Combination of ozone-based advanced oxidation process and nanobubbles generation toward textile wastewater recovery

Abstract

The intricate nature of various textile manufacturing processes introduces colored dyes, surfactants, and toxic chemicals that have been harmful to ecosystems in recent years. Here, a combination ozone-based advanced oxidation process (AOP) is coupled with a nanobubbles generator for the generation of ozone nanobubbles (NB) utilized the same to treat the primary effluent acquired from textile wastewaters. Here we find several key parameters such as chemical oxygen demand ammonia content (NH3), and total suspended solids indicating a substantial recovery in which the respective percentages of 81.1%, 30.81%, and 41.98%, upon 300 min residence time are achieved. On the other hand, the pH is shifted from 7.93 to 7.46, indicating the generation of hydrogen peroxide (H2O2) due to the termination reaction and the self-reaction of reactive oxygen species (ROS). We propose that the reactive oxygen species can be identified from the negative zeta potential measurement (−22.43 ± 0.34 mV) collected in the final state of treatment. The combined method has successfully generated ozone nanobubbles with 99.94% of size distributed in 216.9 nm. This highlights that enhancement of ozone’s reactivity plays a crucial role in improving the water quality of textile wastewater towards being technologically efficient to date.

Full text

Combination of ozone-based
advanced oxidation process and
nanobubbles generation toward
textile wastewater recovery
Sutrisno Salomo Hutagalung
1
,
2
*, Ande Fudja Rafryanto
3
, Wei Sun
4
,
Nurochma Juliasih
5
, Sri Aditia
5
, Jizhou Jiang
4
, Arramel
3
,
Hermawan Kresno Dipojono
6
, Sri Harjati Suhardi
7
,
Nurul Taufiqu Rochman
2
,
3
and Deddy Kurniadi
1
*
1
Engineering Physics, Faculty of Industrial Technology, Institute of Technology Bandung, Bandung, West
Java, Indonesia,
2
National Research and Innovation Agency, South Tangerang, Banten, Indonesia,
3
Nano
Center Indonesia, South Tangerang, Banten, Indonesia,
4
School of Environmental Ecology and Biological
Engineering, Key Laboratory of Green Chemical Engineering Process of Ministry of Education,
Engineering Research Center of Phosphorus Resources Development and Utilization of Ministry of
Education, Wuhan Institute of Technology, Wuhan, Hubei, China,
5
South Pacific Viscose, Purwakarta,
West Java, Indonesia,
6
Research Center for Nanosciences and Nanotechnology, Institute of Technology
Bandung, Bandung, Indonesia,
7
School of Life Sciences and Technology, Institute of Technology
Bandung, Bandung, West Java, Indonesia
The intricate nature of various textile manufacturing processes introduces colored
dyes, surfactants, and toxic chemicals that have been harmful to ecosystems in
recent years. Here, a combination ozone-based advanced oxidation process
(AOP) is coupled with a nanobubbles generator for the generation of ozone
nanobubbles (NB) utilized the same to treat the primary effluent acquired from
textile wastewaters. Here we find several key parameters such as chemical oxygen
demand ammonia content (NH
3
), and total suspended solids indicating a
substantial recovery in which the respective percentages of 81.1%, 30.81%, and
41.98%, upon 300 min residence time are achieved. On the other hand, the pH is
shifted from 7.93 to 7.46, indicating the generation of hydrogen peroxide (H
2
O
2
)
due to the termination reaction and the self-reaction of reactive oxygen species
(ROS). We propose that the reactive oxygen species can be identified from the
negative zeta potential measurement (−22.43 ± 0.34 mV) collected in the final
state of treatment. The combined method has successfully generated ozone
nanobubbles with 99.94% of size distributed in 216.9 nm. This highlights that
enhancement of ozone’s reactivity plays a crucial role in improving the water
quality of textile wastewater towards being technologically efficient to date.
KEYWORDS
textile wastewater, advanced oxidation process, ozone, reactive oxygen species, water
recovery, nanobubble
1 Introduction
An alarming large production of undesired organic contaminants in the textile industry
has become a long-standing issue that deserves special attention (I. Ali et al., 2012;Lapworth
et al., 2012;Salah et al., 2022;Verma et al., 2022). Moreover, the diverse class of organic
contaminants produced by the textile industry triggered a complexity that led to adverse
effects on the human body. This undesired byproduct is highly harmful to environmental
OPEN ACCESS
EDITED BY
Regis Guegan,
Waseda University, Japan
REVIEWED BY
Firdaus Ali,
University of Indonesia, Indonesia
Saravanan P.,
Indian Institute of Technology Dhanbad,
India
*CORRESPONDENCE
Sutrisno Salomo Hutagalung,
33320006@mahasiswa.itb.ac.id
Deddy Kurniadi,
kurniadi@itb.ac.id
SPECIALTY SECTION
This article was submitted to
Water and Wastewater Management,
a section of the journal
Frontiers in Environmental Science
RECEIVED 31 January 2023
ACCEPTED 06 March 2023
PUBLISHED 21 March 2023
CITATION
Hutagalung SS, Rafryanto AF, Sun W,
Juliasih N, Aditia S, Jiang J,
Arramel, Dipojono HK, Suhardi SH,
Rochman NT and Kurniadi D (2023),
Combination of ozone-based advanced
oxidation process and nanobubbles
generation toward textile
wastewater recovery.
Front. Environ. Sci. 11:1154739.
doi: 10.3389/fenvs.2023.1154739
COPYRIGHT
© 2023 Hutagalung, Rafryanto, Sun,
Juliasih, Aditia, Jiang, Arramel, Dipojono,
Suhardi, Rochman and Kurniadi. This is an
open-access article distributed under the
terms of the Creative Commons
Attribution License (CC BY). The use,
distribution or reproduction in other
forums is permitted, provided the original
author(s) and the copyright owner(s) are
credited and that the original publication
in this journal is cited, in accordance with
accepted academic practice. No use,
distribution or reproduction is permitted
which does not comply with these terms.
Frontiers in Environmental Science frontiersin.org01
TYPE Original Research
PUBLISHED 21 March 2023
DOI 10.3389/fenvs.2023.1154739

conditions (I. Ali et al., 2012;Gendy et al., 2022). Both the large
volume of effluents and the type of chemical compositions, to some
extent, create a tip of an iceberg issue that could eventually grow into
complex problems. In the past years, nearly 1 million tons of toxic
dyeing wastewater were produced annually, of which 200,000 tons
were disposed as effluent without proper treatments (Holkar et al.,
2016;Siddique et al., 2017). Therefore, tactical solutions and
preventive measures in handling such environmental issues are
highly pursued to facilitate safe downstream ecosystems and
sustainable human health.
Several approaches have been introduced within the past
2 decades focusing on the treatment of organics-contaminated
wastewater, and the common method in the current applications
is chemical oxidation (Asghar et al., 2015;Sun et al., 2015). Among
other chemical oxidation processes, AOP is categorized to be a
versatile approach in wastewater technology due to the in situ
formation of reactive oxygen species (ROS) such as hydroxyl
radicals (OH•)(Yan et al., 2022;Yasasve et al., 2022;Aljedaani
et al., 2023).
In the recent decade, different studies have formulated that the
characteristic outcome of ozone-based AOP method is influenced by
the chemical and physical parameters, such as pH of the solution
(Deng and Zhao, 2015;Boczkaj and Fernandes, 2017;Koc-Jurczyk
and Jurczyk, 2019).There is an ongoing discussion that pH is an
essential parameter since it plays an important role during O
3
decomposition process (Temesgen et al., 2017). The ozonation
combined with UV-radiation (O
3
/UV) is an efficient catalytic
wastewater system for the degradation of refractory pollutants. In
principle, the catalytic process is started by ozone photolysis,
followed by ROS production (Emam, 2012). Despite renewed
interest of the AOP operational method in the last two decades
(Zhang et al., 2021), few reports have compared to degrade various
textile wastewaters using different AOP approaches.
Nanobubbles (NB) technology has been widely used to degrade
organic contaminants in solutions (Ali et al., 2023;Aluthgun
Hewage et al., 2021;Wang et al., 2019). The physicochemical
concept of NB technology is slightly comparable to the AOP
method. At a glance, NB facilitates the formation of a physical
barrier such that a contaminant layers can be encapsulated by the
bubble surface (Pan et al., 2021;Suvira and Zhang, 2021;Inoue et al.,
2022;Selihin and Tay, 2022) thereby generating ROS efficiently (Fan
et al., 2023). The formation of ROS plays a vital role in degrading
organic compounds that are present in the wastewater (Mishra et al.,
2017). However, the underlying mechanism in the NB technology is
based on the realization of gas bubbling rather than a chemical
treatment approach. Due to this reason, the ozone-based NB
produced by the NB generator could extend its lifetime in the
water column for a longer time compared to its counterparts
(Hu and Xia, 2018). According to Meegoda and coworkers
(J. Meegoda and Batagoda, 2016), when dissolved ozone interacts
with the pollutants, the treated water remains in the saturated ozone.
Therefore, NB retains its physical properties within the water
column for several months under normal pressure. To the best of
our knowledge, a few combinations of the NB applications and other
AOP are introduced. For example, the work carried out by Tasaki
et al. (2009) indicated that oxygen microbubbles (MB) could be
implemented for the methyl orange removal in the presence of a
photocatalyst under UV irradiation. In this study, we employ a
combination of ozone-based AOP and NB to degrade textile
wastewater samples during 300 min of residence time. To assess
the water quality parameters, such as chemical oxygen demand
(COD), ammonia content (NH
3
), and total suspended solids (TSS)
are compared to find the integrated system performance. The NB
formation is then analyzed using Dynamic Light Scatering (DLS)
method to find the size, stability, and surface charge of our NB.
2 Experimental materials
2.1 Experimental set-up
Our system consists of three processes: ozone injection, NB
treatment, and circulation. Figure 1 depicts a technical schematic of
the NB-AOP procedure’s piping and instrumentation’s diagram.
At a glance, initial textile wastewater was circulated via a
distribution pump and controlled using a rotameter. Textile
wastewater was collected from the Indonesian textile industry
and utilized as received. The initial state of textile wastewater is
indicated in Table 1. The water stream was then forced via a venturi
tube, introducing ozone created by an ozone generator. In this
investigation, the commercialized product of an ozone generator
from PT. Nanobubbles Karya Indonesia (NKI) was employed to
develop adequate ozone using the corona discharge method. The
system received around 30 g L
-1
ozone.
After injecting ozone into the water stream, the mixture containing
fluids and gases is passed through the four NB generators schematically
arranged in parallel geometry/framework. The commercialized NB
generator from PT. NKI was employed in this investigation. The
NB generator used in this study is almost similar to the orifice plate
principle conceptually formulated in the previous finding (Rahmawati
et al., 2021). NB generator plate dimensions are 130 mm long and
50.8 mm in diameter. The NB generator comprises of 127 honeycomb-
shaped holes with a diameter of 2.8 mm with a spacing of 1 mm.
Figure 2 depicts the cross-section of an NB generator used in this
investigation.
The water was then passed into the UV-source device situated in
series with respect to the NB generator. Then the treated water is
distributed via a circulating pump continuously. In our study, the
process was conducted from 60 to 300 min. The whole system is
regulated using a wireless system with a Programmable Logic
Controller (PLC).
We note that the NB technology typically operates under the
cavitation phenomenon. The cavitation behaviour is considered as a
direct consequence of a significant reduction in the pressure stream.
To elaborate the hydroxyl radical formations that are generated by
the cavitation, we introduce the cavitation number (Cv) as shown in
Eq. 1to describe the degree of cavitation in the hydraulic devices
such as NB unit.
Cv1/2p2
–pv

ρV2
0
 (1)
We assign p
2
,p
v
,ρ,andVo to fully recovered downstream pressure,
liquid vapor pressure, liquid density, and velocity at the cavitating
constriction’s throat, respectively (Saharan et al., 2013;Wang et al.,
2020). The opening area of throat is defined as the whole area of the
plate’s hole, which is close to 6.46 × 10
−4
m
2
.Meanwhile,theliquid
vapor pressure in 25°C (the actual temperature in process) is 3.165 kPa.
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We note that the incoming flow rate of 570 LPM and the input pressure
of around 192 kPa were used. A pressure drops around 101 kPa was
recorded upon the initialization NB procedure, indicating the effect of
cavitation takes place. As a result, the cavitation number of 0.909 for our
NB system can be determined using Eq. 1.AccordingtoSaharan et al.
(2013), the cavitation phenomenon often occurs when Cv ≤1
(Montalvo Andia et al., 2021;Wang et al., 2020).
2.2 Characterizations
Measurement of size, zeta potential, and stability of NB:
Ozone NB size distribution and zeta potential in water were
analyzed using a particle size analyzer (Zetasizer Pro Blue,
Malvern) based on the principle of dynamic light scattering
method. The zetasizer is capable of determining the bubble sizes
in the range of 0.3 nm–10 μm.
Measurement of pH: pH water was measured every 60 min of
the process using the pH sensor CSIM11 by immersing the probe
into the solution.
Measurement of COD: The measurement of COD in this
experiment was based on standard methods for the examination
of water and wastewater (Methods: 5220 C. Closed Reflux
Titrimetric Method) from APPA, AWWA, and WEF (Baird
et al., 2017). The heating tube was filled with textile effluent
samples and 10 mL of 0.12 N K
2
Cr
2
O
7
solution. After adding
25 mL of H
2
SO
4
solution containing Ag
2
SO
4
to a cooling bath,
the heating tube was removed and dried. The following stage is to
heat for 120 min at 155°C. After cooling, excess K
2
Cr
2
O
7
was
neutralized with a 0.12 N solution of ammonium ferrosulfate
(FAS) and ferroin indicator (2 drops) until the color changed
from blue-green to red-brown.
According to the redox back-titration method’s principles, there
is a stoichiometric correlation between the quantity of oxidant
remaining and the amount of reductive titrant added, which can
be estimated from the end-point volume of FAS used in the titration
process. By taking into consideration that the Fe
2+
/Fe
3+
couple only
FIGURE 1
Piping and Instrumentation’s diagram of the combined ozone-based AOP and NB method. 1. Textile wastewater sample material, 2. Distribution
pump, 3. Oxygen generator, 4. Ozone generator, 5. Rotameter A, 6. Venturi tube, 7. NB generator, 8. Ultraviolet device, 9. Tank 1, 10. Tank 2, 11. Circulation
pump, 12. Reservoir Tank, 13. Outlet pump, 14. Rotameter B.
TABLE 1 The initial condition of textile wastewater.
pH COD (mg L
-1
) NH
3
(mg L
−1
) TSS (mg L
-1
)
Initial
condition
7.93 114.30 1.67 33.73
FIGURE 2
The cross-section of the NB Generator.
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transfers one electron, the COD value may be determined using the
following equation (Rekhate and Shrivastava, 2020).
COD mg O2L−2

V
(ep
0−Vep)xC
FAS x8000
Vs
(2)
where Vep
0and Vep (mL) are the FAS terminal titration volumes for
the blank and the actual aqueous samples, respectively. The first
derivative plot of the recorded potentiometric curve identifies the
terminal titration volume of FAS, C
FAS
is the concentration of
ferrous ammonium sulfate solution (M), and V
S
is the sampling
volume of the aqueous sample (mL).
Measurement of TSS: Standard methods for water and
wastewater examination (Method: 2540 D) from APHA,
AWWA, and WEF were employed to measure the TSS of our
sample (Baird et al., 2017). The 250 mL of textile wastewater
samples were poured into beaker glass. The samples were then
shaken homogeneously. Afterwards, the sample was filtered using
Whatman 934-AH filter paper or its equivalent, which had
predetermined its empty weight. The filter paper was stored in
an oven at 105°C for 2 h or until dry, put in a desiccator for
15 min, and then weighed.
Measurement of NH
3
:NH
3
analysis was conducted based on
standard methods for examining water and wastewater (Method:
4500-NH
3
Nitrogen) from APHA, AWWA, and EWF (Baird
et al., 2017). The Kjeldahl flask containing the sample was
slowly incorporated with boric acid, ensuring that the end of
the condenser was submerged in the container solution in the
beaker. The process is allowed to run for ±5 min to see the steam
enter the reservoir. Subsequently, 25 mL of 50% NaOH solution
was added through the funnel in the Kjeldahl apparatus. The
distillation is allowed to run for 30–40 min. The result of
distillation in the beaker glass is titrated with HCl solution
until the color changes from red-purple to yellow.
3 Results and discussions
To represents the significance of our combined method, the
decoloration of textile wastewater is displayed in Figure 3.Wefind
that the textile wastewater progressively changed its color solution as
a function of residence time during the treatment process. Within
the initial treatment of 60 and 120 min, we found that visual
observation of the treated wastewater in tank one was
significantly different compare to the tank 2. It was noticeable
that within 120 min of residence time, the water color was
changed from brownish to the pale-green, indicating an
improvement in the water quality was achieved via extending the
residence time and continuous circulation of the water line along the
recovery water. At the end of the process (300 min), the textile
wastewater color was unambiguously improved, showing a semi-
transparent solution. Furthermore, the changes in several
parameters control, namely, COD, NH
3,
and TSS are depicted in
Figure 4.
The evolution of COD values as a function of residence time is
presented in Figure 4. According to the findings, the COD
concentration is reduced along the residence time with the final
state reach approximately 80% removal. Further details of COD
FIGURE 3
Photograph images of the reactor tanks upon AOP-NB treatment with different residence times (A) 60 min, (B) 120 min, (C) 180 min, (D) 240 min,
and (E) 300 min, respectively.
FIGURE 4
The characteristic of water quality parameters of textile
wastewater; COD (purple), TSS (green), and NH
3
(orange) during
300 min of treatment.
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evolution during 300 min of the treatment process can be found in
Supplementary Table S1.
We compare the COD parameter of wastewater that was treated
by the combined method of ozone based AOP + NB and ozone-
based AOP alone, as depicted in Figure 5. In general, both systems
have demonstrated their potential for lowering COD levels in water.
As a result of the ozone exposure, the AOP system alone exhibited a
sufficient removal rate to degrade the organic contaminants within
the range of ~10% upon 300 min treatment time. Interestingly, the
combined method of NB + AOP system yielded substantial
improvement in which the COD reduction rate was significantly
greater than the AOP reduction (shown in red lines). Here, we
postulated that such a high COD reduction rate might be attributed
due to the ozone size modification, as those compounds are
subsequently through the NB compartment. Ozone has an
undesired physical characteristic such as poor solubility in the
water. It led to a condition in which ozone is easily escape into
the atmosphere from its solution phase. Therefore, one solution is
required to enhance the persistence time by reducing the material
size or shape. For instance, the size modification of a nanobubble
with a smaller size could improve to a longer persistent time (Tekile
et al., 2016;Nirmalkar et al., 2018).
Figure 6A Shows the bubble size findings that acquired using the
DLS instrument according to the number-distribution data. The
peak distribution value, approximated from 99.94% of the ozone-
based NB was 216.9 nm in size. Here we noted that the quantity and
size of NB could affect their mass transfer efficiency. A significant
NB production is desirable in a large total surface area that would
enhance the mass transfer flow from the bubbles toward the
wastewater solution. However, we should also consider that non-
negligible bubbles with a smaller size could arise under the higher
internal pressure and larger specific area (Peng and Yu, 2015;
Ulatowski et al., 2019;Rahmawati et al., 2021). Here, we revisit
the contact principle used in this study in which multiphase gas-
liquid flow was employed to generate NB. In order to produce a
multiphase flow, we therefore consider that the water flow was
injected with a pressurized ozone (Levitsky et al., 2015;Rahmawati
et al., 2021). When the multiphase flow passed through the
honeycomb structure of NB generator, then the ozone NBs are
subsequently formed due to the multiphase flow breakdown (Ren
et al., 2018;Rahmawati et al., 2021).
The zeta potential in the final residence time of 300 min is
reported to be −21.08 ± 0.35 mV with the corresponding
distribution is depicted in Figure 6B. The negative value here is
assigned to the generated ROS at the bubble interface, which leads to
the bubble stability (Ushikubo et al., 2010;Selihin and Tay, 2022;
Zhou et al., 2022).Typically, zeta potential value can be associated to
the charge of the bubble rather than its density in the water. The
amount of zeta potential indirectly reflects how stable a colloidal
system is. The electrically-charged surface causes particles to repel
one another, preventing the bubbles from emerging on the other
bubble surface (J. N. Meegoda et al., 2018;Ushikubo et al., 2010).
The details of NB’s zeta potential in 300 min of the process can be
found in Table S5.
To interpret such COD reduction in ozone based AOP and NB
system, we propose that this finding could be driven by the enhanced
synergistic effects promoting an efficient chemical degradation
reaction pathway to remove the organic contaminants from the
water.
The pollutant degradation mechanism can be understood in two
consecutive processes. The first stage of the water recovery process is
related to accumulating ozone NB (see arrow red in Figure 7), which
FIGURE 5
Comparison of COD removal percentage between ozone based
AOP + NB versus AOP system.
FIGURE 6
(A) Particle size distribution of ozone NB, and (B) Zeta potential
value of ozone NB.
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leads to the first reaction with the incoming inlet wastewater. Ozone
NB has higher surface contact compared to macro-bubble, so it
would be more persistent in the solution. It also strongly influence
both the pollutant degradation and ROS generation at the following
treatment stage. Here, we found that fine control of ozone exposure
under pH-neutral media accommodates the organic contaminants’
physical degradation. Our finding regarding COD reduction that
operated at neutral pH in agreement with the previous reports
(Lucas et al., 2009;Takahashi et al., 2012;Yang et al., 2012;
Temesgen et al., 2017).
The second recovery stage corresponds to the utilization of
ozone-based AOP as indicated by the blue arrow in Figure 7. In this
section, the generation of ROS or negatively-charged NB
dominantly contributed to degrade the organic contaminants. We
revisit the conventional thermodynamics of bubble collapse
phenomenon to describe the ROS generation (Ghadimkhani
et al., 2016) and ozone breakdown (Ikhlaq et al., 2013;Oh and
Nguyen, 2022). In the ozone decomposition, we attribute the ROS is
a chain reaction comprised of initiation, propagation, and
termination (Khuntia et al., 2015).
Furthermore, ROS via collapsing cavity bubbles can be
described based on the Young-Laplace equation (Kimura et
al., 2004;Takahashi et al., 2007;Ghadimkhani et al., 2016).
PP1+4σd(3)
where Pdenotes gas pressure, P
l
denotes liquid pressure, σdenotes
liquid surface tension, and ddenotes the bubble diameter. The
internal pressure of a bubble is substantially determined by its size,
according to Eq. 3. Since the dis inversely proportional to the gas
pressure, the impact on the internal pressure for the NB differs
significantly compared to the macro-bubbles. Thus, once NB is
defragmented into molecular gas, thus the additional surface energy
can rupture the hydrogen-oxygen bond and therefore generate ROS
(Bandala and Rodriguez-Narvaez, 2019).
In addition, we present a semiempirical approach concerning
the complete chemistry of the degradation pathway in our system.
Based on our understanding, the chemical process in this study
related to the concerted scheme of direct and indirect oxidation
routes. Here, we consider that a direct route is related to a
simultaneous chemical reaction between pollutant target and
ozone, while the indirect route utilized ROS to degrade the
targeted contaminants.
In terms of temporal consideration, a direct reaction of
ozonation between specific compounds and functional groups
lasted in relatively slow reaction time (a constant rate in the
range of K
d
= 1.0–10
6
M
−1
s
−1
)(Gottschalk et al., 2010). Ozone
reacts with a faster degradation rate when it interacts to certain types
of aromatic and aliphatic compounds.
The indirect oxidation route can be attributed to the formation
of ozone radical chains that comprise of three different steps. The
first step involving a decay process of ozone which is accelerated by
initiators. In our scenario, the hydroxyl anion (OH
−
) is converted
into ROS constituent such as OH•. Based on their kinetic argument,
the chemical specificity to the targeted pollutant is non-selective
with the rate of K
d
=10
8
–10
10
M
−1
s
−1
(Gottschalk et al., 2010). Let us
consider that the OH•regain their missing electron by removing an
electron from the hydrogen atom of the target pollutant molecules.
This in turns converted to targeted water product (H
2
O).
FIGURE 7
Schematic illustration of pollutant degradation in our system.
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Consequently, the targeted molecules converted to ROS that
propagates the chain reactions. However, another scheme could
also arise in which a radical reacts to the second radical and
terminated the reaction. A formulated pathway (Voigt et al.,
2020) is described in Figure 8.
We revisit the previous report that the removal contaminant rate
can be maximized if the magnitude of ROS and targeted pollutants
are similar, as previously reported by Bui and coworkers (Bui and
Han, 2020). We note that in this study ROS was not analyzed
quantitatively, nevertheless we predicted that our system able to
produce ROS numbers that higher than the number of pollutants. It
is an interesting fact that the COD removal during the first 60 min of
the process shown a significant reduction around 40% removal. It is
noticeable that our system was able to complete half of the final
reduction within 60 min of the treatment process. In addition, this
phenomenon can be realized with the stability and number of ROS.
Upon simultaneous radicals’formation, when the ROS number is
higher than the pollutant number, the ROS will react with others
through a terminating pathway to generate secondary ROS (with
low reduction potential) and hydrogen peroxide (H
2
O
2
). Those
secondary ROS then predicted it would dominate the oxidation
process rather than the primary radicals (Esfahani et al., 2019),while
H
2
O
2
will contribute to reducing pH value, as seen in Figure 9.
By altering the residence time, the pH solution gradually
decreased from 7.93 to 7.46, as shown in Supplementary Table
S2 Based on previous study, pH in the initial state influences the rate
of ozone oxidation (Patel et al., 2021;Jabesa and Ghosh, 2022;Jesus
et al., 2022;John et al., 2022). When the pH of a solution is raised, the
direct consequence is that the oxidation rate increases. However,
there is a drawback in which the ozone concentration would
decrease (Patel et al., 2021;Jabesa and Ghosh, 2022;Jesus et al.,
2022;John et al., 2022). Furthermore, aqueous solutions containing
high pH induced ozone autodecomposition since it led to indirect
ozone breakdown, resulting in ROS such as peroxy radicals, HO
2
•,
and OH•(Jabesa and Ghosh, 2022;Jesus et al., 2022).However,
when the pH is more than 8, this eventually hampers the process
with the decay of ozone half-lives (Gardoni et al., 2012).
Furthermore, we consider that our UV in the AOP-Ozone
combination promotes the chemical breakdown of organic
pollutants, as previously described by Rekhate et al. (Rekhate and
Shrivastava, 2020) and Wu et al. (Wu, 2008).
Similarly, the elimination of TSS and NH
3
resulted in a
significant shift throughout the residence time. In TSS
removal, the large contaminants tended to adhere to the NB
surface while floating to the outmost water surface (Ali et al.,
2021). It will cluster the solid suspension in solution. As a result,
the removal process of suspension would last rapidly (F. Ali et al.,
2021). Detailed information regarding TSS values in different
sampling regimes during 300 min can be found in Supplementary
Table S3.
On the other hand, the pH of the solution had a considerable
influence on NH
3
decomposition. At pH 9, direct oxidation of NH
3
occurs. In our study, the pH of the solution was determined to be 7-
8, which is comparable to the initial conditions. In this pH range, we
propose that direct oxidation is likely to occur. At low pH (lower
than 7), the quantity of free NH
3
was minimal, resulting in a sluggish
rate of NH
3
oxidation. As the pH gradually increases, the number of
NH
3
generated is continuously enhanced. Consequently, the
availability of free NH
3
increased the oxidation rate (Khuntia
et al., 2013). The maximum reduction of NH
3
in textile
wastewater is achieved at 300 min residence time; further details
can be found in Supplementary Table S4.
4 Conclusion
We summarized that approximately 81.1% of COD recovery in the
water purification system is achieved by utilizing the combination
oxidation methods of AOP and NB for textile wastewater samples.
Here we emphasize the role of NB method in this investigation is
FIGURE 8
Proposed ozonation reaction through direct and indirect
approaches.
FIGURE 9
pH trend in solution probed during the treatment process.
Frontiers in Environmental Science frontiersin.org07
Hutagalung et al. 10.3389/fenvs.2023.1154739

essential to improve the gas–liquid phase efficiency processes.
Moreover, the realization of ROS generation mediates the chemical
conversion of the polluted water to the environmentally-safe effluent
with low level of COD, NH
3,
and TSS. To statistically quantify the NB
size of our system, we reported that 99.94% of the ozone bubble
population is recorded with the size of 216.9 nm upon 300 min. The
zeta potential values unambiguously shed some light on the existence of
ROS at −22.43 ± 0.34 mV. An abrupt reduction of the COD is observed
with the removal percentage of around 40% within the first 60 min
residence time. Here, we consider that the initial reaction kinetics takes
place in both processes facilitating the faster reaction rate compared to
the individual stage of AOP or NB method.
Data availability statement
The original contributions presented in the study are included in
the article/Supplementary Material, further inquiries can be directed
to the corresponding authors.
Author contributions
SH and DK conceived the project. SH formulated the recovery
plant design and carried out the experiments with SS and NJ. AFR,
SA, HD, and WS contributed to data analysis. SH, AR, and AFR.
write the manuscript. NR, JJ, and DK. discussed the results and
supervised the project. All the authors were involved in the
discussion and manuscript preparation. All authors have
approved the final version of the manuscript.
Funding
This research was supported under a doctoral research program
(No. 183/H/2020) funded by the Indonesian Institute of Sciences
and recently inaugurated as the National Research and Innovation
Agency, Indonesia. This research was fully supported by the
Institute of Technology Bandung (ITB) in collaboration with the
National Research and Innovation Agency (BRIN), Indonesia. The
authors declare that this study received funding from PT. Nanotech
Indonesia Global, Tbk. The funder was not involved in the study
design, collection, analysis, interpretation of data, the writing of this
article, or the decision to submit it for publication.
Acknowledgments
AR and AFR. express gratitude to PT. Nanotech Indonesia
Global, Tbk. for the start-up research grant.
Conflict of interest
Authors NJ and SA were employed by the company South
Pacific Viscose.
The remaining authors declare that the research was conducted
in the absence of any commercial or financial relationships that
could be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors
and do not necessarily represent those of their affiliated
organizations, or those of the publisher, the editors and the
reviewers. Any product that may be evaluated in this article, or
claim that may be made by its manufacturer, is not guaranteed or
endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online
at: https://www.frontiersin.org/articles/10.3389/fenvs.2023.1154739/
full#supplementary-material
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