178
Tekstilec, 2023, Vol. 66(3), 178–198    |    DOI: 10.14502/tekstilec.66.2023045
TiO
2
 and ZnO as Advanced Photocatalysts for Effective 
Dye Degradation in Textile Wastewater 
TiO
2
 in ZnO kot napredna fotokatalizatorja za učinkovito razgradnjo barvil v 
tekstilnih odpadnih vodah
Scientific Review/ Pregledni znanstveni članek
Corresponding author/Korespondenčna avtorica:
prof. dr. Barbara Simončič
E-mail: barbara.simoncic@ntf.uni-lj.si
ORCID ID: 0000-0002-6071-8829
Abstract
Textile wastewater, which consist of a complex mixture of synthetic dyes and other organic and inorganic 
compounds derived from various wet chemical textile processes, can have a harmful effect on the environ-
ment; therefore, it must be properly treated before being discharged into municipal wastewater treatment 
plants and natural water bodies. In this scientific review, the main physical, chemical and biological processes 
for the removal of dyes from textile wastewater are presented, focusing on photocatalysis, which is a promi-
sing advanced oxidation process. The mechanism of photocatalysis is described and the methods used to 
determine the efficiency of photocatalytic degradation of dyes are presented. Recent studies involving single 
photocatalytic treatments of real textile wastewaters in the presence of TiO
2
 and ZnO as catalysts are presen-
ted. The advantages of combined processes of photocatalysis in conjunction with other chemical, physical 
and biological treatments to increase the efficiency of wastewater treatment are discussed. Accordingly, pho-
tocatalysis combined with H
2
O
2
, photocatalytic ozonation, a hybrid system of photocatalysis and membrane 
filtration, and coupled photocatalytic-biological processes are described.
Keywords: titanium dioxide, zinc oxide, photocatalysis, dye degradation, textile wastewater
Izvleček
Tekstilne odpadne vode, ki vključujejo kompleksno mešanico sintetičnih barvil in drugih organskih in anorganskih 
spojin, ki izhajajo iz različnih mokrih kemijskih tekstilnih postopkov, lahko škodljivo vplivajo na okolje, zato jih je po­ trebno pred izpustom v komunalne čistilne naprave in naravno vodno okolje ustrezno očistiti. V preglednem članku 
so predstavljeni najpomembnejši fizikalni, kemijski in biološki postopki za odstranitev barvil iz tekstilnih odpadnih 
vod s poudarkom na fotokatalizi, ki je obetavni napredni oksidacijski proces. Opisan je mehanizem fotokatalize in 
predstavljene so metode za določitev učinkovitosti fotokatalitske razgradnje barvil. Izpostavljene so najsodobnejše 
raziskave, ki vključujejo samostojno fotokatalitsko obdelavo realnih tekstilnih odpadnih voda v prisotnosti TiO
2
 in 
ZnO kot fotokatalizatorjev. Predstavljene so prednosti kombiniranih postopkov, ki vključujejo fotokatalizo v povezavi 
z drugimi kemijskimi, fizikalnimi in biološkimi procesi. Med njimi so opisani fotokataliza v kombinaciji s H
2
O
2
, foto­ katalitska ozonacija, hibridni sistem fotokatalize in membranske filtracije ter združeni fotokatalitski­biološki procesi.
Ključne besede: titanov dioksid, cinkov oksid, fotokataliza, razgradnja barvila, tekstilna odpadna voda
Dominika Glažar, Barbara Simončič
University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Textiles, Graphic Arts and 
Design, Ljubljana, Snežniška 5, 1000 Ljubljana, Slovenia

TiO 2 and ZnO as Advanced Photocatalysts for Effective Dye Degradation in Textile Wastewater
179
1  Introduction
The textile industry is considered one of the largest 
water pollutants and is responsible for about 20% of 
global clean water pollution from various wet chem­ ical production processes [1–3]. Wastewaters from 
pretreatment, dyeing, printing and finishing pro­ cesses are highly polluted by complex mixtures of 
synthetic dyes and pigments, finishing agents, auxi­
liaries, heavy metals, surfactants and other chemi­ cals, which can result in harmful effluents in the envi­ ronment [4–6]. Among the effluents, synthetic dyes 
are classified as one of the most hazardous pollutants 
as they are potentially toxic, non­biodegrada ble and 
persistent [7–9]. Due to light absorption, the pres­ ence of dyes in wastewater reduces sunlight penetra­ tion, which negatively affects flora and fauna [4, 8]. 
Accordingly, the removal/degradation of dyes from 
textile wastewater is a challenging research topic for 
which various physical, chemical and biological pro­ cesses, and their combinations have been developed 
and introduced (Figure 1) [4–7, 9–12].
Physical methods for removing dyes from textile 
effluents primarily include adsorption and mem­ brane filtration, in which dye removal is advanta­ geously accomplished by forces such as electrical 
Figure 1: Methods for removal/degradation of dyes from textile wastewater
attraction, gravity, and V an der W aals forces or physi­
cal barriers [6]. In the adsorption method, numerous 
suitable adsorbents are used, the best known of which 
is activated carbon. In addition, polymer resins and 
low­cost agricultural and industrial by­products such 
as peat, chitin, clays and fly ash are used. Membrane 
filtration includes microfiltration, ultrafiltration, 
nanofiltration and reverse osmosis. Since nanofiltra­ tion is more effective than microfiltration and ultra­ filtration, reverse osmosis is the most effective as it 
retains almost all substances from water [6].
Chemical methods include coagulation/floccu­ lation, electrochemical processes, classical oxidation 
and advanced oxidation processes (AOP) [4, 6, 13]. 
Coagulation/flocculation is commonly used to de­ stabilise particles with various coagulants such as in­ organic coagulants, inorganic­organic double coagu­
lants and synthetic polymer flocculants. A complete 
decolourisation is difficult to achieve with this meth­ od. Inorganic coagulants such as iron and alumini­ um salts are widely used in the treatment of textile 
wastewater; however, they have negative effects on 
the environment and human health [6, 14]. There are 
various electrochemical processes such as electroki­ netic coagulation, electroflotation, electrodecan­
tation and electrooxidation [15]. Electrons are used 

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as “primary reagents, ” which are referred to as “clean 
reagents” . In most cases, high concentrations of sup­ porting electrolytes, especially NaCl, are required 
to achieve acceptable results; however, this leads to 
the generation of large amounts of environmentally 
harmful products [6, 15]. In the classical oxidation 
method, ozone (O
3
), hydrogen peroxide (H
2
O
2
), po­ tassium permanganate (KMnO
4
), chlorine dioxide 
(ClO
2
), chlorine (Cl
2
), sodium hypochlorite (NaOCl)  
and oxygen (O
2
) are used as oxidising agents that 
change the chemical composition of the compound 
[13]. In advanced oxidation processes (AOP), reac­ tive oxygen species (ROS) such as hydroxyl radicals 
(
•OH) and superoxide radicals (•O
2
–
) are usually 
generated and utilised. These include various me­
thods such as the photocatalytic ozonation, Fenton 
process, photo­, electro­ and sono­Fenton processes, 
and photocatalysis [2, 6, 13, 16–19]. The best known 
advanced oxidation process is the Fenton process, 
which uses a mixture of ferrous iron (typically Fe(II)) 
and hydrogen peroxide (H
2
O
2
) to generate 
•OH in 
an acidic medium. However, the Fenton process can 
produce chemical sludges that must be properly dis­ posed of. Compared to the Fenton process, the ad­ vantage of the photo­, electro­ and sono­Fenton pro ­ cesses, which combine the Fenton reaction with light 
radiation, electrochemical processes and ultrasound, 
respectively, is a higher pollutant removal rate with 
a lower iron dose [19–24]. Photocatalysis is conside­
red a sustainable treatment process for the degrada­ tion of dyes from textile wastewater in the presence 
of photocatalysts. In this process, high efficiency of 
photocatalytic degradation can be achieved under 
mild reaction conditions in the presence of oxygen 
and water from the atmosphere and UV/visible light 
radiation without the formation of secondary impu­ rities as the dyes are degraded to carbon dioxide and 
water via intermediates [2, 25].
Biological processes use biomaterials such as in­ dustrial enzymes and microorganisms for dye deg­ radation. They can be conducted under aerobic or 
anaerobic conditions. These processes consist of two 
main steps, i.e. adsorption of dyes onto biomaterials 
and their degradation to non­toxic products. While 
peroxidase and azo reductase are the most effec­ tive industrial enzymes, bacteria, fungi, algae and 
yeasts are used as microorganisms. Due to the high 
biodegradability of biomaterials and low operating 
costs, biological processes are considered the most 
promising treatment methods for textile wastewater 
from the environmental and economic perspective 
[4, 6, 11]. Despite many advantages of biological 
processes, there are still shortcomings, including the 
non­degradability of biomass­bound dyes and the 
difficult adsorption of some types of dyes such as azo 
and reactive dyes [6, 11].
2 Photocatalysis as AOP for 
synthetic dye degradation
Photocatalysis is a promising AOP that takes place 
in the presence of a photocatalyst, which is activat­ ed by light [16]. Due to its environmental friendli­ ness and high efficiency, this process has attracted 
much attention in various scientific fields, including 
environmental remediation, where various organic 
and inorganic pollutants can be photocatalytically 
degraded. This also applies to dyes contained in tex­ tile wastewater. Various photocatalysts can be used 
in photocatalysis, including metal semiconductors 
such as TiO
2
 and ZnO nanoparticles, which are very 
promising due to their excellent morphological, 
chemical and optical properties [26]. The efficiency 
of photocatalysis is directly influenced by the de­ sign of the photocatalyst, where the surface­to­vol ­ ume ratio of the particles, their crystallinity, surface 
modifications and light absorption capacity play an 
important role.
2.1 Mechanism of semiconductor 
photocatalysis
The mechanism of semiconductor photocatalysis is 
shown in Figure 2 and can be explained as follows 
[27–29]: when a semiconductor absorbs a photon 

TiO 2 and ZnO as Advanced Photocatalysts for Effective Dye Degradation in Textile Wastewater
181
with the energy equal to or higher than the bandgap 
energy (E
g
) under irradiation with UV or visible light, 
the electrons in the valence band (VB) are excited 
into the vacant conduction band (CB), leaving holes 
in VB. The resulting free electrons and holes can 
migrate to the surface of the semiconductor, where 
they participate in the redox reactions. The electrons 
react with atmospheric oxygen to form 
•O
2
–
 in the 
reduction reaction and the holes react with absorbed 
water to form •OH in the oxidation reaction. Both 
reduction and oxidation take place when the edge of 
the semiconductor’s conduction band is more nega­ tive and the edge of its valence band is more positive 
than the standard redox potential of the reactions. 
The formation of •O
2
–
 and 
•OH, which are the main 
ROS formed at the semiconductor surface, is crucial 
for the photocatalytic activity of the semiconduc­ tor since ROS can subsequently react with organic 
pollutants in the oxidation reaction and degrade 
them to carbon dioxide and water via intermediate 
compounds. At the same time, holes with a high ox­ idation potential can directly cause the oxidation of 
pollutants [25, 29–38]. Nevertheless, the recombi­ nation of electrons and holes that can occur during 
their migration to the semiconductor surface is an 
undesirable process as it reduces the photocatalytic 
efficiency of the semiconductor [38].
Figure 2: Schematic representation of fundamental mechanism of photocatalytic activity of semiconductors 
and proposed surface reactions; e
−
 is electron, h
+
 is hole
To improve the separation of electrons and holes 
and thus the photocatalytic efficiency, semiconduc­ tors are doped with different metal and nonmetal 
ions, loaded with noble metals and coupled with 
other semiconductors to form heterojunctions [38, 
39]. Doping with metal and nonmetal ions is based 
on the incorporation of host materials into the semi­ conductor crystal lattice to change the geometric 
and electronic structure and modulate the charge 
carrier density in the doped semiconductor. The 
doped ions introduce additional localised energy 
levels to trap electrons or holes that immobilise the 
charge carriers, hence reducing the recombination 
rate. As the dopant energy levels are formed above 
the VB or below the CB edge positions in the semi­ conductor, they decrease the bang gap energy and 
consequently increase the visible light absorption 
[40–42]. The loading of semiconductors with noble 
metals enables the formation of the Schottky­based 
heterojunction, typical of the semiconductor­metal 
system, where electrons are easily transferred from 
the CB of the semiconductor to the metal, which acts 

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Tekstilec, 2023, Vol. 66(3), 178–198
as an electron trapper. By creating a Schottky bar­ rier, the separation of photoinduced charge carriers 
is maximised and their recombination is prevented 
[38]. At the same time, visible light excites electrons 
in the metal, leading to surface plasmon resonance 
that further enhances photocatalytic activity [43]. 
The fabrication of semiconductor­semiconductor 
heterojunctions is one of the most effective strate­ gies to enhance the photocatalytic performance un­ der visible light. The mechanism of photogenerated 
charge transfer is very complex and depends on the 
design of the heterojunction. However, the most 
photocatalytically active heterojunctions are those in 
which the electrons and holes located in the CB and 
VB with lower redox power, respectively, are recom­ bined, while ROS are formed in the more energeti­ cally favourable CB and VB of semiconductors [44].
2.2 Determination of photocatalytic 
degradation efficiency
The efficiency of the photocatalytic degradation of 
dyes can be determined from the degradation rate of 
the dye [20, 45–57], where the concentration ratio is 
calculated as follows:
c
t
c0
Dye concentration ratio =
                             
(1)
In Equation 1, c
t
 is the dye concentration at a giv­ en time of irradiation and c
0
 is the initial dye con­ centration. The lower the dye concentration ratio at 
a given time, the higher the dye degradation.
The dye degradation efficiency can also be calcu ­ lated as the percentage of dye degradation as follows 
[45–47, 49–51, 57, 58]:
c0
-c
t
c0
Dye degradation percentage = x 100 (%)
        
(2)
The higher the dye degradation percentage, the 
higher the degradation efficiency.
The apparent rate constant, K
app
, of the photocat­ alytic reaction can also be a measure of the efficien­ cy of the photocatalytic degradation of dyes, where 
pseudo first­order kinetics is used as follows [46, 48, 
50, 51, 58]:
ln      = – K
app 
· t (min
–1
)
c
t
c0
                             
(3)
In the treatment of real textile industry waste­ water, the efficiency of dye removal/degradation is 
usually discussed based on the measurements of total 
organic carbon (TOC) and chemical oxygen demand 
(COD) measurements before and after wastewater 
treatment. In this case, the dye concentrations c
0
 and 
c
t
 in Equation 2 are replaced with TOC
0
 and TOC
t
 or 
COD
0
 and COD
t
, and the percentage of TOC or COD 
removal is calculated as a measure of the mineralisa­ tion efficiency of textile wastewater [12, 49, 52].
3 Titanium dioxide and zinc 
oxide as photocatalysts for 
dye degradation in real textile 
wastewater
In the field of textiles, titanium dioxide (TiO
2
) and 
zinc oxide (ZnO) have emerged as the most impor­ tant semiconductor nanomaterials with a variety 
of applications for the functionalisation of textile 
substrates as well as for the effective photocatalytic 
degradation of various dyes in an aqueous solution 
[3, 31, 32, 59–61]. The main advantages of TiO
2
 and 
ZnO are their thermal, chemical and photochemi­ cal stability, non­toxicity, biocompatibility and low 
price [2, 32, 62–64].
TiO
2
 and ZnO are n­type semiconductors with 
E
g
 of about 3.2 eV, which limits their photocataly tic 
activity to irradiation with UV light [65, 66]. Accord­
ingly, surface modification of TiO
2
 and ZnO by dop­ ing with metal and non­metal ions, loading with 
noble metals, such as Ag, coupling with other semi­ conductors, and dye sensitisation is of great impor­ tance to lower E
g
 and thus increase the photocatalytic 
activity in visible light [40, 67].

TiO 2 and ZnO as Advanced Photocatalysts for Effective Dye Degradation in Textile Wastewater
183
In the process of photocatalytic degradation of 
dyes in an aqueous solution, TiO
2
­ and ZnO­based 
nanomaterials were mostly used as photocatalysts 
in powder form, which were mixed into the dye 
solution under the study [22, 49, 50, 54, 68–75] and 
removed after the photocatalytic treatment usually 
with centrifugation [22, 49, 50, 71, 74, 75] or filtra­ tion [68–70, 72, 73]. In addition to powder form, 
TiO
2
 was applied to various substrates such as trans­ parent glass, glazed ceramic tile and stainless steel by 
doctor blade technique and used in photocatalytic 
reactors [45]. In another study, TiO
2
 nanotubes were 
prepared on titanium foil by anodization at 48 V for 
2 hours followed by iron doping with hydrother­ mal treatment at 150 °C for 3 hours and annealing 
at 550 °C for 1.5 hours [52]. In addition, TiO
2
 and 
ZnO were incorporated into glass­ceramic materials 
with a conventional melting technique of glass batch 
followed by heat treatment at 450 °C for 10 hours 
and used in a batch reactor [76]. Ultra long nano­ fibers, including the Bi
2
Ti
4
O
11
/TiO
2
 heterojunction, 
were also produced via electrospinning and used as 
photocatalyst [77].
It should be noted that the efficiency of photocata­
lytic degradation of dye solutions is influenced not 
only by the structure of the photocatalyst, but also by 
the composition and quality of the wastewater [52]. 
A model dye solution containing a single synthetic 
dye at an appropriate concentration cannot simulate 
the real textile wastewater, which consists of a mix­ ture of synthetic dyes of different chemical structure 
and several other organic and inorganic substances 
that can strongly influence the pH, TOC and COD 
of the wastewater; moreover, the parameters are 
highly variable [78]. In addition, these pollutants 
can significantly reduce the degradation rate of dyes 
by hindering the photocatalytic efficiency of semi­ conductors. Therefore, the study of photocatalysis as 
an AOP for the treatment of real textile wastewater 
from the textile industry is of great importance and 
represents a challenging research topic. The perfor­ mance of TiO
2
 and ZnO as photocatalysts in single 
AOP or in combination with other chemical, physi­
cal and biological processes for dye removal in real 
textile wastewater is summarised in Table 1.
Table 1: Treatment systems, photocatalysts, pollutants and experimental performance
Treatment system Photocatalysts Pollutant Experimental performance Ref.
Single photocatalysis
TiO
2
, Al, F co-doped 
TiO
2
 nanoparticles
Wastewater of textile 
factory, Erode, Tamilnadu, 
India
0.0125 mM catalyst in 10 ml wastewater, 
irradiation with visible light for 120 
minutes
79
Fe-doped TiO
2
 
nanotubes on 
titanium foil
Artificially compounded 
textile wastewater
2.5 × 5 cm
2
 foil as photocatalyst in 5 
mg/L Congo red dye in wastewater, 
irradiation with visible light for 180 
minutes
52
ZnO quantum dots 
of different size
Wastewater of dyehouse 
with pH in range of 6.9, 
Egypt
0.1 g catalyst in 100 ml wastewater, 
direct sunlight for 6 hours per day (9 am 
to 3 pm) for 6 months
81
Photocatalysis in 
combination with 
another AOP
TiO
2
 nanoparticles
Wastewater from different 
textile industries in 
Ghaziabad and Gautam 
Buddha Nagar districts, 
Uttar Pradesh, India
TiO
2
 at various concentrations (1.5 g/L 
to 20.0 g/L) in 100 mg/L Remazol Red 
in wastewater without and in presence 
of H
2
O
2
 of different concentrations, 
irradiation with UV light for 60 minutes
20
Cd-doped ZnO 
nanoparticles
Wastewater from dyehouse 
near Erode, Tamilnadu, 
India
Cd-doped ZnO at different 
concentrations (0 to 1 g/L) and pH 
values (3 to 9) in 500 ml of wastewater 
irradiation with UV light for 240 minutes 
in presence of O
3
 of different dose
18

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Tekstilec, 2023, Vol. 66(3), 178–198
Table 1 shows that there are very few studies 
dealing with the photocatalysis of real textile waste­ water. In these studies, TiO
2
 and ZnO are used in a 
single AOP or in combination with other chemical, 
physical and biological processes. These processes 
are very complex and therefore difficult to compare 
as they differ in terms of chemical structure, mor­ phology and concentration of the photocatalyst, the 
composition of the industrial wastewater and the ex­ perimental performances and conditions. They are 
presented in the following sections.
3.1 Photocatalysis as single AOP
Photocatalysis in the presence of TiO
2
 and ZnO as 
semiconductor photocatalysts has already shown 
promise for photodegradation and mineralisation 
of real textile wastewater. The efficiency of photoca ­ talysis is influenced by several factors, of which the 
structure of the photocatalyst and the composition 
of wastewater have been studied in detail.
Treatment system Photocatalysts Pollutant Experimental performance Ref.
Photocatalysis in 
combination with 
membrane filtration
Polyethylene 
glycol capped ZnO 
nanoparticles
Wastewater from textile 
factory performing dyeing, 
printing and finishing in 
Johor, Malaysia
Photocatalysis (0.08–0.30 g/L 
photocatalyst and pH of 4–13) for 240 
minutes under UV irradiation followed 
by membrane ultrafiltration
83
Photocatalysis in 
combination with 
biological treatment
TiO
2
, ZnO 
nanoparticles
Wastewater from dyehouse 
in Santa Catarina, Brazil
Photocatalysis under UV light irradiation 
for 120 minutes (150 mg of catalyst 
in 250 ml of wastewater) followed by 
aerobic bioprocess for 48 hours
22
ZnO/polypyrrole 
nanocomposite
Wastewater from Gama S. 
A., textile industry in Mar 
del Plata, Argentina
Biological treatment for 96 hours 
followed by photocatalysis (0.5–2.0 g/L 
catalyst in 200 ml of wastewater) for 60 
minutes
84
3.1.1   TiO
2
 versus Al and F co-doped TiO
2
Recently, the photocatalytic degradation of real 
textile wastewater (TEWW) compared to the dye 
methyl orange (MO) was studied using TiO
2
 and 
aluminium (Al) and fluorine (F) co­doped TiO
2
 
(TAF10) nanoparticles under visible light irradia­ tion (Figure 3) [79].
The results show that the absorbance of both the 
MO solution and TEWW decreased with increasing 
irradiation time, indicating an efficient decolouri­ sation of the dye by both photocatalysts. It is also 
evident that the degradation efficiency is affected by 
both the dye solution and the structure of the pho­ tocatalyst. A comparison of the spectra in Figure 3a 
and Figure 3b shows that, as expected, the photocata­
lytic activity of TAF10 was higher than that of TiO
2
 
due to the co­doping of Al and F in TiO
2
, resulting in 
an almost complete decolourisation of MO after 120 
minutes of irradiation. This result was also confirmed 
by the calculated apparent rate constant of MO, 
Figure 3: Photocatalytic degradation of MO dye with TiO
2
 (a) and TAF10 (b) and of TEWW (c) with TAF10 
(reprinted with permission from [79]; Copyright 2022, Elsevier)
Continuation of Table 1

TiO 2 and ZnO as Advanced Photocatalysts for Effective Dye Degradation in Textile Wastewater
185
which was higher for TAF10 (K
app
 = 0.0174 min
–1
)  
than for TiO
2
 (K
app
 = 0.0126 min
–1
). Moreover, the 
efficiency of the TEWW degradation with TAF10 
during the first hour of irradiation was much higher 
than that of MO; however, the efficiency decreased 
significantly during the second hour of irradiation 
(Figure 3c), resulting in the Kapp value of TEWW 
of 0.0134 min
–1
, which is lower compared to the K
app
 
value of MO obtained with the same photocatalyst.
3.1.2   Fe-doped TiO
2
To investigate the effect of chemical additives used 
in different steps of textile chemical processes on the 
photocatalytic removal and mineralisation efficiency 
of the dye Congo Red (CR), the real textile waste­ water was imitated by adding glucose as a desizing 
and reducing agent, sodium carbonate as a scouring 
agent, ferric chloride as a colouring agent, magne­ sium sulphate as a printing agent and ammonium 
chloride as a finishing agent to the CR solution (Fig­ ure 4) [52]. For this purpose, iron­doped titanium 
dioxide nanotubes (Fe–TiO
2
) were used as the pho­ tocatalyst and the batch experiments were carried 
out in the laboratory photoreactor under visible light 
irradiation for 180 minutes after the adsorption­de ­ sorption equilibrium was reached in the dark. The 
Figure 4: Impacts of glucose on dye degradation efficiency of CR (a) and removal efficiency of TOC and COD (b); 
impacts of sodium carbonate on degradation efficiency of CR (c) and removal efficiency of TOC and COD (d) 
(concentrations: CR = 5 mg/L, Glucose = 500 mg/L Sodium carbonate = 500 mg/L); DIW stands for deionised 
water (reprinted with permission from [52]; Copyright 2021, Elsevier)

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Tekstilec, 2023, Vol. 66(3), 178–198
results show that the degree of the CR photodegra­ dation increased with the irradiation time and that 
the structure of the additives directly affected the 
photodegradation efficiency.
For example, the addition of a small amount of 
glucose to the CR solution did not hinder the effi­ ciency of the photocatalytic degradation of CR, it 
even improved it (Figure 4a). The most reasonable 
explanation for this phenomenon was that glucose 
acts as a co­substrate that undergoes the oxidation 
reaction and thus affects the degradation of CR. It is 
believed that glucose acts as a scavenger of the pho­ toinduced holes during the photocatalytic reaction 
and prevents the recombination of electron­hole 
pairs on the Fe–TiO
2
 nanotubes. At the same time, 
the oxidation of glucose by holes did not hinder the 
photodegradation of CR, since the main ROS for the 
oxidation of CR was •OH, as shown by the results 
of the degradation mechanism. In contrast, the ad­ dition of glucose to the CR solution did not posi­ tively affect the removal of TOC and COD, as the 
presence of glucose decreased TOC removal by 19% 
and COD removal by 50% (Figure 4b).
The presence of sodium carbonate in the CR 
solution significantly delayed the photodegradation 
of CR, which dropped from 86% to 34% after 180 
minutes of irradiation (Figure 4c). The reason for 
this phenomenon was attributed to the combination 
of the ability of carbonate ions to scavenge •OH and 
the competitive adsorption of carbonate ions on the 
catalyst surface and blocking of the active sites [52, 
80]. The effect of sodium carbonate on TOC and 
COD removal efficiency was opposite. While TOC 
removal decreased with the addition of sodium car­ bonate, COD removal increased (Figure 4d). The 
decrease in the TOC removal efficiency was related 
to the lower CR degradation in the presence of sodi­ um carbonate. However, the concomitant increase in 
COD removal suggests that sodium carbonate trig­ gered the decomposition of inorganic compounds in 
the solution, resulting in a decrease in COD, but not 
TOC [52].
3.1.3   ZnO of different morphologies
To investigate the photodegradation efficiency of a 
real industrial wastewater from an Egyptian dye fac­ tory under sunlight irradiation, four ZnO samples 
of different morphologies and sizes were used for 
the experiment, including two ZnO quantum dots 
(QD) with the average sizes of 7.1 nm and 9.8 nm, 
and two ZnO nanoparticles (Nano) with the average 
sizes of 13.5 nm and 34 nm (Figure 5) [81]. Com­ mercial ZnO powder was used as a reference. The 
experiments were conducted for 6 hours (from 9 am 
to 3 pm) on different study days from May to Octo­ ber 2018, and the solar photocatalytic activity of the 
ZnO samples was investigated by determining COD 
before and after the degradation experiment.
The results show that the COD values of real 
industrial wastewater before the photocatalytic ex­ periments ranged from 4985 mg/L to 6867 mg/L, 
regardless of the study date, and that the COD val­ ues decreased in the presence of ZnO samples after 
6 hours of irradiation (Figure 5a). It is also evident 
that the COD removal increased with the decrease of 
the ZnO particle size, indicating the effectiveness of 
the size effect of ZnO QDs on the photodegradation 
processes. The reusability of ZnO QDs and Nano 
ZnO for 8 times in the photodegradation process 
of wastewater resulted in a decrease in the photode­
gradation rate, and only the mineralisation efficiency 
achieved by ZnO QDs with the particle size of 7.1 nm  
stayed below the COD limit after the 8th recycling 
process (Figure 5b). It is assumed that the size of the 
photocatalyst increases during the recycling process, 
which is a consequence of the accumulation of pho­ tocatalysts with repeated use.
Research shows that doping TiO
2
 with metal and 
non­metal ions and reducing ZnO particle size sig ­ nificantly increase the efficiency of the wastewater 
photodegradation process. It is also obvious that the 
photocatalytic degradation of wastewater is direct­ ly affected by the chemical additives present. If the 
additive ions can scavenge ROS, the photocatalytic 
process will be significantly hindered.

TiO 2 and ZnO as Advanced Photocatalysts for Effective Dye Degradation in Textile Wastewater
187
3.2 Photocatalysis in combination with 
other chemical, physical and biological 
processes
To increase the efficiency of wastewater treatment, 
photocatalysis has already been advantageously com­ bined with other chemical, physical and biological 
processes. A combination of different processes for 
wastewater treatment offers several advantages over 
single treatments, as certain process combinations, 
their proper integration and optimisation can create 
the synergistic effect in their performance that is criti­
cal for efficient, versatile, scalable, cost­effective and 
environmentally sound wastewater treatment.
3.2.1 TiO
2
 in combination with H
2
O
2
 versus 
photo-Fenton
To study the photodegradation activity of the dye 
Remazol Red (RR) in textile industry wastewater, a 
nanosized TiO
2
 photocatalyst was used in combina­ tion with H
2
O
2
 under UV irradiation, and the results 
were compared with the photo­Fenton process as 
Figure 5: COD limits and situation of real industrial wastewater for six months using ZnO QDs, Nano ZnO and 
commercial ZnO during photocatalysis by sunlight (a); COD limits for recycling process of real industrial waste -
water in October 2019 in presence of ZnO NDs and Nano ZnO during photocatalysis by sunlight (b) (reprinted 
with permission from [81]; Copyright 2020, Elsevier)
a rapid and cost­effective AOP [20]. The degree of 
dye degradation was calculated based on the initial 
and final TOC values and presented as TOC removal 
(Figure 6).
The results show that the presence of 5 mM H
2
O
2
 
increased the photocatalytic activity of TiO
2
 com­ pared with that obtained in the absence of H
2
O
2
 and 
that the photocatalytic activity also increased when 
the concentration of TiO
2
 increased from 0.20 g/L 
to 0.50 g/L (Figure 6a). This resulted in an RR dye 
degradation efficiency of 90% in 210 minutes in the 
presence of 0.5 g/L TiO
2
 and 5 mM H
2
O
2
. UV irradi­ ation is thought to cause photolysis of H
2
O
2
, gener­ ating additional 
•OH radicals that have a synergistic 
effect on the photocatalytic activity of TiO
2
 and con­ sequently on the photodegradation of the RR dye. A 
further increase in the TiO
2
 concentration to 1.0 g/L 
increased the rate of dye degradation and resulted in 
an almost complete degradation ( ≈ 98%) in 60 min­ utes. However, a comparison of these results with the 
RR dye degradation in the photo­Fenton treatment 
revealed that a complete dye degradation (100%) 

188
Tekstilec, 2023, Vol. 66(3), 178–198
Figure 6: Photocatalytic degradation of RR dye with TiO
2
 in presence and absence of H
2
O
2
 (a); photo-Fenton 
treatment of RR dye at Fe
2+
 concentration varying H
2
O
2
 concentration (mM) and pH 3.0 (b) (reprinted with 
permission from [20]; Copyright 2021, Springer)
was achieved in the experiment with 0.5 mM Fe
2+
 
and 5.0 mM H
2
O
2
 at pH 3 in only 8 minutes (Figure 
6b). An economic comparison of the two processes 
also shows that the photo­Fenton process is not only 
faster, but also less expensive.
3.2.2 Cd-doped ZnO in combination with O
3
In another study, photocatalytic ozonation (PCO), 
which integrates photocatalysis in the presence of 
ozonation, was described as an effective approach for 
the degradation of real textile wastewater under UV 
irradiation (Figure 7) [18]. For this purpose, ZnO 
nanocatalyst doped with cadmium (Cd–ZnO) was 
synthesised and used in a photoreactor connected 
to an ozone (O
3
) generator. The operating parame­ ters such as O
3
 dose, pH, and Cd–ZnO amount were 
studied to achieve the optimal conditions for PCO, 
i.e. O
3
 dose of 0.44 g/h, pH of 7 and 0.2 g/L Cd–ZnO. 
The efficiency of the mineralisation of textile waste ­ water with PCO (Cd–ZnO/UV/O
3
) was determined 
based on COD determination at different times of 
wastewater treatment and compared with that of 
separate photocatalysis (Cd–ZnO/UV) and ozona­ tion (O
3
/UV) processes (Figure 7a).
Figure 7: Mineralisation of textile wastewater with different processes (a); mechanism of Cd–ZnO/UV/O
3
 for 
degradation of textile wastewater (b) (reprinted with permission from [18]; Copyright 2019, OIP Publishing)

TiO 2 and ZnO as Advanced Photocatalysts for Effective Dye Degradation in Textile Wastewater
189
The results show that the mineralisation rate ob­ served with Cd–ZnO/UV/O
3
 was by 4.2 times and 3.5 
times higher than that of Cd–ZnO/UV and O
3
/UV ,  
respectively, indicating a synergistic effect between 
O
3
 and Cd–ZnO/UV in PCO. This was due to the 
efficient trapping of generated electrons with O
3
, 
resulting in the formation of ozonide radical anions 
(
•O
3
−
). The radicals react rapidly with protons in the 
solution to form perhydroxyl radicals (HO
3
•), which 
then contribute to the formation of 
• OH (Figure 7b). 
Due to the more efficient trapping of photogenera ted 
electrons with O
3
, a recombination between holes 
and electrons is minimised, leading to the formation 
of a larger number of 
•OH, which accelerates the 
photocatalytic reaction [82].
3.2.3 Polyethylene glycol capped ZnO in 
combination with membrane filtration
A membrane photocatalytic reactor (MPR) (Figure 8a),  
which is a hybrid system of photocatalysis process 
and membrane filtration system, was used as an 
environmentally friendly approach for industri­ al textile wastewater treatment [83]. In MPR, the 
photocatalytic degradation of the wastewater was 
performed under UV­C irradiation in the photoca­
talytic reactor in the presence of polyethylene glycol 
capped ZnO (ZnO­PEG) nanoparticles as the initial 
treatment, followed by filtration through the polyp­ iperazine­amide (PPA) tight ultrafiltration mem ­ brane (UF­PPA). The photocatalytic efficiency of the 
ZnO­PEG nanoparticles was estimated by analysing 
the flux decline during membrane filtration, where 
the normalised flux was calculated as the ratio be­ tween wastewater flux and pure water flux.
The results show that a photocatalytic degra­ dation of wastewater with ZnO­PEG significant ­ ly reduced the pollutants filtered by the UF­PPA 
membrane, which prevented pore plugging of the 
Figure 8: Schematic diagram of MPR (a) (Legend: a − water chiller, b − overhead stirrer with stand, c − photocata-
lytic reactor, d − UV lamp, e − feed, f − cooling jacket, g − pump, h, i − pressure gauge, j − flow meter, k − recycle 
flow, l − membrane filtration system, m − measuring cylinder); normalised flux of UF-PPA membrane against time 
under different pH of industrial wastewater at loading of ZnO-PEG = 0.10 g/L (b); normalised flux of UF-PPA 
membrane against time under different loading of ZnO-PEG nanoparticles at pH = 11 (c); reaction conditions: 
dilution of wastewater = 75%, pressure = 6 bars (reprinted with permission from [83]; Copyright 2019, Elsevier)

190
Tekstilec, 2023, Vol. 66(3), 178–198
membrane for its permeability to be maintained and 
permeate flux through it sustained. The influence of 
the initial wastewater pH and the ZnO­PEG loading 
on the process performance was investigated, and 
the optimal operating conditions of ZnO­PEG in the 
MPR system were determined at pH 11 (Figure 8b), 
0.10 g/L ZnO­PEG nanoparticles (Figure 8c), and 
75% dilution of the textile wastewater. Under these 
conditions, the presence of ZnO­PEG nanoparticles 
as a photocatalyst significantly improved the effec­ tiveness of the MPR system, resulting in maximal 
photocatalytic degradation efficiency and minimal 
membrane fouling.
3.2.4 TiO
2
 and ZnO in combination with 
biological system
The coupled photocatalytic and biological process 
was applied to the treatment of industrial textile 
wastewater, including photocatalytic degradation of 
wastewater in the presence of ZnO or TiO
2
 as photo­ catalysts under UV irradiation, followed by an aerobic 
bioprocess using sludge microorganisms acclimated 
to textile wastewater (Figure 9) [22]. The photocata­ lytic process was performed in the reactor for 2 hours 
and the biological test was performed in the incubator 
under suitable conditions for 12, 24 and 28 hours.
The results show that the absorption peak of 
the wastewater decreased significantly during the 
UV­assisted photocatalysis in the presence of TiO
2
, 
resulting in a 44% decolourisation of the wastewa­ ter and that the subsequent bioprocess additionally 
contributed to the decolourisation of the waste­ water, resulting in an 88% colour removal after 12 
hours and a nearly complete decolourisation of 97% 
within 48 hours of biological treatment (Figure 9a). 
These results indicate that the combined TiO
2
/UV 
and biological system is suitable for the decolouri­ sation of real textile wastewater. In contrast to TiO
2
, 
photocatalysis with ZnO was much less effective and 
caused virtually no changes in the absorption spec­ trum after 2 hours of photocatalysis (Figure 9b). The 
lower photocatalytic efficiency of ZnO compared to 
TiO
2
 was attributed to the lower surface area of ZnO 
particles. The subsequent biological process did not 
contribute to the efficiency of the combined process; 
hance, only 48% of the colour was removed after 48 
hours of treatment. These results demonstrate the 
importance of photocatalysis for the decolourisation 
efficiency of the combined photocatalytic­biological 
process.
Figure 9: Absorption spectra of industrial wastewater before (IW) and after photocatalytic treatment (TiO
2
/UV 
and ZnO/UV) and combined photocatalytic-biological treatment (TiO
2
/UV + Bio and ZnO/UV + Bio), inclu-
ding percentage of colour removal after each treatment step; photocatalysis with TiO
2
 (a), and ZnO (b) (reprinted 
with permission from [22]; Copyright 2019, John Wiley and Sons

TiO 2 and ZnO as Advanced Photocatalysts for Effective Dye Degradation in Textile Wastewater
191
3.2.5 ZnO/polypyrrole in combination with 
biological system
In another proposed coupled photocatalytic­biologi ­ cal process, biological treatment of textile wastewater 
was conducted as a pretreatment and photocatalysis 
as a subsequent process using a ZnO/polypyrrole 
(ZnO/PPy) composite [84]. Previously, the bacterial 
consortium was collected from the inspection cham­ ber of the factory sewer and enriched for the biologi­
cal treatment, and the optimal amount of ZnO/PPy 
photocatalyst and its recyclability were determined 
for photocatalysis. In the combined process, real tex­ tile wastewater containing the azo dye Direct Black 
22 was pretreated with a bacterial consortium for 96 
hours and then photodegraded in the presence of 
ZnO/PPy for one hour under UV irradiation (Figure 
10a). The time dependence of Direct Black 22 was 
observed in both treatments. The results show that 
when the two process steps were applied separately, 
the biological treatment resulted in 71.3% decolouri­ sation of the dye and 80.0% removal of TOC, while 
the photocatalysis resulted in 83.6% decolourisation 
of the dye and 88.4% removal of TOC (Figure 10b). 
Coupling the two treatment processes resulted in a 
much higher decolourisation efficiency of 95.7%, 
while the final TOC removal reached remarkable 
99.9% (Figure 10b).
Figure 10: Sequential biological treatment and photocatalysis of real textile wastewater containing azo dye Direct 
Black 22 (a); decolorisation and TOC removal efficiencies of individual steps of separately applied treatments 
and coupled treatment (b); DB22 stands for Direct Black 22, BT stands for biological treatment, PC stands for 
photocatalytic treatment (reprinted with permission from [84]; Copyright 2020, Elsevier)
The presented combined processes have proved 
the importance of their performance for wastewater 
treatment. It is obvious that the efficiency of photo­ catalysis is significantly increased in the presence of 
oxidants such as H
2
O
2
 and O
3
. In fact, the addition 
of H
2
O
2
 was found to have a synergistic effect on the 
photocatalytic activity of nanosized TiO
2
, as H
2
O
2
 
generates additional •OH radicals under UV irradi­ ation. The synergistic effect between the ZnO­based 
nanocomposite and O
3
 was attributed to the efficient 
capture of the generated electrons by O
3
, leading to 
the formation of •O
3
−
 and HO
3
• , which then contrib­ ute to the formation of 
•OH. In a hybrid system of 
photocatalysis and membrane filtration, the initial 
photocatalytic treatment of wastewater with ZnO 
under UV­C irradiation significantly improved the 
efficiency of the ultrafiltration membrane system, re­ sulting in maximum photocatalytic degradation effi­ ciency and minimal membrane fouling. The coupled 
photocatalytic and biological processes also proved 
to be promising treatment methods, with photo­ catalysis performed as a pretreatment or as a sub­ sequent process. The coupling of the two processes 
resulted in significantly higher decolourisation effi­ ciency and TOC removal compared to the processes 
performed separately.

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4 Conclusion
The treatment of real textile wastewater to remove 
synthetic dyes prior to disposal to the municipal 
wastewater treatment plant or the environment re­ mains a major challenge. Various physical, chemical 
and biological processes have been used for this pur­ pose, among which photocatalysis has already estab­ lished itself as one of the most challenging ones.
Both TiO
2
­ and ZnO­based photocatalysis have 
unique advantages that make them an important 
AOP for textile wastewater treatment. One of the 
key advantages is environmental sustainability, as 
TiO
2
 and ZnO are recognised as biocompatible, 
non­toxic, and chemically inert nanomaterials on 
the one hand, and the ability of photocatalysis to 
degrade the pollutants to water and carbon dioxide 
without hazardous by­products on the other hand. 
It should be emphasised that photocatalysis can 
be used in a variety of environmental remediation 
processes to convert toxic pollutants into harmless 
products, which would not be possible with conven­ tional wastewater treatment processes. However, in 
addition to the advantages, there are also some lim­ itations of photocatalysis. One of them is its narrow 
spectral response, mostly in the UV range, which 
limits its ability to utilise a broader spectrum of sun­ light. In addition, the introduction of photocatalysis 
for large­scale applications is still a challenging re ­ search topic as it is usually studied under ideal lab­ oratory conditions. To improve the applicability of 
photosynthesis in real­world scenarios and to ensure 
the long­term stability of the photocatalysis system, 
further research and development efforts are needed 
for a careful construction and design of large­scale 
photocatalytic reactors.
In addition to single photocatalytic processes, 
combined processes in which photocatalysis is cou­ pled with other chemical, physical and biological 
processes have attracted a considerable interest due 
to their synergistic effects in wastewater treatment. 
Photocatalysis has been successfully performed in 
the presence of other oxidants such as H
2
O
2
 and O
3
, 
and in combination with ultrafiltration and biologi­ cal processes. In these studies, a proper system design 
and determination of optimal treatment parameters 
are of great importance to take advantage of each 
process and maximise treatment performance. The 
complementation of the coupled processes and the 
creation of a synergistic effect resulted in more effi­ cient comprehensive and diverse pollutant removal 
compared to a single wastewater treatment. When 
photocatalysis is coupled with membrane filtration 
as pretreatment, fouling can be reduced, which 
improves and stabilises filtration performance. By 
combining photocatalysis as a pretreatment with a 
biological process, organic load is reduced, which 
lowers energy consumption and operational costs.
Acknowledgments
The research was conducted as part of the course En -
vironmental Aspects in Textiles and Graphics within 
the doctoral study programme Textile Engineering, 
Graphic Communication and Textile Design at the 
University of Ljubljana, Faculty of Natural Science 
and Engineering, Department of Textiles, Graphic 
Arts and Design. The authors sincerely thank the pro -
gramme coordinator Prof. Dr. Petra Forte Tavčer for 
her constructive comments and guidance during the 
research work.
Funding
This research was funded by the Slovenian Research 
Agency, Slovenia (Programme P2-0213 Textiles and 
Ecology, Infrastructural Centre RIC UL-NTF, and a 
grant for the doctoral student D.G.).
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