Journal of Water Process Engineering 44 (2021) 102325

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Journal of Water Process Engineering

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Ozonation of diclofenac in a laboratory scale bubble column: Intermediates,

mechanism, and mass transfer study
Surabhi Patel *, Subrata Kumar Majumder *, Pallab Ghosh
Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India

A R T I C L E I N F O A B S T R A C T

Keywords: Diclofenac (DCF), a non-steroidal analgesic drug, has been recurrently detected in surface and groundwater
Diclofenac during the past two decades. Traces of DCF pass through the conventional wastewater treatment facilities
Hydroxyl radical without any alteration due to its resistance and high stability. This study aims the complete degradation and
Kinetics
partial mineralization of DCF by an ozonation process. The findings of the study suggest that increasing pH of the
Mass transfer
Oxidation intermediates
system and ozone supply rate, and generation of hydroxyl radicals in situ enhance the degradation process. In
Ozone contrast, the increasing initial concentration of DCF slows down the process. The pseudo-first-order reaction rate
Pharmaceutical waste treatment constants were in the range of 0.0742 – –0.0979 min− 1. A model has been developed to predict the behavior of
the rate constant with various operating variables. High volumetric mass transfer coefficients (i.e., 1.150–2.717
× 103 s− 1) was observed during the mass transfer study of ozone in water. A probable mechanism for ozonation is
suggested along with the structures of major intermediates formed during the oxidation. Distinct orange pig­
ments appeared during the reaction, which confirmed the production of DCF-2, 5-iminoquinone. Di-chloro an­
iline, 5-hydroxy DCF. The carboxylic acids were mainly detected as the metabolites. The treatment cost was
estimated to be USD 0.80 for treating 1 m3 of the solution containing 50 mg dm− 3 DCF. In addition, the effect of
other water matrices on DCF degradation was also investigated and reported.

1. Introduction Although its concentration detected in the water bodies has been found
to be much lower than the effective concentration limit, it was proven
Reports on elevating concentrations of pharmaceuticals in ground previously that DCF can exhibit acute toxicity to the organisms due to
and surface water have been raising concern from the last decade [1–4]. the cocktail effect in the presence of other pharmaceuticals [17]. In
Pharmaceuticals are specially designed to be therapeutically active until addition, there are evidences that the persistent exposure of DCF can
the execution of a physiological action on the mammals. Traces of these affect the health of fish, including the development of tumor and
active pharmaceuticals (in the range of in ng dm− 3 to μg dm− 3) have distortion of gills even at the lowest detected concentration (i.e., 5 μg
been found in various water bodies in recent studies. Presence of these dm− 3) [18]. The removal efficiencies of DCF in conventional biode­
active organic compounds has become a major concern owing to their gradable were found to be very low (20–40%) in WWTPs due to poor
toxicity towards aquatic and human life [5–7]. Excretion and improper biodegradability and low sorption efficiency [19]. Frequent detection of
disposal by the manufacturers are the main contributors. Most of the DCF in aquatic environment and its acute toxicity towards the organisms
pharmaceuticals are found in the aquatic environment in its original or necessitates an economic and efficient removal technique. Various
slightly modified form due to ineffective treatment in the wastewater removal processes based on physical or chemical methods, including
treatment plants (WWTPs) [8,9]. Diclofenac (DCF) is one of the exten­ membrane filtration [20], adsorption [21–24], coagulation [25], ion
sively used analgesic, antiarthritic, and anti-inflammatory non-steroidal exchange [26], activated sludge [27,28], and photocatalytic oxidation
drugs (NSAID). Although it is proven the fact that DCF can be removed [29,30] have been extensively studied for the degradation of DCF in
by natural photolysis [10,11], yet it is one of the most frequently wastewater. DCF was pointed out by Water Frame Work Directive in a
detected pharmaceuticals in water bodies such as groundwater [11,12] list of 33 active chemicals which can be a possible threat to the aquatic
and surface water [13,14], at concentrations up to 1.2 μg dm− 3 [15,16]. environment in the next ten years [31].

* Corresponding authors.
E-mail addresses: p.surabhi@iitg.ac.in (S. Patel), skmaju@iitg.ac.in (S.K. Majumder).

https://doi.org/10.1016/j.jwpe.2021.102325
Received 23 February 2021; Received in revised form 23 July 2021; Accepted 12 September 2021
Available online 22 September 2021
2214-7144/© 2021 Elsevier Ltd. All rights reserved.
S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

Fig. 1. Schematic of the experimental setup used for the ozonation of DCF.

Conventional wastewater treatment facilities are designed to elimi­ sodium (98% assay, MP biomedicals, France), hydrogen peroxide (30%
nate BOD (biological oxygen demand), pathogens, and suspended solids assay, Merck, India), HPLC grade acetonitrile and methanol (99.8%
mainly. In recent years, due to the conventional wastewater treatment assay, Merck, India), HPLC grade glacial ammonium acetate (99.6%
methods' inefficiency for removal of active organic compounds, alter­ assay, Merck, India), sodium thiosulfate pentahydrate (99.9% assay,
native and more efficient techniques [such as advanced oxidation pro­ Merck, India), diethyl-p-phenylenediamine-4 tablets (Water analyst
cesses (AOPs)] have been studied, especially for pharmaceutical technology, UK), hydrochloric acid (35% assay, Merck, India), and so­
degradation. Photo-Fenton, O3, O3/H2O2, catalytic ozonation, and UV/ dium hydroxide (>98% assay, Rankem, India). For the measurement of
H2O2 are some of the proven tools for removing stubborn organic chloride ion concentration, the reagent was purchased from Palintest
compounds from wastewater with a good mineralization efficiency. (UK).
Ozone, a strong oxidant can vitiate the refractory material into the
biodegradable compounds, which can be further eliminated in WWTPs 2.2. Plant prototype for ozonation
[32–34]. Complete mineralization was found to be expensive due to the
extreme reaction conditions. However, in partial degradation, more An ozone generation unit consisted of an oxygen concentrator
biodegradable and less toxic intermediates were formed, which was the (model: HG 03, make: Oz-air, country: India) and ozone generator
key to the removal of toxicity. The potential of the AOPs for degradation (model: ISM 10 oxy, make: Oz-air, country: India) was used. Ozone
of pharmaceuticals from wastewater has been well established in the concentrator uses the atmospheric air as feed and converts it into the
previous studies [35–40]. Recently, ozonation of DCF has been reported pure oxygen (99% purity). The oxygen concentrator works on the
by many researchers [25,41–44]. However, only a limited extent of principle of pressure swing adsorption, in which oxygen is isolated from
studies are available for the intermediates, and the mechanism of air on the basis of its molecular properties and affinity towards the
degradation. adsorbent. Oxygen generated from the oxygen concentrator was fed to
The present study aims to predict the degradation pathways of DCF the ozone generator in order to generate a gaseous mixture of ozone and
during ozonation and detect the metabolites formed. The effects of oxygen. The ozone generator works on the corona discharge method
system pH, ozone supply rate, and initial concentration of the substrate (dielectric barrier discharge method [45]), in which nascent oxygen is
were studied in detail. The kinetic parameters for the ozonation of DCF generated due to the voltage applied, which combines with the oxygen
were determined, and a kinetic model was developed for the ozonation molecule and generates ozone. It consists of stainless steel electrode and
process. Involvement of the hydroxyl radicals in the degradation process quartz as dielectric and generates the ozone in the range of 0–2.78 mg
was also investigated. Mass transfer of ozone in the aqueous phase was s− 1. The desired ozone supply can be controlled and regulated. The
analyzed and the parameters for mass transfer were calculated. For the ozone generated was fed to a 1 dm3 glass reactor with the help of a
toxicity analysis, the seed germination technique was used. Removal of sparger, which generated fine bubbles. A syringe was installed into the
total organic carbon (TOC) and release of chloride ion during the reac­ reactor for sample collection. Unreacted ozone, released from the
tion was also studied. reactor, was converted into oxygen by an ozone destructor (model: Dest
50, make: Oz-air, country: India). The schematic of the experimental
2. Materials and methods setup is given in Fig. 1.

2.1. Reagents and standards 2.3. Ozonation and sample analysis

The reagents and chemicals used in this study were diclofenac Ozonation of DCF was carried out in 1-dm3 glass reactor equipped

2
S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

Fig. 2. Solubility profile of ozone in water at pH 8 at different ozone supply rates.

with a sparger of pore size of 40 μm that was capable of generating fine 2.4. Mass transfer of ozone in the aqueous medium
bubbles [7]. The reaction time was 10–60 min that varied with the
ozonation system and the pH. The pH of the aqueous solutions varied Degradation of DCF is highly dependent on the ozone available in the
from 4 to 9. Three different ozone supply rates (i.e., 0.44, 0.48, and 0.50 aqueous phase. Availability of molecular ozone and generation of hy­
mg s− 1) were applied for ozonation. The samples were withdrawn after a droxyl radicals is a function of mass transfer of ozone from the gas
certain time, depending on the ozone supply rate. All the experiments mixture to the aqueous phase. To develop a better understanding of the
were repeated 3–4 times to ensure their repeatability. The experiments behavior of ozone in water, mass transfer of ozone was studied for
were carried out at room temperature (i.e., 298 K). For ozone concen­ different ozone supply rate and pH. The pH of the system and ozone fed
tration measurement, 10 cm3 of the sample was collected from the to the reactor plays an important role during the mass transfer phe­
reactor and immediately analyzed in the colourimeter. For quantifica­ nomena. It was recorded that the mass transfer and dissociation rates of
tion of DCF, 5 cm3 of the sample was withdrawn from the reactor and ozone were higher in the alkaline medium than the acidic medium,
quenched with bubbling nitrogen gas to remove the residual ozone. which is in agreement with the previous studies [46,47]. The size of
Then, the sample was analyzed in the high-pressure liquid ozone bubble generated and its distribution affect the gas to liquid mass
chromatograph-UV (HPLC-UV) (model: Shimadzu, make: LC-20AD, transfer. The measurement of bubble size and its size distribution were
country: Japan), equipped with a C18 column (model: XDB C18, performed in our previous study [7]. The bubble generated from sparger
make: Agilent, country: USA) for quantitative analysis of DCF. The di­ was in the range of 0.044–0.45 mm and Sauter mean diameter was 0.26
mensions of the column were 5 mm × 4.6 mm × 250 mm. A mixture of mm. When ozone is in the liquid phase, it simultaneously undergoes into
50 mol m− 3 acetic acid and acetonitrile (in the 30:70 volume ratio) was the decomposition in a mixed batch reactor, so the mass balance equa­
used as the mobile phase with a sample injection volume of 20 × 10− 3 tion for ozone can be written as
μm3. The detection wavelength for DCF was 278 nm. For mass- ( )
dco3
spectrophotometry applied capillary and nozzle voltages were 3500 = kl a c*O3 − cO3 − kd cO3 (1)
dt
and 1000 V, respectively. Gas and sheath gas flowrates were 13 and 11
dm3 min− 1, respectively at nebulizer pressure of 2.4 bar. The ozone where, kd is the first-order rate constant for decomposition and kla is the
concentration and chemical oxygen demand (COD) in the aqueous so­ volumetric mass transfer coefficient of ozone.
lution was measured by a colourimeter (model: 7100, make: Palintest, Mass transfer resistance in the gas phase is negligible in comparison
country: UK) by using the DPD-4 tablets and COD reagents vials, with the same in the liquid phase. The equilibrium concentration of
respectively. The pH of the system was measured by a pH meter (model: ozone in water can be written as
EQ 610, make: Equiptronics, country: India). For identification of in­ ( )
termediates formed during ozonation, high-resolution liquid chroma­ cs =
kl a
c* (2)
tography coupled with a mass spectrophotometer (HR-LCMS) (model: kl a + kd O3
6550 iFunnel Q-TOF, make: Agilent Technologies, country: USA) from From Eqs. (1) and (2) we get
sophisticated analytical instrument facility (SAIF) of IIT Bombay (India)
was used. The total organic carbon (TOC) was measured by a TOC dcO3 ( )
= (kl a + kd ) cs − cO3 (3)
analyzer (model: Aurora 1030, make: O.I. Analytical, country; USA). dt
Upon integration of Eq. (3) with the initial condition: at t = 0, CO3 =
0, we get

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S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

( )
Fig. 3. The plot of ln cs − ccs O versus t, to determine (kd + kla).
3

( )
cs
Table 1 From the plot of ln cs − cO3 versus t, the value of (kd + kla) can be
Volumetric mass transfer coefficients of ozone in the aqueous solution at
different pH.
determined from the slope, as shown in Fig. 3.
The values of kd at different pH were taken from a previous study
pH Volumetric mass transfer coefficient First-order decomposition rate
[48], and the values of the volumetric mass transfer coefficient for ozone
kla × 103 (s− 1) constant of ozone
kd × 104 (s− 1) (i.e., kla) were calculated by using them. Table 1 presents the values of
O3 supply O3 supply O3 supply
48 kla of ozone in water. It was observed that kla increased with the pH of
rate rate rate
0.44 mg 0.48 mg 0.50 mg
the medium as well as the ozone supply to the reactor. Value of the
s− 1 s− 1 s− 1 volumetric mass transfer coefficient was increased by 2.5 folds when pH
of the system was increased from 4 to 9. At the higher pH, the decom­
6 1.150 1.370 1.650 2.50
7 1.234 1.384 1.784 3.16 position rate constant of ozone was higher than that in the acidic me­
8 1.317 1.517 2.317 7.83 dium, as reported in a previous study [48]. The value of the
9 1.617 1.777 2.717 13.3 decomposition rate constant was increased from 2.5 to 13.3, when the
pH was increased from 6 to 9. Higher decomposition of ozone leads to
( ) the generation of hydroxyl radicals, which enhances the degradation of
cs
ln = (kl a + kd ) t (4) DCF.
cs − cO3

With continuous ozone supply, the concentration of ozone in the

2.5. Uncertainty analysis
aqueous phase was found to be constant after some time, which can be
considered as the saturation concentration of ozone. The solubility
The ozonation experiments were replicated 3–4 times. The
profiles of ozone in water for different ozone supply rate are shown in
decreasing concentration of DCF during ozonation was estimated from
Fig. 2.
the peaks obtained from HPLC. Various factors (i.e., instrumental,

Table 2
Mean, standard deviation, and standard and relative uncertainty for the DCF concentration.
Time [DCF] [DCF] [DCF] Mean C Standard deviation (SD) Standard uncertainty (U) Relative uncertainty (Ur)
(min) [DCF]0 [DCF]0 [DCF]0 (− ) (− ) (− ) (%)
(− ) (− ) (− )

1 0.803 0.826 0.856 0.827 0.025 0.015 1.78

2 0.543 0.583 0.609 0.578 0.033 0.019 3.35
3 0.428 0.474 0.441 0.447 0.023 0.014 3.08
4 0.262 0.285 0.251 0.266 0.016 0.009 3.65
5 0.156 0.162 0.187 0.168 0.016 0.009 5.67
6 0.077 0.085 0.096 0.087 0.008 0.005 5.83
7 0.064 0.051 0.054 0.056 0.007 0.004 7.13
8 0.030 0.025 0.026 0.027 0.002 0.001 4.03
9 0.017 0.016 0.017 0.016 0.001 0.001 2.64
10 0.012 0.014 0.015 0.013 0.002 0.001 6.83

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S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

Fig. 5. Variation of the pseudo-first-order rate constant (kapp) with the pH of

the medium for the ozone supply rate of 0.44 mg s− 1 and initial DCF concen­
tration of 50 mg dm− 3.

human, and environmental conditions) led to the errors in the experi­

ments during replication. Therefore, it is essential to conduct an analysis
of uncertainty on each data set.
The standard deviation (SD) of the data obtained was calculated from
Eq. (5).
√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
√∑
√n ( )2
√ Ci − C
√1
SD = (5)
n− 1

where Ci is the estimated value of the DCF concentration, n is the

number of replications of the experiments and C is the mean value of
DCF concentration, which can be calculated by

1∑ n
C= Ci (6)
n i=1

The standard value for uncertainty (U) and relative uncertainty (Ur)
are estimated from Eqs. (7) and (8), respectively.
SD
U = √̅̅̅ (7)
n

U
Ur (%) = × 100 (8)
C
The calculated values of the mean, standard deviation, and standard
and relative uncertainty for the concentration of DCF are given in
Table 2.
Therefore, it is observed that the relative uncertainty of data sets was
in the range on 1–7%.

3. Results and discussion

3.1. Effect of pH of the medium

The oxidation of DCF was conducted in the pH range of 4–9. The

results are shown in Fig. 4(a)–(c). The degradation efficiency was usu­
Fig. 4. Removal of DCF by ozonation in the pH range of 4–9 at the ozone ally above 90%. The pH of the solution had a strong effect on degra­
supply rate of (a) 0.44, (b) 0.48, and (c) 0.50 mg s− 1. Initial concentration of dation. Effect of the pH on ozone lifetime and its decomposition rate
DCF = 50 mg dm− 3. were enunciated by Von Sonntag and Von Gunten [49]. It has been re­
ported that the lifetime of ozone decreases with increasing pH of the
aqueous phase [50]. This was also observed in the present study as at pH
9, the rate constant for decomposition was the highest (see Table 1).

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S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

3.2. Effect of hydroxyl radical generation in situ

H2O2 was added for the generation of hydroxyl radical and isopropyl
alcohol was used for scavenging it. Fig. 6 shows the effect of hydroxyl
radicals present in the reaction system. Addition of isopropyl alcohol
slowed down the reaction by consuming the hydroxyl radicals. This led
to a lower value of the reaction rate constant. kapp dropped from 0.4485
to 0.3947 min− 1 when isopropyl alcohol at a concentration of 13 mol
m− 3 was added to the reaction medium. Addition of 42 mol m− 3 H2O2
led to an increase in kapp by 7%. This variation in the rate constant
confirmed the involvement of hydroxyl radicals in the degradation of
DCF.

3.3. Effect of initial concentration of DCF

To analyze the effect of loading of the target pollutant, the concen­

tration of DCF was varied in the range of 50–125 mg dm− 3 at pH 9 for
the ozone supply rate of 0.44 mg s− 1. It was observed that the degra­
dation efficiency of DCF dropped to 90% from 99%, when the initial
concentration of DCF was increased from 50 to 125 mg dm− 3 (see
Fig. 7). These results indicate that the ozone demand increased as the
Fig. 6. Effect of addition of H2O2 and IPA on the degradation of DCF at pH 7.
initial concentration of DCF increased. A lesser degradation efficiency at
Initial concentration of DCF = 50 mg dm− 3, [H2O2] = 42 mmol dm− 3, and
[IPA] = 13 mmol dm− 3. the higher concentration of the target pollutant suggests the unavail­
ability of a sufficient amount of the oxidants (i.e., molecular ozone and
hydroxyl radicals) in the aqueous phase for reacting with DCF. To ach­
ieve a higher degree of removal of DCF, an optimized ratio of ozone dose
and DCF loading is required. Although, the degradation efficiency for
125 mg dm− 3 DCF was decreased by only 10%, a low mineralization
efficiency (i.e., 9%) was recorded for this system. It implies that the
intermediates formed during ozonation required a higher dose of the
oxidant(s) for complete mineralization.

3.4. Effect of ozone supply rate

Ozonation of DCF was conducted at three different ozone supply

rates, i.e., 0.44, 0.48, and 0.50 mg s− 1. It was found that the removal of
DCF was highly sensitive to the ozone supply rate. A slight increase in
the ozone supply rate led to a shorter reaction time for the removal DCF.
For all ozone supply rates, complete removal of DCF was achieved.
However, the time required for the removal showed significant differ­
ences. When the ozone dose was increased by 0.04 mg s− 1, the time
required for complete removal of DCF was reduced to 15 min from 70
min. When the ozone dose was increased further by 0.02 mg s− 1, the
reaction time was 6–10 min. An identical behavior was observed for the
pH range of 4–7. Higher ozone supply rate led to the higher values of the
apparent rate constant. The increase in the rate constant with ozone
Fig. 7. Effect of the initial concentration of DCF on its removal at pH 7 and
supply can be described based on two-film theory [52]. According to
0.44 mg s− 1 ozone supply rate.
which, the higher concentration of ozone at the interface led to increase
in the mass transfer coefficient. However, at a certain value of ozone
Although ozone was short-lived in the alkaline medium, the time
supply, the water will be saturated with ozone, completely. At this point,
required to achieve 99% degradation was lesser than that in the acidic
ozone concentration in water will be constant and further increase in
medium. The increase in the rate of degradation of DCF with the pH of
ozone dose will not affect the mass transfer. After achieving the ozone
the medium suggests in situ generation of the hydroxyl radicals. Non-
saturation, the degradation process would become dependent on the
selectivity and higher oxidizing power of the hydroxyl radicals make
reaction rate constant. Hence, a further increase in ozone dose does not
them a better oxidizing agent than molecular ozone for the oxidation of
affect the degradation efficiency. The other major factor is enhanced
DCF. The pseudo-first-order rate constant was increased by 32% when
hydroxyl radical production, which takes part actively in the degrada­
the pH was increased from 4 to 9 (see Fig. 5). Involvement of the hy­
tion process. The effect of ozone supply rate on DCF degradation effi­
droxyl radical in the oxidation of DCF was also confirmed by adding a
ciency at different pH is shown in Fig. 8.
catalyst and a scavenging agent for the hydroxyl radical (see Section
3.2).
3.5. Production ammonia and chloride ion during DCF degradation
However, system pH was found to be declining during the ozonation
process due to generation of acidic metabolites and NOx as reaction
Fig. 9 shows the concentration of ammonia and chloride ion detected
solutions were not buffered during the ozonation. It was also reported in
in the reaction medium during the ozonation of DCF. 88% of the total
previous study that the during ozone generation, NOx production takes
chlorine and 72% of the total nitrogen were released during the
place due to electric discharge, which also contributes into pH decline
mineralization process. In the first 25 min of the reaction, the chloride
[51].
concentration increased rapidly due to the probable substitution of

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S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

Fig. 8. Effect of ozone supply rate on the degradation efficiency of DCF for (a) pH 4, (b) pH 5, (c) pH 6, (d) pH 7, (e) pH 8, and (f) pH 9 at the initial DCF con­
centration of 50 mg dm− 3.

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S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

1 3
Fig. 9. Production of ammonia and chloride ion during the ozonation of DCF at pH 7, 0.44 mg s− ozone supply rate, and 50 mg dm− initial DCF concentration.

chlorine by the oxygen-containing groups. After 25 min of reaction, the Although the metabolites formed during oxidation depend on the
concentration dropped slightly as some chloride ions reacted with the technique applied for degradation, the metabolites detected for sonol­
other metabolites and formed products such as dichloro aniline, which is ysis, Fenton, and ozonation are common [41,55,56]. During the reac­
a major intermediate that was detected. Various chlorine-containing tion, the solution turned to orange after 10 min, which disappeared
intermediates were detected in the proposed mechanism (see Section within 10 min. It was found that the production of diclofenac-2, 5-imi­
3.6). Detection of ammonia reflects the loss of nitrogen atom in the noquinone (M3) was responsible for this color, which also agrees with
structure of DCF. The elimination of nitrogen is considered as an the recent studies [42,57]. Disappearance of M3 suggests its further
essential step in the mineralization process because it signifies the degradation. In the HR-LCMS spectrum, total eight distinct peaks were
rupture of two aromatic rings. Although the final products did not visible, out of which five peaks had low intensity. The positive ion
contain nitrogen, a loss of only 72% of the nitrogen illustrates the for­ spectrum from the LC-MS shows a peak at 9.93 min (m/z = 312) for [M
mation of intermediate nitrogen-containing compounds, which went + H]. Further fragmentation shows the loss of a water molecule (m/z =
undetected. 294) as [M-H2O]. Both the mass spectroscopic results indicate the
presence of 5-hydroxy DCF (M2). 5-hydroxy DCF is an unstable com­
pound in the oxidative atmosphere. It tends to be oxidized into DCF-2, 5-
3.6. Mineralization (TOC) and COD removal during ozonation
iminoquinone, which was also detected as a metabolite in the present
study. M2 may also produce the derivatives of phenyl acetic acid (M7
Fig. 10a and b show TOC and COD removal from DCF solution for
and M11, m/z = 152.1). DCF-2, 5-iminoquinone (M3) was detected at
various ozone supply. Maximum TOC removal was 60% achieved at
RT = 9.8 min (m/z = 310). Probable attack of ozone at the acetate group
ozone supply of 0.50 mg s− 1. For lower ozone supply (0.44 mg s− 1), the
attached to the benzene ring may release a CO2 molecule and oxidize the
TOC removal was only 20% it indicates that at lower ozone supply,
carbon attached to ring into the aldehyde group (M1). M1 was converted
recalcitrant intermediates were formed after 30 min of ozonation. It can
to M4 (i.e., N acetyl 2-amino salicylic acid, m/z = 218, RT = 7.58 min)
also be observed that at initial stage of ozonation (0.48 and 0.50 mg s− 1),
and 2-chloro benzoate (i.e., M5, m/z = 177, RT = 7.10 min) at the
mineralization rate was higher due to availability of easily degradable
advanced stages of oxidation. Further oxidation breaks the C–N bond
parent compound. Behavior of COD removal also follows the same
and produces various products containing one benzene ring. Cleavage of
pattern as it was found to be maximum i.e. 80% for highest ozone supply
the C–N bond leads to the generation of a major product, i.e., dichloro
(0.50 mg s− 1). For lower ozone supply, the COD removal 40–50%, only.
aniline (i.e., M6, m/z = 161.9, RT = 7.024 min). Dichloro aniline was
Direct ozonation of parent compound led to production of less degrad­
commonly found as a metabolite in the previous studies as well [56].
able intermediates, which can be partially removed by hydroxyl radicals
Further degradation of M6 produced 2, 6-dichloro quinone (i.e., M8, m/
and partial TOC elimination and lower COD removal [53,54].
z = 178), 4-amino, 3, 5-di chloro phenol (m/z = 177), and 2-chloro
aniline (m/z = 127). After the C–N cleavage, ring-opening reactions
3.7. Identification of the intermediates and the mechanism proposed take place and generate smaller acids (i.e., M13, M14, and M15). Thus,
the ozonation of DCF involves the decarboxylation, dechlorination, and
Metabolites formed during the ozonation of DCF were identified in hydroxylation steps. Cleavage of the C–N bond and opening of the
order to propose the probable degradation pathways. It is important to benzene rings generated the compounds of lower molecular weight.
find the final products and intermediates (formed during ozonation) to Smaller carboxylic acids such as acetic acid, formic acid, and oxalic acid
ensure the mineralization and toxicity. For the identification study, a were produced at the final stage of ozonation, which further mineralized
separate experiment was conducted without altering the pH. The ozone to carbon dioxide and water. Dechlorination was also confirmed by the
supply rate was 0.50 mg s− 1 and the concentration of H2O2 was 42 mol presence of chloride ion in the reaction medium. Fig. 12 represents the
m− 3. For identification and detection of the intermediates, HR-LCMS probable mechanism for DCF ozonation.
(coupled with the library) was used, and the spectrum obtained is
shown in Fig. 11.

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S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

pseudo-first-order.
d[DCF] ( [ ])
= − kO3 [O3 ] + k• OH ⋅ OH [DCF] (10)
dt

d[DCF]
= − kapp [DCF] (11)
dt
Upon integration, we get
( )
[DCF]
ln = − kapp t (12)
[DCF]0

The value of kapp can be determined from the slope of the plot of ln
( )
[DCF]
[DCF]
versus t. The experimental data for the ozonation of DCF fitted
0

Eq. (12) well for different operating conditions. The coefficient of

determination was greater than 0.93. Therefore, it can be concluded that
the degradation of DCF by ozone and hydroxyl radical follows the
pseudo-first-order kinetics. The values of kapp for DCF degradation were
in the range of 0.0742–0.48 min− 1. The highest value of kapp was found
to be 0.48 min− 1 at the ozone supply rate of 0.50 mg s− 1 and pH 9.
Degradation of DCF by ozone is a complex process that involves
various direct and indirect parameters. Due to the unstable nature of the
ozone and the constraints encountered during the measurement of hy­
droxyl radicals in the reaction medium, the ozone supply was considered
an independent variable. System pH, ozone dosage, and the initial
concentration of DCF were considered as the main affecting parameters
for kapp [58]. The nature of dependency of all three parameters was
estimated and an empirical model was developed for kapp. This model is
given as
( )( )a
Ea QO3 × tR
kapp = Aexp − (pH)b (13)
RT [DCF]0 × VR

where QO3 is the ozone supply rate (mg s− 1) to the system [DCF]0 is the
( )
initial concentration of DCF (mg dm− 3) and − RT Ea
was considered as a

constant since all experiments were performed at 298 K. After taking

logarithm on both sides
( )
( ) QO3 × tR
(14)
′
ln kapp = lnA + aln + bln(pH)
Fig. 10. a. TOC removal during ozonation for various ozone supply rate (0.44, [DCF]0 × VR
0.48, and 0.50 mg s− 1) at pH 9.
b. COD removal during ozonation for various ozone supply rate (0.44, 0.48, and ( )
Ea
where A = Aexp − .
′
0.50 mg s− 1) at pH 9. RT

Values of a and b were calculated by performing the multiple

3.8. Kinetic study of ozonation of DCF (
QO3 ×tR
)
regression analysis between ln(kapp), [DCF] ×VR , and ln(pH).
0

The reaction of the target pollutant with ozone is a function of From the multiple regression, the values of a and b were estimated as
various macroscopic parameters, i.e., ozone supply rate, pollutant 0.59 and 0.35 with R2 = 0.922, respectively. Value of lnA′ was found to
loading, and system pH. Availability of ozone in the aqueous phase be − 5.16. Therefore, Eq. (13) can be further written as
depends on the extent of mass transfer of the ozone from the gas to the
( )0.59
liquid phase. The pH of the system also plays a vital role in ozone QO3 × tR
kapp = 5.71 × 10− 3 (pH)0.35 (15)
dissociation and mass transfer, as described in details in Section 2.4. [DCF]0 × VR
Thus, the kinetics of the DCF degradation reaction was studied under
different operating parameters. Dependence of the pseudo-first-order Final equation for the DCF degradation can be written as
rate constant on the operational parameters was also studied. ( )
( )0.59
Since it is an established fact that both molecular ozone and hydroxyl QO3 × tR
[DCF] = [DCF]0 exp 5.71 × 10− 3 (pH)0.35 t (16)
radical act as the oxidants, the rate of oxidation of DCF can be written as [DCF]0 × VR
d[DCF] [ ]
= − kO3 [DCF][O3 ] − k• OH [DCF] ⋅ OH (9) In order to verify the developed model, the theoretical data were
dt calculated with the help of model and compared with experimental data.
where [DCF], [•OH], and [O3] denote the concentration of diclofenac, It was found that the relative error for model was <7%. Therefore, the
hydroxyl radical, and ozone, respectively at time t. kO3and k•OH are the developed kinetic model for DCF degradation was considered accurate
rate constants for the reactions involving ozone and hydroxyl radical, for the operating parameters, i.e. ozone supply of 0.44–0.50 mg s− 1,
respectively. system pH of 4–9, and initial concentration of DCF 50–125 mg dm− 3.
Eq. (9) can be simplified to Eq. (10) by considering the reaction as

9
S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

Intermediates

Diclofenac

Fig. 11. HR-LCMS chromatograph of the intermediates produced during the ozonation of DCF.

Fig. 12. Mechanism of the ozonation of DCF and the proposed structure of the metabolites.

3.9. Effect of water matrix on the degradation of DCF on the DCF concentrations present in the water bodies [59–61]. The
effect of water matrix on the removal efficiency and mineralization is
To analyze the effect of water matrix, a separate experiment was shown in Fig. 13. It was observed that the organic and inorganic ma­
conducted with a real wastewater (COD = 50–55 mg dm− 3) spiked with terials present in the real wastewater inhibit the ozonation of DCF to
DCF and compared with the degradation efficiency achieved in ultra- some extent. The removal efficiency was dropped by 17% when the
pure water (COD = 0 mg dm− 3 and electrical resistivity = 0.0055 μS ozonation was conducted in wastewater instead of ultrapure water.
cm− 1). One run was conducted with a lower concentration of DCF, based Although the removal efficiency was lower, 83% removal of DCF was

10
S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

From these calculations, it is evident that the process was more

1.0
economical at the ozone supply rate of 0.50 mg s− 1 than 0.48 mg s− 1,
DCF in wastewater
due to the lower reaction time. The cost of the process was decreased by
DCF in ultra-pure water
19%, when the ozone supply rate was higher by 1.13%. The lowest cost
0.8
for processing 1 dm3 DCF solution was USD 0.80 at the ozone supply rate
of 0.5 mg s− 1.
[DCF]/[DCF]0 (-)

0.6 4. Conclusions

Ozonation was found to be a very effective method for removing DCF

0.4 from water. The removal efficiency was in the range of 95–99% in the
alkaline as well as acidic media. However, the reaction time required for
complete removal of DCF was less in the alkaline medium. The values of the
0.2 volumetric mass transfer coefficient were in the range of 1.150 × 103–2.717
× 103 s− 1 for pH 6–9 and ozone supply rate of 0.44–0.5 mg s− 1. The values of
the pseudo-first-order rate constant were in range of 0.0742–0.0979 min− 1.
0.0 The results also confirmed the active participation of the hydroxyl radicals
0 10 20 30 40 50 60 70 80 in the degradation process. The rate constant dropped by 11% when 13
Time (min) mmol dm− 3 IPA was added as the radical scavenger. The rate of ozone supply
to the reactor was proved to be a key factor for the degradation time.
Fig. 13. Effect of wastewater matrix on the degradation of DCF at pH 7. Ozone An increase of 0.06 mg s− 1 in the ozone supply rate led to a rapid fall in the
supply rate = 0.44 mg s− 1; Initial concentration of DCF = 50 mg dm− 3. reaction time (i.e., from 60 min to 6 min). 88% of the total chlorine and
72% of the total nitrogen were released during the degradation process.
The final model for DCF ozonation developed was [DCF] = [DCF]0
Table 3 ( ( ) 0.59
)
QO3 ×tR
Cost associated with electricity consumption at different ozone supply rates. exp 5.71 ×10− 3
[DCF]0 ×VR (pH)0.35 t in which the effect of system
Cost calculations Ozone supply
rate (mg s− 1) pH, initial concentration of DCF, and ozone supply rate were considered as
the main variables. With a real effluent having 50 mg dm− 3 COD, the removal
0.44 0.50
efficiency was dropped by 17%. The mineralization of DCF by ozone mainly
Total time required for the complete removal of DCF from 1 dm3 1.00 0.17 consisted of three steps, i.e., attack of the oxidant, cleavage of the C–N bond,
solution (h)
Rate of energy consumption (W) 32.60 37.00
and ring opening. The major intermediates detected in HR-LCMS were di
Energy required for complete removal of DCF (Wh) 32.60 6.16 chloro aniline, 5-hydroxy DCF, DCF-2 5-iminoquinone, 2-chloro benzoate,
Cost of energy (USD)a 4.238 0.80 derivatives of phenyl acetic acid, and carboxylic acids (e.g., acetic, formic,
a
Calculated as per the current rate of INR. and oxalic acids). The main mechanism involved in the degradation process
included decarboxylation, dechlorination, and hydroxylation. The cost of
treatment for 1 dm3 DCF was USD 0.80 at the ozone supply rate of 0.5 mg s− 1.
achieved in 60 min of the reaction. It can be concluded that the sub­
stances present in the wastewater competed with DCF for consuming the
oxidant present in the solution. Therefore, the availability of ozone and Nomenclature
hydroxyl radical to DCF was decreased, and hence, a reduced removal
rate was observed. It can be anticipated that the presence of other A Pre exponential factor of Arrhenius equation (s− 1)
contaminants inhibits the degradation process, so the removal efficiency A′ Correlation coefficient (s− 1)
achieved in the case of ultra-pure water cannot be achieved in the real a Correlation coefficient (− )
wastewater. b Correlation coefficient (− )
CO3 Concentration of ozone in aqueous phase (mg dm− 3)
Cs Saturation concentrations of ozone in aqueous phase (mg
3.10. Energy consumption dm− 3)
Ci Concentration of DCF (mg dm− 3)
To implement the ozonation process at the industrial level, it is very C Mean value of DCF concentration (mg dm− 3)
important to analyze and minimize the cost of the process. In the present Ea Activation energy (kJ mol− 1)
study, the main contributor to the operating cost was energy con­ kla Volumetric mass transfer coefficient of ozone (s− 1)
sumption. The fixed cost involves the cost of the machinery. The oper­ kd First-order rate constant for ozone decomposition (s− 1)
ating cost involves the cost of energy used, inasmuch as there is no kapp Pseudo-first-order rate constant for DCF ozonation (s− 1)
chemical consumption. The operating cost is discussed in this section. k•OH Rate constant for oxidation of DCF by ozone (M− 1 s− 1)
1
Operation cost = α Q (17) kO3 Rate constant for oxidation of DCF by hydroxyl radical (M−
s− 1)
where Q and α are the energy consumption rate and the cost of elec­ n Number of replication of the experiments (− )
tricity, respectively. The electricity consumed by the ozone generator is Q Energy consumption rate (kWh)
directly proportional to the ozone supply rate. The experiments were QO3 Ozone supply rate (mg s− 1)
conducted at three different ozone supply rates. For the ozone supply SD Standard deviation (− )
rates of 0.44 and 0.5 mg s− 1, the energy consumption rates are 32.6 and R Universal Gas constant (kJ K− 1 mol− 1)
37 W, respectively (according to the specifications provided by the T Temperature (K)
equipment manufacturer). The required reaction times for the complete t Time (s)
removal of DCF were 60 and 10 min, respectively, for these ozone supply tR Residence time for DCF degradation (s)
rates. The total energy consumed and the associated cost are given in U Standard value for uncertainty (− )
Table 3 based on the present cost of electricity (i.e., INR 9.45 per kWh). Ur Relative value of uncertainty (− )

11
S. Patel et al. Journal of Water Process Engineering 44 (2021) 102325

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