Journal of Photochemistry and Photobiology B: Biology 116 (2012) 66–74
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Journal of Photochemistry and Photobiology B: Biology
journal homepage: www.elsevier.com/locate/jphotobiol
Hydroxyl radical’s role in the remediation of wastewater
S.S. Shinde, C.H. Bhosale, K.Y. Rajpure ⇑
Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India
a r t i c l e
i n f o
Article history:
Received 27 June 2012
Received in revised form 18 July 2012
Accepted 7 August 2012
Available online 15 August 2012
Keywords:
Advanced oxidation processes
Wastewater
Hydroxyl radicals
ZnO
a b s t r a c t
The photocatalytic degradation of wastewater with ZnO based photocatalysts under solar illumination
has been investigated. Advanced oxidation processes such as photoelectrocatalysis, sonolysis and H2O2
treatment show promise in eliminating the dangers of exposure to wastewater and the products of their
natural breakdown. A basic understanding of the mechanistic details involved in the oxidative transformations remains the key for improving the effectiveness of the advanced oxidation processes. The role of
hydroxyl radical in the breakdown of the wastewater is elucidated through determining the degradation
rates, analyzing transformation intermediates and studies using computational chemistry methods. In
order to realize a complete mineralization of wastewater COD, BOD and TOC analysis has been carried
out.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Salvage of wastewater sewage is recognized to be a strategic approach in a sustainable water management portfolio in order to
minimize the growing water demand in a water-scarce environment [1,2]. The toxicity and persistence of pollutants can directly
impact the health of ecosystems and present a threat to humans
through contamination of drinking water supplies [3]. In response
it has become a challenge to achieve the effective removal of persistent organic pollutants from waste water effluent to minimize
the risk of pollution problems. Consequently, considerable efforts
have been devoted to developing a suitable purification method
that can easily destroy these bio-recalcitrant organic contaminants. Due to their incomplete removal during wastewater treatment, they are ubiquitous in secondary wastewater effluents,
rivers and lakes at low concentration. Despite their low concentration, these contaminants are a major health concern because of
their extremely high endocrine disrupting potency and genotoxicity [4]. These findings enunciate the necessity for further research
on the removal of trace contaminants to minimize their accumulation, particularly prior to indirect or direct reuse of reclaimed
water.
Conventional wastewater purification systems are generating
wastes during the treatment of contaminated water, which requires additional steps and cost. Heterogeneous photocatalysis
(using ZnO and ZnO based photocatalyst) is a promising new alternative method among advanced oxidation processes (AOPs) which
generally includes UV/H2O2, UV/O3 or UV/Fenton’s reagent for oxi⇑ Corresponding author. Tel.: +91 231 2609435; fax: +91 231 2691533.
E-mail address: [email protected] (K.Y. Rajpure).
1011-1344/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotobiol.2012.08.003
dative removal of organic pollutants and inactivation of microorganisms in water [5,6]. Heterogeneous semiconductors in the
field of photocatalysis were investigated deeply because of their
high efficiency, commercial availability and high chemical stability. When the semiconductor particles are illuminated with UVlight, an electron promotes from the valence band to the conduction band due to photo-excitation, thus leaving an electron deficiency or hole in the valence band; in this way, electron/hole
pairs are generated. These electron hole pairs can either recombine
or can interact separately with other molecules. Both reductive and
oxidative processes can occur at/or near the surface of the photoexcited semiconductor particle [7]. In aerated aqueous suspensions, oxygen adsorbed on the surface of the catalyst acts as an
electron trap on the conduction band and electron/hole recombination can be effectively prevented and lifetime of holes is prolonged. In this process, destruction of recalcitrant organics is
governed by the combined actions of a semiconductor photocatalyst, an energetic radiation source and an oxidizing agent. Moreover, the process can be driven by solar UV or visible light. Near
the earth’s surface, the sun produces 0.2–0.3 mol photons m2 h1
in the range of 300–400 nm with a typical UV flux of 20–30 W m2.
This suggests using sunlight as an economically and ecologically
sensible light source [8].
ZnO as a semiconductor oxide has been found to be very efficient
photocatalyst due to its abundant availability, cost-effectiveness
and chemical stability in both acidic and basic medium. In this paper, we reported the photoelectrocatalytic detoxification of sugarcane factory wastewater under electrical bias using ZnO and Ga,
N doped ZnO electrodes. Also, the effect of H2O2 and sonolysis treatment into photocatalysis of wastewater has been studied. Analysis
of water sample is carried out for their chemical oxygen demand,
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S.S. Shinde et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 66–74
biological oxygen demand and total organic carbon tests in order to
analyze extent of the degree of complete mineralization.
18
16
14
2. Experimental
3. Results and discussion
3.1. Under ZnO photocatalyst
A current–voltage curve of a 64 cm2 ZnO electrode in WWS
using a steel counter electrode at a distance of 1 mm under UVA
illumination is shown in Fig. 1. The current reached to its saturation at about 15.74 mA at a bias voltage of 1.5 V. It achieves the
current plateau around 1.5 V in the dilute solution for large electrode, due to electrolyte resistance (iR drop). The iR drop, however,
is kept small by using a very small distance between the two electrodes. To validate the versatility of the technique, the performance
of ZnO electrode is investigated to see the photoelectrocatalytic
degradation of wastewater. The initial experiments showed that
WWS does not undergo any degradation under direct solar light
illumination in the absence of catalyst. No degradation of WWS
is also noticed over ZnO electrode in the absence of solar light,
12
i (mA)
Firstly, fluorine doped tin oxide (FTO) conducting substrates
were prepared onto the corning glass substrates. Initial ingredients
used to deposit FTO thin films are as follows: stannic chloride
pentahydrate (SnCl45H2O), ammonium fluoride (NH4F) supplied
by HIMEDIA, Pvt. Ltd., Mumbai, oxalic acid ((COOH)22H2O), propane-2-ol (Iso-propyl alcohol) (CH3CHOHCH3) supplied by s d
FINE-CHEM, Ltd., Mumbai. A total of 100 ml of 2 M stannic chloride
(35.7 g) solution was prepared in doubled distilled water and
14.285 g of ammonium fluoride was dissolved in it, to obtain the
20 wt.% doping concentration of fluorine. A few drops of 0.1 M oxalic acid were added to it to clear off some of the precipitated traces
and finally it gives stock solution. From this stock solution, 10 ml
solution was taken and 10 ml of propane-2-ol was added in it to
have a total of 20 ml spraying solution. The resulting precursor
solution was sprayed onto the corning glass substrates of size
10 10 0.15 cm3 through the specially designed glass nozzle
held at optimized substrate temperature 475 °C. The compressed
air (2.3 kg cm2) was used as carrier gas at a constant spray rate
of 5 cc min1 keeping nozzle-to-substrate distance is of 32 cm.
We achieved large area uniform FTO substrates exhibited sheet
resistance of 5–15 X1 and transparency of 90–95%.
The substrates of size (10 10 0.15 cm3) were cleaned using
chromic acid and methanol treatment and again rinsed with double
distilled water. Prior to using FTO as substrates for ZnO deposition,
they were first etched in HCl and finally cleaned with acetone. Pure,
Ga, N-doped zinc oxide thin films were prepared onto the corning
glass substrates by using chemical spray pyrolysis technique in
aqueous medium. To deposit Ga (2 at.%), N (10 at.%) doped ZnO thin
films, zinc acetate (Zn(CH3COO)22H2O, AR grade, 98.8% pure) supplied by HIMEDIA, Gallium nitrate (Ga(NO3)3H2O, (AR grade,
99.9% pure) supplied by ALFA ASER and N,N-dimethylformamide
(HCON(CH3)2, AR grade, 99% pure) supplied by THOMAS BAKER
were used as initial ingredients. To attain Ga, N doping, gallium nitrate and N,N-dimethylformamide (HCON(CH3)2) was mixed into
the solution. The physic-chemical properties of Ga and N doped
ZnO thin films have been explained elsewhere [9–11]. Analytical reagent grade potassium dichromate (K2Cr2O7), sulphuric acid
(H2SO4) and silver sulphate all obtained from Loba Chemie were
used with required concentrations without further purification.
Single cell PEC reactor consists ZnO, Ga:ZnO and N:ZnO electrodes
coated on a conducting glass substrate was used to record and test
the deposited photoelectrodes indoor [12].
10
8
6
4
2
0
0.0
0.5
1.0
1.5
2.0
E vs steel (mV)
Fig. 1. The i-E curve for a ZnO electrode (64 cm2) under UVA illumination in NaOH
against applied voltage w.r.t. steel counter with a flow rate of 8.4 l h1.
although there is slight decrease in respective extinction peaks
which is likely due to adsorption loss of solution through circulation pipes and photoelectrochemical cells. The very low levels of
contaminant would results low solution conductivity leading to
high series resistance and in turn to a high iR drop to be compensated by application of bias voltage. Experiments are carried out in
batch mode using recirculation of the liquid with a total duration
of up to 5.5 h under solar light illumination. The photocatalytic
degradation follows a pseudo first order reaction and its kinetics
can be expressed using relations [13].
Fig. 2 describes the photoelectrocatalytic detoxification of
wastewater sugarcane factory (WWS) using ZnO photoanode (active area of 64 cm2) under sunlight illumination. Fig. 2a shows
the variation of photocurrent as a function of time during detoxification of WWS. Although photocurrent decays over the course of
time, an average of 0.0154 A photocurrent is drawn from detoxification of WWS using ZnO. The photocurrent decreases in later
stages is due to the natural decrease in sunlight intensity in the
afternoon as well as oxidation of organic species. Fig. 2b shows
the change in extinction spectra of WWS collected at various intervals during its photocatalytic detoxification recorded in the wavelength range from 200 to 500 nm. During the course of the
degradation experiments, the concentration of WWS decreases
due to its decomposition (photochemical oxidation). It is further
used to plot variations in ln (c/c0) as a function of reaction time
as shown in Fig. 2c that shows kinetics of degradation (extinction
taken at 276 nm). The linear portion in this plot has a slope of rate
constant (k) reveals the apparent first order reaction kinetics.
It is generally accepted that, when semiconductor photocatalyst
are irradiated by light with energy higher than or equal to the band
gap, an electron (e) in the valence band (VB) can be excited to the
conduction band (CB) with the simultaneous generation of a hole
(h+) in the VB. The photoelectron can be easily trapped by electronic acceptors like adsorbed O2, to further produce a superoxide
radical anion (O
2 ), whereas the photo-induced holes can be easily
trapped by electronic donors, such as organic pollutants, to further
oxidize organic pollutants [14,15]. However, if the photo-generated electrons recombined with the photo-induced holes, the photocatalytic activity would be decreased. In general, oxygen
vacancies in ZnO catalyst can act as the active centers to capture
photo-induced electrons and the recombination of photo-induced
electrons and holes can be effectively inhibited [14], so that the
photocatalytic activity can be greatly improved. The values of the
parameters k0 , k00 and p (k000 ) are found to be 2.1 102 cm3 s1,
3.28 104 cm s1 and 131.6 M1, respectively. Moreover, k is
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S.S. Shinde et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 66–74
20
2.5
(a)
18
16
(b)
2.0
Extinction
iPh (mA)
14
12
10
8
1.5
0 min
20 min
40 min
80 min
160 min
240 min
320 min
1.0
6
4
0.5
2
0
0
50
100
150
200
250
0.0
200
300
250
300
Time (min)
350
400
450
500
Time (min)
0.0
(c)
-0.2
-0.4
ln (c/c 0 )
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
0
50
100
150
200
250
300
Time (min)
Fig. 2. Detoxification of WWS with ZnO under solar light illumination (a) plot of photocurrent as a function of reaction time, (b) extinction spectra with reaction time and (c)
kinetics of detoxification (extinction taken at 276 nm).
Table 1
Various kinetic parameters and conditions for degradation of WWS with ZnO based photocatalyst with different advanced oxidation processes under solar illumination.
Advanced oxidation process
k, 104 (s1)
k0 , 102 (cm3 s1)
k00 , 104 (cm/s)
iph (A)
k000 (M1)
p = 1/k000 , 102 (M)
Efficiency (%)
Sonolysis
Photocatalysis
Photocatalysis
Photocatalysis
Photocatalysis
Photocatalysis
Photocatalysis
0.27
0.84
0.94
1.60
1.78
2.00
2.33
0.68
2.10
2.35
4.00
4.46
5.00
5.82
1.05
3.28
3.67
6.25
6.96
7.81
9.10
–
0.0154
0.0170
0.0207
0.0204
0.0208
0.0207
–
131.6
133.4
186.5
210.9
232
271.6
–
0.760
0.750
0.536
0.474
0.431
0.368
18.7
80.6
84.7
89.0
90.5
92.2
94.5
(with ZnO)
(with GZO)
(with NZO)
and Sonolysis (with NZO)
and H2O2 (with NZO)
and H2O2 and sonolysis (with NZO)
proportional to the area of the electrode, if a sufficiently well collimated light source is used and to its intensity and therefore to
the photocurrent. The unlikely higher values of k00 and k000 could
be explained by an improvement of the photoanode surface by
prolonged illumination and interfacial electron transfer. In this
experiment, it is possible to degrade WWS with a ZnO photocatalyst about 80.6% in 320 min under sunlight illumination. The various kinetic parameters associated with WWS detoxification by ZnO
photocatalyst under solar illumination in typical experiment are
listed in Table 1. Rate constants k00 reflects the properties of light
source and the efficiency of photocatalyst/solute interaction and
is useful for comparing results obtained with different electrode
sizes (surface areas). In the case of sunlight, they can be compared
directly with values obtained with other reactor geometries and
experimental conditions carried out with this light source. The k000
finally reflects the interaction of the photocatalyst (surface
properties) and the solute alone, independent of light source.
Photocatalytic activity of ZnO is attributed both to the donor states
caused by the large number of defect sites such as oxygen vacancies and interstitial zinc atom and to the acceptor states which
arise from zinc vacancies and interstitial oxygen atoms [16].
3.2. Using Ga-doped ZnO (GZO) photocatalyst
Recently, much effort has been devoted to study ZnO as a very
promising semiconductor for photocatalytic degradation of water
pollutants. But, it has numerous shortcomings such as recombination of photogenerated electron–hole pair, low quantum yield, surface morphology of the films, which obstruct commercialization of
the photocatalytic process [17]. Consequently, it has enormous
attention in improving the photocatalytic activity by appropriate
modification of semiconductors for the degradation of wastewater.
Therefore, various methods have been developed to reduce the
electron–hole recombination rate of ZnO in the photocatalytic
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20
20
15
i (mA)
i (mA)
15
10
5
10
5
0
0
0.0
0.5
1.0
1.5
0.0
2.0
0.5
1.0
1.5
2.0
E vs steel (mV)
E vs steel (mV)
Fig. 3. Dark and light current for a GZO electrode (64 cm2).
Fig. 5. Dark and light current for a NZO electrode (64 cm2).
processes. One approach is to fabricate them with various nanostructures such as nanoparticles, nanosheets, nanotubes, nanoplates. with high specific surface areas [18,19]. Other interesting
approach is to dope the transition metals (Ga, Al, In), non-metals
(N, F, C), alkaline and rare earth metals in order to reduce the band
gap energy and improve charge separation between photogenerated electrons and holes [20]. However, insufficient light harvesting
especially in the visible-light region and inefficient energy conversion [21,22], still remain two great challenges to us. So development of novel and efficient photocatalysts would be a major
advance in photochemistry and a critical breakthrough with respect to the rising concern of global energy and environmental issues. Very recently, Ga-doped ZnO nanocrystalline thin films have
attracted much attention in the photocatalytic processes owing to
its high photocatalytic activity in the degradation of organic contaminants because of the large content of oxygen vacancies and
strong absorption of OH ions on the surface of the catalyst [23,24].
The testing of GZO photocatalyst has been carried out by measuring i-E curve as shown in Fig. 3 with saturation current of about
17.25 mA at a bias voltage of 1.5 V. Fig. 4 illustrates the photoelectrocatalytic degradation of WWS using GZO photocatalyst under
sunlight illumination. Fig. 4a shows the improvement of photocurrent as a function of time during degradation of WWS. Although
photocurrent decays over the course of time, an average of
0.0170 A photocurrent has drawn from degradation of WWS using
GZO. The decrease of the photocurrent in later stages of the experiment was due to the natural decrease in sunlight intensity in the
afternoon as well as degradation of organic species. The extinction
spectra of WWS decrease with illumination time. Variation in ln (c/
c0) as a function of illumination time is as shown in Fig. 4(b) that
shows degradation kinetics (extinction taken at 276 nm). The rate
constants k0 , k00 and k000 are found to be 2.35 102 cm3 s1,
3.67 104 cm s1 and 133.4 M1, respectively. With the help of
GZO photocatalyst, it is possible to degrade SWW up to 84.7% in
320 min under sunlight illumination.
It is clearly seen from Table 1 that the rate of photocatalytic
degradation of Ga-doped ZnO catalysts is higher as compared to
pure ZnO. The rate constant of the catalyst increases due to Ga doping. The reason for the high photocatalytic activity of Ga-doped
ZnO can be explained as follows: Pleskov [25] reported that the value of space charge region potential for efficient separation of electron–hole pairs should not be lower than 0.2 V. The increase in
Ga3+ ion concentration the surface barrier becomes higher, the
space charge region becomes narrower and hence the electron–
hole pairs are efficiently separated by the large electric field. It
was also reported that the recombination of electron–hole pair in
pure ZnO is very fast [26] which inhibits the formation of hydroxyl
radicals required for the degradation of the pollutant. The Ga doping in ZnO helps to reduce the potential barrier and significantly
separates the electron–hole pairs due to the large electric field.
20
0.0
(a)
18
16
(b)
-0.5
12
ln (c/c0 )
iPh (mA)
14
10
8
6
-1.0
-1.5
4
-2.0
2
0
0
50
100
150
200
Time (min)
250
300
0
50
100
150
200
250
300
Time (min)
Fig. 4. Detoxification of WWS with GZO under solar light illumination (a) plot of photocurrent as a function of reaction time, (b) kinetics of detoxification.
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0.0
(a)
20
(b)
-0.5
ln (c/c0 )
iPh (mA)
15
10
5
0
-1.0
-1.5
-2.0
0
50
100
150
200
250
300
-2.5
0
50
Time (min)
100
150
200
250
300
Time (Min)
Fig. 6. Detoxification of WWS with NZO under solar light illumination (a) plot of photocurrent as a function of reaction time, (b) kinetics of detoxification.
Moreover, the penetration depth of light into ZnO is greatly enhanced with Ga doping and even exceeds the space charge layer.
Therefore, the recombination of photogenerated electron–hole
pairs becomes difficult in ZnO for the Ga-doped catalyst. It should
be noted that the electrical field of the ZnO particles are very small
while the band gap energy of Ga2O3 in particular are not sufficient
for photocatalytic reactions. Thus, the combination of both ZnO
and gallium oxide is imperative to match the thickness of charge
layer and the depth of the light penetration for separating photoinduces electron–hole pairs. However, an optimum doping of Ga
(2 at%) is highly required for enhancing the potential difference between the surface and the center of the particles so as to efficiently
separate the photo-induced electron–hole pairs [26] because the
excess Ga2O3 covering the most of the surface of ZnO may inhibit
the direct exposure of UV light on the surface of ZnO which limits
the excitation of electron from valence band to conduction band
and increase the recombination of photogenerated electron–hole
pairs. The adsorption of the WWS on the surface of the catalyst also
plays an important role on their photocatalytic activity. The
adsorption capacity of GZO is higher than pure ZnO because of
the smaller particle size which provides high surface area and enough active sites to adsorb the WWS on the surface of the catalyst.
3.3. Using N doped ZnO photocatalyst
Nature provides a multiplicity of materials, architectures, systems and functions with many inspiring properties such as sophistication, miniaturization, hierarchical organizations, adaptability
and environment-response. Mimicking the elaborate architectures
and basic principles to design and make more reliable and efficient
materials or systems is highly appealing. One of the most promising materials that have been synthesized is N-doped ZnO (NZO).
Since the pioneering work nitrogen-doped ZnO has received a lot
of attention because the implantation of nitrogen modifies the
electronic structure by introducing localized states to the top of
the valence band, narrowing the band gap. This reduction of the
band gap makes possible the photocatalytic activity in a number
of reactions under visible light.
The general behaviour of photocatalyst performance can be accounted for by considering three factors: (1) the nature of surface
coating and surface active area left available for catalysis; (2) the
surface-to-volume ratio; (3) the density of surface –OH groups, related to nature of the reaction involved in the synthesis of the
material. The presence of –OH groups is directly related to the local
production of hydroxyl radicals. These functionalities also provide
sites for adsorption of the substrates in addition to unsaturated
surface metal atoms. The testing of NZO photocatalyst has been
carried out by measuring i-E curve (Fig. 5). The current reached
to its saturation at about 20.73 mA at a bias voltage of 1.5 V.
Fig. 6 depicts the WWS degradation on NZO under sunlight illumination (a) plot of photocurrent as a function of degradation time,
(b) kinetics of degradation (extinction taken at 276 nm). For the
degradation of WWS using NZO catalyst, similar observations are
made and similar conclusions have been drawn. Fig. 6a shows
the variation photocurrent as a function of time during degradation of WWS. A high photocurrent of 0.0207 A was drawn during
degradation experiment. The degradation of WWS under NZO catalyst also agreed remarkably with the first-order kinetics. The
applicability of the first-order kinetics to this study has been confirmed by the linearity of the plot of ln (c/c0) against irradiation
time for various experiments (Fig. 6b). The analysis of kinetic
parameters due to surface trapping and recombination has been
shown in Table 1. The photocatalytic reaction is totally inhibited
due to strong binding of the anion to the active sites thereby preventing the adsorption of WWS. Previous literature has shown that
this in situ generated radical can sufficiently act as strong oxidizing
agent or initiate the formation of hydroxyl radical [27,28]. Using
NZO photocatalyst we have degraded the WWS up to 89% in
320 min.
A high activity of photocatalyst should satisfy two requirements, namely, large surface area for absorbing substrates and high
crystallinity to reduce photoexcited electron–hole recombination
rate. We ascribe the improvement mechanism to a defect energy
state, newly formed by N-doping between the valence (VB) and
the conduction (CB) bands in the ZnO lattice. The electrons, generated in the VB from the light irradiation, can first transfer to the defect energy state, and then further transfer to the CB by absorbing
less energy than that of the first step transition. This means that
the electron transition from VB to CB in ZnO semiconductor, generally produced by UV irradiation, can be fulfilled even with the lower energy of visible irradiation since a defect energy state is
formed. In our case, oxygen vacancies should have existed when
oxygen atoms are substituted with nitrogen atoms since nitrogen
and oxygen have different valance states.
3.4. Sonolysis
When high-frequency ultrasound waves are introduced into an
aqueous solution, bubbles rapidly form and develop through rarefaction/compression cycles. This ultrasound-induced implosion
process is known as cavitation. Very high temperatures and pressures accompany the implosion of the cavitation bubbles, forming
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0.00
ln (c/c0 )
-0.05
-0.10
-0.15
-0.20
-0.25
0
50
100
150
200
250
300
Time (min)
Fig. 7. Kinetics of degradation of WWS under sonolysis process.
microscopic areas of extremely high energy. High-frequency sonolysis can induce the degradation of wastewater by two main avenues. Upon collapse of the cavitation bubble, vaporized volatile
compounds are destroyed via pyrolytic or combustive reactions
because of the extreme temperature and pressure conditions.
Small molecular weight hydrocarbons and other volatile compounds form intermediates and products that mirror pyrolysis or
combustion reaction products [29]. The second type of reaction
pathway consists of the chemical processes at the interface of
the bubble, induced by hydrogen atoms and hydroxyl radicals
formed from the homolysis of water, promoted by the implosion
conditions.
To treat wastewater, many types of techniques are being actively studied: adsorption treatments with activated carbon, biological treatments and advanced oxidation processes (AOPs) such
as ozonation treatment, Fenton reactions, and photolysis radiolysis. [30,31]. Strong ultrasonic wave irradiation in water brings
about the formation and collapse of small gas bubbles. During
the collapse, local reaction site of several thousand degrees and
several hundred atmospheres are produced due to the quasi-adiabatic collapse [31,32], while the bulk liquid temperature hardly
changes. This process is known as cavitation which accompanies
the generation of a shockwave, the emission of light, etc. Compounds in the cavitating bubbles undergo thermal reactions. The
chemical changes that are entailed by these reactions are known
as sonochemistry. In general, sonochemical degradation proceeds
via a reaction with OH radicals, which are formed from water pyrolysis in the collapsing hot bubbles and/or at the interface region of
the hot bubbles. For the volatile or hydrophobic pollutants, the
degradation proceeds not only via the OH radical reaction, but also
via a direct pyrolysis reaction in the collapsing hot bubbles and at
the interface of the bubbles.
Fig. 7 shows the kinetics of degradation of WWS under sonolysis treatment. The degradation of WWS under sonochemical reactions agreed remarkably first-order kinetics. It is confirmed from
linearity of the plot of ln (c/c0) against irradiation time for various
experiments. The results are due to the accumulation of the solute
in the bubbles and/or at the gas–liquid interface of the bubbles,
indicating that the concentration of solutes becomes inhomogeneous in solution [33]. It was considered that most of the solutes
are relatively hydrophobic compared with water, so they tend to
accumulate at the gas–liquid interface of the bubbles and decrease
the surface free energy of the bubbles. Sonolysis treatment degrades the WWS up to 18.7% after 320 min i.e. it is very slow process as compare to other process. Influence of H2O2 in sonolysis
process enhances the degradation efficiency of WWS up to 25%
while only H2O2 treatment can degrade up to 21.3%.
3.5. Photoelectrocatalysis coupled with sonolysis using NZO
photocatalyst
Fig. 8 describes the results on photoelectrocatalysis coupled
with sonolysis of WWS using NZO photocatalyst under solar illumination. Fig. 8a shows the variation of photocurrent as a function
of reaction time during detoxification of WWS. Although photocurrent decays over the course of time, an average of 0.0204 A photocurrent is drawn during purification of WWS using NZO catalyst.
During the course of the degradation experiments, the concentration of WWS decreases due to its decomposition (photochemical
and sonochemical oxidation). The degradation kinetics (Fig. 8b)
of WWS shows initial values of kinetic parameters to be
4.46 102 cm3 s1, 6.96 104 cm s1 and 210.9 M1 (Table 1),
respectively. With the application of both processes with NZO photocatalyst, it is possible to degrade WWS up to 90.5% in 320 min
under sunlight illumination.
Combining the two methods had a pronounced effect on the mineralization. The extent with which we achieved mineralization in
this simultaneous combination method is more than an additive effect. The treatment using photocatalysis, sonolysis, or the sequential
0.0
(a)
20
(b)
-0.5
ln (c/c0)
iPh (mA)
15
10
-1.0
-1.5
-2.0
5
-2.5
0
0
50
100
150
200
Time (min)
250
300
0
50
100
150
200
250
300
Time (min)
Fig. 8. Photoelectrocatalysis coupled with sonolysis of WWS with NZO catalyst under solar illumination (a) plot of photocurrent as a function of reaction time, (b) kinetics of
degradation.
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0.0
(a)
20
(b)
-0.5
-1.0
ln (c/c0 )
iPh (mA)
15
10
-1.5
-2.0
5
-2.5
0
-3.0
0
50
100
150
200
250
300
Time (min)
0
50
100
150
200
250
300
Time (min)
Fig. 9. Photoelectrocatalysis and H2O2 treatment of WWS with NZO catalyst under solar illumination (a) plot of photocurrent as a function of reaction time, (b) kinetics of
degradation.
combination of the two, each resulted in less than 81%, 21% and 90%
mineralization of the WWS. These results further indicate that intermediates formed during oxidation of WWS are quickly mineralized
in the simultaneous sonolysis and photocatalysis experiments and
thus validates its usefulness for environmental remediation. During
the initial stages of WWS degradation in sonolysis experiment, we
observe a small increase in the UV absorption. With increasing time,
the transient absorbing builds up, while it is quite efficiently removed in the combined sonolysis and photocatalysis experiment.
These results explain why sonolysis alone cannot be effective in
achieving complete mineralization of WWS in a short time. Among
the two combinations that we attempted, the simultaneous sonolysis and photocatalysis experiment clearly stands out to be better
than sequential combination. There have been suggestions that
the sonication prevents the catalyst from aggregation. While this
argument is certainly valid in a general sense, it alone cannot explain
the difference in two combinative techniques. The added advantage
of sonication on photocatalysis stems during simultaneous sonolysis + photocatalysis experiment that the photocatalyst surface to
be constantly refreshed. Also, the mass transport of reactants and
products to and from the catalyst surface improves as the slurry is
constantly agitated.
3.6. Photoelectrocatalysis coupled with H2O2 treatment using NZO
photocatalyst
The effect of H2O2 treatment onto the NZO photocatalytic reaction is also carried out under solar light irradiation, which focused
mainly on attempting to enhance the efficiency in the removal of
organic pollutants by adding (30% M/v, 5 cc) H2O2 in solution for
reducing recombination of the conduction band electron and the
valance band hole. Hydroxyl radicals can be formed through various chemical reaction pathways, such as: (i) irradiation of H2O2;
(ii) photolysis of ozone, either through the generation of singlet
oxygen atoms which then react with water to generate OH; (iii)
photolysis of Fe3+ or [34]; (iv) Fenton type reaction of Fe+2; (v) radiolysis of water [35]. In the case of natural water, present protonic
forms of nitrate and nitrous ions are sources of hydroxyl radicals
[36]. The organic matter dissolved in aquatic environment, especially humic acids, absorbed a large portion of photons, and formation of hydroxyl radicals can also occur. Hydroxyl radicals can react
with organic substances by: electron transfer, H abstraction, or
OH addition to bond [37].
It is found that H2O2 concentration influences the rate of
photo-oxidation of WWS with NZO. In the presence of solar
radiation, hydrogen peroxide photo-dissociates to form OH free
radicals, which attack the organic compounds and undergo very
rapid and effective substitution reactions to form oxygenated
intermediates. The rate of photocatalytic degradation of organic
compounds is significantly improved either in the presence of oxygen or by addition of hydrogen peroxide. The rate of photocatalytic
degradation of WWS first increased when hydrogen peroxide concentration increased and reached to a maximum (5 cc) but above
an optimum value increasing H2O2 concentration retards the
reaction. This dual effect of H2O2 can be explained by radical
reaction mechanisms. The added H2O2 could accelerate the reaction by producing hydroxyl radicals from scavenging the electrons
and absorption of light. By addition of excess H2O2, it acts as hydroxyl radical or hole scavenger to form the per-hydroxyl radicals
ðHO2 Þ which is a much weaker oxidant than hydroxyl radicals
[38]. Therefore, high concentration of hydrogen peroxide inhibited
the reaction rate of WWS degradation for available hydroxyl
radicals.
Fig. 9 describes the photoelectrocatalysis and H2O2 treatment of
WWS degradation using NZO photocatalyst under solar illumination. Fig. 9a shows the variation of photocurrent as a function of
time during detoxification of WWS. Although photocurrent decays
over the course of time, an average of 0.0208 A photocurrent is
drawn during degradation of WWS with NZO catalyst. During the
course of the degradation experiments, the concentration of
SWW decreases with reaction time due to generation of more hydroxyl radicals. The degradation kinetics (extinction at 276 nm) of
WWS is analyzed from plot of ln (c/c0) as a function of reaction time
(Fig. 9b). The initial values of the rate constants are found to be
5.0 102 cm3 s1, 7.81 104 cm s1 and 232 M1 (Table 1),
respectively. With the application of H2O2 with photoelectrocatalysis under NZO catalyst, it is possible to degrade WWS up to 92.2%
in 320 min under sunlight illumination.
3.7. Photocatalysis coupled with sonolysis and H2O2 treatment under
NZO photocatalyst
Finally, in order to enhance the degradation efficiency of WWS,
we carried three advanced oxidation processes simultaneously i.e.
photoelectrocatlysis, sonolysis and H2O2 irradiation. Fig. 10 describes the degradation of WWS with three AOPs (photoelectrocatalysis, sonolysis and H2O2 treatment) concurrently with NZO
photocatalyst under solar illumination. The variation of photocurrent as a function of reaction time during degradation of WWS is
shown in Fig. 10a. Although photocurrent decays over the course
73
S.S. Shinde et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 66–74
0.0
(a)
20
-1.0
ln (c/c0)
15
iPh (mA)
(b)
-0.5
10
-1.5
-2.0
-2.5
5
-3.0
0
0
50
100
150
200
250
300
Time (min)
0
50
100
150
200
250
300
Time (min)
Fig. 10. Photoelectrocatalysis coupled with sonolysis and H2O2 treatment of WWS with NZO catalyst under solar illumination (a) plot of photocurrent as a function of
reaction time, (b) kinetics of degradation.
(a)
(b)
Fig. 11. Extent of mineralization of WWS under ZnO based photocatalyst.
of time, an average of 0.0207 A photocurrent is drawn during
degradation of WWS with three AOPs under NZO catalyst. During
the course of the degradation experiments, the concentration of
WWS decreases with reaction time due to photochemical, sonochemical and thermochemical oxidation. The degradation kinetics
is studied from plot (Fig. 10b) of ln (c/c0) as a function of reaction
time. The values of the kinetic parameters are found to be
5.82 102 cm3 s1, 9.10 104 cm s1 and 271.6 M1 (Table 1),
respectively. With relevance of three advanced oxidation processes
(photoelectrocatalysis, sonolysis and H2O2 treatment) under NZO
catalyst, it is possible to degrade WWS up to 94.5% in 320 min
under solar illumination.
74
S.S. Shinde et al. / Journal of Photochemistry and Photobiology B: Biology 116 (2012) 66–74
3.8. Extent of mineralization
Apart from extinction study, the extent of mineralization of
SWW, is analyzed by measuring COD, TOC and BOD values of the
solution as a function of time for different advanced oxidation processes. Fig. 11a–c compares the information obtained from chemical oxygen demand (COD), total organic carbon (TOC) and
biological oxygen demand measurements. COD study as a function
of illumination time give the concentration of oxidizable matter
left in the solution. One can clearly see from the figure that
SWW degrades as illumination time increases. It could be concluded that the suppression of electron–hole recombination and
generation of more OH radicals in samples play an important role
in the enhanced rate of photo-mineralization. The COD, TOC and
BOD decreases from 4566 to 260, 164 to 9.35 and 80 to 4.5 mg/L
respectively. The observed decay constants are indicating the
destruction of main elements of WWS. Measurements of TOC at
the beginning and at the end of degradation experiments show
that the decay rate of the extinction at 276 nm, ext276, and the
TOC are directly correlated, with d(ext276)/dt d(TOC)/dt. In excess
oxygen, photocatalytic reduction is less frequently encountered
than the oxidation, presumably because the reducing power of
photogenerated electrons is significantly lower than the oxidizing
power of photoholes and also most reducible substrates do not
compete kinetically with oxygen in the trapping of conduction
band electrons [39]. It is seen that coupling of photocatalysis, sonolysis and H2O2 treatment shows almost 94.5% of removal of
impurities.
4. Conclusions
The photocatalysis of WWS with different surface trapping defects under solar illumination have been investigated. With simultaneous exploitation of photoelectrocatalysis, sonolysis and H2O2
treatment together with NZO photocatalyst highest degradation
efficiency of 94.5% has been achieved under solar illumination.
Acknowledgement
The authors are very much thankful to the Defense Research
and Development Organization (DRDO), New Delhi, for the financial support through its Project No. ‘‘ERIP/ER/0503504/M/01/
1007’’.
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