Removal of rhodamine B dye by plasma jet oxidation process

 Communications in Physics, Vol.31, No. 1 (2021), pp. 95-102
 DOI:10.15625/0868-3166/15558
 REMOVAL OF RHODAMINE B DYE BY PLASMA JET OXIDATION PROCESS
 LE THI QUYNH XUAN1,2,+, TRAN HONG QUAN1,2,+, TRINH THU HA3
 AND DAO NGUYEN THUAN1,†
1Laboratory of Plasma Technology, Institute of Materials Sciences (IMS), Vietnam Academy of
 Science and Technology (VAST), Hanoi, Vietnam
2Graduate University of Science and Technology (GUST), VAST
3Laboratory of Environmental and Bioorganic Chemistry, Institute of Chemistry, VAST
 E-mail: †thuandn@ims.vast.ac.vn
 +These authors contributed equally to this work.
 Received 2 October 2020
 Accepted for publication 23 November 2020
 Published 05 April 2021
 Abstract. Recently, nonthermal atmospheric pressure plasma has been developed as a novel tool
 in removal of water pollutants. Rhodamine B dye, widely used in textiles and biology, is toxic to
 both humans and animals, hence removing residual Rhodamine B in solution is necessary. In this
 work, we set up a cold plasma jet system and used it to effectively remove Rhodamine B in the
 solution. We showed that the main oxidation substance responsible to remove Rhodamine B is the
 hydroxyl radical (•OH). By studying the effects of – the hydrogen peroxide (H2O2) concentration
 in the initial and later produced in the solution, the plasma power, and the Ar gas flow speed – on
 the dye remove rate, we identified 2 main reaction pathways to generate hydroxyl radical (•OH).
 Both of these reaction pathways involved high-energy electrons interacting with water and O2 in
 the solution and in the ambient air. Our work provided important information to understand the
 mechanism of dye removal by cold plasma treatment.
 Keywords: cold plasma, plasma jet, oxidation, organic dye removal, Rhodamine B.
 Classification numbers: 52.77.-j; 82.30.Lp; 81.65.Mq; 82.80.Dx.
 I. INTRODUCTION
 The organic dyes, have been widely used in the textile industry for decades, are known as
 toxic and polluted compounds because of their high persistence in soil and water resources. Rho-
 damine B dye, belonging to the class of Xanthene, is used as a colorant in textiles and a staining
 ©2021 Vietnam Academy of Science and Technology
96 REMOVAL OF RHODAMINE B DYE BY PLASMA JET OXIDATION PROCESS
in biology. It can cause skin irritation, eyes and respiratory infection, chronic and neuro toxicity
in both humans and animals, thus demanding in removing residue Rhodamine B in solution is in-
evitable [1]. Recent works on removing Rhodamine B pay a great attention to advanced oxidation
technologies, including photocatalysis [2] and plasma treatment [3–5].
 Non-thermal atmospheric pressure plasma (ATP) has been developed as a novel tool in re-
moval of water pollutants [6] owing to its advantages of working in daily life conditions, quick and
energy efficient. Atmospheric plasma produces a great amount of reactive oxygen species (ROS),
high energy particles (electrons and ions). Various ROS species as hydroxyl radical (•OH), hy-
drogen peroxide (H2O2), ozone (O3), hydroperoxyl radical (•HO2) generated by plasma processes
can acts as strong oxidizers to remove organic dyes [7–13]. Although many researches have sug-
gested •OH as one of the main oxidation substances, its generation pathways [8–12] as well as
its conversion reactions [8, 9], during cold plasma interaction with solution, are complicated and
remain unclear.
 In this work, a plasma jet system was set up for removal of organic dye and demonstrated
its operating in removing Rhodamine B. We quantified the amount of •OH and H2O2 generating
during plasma processes by UV-VIS absorption and chemical measurements, and identified •OH as
the key oxidation substance. By studying effect of – the hydrogen peroxide (H2O2) concentration
in the initial and later generating in the solution, the plasma power, and the Ar gas flow speed – on
the dye remove rate, we proposed there were only 2 main reaction pathways to generate •OH.
II. EXPERIMENT
II.1. Materials
 Rhodamine B (≥95%) was purchased from Sigma Aldrich (SKU: 6626) and hydrogen
peroxide H2O2 (30%) was purchased from Xilong Scientific Co., Ltd. Rhodamine B was diluted
in double distilled water to a concentration of 50 mg/l and 10ml-sample was used for each plasma
treatment experiment.
II.1.1. Setup of the plasma jet system for removal of Rhodamine B
 Fig. 1. Experiment setup of the plasma jet system for removal of Rhodamine B.
 LE THI QUYNH XUAN et al. 97
 The schematic representation of the plasma jet system for removal of Rhodamine B is
shown in Fig. 1. A high frequency, high voltage power supply was applied between a needle
electrode (inside a 5cm-length quartz tube) and a copper plate (below a beaker containing dye
solution) to generate non-thermal plasma [14]. The Argon gas was fed through the needle with a
gas flow rate of 4-8 L/minute. A stable plasma jet with a length of 1.5–2 cm could be achieved
at output voltage ∼3–6 kV and oscillating frequency ∼ 55 KHz. Distance from the plasma head
to the Rhodamine B solution was 0.5 cm. A magnetic stirrer was used to stir the Rhodamine B
solution at 450 rpm to ensure even plasma treatment. A high voltage probe and a micro current
probe were connected to a Tektronix TBS1154 oscilloscope to measure the applied voltage and the
discharged current, respectively. We integrated the multiply of the discharged current and voltage
over a duty cycle to obtain a Lissajous plot and used it to calculate dissipated power of the plasma
system [15].
II.1.2. UV-VIS absorption spectroscopy
 UV-VIS absorption measurements were performed with a quartz cuvette with an optical
path length of 10 mm and a sample volume of at least 300 µl. A single-beam Ocean Optics system
included a DH-2000-BAL light source, a cuvette holder and a USB4000 spectrometer connected
by optical fibers. Typically, 300-500 µl solution after plasma treatment was loaded to the quartz
cuvette, which was put in the cuvette holder. The beam, which was generated by the UV-VIS
light source, passed through the sample and the transmitted light was collected and driven into
the spectrometer. The Oceanview software were used to control the spectrometer and acquire data
with an integration time of 500 ms. Three continuous scans were acquired and averaged to give
an absorption spectrum in all cases.
II.1.3. Detection and quantification of H2O2
 We used two methods to detect and measure H2O2concentration: (1) monitoring absorp-
tion peak at 230 nm and using Beer-Lambert’s law to calculate; (2) monitoring decolorization of
KMnO4 0.1N when titrating to H2O2 solution to estimate the amount of H2O2 (1 ml KMnO4 0.1N
corresponds to 1.7 mg H2O2).
III. RESULTS AND DISSCUSION
III.1. Removal of Rhodamine B by plasma jet treatment
III.1.1. Decolorization and degradation of Rhodamine B
 Figure 2 demonstrates that 10 ml of Rhodamine B (50 mg/l) (Fig. 2(A)) can be removed
by 24 minutes plasma treatment. The solution after plasma treatment was completely transparent
and no residue remained on the inner side of the beaker (Fig. 2(B)). Fig. 2(C) shows absorption
spectra of the Rhodamine B solution in a broad range from 220 nm to 700 nm, before (0 minute)
and after (24 minutes) plasma treatment. The absorption peak of Rhodamine B solution at 554
nm is due to n → π* transitions of (C = O,C = N) that yield the color of the solution. And the
absorption peak at 259 nm is due to π → π* transitions of the aromatic ring groups that yield the
aromatic content of the solution [7, 8]. We observed only H2O2 absorption peak at 230 nm from
Rhodamine B solution after 24 minutes plasma treatment, that confirmed the dye was completely
removed. Because the absorption peak at 259 nm of the aromatic ring could be interfered by H2O2
98 REMOVAL OF RHODAMINE B DYE BY PLASMA JET OXIDATION PROCESS
absorption peak at 230 nm, we used the absorption peak at 554 nm to monitor the Rhodamine B
removal effect of our plasma treatment.
 Fig. 2. Picture of 10 ml Rhodamine B (50mg/l) solution (A) before plasma treatment
 and (B) after 24-minutes plasma treatment. (C) Absorption spectra of Rhodamine B (0
 min, black curve) before and (24 min, red curve) after plasma treatment. The inset is the
 chemical structure of Rhodamine B.
III.1.2. Removal rate of Rhodamine B by plasma jet
 Fig. 3. (A) Dependence of the absorption band of Rhodamine B at 554 nm on plasma
 treatment time. (B) Percentage of the remained dye (black dots) as a function of plasma
 treatment time and the fitting (red curve) by a first-order reaction kinetic.
 To determine the efficiency of removing Rhodamine B by plasma treatment, we measured
the absorption band of Rhodamine B around 554 nm at different plasma treatment time (Fig. 3(A)).
 LE THI QUYNH XUAN et al. 99
We estimated the percentage of the remained dye by the ratio of the integrated area in absorption
spectrum (from 450 nm to 600 nm) between a particular treatment time and before plasma treat-
ment (Fig 3(B)). Typically, the dependence of the concentration of remained dye on the oxidation
treatment time can be described by a first-order reaction kinetic [10] as:
 −kt
 C = C0e , (1)
with C0 is the initial dye concentration, C is the dye concentration after t minutes treatment time
and k is the rate constant. Fitting our data in figure 3B with an exponential decay gave k = 0.18
with the coefficient of determination R2 = 0.98.
III.2. Mechanism of the Rhodamine B degradation process by plasma jet
III.2.1. Hydroxyl radical (•OH) is the crucial oxidation substance
 In most oxidation process of organic dye by plasma discharge, the hydroxyl radical (•OH)
has been shown to be one of the main oxidation factors [3,9,13,16]. The hydroxyl radical (•OH),
which is the neutral form of the hydroxide ion (OH−), is highly reactive and consequently short-
lived. Various reaction pathways to generate •OH by plasma processes have been reported [8-12]
and can be summarized as following:
 ∗e + H2O −→ ·OH + ·H + e, (2)
 3O3 + H2O −→ 2 · OH + 4O2, (3)
 H2O2 + 2O3 −→ 2 · OH + 3O2, (4)
 H2O2 + ∗e −→ 2 · OH + e, (5)
with ∗e is the high-energy electron.
 Those •OH generation pathways can be divided in 2 groups: directly (Eq. (2)) when the
high energy electrons from plasma interact with water in the solution; or indirectly (Eqs. (3)–(5))
through intermediate products like Ozone (O3) or hydrogen peroxide (H2O2). The O3 in Eqs. (3)
and (4) are produced by these reactions:
 ∗e + O2 −→ 2∗O + e, (6)
 ∗O + O2 −→ O3, (7)
with ∗O is the high-energy oxygen atom.
 Meanwhile, parts of the hydroxyl radical (•OH) can be converted to hydrogen peroxide
(H2O2) and hydroperoxyl radical (•HO2) at different reaction rates [8, 11, 17] as:
 k1 9 −1 −1
 ·OH + ·OH −→ H2O2,k1 = 5.5∗10 M s , (8)
 k2 7 −1 −1
 H2O2 + ·OH −→ · HO2 + H2O,k2 = 2.7∗10 M s . (9)
 Since k1 is two orders of magnitude higher than k2, reaction in Eq. (8) happens at much
slower rate than reaction in Eq. (9). However, •HO2 radical is much less reactive than •OH, hence
increasing •HO2 should result in a negligible contribution to the dye removal rate [8].
III.2.2. Hydrogen peroxide (H2O2) generation by plasma jet
 Although the hydroxyl radical (•OH) has been proved as one of the main oxidation agent
to remove dye in plasma treatment [3, 9, 13, 16], the contribution of each reaction pathway to
generate •OH (Eqs. (2)–(5)) has not yet been determined. Since reactions in Eqs. (4) and (5)
100 REMOVAL OF RHODAMINE B DYE BY PLASMA JET OXIDATION PROCESS
require hydrogen peroxide (H2O2), we quantified amount of H2O2 produced in the solution by
plasma and investigated the effect of H2O2 concentration on the dye removal rate.
 Fig. 4. (A) Hydrogen peroxide (H2O2) concentration detected by 2 different methods
 after several plasma treatment time. (B) Plot of percentage of the remained dye as a
 function of plasma treatment time in 3 different initial H2O2 concentration.
 We employed 2 methods to quantify H2O2 in the solution as described in the Method sec-
tion. The first one used the absorption peak at 230 nm while the second one used titration to
decolorize certain amount of KMnO4. Figure 4A shows a very good agreement between these
2 methods to quantify amount of H2O2 generated in the solution by the plasma jet. The H2O2
concentration increased with plasma treatment time and was saturated at 22 mg/l after 24 min-
utes. The pH of the solution (measured by an ADWA AD11 pH meter with the error ±0.1) also
changed from 7.0 to 4.5 due to the amount of H2O2 present in the solution. These results validated
the reaction in Eq. (8).
 We performed plasma treatment experiments with 3 different concentrations of the initial
hydrogen peroxide (H2O2): 0, 3 and 50 mg/l to study effect of H2O2on the reaction rate. Figure 4B
shows that the reaction rate is independence of the initial H2O2 concentration, hence reactions in
Eqs. (4) and (5) contribute very little to the •OH generation during plasma treatment. These results
agree with previous work by Daneshvar and co-authors, where H2O2 only reacted efficiently with
Rhodamine B under a strong UV radiation (5–30 W/m2) [8]. The UV power measured at a distance
of 5 mm from our plasma nozzle was only 0.3 W/m2.
 By excluding reactions in Eqs. (4) and (5), we can validate the only pathways to generate
the hydroxyl radical (•OH) are reactions in Eqs. (2) and (3). The direct reaction in Eq. (2) requires
only high-energy electron ∗e reacting with water, while the indirect reaction in Eq. (3) requires
Ozone (O3), an intermediate substance produced by ∗e reacting with Oxygen (O2) in ambient air
and inside the solution. To resolve how much each reaction contributes to the amount of •OH
generation during plasma treatment, further works including mixing additional Oxygen gas or
employing an scavenger substance [12] need to be done.
 LE THI QUYNH XUAN et al. 101
 Fig. 5. Comparison of the Rhodamine B removal rate at (A) different plasma powers:
 1.3, 1 and 0.7 W; and (B) different gas flow speed: 4 and 8 L/min.
III.3. Effect of the plasma power and Ar gas flow speed
 To find an optimal condition for Rhodamine B removal by plasma treatment, we studied
the effect of the plasma power and the gas flow speed on the removal rate. Figure 5A shows that
the removal rate increases linearly with the plasma power: the rate constants are 0.24 min−1, 0.18
min−1 and 0.13 min−1 with plasma power of 1.3 W, 1 W and 0.7 W (Table 1), respectively. This
effect also is in agreement with reactions in Eqs. (2) and (3), since both these reaction pathways
require high-energy electron ∗e. The amount of high-energy electron ∗e generated by plasma
jet should be linear with the plasma power. Although higher plasma power will lead to faster dye
removal rate, the heating problem and instability of the power supply will limit the actual operating
power.
 Table 1. Dependence of the percentage of the remained dye after 10 minutes plasma
 treatment, the rate constant and the coefficient of determination R2 versus the plasma
 powers.
 Plasma Percentage of the remained Rate
 power dye after 10 minutes constant R2
 (W) plasma treatment (%) k (min−1)
 1.3 8 0.24 0.99
 1 13 0.18 0.98
 0.7 26 0.13 0.98
 Figure 5(B) shows that the Ar gas flow speed (4 and 8 L/min) does not affect the dye
removal rate. This result is in agreement not only with previous work [18], but also with the
reactions in Eqs. (2) and (3). Both these reactions are independent of the Ar gas flow rate.
102 REMOVAL OF RHODAMINE B DYE BY PLASMA JET OXIDATION PROCESS
IV. CONCLUSION
 We set up a cold plasma jet system for dye removal and demonstrated it effect in removing
Rhodamine B by plasma treatment. 10ml of Rhodamine B (50mg/l) could be removed completely
by 24 minutes plasma treatment with 1 W plasma power. We showed that the main oxidation
substance to remove Rhodamine B was hydroxyl radical (•OH) generated by plasma. By studying
the effect of - the hydrogen peroxide (H2O2) concentration in the initial and later generated in
the solution, the plasma power, and the Ar gas flow speed - on the dye remove rate, we identified
2 main reaction pathways (Eqs (2) and (3)) to generate hydroxyl radical (•OH). Both of these
reaction pathways involved high-energy electrons interacting with water and O2 in the solution
and in the ambient air. Our work provided important information to understand the mechanism of
dye removal by cold plasma treatment.
ACKNOWLEDGMENT
 This research was supported by Institute of Materials Sciences (IMS), Vietnam Academy
of Science and Technology (VAST) under grant number HTCBT.04/20-20, and by Vingroup Joint
Stock Company and supported by the Domestic Master/ PhD Scholarship Programme of Vingroup
Innovation Foundation (VINIF), Vingroup Big Data Institute (VINBIGDATA) for Le Thi Quynh
Xuan.
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