11
Fenton- and ozone-based AOP
processes for industrial effluent
treatment
Q.Q. Cai, L. Jothinathan, S.H. Deng, S.L. Ong, H.Y. Ng, J.Y. Hu
Department of Civil & Environmental Engineering, Faculty of Engineering,
National University of Singapore, Singapore, Singapore
1
Introduction
One of the major threats to water quality is chemical pollution
to receiving water bodies, primarily caused by the discharge of
inadequately treated industrial effluent. Owing to the adverse
effect of industrial effluent containing high levels of hazardous
pollutants, the discharge of industrial effluents is regulated by
stringent and specific guidelines. The treatment of such industrial
effluent is especially challenging due to the inhibitory properties
of recalcitrant organic pollutants, the need to meet stringent discharge standards, and the typical variable composition of industrial effluents. Extensive research efforts have been invested, and
are still being actively pursued, to explore effective and affordable
treatment technologies for the remediation of hazardous pollutants. A widely recognized efficient treatment alternative of industrial effluent containing recalcitrant compounds is the use of
advanced technologies based on chemical oxidation, such as
the advanced oxidation processes (AOPs). Generally, AOPs are
water treatment processes operating at near ambient temperature
and pressure conditions (Glaze et al., 1987). AOPs typically involve
the generation of hydroxyl radicals (•OH), which are capable of
oxidizing most organic compounds without restriction to specific
classes or groups of compounds. The classification of conventional AOPs is based on the source used for •OH generation.
The Fenton reaction was discovered by H.J.H. Fenton in 1894,
when he reported the activation of hydrogen peroxide (H2O2) by
Advanced Oxidation Processes for Effluent Treatment Plants. https://doi.org/10.1016/B978-0-12-821011-6.00011-6
# 2021 Elsevier Inc. All rights reserved.
199
200
Chapter 11 Fenton- and ozone-based AOP processes
ferrous ion (Fe2+) for tartaric acid oxidation (Fenton, 1894). The
core reaction involved in Fenton-based AOPs is the interaction
between peroxides (usually H2O2) and iron ions for •OH generation. In recent decades, Fenton-based AOPs have been widely
used in cost-effective wastewater treatment processes for removing recalcitrant organic pollutants (Neyens and Baeyens, 2003;
Bautista et al., 2008). Technologies for enhanced Fenton reaction
and reduced sludge production were also explored and implemented in Fenton-based AOPs, including heterogeneous Fenton,
thermal-enhanced Fenton, photo-Fenton, electro-Fenton, and
sono-Fenton processes.
Ozone-based AOPs are cost-effective industrial effluent treatment technologies that are suitable for full-scale applications.
With a redox potential of 2.07 V, ozone is effective in splitting
organic bonds and dissociating aromatic rings in recalcitrant
organic compounds (Zouboulis et al., 2007). Once ozone molecules are dissolved in water, their decomposition through a series
of chain reactions would occur for the generation of various radical species such as •OH. The effectiveness of ozonation is affected
by both the reaction kinetics and the ozone mass transfer; low gasliquid ozone mass transfer is the key step that limits the overall
reaction rate (Danckwerts, 1970). The techniques adopted to
enhance ozonation efficiency mainly include heterogeneous catalytic ozonation, microbubble ozonation, and a combination of
the two improvement methods.
In this chapter, the fundamentals of conventional Fenton/
ozonation and improved Fenton/ozonation processes, such as
fluidized bed reactor Fenton, photo-Fenton, and electro-Fenton
processes as well as heterogeneous catalytic ozonation, microbubble ozonation, and microbubble-catalytic ozonation, are
described. A historical review on recent developments of
Fenton- and ozone-based AOPs for industrial effluent treatment
is conducted. Case studies demonstrating industrial applications
of Fenton- and ozone-based AOPs are also discussed.
2 Fenton-based AOP processes
A range of Fenton-based AOP processes are discussed here,
including their main reaction mechanisms, important operating
factors, and process performances for industrial effluent treatments. The Fenton process, the base for many other modified
Fenton processes, is first introduced, followed by the fluidized
bed reactor Fenton, photo-Fenton, and electro-Fenton processes.
Chapter 11 Fenton- and ozone-based AOP processes
2.1
Fenton process
Fundamental to the Fenton process is the Fenton reaction,
which generates hydroxyl radicals at atmospheric pressure and
room temperature. H2O2 and ferrous ions are called Fenton
reagents. The classic Fenton reaction occurs when these two
reagents are brought into contact in aqueous solution at acidic
pH. Eqs. (1)–(9) represent the key reaction steps that occur during
the Fenton oxidation process with their reported reaction rates
(Sychev and Isak, 1995). The ferrous ion initiates and catalyzes
H2O2 decomposition, and results in the generation of highly reactive •OH (Eq. 1). Meanwhile, the generated ferric ion could be
reduced by excess H2O2 to ferrous ion (Eq. 2), enabling an effective
cyclic mechanism for ferrous regeneration. In the reaction
described in Eq. (2), another radical species, the hydroperoxyl
radical (HO2 ), is also produced; it is less effective than •OH for
organic oxidation due to its lower redox potential. Reaction (2)
is usually referred to as a Fenton-like reaction, and it is much
slower than the Fenton reaction. Thus the produced Ferric ions
in Reaction (1) would be progressively accumulated, leading to
ferric sludge production in the Fenton system.
H2 O2 + Fe2 + ! Fe3 + + HO + OH , k3:1 ¼ 40 80 M1 s1
(1)
H2 O2 + Fe3 + ! Fe2 + + HO2 + H + , k3:2 ¼ 9:1 107 M1 s1
(2)
Eqs. (3)–(5) depict other rate-limiting steps for ferrous ion regeneration and hydroxyl radical production. Excess ferrous ions could
scavenge the produced radical species (Eqs. 3, 4), and HO2 could
react with ferric ions for additional ferrous ion regeneration
(Eq. 5). It shall be noted that in the above reactions, the ferrous
ion acts as a catalyst and H2O2 is continuously consumed for
•
OH production.
Fe2 + + HO ! Fe3 + + OH , k3:3 ¼ 2:5 5 108 M1 s1
1 1
6
Fe2 + + HO2 ! Fe3 + + HO
s
2 , k3:4 ¼ 0:72 1:5 10 M
(3)
(4)
Fe3 + + HO2 ! Fe2 + + O2 + H + , k3:5 ¼ 0:33 2:1 106 M1 s1 (5)
Eqs. (6)–(8) summarize other radical scavenging reactions that
could occur during the Fenton process.
HO + H2 O2 ! HO2 + H2 O, k3:6 ¼ 1:7 4:5 107 M1 s1
HO + HO2 ! H2 O + O2 , k3:7 ¼ 1:4 1010 M1 s1
HO + HO ! H2 O2 , k3:8 ¼ 5 8 10 M
9
1 1
s
(6)
(7)
(8)
201
202
Chapter 11 Fenton- and ozone-based AOP processes
The whole Fenton process involves a complex reaction mechanism. Hydroxyl radicals are generated in the chain initiation reaction (Eq. 1) while they could be scavenged by ferrous ions (Eq. 3),
hydrogen peroxide (Eq. 6), hydroperoxyl radical (Eq. 7), and even
themselves. Hydrogen peroxide could be the initiator for radical
production, but also could be the inhibitor for the subsequent oxidation process. Ferrous ions could accelerate hydrogen peroxide
consumption, but also scavenge the produced radicals. In the
presence of organic molecules (RH), the produced hydroxyl radicals will attack them through proton abstraction and produce
highly reactive organic radicals (R•) (Walling and Kato, 1971),
which could be further oxidized (Eq. 9).
RH + HO ! H2 O + R , k3:9 ¼ 108 M1 s1
(9)
Based on an exhaustive review of existing studies on the application of Fenton processes for industrial effluent treatment, the
main factors affecting process efficiency include pH, temperature,
Fenton reagent dosage, and the Fe2+/H2O2 ratio.
A classic Fenton system is highly dependent on reaction pH
for achieving effective organic oxidation. The optimum pH used
in Fenton oxidation for industrial effluent treatment is mostly
reported to be 3 (Balcik-Canbolat et al., 2016; Yang et al., 2019;
Zeng et al., 2015; Zhang et al., 2019; Cui et al., 2015; Xiao et al.,
2017; Yongrui et al., 2015; Methatham et al., 2016). Either lower
or higher pH values could adversely affect Fenton process performance. At lower pH values, H2O2 would react with a larger
amount of H+ ions to form stable oxonium ions, H3O+2 (Chen
et al., 2009), and free ferrous ions would form iron complex species [Fe(H2O)6]2+ (Xu et al., 2009). Due to the lower activity of the
Fenton reagent, •OH production is inhibited. The reaction between
Fe3+ and H2O2 would also be inhibited under lower pH value conditions. In addition, the steady-state concentration of •OH could be
reduced due to the scavenging effect from H+ ions. At higher pH,
free iron ion concentration could be dramatically reduced with ferric oxohydroxide formation and ferric hydroxide precipitation,
which could inhibit both •OH production and Fe2+ regeneration
(Shemer et al., 2006). H2O2 self-decomposition to H2O2 and O2
would be accelerated at a pH level higher than 5 (Szpyrkowicz
et al., 2001). Moreover, the redox potential of •OH is reported to
be lower at a higher pH (E0 ¼ 2.8–1.95 V at pH 0–14) (Kim et al.,
1998). Therefore, adequate pH control along the Fenton reaction
is crucial to maintain satisfactory system performance.
The Fenton reaction can be initiated in a wide range of ambient
temperatures, but is more efficient at temperatures above 20°C.
A temperature increase would certainly favor the rate of Fenton
Chapter 11 Fenton- and ozone-based AOP processes
reaction while temperatures above 50°C would also promote
hydrogen peroxide’s thermal decomposition to water and oxygen
(Muthukumari et al., 2009). Several studies have reported the
optimum temperature for Fenton processes carried out at atmospheric pressure. The optimum temperature in the Fenton process
for partially hydrolyzed polyacrylamide wastewater treatment was
reported to be 30°C by Yongrui et al. (2015). Bampalioutas et al.
(2019) reported the optimum temperature for treating olive mill
wastewater as 35°C. According to Xiao et al. (2017), the optimum
operating temperature for treating textile wastewater was as high
as 40°C. Although these lab-scale studies demonstrated a more
effective Fenton reaction at high temperature, ambient temperatures were usually adopted in full-scale operation because this
was practically and economically applicable.
The Fenton process performance is generally correlated with
the amount of •OH generated, and •OH production is directly
affected by the dosage of Fenton reagents. Therefore, the optimal
dosage of H2O2 and Fe2+ required for the Fenton oxidation of a
given wastewater must be established empirically, based on specific lab-scale or pilot-scale studies. The optimal dosage is dependent on wastewater characteristics, the nature and concentration
of the target pollutants, treatment conditions, and objectives.
Usually, as a rule, H2O2 dosage plays a crucial role in determining
the overall efficiency of the Fenton process. The process oxidation
efficiencies were generally observed to be increased with higher
H2O2 dosage (Kang and Hwang, 2000; Lin et al., 1999). However,
it shall be noted that unconsumed H2O2 during the Fenton process may contribute to COD in the treated effluent (Lin and Lo,
1997). An excess amount of H2O2 also scavenges the generated
•
OH (Eq. 6), thereby inhibiting the entire process performance.
The H2O2 dosage shall be optimized so that the total supplied
amount is consumed effectively during the Fenton process, and
this optimal dosage can only be obtained through testing with
specific target wastewater. A higher concentration of Fe2+ usually
induces a higher oxidation efficiency of the Fenton process while
the efficiency enhancement was observed to be marginal above
an excessive concentration of Fe2+ (Kang and Hwang, 2000;
Rivas et al., 2001). This is due to the reaction between Fe2+ and
•
OH (Eq. 3) that reduces the available active radicals for organic
oxidation. The optimal Fe2+ dosage could be determined based
on the optimal H2O2 dosage; they are related via the optimal
molar ratio of Fenton reagents [H2O2]/[Fe2+] for each specific
wastewater treatment.
The optimal molar ratio between H2O2 and Fe2+ is one of the
determinants of the Fenton reaction efficiency as well as one of
203
204
Chapter 11 Fenton- and ozone-based AOP processes
the main control parameters of this process. In spite of many
research studies and industrial applications of the Fenton process,
there is no agreement on a universal value of optimal [H2O2]/
[Fe2+]. Nevertheless, some optimum values of [H2O2]/[Fe2+] were
reported from 1.42 to 20.00 for industrial effluent treatment with
the Fenton process (Fenton, 1894; Yang et al., 2019; Xiao et al.,
2017; Chen et al., 2009; Szpyrkowicz et al., 2001; Bampalioutas
et al., 2019; Lin et al., 1999; Lin and Lo, 1997). In the scenario where
Fe2+ is overdosed and the molar ratio between H2O2 and Fe2+ is far
less than 1, H2O2 is quickly and fully consumed for •OH production
(Yoon et al., 2001). The excess amount of Fe2+ reacts with •OH
(Eq. 3), which could lead to the termination of the chain reactions.
Reactions for Fe2+ regeneration (Eqs. 2, 5) might be inhibited, and
under this Fenton reaction condition, Fe2+ is used as a major reactant instead of a catalyst. In addition, the organic compounds
would have to compete with the residual Fe2+ for •OH, and the
Fenton oxidation efficiency could be limited. Therefore, overdosing of Fe2+ is undesirable as it has an inhibitory nature on the
oxidation process. When [H2O2]/[Fe2+] is higher than 1, Fe2+ is rapidly depleted to decompose H2O2 for •OH generation (Yoon et al.,
2001). Instead of slowly reacting with Fe3+, the residual H2O2
would react with •OH to produce HO2 (Eq. 6). The generated
HO2 could participate in ferrous ion regeneration by reducing
Fe3+ to Fe2+ (Eq. 5). In general, higher [H2O2]/[Fe2+] results in
increased oxidation efficiency. However, an excess amount of
H2O2 has to be considered with care due to its radical scavenging
ability, which could limit the organic oxidation efficiency.
Compared to other AOPs, the classic Fenton process has simpler equipment/facility design requirements, greater operational
simplicity, and process scalability. It has been applied for various
types of industrial effluent treatment and contaminant abatement. Table 1 depicts recent research works on industrial effluent
treatment with the classic Fenton process.
The typical scheme of the Fenton process consists of acidification, oxidation, neutralization, flocculation, and settling. From an
operational point of view, the classic Fenton process has three
main drawbacks: pH control in the acidification stage, Fenton
reagent consumption in the oxidation stage, and sludge production in the neutralization and flocculation stages. A continuous
supply of acid and Fenton reagents may excessively increase the
operating costs in case of treating wastewater with a high organic
loading or flowrate. The reddish-brown ferric precipitate must be
separated from the liquid mixture, properly processed, and disposed as solid waste. In real practice, sludge generation is the fundamental reason for improving the Fenton process design, which is
generally aiming for two objectives: (1) Reducing ferric sludge
Table 1 Recent research works on industrial effluent treatment with the classic Fenton process.
Operating condition
Wastewaters/
compounds
Benzene dye
intermediates
wastewaters
Medium density
fiberboard
wastewater/
formaldehyde
Olive mill wastewater
Pharmaceutical
wastewater
Hydrolyzed
polyacrylamide
wastewater
Hydrolyzed
polyacrylamide
wastewater
Sulfide mineral
processing
wastewater
Textile dye
wastewater
TFT-LCD wastewater
Triethyl phosphate
wastewater
Initial level
(mg/L)
H2O2
(mM)
Fe2+
(mM)
pH
Process performance
References
COD0: 4000
1000
360
4.13
85% COD removal in 60 min, enhanced BOD5/COD
ratio from 0.08 to 0.49
Guo et al.
(2018)
COD0: 395
CH2O0: 59
23.26
9.29
3
58% COD and 77% CH2O removal in 70 min
BalcikCanbolat et al.
(2016)
Total phenols:
9740
COD0: 4061
Berberine0: 709
147
18
4
89% Total phenol removal in 120 min
[H2O2]/
[COD]:
1.25
750
[H2O2]/
[Fe2+]:
10
37.5
3
36% COD removal and 91% berberine removal
in pilot system with HRT of 2.5 h
Bampalioutas
et al. (2019)
Cui et al.
(2015)
3
90% COD removal, enhanced BOD5/COD ratio
from 0.078 to 0.463
Zhang et al.
(2019)
COD0:
1500–1700
5.30
1.44
3
72% COD removal
Yongrui et al.
(2015)
COD0: 130
8.82
1.76
2–4
77% COD removal
Zeng et al.
(2015)
COD0: 522
11.77
7.14
3
Xiao et al.
(2017)
COD0:
3800–4500
Organic
phosphorus0: 58
291
4
3
95% COD removal with 60 min Fenton oxidation
and subsequent flocculation with 60–100 mg/L PAC
and 10 mg/L CPAM
70% COD removal in 120 min
20
14
3
COD0: 10,381
Organic phosphorus was reduced to 5 mg/L with
120 min Fenton oxidation
Methatham
et al. (2016)
Yang et al.
(2019)
206
Chapter 11 Fenton- and ozone-based AOP processes
production; and (2) enhancing ferrous ion regeneration. Several
improvements to the classic Fenton process for industrial effluent
treatment are discussed in the following sections.
2.2
Fluidized bed reactor Fenton (FBR-Fenton)
process
To address the problem of sludge production in the classic
Fenton process, researchers have explored the possibility of conducting the Fenton reaction in a fluidized bed reactor, which could
synergistically combine the effectiveness of homogeneous Fenton
and the sludge reduction of heterogeneous Fenton. This modified
process is called fluidized bed reactor Fenton (FBR-Fenton).
A fluidized bed reactor is a contacting device that uses the principle
of fluidization for its operation, so that the packed material could
be expanded by the upward or downward movement of the fluid.
Most fluidized materials used in FBR-Fenton systems are inert
solids. They are defined as carriers that provide available sites for
the crystallization and precipitation of iron hydroxides, and hence
reduce sludge production in the Fenton reaction.
The FBR-Fenton process involves five main reaction mechanisms that contribute to its efficient oxidation performance and
reduced sludge generation (Fig. 1). The first mechanism relates
to the homogenous Fenton reaction for •OH generation and radical chain reactions for ferrous ion regeneration (Eqs. 1–5).
Organic pollutants are subsequently oxidized to form less-toxic
intermediates. The third mechanism is deposition of ferric
hydroxide on the carrier surface via heterogeneous nucleation
(Boonrattanakij et al., 2011). The ferric hydroxide-coated carrier
may possess some extent of catalytic activity and initiate a
Fig. 1 Scheme of main reaction
mechanisms involved in the
FBR-Fenton process.
Chapter 11 Fenton- and ozone-based AOP processes
207
Fig. 2 Schematic diagram of an
FBR-Fenton system.
heterogeneous Fenton reaction for extra •OH generation. Meanwhile, the presented organic oxidation intermediates, particularly
volatile fatty acids, could form soluble complexes with ferric ion,
and hence interfere with ferric ion solubility and crystallization
(Boonrattanakij et al., 2011).
A schematic FBR-Fenton system is shown in Fig. 2; it consists
of a cylindrical reactor packed with carriers and their support
material. Glass beads and porous ceramsite are usually used to
support the carriers as well as provide even distributions of the
incoming flow. After wastewater is acidified with sulfuric acid,
Fenton reagents are dosed into the influent line and pumped
together with wastewater into the FBR-Fenton reactor from the
bottom. Internal circulation of the reaction mixture is subsequently initiated to fluidize the carriers with the desired bedexpansion rate. The treated wastewater is collected from the
effluent outlet at the top, and has gone through the neutralization,
flocculation, and settling processes.
As the main mechanism involved in FBR-Fenton for organic
oxidation is the classic Fenton process, the operating factors
affecting the FBR-Fenton process efficiency are the same as those
discussed in the classic Fenton process, including pH, temperature, Fenton reagent dosage, and the Fe2+/H2O2 ratio. An extra
parameter that shall be considered in the FBR-Fenton process
is the carrier’s material. After the Fenton reaction is initiated, ferric
ions would be formed over a very short period of time. Without the
presence of carriers, ferric hydroxide would agglomerate via
homogeneous nucleation and form ferric sludge. When the Fenton reaction mixture is seeded with foreign particles (carriers),
208
Chapter 11 Fenton- and ozone-based AOP processes
ferric hydroxide nuclei can be deposited on these carriers via heterogeneous nucleation. Effective iron crystallization requires the
notable dominancy of the heterogeneous nucleation process,
which is also one of the key mechanisms for reduced sludge formation. The surface properties and size distribution of the carrier’s material play important roles in iron crystallization
kinetics. In this context, different carriers have been adopted in
the FBR-Fenton process for industrial effluent treatment, including SiO2, sand, Al2O3, Fe2O3, granular activated carbon, ceramsite,
and brick particles (Boonrattanakij et al., 2011, 2018; Briones et al.,
2012; Matira et al., 2015; Chen et al., 2015a; Lyu et al., 2016; Liu
et al., 2014; Bello et al., 2019; Su et al., 2011, 2013; Anotai et al.,
2012). SiO2 is the most popular carrier used in the FBR-Fenton
process due to its proved capability to promote iron crystallization. Chen et al. compared the performance of SiO2, Al2O3, and
Fe2O3 and reported that SiO2 was the most suitable carrier for
the highest organic removal and total iron removal. According
to Lyu et al., granular activated carbon (GAC) exhibited a relatively
higher iron removal capability than ceramsite. Liu et al. reported
sand to be the best carrier for silicon wastewater treatment compared to GAC and brick particles. Sand material was able to
achieve better reactor bed expansion, fluidized state, and pollutant removal rate. Boonrattanakij et al. studied iron crystallization
onto construction sand and SiO2 the FBR-Fenton process. It was
found that iron crystallization onto the construction sand was faster than crystallization onto SiO2, although the final iron removal
efficiencies were comparable. Besides the carrier material, the pH
also affects the iron crystallization kinetics. Under the most commonly used pH condition in the Fenton process, pH 3, the homogenous nucleation of ferric hydroxide occurs slowly, and hence
heterogeneous nucleation could induce effective iron crystallization. As a result, most ferric precipitates are deposited on the carrier’s surface and the remaining iron species for sludge formation
are reduced. Diz and Novak (1998) studied iron crystallization on
quartz sand in a fluidized bed reactor; it was found that the optimum pH for iron crystallization was between 3 and 4. This pH
range matches well with the recommended pH range for conducting classic Fenton oxidation, which makes it possible to combine
the classic Fenton process for organic oxidation and the iron crystallization process for sludge reduction.
In recent years, the FBR-Fenton process has been adopted for
pharmaceutical removal, thin-film transistor liquid crystal displays (TFT-LCD) wastewater, textile wastewater, and silicon
wastewater treatment. Table 2 summarizes these research works
with detailed information on operating conditions and process
performance.
Table 2 Recent research works on industrial effluent treatment with the FBR-Fenton process.
Operating condition
Wastewaters/
compounds
Initial level
(mg/L)
H2O2
(mM)
Fe2+
(mM)
pH
Carrier
Acetaminophen
(ACT)
ACT0: 758.15
19.87
0.06
3.22
SiO2
Dimethyl
sulfoxide (DMSO)
Flax wastewater
DMSO0: 390.65
32.5
5
3
TOC0: 350
17.65
5.35
3
Multidye
wastewater
Acid yellow (AY):
80 mg/L
Disperse red (DR):
50 mg/L
Reactive brilliant
blue (RBB): 80 mg/L
COD0: 800
TOC0: 133
[H2O2]/
[COD]:
1
[H2O2]/
[Fe2+]:
10
3
68.97 g/L
SiO2
74.07 g/L
SiO2
GAC and
ceramsite
[H2O2]/
[COD]:
2.6
46
[H2O2]/
[Fe2+]:
13.6
5
3.5
[H2O2]/
[COD]:
7.94
60
[H2O2]/
[Fe2+]:
8.36
5
Silicone
wastewater
Screw
manufacturing
wastewater
Textile
wastewater
COD0: 400
TFT-LCD
wastewater
Monoethanolamine
(MEA): 305.4
COD0: 314–404
Process performance
97.8% ACT removal, 62.92% in 120 min,
total iron removal compared to 9.06%
total iron removal in classic Fenton
95.22% DMSO and 34.38% TOC removal
in 120 min
89% TOC removal at 50% bed expansion
rate in 100 min
100% AY, 80% DR, 85% RBB,
and 93% COD removal in 10 min
References
Guo et al.
(2018)
Matira et al.
(2015)
Chen et al.
(2015a)
Lyu et al.
(2016)
95% COD and 85% TOC removal with
HRT of 60 min, total iron removal rate
was 26%
80% COD removal in 40 min with 50%
bed expansion rate
Liu et al. (2014)
3.4
Quartz sand
with 35%
filling rate
SiO2
3
SiO2
Su et al. (2011)
3
SiO2
86.7% COD removal and 97% color
decay in 10 min, with 50% bed expansion
rate
98.9% MEA, 64.7% COD, 62% TOC,
and 43.5% total iron removals in 2 h
Boonrattanakij
et al. (2018)
Anotai et al.
(2012)
210
Chapter 11 Fenton- and ozone-based AOP processes
2.3
Photo-Fenton process
The photo-Fenton process is another modification of the
classic Fenton process that has been brought to the actual
practice of industrial effluent treatment. By combining Fenton
reagents and UV-vis irradiation, the photoreduction of Fe3+
described in Eq. (10) could facilitate Fe2+ regeneration and simultaneously promote •OH generation through the Fenton reaction.
Extra •OH could also be generated through the direct photolysis of
H2O2 (Eq. 11). As a result, photo-Fenton achieves a higher oxidation rate and efficiency than the classic Fenton process (Ortegabana et al., 2012). In addition, there are remarkable reductions
Lie
of total iron usage and ferric sludge formation in the photoFenton process (Hermosilla et al., 2009).
FeðOHÞ2 + + hv ! Fe2 + + HO
(10)
H2 O2 + hv ! 2HO
(11)
The photo-Fenton process could be conducted in both homogeneous and heterogeneous systems, and the main difference
between the two systems is the form of iron source. In the homogenous system, the used iron species exist in the same phase with
reactants, and the most common and convenient iron source used
for homogeneous photo-Fenton is FeSO47H2O. Due to good mass
transfer, the homogeneous Fenton system usually achieves a
higher reaction rate and oxidation efficiency. However, strict pH
control during the oxidation process and the formation of a large
quantity of ferric sludge after the treatment process are the main
drawbacks. Hence, the heterogeneous photo-Fenton process was
adopted for overcoming these limitations. In the heterogeneous
system, iron sources are immobilized within/on the catalyst
structure, and they could induce catalytic degradation of the target pollutants without the generation of ferric sludge. Various natural iron minerals and iron oxides were applied for organic
pollutant removal in a wider pH range (Rusevova et al., 2012;
Gonzalez-Olmos et al., 2012; An et al., 2013). Besides iron (Fe),
other transition metals could also catalyze the oxidation process
with UV-vis irradiation, such as copper (Cu) and manganese (Mn)
(Yip et al., 2005; Lai et al., 2019). The reaction systems using Cu
and Mn as catalysts follow a similar network as that of Fe.
Photo-Fenton with heterogeneous catalysts is also referred to as
a photo-Fenton-like reaction, and recent research studies have
mostly focused on specific recalcitrant organic removal with
heterogeneous photo-Fenton treatment, such as synthetic dyes,
Chapter 11 Fenton- and ozone-based AOP processes
phenol, antibiotics, and 1,4-dioxane (Avetta et al., 2015; Lima
et al., 2017; Liu et al., 2017; Barndõk et al., 2016; Gao et al.,
2015; Lai et al., 2019). It was also found that the heterogeneous
photo-Fenton process could be carried out under neutral or unaltered pH conditions. Nevertheless, compared to the homogeneous reaction, the heterogeneous photo-Fenton reaction has a
slower oxidation rate due to limited catalyst surface and mass
transfer rate. It may not be practically efficient for treating
high-strength industrial effluents on a large scale. Industrial effluent treatment with the homogeneous photo-Fenton process will
be mainly discussed in the later sections.
In the context of the homogenous photo-Fenton process, the
key factors affecting the process efficiency include pH, temperature, Fenton reagent dosage, Fe2+/H2O2 ratio, light source, and
chelating agents. As the first four factors have been discussed in
the classic Fenton process, the effects of light source and chelating
agents will be mainly reviewed here.
It should be noted that under the optimum Fenton reaction pH
conditions, the predominant ion species Fe(OH)2+ has an absorption band between 200 and 410 nm. The maximum quantum yield
of Fe3+ photoreduction is obtained at an irradiation wavelength
around 313 nm (Faust and Hoign
e, 1990). Light irradiation at a
shorter wavelength would promote H2O2 photolysis, which is
not the actual photo-Fenton reaction. Both UV and visible light
irradiation could be adopted for conducting the photo-Fenton
process. Generally, the photo-Fenton process with UV irradiation
achieves a higher oxidation rate and efficiency than the visible
light-assisted Fenton process. Kitsiou et al. (2014) investigated
sulfamethazine (SMT) mineralization in UVA- and visible lightassisted photo-Fenton systems. UVA irradiation at a light flux of
0.94 104 Einstein/L min was provided by a 9 W lamp with a peak
emission wavelength at 366 nm, and visible light irradiation at a
light flux of 0.88 104 Einstein/L min was provided by a 9 W lamp
with a peak emission wavelength at 440 nm. The UVA-assisted
photo-Fenton process achieved better mineralized efficiency after
a 180 min oxidation reaction, with 87% TOC removal compared to
70% TOC removal in the visible light-assisted photo-Fenton system. A pilot-scale study was subsequently conducted with natural
solar irradiation at a total light intensity of 2 mW/cm2. The SMT
concentration was reduced from 20 to 6 mg/L in 120 min, suggesting that process scale-up was feasible. Martı́nez-Costa et al. (2018)
reported the degradation of sulfamethoxazole (SMX) and trimethoprim (TMP) in photo-Fenton processes using UV and solar irradiation. UV irradiation was provided by a 700 W low-pressure
mercury lamp (λ ¼ 254 nm) at a light flux of 3.75 107 Einstein/
211
212
Chapter 11 Fenton- and ozone-based AOP processes
s, and solar irradiation was provided by a solar simulator
equipped with a 1500 W xenon lamp (λ ¼ 290–800 nm) at an output
intensity of 450 W/m2. With a reaction time of 50 min, the solar
light-assisted photo-Fenton process removed 97% and 52% of
SMX and TMP, respectively. The UV-assisted photo-Fenton process achieved higher antibiotic removal efficiencies, with 100%
and 79% removal of SMX and TMP, respectively. In spite of the
higher oxidation efficiency in the UV-Fenton systems, the acquisition and operation of the UV system involve high capital and
operating costs. Moreover, the excessive effluent turbidity and
coloration would significantly hinder the photo-Fenton process
efficiency. Therefore, UV-Fenton systems are usually adopted
for the low flow rate tertiary treatment of effluents subjected to
stringent discharge standards. To avoid high investment and
maintenance costs, visible light from natural solar irradiation is
suggested as the alternative replacing UV irradiation, and the
solar-Fenton process is of great interest in the field of industrial
effluent treatment.
To avoid the strict pH control required in the conventional
photo-Fenton process, chelating agents are introduced into the
system for forming stable complexes with ferric ions and preventing ferric precipitation at neutral pH. The ferric complexes could
significantly absorb UV-vis light and also undergo photolytic
decomposition, which is broadly described in Eq. (12) (Clarizia
et al., 2017). Fe 3+ photo-reduction and Fe2+ generation could
be initiated through ligand-to-metal charge transfer (LMCT).
Fe3 + L + hv ! Fe3 + L ∗ ! Fe2 + + L
(12)
Clarizia et al. (2017) reported that the most commonly used chelating agents in the photo-Fenton process were oxalate, citrate,
ethylenediamine-N,N0 -disuccinic acid (EDDS), ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid (NTA). When
EDTA was adopted, the formed ferric complexes allowed for a
pH value shifting to 12 with no ferric precipitation. The adoption
of oxalate, citrate, EDDS, and NTA could make soluble Fe3+ available at near-neutral pH values. Each ferric-ligand complex
exhibits different light absorption properties at varying pH and
irradiation wavelength, which thereby affects the LMCT reaction
rate. Weller et al. (2013) reported that the ferric-bisoxalate
complex had a higher quantum yield (1.23) at 313 nm, and the
ferric-trisoxalate complex had a higher quantum yield (1.00) at
436 nm. Kocot et al. (2006) found that the ferric-EDTA complex
obtained the highest quantum yield (0.05) at 313 nm and pH of 4.
The quantum yields of the ferric-NTA complex were estimated
Chapter 11 Fenton- and ozone-based AOP processes
at two pH conditions (4 and 6) and irradiation wavelengths (325
and 313 nm) by Abida et al. (2006). The highest quantum yield of
0.46 was obtained at 313 nm and pH of 4. The ferric-citrate complex could achieve good quantum yield values (0.21–28) at a longer irradiation wavelength of 436 nm (Faust and Zepp, 1993). To
develop a cost-effective photo-Fenton process at near-neutral
pH, the chelating agents should be carefully selected with a minimal increase in chemical cost and initial organic loading. The
chelator dosage, operating condition, biodegradability, and ecotoxicity of the species formed should all be evaluated.
Table 3 summarizes recent research works investigating the
feasibility of the photo-Fenton process for industrial effluent
treatment.
2.4
Electro-Fenton process
The electro-Fenton (EF) process is another Fenton-based AOP
that has been successfully applied for recalcitrant organic removal
(Panizza and Cerisola, 2005; Nidheesh and Gandhimathi, 2012;
Barhoumi et al., 2015). In the EF process, organic matters could
be destroyed by both the action of Fenton reagents in the bulk
and anodic oxidation at the anode surface (Brillas et al., 2009;
Oturan and Aaron, 2014), where four reaction mechanisms could
be involved (Fig. 3): (1) in situ electrical generation of H2O2 via oxygen reduction on the cathode (Guandao et al., 2015); (2) •OH generation via the Fenton reaction between the in situ generated H2O2
and Fe2+ ( Jiang et al., 2018; Zhou et al., 2012); (3) promotion of •OH
generation on the anode surface when special materials are utilized
as the anode (e.g., boron-doped diamond, BDD) (Panizza and
Cerisola, 2005; Barhoumi et al., 2017); and (4) direct reduction of
Fe3+ on the cathode for Fe2+ regeneration (El-Ghenymy et al.,
2014; Qiu et al., 2015; Trellu et al., 2018).
Based on the method of Fenton reagent addition and formation, the EF process could be classified into four categories
(Kurt et al., 2007; Isarain-Chávez et al., 2011; Rahim Pouran
et al., 2015) (Fig. 4): (1) Fe2+ is externally added and H2O2 is generated using an oxygen sparging cathode; (2) H2O2 is externally
added and Fe2+ is generated using a sacrificial anode; (3) H2O2
and Fe2+ are in situ generated using a sacrificial anode and an oxygen sparging cathode, respectively; and (4) H2O2 and Fe2+ are
externally added for •OH production, and Fe2+ is regenerated
through Fe3+ reduction on the cathode. Among the four categories, the type 4 EF process is the most promising for industrialscale wastewater treatment, as using sacrificial anodes to provide
Fe2+ is not economical for long-term practical applications and
213
Table 3 Recent research works on industrial effluent treatment with the photo-Fenton process.
Wastewaters/
compounds
Initial
level
(mg/L)
Operating condition
H2O2
(mM)
2+
Fe
(mM)
pH
Light source
Process performance
References
TOC0: 501
117.65
6.70
2.9
55 W UVC lamp, 200–280 nm
53% TOC removal in 2 h
Expósito et al.
(2016)
COD0:
10,019
COD0: 800
468.74
3.8
Natural solar light, 2424 kJ/m2
77% COD removal in 30 min
29.41
[Fe3+] ¼
9.11
1.79
3
74% COD removal in 120 min
COD0:
35,369
160.56
5.11
3
Antibiotic
wastewater
Oxacillin0:
81.49
10
0.09
6
150 W mercury lamp,
200–580 nm
(a) 80 mW UVA LED, 370 nm,
23 W/m2; (b) 1.4 W UVA LED,
365 nm, 70 W/m2; (c) 1.4 W
UVA LED, 365 nm, 85 W/m2
30 W UVA lamp, 365 nm,
1.5 mW/cm2
Guzmán et al.
(2016)
Marcinowski
et al. (2014)
Rodrı́guezChueca et al.
(2016)
Coagulated
textile
wastewater
Pharmaceutical
laboratory
effluent
COD0: 450
TOC0: 151
19.60
1
3
6 W black-light lamp, 365 nm,
0.5 mW/cm2
75% COD removal and 62%
TOC removal in 90 min
Antipyrine:
389
TOC0: 1914
73.53
0.36
2.7
Solar CPC reactor, 30 W/m2
100% Antipyrine removal
and 21% TOC removal in
120 min
Beverage
industrial
wastewater
Citrus
wastewater
Cosmetic
wastewater
Crystallized-fruit
wastewater
(a) 45% COD removal in
360 min; (b) 64% COD
removal in 360 min; (c) 74%
COD removal in 360 min
100% Oxacillin removal in
50 min
GiraldoAguirre et al.
(2018)
GilPavas et al.
(2017)
Foteinis et al.
(2018)
Chapter 11 Fenton- and ozone-based AOP processes
e–
+
–
OH –
Fe 2+
Fe 2+
H 2O 2
Fe 3+
Fe 3+
H + + OH
e–
H 2O
Anode
215
OH
RH
Products
e–
H 2O 2
2e–
O2 + H+
Cathode
Fig. 4 Conceptual classification of electro-Fenton processes.
the cathodic production of H2O2 from oxygen is significantly limited by low oxygen solubility in water. The type 4 EF process is also
called the Fered-Fenton process, in which organic oxidation is
carried out mainly through the homogeneous Fenton reaction,
and electric current is applied to the aqueous mixture for provoking the cathodic reduction of Fe3+. The electrolytic regeneration of
Fe2+ allows for considerable reduction in sludge production, overcoming the main drawbacks of the classic Fenton process.
To enhance the EF process performance, a number of process
modifications were developed by incorporating other technologies
into the electrochemical process. The photo-electro-Fenton (PEF)
process is a combination between the electrochemical process
and the photochemical process. With additional light irradiation,
Fe2+ regeneration could be facilitated through the photoreduction
of Fe3+ (Eq. 10), and •OH generation through the Fenton reaction
Fig. 3 Reaction mechanisms of
the electro-Fenton process.
216
Chapter 11 Fenton- and ozone-based AOP processes
could be simultaneously promoted. Extra hydroxyl radicals would
also be produced through peroxide photolysis (Eq. 11). The sonoelectro-Fenton (SEF) process is another type of modified EF
process by incorporating the sonochemical process into the EF
process. Under the ultrasound wave, more hydroxyl radicals could
be generated by water pyrolysis, and hence the degradation of
organic pollutants is improved. In addition, the ultrasound wave
could benefit the regeneration of Fe2+ from the iron intermediate
complexes (Babuponnusami and Muthukumar, 2012). In terms
of physical properties, ultrasound wave could effectively clean
the electrodes’surface by dispersing the passivated layers, and provide efficient mass transfer in the SEF reaction system (Chen and
Huang, 2014).
Heterogeneous catalytic electro-Fenton is another technique
used to enhance Fenton oxidation efficiency. In most cases, the
catalytic effect involves promoting H2O2 decomposition for
hydroxyl radical generation, and facilitating oxidative degradation
of organic contaminants. Furthermore, the heterogeneous catalytic process could widen the pH range for EF process operation
and reduce sludge production. The most adopted heterogeneous
catalyst in the EF process could be classified into two categories:
(1) natural minerals, and (2) synthetic catalysts. The group of natural minerals mainly includes hematite (α-Fe2O3), wustite (FeO),
magnetite (Fe3O4), and goetite (α-FeOOH) (Sánchez-Sánchez
et al., 2007; Expósito et al., 2007), and the synthetic catalyst group
mainly consists of nanoscale iron minerals (e.g., nano-Fe3O4; Hou
et al., 2015) and metal-doped iron catalysts (e.g., Fe-C (Zhang
et al., 2015), Pd-Fe3O4 (Luo et al., 2014), and FexCuyOz (GarridoRamı́rez et al., 2016; Ganiyu et al., 2018)). Fig. 5A depicts the dominant catalytic activities in a heterogeneous catalytic EF system.
Fig 5B illustrates the promotion of hydroxyl radical generation
in the presence of transition metals.
Besides process modification, numerous efforts have also been
made to enhance the EF process efficiency by optimizing various
operating parameters, including pH, dissolved oxygen level, current density, temperature, and the electrode and catalyst types.
Recent works on recalcitrant organic compound degradation with
different electro-Fenton processes are summarized in Table 4.
Table 5 depicts recent research works on industrial effluent
treatment with electro-Fenton processes. It was found that the
Fered-Fenton and heterogeneous EF processes were more widely
applied in real industrial effluent treatment. Real industrial effluents are complex mixtures loaded with high concentrations of
pollutants, and they usually have water matrixes with diverse
Chapter 11 Fenton- and ozone-based AOP processes
-
+
pH adjustment
H+
H
pH adjustment
+
e–
a-FeOOH
Fe3O4
Fe2+
H+
e
O2
Fe
OH–
–
Pd/Fe3O4
FexCuyOz
H
-
+
H+
a-Fe2O3
+
+
H2O2
OH
Mn+
M(n+1)+
e–
H2O2
2+
Mn+
[H] + H2O
Fe2+
2+
H+
–
OH
OH
Fe
Fe-C
H+
FeO
OH
Fe3+
Fe2+ H2O2
e
H2O
OH
R
Anode
OH
[Byproducts]
R
(A)
OH
217
e–
Fe3+
Fe2+
Fe3+
e–
e–
e–
OH– H2O2
O2
+
OH
e–
O2
CO2 + H2O
Cathode
Anode
(B)
Cathode
Fig. 5 Mechanisms of heterogeneous catalytic EF process (A) in the presence of iron catalysts; (B) in the presence of
transition metals.
pH values and significant radical scavenging effects. FeredFenton could obtain the effectiveness of homogeneous Fenton
oxidation yet with less sludge production; it also requires simple
process design and operation procedures. The heterogeneous EF
process could be carried out under a wider pH operating range as
well as with chemical consumption. Hence, these two EF processes were frequently adopted for industrial effluent treatment.
3
Ozonation process
Ozonation is one of the most well-known and matured
advanced oxidation processes (AOPs). It has been widely applied
for the treatment of recalcitrant/toxic organics in industrial effluent (Oturan and Aaron, 2014; Jothinathan and Hu, 2018; Chen
et al., 2015b). Ozone molecules are effective oxidants with a high
oxidation potential (E° ¼ +2.07 eV). They react with various
organic and inorganic compounds either by a direct reaction of
molecular ozone or through an indirect radical-type reaction
Table 4 Recent research works on recalcitrant organic degradation with different electro-Fenton processes.
Target
organic
compounds
Phenol/activated
carbon fiber
regeneration
Florfenicol
Sulfomethaxazole
Indigo carmine
dye
Dinitrotoluene
(DNT)/2,4,6trinitrotoluene
(TNT)
Process/
reactor setup
Operating conditions
Process performance
References
Lab-scale EF
process, open
cylindrical, and
undivided
electrochemical
cell
Lab-scale EF
process
Activated carbon (AC) fiber as cathode
and BDD as anode, room temperature; pH:
3.0, current: 300 mA
91% of phenol removal, morphological
and chemical characteristics of AC were
not affected
Trellu et al.
(2018)
A graphene-modified EF catalytic
membrane (EFCM) as cathode, Pt foil as
counter electrode, T: 25°C, pH: 5.9 0.07
Operating potential: 0.6 V
Jiang et al.
(2018)
Lab-scale PEF
process, 3 L
undivided twoelectrode quartz
cell
Lab-scale PEF,
thermostatic
cylindrical glass
cell
Lab-scale SEF,
450 mL batch
reactor
UV lamp, 254/365 nm, RuO2/Ti mesh as
anode and ACF felt as cathode, room
temperature, pH: 3.0; Fe2+: 1 mM, current:
0.36 A
90% Florfenicol removal, EFCM could
not only act as cathode but also as
membrane barrier to concentrate and
enhance the mass transfer of florfenicol,
increasing its oxidation chances
80% TOC removal, PEF could efficiently
reduce carboxylic acid production for
quicker photodegradation by UV light
6 W fluorescent bulb, 360 nm, BDD as
anode and carbon-PTFE as cathode, 35°C,
pH: 3.0, Fe2+: 1 mM
Ultrasonic frequency: 120 kHz, platinum
plates were used as both anode and
cathode, T: 30°C, pH: 1, O2 flow rate:
150 mL/min, Fe2+: 150 mg/L, electrode
potentials: 6 V
Complete mineralization of indigo
carmine dye, Fe3+-oxalate complexes
were destructed under the action of
ultraviolet light
100% DNT and TNT removal in 7 h
Wang et al.
(2011)
Flox et al. (2006)
Chen and Huang
(2014)
Table 4 Recent research works on recalcitrant organic degradation with different electro-Fenton processes—cont’d
Target
organic
compounds
Process/
reactor setup
Phenol
Lab-scale SEF, 5 L
ultrasonic reactor
(type: DUMAN-120)
Catechol
Lab-scale
heterogeneous EF,
one-compartment
electrochemical
cell
Lab-scale
heterogeneous EF,
0.3 L undivided cell
2,4Dichlorophenol
Phenol
Phenol
Lab-scale
heterogeneous EF,
200 mL undivided
reactor
Lab-scale
heterogeneous EF,
singlecompartment
electrochemical
glass cell
Operating conditions
Process performance
References
Ultrasonic frequency: 34 kHz, stainless
steel used as both anode and cathode,
electrode distance: 5 cm, room
temperature; pH: 3.0, Fe2+: 4 mg/L
H2O2: 500 mg/L
Current density: 12 mA/cm2
Pt sheet as anode and f active carbon
fiber as cathode, T: 25°C; pH: 3; nanoFe3O4: 1 g/L
Current density: 10 mA/cm2
Phenol removal rate constants of
0.0683 min1 under the optimal
condition
Babuponnusami
and
Muthukumar
(2012)
100% Catechol removal in 2 h, verified
stability and reuse of the catalyst
Hou et al. (2015)
Ti/IrO2-RuO2 as anode and PTFE as
cathode, room temperature, pH: 6.7
Fe-C loading: 6 g/L, current: 100 mA
95% 2,4-Dichlorophenol removal in 2 h,
catalytic activity of Fe-C was well
maintained with much lower iron
leaching after PTFE coating
98% Phenol removal in 60 min with an
initial concentration of 20 mg/L
Zhang et al.
(2015)
Pt as both anode and cathode, T: 25°C,
Pd-Fe3O4 loading: 1 g/L, pH: 3, current:
50 mA
Modified glassy carbon as working
electrode, platinum wire as counter
electrode, Bimetallic (FexCuy) allophane
nanoclays as catalyst
T: 25°C, pH: 3, oxygen reduction potential:
0.6 V
100% Phenol and 80% COD removals in
2 h, 100% phenol removal at pH 5.5 in 4 h
Luo et al. (2014)
Garrido-Ramı́rez
et al. (2016)
Table 5 Recent research works on industrial effluent treatment with electro-Fenton processes.
Wastewaters/process
Initial
level
(mg/L)
Operating condition
H2O2 (mM)
Fe2+
(mM)
pH
Process performance
References
75
7.5
3
Ti sheet as cathode and RuO2/Ti as anode, current
density of 5.1 mA/cm2, 49.7% TOC removal in 2 h,
BOD5/COD ratio was increased from 0.11 to 0.31
Fan et al.
(2015)
Electronic wastewater/
Fered-Fenton
COD0:
580
TOC0:
198
COD0:
190
14.56
0.2
3
Pharmaceutical wastewater/
Fered-Fenton
COD0:
565
[H2O2]/
[Fe2+]:
3.6
2.89
GarciaRodriguez
et al. (2018)
Davarnejad
and Sabzehei
(2019)
Textile effluents/iron-loaded
sepiolite heterogeneous EF
COD0:
800
H2O2/
wastewater
volume ratio:
0.32 mL/L
In situ
generated
Graphene-based gas diffusion cathode and BDD
anode, current density of 29 mA/cm2, air flow of
0.2 L/min, 80% TOC removal in 3 h
Ferrous plate electrodes, current density of
58.47 mA/cm2, 97.21% COD removal in 59.68 min
20 g/L
catalyst
2
Iglesias et al.
(2013)
Winery wastewater/Feloaded activated carbon
heterogeneous EF
COD0:
52.8
In situ
generated
0.72
3.1
Coal gasification
wastewater/Fe-loaded
sludge derived activated
carbon heterogeneous EF
COD0:
173.3
TOC0:
57.6
Total
phenol:
48.3
In situ
generated
5 g/L
catalyst
6.8
Boron-doped diamond as anode and graphite sheet
as cathode, air flow rate of 1 L/min, cell voltage of
5 V, 86.7% COD removal in 90 min
Boron-doped diamond as anode and thick nickel
foam as cathode, air flow rate of 1 L/min, cell
voltage of 15 V, 100% of decolorization, and 82% of
COD removal
Ti as anode and activated carbon fiber as cathode,
air flow rate of 4.0 L/min, cell voltage of 3 V,
78.1% COD, 93.5% phenol, and 65.5% TOC removal
in 2 h
Reverse osmosis
concentrate/Fered-Fenton
Iglesias et al.
(2015)
Hou et al.
(2016)
Chapter 11 Fenton- and ozone-based AOP processes
Fig. 6 Reaction pathway of ozone and hydroxyl radicals (•OH).
involving the hydroxyl radicals induced by the ozone decomposition in water (Fig. 6) (Yong and Lin, 2012; Zimmermann et al.,
2011). In the direct reaction, ozone molecules could destroy aromatic structured organic compounds through reactions such as
cycloaddition, electrophilic addition, and nucleophilic addition
(Beltrán, 2004). Direct ozonation is a highly selective oxidation
process, that is, it is relatively unreactive toward many inorganic
species and some classes of organic compounds (e.g., carboxylic
acids) (Buxton et al., 1988; Gottschalk et al., 2010). Additionally,
the selective reactions between ozone molecules and unsaturated
bonds or amino groups in organics would lead to the formation of
aldehydes and carboxylic acid, which results in incomplete mineralization (Rosenfeldt et al., 2006). Indirect ozonation usually
occurs through radical chain reactions with the formation of
•
OH (E° ¼ + 2.80 eV), which is capable of oxidizing saturated aliphatic or aromatic compounds to form stable oxidized end products (Gottschalk et al., 2010).
The conventional ozonation process has some drawbacks such
as high-energy consumption due to poor utilization of gaseous
ozone, process selectivity, incomplete oxidation due to unfavorable reaction kinetics, and incomplete mineralization of recalcitrant organics (Xiong et al., 2018; Xia and Hu, 2018). Hence, it
becomes more imperative to develop modified ozonation process
that could enhance the ozone mass transfer and promote •OH
generation. Techniques for efficient ozone transfer are especially
desired for the cost-effective treatment of wastewater containing
high concentrations of organic pollutants. Techniques such as
catalytic ozonation processes, microbubble ozonation, and
catalytic-microbubble ozonation have emerged as efficient treatment methods to enhance ozonation efficiency by improving
ozone mass transfer and •OH generation.
221
Chapter 11 Fenton- and ozone-based AOP processes
3.1
Catalytic ozonation process
Catalytic ozonation processes have emerged to overcome the
drawbacks of the conventional ozonation process. The addition
of either a homogeneous or heterogeneous catalyst into the ozonation process could promote •OH generation, thereby enhancing
the removal efficiency of organic compounds ( Jung and Choi,
2006; Park et al., 2004; Wang and Bai, 2017). Heterogeneous catalysts are generally preferred when compared to homogeneous catalysts because of the sustained reactions of ozone through the
active sites of the catalyst. Additionally, heterogeneous catalysts
are easy to recover and reuse, even after several treatment cycles
(Wang et al., 2019a; Legube and Karpel Vel, 1999). In most cases,
the catalytic effect mainly involves enhancing the ozone molecular decomposition for hydroxyl radical generation and promoting
the oxidative degradation of organic contaminants. The catalytic
effect only occurs when at least one of the three conditions is fulfilled: (1) ozone is adsorbed on the catalyst surface; (2) Organic
molecules are adsorbed on the catalyst surface; or (3) both ozone
and organic molecules are adsorbed on the catalyst surface
(Fig. 7). An efficient catalyst requires high physical adsorption
capacity and excellent catalytic activity, and the catalytic activity
highly depends on the catalyst’s chemical-physical properties
such as pore volume, pore size, surface area, and mechanical
n
actio
ct re
Dire
Organics
Oxidation
byproducts
O3
t
irec
Ind tion
c
rea
222
Reactive species
HO2∑, O2-∑, and O3-∑
∑OH
Organics
Oxidation byproducts
and CO2
Organics
Adsorption
Fig. 7 Schematic representation of catalytic ozonation mechanism.
Chapter 11 Fenton- and ozone-based AOP processes
strength (Legube and Karpel Vel, 1999; Nawrocki and KasprzykHordern, 2010).
The most widely used catalysts in heterogeneous catalytic
ozonation are supported or unsupported metal oxides and
carbon-based heterogeneous catalysts (Nawrocki and KasprzykHordern, 2010). Several studies have been performed using catalysts such as activated carbon, metal oxides, and minerals to prove
their effectiveness in initiating O3 decomposition to form •OH,
simultaneously improving organic oxidation (Beltrán et al.,
2005; Zhang et al., 2008; Kasprzyk-Hordern et al., 2006; Rosal
et al., 2009). A literature review has provided evidence that
carbon-based heterogeneous catalysts such as activated carbons
(ACs) and carbon nanotubes (CNTs) considerably improve the
removal rate of organic pollutants due to their physical stability
and mesoporous structure (Nawrocki and Kasprzyk-Hordern,
2010; Alvárez’ et al., 2009).
Many studies have reported that ACs and CNTs could participate in ozone radical reactions and initiate O3 decomposition for
•
OH generation (Liu et al., 2009; Beltrán et al., 2009). It has been
demonstrated that the main factor responsible for ozone decomposition in the presence of ACs is the combination of basic surface
groups and metal centers. Leili et al. (2013) demonstrated the degradation of furfural using AC as a catalyst. It was found that 80.2%
of furfural removal was mainly achieved by •OH oxidation via
catalytic ozonation. A study from Faria et al. (2008) reported that
the AC-initiated catalytic ozonation could achieve significant
sulfonated aromatic compound degradation due to the combined
effects of direct ozonation, adsorption, and free radical oxidation.
In another study (Alvárez’ et al., 2009), the authors demonstrated
that using granular activated carbon (GAC) as the catalyst induced
the effective removal of gallic acid in a secondary effluent. They
observed that GAC could adsorb gallic acid very efficiently, but
exhibited limited capacity for COD removal in the secondary
effluent due to the substantial fraction of organic matter not
adsorbed onto the GAC surface.
Similar to ACs, carbon nanotubes promote •OH formation during ozonation to levels exceeding ozone and almost equivalent to
O3/H2O2 (Oulton et al., 2015). The alternation of a carbon catalyst
surface using HNO3 has exhibited vastly greater rates of ozone
consumption and •OH formation (Cho et al., 2011; Sui et al.,
2012; Upadhyayula et al., 2009). Tizaoui et al. (2015) demonstrated
the degradation of methyl orange using carbon nanotubes as the
catalyst. It was found that the addition of CNTs significantly
improved the decolorization of methyl orange as compared to
the ozone process alone. Liu et al. (2011) studied the degradation
223
224
Chapter 11 Fenton- and ozone-based AOP processes
of oxalic acid using multiwalled carbon nanotubes (MWCNTs),
where they found that a higher MWCNT dosage resulted in better
removal of oxalic acid. Recently, researchers have shown interest
in graphene-based carbon material for water treatment as an
alternative to CNTs and ACs due to its functional group, high surface area, and distinct mechanical and electrical properties (Yang
et al., 2014). Graphene oxide (GO) is an oxidized form of graphene
with carboxyl, hydroxyl, carbonyl, and epoxy functional groups in
its carbon lattice (Perreault et al., 2015). Jothinathan and Hu
(2018) proposed that GO without any surface modification could
act as an efficient •OH scavenger and inhibit the reaction between
•
OH and organics, and surface-modified GO was adopted in this
study for effective ibuprofen degradation.
Recently, metal-doped catalysts with carbon substrates have
become popular due to their better catalyst activity and stability
(Jothinathan and Hu, 2018; Le et al., 2015; Shahamat et al., 2014).
Huang et al. (2017a) reported that manganese oxide-doped sewage
sludge-derived activated carbon (MnOx/SAC) exhibited the highest catalytic activity at acidic pH for the degradation of oxalic acid.
Wang et al. (2019b) adopted Fe-supported regenerated granular
activated carbon (Fe/rGAC) as the catalyst in textile wastewater
treatment. It was found that Fe/rGAC improved the performance
of the catalytic ozonation process by 14%–25% when compared
to using rGAC as the catalyst. In addition, they demonstrated that
•
OH generation was significantly improved by Fe/rGAC, contributing to a biodegradability enhancement of 0.24–0.55 times when
€ et al. (2015)
compared to the conventional ozonation process. Lu
studied the degradation of 2,4-dichlorophenoxyacetic acid by
using bimetal coated activated carbon (Fe-Ni/AC) in the catalytic
ozonation process. The target pollutant’s degradation rate constant was 1.9 times higher than that in the conventional ozonation
process. Zhuang et al. (2018) reported that manganese oxidesupported activated carbon was highly effective for treating
paper-making wastewater. They observed the significant inhibition of radical scavengers in the catalytic ozonation process, which
is attributed to the catalyst-promoting ozone decomposition for
generating more •OH. Table 6 depicts recent research works on
carbon-based heterogeneous catalytic ozonation for industrial
effluent treatment.
Similar to carbon-based materials, minerals (Al2O3, TiO2,
ceramics, etc.) and natural solid minerals (zeolites, soils, sandbased material, and goethite) have been widely used in heterogeneous catalytic ozonation (Wang et al., 2019a). Yuan et al. (2016)
reported the effective degradation of p-chloronitrobenzene
(p-CNB) during the iron-doped pumice (Fe/Pumice) initiated
Table 6 Recent research works on industrial effluent treatment with the carbon-based heterogeneous catalytic
ozonation process.
Wastewater/
organics
Catalyst
Operating condition
Process performance
References
Secondary
effluent
Activated carbon
TOC removal: 50%
Alvárez’ et al. (2009)
Phenol in
industrial
wastewater
Fe-Mn/granular
activated carbon
Phenol removal: 100%
TOC removal: 60.1%
Xiong et al. (2019)
Reverse
osmosis
concentrate
Activated carbon
TOC removal: 58%
Fang and Han (2018)
Bio-treated
textile
wastewater
Regenerated
activated carbon
(rGAC) and
Fe-rGAC
Catalyst dosage: 2 g
O3 concentration: 40 mg/L
[TOC]0: 171 12 mg/L
pH: 6.12
BOD5: 150 100 mg/L
Catalyst dosage: 1.0 g/L
O3 concentration: 22 mg/L
[TOC]0: 149 mg/L
[Phenol]0: 200 mg/L
BOD5/COD: 0.011
[TOC]0: 67.4 mg/L
Catalyst dosage: 2.0 g/L
Ozone dosage: 120 mg/L
pH: 6–7
COD: 50.5 mg/L
pH: 7.8
DOC: 17.8 mg/L
UV254: 0.64
Color: 30 (unit)
Ozone dosage: 18 mg/L
Wang et al. (2019b)
Paper-making
wastewater
MnOx/activated
carbon
COD removal:
Fe- rGAC: 28%
rGAC: 21%
Color removal:
Fe- rGAC: 81.7%
rGAC: 56.7%
UV254 removal:
Fe- rGAC: 59.4%
rGAC: 45.3%
COD removal: 77.5%
BOD5/COD: 0.46
Color: 58.5
COD: 210 20 mg/L
Color: 110 10
BOD5/COD: 0.16
Catalyst dosage: 1.0 g/L
Ozone dosage: 10 mg/L
Zhuang et al. (2018)
Continued
Table 6 Recent research works on industrial effluent treatment with the carbon-based heterogeneous catalytic
ozonation process—cont’d
Wastewater/
organics
Catalyst
Operating condition
Process performance
References
Oxalic acid
Fe2O3/activated
carbon
Oxalic acid removal efficiency: 89%
Li et al. (2018)
Phenolic
wastewater
Nano Fe3O4/
activated carbon
BOD5/COD: 0.52
Phenol removal: 98.5%
COD removal: 69.8%
Farzadkia et al.
(2014)
p-Hydroxyl
benzoic acid
(PHBA)
Reduced graphene
oxide (rGO)
PHBA removal efficiency: 95%
Remarks: carbonyl groups in rGO acted as the
active sites to generate reactive oxygen species
for catalytic reaction
Wang et al. (2016)
Oxalic acid
Multiwalled
carbon nanotubes
(MWCNTs)
Multiwalled
carbon nanotubes
(MWCNTs)
Catalyst: 0.71 g/L
[Oxalic acid]0: 10 g/L
Contact time: 60 min
Ozone dosage: 0.8 mg/min
Catalyst: 2 g/L
pH: 8.0
[Phenol]0: 500 mg/L
BOD5/COD: 0.3
Ozone dosage: 33 mg/L min
Catalyst: 0.2 g/L
[PHBA]0: 5 mg/L
Contact time: 60 min
Ozone dosage: 20 mg/L
Initial pH: 3.5
Catalyst: 0.14 mg/L
Initial pH: 3.0
[DMP]0: 90 mg/L
Catalyst: 20 mg/L
Initial ozone dosage: 10 mg/L
[pCBA]0: 2 mM
Tert-butanol: 320 mM
pH: 7
Catalyst: 10 mg/L
[MO]0: 20 mg/L
Contact time: 2 min
Ozone dosage: 2 mg/L
Initial pH: 3.0
Catalyst: 0.5 g/L
[BPA]0: 50 mg/L
Contact time: 40 min
Ozone dosage: 3.0 mg/L
Removal efficiency: 90%–100% (based on ballmilled MWCNT duration)
Soares et al. (2015)
Modified MWCNT oxidized with 70% HNO3
performed better
pCBA removal efficiency: 80%
Oulton et al. (2015)
MO removal efficiency: 60%
Tizaoui et al. (2015)
BPA removal efficiency: 90%
Huang et al. (2017b)
paraChlorobenzoic
acid (pCBA)
Methyl orange
(MO)
Carbon nanotubes
Bisphenol A
(BPA)
MWCNTs/Fe3O4
Chapter 11 Fenton- and ozone-based AOP processes
catalytic ozonation process. Liu et al. (2016) reported that Mn-Fe/
Al2O3 exhibited the strongest catalytic activity for bisphenol degradation with good stability and reusability. Roshani et al. (2014)
investigated the degradation of benzotriazole by using Cu/Al2O3,
Mn/Al2O3, and Mn-Cu/Al2O3 as catalysts. At alkaline pH condition, Mn-Cu/Al2O3 exhibited the highest catalytic activity.
Table 7 presents recent studies on the catalytic ozonation processes that adopt metal-doped minerals for industrial effluent
treatment.
3.2
Microbubble ozonation process
In the ozonation process, the dissolution rate of ozone gas into
the liquid phase mainly depends on the gas-liquid interfacial area.
In order to enhance the ozone mass transfer rate, it becomes necessary to increase the gas-liquid interfacial area by decreasing the
ozone bubble size. Microbubble ozonation technology has drawn
great attention as it could provide small bubble size with diameters between 10 and 50 μm. Compared to a macrobubble, the
microbubble obtains a huge interfacial area, a long stagnation
time, a low bubble rising speed, and a high interior pressure
(Xiong et al., 2018; Takahashi et al., 2007). The most distinct characteristic of microbubbles is that they could shrink and collapse in
the liquid phase due to high interior pressure while macrobubbles
could only burst after rising to the surface of the liquid phase. The
size reduction of the microbubbles during the shrinking stage is
due to the dissolution of gas present inside the microbubbles; this
gas will eventually disappear after the microbubbles collapse
(Fig. 8).
Takahashi et al. (2007) demonstrated that microbubbles could
enhance gas hydrate formation due to their ability to alter the
nucleation condition and gas solubility. The correlation between
the microbubble size and the interior gas pressure could be presented by the Young-Laplace equation:
P ¼ PI +
2σ
r
(13)
where P and PI are the gas and liquid pressure, respectively, σ is the
liquid surface tension, and r is the radius of the microbubble.
According to Eq. (13), the interior pressure of the microbubbles
is inversely proportional to the bubble radius, thus a reduction
of bubble size would lead to a sharp increment in the interior pressure. According to Henry’s law, the amount of dissolved gas
around the shrinking microbubbles would increase with higher
microbubble interior pressure. When the interior pressure’s
227
Table 7 Recent research works on industrial effluent treatment with the metal oxide-based heterogeneous catalytic
ozonation process.
Wastewater/organics
Catalyst
Diclofenac
Iron silicateloaded pumice
Acid Red B
Cu-Mn/Al2O3
Coal chemical wastewater-biotreated effluent
Cu-Mn-Ce/Al2O3
Petroleum refinery wastewater
Mn-Fe-Cu/Al2O3
Nitrobenzene benzoic acid (recalcitrant organic
in chemical wastewater)
Vanadium oxides/
zeolites
Synthetic petroleum effluent
g-Al2O3
Operating
condition
Catalyst: 0.8 g/L
Initial O3 concentration:
5.52 mg/L
[Diclofenac]0: 29.6 mg/L
Catalyst: 4.0 g/L
[Acid Red B]0: 250 mg/L
Contact time: 20 min
Ozone dosage:
4.26 mg/min
Initial pH: 8.5
Catalyst: 20 mg/L
Initial pH: 7.0
[COD]0: 180 mg/L
Ozone dosage: 80 mg/L
Catalyst: 20 g
Initial pH: 8.2
[COD]0: 2825 mg/L
BOD5/COD: 0.098
Catalyst: 0.5 g
Initial ozone dosage:
10 mg/L
Nitrobenzene:
55.9 mg/L
Benzoic acid: 67.5 mg/L
Nitrobenzene: 6.5
Benzoic acid: 3.5
Catalyst: 2 g/L
[TOC]0: 250 mg/L
Ozone dosage: 5 mg/L
Process performance
References
Diclofenac removal: 73.3%
Yuan et al.
(2012)
Acid Red B removal: 99.35%
Li et al. (2015)
COD removal: 70%
Teng et al.
(2019)
COD removal: 67.1%
BOD5/COD: 0.330
Chen et al.
(2015b)
TOC removal:
Nitrobenzene: 85.4%
Benzoic acid: 88.3%
Xu et al.
(2019)
TOC removal: 90% with
addition of NaCl
Vittenet et al.
(2015)
Chapter 11 Fenton- and ozone-based AOP processes
Fig. 8 Pathways of microbubbles for •OH formation.
increasing rate is sufficiently high, the temperature within the
microbubble would rise sharply because of adiabatic compression, leading to the collapse of the microbubbles (Takahashi
et al., 2007, 2003). Takahashi et al. (2007) demonstrated the generation of free radicals (especially •OH) from the collapsing of
microbubbles. Possible pathways for •OH formation during the
collapsing of microbubbles were proposed as following: (1) excess
ions trapped on the surface of the microbubble interface; (2) collapsing of the microbubble leading to a sudden disappearance of
the gas-liquid interface; (3) significant increase in ion concentration around the shrinking microbubble gas-liquid interface;
and (4) generation of radicals through adiabatic compression.
Xu et al. (2012) investigated •OH generation during the fewmicrosecond collapse of microbubbles through dynamic stimuli
such as ultrasound and shock waves. It was found that dynamic
stimuli induced more efficient free radical generation when
compared to the process involving microbubble shrinking and
collapsing.
In recent years, microbubble ozonation has been recognized as
a promising AOP technology for industrial effluent treatment, as it
could overcome the major drawbacks of conventional ozonation
processes. The distinctive physicochemical properties of microbubbles could enhance the ozone gas mass transfer and •OH formation, thereby increasing the ozone utilization rate and
promoting organic oxidation. It was reported that smaller ozone
microbubbles could induce a larger mass transfer coefficient
and higher ozone saturation concentration (Xiong et al., 2018).
In the microbubble ozonation process, the reactive oxygen species (especially •OH, HO2 , O2 , and O3 ) could be generated
229
230
Chapter 11 Fenton- and ozone-based AOP processes
through two major mechanisms: (1) shrinking and collapsing of
O3 microbubbles; and (2) self-decomposition of O3 and radical
chain reactions. The additional •OH production from microbubble shrinkage and collapse has huge benefits on the oxidation efficiencies of recalcitrant organic compounds. Xia and Hu (2018)
pointed out that the collapse of ozone microbubbles could also
induce local turbulence and facilitate the reaction between
O3/•OH and organic pollutants, thereby enhancing the oxidation
efficiency. Table 8 depicts recent research works on industrial
effluent treatment with the microbubble ozonation process. Li
et al. (2009) studied the oxidation efficiency of free radicals generated from air, oxygen gas, and nitrogen microbubbles in terms
of phenol removal The results showed that with 2 h contact time,
the phenol removal rates in the air, oxygen gas, and nitrogen
microbubble-mediated reaction system were 36%, 59%, and
83%, respectively. Zheng et al. (2015) compared the performance
of macrobubble and microbubble ozonation for treating acrylic
fiber wastewater. It was reported that the microbubble ozonation
process could enhance the COD, NH3-N, and UV254 removal by
25%, 35%, and 9% when compared to the macrobubble ozonation
process. Chu et al. (2008) studied the treatment of textile wastewater with microbubble ozonation and conventional ozonation processes, and it was indicated that the decolorization rate and COD
reduction were both significantly improved in the microbubble
ozonation process. The literatures shows that a combination of
ultrasound and microbubble ozonation could further improve
the ozone mass transfer and •OH formation. Ultrasonication
could induce several turbulences in the aqueous phase and benefit the mixing of ozone molecules and target pollutants. Additionally, the ultrasound waves could intensify •OH generation
via pyrolysis. •OH could be generated inside and/or near the interface of bubble cavity. According to Guo et al. (2016), sulfamethoxazole degradation was mainly induced by the direct oxidation of
ozone molecules rather than the •OH reaction. Zhao et al. (2015)
demonstrated that ultrasound could intensify the ozone decomposition and accelerate the initiation of •OH generation, thereby
enhancing nitrobenzene degradation.
3.3
Heterogeneous catalytic-microbubble
ozonation process
Similar to the microbubble ozone/ultrasound process, the
hybrid technology of combining the heterogeneous catalyticmicrobubble ozonation process could improve both ozone mass
Table 8 Recent research works on industrial effluent treatment with the microbubble ozonation process.
Wastewater/organics
System
Operating condition
Acrylic fiber wastewater
O3 macrobubbles
O3 microbubbles (M-O3)
O3 dosage: 5 g/h
Temperature: 20°C
Organic-contaminated wastewater
with high salinity
O3 micronanobubbles
[COD]0: 4500 mg/L
Cl: 123,000 mg/L
Dimethyl phthalates (DMP)
O3 microbubbles
Textile wastewater
O3 microbubbles
Phenol containing synthetic water
O3 microbubbles
Dimethyl phthalate
Microbubble-O3/
ultrasound
Reactive Red X-3B dye
O3/ultrasound
SO4 2 : 58,500 mg/L
Na+: 114,000 mg/L
O3 gas flowrate: 3.5 g/h
Initial pH: 9
Contact time: 1 h
[DMP]0: 1.03 mol/m3
[O3]0: 132 mg/L
Initial pH: 8.7
[COD]0: 530–600 mg/L
BOD5/COD: 0.15–0.22
[O3]0: 20 mg/L
[Phenol]0: 0.1 mol/L
[O3]0: 1.11 mg/s
[DMP]0: 1.029 mol/m3
pH: 7
[Red X-3B]0: 100 mg/L
pH: 6.5
Ultrasonic intensity:
200 W L1
Process
performance
References
O3 macrobubbles
COD removal: 17%
UV254 removal: 7%
NH3-N removal: 12%
O3 microbubbles
COD removal: 42%
UV254 removal: 42%
NH3-N removal: 21%
COD removal: 63%
Zheng et al. (2015)
DMP removal: 95%
Jabesa and Ghosh
(2017)
COD removal: 70%
Color removal: 80%
Chu et al. (2008)
Phenol removal: 80%
Ozone consumption:
Microbubble: 10 mg/L
Macrobubble: 40 mg/L
DMP removal: 95%
Wu et al. (2019)
Dye removal: 99.2%
Jabesa and Ghosh
(2017)
Xia and Hu (2018)
Jabesa and Ghosh
(2017)
232
Chapter 11 Fenton- and ozone-based AOP processes
transfer and •OH generation for better oxidation efficiency. As previously mentioned, external dynamic stimuli such as ultrasound
and shock waves could catalyze microbubble ozonation via the
collapsing of microbubbles. Besides dynamic stimuli, incorporating a heterogeneous catalyst into the microbubble ozonation system could also enhance the generation of •OH. Many studies have
demonstrated effective recalcitrant organic removal in the catalytic ozonation process (Yong et al., 2005; Gharbani and
Mehrizad, 2014; Deng et al., 2015) while it was highlighted that
the incorporation of a heterogeneous catalyst in the conventional
catalytic ozonation process was not adequate for cost-effective
recalcitrant organic degradation (Liu et al., 2018; Cheng
et al., 2018).
The main mechanisms for enhanced organic degradation in
the heterogeneous catalytic-microbubble ozonation process
might include: (1) enhanced ozone mass transfer contributed
by ozone microbubbles; and (2) the surface-mediated process
of a heterogeneous catalyst enhancing •OH formation (Fig. 9).
Liu et al. (2018) adopted the activated carbon catalyticmicrobubble ozonation process for treating bio-treated coal.
Organics
Adsorption
Catalyst
Organics
·OH
Oxidation
bypoducts and CO2
Microbubble-catalytic O3
·OH
Oxidation byproducts from
M-O3 mechanism
Microbubble ozone
(M-O3)
Oxidation byproducts
Fig. 9 Mechanism of
heterogeneous catalyticmicrobubble ozonation process.
O3,·OH
Organics
Chapter 11 Fenton- and ozone-based AOP processes
233
The results show that this ozonation process achieved efficient
degradation of refractory compounds in bio-treated coal wastewater with a higher ozone utilization rate (98%), and the residual
ozone concentration was negligible. Similarly, Ma et al. (2019)
adopted Fe3O4 nanoparticles@cow dung ash as the catalyst for
carrying out catalytic-microbubble ozonation and treating biologically pretreated leachate. An enhanced generating rate of
•
OH, effective refractory pollutant degradation, and biodegradability enhancement were observed. Huang et al. (2017b) reported
a higher ozone utilization rate during the treatment of biologically
treated leachate from municipal solid waste by incorporating activated carbon into a microbubble ozonation system. Table 9 summarizes recent research works on industrial effluent treatment
with the heterogeneous catalytic-microbubble ozonation process.
Table 9 Recent research works on industrial effluent treatment with the heterogeneous
catalytic-microbubble ozonation process.
Wastewater/
organics
System
Operating condition
Leachate from municipal
solid waste
M-O3/
activated
carbon
Biologically pretreated
leachate
M-O3/Fe3O4
Bio-treated coal
wastewater
M-O3/
activated
carbon
Phenol-containing
wastewater
M-O3/Ca(OH)2
COD: 817 50 mg/L
pH: 8
Color: 350–400
Catalyst dosage: 350 g
Ozone dosage: 60 mg/L
COD: 1270 50 mg/L
TOC: 492 25 mg/L
BOD5/COD: 0.05
Catalyst dosage: 0.8 g/L
Ozone dosage: 3.0 g/L
COD: 283.8 mg/L
pH: 8
TOC: 99.8 mg/L
BOD5/COD: 0.038
Catalyst dosage: 28% (filling
ratio in reactor)
Ozone dosage: 30 mg/L
[Phenol]0: 450 mg/L
Ozone dosage: 65 mg/L
Catalyst dosage: 2 g/L
TOC: 365 mg/L
Process
performance
References
COD removal:
89.9%
Huang et al.
(2019)
COD removal:
53%
BOD5/COD: 0.32
Ma et al.
(2019)
COD removal:
42%
Ozone utilization
rate: 98%
BOD5/COD: 0.30
Liu et al.
(2018)
Phenol removal:
100%
Cheng et al.
(2018)
Continued
234
Chapter 11 Fenton- and ozone-based AOP processes
Table 9 Recent research works on industrial effluent treatment with the heterogeneous
catalytic-microbubble ozonation process—cont’d
Wastewater/
organics
System
Operating condition
Synthetic dye
wastewater (Acid
Red 3R)
Acid Red 18 wastewater
M-O3/
activated
carbon
M-O3/Ca(OH)2
Catalyst: 2 g/L
Ozone dosage: 12.5 mg/min
[Acid Red 3R]0: 100 mg/L
TOC: 145 mg/L
[Acid red 18]0: 450 mg/L
Catalyst dosage: 3 g/L
Ozone dosage: 65 mg/L
Process
performance
References
TOC removal:
75%
Zhang et al.
(2018)
Acid Red 18
removal: 100%
TOC removal:
100%
Quan et al.
(2017)
4 Applications of Fenton and ozone-based
AOPs for industrial effluent treatment
Fenton- and ozone-based AOPs are versatile technologies for
industrial effluent treatment. They can be applied as pretreatment
processes for recalcitrant organic abatement and biodegradability
enhancement. They can also be applied as tertiary treatment processes for meeting stringent discharge standards. In this section,
recent pilot-scale and full-scale applications of Fenton- and
ozone-based AOPs for industrial effluent treatment are discussed.
4.1
Fenton-based AOPs
4.1.1
Fenton process
The pilot-scale treatment of pharmaceutical wastewater with
the Fenton process was studied by Cui et al. (2015). Raw pharmaceutical wastewater had an initial COD level of 4061 mg/L and a
target pollutant berberine level of 709 mg/L. The maximum treatment capacity of the pilot system was 5 tons/d while the Fenton
process operating conditions were first optimized by response
surface methodology (RSM), and the pilot system was continuously operated for 56 days. The RSM results suggested the optimum operating conditions to be pH 3, an H2O2/COD molar
ratio of 1.25, an Fe2+/H2O2 molar ratio of 0.1, and a system flow
rate of 100 L/h. During the 56-day stable operation, the average
removal efficiencies of COD and berberine were 28.3% and
77.7%, respectively. The BOD5/COD ratio of the pharmaceutical
wastewater was enhanced from less than 0.1 to 0.3.
Chapter 11 Fenton- and ozone-based AOP processes
Bae et al. (2015) reported a full-scale application of the Fenton
process in a dyeing wastewater pretreatment plant, which
receives wastewater from 61 dyeing factories. The total treatment
capacity of the plant was 100,000 m3/day, and the plant influent
had a pH value of 10–12 and an average soluble COD (SCOD) level
of 1100 mg/L. The main treatment processes consisted of primary
sedimentation, a pure oxygen-activated sludge process (ASP), the
Fenton process, coagulation, and final sedimentation. Plant effluent was discharged to a regional municipal wastewater treatment
plant. The Fenton process operating pH was 3.5 and the HRT was
30 min. The adopted H2O2 and FeSO47H2O dosages were 4.0 and
4.2 mM, respectively, and H2O2 was injected with a two-step dosing mode (30% at 12 min and 70% at 16 min). In the coupled
biological-AOP processes, pure oxygen ASP achieved 53% SCOD
and 13% color removal while the Fenton process removed 66%
of SCOD and 73% of color. The effluent COD after final sedimentation was 128 mg/L. A full-scale application of rapid Fenton oxidation for bio-treated dyeing and finishing wastewater (BDFW)
treatment was reported by Chen et al. (Chen et al., 2019). The
highlight of this study is that the rapid Fenton oxidation process
was conducted in second-scale intervals using pipeline reactors.
The full-scale plant with a treatment capacity of 400,000 m3/day
has been continuously operated in 16 pipeline reactors since
2014. Each pipeline reactor had a length of 13.6 m and a working
volume of 18.32 m3; rapid Fenton oxidation, neutralization, and
flocculation were carried out in sequence inside the pipeline
reactor. The estimated HRT of Fenton oxidation was 24 s, which
is significantly shorter than the conventional Fenton process.
After Fenton oxidation, the COD, SCOD, and DOC levels in BDFW
were reduced from 140, 110, and 35 to 77, 71, and 26 mg/L,
respectively.
4.1.2
FBR-Fenton process
The FBR-Fenton technology has been exploited in full-scale
industrial effluent treatment in Taiwan since 2000. It is usually
applied to achieve further COD reduction in bio-treated wastewaters containing recalcitrant organic pollutants. Table 10 summarizes several completed projects from Greentec Environmental
Protection Technology (GreenTec, www.biogreentec.com.tw)
and EVER-CLEAR Environmental Eng. Corp. (EVER-CLEAR,
www.ever-clear.com.tw). These projects demonstrate the fullscale applications of FBR-Fenton in different industrial sectors,
including the paper mill, yeast production, leather manufacturing, and electronic industries.
235
236
Chapter 11 Fenton- and ozone-based AOP processes
Table 10 Full-scale application of FBR-Fenton technology for treating different industrial
effluents.
Wastewater
Treatment
capacity (m3/day)
Water characteristics
before treatment
Water characteristics
after treatment
Pulp and paper effluent
86,000
Yeast production
effluent
Leather manufacturing
effluent
Electronic industry
effluent
ABS resin production
effluent
8300
COD 800 mg/L
SS 100 mg/L
COD 1000 mg/L
COD 100 mg/L
SS 40 mg/L
COD 200 mg/L
1200
COD 300 mg/L
COD 100 mg/L
3000
COD 210 mg/L
COD 80 mg/L
5000
COD 140 mg/L
COD 60 mg/L
4.1.3
Photo-Fenton process
Despite the excellent performance of photo-Fenton reported in
recent lab-scale treatability studies, process cost-effectiveness
studies in large-scale applications are quite limited. The most
promising studies were carried out in at the pilot scale using the
solar photo-Fenton process. Ruı́z-Delgado et al. (2019) investigated the degradation of four micropollutants (terbutryn, chlorfenvinphos, pentachlorophenol, and diclofenac) in olive mill
wastewater (OMW) with the photo-Fenton process. The polyphenols presented in OMW could act as iron chelators and allow for
effective Fenton oxidation without water acidification. The
photo-Fenton process was carried out in a compound parabolic
collector (CPC) solar pilot plant, with a total irradiation area of
3.08 m2 and a reactor volume of 39 L. With optimum dosages of
0.1 mM Fe2+ and 1.47 mM H2O2, more than 90% removal of the four
micropollutants was achieved by the solar photo-Fenton pilot system. Foteinis et al. (2018) conducted a life cycle analysis (LCA) of a
semiindustrial solar photo-Fenton system for treating pharmaceutical laboratory effluent. The solar photo-Fenton process was conducted in a CPC reactor with a mean solar intensity of 30 W/m2
and a treatment capacity of 0.7 m3/h, The pharmaceutical wastewater contained 1914 mg/L TOC and 389 mg/L antipyrine, and it was
acidified to a pH value of 2.7 before treatment. Under the optimal
operation conditions with 73.53 mM H2O2 and 0.36 mM Fe2+, the
Chapter 11 Fenton- and ozone-based AOP processes
237
CPC plant achieved 100% removal of antipyrine and 21% mineralization efficiency. The LCA results revealed that chemical reagent
consumption (mainly H2O2) was the most significant environmental hotspot for CO2 emission. Also, this semiindustrial solar
photo-Fenton system was a sustainable technology for treating
micropollutant-containing wastewater.
4.1.4
Electro-Fenton process
The Fered-Fenton technology has been exploited in full-scale
industrial effluent treatment. A commercial Fered-Fenton system
generally consists of an electrolytic reactor, electric supplies, and
a control system. The Fered-Fenton system is typically operated in
batch mode by continuously recirculating the reaction mixture
between the oxidation tank and the electrolytic reactor, wherein
Fe2+ regeneration occurs by the cathodic reduction of Fe3+, being
returned to the oxidation tank as a catalyst. This cycle is repeated
enough to complete the reaction in the oxidation tank. Table 11
summarizes several completed projects from GreenTec Environmental Protection Technology These projects demonstrate the
full-scale applications of Fered-Fenton in different industrial sectors, including the printboard manufacturing, fiber production,
and electronic industries. Compared to FBR-Fenton, the FeredFenton system is primarily aimed at wastewater treatment with
a medium to high organic load, and its treatment capacity is relatively lower.
Table 11 Full-scale application of Fered-Fenton technology for treating different industrial
effluents.
Wastewater
Printboard
manufacturer
wastewater
Synthetic fiber
production wastewater
Electronic industry
wastewater
Surface treatment
wastewater
Treatment
capacity (m3/day)
Water characteristics
before treatment
Water characteristics
after treatment
30
COD 2000 mg/L
COD 400 mg/L
100
COD 1000 mg/L
COD 100 mg/L
1
COD 18,000 mg/L
COD 1800 mg/L
10
COD 10,000 mg/L
COD 2000 mg/L
238
Chapter 11 Fenton- and ozone-based AOP processes
4.2
Ozone-based AOPs
4.2.1
Catalytic ozonation process
A pilot-scale treatment of coal gasification wastewater with the
catalytic ozonation process was conducted by Wei et al. (2019),
where a novel catalyst Cu-Co bimetal induced NiCAF (CuCO/
NiCAF) was developed and adopted in the treatment process.
The long-term operation of the catalytic ozonation system was
carried out with a treatment capacity of 5 m3/day. In the presence
of a developed catalyst, the ozone utilization efficiency was
increased by 120% with a higher ΔCOD/ΔO3 ratio of 2.12, compared to a ΔCOD/ΔO3 ratio of 0.96 in the conventional ozonation
process. Furthermore, 75% power consumption reduction was
achieved in the catalytic ozonation process. The total cost for
CuCO/NiCAF catalyst preparation was estimated to be USD
1.26/kg and the operating cost for removing 1 kg COD was USD
0.4, whereas the operating cost in the conventional ozonation process was estimated to be USD 1.6/kg COD. The considerable
reduction of operating cost was due to lower ozone consumption
in the catalytic ozonation process, where the required ozone dosages were 25 and 100 mg/L in the catalytic and conventional ozonation processes, respectively. Additionally, it was highlighted that
the developed catalyst was able to achieve both high catalytic
activity and excellent structural stability, which are feasible for
large industrial-scale applications. Ma et al. (2018) conducted a
pilot-scale study on treating bio-treated dyeing and finishing
wastewater. Recycled iron shavings were used in the catalytic
ozonation system, and the effective reaction volume was 2.9 m3.
The influent COD levels ranged from 132 to 148 mg/L, and an
average effluent COD level below 80 mg/L was achieved with
30 min HRT. Zhuang et al. (2017) developed a metal-doped
activated carbon catalyst (MnOx/activated carbon) for conducting the pilot-scale treatment of coal gasification wastewater.
An MnOx/activated carbon-mediated catalytic ozonation process
was able to achieve 72% COD removal (average influent
COD: 150 mg/L) with an effective ozonation utilization rate
(ΔCOD/ΔO3: 1.21). Wu et al. (2017) integrated the catalytic ozonation and persulfate oxidation into a pilot-scale pretreatment
technology for dry-spun acrylic fiber wastewater treatment. The
optimum operating conditions were identified as 40 g/h ozone
dosage, 4.44 h HRT, 61.8°C reaction temperature, and 1.3 kg/ton
persulfate. The maximum COD removal efficiency was 44.36%,
and the wastewater biodegradability was enhanced from 0.078
to 0.315 in terms of the BOD5/COD ratio.
Chapter 11 Fenton- and ozone-based AOP processes
4.2.2
Microbubble ozonation process
Khuntia et al. (2015) investigated the degradation of brilliant
green dye in a pilot-scale microbubble ozonation system. The
observed TOC and dye removal efficiencies were 80% and 90%,
respectively. By using 25 μm ozone microbubbles, an effective
wastewater treatment performance could be obtained at a very
low ozone generation rate due to the enhanced mass transfer of
microbubble ozone gas in the aqueous phase. Jabesa and
Ghosh (2016) adopted microbubble ozonation technology for
the pilot-scale degradation of diethyl phthalate, and a final TOC
removal efficiency of 94% was achieved with an ozone generation
rate of 1.94 mg/s. Gao et al. (2019) developed a pilot system by
combining microbubble ozonation and UV irradiation for removing refractory organics in a secondary wastewater effluent. The
combined system enhanced the •OH generation up to 2–6 times
higher than that in the conventional ozonation and UV irradiation
processes alone. Table 12 summarizes recent pilot-scale studies
on industrial effluent treatment with the catalytic and microbubble ozonation processes.
5
Concluding remarks
In recent years, many research efforts have been made toward
improvement of the Fenton and ozone-based AOPs. The system
performances of all developed technologies have been successfully validated through their lab-scale, pilot-scale, and even fullscale applications in treating real industrial effluents.
The classic Fenton process has a wide range of applications to
various types of industrial effluents, and it is the earliest Fenton
technology demonstrated on an industrial scale. The classic Fenton
process usually has a simple process design and operation protocol,
low capital cost, and good scalability. It also has a number of drawbacks that limit its widespread acceptance, including high chemical
consumption, low efficiency with the presence of radical scavengers, requirement for mildly acidic conditions, and the production
of ferric sludge. Some of these drawbacks are intrinsic while others
could be alleviated through technology development.
FBR-Fenton is a successful modification of the classic Fenton
process by conducting a homogenous Fenton reaction in a fluidized bed reactor. Both lab-scale and industrial-scale applications
show that FBR-Fenton could overcome the problem of excessive
sludge generation in classic Fenton oxidation. In spite of encouraging results in this field, more studies are needed to address
239
Table 12 Recent pilot-scale studies on industrial effluent treatment with catalytic and microbubble ozonation processes.
Wastewater/organics
System
Operating condition
Performance efficiency
References
Bio-treated dyeing and
finishing wastewater
Recycled iron shaving
catalyst
COD final: 54 5 mg/L
DOC final: 16 3 mg/L
Ozone utilization efficiency
DCOD/DO3: 0.64
Ma et al.
(2018)
Coal gasification wastewater
1. Ni-induced C-Al2O3 (NiCAF)
2. Cu-Co bimetal induced
NiCAF (CuCO/ NiCAF)
COD final: 22.1 mg/L
DOC final: 6.3 mg/L
Ozone utilization efficiency
DCOD/DO3: 2.12
75% reduction of energy
consumption compared to ozone
alone
Wei et al.
(2019)
Coal gasification wastewater
MnOx/sewage sludge-based
activated carbon
[COD]0: 165 20 mg/L
[DOC]0: 76 6 mg/L
Reactor volume 2.93 m3
pH: 7.08 0.20
Influent flowrates:
2.9–17.6 m3/h
HRT: 30 min
Ozone concentration:
80 mg/L
Catalyst dosage: 2 mg/L
Ozone dosage: 10.2 gO3/min
Catalyst loading
CuCO/NiCAF: 150 kg
[COD]0: 70–80 mg/L
[TOC]0: 20–25 mg/L
pH: 6.5–7.0
Feed flux: 5 m3/day
HRT: 30 min
Ozone dosage: 25 mg/L
[COD]0: 150 25 mg/L
[TOC]0: 50 10
BOD5/COD: 0.06
Ozone dosage: 90 mg/L
Zhuang et al.
(2017)
Brilliant green dye
Ozone microbubbles
COD final: 41 4 mg/L
TOC final: 19 1 mg/L
BOD5/COD: 0.44
Ozone utilization efficiency
DCOD/DO3: 1.21
TOC removal: 80%
Dye removal: 90%
[Dye]0: 0.0274–0.164 mM
Microbubble generator gas
intake capacity: 1.7 mL/s
Reactor capacity: 20 L
Ozone dosage: 6.0 g/h
Khuntia et al.
(2015)
Table 12 Recent pilot-scale studies on industrial effluent treatment with catalytic and microbubble ozonation
processes—cont’d
Wastewater/organics
System
Diethyl phthalate (DEP)
Ozone microbubbles
Recirculating aquaculture
wastewater
Ti-Mn/TiO2/Al2O3 membrane
Pretreatment of dry-spun
acrylic fiber wastewater
Catalytic ozone-persulfate
Biologically treated
wastewater (micropollutant
removal)
Granular activated carbon
Secondary effluent from
wastewater treatment plant
Microbubble ozonation-UV
process
Operating condition
3
[DEP]0: 0.18 mol/m
Reactor capacity: 20 L
pH: 7
[TOC]0: 23 mg/L
Ozone dosage: 10 g/h
[CODMn]0: 2.1–4.1 mg/L
pH: 7.7–8.1
UV254: 0.073–0.095
Ozone dose: 52 mg/min
Recirculation rate: 4000 L/h
[COD]0: 2056 mg/L
BOD5/COD: 0.08
pH: 6.9
Ozone dosage: 40 g/h
Persulfate: 1.3 kg/ton
Catalyst: 120 L
Total nitrogen (TN):
452 mg/L
Reactor effective volume:
180 L
[COD]0: 252.7 60.4 mg/L
[DOC]0: 68.3 25.8 mg/L
UV254: 62.5 11.6 mg/L
Average flow rate: 0.8 m3/h
[COD]0: 22.50 0.50 mg/L
[TOC]0: 43.79 0.44 mg/L
pH: 7.61–8.25
UV254: 0.119 0.03
Reactor volume: 20 L
HRT: 30 min
Performance efficiency
References
DEP removal: 78%
TOC removal: 53%
Jabesa and
Ghosh (2016)
CODMn removal: 52%
Chen et al.
(2015c)
COD removal: 42.4%
BOD5/COD: 0.315
TN removal: 28.5%
Wu et al.
(2017)
COD removal: 91%
DOC removal: 85%
HRT: 17 3 min
Ozone consumption: 0.89 0.29
gO3/gDOC
COD removal: 37.50%
UV254: 81.15%
Knopp et al.
(2016)
Gao et al.
(2019)
242
Chapter 11 Fenton- and ozone-based AOP processes
some research gaps. Existing studies have mostly observed the
effects of conventional operational parameters (pH, Fenton
reagents, HRT, and carrier) on FBR-Fenton system performance.
Comprehensive process optimization and modeling are somewhat limited. As the FBR-Fenton is operated in a liquid-solid system, process modeling and optimization could be relatively
challenging due to the additional parameters. More studies are
needed to address this part and take the system hydrodynamics
into consideration.
One main concern of the photo-Fenton process is its cost effectiveness, and many lab-scale studies have been conducted to
address this issue such as the utilization of heterogeneous catalysts, chelating agents, and solar energy. The heterogeneous
photo-Fenton process could overcome pH limitations in a homogenous system. Iron oxides are good candidates for the heterogeneous catalyst due to their wide availability and cheap price. In
terms of system scaling up, both the catalyst’s lifetime and the relatively low oxidation efficiency of the heterogeneous system
should be carefully considered. Chelating agents could prevent ferric precipitation at near-neutral or even alkaline pH conditions
while their addition could also increase the process’s chemical cost
and wastewater organic content. Replacing UV irradiation with
natural solar irradiation could significantly reduce the process capital cost and power consumption. However, by using this natural
resource, the process efficiency is also affected by the location latitudes, atmospheric conditions, and sunshine duration. To achieve
an adequate irradiation area and solar intensity, the design of the
solar photo-Fenton reactor is relatively complex. Furthermore,
the applicability and performance of solar photo-Fenton is subject
to the transmissivity properties of treated water. More pilot-scale
and full-scale studies are needed to prove the cost-effectiveness
of the photo-Fenton process for industrial effluent treatment.
The electro-Fenton process is also a successful process
improvement by combining the Fenton process with the electrochemical process. Both lab-scale and industrial-scale studies
demonstrate that electro-Fenton could enhance Fe2+ regeneration and reduce sludge production. Unlike with other Fentonbased AOPs, the electro-Fenton process is particularly suitable
for treating high COD and hardly biodegradable organiccontaining wastewater. Because the electro-Fenton reactors are
typically operated in batch mode, their treatment capacities in
full-scale applications are relatively lower than those of other
Fenton processes. Furthermore, the high electricity cost involved
during their operation could reduce the cost effectiveness for
large-scale industrial effluent treatment.
Chapter 11 Fenton- and ozone-based AOP processes
Several achievements of the ozonation process have encouraged researchers to improve the drawbacks of ozonation technology, especially on the enhancement of the ozone utilization rate.
The low ozone solubility and instability of ozone molecules in the
aqueous phase in the conventional ozonation process lead to the
development of technologies such as microbubble ozonation, catalytic ozonation, and combined microbubble ozonation. The literatures reveals that catalytic ozonation processes are feasible
ozone AOPs for large-scale industrial effluent treatment while
the catalyst selection for practical application becomes crucial,
as many parameters such as catalyst leaching, dosage, regeneration, cost, etc., need to be considered.
Introducing ozone gas into an aqueous system with microbubbles could facilitate gas-liquid ozone mass transfer and ozone dissolution. The application of microbubble technology in ozonation
processes for efficiency improvement has been validated in both
lab- and pilot-scale studies. In the context of large-scale industrial
effluent treatment, microbubble technology is a straightforward
modification method that could be retrofitted into the existing
conventional ozonation process with minimal design alteration
and additional operating cost. However, its ability for enhancing
ozonation efficiency could be relatively limited when the target
wastewater exhibits strong radical scavenging effects.
Lab-scale studies demonstrated that combined catalyticmicrobubble ozonation exhibited remarkable synergistic effects
on ozonation efficiency enhancement and organic pollutant
abatement while pilot- or full-scale applications of this combined
process are rather limited. More research efforts are needed
before its exploitation in large-scale industrial effluent treatment.
Some research gaps that should be addressed are the following: (1)
the economic feasibility of the combined catalytic-microbubble
ozonation process for industrial-scale applications; (2) the reaction mechanisms involved in the combined ozonation process;
and (3) the impacts of water quality parameters such as pH,
organic matter, and inorganic species.
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