 0.25, 0.50, 0.75 and
1.0%), light source: halogen linear lamp (500 W, 9500 Lumens) continuous stirring and air purging, irra-
diation time: 300 min
0.0008
Chlorothalonil/doped TiO2 (ssg)
Chlorothalonil/doped TiO2 (msg)
0.0006
0.0004
0.0002
0.0000
1.00
0.75
0.25
0.50
0.0
Mn-Dopant concentration (%)
Fig. 6 Photocatalytic activity of undoped and Mn-doped TiO2 [synthesized by simple sol–gel method
(ssg) and modifed sol–gel method (msg)] for the mineralization of ketoprofen and chlorothalonil. Exper-
imental condition: reaction vessel: immersion well photochemical reactor made of Pyrex glass, Chloro-
thalonil (0.15 mM), volume (250 mL), Dopant: Mn, Dopant concentration (0, 0.25, 0.50, 0.75 and 1.0%),
light source: halogen linear lamp (500 W, 9500 Lumens) continuous stirring and air purging, irradiation
time: 300 min
1 3

K. Umar et al.
gap energy. Secondly, also they have small crystallite size which is accountable to
increase surface area and the better activity observed.
In all following experiments, Mn-doped TiO2 (0.75%, msg) was employed as the
photocatalyst, meanwhile this material showed the maximum whole action for the
mineralization of the organic pollutants.
Photolysis of aqueous suspension of organic pollutants in the presence of Mn‑doped
TiO2
Aqueous suspension irradiation of desired compound (ketoprofen or chlorothalonil)
in the presence of Mn-doped TiO2 (0.75%) with continuous bubbling of atmospheric
oxygen lead to reduction in content of TOC as a time function. Figures 7 and 8
depict the reduction in content of TOC as a time function on irradiation of aque-
ous suspensions of ketoprofen or chlorothalonil in the nonexistence and existence of
doped TiO2. Controlled experiments were performed by irradiating a solution of the
compounds in the absence of photocatalyst where no visible depletion in the TOC
content takes place as shown in the fgure whereas in the presence of Mn-doped
TiO2 a mineralization of 83% and 72% take place in ketoprofen and chlorothalonil,
respectively, in 300 min.
For each irradiation process, the rate constant for the mineralization of ketoprofen
or chlorothalonil was measured from the natural logarithm plot of the TOC content
as a function of time irradiation.
Fig. 7 Change in concentration as a function of time on irradiation of an aqueous solution of ketopro-
fen in the absence and presence of Mn-doped TiO2 (0.75%), (simple sol–gel method, modifed sol–gel
method) respectively. Experimental conditions: reaction vessel: immersion well photochemical reactor
made of Pyrex glass, ketoprofen (0.12 mM), volume (250 mL), light source: halogen linear lamp (500 W,
9500 Lumens) continuous stirring and air purging, irradiation time: 300 min
1 3

Synthesis of Mn-doped TiO2 by novel route and photocatalytic…
Fig. 8 Change in concentration as a function of time on irradiation of an aqueous solution of chlorotha-
lonil in the absence and presence of Mn-doped TiO2 (0.75%), (simple sol–gel method, modifed sol–gel
method) respectively. Experimental conditions: reaction vessel: immersion well photochemical reac-
tor made of Pyrex glass, chlorothalonil (0.15 mM) volume (250 mL), light source: halogen linear lamp
(500 W, 9500 Lumens) continuous stirring and air purging, irradiation time: 300 min
As a representative example, the mineralization curve (ln C vs time) of ketopro-
fen in the existence of doped TiO2 shown in Fig. 9 which can be plotted ration-
ally well by an exponential curve decay signifying pseudo frst order kinetics. The
5
Ketoprofen
Y = -0.004x - 3.651 R² = 0.989
4
3
2
1
0
0
100
200
300
400
Irradiation Time (min)
Fig. 9 Plot showing the linear regression curve ft for the natural logarithm of concentration of ketopro-
fen as a function of time on irradiation of an aqueous suspension of the pollutant in the presence of Mn-
doped TiO2 (0.75%). (modifed sol–gel method)
1 3

K. Umar et al.
mineralization rate for the compounds was measured in term of mol L−1 min using
formulas such as the following;
−d[TOC]∕dt = kC1
(3)
TOC=Total Organic Carbon, C=pollutant concentration, k=constant rate, n=1.
Efect of initial pH
The pH of an aqueous solution of organic pollutants shows a signifcant role, as it
efects the charge on the catalyst surface, the aggregation particles number and the
position of conductance and valance band. To see the pH efect on the mineraliza-
tion kinetics, the mineralization of both the pollutants was investigated at various pH
values employing Mn-doped TiO2 (0.75%) as photocatalyst. The mineralization rate
for organic pollutants ketoprofen and chlorothalonil was investigated in the range
of pH from 4 to 13. Figure 10 shows the mineralization rate for the organic pollut-
ants ketoprofen and chlorothalonil as a function of pH. The solution initial pH was
adjusted by adding the required amount of dilute HNO3 or NaOH solution before
irradiation.
The understanding of pH infuence on the process of photocatalytic is very dif-
fcult because of its numerous roles like electrostatic interactions among solvent
molecules, semiconductor surface, charged radicals and substrate produced dur-
ing the reaction process. The surface ionization state of the photocatalyst can be
0.0007
Ketoprofen
Chlorothalonil
0.0006
0.0005
0.0004
0.0003
0.0002
0.0001
2
4
6
8
10
12
14
pH
Fig. 10 Infuence of pH on the rate for the mineralization of ketoprofen, chlorothalonil in the presence of
Mn-doped TiO2 (0.75%). Experimental condition: reaction vessel: immersion well photochemical reactor
made of Pyrex glass, ketoprofen (0.12 mM), chlorothalonil (0.15 mM), volume (250 mL), reaction pH
(4.7, 6.4, 8.7 10.8 and 12.9), light source: halogen linear lamp (500 W, 9500 Lumens) continuous stirring
and air purging, irradiation time: 300 min
1 3

Synthesis of Mn-doped TiO2 by novel route and photocatalytic…
0.0008
Ketoprofen
Chlorothalonil
0.0006
0.0004
0.0002
0.0000
0
1
2
3
4
5
6
Catalyst Concentration (gL-1)
Fig. 11 Infuence of catalyst concentration on the rate for the mineralization of ketoprofen, chlorothalonil
in the presence of Mn-doped TiO2 (0.75%). Experimental conditions: reaction vessel: immersion well
photochemical reactor made of Pyrex glass, ketoprofen (0.12 mM), chlorothalonil (0.15 mM), volume
(250 mL), photocatalyst: Mn-doped TiO2 (0.5, 1.0, 2.0 3.0, 4.0 and 5.0 g L−1), light source: halogen lin-
ear lamp (500 W, 9500 Lumens) continuous stirring and air purging, irradiation time: 300 min
deprotonated and protonated under alkaline and acidic conditions, respectively, as
indicated in the given equations:
TiOH + H+ → TiOH+2 ,
(4)
TiOH + −OH → TiO− + H2O.
(5)
In this study it has been shown that the rate of mineralization for both the pollut-
ants was better under acidic pH. The zero point of charge (pHzpc) of Degussa P25
has been stated as 6.25 [32]. Hence, the surface of the particle is positively charged
as pH moves to an acidic range whereas it is negatively charged at pH values above
pHzpc. For ketoprofen because of low pKa value (5.0) [33], this compound will be
present in an anionic state within the studied pH range. The negative charges of
TiO2 would repel with pH rising, resulting in a reduction in the photomineralization
competence. The better mineralization rate of chlorothalonil in acidic pH may be
ascribed to the fact that the morphological alignment of the molecule is favored for
reactive species attack under this condition.
Infuence of catalyst dosage
In order to prevent extra amounts of catalyst, the maximum catalyst concentra-
tion has to be identifed, which is important for any feld application. The efect of
catalyst concentration on the mineralization kinetics of organic pollutants ketopro-
fen and chlorothalonil was studied using various concentrations of Mn-doped TiO2
1 3

K. Umar et al.
Fig. 12 Gas chromatogram of ketoprofen [(a) unirradiated mixture (0 h)], [(b) irradiated mixture (9 h)].
Experimental conditions: ketoprofen (0.12 mM), V=250 mL, photocatalyst: Mn-doped TiO2 (0.75%)
(3 g L−1
)
ranging from 0.5 to 5 g L−1. The result on the rate of mineralization of ketoprofen
and chlorothalonil as a role of catalyst concentration is revealed in Fig. 11. The rate
of both organic pollutants (ketoprofen and chlorothalonil) with Mn-doped TiO2 was
1 3

Synthesis of Mn-doped TiO2 by novel route and photocatalytic…
O
O
O
O
Doped-TiO2 / Visible light
H2O/ O2 / +e
OH
OH
2
1
+ OH
O
OH
O
OH
O
O
+H
OH
OH
4
3
5
Fig. 13 Proposed photocatalytic degradation pathway of ketoprofen in presence of Mn-doped TiO2
(0.75%)
to indicate increase with rise in loading catalyst from 0.5 to 4 g L−1 for ketoprofen
and from 0.5 to 3 g L−1 of chlorothalonil. Moreover, increase in loading catalyst
moves to a slight reduction in the mineralization rate. The reason for the decrease
in mineralization rate at higher catalyst dosage may be due to the fact that, at higher
catalyst loading the interaction between particle-to-particle increases which is ulti-
mately responsible for a agglomeration. This agglomeration reduces the surface
area available for absorption of light and, hence, a drop in the mineralization rate
occurred [34, 35].
Intermediate products
The unirradiated and irradiated samples of both the compounds under investigation
were measured employing the GC–MS analysis method with a purpose to recognize
the transitional yields generated during the process of photooxidation.
Transitional products of ketoprofen
An aqueous solution of ketoprofen (0.2 mM) with Mn-doped TiO2 (3 g L−1) was
irradiated through a linear lamp of visible light halogen (500 W, 9500 Lumens) at
diferent time intervals with continuous bubbling and stirring of atmospheric O2.
The GC–MS analysis of an unirradiated solution of Ketoprofen shows a single peak
seeming at the time of retention (Rt) 14.62 min (Fig. 12a) which was categorized by
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K. Umar et al.
Fig. 14 Gas chromatogram of chlorothalonil [(a) unirradiated mixture (0 h)], (b) irradiated mixture (9 h).
Experimental conditions: chlorothalonil (0.15) mM, V=250 mL, photocatalyst: Mn-doped TiO2 (0.75%)
(3 g L−1
)
paralleling the pattern of mass fragmentation and molecular ions. The analysis of a
9 h irradiated mixture of ketoprofen indicates the generation of several transitional
products seeming at times of retention (Rt) 4.07, 5.28, 7.27, 10.75, 13.20 and 17.25
besides unafected initial material Ketoprofen (1) appearing at Rt =14.62 as revealed
in Fig. 12b. Two products appearing at Rt =5.28 and 10.75 min are characterized as
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Synthesis of Mn-doped TiO2 by novel route and photocatalytic…
Fig. 15 Proposed photocatalytic degradation pathway of chlorothalonil in presence of Mn-doped TiO2
(0.75%)
benzoic acid (3) and 2-phenylpropanoic acid (4) on the basis of a pattern of mass
fragmentation and the molecular ions. The generation of products 3 and 4 in the
photocatalytic ketoprofen degradation could be agreed to be as pathways revealed in
Fig. 13. The formation of these products from 1 involves the attack of electrons and
followed by hydroxyl radicals which may in turn abstract hydrogen atom from the
solvent to form the observed products 3 and 4.
Intermediate products of chlorothalonil
Aqueous solution irradiation of chlorothalonil (0.30 mM) was carried out with Mn-
doped TiO2 (3 g L−1) with a linear lamp of a visible light halogen (500 W, 9500
Lumens) at diferent time intervals with continuous stirring and bubbling with
atmospheric O2. Figure 14a, b shows the GC examination of unirradiated (0 h) and
irradiated (9 h) mixture of chlorothalonil, respectively. Analysis of 9 h irradiated
mixture of chlorothalonil indicates the generation of many transitional yields, of
which two products seeming at retention time 14.15 and 17.01 has been character-
ized as 9 and 8 on the basis of the pattern of mass fragmentation and molecular ions,
indicating the replacement of a cyanide group with a hydroxyl group in the benzene
ring as revealed in Fig. 15.
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K. Umar et al.
Conclusion
The outcomes of these studies evidently designate that Mn-doped TiO2 was suc-
cessfully synthesized by two diferent routes, characterized and also catalyzed
the photomineralization of organic pollutants ketoprofen and chlorothalonil. The
modifed sol–gel method proved to more efcient. Due to the lower value of band
gap energy and small crystallite size, the photocatalysts, which were synthesized
by modifed sol–gel technique, exhibit better activity photocatalysis. The photo-
catalyst Mn-doped TiO2 with 0.75% was found to be more efcient as compared
to other dopant concentrations using visible light.
These investigations clearly shows that the selection of optimum degrad-
ing parameters is important to get a high mineralization rate, which is vital for
any application of photocatalytic oxidation processes at the feld level. The dif-
ferent transitional yields generated during the photo- mineralization processes
of organic pollutants (ketoprofen and chlorothalonil) were refected in valuable
information for the pathways of degradation in heterogeneous photocatalysed
reactions to understand the mechanism of reaction.
Acknowledgements The authors gratefully acknowledge the post doctoral fnancial support (USM/
PPSK/FPD(BW) 1/06 (2018) and a research grant (304/PKIMIA/6316174) by School of Chemical Sci-
ence, Universiti Sains Malaysia.
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