Degradation of a cationic dye (Rhodamine 6G) using hydrodynamic cavitation coupled with other oxidative agents: Reaction mechanism and pathway

Article abstract of DOI:10.1016/j.ultsonch.2016.05.028

In the present study, decolorization and mineralization of a cationic dye, Rhodamine 6G (Rh6G), has been carried out using hydrodynamic cavitation (HC). Two cavitating devices such as slit and circular venturi were used to generate cavitation in HC reactor. The process parameters such as initial dye concentration, solution pH, operating inlet pressure, and cavitation number were investigated in detail to evaluate their effects on the decolorization efficiency of Rh6G. Decolorization of Rh6G was marginally higher in the case of slit venturi as compared to circular venturi. The kinetic study showed that decolorization and mineralization of the dye fitted first-order kinetics. The loadings of H₂O₂ and ozone have been optimized to intensify the decolorization and mineralization efficiency of Rh6G using HC. Nearly 54% decolorization of Rh6G was obtained using a combination of HC and H₂O₂ at a dye to H₂O₂ molar ratio of 1:30. The combination of HC with ozone resulted in 100% decolorization in almost 5-10 min of processing time depending upon the initial dye concentration. To quantify the extent of mineralization, total organic carbon (TOC) analysis was also performed using various processes and almost 84% TOC removal was obtained using HC coupled with 3 g/h of ozone. The degradation by-products formed during the complete degradation process were qualitatively identified by liquid chromatography-mass spectrometry (LC-MS) and a detailed degradation pathway has been proposed.

Full text of DOI:10.1016/j.ultsonch.2016.05.028

Accepted Manuscript
Degradation of a cationic dye (Rhodamine 6G) using hydrodynamic cavitation
coupled with other oxidative agents: reaction mechanism and pathway
Sunil Rajoriya, Swapnil Bargole, Virendra Kumar Saharan
PII:
S1350-4177(16)30170-5
DOI:
http://dx.doi.org/10.1016/j.ultsonch.2016.05.028
ULTSON 3238
Reference:
Ultrasonics Sonochemistry
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Accepted Date:
1 April 2016
15 May 2016
18 May 2016
Please cite this article as: S. Rajoriya, S. Bargole, V.K. Saharan, Degradation of a cationic dye (Rhodamine 6G)
using hydrodynamic cavitation coupled with other oxidative agents: reaction mechanism and pathway, Ultrasonics
Sonochemistry (2016), doi: http://dx.doi.org/10.1016/j.ultsonch.2016.05.028
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Degradation of a cationic dye (Rhodamine 6G) using hydrodynamic cavitation
coupled with other oxidative agents: reaction mechanism and pathway
Sunil Rajoriya, Swapnil Bargole, Virendra Kumar Saharan*
Chemical Engineering Department, Malaviya National Institute of Technology, Jaipur-302017,
India
*Author to whom correspondence should be addressed.
Email: viru8uict@gmail.com; Tel: +91-141-2713244
1

Abstract
In the present study, decolorization and mineralization of a cationic dye, Rhodamine 6G (Rh6G),
has been carried out using hydrodynamic cavitation (HC). Two cavitating devices such as slit
and circular venturi were used to generate cavitation in HC reactor. The process parameters such
as initial dye concentration, solution pH, operating inlet pressure, and cavitation number were
investigated in detail to evaluate their effects on the decolorization efficiency of Rh6G.
Decolorization of Rh6G was marginally higher in the case of slit venturi as compared to circular
venturi. The kinetic study showed that decolorization and mineralization of the dye fitted first-
order kinetics. The loadings of H2O2 and ozone have been optimized to intensify the
decolorization and mineralization efficiency of Rh6G using HC. Nearly 54% decolorization of
Rh6G was obtained using a combination of HC and H2O2 at a dye to H2O2 molar ratio of 1:30.
The combination of HC with ozone resulted in 100% decolorization in almost 5 to 10 min of
processing time depending upon the initial dye concentration. To quantify the extent of
mineralization, total organic carbon (TOC) analysis was also performed using various processes
and almost 84% TOC removal was obtained using HC coupled with 3g/h of ozone. The
degradation by-products formed during the complete degradation process were qualitatively
identified by liquid chromatography-mass spectrometry (LC-MS) and a detailed degradation
pathway has been proposed.
Keywords: Rhodamine 6G; Hydrodynamic Cavitation; Circular and Slit venturi; Intensification;
Ozonation; Hydrogen peroxide
2

1. Introduction
In recent years, the discharge of wastewater coming from various textile industries is becoming a
major environmental problem due to loading of the enormous quantity of different toxic dyes. It
is estimated that nearly 18-20% of the total production of dyes is discharged into the
environment during dyeing process [1-4]. Among most of the toxic dyes, Rhodamine 6G dye is
mostly used as a colorant in the textile industries. It is a non-volatile compound, highly water
soluble, and dark reddish purple in color. It has some other applications in the biochemistry
research laboratories where this dye is used as a diagnostic tool to detect the antigen in a liquid
sample. Rh6G is also utilized for fluorescence microscopy and fluorescence correlation
spectroscopy in the area of biotechnology [5]. Water containing Rhodamine dyes causes
irritation of the skin, eyes and respiratory system of human beings. It has also medically verified
that drinking water contaminated with Rhodamine dyes is highly carcinogenic and poisonous to
living organisms [6-7]. Hence, these toxic pollutants must be treated before releasing them into
the environment. These pollutants contain larger complex molecules (i.e. aromatic compounds)
which cannot be treated efficiently by biological methods and other conventional methods such
as adsorption, coagulation by chemical agents and membrane filtration. These methods are very
costly and cannot mineralize these pollutants and rather separate them physically from the
wastewater which causes secondary load on the environment. But over the past years, advanced
oxidation processes (AOPs) such as Fenton, Photo-Fenton, photocatalytic process, and cavitation
have been employed successfully by many authors [8-15] for the treatment of such pollutant
molecules. In last decade, cavitation has also emerged as an effective oxidative process to
degrade the organic pollutants present in wastewater. The phenomena of cavitation may be
described in three sections as 1) nucleation: formation of cavities within the liquid; 2) growth of
bubbles/cavities under the fluctuating surrounding pressure field; (3) subsequent collapse of
3

cavities: violent implosion of vapor/gas filled cavities. In recent years, most of the researchers
have focused on the use of acoustic cavitation in the area of wastewater treatment. However, it
was observed that utilization of acoustic cavitation on an industrial scale creates the difficulties
due to the higher maintenance costs and low energy efficiency. In recent years, hydrodynamic
cavitation (HC) is found to be an alternative to replace acoustic cavitation as an efficient
technology for the mineralization of various organic pollutants from wastewater due to its ease of
operation on an industrial level. In HC, cavitation can be produced by pressure variation in a
flowing liquid through cavitating devices. The cavities are formed at the throat of a venturi or
orifice where pressure of the liquid falls below the vapor pressure of the liquid. In the
downstream side of the cavitating device, when pressure recovers these cavities implode
violently resulting high temperatures of 5000K and pressures of 1000atm [16-20]. Under such
•
extreme conditions, water molecules are thermally dissociated into H and OH radicals [14].
•
•
These generated OH radicals (non-selective reactive species) inside the cavitating devices are
capable of oxidizing the organic substances and thus mineralize these compounds [17-19, 21].
There have been many studies reported in literature for the treatment of Rh6G using different
approaches mainly using photo-catalysis [5, 22-25]. In this regard, Bokhale et al. [22] studied
sono-catalytic and sono-photocatalytic degradation of Rh6G and reported that the maximum
degradation as 63.3% was achieved using sono-photocatalytic (US/UV) in the presence of TiO2
(4g/L). Lutic et al. [26] have investigated the photocatalytic degradation of Rh6G and reported
that the maximum removal of 94% was achieved using photoactive ZnO after 90min of
treatment. Asiri et al. [27] have studied the photo-degradation of Rh6G using nano-sized TiO2 in
the presence of irradiation sources. It was observed that nanoparticles of TiO2 with smaller size
were found to be more efficient in the photo-degradation of Rh6G due to their large surface area.
4

Though, the degradation of Rh6G was investigated in details using various processes at a
laboratory scale but still these technologies do not find any application on an industrial scale due
to their lower efficiency and higher cost for the treatment of real industrial effluent [25].
Moreover these studies have reported the degradation of Rh6G only which was evaluated by
observing a reduction in color intensity of the dye and complete mineralization was not studied
using these methods. The mineralization of Rh6G using HC has not been reported yet in the
literature. Hence, the objectives of this study were to evaluate the ability of HC to mineralize the
Rh6G in aqueous solution and to intensify the mineralization of Rh6G by combining HC with
other oxidizing agents to establish HC on an industrial scale for the treatment of real effluent.
The effects of operating parameters like solution pH, initial concentration of dye, inlet pressure,
and cavitation number on the decolorization of Rh6G were investigated. The effect of oxidation
additives such as hydrogen peroxide (H2O2) and ozone has also been studied since these two
oxidizing agents are mostly used for the treatment of an industrial effluent. An attempt has been
made to identify the degradation by-products using LC-MS study and subsequently a possible
degradation pathway of Rh6G has been proposed. Furthermore the mechanism of degradation of
Rh6G using HC was also verified by observing the amount of H2O2 produced during HC.
2. Experimental details
2.1 Reagents
Rhodamine 6G dye (molecular weight: 479.02g/mol; chemical formula: C28H31N2O3Cl) was
procured from Central Drug House Pvt. Ltd (India). The chemical structure of Rh6G is shown
in Table 1. Double distilled water (prepared freshly in the laboratory via distillation unit) was
used to make the solution of Rh6G. Hydrogen Peroxide (30%, w/v) of AR grade was purchased
5

from Lobachemie (India). Solution pH was adjusted using NaOH and H2SO4. All reagents as
received from suppliers were used for the experiments without any further purification.
2.2 Hydrodynamic cavitation reactor setup
Hydrodynamic cavitation reactor set-up used in the present work is shown in Fig. 1. HC reactor
set-up consists: (1) a storage tank with 20L capacity, (2) a high pressure piston pump driven by a
motor (power rating 2.2 kW) with a maximum discharge pressure of 30, (3) flow control valves
(V1–V4), and pressure gauges (P1, P2), (4) a flow meter, (5) cavitating device (slit or circular
venturi) accommodated with flanges. Pipes used in HC reactor have an internal diameter of 19
mm. The suction side of a pump is connected to the bottom of a storage tank. The discharge from
the pump branches into main line and bypass line. The main line consists of a cavitating device
and flow rate of water in the main line was controlled by regulating rpm of motor through
variable frequency drive (VFD) and additionally a bypass line is also provided to control the
flow in main line. Pressure gauges (P1 and P2) are provided in main line to check the fluid
pressure. The dimensions of circular and slit venturi used in this work are given in Table 2.
Schematic view of circular and slit venturi is shown in Fig.2. In this work, dimensions of the
cavitating device selected were recommended by Bashir et al. [28], and Kuldeep and Saharan
[29]. They have studied the effect of different geometrical parameters on the cavitation behavior
inside a cavitating devices using CFD (Computational Fluid Dynamic) study and offered the
optimized parameters of a cavitating device to achieve best cavitational activity.
2.3 Experimental Procedure
Decolorization of Rh6G has been carried out using HC at various conditions with a fixed
aqueous Rh6G solution volume of 6L. All the experiments were performed for 120min and
6

samples were taken from the storage tank at a fixed interval of time for further analysis. During
all the experiments, temperature of the solution (30 2°C) was kept constant and was maintained
by circulating cooling water through the cooling jacket. Initially, effect of initial concentration of
dye on the decolorization rate was studied in the range of 10-50 ppm using slit venturi. Then,
experiments were conducted over the solution pH range of 2.0 to 12.0 to optimize solution pH.
To optimize the cavitating devices, the effect of inlet pressure and cavitation number on the
decolorization rate of Rh6G has been studied at an inlet pressure from 3 to 11 bar using slit and
circular venturi. Two additional experiments were carried out to validate the degradation
mechanism of Rh6G using HC. In the first experiment, effect of radical scavenger was studied
by adding tert-butanol (1g/L) in the system to investigate the scavenging action on •OH radicals.
In the second experiment, doubled distilled water was circulated through HC at an optimized
inlet pressure to determine the concentration of H2O2 produced in the system. During this
experiment, samples were taken at a regular time of interval (15min) and then concentration of
H2O2 was measured. In order to enhance the decolorization of Rh6G, effect of process
intensifying additives (H2O2 and ozone) in combination with HC were also investigated. Five
different molar ratios of concentration of Rh6G to H2O2 as1:10, 1:20, 1:30, 1:40 and 1:50 were
selected to study the combined effect. Effect of H2O2 on the decolorization rate of Rh6G was also
investigated individually i.e. without HC in the present work. The experiments were conducted
using a combination of HC and ozone at various feed rate of ozone from 1 to 7g/h. Ozone was
directly injected at the throat of a venturi thereby exposing ozone directly to the cavitational
effect. Ozone was generated by an ozone generator (maximum feed rate of ozone: 10g/h, make:
Eltech Engineers, India). Initially, Ozonator was calibrated in terms of feed rate of oxygen to
acquire the desired mass flow rate of ozone.
7

2.4 Analytical procedure
Concentration of Rh6G was measured by UV/Vis-Spectrophotometer (Shimadzu-1800) at the
maximum wavelength (λmax) of 540nm. Firstly, calibration chart was prepared for the known
concentrations of Rh6G in the range of 2-12ppm to calculate the concentration of unknown
sample during experiments. A pH meter (Hanna Instruments, USA) was used to determine the
pH of solution throughout the experiment. The degradation by-products of Rh6G during HC
process were confirmed by liquid chromatography–mass spectroscopy (LC-MS) (make: Xevo
G2-S Q-Tof, Waters, USA) using C18 column (4.6mm x 250mm). A mixture of Acetonitrile and
phosphate buffer (45:55) was used as a mobile phase at a flow rate of 0.2 mL/min for 10 min.
Total organic carbon (TOC) content of dye solution was measured using TOC analyzer (make:
GE InnovOx). The amount of H2O2 produced during HC was measured by titrating sample
against potassium permanganate (0.001M KMnO4). Double distilled water was circulated
through HC reactor and 100mL of water sample was collected at a regular interval of 15 min.
4mL of 4M sulfuric acid was added in the collected samples and titrated against 0.001M KMnO4
until the purple color appeared. The end point of titration is colorless to purple. In the present
work, all experiments were repeated at least twice to evaluate the repeatability of the observed
data and the experimental errors were found to be within 3% of the reported average values.
3. Results and discussion
3.1 Effect of initial Rh6G concentration
The effect of initial Rh6G concentration on the decolorization rate was investigated using HC.
The degradation of dye molecules using HC involve mainly two mechanisms i.e. first, the
oxidation of dye molecules by hydroxyl free radicals (•OH) and second, thermal pyrolysis
8

(combustion) of dye molecules which is entrapped inside the cavities [2, 30]. Both mechanisms
can occur at the multiple locations such as at the interface of cavity, in the bulk liquid medium,
and in the core of cavity. Experiments were conducted with various concentrations of dye (10,
20, 30, 40, 50ppm), at an inlet pressure of 5bar and solution pH of 10. The observed results are
shown in Fig. 3 and Table 3. Under the effect of HC, Rh6G gets dissociated into the
•
intermediates which further react with OH radicals and ultimately degrade into final products
(CO2, H2O etc.). The following reaction takes place in HC:
Rh6G + •OH
degradation products
(1)
(2)
ꢁ
ꢆ
ꢀ ꢂꢃꢄꢅ
ꢁ ꢆ
ꢈ= kꢈ Rh6G ꢁ∙ OHꢆ
−
ꢀꢇ
A pseudo first order kinetic was used as a means to evaluate and correlate the observed data. The
concentration of •OH radicals is found to be constant during the decolorization process under the
cavitation effect and also it is highly reactive and non-selective and therefore its concentration
effect can be neglected [31]. The degradation kinetics can be represented by Eq.(3).
Ln ꢈꢁꢂꢃꢄꢅꢆ = kꢈxꢈtꢈꢈꢈꢈ
(3)
ꢉ
ꢁꢂꢃꢄꢅꢆ
Where, [Rh6G]0 and [Rh6G] are initial concentration and concentration of Rh6G (ppm) at time t
respectively, k is the decolorization rate constant (min-1), and t is the processing time (min). It
can be seen from Fig. 3a that percent decolorization of Rh6G decreases from 32.06% to 8.37%
with an increase in initial concentration from 10 to 50ppm. The kinetic analysis shows that
degradation of Rh6G using HC fitted proposed pseudo-first order kinetics (shown in Fig. 3b). It
is observed that pseudo first order rate constant decreases from 2.9 x 10-3min-1 to 0.7 x 10-3min-1
for an increase in the concentration from 10ppm to 50ppm as shown in Table 3. The obtained
lower decolorization rate at initial concentration of Rh6G higher than 10ppm can be attributed to
•
the insufficient generation of OH radicals to degrade the dye, which shows that the relevant
9

proportion of total amount of •OH radicals and dye molecules is a significant factor [10]. Since,
decolorization rate constant was found to be maximum at 10ppm; therefore all the remaining
experiments were carried out at 10ppm concentration of Rh6G. In this study, the observed results
can be validated with some other similar literature reports [2, 15, 32-34]. Saharan et al. [2] have
investigated the degradation of reactive red 120 dye (RR120) using HC and reported that
degradation kinetics of RR120 using HC follows first order reaction kinetic. Patil et al. [32] have
studied the effects of initial concentration on the degradation of imidacloprid using HC and
reported a decrease in the degradation rate with an increase in the initial concentration. Wang et
al. [33] have investigated the degradation of reactive brilliant red K-2BP using HC. They have
observed that, % degradation decreased from 94.2% to 24.7% with an increase in the
concentration from 10 to 50mg/L.
3.2 Quantification of chemical effects (dosimetry study)
In order to validate the degradation mechanism which is driven by the attack of •OH radicals on
the pollutant molecules, an experiment was carried out using HC with the addition of tert-butanol
•
(a type of •OH scavenger). Tert-butanol is capable of neutralizing OH radicals and it reacts with
•
•OH and H radicals and gives less reactive tert-butanol radical. It can be seen from Fig. 3 that
decolorization rate of Rh6G decreased from 3.0 x 10-3 to 2.1 x 10-3 min-1 with the addition of
tert-butanol and 22% decolorization was obtained after 120 min of treatment as against 32%
obtained without tert-butanol at the same experimental condition using HC. The obtained result
•
shows that OH radicals were mainly responsible for the decolorization efficiency. Tert-butanol
•
produces tert-butanol radicals on reacting with OH radicals (in Eq. 4) which is less reactive
species as compared to •OH radicals and therefore it reduces the degradation rate.
10

C4H10O + •OH
C4H9O•+ H2O
(4)
Tert-butanol can also react with •H radicals and it gives n-butanol radical as shown in Eq. (5)
•
CH3CH2CH2 CH2 + H2O
(CH3)3COH + •H
(5)
Similar results have also been reported in literature by Yao et al. [35] for the degradation of
parathion using ultrasonic irradiation. They have observed that free radical reactions are
predominant reactions occurs during the degradation of parathion using ultrasonication.
•
Additionally, one more experiment was performed to check the occurrence of OH
radicals during HC by measuring the quantity of H2O2 produced at 5bar inlet pressure. It can be
seen from Fig. 4 that the amount of H2O2 formed with respect to time increases due to the
•
recombination of OH radicals which are generated under cavitating conditions. This study
further proves that •OH radicals are formed during HC process.
3.3 Effect of solution pH
The solution pH is an important parameter for the decolorization rate of Rh6G using HC. The
effect of solution pH was studied by varying solution pH over the range of 2.0-12.0. Experiments
were carried out at an inlet pressure of 5bar for 120min of treatment time. The observed results
are shown in Fig. 5 and Table 4. It can be seen that extent of decolorization and first order rate
constant increases with an increase in the solution pH from 2.0 up to an optimum value of 10.0.
The decolorization rate decreases after pH 10. These results clearly show that decolorization rate
was higher at basic medium than that obtained under acidic conditions. It has been reported that
Rh6G molecule becomes hydrophobic under basic condition (i.e. at higher pH than its pKa value
of 6.13) as the free electron pair on the amine nitrogen (primary amine group) transfer charges
from amine nitrogen to aromatic rings. Under such condition Rh6G exists in its molecular state
11

and therefore Rh6G locate itself at the cavity-water interface due to its hydrophobic nature. The
•
concentration of OH radicals is maximum at the cavity-water interface and therefore higher
decolorization rate was obtained under basic condition. On the other hand, Rh6G molecule
becomes hydrophilic in nature under acidic pH due to the protonation of nitrogen in the
secondary amine group of the xanthene ring. Due to its hydrophilic nature, it remains in bulk of
the solution where concentration of •OH radicals is minimum. Therefore, a lower decolorization
rate was achieved under acidic conditions. The pH of solution was kept 10.0 for all the remaining
sets of experiments. A few related results can be obtained in the literature where basic conditions
are found to be superior for the degradation of pollutant molecules using cavitation. Eslami and
Eslami [23] have investigated the degradation of Rh6G at different values of pH in the range 3.0-
9.0 using ultrasonic cavitation in presence of H2O2 and zinc oxide and reported that higher
degradation rate constant (2.8 x 10-2 min-1) was achieved at high pH of 9.0. Wang et al. [36] have
studied the degradation of alachlor over the range of pH from 2 to 12 using HC and reported that
the rate constant increases from 4.45 x 10-2 to 5.77 x 10-2 min-1 with an increase in pH value
from 2.0 to 12.0.
3.4 Effect of inlet pressure, cavitation number and geometrical parameters
In HC reactor, cavitation number is a dimensionless parameter used to characterize the
cavitational intensity inside the cavitating device. It is defined as the ratio of pressure drop
between the throat and extreme downstream section of the cavitating device to the kinetic head at
the throat. It is given by the following Eq. (6):
P − P
ꢌ
ꢋ
ꢊꢎ
Cꢊ =ꢈ
ꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈ(6)
1
2
ρvꢍꢌ
12

Where, P2 is the fully recovered downstream pressure, Pv is the vapor pressure of the liquid, v0 is
the velocity at the throat of the cavitating device, and ρ is the density of the liquid. As the inlet
pressure increases, velocity at the throat increases which in turn reduces the cavitation number.
More number of cavities are generated under ideal condition (Cv ≤ 1), but in many cases cavities
can get generate at Cv>1 due to the existence of little-dissolved gases and suspended particles
[37]. It has been reported that the cavitation number in the range of 0.1 to 0.2 gives better
degradation efficiency [2, 15, 31].
In order to examine the effects of inlet pressure and cavitation number on the decolorization rate
of Rh6G, experiments were performed by varying inlet pressure in the range 3-11 at an optimum
solution pH of 10.0 using circular and slit venturi. The obtained results are shown in Table 5. It
has been observed that the decolorization rate increases with an increase in the inlet pressure up
to an optimum value of 5bar for both the venturies and then it decreases beyond an optimum. An
increase in decolorization rate with an increase in inlet pressure or decrease in cavitation number
may be attributed to the enhanced cavitational intensity due to more number of cavities are
•
formed at higher inlet pressure leading to the generation of more OH radicals. At higher inlet
pressure beyond an optimum, it is estimated that choked cavitation takes place reducing the
cavitation intensity due to partial collapse of cavities leading to lower degradation rate [38-41].
Also, inactive (unstable) cavities are formed in the cavitating devices at higher inlet pressure
above an optimum value which reduce the degradation rate of pollutant molecules. An optimum
inlet pressure for circular and slit venturi was found to be 5bar with respective cavitation number
as 0.11 and 0.07. Almost 32% decolorization with first order rate constant of 3.0 x 10-3 min-1 was
observed in the case of slit venturi while about 29% decolorization with first order rate constant
of 2.5 x 10-3 min-1 was found using circular venturi in 120min at an optimum inlet pressure and
13

cavitation number. The possible reason behind the higher decolorization using slit venturi can be
attributed to the fact that velocity of liquid in case of slit venturi is higher as compared to circular
venturi at the same operating inlet pressure. Therefore, number of passes through slit venturi
would be higher as compared to circular venturi. Because of higher number of passes through the
cavitating device, pollutant molecules experience longer exposure of the cavitating conditions
resulting into higher decolorization rate of pollutant molecules [38]. In addition to this, there is
another parameter, known as α (throat perimeter to its cross-sectional area ratio) which also
affects the cavitational activity. A higher value of α provides better cavitational intensity. In the
present work, slit venturi has a higher value of α (2.63) as compared to circular venturi (2.00)
and therefore gives better degradation efficiency than circular venturi. It has been reported that
higher ratio of throat perimeter to throat cross sectional area maximizes the frequency of
turbulence which causes rapid oscillation of a cavity and thereby resulting into higher cavity
collapse pressure [16, 42-43]. Hence, optimized slit venturi has been used for all the remaining
sets of experiments. The cavitation number, velocity, and flow rate through the system with
respect to operating pressure are given in Table 5. Similar results have been reported by Saharan
et al. [15]. They have investigated degradation of Orange-G dye using three cavitating devices
such as circular, slit and orifice and reported that degradation of Orange-G was higher in the case
of slit venturi as compared to circular venturi and orifice.
3.5 Combination of hydrodynamic cavitation and H2O2
The main aim of this study is to generate the additional amount of •OH radicals by adding H2O2
in HC reactor. Hydrogen peroxide due to its high oxidation capacity (1.78eV) can be used as an
oxidizing agent to enhance the decolorization efficiency of dye and also it can easily be
dissociated into more reactive •OH radicals under the effect of HC. The effect of different molar
14

ratios of dye to H2O2 concentration (1:10, 1:20, 1:30, 1:40, and 1:50) on the decolorization rate
of Rh6G has been investigated using slit venturi at 5bar inlet pressure and pH 10.0. The obtained
results are depicted in Fig. 6 and Table 6. It has been observed that decolorization rate constant
of Rh6G was increased from 3.0 x 10-3 min-1 using HC alone to 21.6 x 10-3 min-1 when HC is
combined with H2O2 at an optimum molar ratio of dye to H2O2 (1:30). There was no further
increase in the decolorization rate of dye beyond the molar ratio of 1:30 (dye concentration:
H2O2). Hence, molar ratio of 1:30 has been selected as an optimum loading at which 53.72%
decolorization was achieved. The obtained higher decolorization rate is due to the continuous
dissociation of H2O2 into •OH radicals under HC effect. Beyond an optimal molar ratio of dye to
•
H2O2, reduction in the degradation rate is due to the recombination of OH radicals and the
scavenging of •OH radicals by H2O2 itself [32-33, 38, 44-46]. The following reactions (Eqs 7-14)
can occur during the degradation of pollutant molecules using the combined HC and H2O2
process [39].
H2O2
H2O
2●OH
(7)
(8)
•H + •OH
HC
•OH + •OH
H2O2
(9)
•
HO2 + H2O
•OH + H2O2
(10)
(11)
(12)
(13)
(14)
•
•OH + HO2
H2O + O2
•OH + H2O + O2
•
HO2 + H2O2
Rh6G + •OH
Rh6G + H2O2
CO2 + H2O + degradation products
CO2 + H2O + degradation products
15

It can be concluded that H2O2 should be added at an optimal molar ratio to obtain best
degradation efficiency of pollutant molecules. Similar trends were reported in the literature for
the degradation of different organic pollutants [2, 21, 47-49]. Saharan et al. [2] have studied the
degradation of reactive red 120 using hydrodynamic cavitation in the presence of H2O2. They
have observed that nearly 60% decolorization was obtained using HC alone at a solution pH of
2.0 and an operating inlet pressure of 5bar. Whereas, decolorization has risen to about 100% by
adding hydrogen peroxide at an optimum molar ratio of reactive red 120 to H2O2 as 1:60. Gore at
al. [47] have observed almost complete decolorization of reactive orange 4 at 1:30 molar ratio of
Orange-G to H2O2 and no further enhancement in the degradation rate was achieved at higher
ratio because of the scavenging effect of H2O2.
The study of H2O2 alone was also conducted in a stirred tank mixer at an optimum molar ratio of
dye to H2O2 (1:30) for 2h of operation time. A 10 ppm solution of Rh6G was taken in a stirred
tank and solution pH was kept constant at 10.0. Normal stirring has been used to ensure complete
mixing of H2O2. It has been observed that almost 17% decolorization with a rate constant of 2.10
× 10−3 min−1 was obtained after 2h of treatment time and thus it can be concluded that normal
•
stirring is incapable to dissociate the H2O2 into OH radicals and thereby reducing degradation
efficiency.
To compute the efficiency of hybrid technique (HC + H2O2) as compared to the individual
methods, synergetic coefficient has been determined. First order rate constant increased
significantly from 2.10 × 10−3 min−1 in the case of only H2O2 to 21.6 × 10−3 min−1 for the
combination of HC and H2O2. The synergetic coefficient based on decolorization rate constant
can be calculated for the hybrid technique by the following Eq. (15):
16

ꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈSynergeticꢈcꢏeꢐꢑicient =ꢈꢒꢒ
(15)
(
)
ꢓꢔꢕꢓꢖꢗꢖ
ꢘꢒ
ꢓꢔ
ꢓꢖꢗꢖ
= 21.6 x 10–3/ (3.0 x 10–3 + 2.1 x 10–3)
= 4.24
The obtained synergetic coefficient as 4.24 demonstrates that the synergetic effect of combined
method is better than the individual methods. This is due to enhanced generation of •OH radicals
by the dissociation of H2O2 in presence of HC leading to the higher decolorization efficiency of
Rh6G as compared to that obtained using only H2O2.
3.6 Combination of Hydrodynamic cavitation and ozone
Ozone (O3) is a strong oxidizing agent which is capable in degrading the organic pollutants due
to its high oxidation capacity (2.08eV). The combination of HC and ozone can be used to
intensify the degradation efficiency due to enhanced mass transfer of ozone molecules from gas
phase to the bulk solution to react with organic pollutants under the cavitating conditions.
Moreover, under the cavitational effect, ozone is easily decomposed yielding molecular O2 and
atomic oxygen which reacts with water molecules to form •OH radicals. The following reactions
(Eqs. 16-21) can occur during the process of HC coupled with ozone [39, 47].
HC
O3
O2 + O (3P)
●OH + •H
(16)
(17)
(18)
(19)
HC
H2O
O (3P) + H2O
●OH + ●OH
2●OH
H2O2
17

●OH + Rh6G
O3 + Rh6G
CO2 + H2O + degradation products
CO2 + H2O + degradation products
(20)
(21)
In order to study the effect of combination of HC with ozonation on the decolorization rate of
Rh6G, experiments were performed at various loadings of ozone (1 to 7g/h). A higher
concentration of Rh6G (50ppm) was taken to study the decolorization as well as mineralization
rate using HC combined with ozone, since at lower concentration i.e. 10ppm decolorization rate
was very fast and also, the corresponding initial TOC was only 5ppm. Hence, there may be
chances of higher experimental error. In order to avoid the experimental error and to observe the
distinguishable reduction in TOC using TOC analyzer, a higher concentration of Rh6G was
selected to analyze TOC with respect to time. The value of rate constant for mineralization and
decolorization of Rh6G are shown in Table 7. It can be observed that the decolorization of Rh6G
is 32.06% using HC alone after 120 min and further it is possible to achieve 100% decolorization
in just 10min by combining HC with ozone. As shown in Table 7, decolorization rate constant of
Rh6G increases with an increase in the feed rate of ozone. When ozone feed rate increases, more
•OH radicals may exist for the degradation of organic pollutants due to the dissociation of ozone
under cavitating conditions. But mineralization study shows that reduction in TOC was found to
be maximum at the ozone feed rate of 3g/h. The mineralization rate constant increases with an
increase in ozone feed rate till 3g/h and further decreases. Maximum 73.19% reduction in TOC
with rate constant of 10.9 x 10-3min-1 was obtained in 120min of operation at ozone feed rate of 3
g/h. Almost 10% reduction in TOC was obtained using only HC for the same treatment time.
Hence, it can be concluded that optimum loading of ozone flow rate is 3 g/h for the degradation
of Rh6G using hybrid method (HC + Ozone).
18

At this optimum flow rate of ozone, an experiment was conducted using only ozonation to
evaluate the synergetic effect of combined process. During experimentation using only ozone,
desired volume of Rh6G solution (6L) was filled in the storage tank and ozone was directly
injected into the Rh6G solution. Then, samples were withdrawn at regular time of interval by
sampling valve which was connected at bottom of the storage tank. It can be seen from Table 7
that around 41% TOC reduction was achieved at the ozone flow rate of 3g/h in 120 min of
operation.
From Table 7, it can also be observed that mineralization rate constant (10.9 × 10−3 min−1) for the
HC combined with ozone was higher as compared to that obtained for the case of only ozone
(5.3 × 10−3 min−1) and only HC (1.6 x 10-3 min-1). Synergetic coefficient based on the
mineralization rate constant can be calculated for the combined method (HC + ozone) to examine
the efficiency of this method using following Eq. (22):
ꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈSynergeticꢈcꢏeꢐꢑicient = ꢈꢒꢒ
(22)
(
)
ꢓꢔꢕꢗ
ꢙ
ꢘꢒ
ꢓꢔ
ꢗ
ꢙ
= 10.9 x 10–3/ (1.6 x 10–3 + 5.3 x 10–3)
= 1.58
The combination of HC and ozone indicates synergetic effect over individual method and
observed value of the synergetic coefficient was 1.58. Similar results related to the effect of
ozonation coupled with HC on the degradation rate of organic pollutants such as reactive orange
4 [47] and Triazophos [50] are reported in literature. Gore et al. [47] have investigated the effect
of HC coupled with ozone for the mineralization of reactive orange 4 dye solution at various feed
rate of ozone from 1 to 8 g/h. They have observed that about 76.25 % reduction in TOC obtained
at optimal ozone feed rate of 3 g/h while only 14.67 % was obtained using HC alone in 60 min of
19

treatment time. They have also reported synergetic effect for the combined process (HC +
ozone).
3.7 Mineralization study
Total organic carbon (TOC) test is mostly used as an effective procedure to quantify the organic
strength of wastewater. It is well known that degradation by-products generate throughout the
oxidation of cationic dyes, and some of them could be more toxic and carcinogenic than their
parent molecules. Hence, it is necessary to investigate the extent of mineralization of Rh6G by
applying various processes such as HC alone, HC+H2O2, and HC+Ozone. Rh6G solution of
50ppm (corresponding TOC as 22ppm) was taken to study the effect of various processes on
mineralization. The kinetics of mineralization of Rh6G is depicted in Fig. 7. This figure clearly
shows that mineralization of dye fitted first order kinetic data for all the above mentioned
processes. The obtained results using various processes reveal that the combination of HC and
ozone was more efficient as compared to other processes. The rate constants of mineralization
process and reduction in TOC (%) using various processes are summarized in Table 7. First
experiment was conducted using only HC to mineralize the dye at an optimum inlet pressure of
5bar and pH of 10 using slit venturi. It has been observed that reduction in TOC was mostly
occurred in the initial 60 min and overall 10% reduction in TOC was obtained. After that, slower
mineralization rate was observed which indicates that the primary by-products were not
mineralized easily into CO2 and H2O. After that mineralization study of Rh6G was performed
using HC+H2O2 at an optimum molar ratio of dye to H2O2 (1:30µM). Almost 16% TOC was
reduced after 120min of operation. In another experiment, same amount of H2O2 was added after
every 15min in order to check the effect of higher dosage of H2O2 on the extent of
20

mineralization. It was observed that there was no significant change in TOC at the higher loading
•
of H2O2 due to the scavenging action of OH radicals by the excess amount of H2O2. The
obtained results indicate that degradation byproducts are hardly degraded using these two
processes. Further experiments were carried out using combined HC and ozone at the optimized
feed rate of ozone (3g/h). It has been observed that TOC reduction was significantly increased to
73.19% within 120 min of treatment as compared to that obtained in HC and HC+H2O2. A high
mineralization rate constant of 10.9 x 10-3 min-1 was obtained using HC+Ozone. Thereafter,
further increase in the flow rate of ozone (5g/h) decreases rate of mineralization (10.3 x 10-3 min-
1). At higher feed rate of ozone, scavenging action of un-dissociated ozone molecules under
cavitating conditions may decrease mineralization rate of organic pollutants. Hence, optimum
feed rate of ozone can be considered as 3g/h.
In order to check the extent of mineralization at higher inlet pressure (10, 15, and 20bar),
experiments were also carried out at higher pressure using HC, HC + H2O2, and HC + Ozone. It
was observed that in case of HC and HC + H2O2, there was no significant change in rate of
mineralization at higher pressure. But in the case of HC combined with ozone, mineralization
rate constant of Rh6G increased from 10.9 x 10-3 min-1 at an inlet pressure of 5bar to
22.3 x 10-3 min-1 at 20bar (shown in Fig. 7). It may be due to the enhancement of mass transfer
rate between ozone molecules and bulk liquid solution due to high turbulent and shear created at
higher operating inlet pressure. It may also possible that ozone molecules get easily dissociated
into •OH radicals at higher operating inlet pressure of HC.
From obtained results, it is clear that the combination of HC with ozone contributes more in
mineralization as well as enhances degradation efficiency of Rh6G.
21

3.8 Degradation products and degradation pathway
In order to identify possible degradation by-products of Rh6G, liquid chromatography-mass
spectrometry (LC-MS) analysis was performed on the samples collected during the combined
process of HC and ozone at an optimum feed rate of 3g/h. Since, degradation mechanism of
•
Rh6G is the oxidation of Rh6G by the generated OH radicals in all the three methods i.e. HC,
HC+H2O2, and HC+ Ozone and therefore samples treated through HC+Ozone were taken for the
LC-MS analysis. Moreover, similar mass spectra were observed for the samples treated using
these three different methods. Samples were taken at fixed time interval (after every 15min) and
then analyzed by LC-MS immediately. This is a method to understand the reaction mechanism of
Rh6G during HC. Rh6G was detected merely at m/z = 443 as shown in Fig. 8a. The degradation
by-products after 30min of treatment using HC+Ozone were detected at m/z = 415, 387, 359,
345, 312, 298, 286, 284, 272, and 258 as shown in Fig. 8b. Based on the m/z value of by-
products observed during the course of treatment, a possible degradation pathway of Rh6G has
been proposed in Fig. 9. It was found that the degradation of Rh6G follows oxidation mechanism
which is initiated by the attack of •OH radicals on the Rh6G and then subsequent reaction with
the intermediates under cavitating conditions. LC-MS analysis shows the formation of many
aromatic by-products. It should be noticed that in the N-de-ethylation process, the hypsochromic
shift of the absorption band is a characteristic phenomenon [50, 51]. In path 1 (Fig. 9), the
esterification reaction takes place to convert the ester group into carboxylic acid giving a
•
possible by-product (m/z = 415). In HC, the generated OH radicals may prefer to attack the
central carbon atom with a high negative density charge to destruct the molecule structure of
Rh6G and further degraded by N-de-ethylation process. The two major by-products with m/z as
415 and 387 obtained from mass spectra can be known as N-ethyl, N´-ethyl Rhodamine, and N-
22

ethyl Rhodamine respectively. N-de-ethylated by-products such as N-ethyl Rhodamine (m/z =
387) was degraded into a possible by-products which has a m/z value of 359 and then it can be
break into some other by-products with m/z values of 345 and 286. These formed by-products
were further degraded into the m/z values of 272 and 258. In path 2, N-de-ethylation process as
well as carboxylation process occurs which gives possible degradation by-product corresponding
to the m/z value of 312. After carboxylation process, formed by-products further degraded into
the other possible degradation by-products with m/z values of 298 and 284 by the dealkylation
reactions. The obtained by-products were further oxidized into 3,4-dihydroxybenzoic acid (m/z =
155), adipic acid (m/z = 146), glutaric acid (m/z = 132), benzoic acid (m/z = 122), butane-1,3-
diol (m/z = 90) and ethane 1,2-dioic acid (m/z = 90). Similar byproducts were also observed by
other authors [51, 52] for the photocatalytic degradation of Rhodamine B. The similarity
between the observed and reported data shows that degradation mechanism using HC is also
initiated and propagated by the attack of •OH radicals on the pollutant molecules.
4. Conclusions
The present study has shown the efficiency of HC for the decolorization and mineralization rate
of Rh6G. Hybrid methods were found to be more efficient as compared to an individual process.
At optimum parameters i.e. solution pH, inlet pressure and cavitation number, decolorization
efficiency of Rh6G can be intensified in the presence of different oxidizing agent. The main
conclusions drawn from present study can be summarized as follows:
23

1. The decolorization rate of Rh6G was influenced by the initial concentration of dye,
solution pH, inlet pressure, and cavitation number using HC. The decolorization of Rh6G
using HC process follows a first-order kinetic.
•
2. The concentration of H2O2 produced in the system confirms that the OH radicals are
formed during cavitation. The addition of radical scavenger such as tert-butanol decreases
decolorization rate and thus indicating that decolorization of Rh6G using HC is
dominated by the attack of •OH radicals.
3. The obtained results revealed that decolorization rate of Rh6G increased with an increase
in inlet pressure till 5bar for both the circular and slit venturi.
4. Between the circular and slit venturi (used as cavitating devices in HC), slit venturi gives
higher decolorization rate due to higher perimeter.
5. The decolorization rate of dye was also found to be strongly dependent on the solution
pH. The maximum decolorization of Rh6G using HC alone was found to be 32.06% at
10.0 pH and pressure of 5bar.
6. It was observed that HC coupled with H2O2 enhances the decolorization efficiency of
Rh6G giving 53.72% decolorization at an optimum molar ratio of dye to H2O2 (1:30µM)
as compared 32.06% obtained using HC alone.
7. The combination of ozone with HC gives the complete decolorization of Rh6G in 10 min
of treatment time at 5bar inlet pressure. Almost 84% TOC was removed at higher inlet
pressure (20bar) using HC coupled with ozone (3g/h) in 120min.
8. LC-MS was employed for identification of the degradation intermediates. It was observed
that the degradation of Rh6G into the smaller intermediates and end products occurred
•
through carboxylation and dealkylation process and initiated by the attack of OH
24

radicals. A possible degradation pathway and reaction mechanism of Rh6G was
proposed.
Overall, the obtained results revealed that the combination of HC with ozone can be a better
technique to completely mineralize Rh6G and other similar dye effluents at large scale of operation.
Acknowledgements
Dr. Virendra Kumar Saharan would like to thank DST, GOI for providing financial support
under Inspire Faculty Fellowship (DST/INSPIRE Faculty Award/2013/IFA13-ENG49).
25

Appendix
Sample calculation of cavitation number for slit venturi at an optimum 5bar inlet pressure:
Downstream pressure (p2) = 101325 Pa
Vapor pressure of water at 300C (pv) = 4242.14 Pa
Volumetric flow rate at 5bar pressure (V) = 592 LPH = 1.64 x10-4 m3/s
Dimension of the throat of the venturi: W=3.14mm, H=1mm, L=1mm
Flow area: A0 = 3.14x10-6 m2
Velocity at the throat of venturi:
V0= V/A0 = 1.64 x 10-4/3.14 x 10-6 = 52.23 m/s
Cavitation number is calculated as:
P − P
ꢌ
ꢋ
ꢊꢎ
Cꢊ =ꢈ
1
2
ρvꢍꢌ
= (101325 – 4242.14) / (0.5 x 1000 x (52.23)2)
= 0.07
26

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