Namata 2nd ppr
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Transcripts - Namata 2nd ppr
Journal of Water Process Engineering 7 (2015) 314–327
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Journal of Water Process Engineering
journal homepage: www.elsevier.com/locate/jwpe
Degradation of Reactive Yellow 145 dye by persulfate using
microwave and conventional heating
Namata N. Patil, Sanjeev R. Shukla∗
Department of Fibres and Textile Processing Technology, Institute of Chemical Technology (University under the Section-3 of UGC act 1956) Nathalal Parekh
Marg, Matunga, Mumbai 400019, India
a r t i c l e i n f o
Received 3 May 2015
Received in revised form 4 August 2015
Accepted 12 August 2015
Reactive Yellow 145
a b s t r a c t
Degradation of C. I. Reactive Yellow (RY 145), a bi-functional dye containing azo chromophore has been
attempted in the presence of potassium persulfate (KPS) as an initiator in the presence of microwave
irradiation. The conditions of degradation were optimized and its kinetics was studied. Complete dye
degradation was observed in 3600 and 280 s, respectively, using conventional and microwave heating at
the original pH (5.8) of the dye solution with COD removal of 60 and 66.5%. Under optimized conditions,
efﬁciency of H2O2 was also compared for degrading the dye. Pseudo-ﬁrst-order kinetics with higher
kinetic rate constant (0.0165 × 102
) and maximum energy efﬁciency (67.6%) was achieved through
microwave assisted KPS system. Additional beneﬁt of microwave approach was accrued by the extensive
reduction in electrical energy consumption by 10638.32 kW h/m3
. The effect of addition of SO4
anions to the dye solution has indicated that the former enhances the degradation, whereas the
latter reduces it. The temperature study using Arrhenius equation indicated decrease in the activation
energy for the microwave system along with 2.6 times higher frequency factor than that for conventional
heating. Thus, the microwave radiation assisted KPS system has been found to be the most cost effective
and environmentally benign for the dye degradation process.
© 2015 Published by Elsevier Ltd.
Out of about 0.7 million tons of worldwide consumption of dyes,
the azo chromophore based dyes represent more than half the share
in textile coloration. However, up to 30% of theses dyes remain
unﬁxed, washed away and therefore discharged into wastewater
. The presence of even very small amounts of dyes in water cre-
ates aesthetic problems, hinders the photosynthesis, badly affects
the aquatic life thereby posing signiﬁcant risk to the food chain
[2,3]. Hence, the remediation of dye wastewater has been consid-
ered to be one of the global aim in controlling the environmental
pollution . Well known physico-chemical methods and the bio-
logical treatments suffer from inherent disadvantages related to
secondary pollution, poor efﬁciency and/or longer reaction time
[5,6]. These limitations are overcome by advanced oxidation pro-
Abbreviations: RY 145, C. I. Reactive Yellow 145; KPS, potassium persulfate; MW,
Microwave; k, kinetic rate constant; R2
, correlation coefﬁcient; EE0, electrical energy
per order; COD, chemical oxygen demand; Ea, activation energy.
∗ Corresponding author. Fax: +91 22 33611020.
E-mail addresses: firstname.lastname@example.org (N.N. Patil), email@example.com
cesses (AOPs), which are based on non-selective and powerful
oxidising species [7,8] including sulfate radicals (SO4
Several reports have mentioned the activation of persulfate ion
2−) through various heating sources for SO4
•− generation. The
previously reported work enlists the persulfate activation by con-
ventional heating [11,12], by using UV-radiations [13–15], electron
beam  and ultrasonication . Kusic et al.  have applied
transition metal catalysis for persulfate activation in which low
valent metal ions act as electron donors. Yang et al.  achieved
the microwave assisted decolorization of an azo chromophore
based Acid Orange 7 dye within 5–7 min at 800 W power using
potassium persulfate (KPS).
The parent ion S2O8
2− has several advantages like non-
selectivity in attack, relatively stable nature, cheap  and easy
availability in the form of ammonium, sodium, or potassium salt.
Under neutral pH, KPS gives better results in photo-oxidative
removal of some organic materials than corresponding ammonium
Microwave (MW) irradiation is a well-known approach for
the process intensiﬁcation via selective absorption of the ther-
mal energy by polar molecules resulting in uniform heating of the
reaction liquid. The important advantages of microwave based pro-
cessing over the conventional systems are the development of very
2214-7144/© 2015 Published by Elsevier Ltd.
N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 315
high power densities in the processing zone, less reaction time, less
energy consumption, homogeneous and selective heating as well
as cleaner and greener process .
Reactive Yellow 145 (RY 145), an anionic hetero bi-functional
reactive dye containing azo chromophore, is commonly used in
dyeing of cotton, rayon, polyester blended fabric, printing and
Moroccan tannery factories . It reacts with the cellulosic
hydroxyl group in dual way either by nucleophilic addition to an
activated double bond (sulphatoethylsulphone group) or by nucle-
ophilic substitution of reactive chlorine atom (monochlorotriazine
group) . The presence of these two reactive species enhance
the light fastness property . However, one should not neglect
their mutagenic and carcinogenic effects on human and aquatic life
. Therefore, this dye has been selected in present study as a rep-
resentative refractory pollutant due to its fairly high consumption
(approx. 680 metric tonnes in India) for dyeing. Some researchers
have reported its decolorization by biological treatment (Enterococ-
cus faecalis strain YZ66)  and also its adsorption onto wheat bran
. There are also reports on the decolorization of this dye, which
are based on the multiple combinations of AOPs such as ozona-
tion and UV-C/H2O2 treatment , ozonation involving granular
activated carbon , photocatalytic systems with TiO2 coated on
synthetic non-woven ﬁbres , UV irradiation coupled with per-
sulfate  and UV/TiO2 followed by biological treatment (using
Pseudomonas ﬂuorescens) . However, there are no reports on
use of KPS in the presence of microwave irradiation for degradation
of this dye.
The present work, therefore investigates the efﬁciency of
microwave irradiation for the degradation of Reactive Yellow 145
using KPS. The results have been compared with those obtained
by conventional heating. The kinetics and the effect of presence
of Na2SO4 and Na2CO3, which are essentially present in reactive
dye bath has been investigated. Further, the power dissipation,
percent energy efﬁciency and electrical energy consumption have
also been analyzed. A temperature sensitive Arrhenius equation has
been studied in details that yielded activation energy and frequency
2. Materials and methods
Potassium persulfate (K2S2O8), Na2SO4, Na2CO3, HCl, H2O2 (50%
v/v) etc. were of analytical reagent grade obtained from S.D. Fine
Chem., Pvt., Ltd., Mumbai, India. The commercial dye RY 145 (Fig. 1)
was received from Atul Ltd., India and was used without any further
puriﬁcation. HCl and Na2CO3 were used for adjustment of pH. All
the chemicals were used as received from the supplier and their
solutions were prepared in distilled water.
2.2. Microwave setup
Experiments under microwave were performed in a stoppered
Erlenmeyer ﬂask with a capacity of 200 mL by modifying a domestic
microwave oven (Morphy Richards, Model-MWO 20 MBG) with
rated output power of 800 W as a source of microwave irradiation
which has been described elsewhere . The setup was ﬁtted with
a programmer for temperature setting, time and rate of heating and
a Teﬂon stirrer driven by a motor so as to ensure slow agitation of
the dye solution during the microwave heating. The power function
could be varied up to 100% and the experiments were conducted
at 50% power output i.e., 400 W. The solutions were exposed to
microwave irradiation in a batch process till 280 s. The calorimetric
measurement was carried out to calculate actual power dissipation
in the microwave system and was observed to be 270.4 W for output
power of 400 W, which indicates an energy efﬁciency of around
67.6% (Appendix A).
2.3. Experimental methodology
For all the experiments, the reaction volume (50 mL) and the
dye concentration (50 mg/L) were kept constant, except for studies
on operating volume and dye concentration. KPS was dissolved in
dye solution and exposed to microwave irradiation. The ﬂask was
placed at the centre of the rotating plate directly below the mag-
netron source. The effect of temperature by conventional heating
was also investigated and compared by maintaining the constant
temperature of dye solution at 40, 50, 70 and 90 ◦C, using a ther-
mostatically controlled water bath. After each experiment, 5 mL
solution was withdrawn, cooled immediately to 25 ◦C and analyzed
for the extent of decolorization using UV–vis spectrophotometer.
2.4. Analytical procedures
The extent of degradation of dye as a function of microwave irra-
diation time was analyzed using UV–vis Spectrophotometer (Model
8500, TECHCOMP, Hong Kong). The absorbance at = 419 nm was
measured and the dye degradation was calculated using the fol-
lowing equation (Eq. (1)):
Degradation (%) =
(Ai − At)
× 100 (1)
where Ai and At are the absorbance values before and after the
The samples were also analyzed for COD removal by using closed
reﬂux micro method and measured on Hach Colorimeter (model
DR/850) at a ﬁlter value 610 nm  and percentage COD removal
was calculated using the following equation (Eq. (2)):
COD removal (%) =
(CODi − CODt)
× 100 (2)
where CODi and CODt are the COD (mg/L) of the dye solution before
and after the treatment.
The changes in the functional groups were investigated using
FTIR spectrum 2000 PerkinElmer spectrophotometer in the IR
region of 750–4000 cm−1.
The reproducibility of the results was checked by repeating all
the experiments twice and observed to be reproducible within the
limits of ±2%. Error bars have also been revealed to represent the
variation which was within 2% of the reported average value.
2.5. Kinetics of RY 145 degradation process
Kinetics of the degradation process was evaluated and has been
observed to follow pseudo-ﬁrst-order model (using Eq. (3)):
= −kt (3)
The reaction rate constant (k) was obtained by plotting the values of
ln [At/A0] against time, using experimental data of dye degradation.
3. Results and discussion
3.1. Effect of microwave power dissipation and temperature
study at each power
The effect of power dissipation on the extent of RY 145 dye
degradation has been studied and the corresponding temperature
for each microwave power output at the end of 280 s has been
depicted in Fig. 2. Nominal degradation (7.3%) at power dissipa-
tion of 80 W has been observed due to the kinetically insigniﬁcant
316 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327
Fig. 1. Characteristics of RY 145 dye.
Fig. 2. Effect of microwave power dissipation and temperature on the extent of dye degradation.
thermal decomposition of KPS (3.7 mM) at temperatures lower than
50 ◦C. It has been noticed that the extent of degradation at power
dissipation of 240 W at the end of 80 s reached 35%, which was less
than half as against 85.4% at 400 W under otherwise similar condi-
tions. This has been attributed to much insufﬁcient production of
the sulfate radicals (SO4
•−) at lower temperature (48 ◦C at 240 W)
as compared to that at 68 ◦C and 400 W, which is actually essential
for the radical formation. It can also be observed from Fig. 2 that
the degradation of the dye increased with the time of microwave
irradiation and also with relative increase in the temperature of
microwave system as the power was raised from 80 to 800 W.
This observation is in concurrence with several reports where
efﬁciency of microwave in terms of pollutant degradation has been
enhanced either by increasing the irradiation time [32,33] or by
increasing the microwave power [34,35]. The observed results may
be attributed to the fact that the higher microwave energy provokes
electronic vibrations and raises the temperature of the system 
thereby producing more SO4
•− radicals from the dissolved persul-
fate ions. Fig. 3 explains the kinetic analysis of the degradation
process (using Eq. (3)) and it has been observed that the kinetic
rate constants obtained for varying power dissipation levels at 240,
400, 640 and 800 W were 0.008, 0.045, 0.049 and 0.054 s−1, respec-
tively. Complete degradation of the dye was achieved within 5 min
of the microwave irradiation for power dissipation of 400 W and
above. The rate constant for dye degradation at 800 W (0.054 s−1)
was only marginally higher as compared to that obtained at power
dissipation of 640 W (0.049 s−1) and 400 W (0.045 s−1), indicating
that 400 W power was the optimum.
Jou  has reported similar mechanism for degradation of the
pentachlorophenol solution using zero-valent Fe under microwave
irradiation. It has been reported that the higher microwave power
increases the reaction temperature leading to faster conversion of
persulfate ions to sulphate radicals . Thus, it is essential to opti-
mize the threshold microwave power (operating temperature) and
irradiation time for the effectiveness of the reaction so as to avoid
unnecessarily excessive consumption of energy.
3.2. Effect of initial dye concentration
The effect of RY 145 concentration in the range 50–200 mg/L on
the extent of degradation has been studied at microwave output
power of 400 W, at (original dye solution) pH 5.8 and reaction time
up to 280 s keeping the reaction volume (50 mL) constant. These
results are presented in Fig. 4.
The extent of dye degradation gradually increased after about
120 s, whereas with increasing dye concentration it decreased.
N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 317
Fig. 3. Kinetic study of the degradation process at different microwave power.
Fig. 4. Effect of initial concentration of RY 145 on the extent of degradation.
Effect of initial dye concentration on the extent of degradation.
Concentration (mg/L) Degradation
(%) at 120 s
k × 10−2
50 93.6 2.2 0.988
75 91.8 2.1 0.988
100 90.0 1.7 0.977
150 83.6 1.3 0.972
200 77.0 1.0 0.963
The pseudo-ﬁrst-order reaction kinetics ﬁtted well for this study.
Table 1 indicates that with 120 s irradiation time, the extent of
degradation at 50 mg/L dye concentration was 93.6%, which sig-
niﬁcantly reduced to 77% at 200 mg/L. The pseudo-ﬁrst-order rate
constant (k) also decreased to less than half in its value from
2.2 × 10−2 s−1 to 1 × 10−2 s−1. The reduced degradation at higher
concentration of RY 145 may be due to the insufﬁciency of amount
of KPS, which was kept constant at 3.7 mM. As a result, SO4
radicals generated in the solution were not sufﬁcient enough for
reaction with the dye pollutant.
At the end of 280 s of microwave irradiation, the 50 mg/L dye
solution showed 99.7% degradation and at 200 mg/L concentration
it was little less as 94.6%. The reaction kinetics was observed to
be faster at 50 mg/L. Since the highest rate constant for maximum
degradation was at 50 mg/L and also the fact that the wastewater
after the secondary biological treatment mostly contains less than
50 mg/L dye concentration, rest of the experiments were performed
at 50 mg/L initial concentration of RY 145. Similar degradation
318 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327
Effect of operating volume on the extent of degradation.
Volume (mL) Degradation
(%) at 120 s
k × 10−2
50 94.3 2.1 0.989
75 60.6 0.9 0.981
100 28.7 0.4 0.982
trend was observed by Zhang et al.  for microwave assisted pho-
tocatalytic degradation of Reactive Brilliant Red X-3B dye where the
net degradation was found to increase till 60% with an increase in
the initial concentration up to 1000 ppm and decreased thereafter.
Dafale et al.  have observed that the ﬁrst-order kinetic con-
stant decreased from 2.277 × 10−1 to 0.906 × 10−1 h−1 as the dye
concentration increased from 25 to 100 ppm, during the biological
degradation of azo chromophore based Remazol Black-B dye.
3.3. Effect of operating volume of dye solution
Studies related to operating volume were performed at constant
initial concentration of 50 mg/L of RY 145 at original solution pH of
5.8 by varying the volume (50, 75 and 100 mL). The effect of the
reaction volume on the extent of degradation has been depicted
in Fig. 5, which indicates that the extent of percentage degrada-
tion decreased with an increase in the reaction volume at ﬁxed
microwave power dissipation of 400 W.
Table 2 gives the data related to kinetic rate constants for the
dye degradation at various operating volumes. In 120 s of irra-
diation time, the kinetic rate constant for 50 mL reaction volume
was 2.1 × 10−2 s−1 (94.3% degradation), which was much higher as
compared to that of 0.4 × 10−2 s−1 at 100 mL with 28.7% degrada-
tion. These results indicate that the increase in reaction volume
correspondingly decreases the power dissipation per unit volume,
thereby causing decreased power density of the system. Remya and
Lin  have mentioned the comparative decrease in the degrada-
tion of ammonia as its concentration and volume was increased in
3.4. Effect of KPS concentration
Effect of KPS concentration (0.7–4.4 mM) on dye degradation at
ﬁxed power dissipation of 400 W and constant volume of 50 mL has
been shown in Fig. 6. It may be observed that the dye degradation
increased continuously from 0.7 to 3.7 mM KPS, the reason being
that more sulfate and hydroxyl radicals will be available to attack
the azo bond of the dye molecule thereby enhancing the degra-
dation reaction. However, further increase in KPS concentration to
4.4 mM showed decreased dye degradation.
8 + SO
4 → SO2−
4 + S2O
This may be attributed to the presence of S2O8
2− ion, which itself is
a scavenger of SO4
•− radicals as shown in Eq. (4), mainly under
acidic conditions. Similar results were observed by Hori et al.
 for decomposition of perﬂuorocarboxylic acids. Highly linear
(R2 > 0.95) correlation coefﬁcient values for all KPS concentrations,
given in Table 3, conﬁrm a pseudo-ﬁrst-order kinetics pattern for
this parameter. It may be observed from the calculated rate con-
stants that the k value at KPS concentration of 2.2 mM was 4 times
as high as that at 0.7 mM. Also, dye degradation at 120 s for 2.2 mM
KPS was signiﬁcant with respect to 1.5 mM, the maximum being
93.2% at 3.7 mM KPS concentration. Based on these ﬁndings, the KPS
concentration was ﬁxed at 2.2 mM. The experiments have shown
that KPS alone could give only 12.3% dye degradation at a con-
centration of 2.2 mM, whereas microwave alone had no effect at
Pseudo-ﬁrst-order kinetics details for various KPS concentrations.
KPS (mM) Degradation
(%) at 120 s
k × 10−2
0.7 43.6 0.4 0.964
1.5 46.3 0.6 0.965
2.2 81.8 1.6 0.976
3.0 91.2 1.9 0.978
3.7 93.2 2.3 0.996
4.4 80.9 1.3 0.990
all. Microwave irradiation is known to change the thermodynamic
function and reduce the activation energy of the reaction system
. The temperature effect of microwave plays role in fast sul-
fate radical generation in less reaction time. The reason behind this
is microwave irradiation can produce efﬁcient internal heating by
direct coupling with the molecules of solvents via rapid rotation
and selective thermal energy absorption of polar molecules in the
reaction . Only on combination of the two, dye degradation was
achieved at 68 ◦C.
3.5. Synergistic index
The synergistic index (f) of the combined microwave assisted
KPS system was calculated using the Eq. (5), in order to estimate its
effectiveness over the individual systems .
k(PS) + k(MW)
(1.6 × 10−2
(0.03 × 10−2
) + (0.01 × 10−2
= 40 (5)
where kPS and kMW are the kinetic rate constants in the presence of
KPS (2.2 mM) and microwave for individual reactions, respectively.
These values were obtained from the reaction kinetics. Synergistic
index calculated for microwave process was found to be 40, which
was 15 in case of conventional heating, hence both processes con-
ﬁrmed the synergistic effect. Higher the value of the synergistic
index, greater is the effectiveness of the assisted process. Signiﬁ-
cantly higher value of f in case of microwave process indicates the
interdependence of KPS and microwave on each other, as enhanced
formation of the SO4
•− radicals took place under the microwave
3.6. Effect of operating pH on degradation as a function of
relative concentration of persulfate ions and hydroxyl radicals
The rate of degradation of organic pollutants is usually inﬂu-
enced by the pH of reaction. Microwave assisted degradation of
RY 145 (50 mg/L) was investigated at different pH between 3 and
10.5 using 2.2 mM KPS under microwave irradiation of 280 s. The
decolorized solutions were iodometrically titrated to a colorless
endpoint so as to measure residual persulfate concentration 
followed by a qualitative ‘starch iodide paper’ test to indirectly
study OH radical generation at each pH. It may be observed from
Fig. 7 that the degradation at 120 s of irradiation gradually increased
from 71 to 89% as the pH of the solution decreased from 10.5 to 3,
maximum being at pH 3. Table 4 gives the corresponding values of
the rate constants and correlation coefﬁcients at different initial pH.
The kinetic rate constant increased from 1 × 10−2 to 2 × 10−2 s−1
with this decrease in pH. However, the pH 5 was chosen with little
compromise and used in further experiments, since it was nearer
to the original RY 145 dye solution (pH 5.8), and no addition of HCl
required to adjust the pH. Iodometric titration resulted in 0.03, 0.05,
0.07, 0.10 and 0.13 mM residual persulfate for solution pH 3, 5, 7, 9
and 10.5, respectively. It could be seen in Fig. 8 that starch iodide
N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 319
Fig. 5. Effect of the reaction volume on the extent of degradation.
Fig. 6. Effect of initial KPS concentration on the extent of degradation.
Pseudo-ﬁrst-order kinetics details for operating pH.
(%) at 120 s
k × 10−2
Change in pH after
3 89.0 2.0 0.985 2.1
5 85.7 1.7 0.980 2.2
7 82.8 1.5 0.980 2.4
9 79.1 1.0 0.975 5.4
10.5 71.0 1.0 0.980 9.6
paper turned darker (blackish blue) as the solution pH increased to
Enhanced degradation of RY 145 under acidic conditions could
be due to improved generation of SO4
•− radicals that can also be
correlated to above two tests i.e., the least residual persulfate ion
concentration being 0.03 mM at pH 3 and no color change in starch
iodide paper. This conﬁrms almost complete conversion of persul-
fate ion into SO4
•− radical , as shown in Eqs. (6) and (7).
8 + H+
8 (at acidic pH) (6)
8 → SO
4 + SO2−
4 + H+
Higher concentration of residual persulfate ions as the pH
increases might possibly be due to scavenging action of OH− ions
•− radical resulting in the formation of OH• as a dominant
radical species along with SO4
2− ions  as shown in Eq. (8) which
could visibly be conﬁrmed by turning of starch iodide paper more
4 + OH−
4 + OH
(at alkaline pH) (8)
Based on the obtained persulfate ion and OH• concentrations,
predominant radical species at different pH levels can be conﬁrmed
320 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327
Fig. 7. Effect of operating pH on the extent of degradation.
•− at pH 3; SO4
•− and OH• at pH 7; OH• at pH 10.5). These
observations are in agreement with the ESR results reported by
Norman et al.  who stated that, as the pH increases, the conver-
sion of SO4
•− to HO• becomes signiﬁcant. As far as degradation is
concerned, similar results were observed in earlier ﬁndings where
decolorization efﬁciency of the dye Direct Red 16 was highest at
its original solution pH of 6 as compared to alkaline pH . Lee
et al.  investigated effect of initial pH on the extent of perﬂuo-
rooctanoic acid (PFOA) degradation using microwave-induced KPS
activation over the pH range 2–11.5 and reported that the pseudo-
ﬁrst-order rate constant enhanced 7.4 times at pH 2 compared to pH
11.5. In 0.5 h of reaction time, the PFOA decomposition efﬁciency
increased from 0% (pH 11.5) to 34.5% (pH 2). However, no signiﬁcant
difference in the decomposition efﬁciency was observed when the
solution pH was between 3.6 (k = 0.33 h−1) and 8.8 (k = 0.32 h−1).
In our study, initial pH value of RY 145 solution was 5.8 which
remained unchanged after the addition of KPS. However, at the end
of 280 s of microwave irradiation, the ﬁnal pH value decreased to
2.5, possibly due to the protons released in the reaction when SO4
radicals react with water . Hence KPS treated efﬂuent (which is
toward acidic range) shows probable commercial utilization in the
neutralization of cotton textile industry wastewater, which is alka-
line in nature. Similarly, optimum pH for speciﬁc pollutant under
question can be established using laboratory scale characterization
studies for its probable reuse.
3.7. Effect of carbonate and sulphate anions at various loadings
The textile efﬂuent generated from reactive dyeing is rich in
inorganic salts like sodium sulphate (SO4
2− anion) and sodium car-
2− anion). Sodium sulfate is used for enhanced pickup
of dye on cotton, whereas sodium carbonate provides alkaline pH
for covalent ﬁxation of the dye on cotton . There is no detailed
study reported on addition of SO4
2− and CO3
2− anions to the dye
solution over a broad range of their concentrations. These condi-
tions were simulated in the present work by separately adding
these salts over the concentration range of 1–100 mM/L each to
the 50 mg/L dye solution. The data on dye degradation is given in
Table 5 along with corresponding change in the pH for each case.
Under optimized operating conditions, the control sample resulted
Fig. 8. OH radical conﬁrmation by starch iodide paper as a function of solution
pH (For interpretation of the references to color in this ﬁgure legend, the reader is
referred to the web version of this article.).
in 82.0% degradation after 120 s of microwave irradiation. In the
case of SO4
2− anions the degradation enhanced to 88.1% at 1 mM/L
but decreased to 83.5% with further increase in concentration to
100 mM/L. The pH decreased to 2.7 from initial 5.8.
This could be due to the fact that the added SO4
2− anions are
converted to stronger SO4
•− radicals either by abstraction of proton
from the solution (as shown in Eq. (9)) or due to interaction of SO4
anions with hydroxyl radical (as shown in Eq. (10)).
4 + H+
4 + OH
4 + OH−
On the other hand, the presence of CO3
2− anions decreased
slightly the extent of degradation from 78.5% (1 mM/L) to 76.1%
(100 mM/L) and increased the pH of the dye solution. The hin-
dered degradation can be explained in terms of the change in the
solution pH as observed earlier for the effect of pH on dye degra-
dation in section 3.6. The addition of Na2CO3 (100 mM/L) gave the
solution pH 11.5. The solution will have CO3
2− anions at the solu-
tion pH > 7.5, which could initiate decomposition of SO4
N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 321
The extent of RY 145 degradation in the presence of SO4
Solution Concentration (mM/L) Degradation (%) Initial pH Change in pH
after 120 s of MW
RY 145 – 82.0 5.8 3.1
ions 1 88.1 5.5 2.7
10 85.6 6.1 3.4
100 83.5 6.3 4.4
ions 1 78.5 9.6 8.4
10 77.2 11 10.7
100 76.1 11.7 11.6
Comparison of dye degradation efﬁciencies of KPS and H2O2 under microwave.
Time (s) Dye degradation (%)
2.2 mM 150 mM 300 mM
0 12.3 0.0 0.0
40 20.1 0.4 1.6
80 55.6 1.2 2.5
120 81.8 1.8 3.8
160 91.3 2.2 4.8
200 94.7 2.7 5.9
240 96.9 3.3 6.7
280 99.1 3.8 8.0
thereby rapidly transforming them into weaker SO4
2−  anions
as shown in the Eq. (11).
4 + CO2−
3 → SO2−
4 + CO
Thus, as the solution pH approaches to 11.5, CO3
become more dominant and act as a scavenger for SO4
hindering the dye degradation. Same was the observation by Zhang
et al.  for the presence of CO3
2− anions, which caused a sig-
niﬁcant inhibition effect on photocatalytic degradation of the dye
Reactive Brilliant Red X-3B followed by the kinetic k drop from
0.0203 to 0.0062 min−1.
3.8. Comparative study of microwave assisted H2O2 process
50 mL of 50 mg/L RY 145 dye concentration was treated with
hydrogen peroxide (150 and 300 mM) under microwave power of
400 W for 280 s at original solution pH and the results are given in
Table 6. Only 4 and 8% dye degradation was observed with these
dosages of H2O2. Thus, peroxide was found inefﬁcient in degrading
RY 145. The probable reason for poor efﬁciency of peroxide has been
explained by Saien et al.  by relating decomposition of peroxide
at higher temperature simply into water and oxygen. Yang et al. 
have also reported similar inactivity of peroxide in degrading Acid
Orange 7 azo dye at higher temperature, which has been attributed
to extremely large O O bond energy of H2O2 (213.3 kJ/mol) with
respect to persulfate (140 kJ/mol).
3.9. Comparative performance of conventional and microwave
heating for KPS activation
Exact reasons behind microwave enhanced degradation of RY
145 among thermal and other non-thermal effects of microwave
were the main cause of the debate. In order to investigate the same,
the conventional heating was used for KPS activation. Heating the
RY 145 dye solution alone (in absence of KPS) even up to 90 ◦C had
no degradative effect indicating that the dye does not decompose
on its own even at elevated temperature, which was the obser-
vation with microwave system also. The optimized 2.2 mM KPS
was dissolved in 50 mL of 50 mg/L aqueous solution of RY 145 and
heated at constant temperature of 70 ◦C, on a conventional ther-
mostatically controlled water bath for the period up to 60 min. The
selection of temperature 70 ◦C was corresponding to the tempera-
ture 68 ◦C achieved by the optimized microwave power of 400 W.
The calorimetric measurement was carried out to calculate actual
power dissipation in the conventional heating and was observed
to be as low as 217.6 W for power of 1000 W, which indicates an
energy efﬁciency of 21.8% (Appendix A). The extent of RY 145 degra-
dation using conventional heating in the presence of 2.2 mM KPS
is shown in Fig. 9 from which it may be observed that after 60 min
(3600 s) reaction at 40 ◦C, the extent of degradation was as poor as
17.3%, which enhanced to 61.2% as the temperature was raised to
50 ◦C. The rate of RY 145 degradation was faster at 90 ◦C as com-
pared to 70 ◦C, which can be correlated to the temperatures of the
microwave system at the power output of 800 and 400 W, respec-
tively. In terms of the reaction time, conventional system needed
50 min (at 70 ◦C) and 15 min (at 90 ◦C) to reach 90% degradation.
The extent of COD removal for microwave system was 66.5%,
which was relatively higher with respect to 60% COD removal for
conventional heating process (at the end of 60 min for temperature
of 70 ◦C).
Since electric energy represents a major portion of the operating
costs, simple ﬁgures-of-merit based on electric energy consump-
tion could be very useful to compare electrical efﬁciency of the two
processes. The appropriate ﬁgure-of-merit is the electrical energy
per order (EE0), deﬁned as the number of kW h of the electrical
energy required for reducing the concentration of a pollutant by
1 order of magnitude in 1 m3 of the contaminated water . EE0
(kW h/m3) can be calculated from the following equation (Eq. (12)):
P × t × 1000
V × 60 × log (Ci/Cf )
where P is the power (kW) used in the process, t is the irradi-
ation time (min), V is the volume (L) of the RY 145 solution in
the reactor along with Ci and Cf as the initial and ﬁnal concentra-
tions. The electric energy (EEO, kW h/m3) required for degradation
of 50 mg/L of RY 145 in microwave and conventional processes at
same (≈70 ◦C) temperature was 305.7 and 10989 kW h/m3, respec-
tively (Appendix B). EEO can be correlated to the energy costs.
Considering the cost of electricity and the time taken for the pro-
cess, the microwave treatment will prove highly cost effective,
which save up to 10683.32 kW h/m3 of electrical energy (Appendix
The comparative analysis of the processes in terms of the
ﬁrst-order kinetic rate constant, percentage degradation and time
required for degradation, presented in Table 7, indicates that
microwave irradiation was much more efﬁcient compared to the
322 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327
Fig. 9. Effect of conventional heating on the extent of dye degradation.
Kinetic rate constant, percentage degradation and time requirement for RY 145 degradation at 50 mg/L concentration.
Process k × 10−2
) Degradation (%) Reaction time R2
Microwave 1.65 99.1 280 0.9769
Conventionalheating 0.3 98.5 3600 0.9921
3.10. Activation energy and frequency factor
Activation energy (Ea) is an important parameter associated
with the changes in kinetics and thermodynamics of the two heat-
ing processes, microwave and conventional, (Arrhenius equation
(Eq. (13)). Ea is the threshold energy that the reactant(s) must
acquire before reaching the transition state. Ea values for both the
processes were calculated by plotting ln (k) versus 1/T and the slope
Ea/T was obtained by the linear regression. This ratio was multiplied
by the universal gas constant (R = 8314 −1 kJ/mol−1) to obtain the Ea
k = Aexp −
The comparative Ea plots for microwave and conventional pro-
cess have been given in Fig. 10. The high degree of linearity
(R2 > 0.90) between the two variables yields reliable estimations
of the activation energy (Ea). It is well known that a reaction with
large activation energy needs much more energy to reach the tran-
sition state. The obtained activation energy values were 41.4 and
44.4 kJ/mol for microwave and conventional process, respectively
(Appendix C). It can be realized that the higher temperature was
favorable to enhance the degradation and the activation energy was
reduced by using the microwave irradiation.
The microwave irradiation decreases the threshold energy
needed for the oxidation of the pollutants but does not alter the
nature of the reaction. The term, e−Ea/RT, in the Arrhenius equation
is known as the frequency factor and it represents the molecules
with energy equal to or greater than the activation energy. Fre-
quency factor (4.5 × 10−7) for the microwave assisted degradation
was calculated (Appendix D) and found to be 2.6 times higher than
that for conventional heating (1.7 × 10−7). This conﬁrms that the
microwaves facilitate the reaction kinetics since nearly 2.6 times
more number of molecules get involved in microwave process
whose energy is equal to or greater than Ea. This proves that the
non-thermal effects of microwave are responsible to increase the
reactant mobility and diffusion exchange rate between the pollu-
tant and sulfate radicals. This study demonstrated that microwave
assisted system can signiﬁcantly lower the energy barrier with 2.6
times increase in the reaction rate.
The weakening of the chromophoric bond of the RY 145 dye by
the combination of the polarization effect and the thermal effect
(that enhanced production of SO4
•− radicals) improved the overall
rate of degradation. The present work has, thus, established that
microwave greatly raises the overall process efﬁciency as an alter-
native energy source of KPS activation, which drastically shortened
the reaction time (to 4 min) by almost 15 times.
3.11. Elucidation of probable dye degradation mechanism using
Probable degradation mechanism for RY 145 is given in Fig. 11.
The dye RY 145 and its KPS-microwave assisted degradation prod-
ucts were analyzed by FTIR (Fig. 12). It can be seen from Fig. 12
that the spectrum for RY 145 has a sharp characteristic peak
at 1655 cm−1 conﬁrming presence of N N chromophore ,
which completely disappeared in the spectrum of degraded sam-
ple, conﬁrming oxidative cleavage of the chromophoric group by
attack of SO4
•− radicals. Similarly, the peak at 1256 cm−1 for C O
stretching is absent in treated sample as a result of deprotona-
tion of SO2CH2CH2OSO3Na group in the dye molecule. Besides,
the dye spectrum shows a hump in the region of 3330–3550 cm−1
indicating the presence of primary, secondary and tertiary amines
(aliphatic as well as aromatic) in the structure, which diminished
in spectra of KPS-microwave treated dye indicating further degra-
The intensity of peak at 1050.62 cm−1, for meta/ortho substi-
tuted aromatic SO3 bending vibration in the dye decreased after
the degradation, indicating desulfonation of the dye by the attack
•− radicals . The peak at 1496.99 cm−1 in the dye spec-
N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 323
Fig. 10. Comparative Ea plots between microwave and conventional process.
trum corresponds to conjugated cyclic system of C N for stretching
vibration, which disappeared on treatment. This indicates attack on
polar C N bonds linked to the benzene ring, conﬁrming the cleav-
age of recalcitrant triazine ring. Similarly, a peak between 720 and
600 cm−1 in the dye molecule is observed for the C S bond of sul-
fonate group linked to the benzene ring that was absent in treated
spectrum as a result of C S bond cleavage to form organic acids
with or without hydroxyl group, sulfate, ammonium and nitrate
Fig. 11. Probable degradation mechanism of RY 145 using FTIR.
324 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327
Fig. 12. FTIR spectra of RY 145 and RY 145 subjected to microwave degradation.
According to the literature  and our study elaborated in
section 3.6, decrease in pH of treated sample represents the forma-
tion of acid as one of dye degradation by-product, which usually
is further oxidized to carboxylic acids and CO2. The results clearly
establish the fragmentation of the RY 145 dye into smaller compo-
nents by KPS oxidation.
Two different KPS activation approaches based on microwave
and conventional heating have been observed to follow pseudo-
ﬁrst-order kinetics for degradation of azo dye RY 145. The
microwave approach was much faster at 400 W (68 ◦C, 4 min) with
respect to conventional heating (70 ◦C, 60 min), which has been
attributed to additional non-thermal effects of microwave respon-
sible for polarization of dye molecules that enhance the kinetics
of the system by keeping the related thermodynamics unchanged.
Signiﬁcance of this non-thermal effect was further proved by high
synergistic factor of 40 as against 15 for conventional heating. There
was extensive reduction in activation energy of the system and
the electrical energy consumption. Effect of addition of SO4
has enhanced the degradation whereas CO3
2− ions hindered the
KPS activity. Probable degradation mechanism of RY 145 has been
proposed which was conﬁrmed by FTIR analysis and supported by
enhanced COD removal. The present lab-scale study leads to a path-
way for use of KPS activation by designing a pilot-scale reactor for
degradation of various harmful pollutants.
Authors would like to express their gratitude to University
Grants Commission, New Delhi, India, for funding of this project
under Major Research Project grant. Namata Patil is also thankful
for fellowship under UGC-SAP.
Appendix A. Calorimetric energy efﬁciency calculations
Actual power dissipation (W) = mCp dT/dt (A.1)
where Cp is the heat capacity of the solvent (J kg−1 K−1), m is the
mass of solvent (kg), dT is the temperature difference between
the initial temperature and the ﬁnal temperature after a speciﬁc
reaction time (K), and dt is the time (s).
Calorimetric energy efﬁciency can then be calculated as follows
(using Eq. (A.2)):
Calorimetric energy efﬁciency
Actual power dissipation (from Eq. (A.1))
Electric power supplied to the system
A1. Calorimetric energy efﬁciency calculations for microwave
Basis: 50 mL (0.050 L) of aqueous RY 145 solution.
Power of microwave used = 400W
Initial temperature of the system = 31◦
Final temperature of the microwave system after240s = 68◦
Temperature difference (dT) = (68 − 31) = 37◦
C = 310K
dT/dt = 310/240 = 1.292
Actual power dissipation (W) = mCp dT/dt
∴ Actual power dissipation (W) = (0.050 × 4185 × 1.292)
Actual power dissipation (W) = 270.35W
N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 325
∴ Calorimetric energy efﬁciency = 270.35/400
Calorimetric energy efﬁciency = 67.6%for microwave system
A.2. Calorimetric energy efﬁciency calculations for conventional
Basis: 50 mL (0.050 L) of aqueous RY 145 solution
Power of conventional heating used = 1000W
Initial temperature of the system = 31◦
Final temperature of the microwave system after300s = 70◦
Temperature difference(dT) = (70 − 31) = 39◦
C = 312K
dT/dt = 312/300 = 1.04
Actual power dissipation (W) = mCp dT/dt
∴ Actual power dissipation (W) = (0.050 × 4185 × 1.04)
Actual power dissipation (W) = 217.62W
∴ Calorimetric energy efﬁciency = 217.62/1000
Calorimetric energy efﬁciency = 21.8% for conventional heating
Appendix B. Electrical energy per order (EEO) calculations
Sample calculation for the electrical energy per order (EEO), for
complete degradation using microwave system is as follows (using
P × t × 1000
V × 60 × log(Ci/Cf)
B.1. EEO calculations for microwave system
Basis: 50 mL (0.050 L) of aqueous RY 145 solution
Power of microwave used = 400W = 0.4kW
Time requuired for degradation = 4.7 min = 280s
Initial concentration ofRY145 = Ci = 50mg/L
Final concentration ofRY145 = Cf = 0.45mg/L
∴ log(50/0.45) = 2.05
0.4 × 4.7 × 1000
0.050 × 60 × 2.05
EE0 = 305.69kW h/m3
) = (A)
Similar EEO calculation has been performed for the conventional
B.2. Electrical energy calculations for conventional heating
Basis: 50 mL (0.050 L) of aqueous RY 145 solution
Power of water bath used = 500W = 0.5kW
Time required for degradation = 600 min = 3600s
Initial concentration of RY145 = Ci = 50mg/L
Final concentration of RY145 = Cf = 0.75mg/L
∴ log(50/0.75) = 1.82
1 × 60 × 1000
0.050 × 60 × 1.82
EE0 = 10989.01(kW h/m3
) = (B)
B.3. Net electrical energy saved (using Eq. (A.4))
Net electrical energy saved = (A) − (B) (A.4)
where A corresponds to Electrical energy needed for conven-
tional heating.where B corresponds to Electrical energy needed for
Net electrical energy saved = (10989.01 − 305.69)kW h/m3
Net electrical energy saved = 10683.32kW h/m3
Appendix C. Activation energy calculations
Activation energy can be calculated using Eq. (A.5). The slope
values for calculating C.1 and C.2 are applied from Fig. 9 in the
Activation energy = slope × universal gas constant(R) = slope
C.1. Activation energy calculations for microwave system
Activation energy = 4983 × 8314
∴ Activation energy of the microwaves ystem = 41400J/mol
Appendix D. Frequency factor calculations
Frequency factor: it can be calculated using the equation given
below (Eq. (A.6)):
k = Ae−
D.1. Frequency factor calculations for microwave system
Activation energy for microwave system = 41.400 J/mol
Temperature at 400 W power at the end of 280 s = 68 ◦C = 341 K
RT = e
∴ Frequency factor for microwave system = 4.5 × 10−7
326 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327
D.2. Frequency factor calculations for conventional heating
Activation energy for conventional heating = 44.400 J/mol
Temperature at the end of reaction = 70 ◦C = 343 K
RT = e
∴ Frequency factor for conventional heating = 1.7 × 10−7
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