applied
sciences
Review
Photocatalytic Inactivation as a Method of Elimination of E. coli
from Drinking Water
Timothy O. Ajiboye 1,2 , Stephen O. Babalola 3 and Damian C. Onwudiwe 1,2, *
1
2
3
*
Citation: Ajiboye, T.O.; Babalola,
S.O.; Onwudiwe, D.C. Photocatalytic
Inactivation as a Method of
Material Science Innovation and Modelling (MaSIM) Research Focus Area, Faculty of Natural and
Agricultural Science, North-West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735,
South Africa; 32480342@student.g.nwu.ac.za
Department of Chemistry, Faculty of Natural and Agricultural Science, North-West University (Mafikeng
Campus), Private Bag X2046, Mmabatho 2735, South Africa
Department of Biology/Microbiology/Biotechnology, Alex Ekwueme Federal University, Ndufu-Alike Ikwo,
Ebonyi State 84001, Nigeria; Stephen.babalola@funai.edu.ng
Correspondence: Damian.Onwudiwe@nwu.ac.za; Tel.: +27-18-389-2545; Fax: +27-18-389-2420
Abstract: The presence of microorganisms, specifically the Escherichia coli, in drinking water is of
global concern. This is mainly due to the health implications of these pathogens. Several conventional
methods have been developed for their removal; however, this pathogen is still found in most
drinking water. In the continuous quest for a more effective removal approach, photocatalysis has
been considered as an alternative method for the elimination of pathogens including E. coli from
water. Photocatalysis has many advantages compared to the conventional methods. It offers the
advantage of non-toxicity and utilizes the energy from sunlight, thereby making it a completely
green route. Since most photocatalysts could only be active in the ultraviolet region of the solar
spectrum, which is less than 5% of the entire spectrum, the challenge associated with photocatalysis
is the design of a system for the effective harvest and complete utilization of the solar energy for the
photocatalytic process. In this review, different photocatalysts for effective inactivation of E. coli and
the mechanism involved in the process were reviewed. Various strategies that have been adopted
in order to modulate the band gap energy of these photocatalysts have been explored. In addition,
different methods of estimating and detecting E. coli in drinking water were presented. Furthermore,
different photocatalytic reactor designs for photocatalytic inactivation of E. coli were examined.
Finally, the kinetics of E. coli inactivation was discussed.
Elimination of E. coli from Drinking
Water. Appl. Sci. 2021, 11, 1313.
https://doi.org/10.3390/app11031313
Keywords: photocatalysis; water treatment; Escherichia coli; bacteria inactivation
Received: 29 November 2020
Accepted: 2 January 2021
Published: 1 February 2021
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Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1. Introduction
Water is an essential natural resource and, perhaps, the greatest need of humanity
for health, wellbeing, development, and sustenance of life [1–3]. Its availability and
accessibility plays an important role in food production, poverty reduction, social welfare
and economic development [1]. Water has so many functions in the body, such as the
maintenance of the normal volume and consistency of blood and lymph, modulation of
moisture content of internal organs of the body, removal of poisons or toxins from the body
through urine, sweat and breathing, the regulation of body temperature. It is also essential
for the regulation of the normal structure and functions of the skin [1]. Unfortunately,
about 20% of the world’s population has no access to clean drinking water, a problem
which is further complicated by the increasing world population [4]. According to the
World Health Organization report of 2015, more than 2.6 billion people lack safe drinking
water, leading to the death of about 3.4 million people from various water related diseases
each year. Of these deaths, 1.4 million were reported to be children [5,6].
The exposure to preventable health risks has been attributed to the absence, inadequacy, or inappropriately managed water and sanitation services. Some of the health risks
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and diseases associated with the lack or insufficient clean water supply include hepatitis,
cholera, typhus, polio, tuberculosis, diphtheria and diarrhea. These have become a problem
of global concern [7,8]. Diarrhea is largely preventable, but it remains the most widely
known disease linked to contaminated food and water. About 829,000 yearly deaths are
attributed to diarrhea, and this is due to unsafe drinking-water, poor sanitation, and inadequate hand hygiene [7]. Although these water-related diseases are common in developing
countries, they are also a challenging health concern in the developed world [3,6]. Thus,
water quality control from water treatment plants continues to be an important global topic
of public health concern in the water industry. Shortage of fresh and portable water supply
is also combated through water recycling and reuse. However, this is also constrained
due to the need to completely eliminate contaminants in waste water [9]. Hence, the
decontamination of water from both chemical and microbial (biological) contaminants is a
major problem that must be solved especially in developing nations of the world. Microbial
contaminants are often considered a higher priority than chemical contaminants because
the adverse health effects of microbial contaminants are immediate, while that of chemical
contaminants are connected with long-term exposures [2]. Most biological agents that
cause water borne diseases are transmitted to humans via sewage discharge into water
sources [8].
In water treatment plants, a simple and reliable microbiological water quality control
of drinking water is often required to assess and monitor the presence of these pathogens,
especially the feacal coliforms [1,6]. These are a group of bacteria that emanate from the
feacal droppings of livestock, wildlife and humans. Escherichia coli (E. coli) is the most
abundant of this category, and it is often associated with the health risk of water [3,6].
There is a high possibility of the presence of other harmful viruses and bacteria of feacal
origin when E. coli is present in water samples. Pathogenic microbes such as Giardia, Cryptosporidium, and Shigella may also be found in water samples where E. coli is present [3].
This is why E. coli is often used as an indicator organism or a benchmark for determining
water purity and for the assessment of water samples containing feacal contaminants
beyond acceptable levels [1,3,6,8]. Two key factors are responsible for the use of E. coli as
the preferred indicator for the detection of faecal contamination in drinking water: firstly,
some faecal coliforms are non faecal in origin; and, secondly, there are well-developed and
improved testing methods for E. coli [1]. Consequently, E. coli is also used to evaluate the
efficiency of water disinfection methods. If the water sample is free from E. coli after a
disinfection process, it is concluded that the method is efficient and the water is free from
feacal contamination—thus making it unnecessary to analyze the water sample for other
pathogens [8,10–12].
Several conventional water disinfection methods such as chlorination, ozonation,
radiation, advanced filtration, and the use of ultraviolent (UV) light have been used over
the years [3,13]. Water decontamination by chlorination, for example, uses chlorine as a
strong oxidant, and it is one of the important, most common, relatively simple, effective
and inexpensive conventional ways to provide biologically potable water [3,4,10,13,14].
Unfortunately, conventional water purification technologies have several drawbacks, some
of which include:
•
•
Formation of Disinfection Byproducts (DBPs) and their consequent carcinogenicity:
Chemical disinfection methods such as chlorination and ozonation could also lead to
the formation of toxic and corrosive disinfection byproducts such as trihalomethanes
(THM), chloroform, chlorite, and haloacetic acids. This is due to high amount of
content of free residual chemicals after the disinfection process, and these compounds
have been proven to be extremely carcinogenic [3,4,8,9,13,15,16].
High operation and maintenance cost: Expensive chemicals or equipment are required
for the operation and maintenance of many of these methods. For instance, water
decontamination with ultraviolet light applies shortwave radiation (<280 nm) and is
associated with increased energy utilization. This requires expensive equipment for
the production [13,15,16].
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•
•
Antibiotic drug resistance: Microorganisms soon become resistant to some of these
conventional methods with prolonged use, thereby making the method ineffective
for water disinfection over time. Some biohazards are even naturally resistant to
chlorination and UV treatments [9,16–18].
Low disinfection efficiency: For example, chlorination is not able to eliminate all
pathogens. This is due to its slow biodegradation kinetics [8,13].
Moreover, the conventional disinfection technologies are often met with implementation challenges. A high concentration of residual chemicals could cause an unpleasant taste
and smell in the drinking water [13,16]. Ozonation, chlorination and other techniques have
been used for removing these bacteria, but they usually produce toxic by-products and
some of these byproducts may be mutagenic or carcinogenic [19,20]. In fact, disinfection
with the help of UV radiation alone, which is considered as a good alternative due to
the fact that the introduction of chemicals is not required, has now been discovered to be
disadvantageous. This is because some bacteria are resistant to UV radiation since they
regenerate after a few hours of irradiation [19,20]. Hence, there is a great need for alternative water disinfection technologies that could make up for the inherent limitations of the
conventional water decontamination methods [3]. Recent studies have been focused on
the development of an advanced antibacterial technology against microbial contaminants
in water. Specifically, a more efficient method of inactivating bacteria such as E. coli is
desirable.
Photocatalytic disinfection is considered as one of the innovative and promising options with high disinfection potential for water purification [13,15]. It is regarded as one of
the most prominent advanced oxidation technologies (AOT) and has so many applications
in water and air purification, viral and bacterial inactivation, and deodorization [16,21,22].
It has been an area of great interest in recent years, since it is applicable in many fields
including environmental and energy related research areas [13]. Photocatalysis has so
many advantages over the conventional water disinfection methods. Compared to the
other conventional methods of inactivating E. coli, photocatalysis is better and promising
because of its cost effectiveness [23]. Photocatalytic materials used for disinfection could be
recycled, while the conventional chemical methods consume the chemical disinfectants [4].
Finally, the photocatalytic materials used are non-hazardous and environmentally friendly,
yet are effective in the inactivation of pathogenic microorganisms in water [4,8].
1.1. The Structure of Escherichia coli
In 1885, Theodore Escherich discovered and described Bacillus coli. It was then renamed Escherichia coli after his name [1,24] E. coli is a diverse group of gram-negative
bacteria commonly found in the environment. They are also a normal part of the lower
intestinal tract microbiota of mammals including humans [1,24,25]. E. coli are typically
rod-like in shape with about 2 micrometres (µm) length, a diameter of 0.5 µm and a cell
volume of about 0.6–0.7 µm3 [1,26]. They are non-sporulating and use mixed-acid anaerobic fermentation to produce carbon dioxide, ethanol, lactate, acetate, and succinate. Some
strains of E. coli could grow at temperatures up to 49 ◦ C, but the temperature for optimal
growth is about 37 ◦ C [1,26]. The cell wall of E. coli is made of a thick layer of protein and
sugar that prevents the cell from bursting. The plasma membrane, made of lipids and
proteins, regulates the movement of molecules in and out of the cell. The fimbriae help the
E. coli to attach itself to surfaces, while ribosomes aid in the production of proteins [1]. The
flagella are composed of proteins, giving cells the capability to move (Figure 1).
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Figure 1. Structure of Escherichia coli. Reprinted from [24].
1.2. The Different Strains of Escherichia coli
E. coli is one of the most studied living organisms due to the enormous clinical
and scientific interests in the organism. It has been used as the model organism for
studies in a lot of biological research including studies in genetics, molecular biology,
and evolution [25]. Depending on the strain, E. coli could either be harmful or harmless.
Both the pathogenic and non-pathogenic E. coli behave ecologically and biochemically
alike, making it difficult to detect the pathogenic ones among commensal E. coli [25,27].
Meanwhile, the harmful pathogenic strains are responsible for diarrheal and other diseases
in humans and animals [27,28]. The pathogenic strains have virulence factors involved in
their pathogenesis and are used to group them into various pathotypes or virotypes [28].
There are two broad groups of pathogenic strains of E. coli depending on the site of infection:
•
•
Intestinal Pathogenic E. coli (IPEC): Most pathogenic strains of E. coli are Intestinal
pathogenic E. coli. They are also known as Enteric E. coli, and are the group of E. coli
that elicit their pathological effects and cause diseases in the gastrointestinal tract [1,28].
Examples of Intestinal pathogenic E. coli include enteropathogenic E. coli (EPEC),
enterohaemorrhagic E. coli (EHEC), enterotoxigenic E. coli (ETEC), enteroaggregative
E. coli (EAEC), enteroinvasive E. coli (EIEC) and diffusely adherent E. coli (DAEC).
Extra-Intestinal Pathogenic E. coli (EXPEC): Any other group of E. coli that cause diseases or exert their pathogenic syndromes in systems other than gastrointestinal tract
are known as Extra-intestinal pathogenic E. coli [28]. Some of the examples of Extraintestinal pathogenic E. coli includes sepsis-causing E. coli (SEPEC), uropathogenic
E. coli (UPEC) and neonatal meningitis-associated E. coli (NMEC). The genomes of
several strains of both pathogenic and non-pathogenic E. coli have been completely
sequenced, others are being sequenced while many other strains will be sequenced
in the future as new strains emerge through the natural biological process of mutation [1,25,27,28]. However, this review is limited to three of the most common strains of
E. coli including: the non-pathogenic commensal, K-12 strain; the enterohemorrhagic
O157:H7 strain, and the uropathogenic CFT073 strain.
1.2.1. The Non-Pathogenic K-12 Strain
Virulence genes found in E. coli are responsible for their pathogenic effects. The K-12
strains of E. coli lack all known E. coli virulence genes and are therefore considered to be
a safe, nonpathogenic bacterial strain [29]. The harmless strains produce vitamin K2 and
prevent the invasion of pathogenic bacteria within the intestine as they are often part of the
normal flora of the gut [1]. E. coli K-12 strains are rough and the O antigen, which is part of
the lipopolysaccharide, is absent [29]. In gene cloning experiments, Escherichia coli K-12
strains are the most commonly used host strains because of the following advantages:
•
•
Genetically, they are a well understood group of living organisms.
They could be modified easily by many genetic methods, and
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•
They are grouped as biologically safe vehicles for the propagation of many efficient
gene cloning and expression vectors.
1.2.2. The Enterohaemorrhagic O157:H7 Strain
After the outbreak in 1982, E. coli O157:H7 became the most widely known enterohaemorrhagic E. coli (EHEC) strain [26]. Shiga toxin production, a characteristic feature of the
pathogenesis of bloody diarrhea, was reported in Hemorrhagic Colitis outbreak caused by
E. coli O157:H7 strain in the United States [26,28,30]. The ability to produce Shiga toxins is the
common characteristic of all EHEC, and they are often referred to as Shiga toxin-producing
E. coli (STEC) [26]. E. coli O157:H7 has been implicated in the death of the elderly, the very
young, and the immunocompromised including serious food poisoning leading to product
recalls [1]. Its pathogenicity coupled with its ability to survive environmental stress make
it a powerful threat to public health efforts. It is resistant to commonly used methods for
controlling bacteria such as boiling. The conventional means of isolating other E. coli are not
effective in the isolation and identification of enterohemorrhagic E. coli 0157:H7 because it has
some biochemical differences from most other E. coli strains [1,26,28,30]. For example,
•
•
•
Ninety percent of other strains of E. coli cannot ferment sorbitol but 0157:H7 strain
can ferment it [1,26,28,30].
It does not produce functional β-glucuronidase, whereas most of the other E. coli
strains are positive for this test [1,26,28,30].
Unlike the other 60% non-sorbitol fermenting E. coli, the 0157:H7 strains will not
ferment rhamnose on nutrient agar [1].
Therefore, special techniques such as DNA probes and polymerase chain reaction
(PCR), ELISA procedure utilizing the monoclonal antibody (4E8C12) are used in the isolation of E. coli 0157:H7 [1].
1.2.3. The Uropathogenic CFT073 Strain
About seventy to ninety percent of the estimated 150 million community acquired
urinary tract infections are caused by the uropathogenic E. coli [31]. In the United States,
7 million cases of acute cystitis and 250,000 cases of pyelonephritis are reported annually,
70–90% of which are also caused by uropathogenic E. coli [31]. A lot of genes that code
for fimbrial adhesins, phase-switch recombinases, autotransporter and iron sequestration
systems are found in the genome of the uropathogenic E. coli CFT073 strain. These features
predispose the strain to colonize the urinary tracts and elicit its pathological effects [32].
2. Mechanism of the Photocatalytic Inactivation of E. coli
Several studies have already discussed the mechanism of photocatalysis. Hence, only
the summary is given in this review [33–36]. Photocatalysis is easy to implement as it
essentially involves the use of a light source and a photocatalyst. An interaction between
light and the semiconductor solid catalyst results in the absorption of light with energy
equal to or greater than the band gap energy of the semiconductor photocatalyst. This
leads to the excitation of electron(s) from the lower energy level, the valence band (VB), to
the higher energy level, the conduction band (CB). Generally, semiconductors consist of an
electron filled valence band and an empty conduction band [37]. The migration of electrons
from the occupied VB leaves positively charged holes on the VB (electron vacancy). It is
possible for both the electrons generated and the holes to recombine, which is not desirable
because their combination will lead to the loss of energy informing of heat energy. The
second possibility is the production of hydroxyl radicals as a result of the interaction of
holes with water and superoxides are formed as a result of the interaction of the generated
electron with oxygen [34,38] as shown in Figure 2. These photogenerated highly reactive
species (ROS) or free radicals (such as hydroxyl radicals (OH. ) and superoxide (O2− )) are
capable of eliminating biological and chemical water contaminants [8,15,16]. The free
radicals produced are highly effective antibacterial agents within aqueous environments.
−
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Figure 2. Mechanism of photocatalytic inactivation of E. coli.
Three common theories have been proposed for the mechanism of inactivation of
bacteria. The first is the destruction of the DNA, the second is the inhibition of the respiratory activities as a result of the oxidation of coenzyme A by the photogenerated reactive
species, which leads to their loss from the cells. The third mechanism is the destruction
of the membranes by reactive species, thereby leading to cell content’s leakage [35,39,40].
Irrespective of the mechanism, the inactivation is affected by the photogenerated holes,
electrons, and the reactive oxygen species (such as hydroxyl radicals, peroxyl radicals and
hydroperoxyl radicals) generated during the mechanism of photocatalysis.
As the E. coli inactivation proceeds, the shape of E. coli cells begins to get disfigured
and the shape becomes different from the original shape before the inactivation process.
This is followed with the rupturing of the cell wall, which permits the reactive species to
permeate the cell and the size of the remaining part of the cell decreases noticeably [41]. The
inorganic species used to monitor the destruction is potassium ion, since it is responsible
for regulating polysomes and proteins. Rupturing of cell membrane leads to the leaking of
potassium ions. The concentration of potassium ion has been discovered to increase with
an increase in the time of photocatalytic inactivation [9,41].
3. Inactivation Process by Photocatalysts
Several photocatalysts have been used to inactivate E. coli, and they include metal
oxides, sulphides, halides, phosphide or other metal free photocatalysts. The type of
light source used along with these photocatalysts depends on the band gap energy of
the photocatalysts used [42]. These photocatalysts are not only photoactive, but they
are non-toxic, of moderate band gap and inert, which are the desirable properties of the
photocatalyst [33]. The redox potential of both the valence band and conduction band of
different photocatalysts at neutral pH is shown in Figure 3.
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Figure 3. Redox potential and band gap energies of some selected semiconductor photocatalysts at pH 7. Reprinted with
permission from [43], Copyright (2016) American Chemical Society.
The different compounds that have been utilized as semiconductor photocatalysts in
the inactivation of E. coli will be highlighted and their specific properties discussed.
3.1. Metal Oxides as Photocatalysts for the Inactivation of E. coli
Metal oxides such as TiO2 , ZnO, ZrO2 , CeO2 , Fe2 O3 , SnO2 and WO3 have photocatalytic properties and have been reported in different studies [8,15,44–48]. The photocatalytic properties of some of these metal oxides have been harnessed for the conversion of
solar energy to chemical energy in order to oxidize or reduce materials, degrade pollutants
and inactivate pathogens [13]. Nanocomposite made from two oxides (zinc oxide and
silver oxide) has been used to inactivate E. coli under both visible and ultraviolet light
radiation. Despite pure zinc oxide being reported to be effective against bacteria [49], its
photocatalytic activities were lower than that of the nanocomposite made from zinc oxide
and silver oxide [50]. In a bid to reducing the time needed for the inactivation of bacteria,
zinc oxide nanoarrays were fabricated and used to inactivate 97.5% of E. coli within 60 s
under ultraviolet light. However, after several cycles of use, the time taken for inactivation
to take place was 600 s under similar condition [51]. Oxides of copper nanoparticles derived from citrus has been used as light-sensitive catalysts for decontaminating wastewater
containing E. coli [52]. Magnesium oxide nanoparticles obtained via green synthesis from
plant biomass was also used to inhibit E. coli [53]. The synthesis of magnesium oxide
from plant biomass is advantageous because it is not poisonous, is environmentally sustainable and cheaply available [54,55]. In addition, the biomass contains stabilizing and
reducing agents such as phenolic compounds, natural acids, alkaloids, and terpenoids,
which aid the formation of the nanoparticles of metal oxides [54,55]. The inactivation of
60% of Escherichia coli was achieved in the presence of light by using chitosan and sodium
alginate functionalized by nanoparticles of ZnO and CuO. The functionalization led to the
generation of more reactive oxygen species under light [56].
Titanium oxide is one of the earliest discovered catalysts and has remained the photocatalyst of choice due to its non-toxic nature, relative cheapness, good photocatalytic
performance and high chemical stability [57]. It is an important nanomaterial with optical, catalytic and dielectric properties [58]. Titanium dioxide has attracted a considerable
attention because of its wide and growing industrial applications in the production of
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solar cells, fillers, pigments including catalyst supports and as photocatalysts [8,58]. It
has three crystal forms, namely brookite, anatase and routile. The anatase form induces
maximum speed in the formation of free radicals [8]. Several studies have shown the
potential of the TiO2 catalyst in the successful and complete deactivation of microbial
cells including cancerous cells, bacteria, viruses, fungi and algae under the irradiation of
UV light source [58]. One of the numerous investigations that utilized titanium oxide as
photocatalyst for inactivating E. coli was reported by Kikuchi et al. [59]. In the study, a
transparent film of titanium oxide was used to inactivate E. coli in the presence of light.
Hydrogen peroxide was introduced to increase the amount of reactive oxygen species that
is present in the photocatalytic system. Despite the wide acceptance of titanium oxide as a
good photocatalyst for inactivating bacteria, the limitation of titanium oxide photocatalyst
is that it is only active in the ultraviolet (UV) region of the solar spectrum due to the wide
band gap energy of 3.2 eV. This UV region accounts for less than 5% of the solar spectrum,
which implies that pure titanium oxide under-utilizes the solar spectrum. Thus, different
structural modifications of titanium oxide have been carried out to reduce the band gap
energy of titanium oxide in order to absorb in the visible region of the solar spectrum,
which accounts for the larger percentage of the spectrum. Titanium oxide also displays
better bactericidal performance against E. coli when jointly doped with silver and nitrogen.
The death of E. coli was ascribed to the disconfiguration of the cell and cell wall thinning
through oxidative damage under visible light [60]. Ribeiro et al. [13], for example, reported
that TiO2 inactivated all E. coli cells within 60 min with UV light.
3.2. Metal Sulphides as Photocatalysts for the Inactivation of E. coli
Different types of sulphides have been reported for the inactivation of E. coli. For
example, binary indium sulphide with band gap energy of 2.25 eV was used as the visible
light sensitive photocatalyst for inactivating E. coli. Instead of hydroxyl radical causing
the inactivation, hydrogen peroxide, peroxyl radicals, holes and electrons were reported
as the cause of inactivation as confirmed by the scavenger investigation. These reactive
species caused 5 log inactivation of bacteria cell by destroying enzymes and DNA, which
are cytoplasmic components of the bacteria after 180 min of visible light irradiation [61].
Zinc sulphide has also been used for E. coli inactivation. When the photocatalyst was
exposed to UV light, 99% of E. coli was inactivated within 120 min [62]. Apart from the
binary sulphides, ternary sulphides were also used for E. coli inactivation. Wang et al. [63]
reported the use of ternary CdIn2 S4 as a visible light-active photocatalyst. The ternary
compound was found to be effective because it could be recycled via partition without
losing its performance. This photocatalyst was able to inactivate 7 log of the bacteria within
180 min. The efficiency of the bacteria inactivation was also found to improve with increase
in the photocatalyst loading. A novel nanocrystal ternary sulphide (Cu2 WS4 ) was used to
inactivate more than 99% (5 log) of Gram-negative E. coli in the presence of light. Further
investigation into the mechanism of antibacterial inactivation of this ternary sulphide
photocatalyst showed that it has the ability to selectively bound to bacteria. In addition, it
has peroxidase and oxidase properties, which enhanced the generation of reactive oxygen
species that are active against E. coli [64].
3.3. Metal Halides as Photocatalyst for the Inactivation of E. coli
One of the metal halides that has been extensively studied for the inactivation of E. coli
is the silver halides. Silver iodide has been decorated on an organic framework made
from 1,3,5-triformylphloroglucinol and 2,5-diaminopyridine precursors and was used for
inactivating E. coli. The application of silver iodide as photocatalyst against bacteria was
found to be a good option because of its stability after four consecutive runs without losing
its photocatalytic performance [65]. To inactivate E. coli, silver iodide has also been used
along with cerium oxide [66], copper ferrite [67], bismuth oxyiodide [68], zinc ferrite [69],
bismuth stannate (Bi2 Sn2 O7 ) [70] and bismuth vanadate [70]. Silver bromide has also been
an active photocatalyst against E. coli. Yu et al. [70] utilized silver bromide decorated
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on graphitic carbon nitride to inactivate the bacteria in the presence of visible light. The
effectiveness of silver bromide was obvious because 4.8 log of the bacteria was inactivated
in the presence of silver bromide unlike 4.2 log that was inactivated when pure graphitic
carbon nitride was utilized without silver bromide. More than 99% of the E. coli was
inactivated by silver bromide when it was used along with silver vanadate for bacteria
inactivation in the presence of light in less than an hour [71]. In addition, the elimination
of E. coli by using silver bromide assisted with bismuth oxybromide has been reported [72].
The photocatalytic efficiency of silver bromide was boosted when it was used along with
graphitic carbon nitride for eliminating E. coli [73]. Krutyakov et al. also reported the use of
nanocomposite photocatalysts made from silver chlorides for the inactivation of E. coli. The
photocatalyst was stabilized by amphopolycarboxyglycinate, which is sodium-containing
amphoteric surfactant [74].
3.4. Metal Phosphides as Photocatalysts for the Inactivation of E. coli
Relatively less attention has been given to the use of phosphides as a photocatalysts,
but the use of zinc phosphide has been well investigated [75,76]. The large abundance
of zinc and phosphorus in the earth crust and the possibility of fabricating different
morphologies with band gap that is as low as 1.5 eV makes zinc phosphide one of the
photocatalysts that could be considered for the inactivation of E. coli [77]. The major
drawback for the utilization of zinc phosphide is that it has low stability in water [75,77].
One of the adopted strategies for the improvement of its stability is to functionalize it
with organic molecules [76]. Vance et al. used this strategy to improve the stability
of zinc phosphide nanowire and boron nitride was used for the functionalization. The
functionalized zinc phosphide was used for inactivating E. coli with 5-log of the bacteria
inactivated within 300 s under visible light irradiation [76]. However, the photocatalytic
activities of the un-functionalized phosphide were found to be better than that of the
functionalized phosphide.
3.5. Metal Free Photocatalysts for the Inactivation of E. coli
One of the metal-free materials that has been used for inactivating E. coli is threedimensional reduced graphene oxide and graphene oxide under visible light irradiation.
The inactivation rate was boosted by five times when the peroxymonosulphate was introduced as an activator [78]. The inactivation was carried out through non-radical and radical
reaction pathways. Holes, hydroxyl radicals, hydroperoxyl radicals and peroxyl radicals
were found to be the active radicals against the bacteria as revealed by the scavenger
investigations carried out by the non-toxic scavengers. These non-toxic scavengers were
selected to prevent damages to bacteria cells so as to obtain accurate quenching effects [78].
Graphene oxide quantum dots were also effective against E. coli, but when it was functionalized with nanorods and nanoflakes of zinc oxide, it was observed that these functionalized
photocatalysts displayed better antibacterial efficiency than when ordinary graphene oxide
quantum dots was utilized as photocatalysts [79]. Another metal-free photocatalyst that
was used to inactivate E. coli is graphitic carbon nitride, and it was found to be effective
against E. coli. However, the rate of inactivation was found to be better when graphitic
carbon nitride was composited with molybdenium sulphide and bismuth oxide [80]. The
graphitic carbon nitride was coupled with copper (I) oxide and used as photocatalyst under
visible light to inactivate E. coli. Investigations by electron spin resonance and reactive
species trapping revealed that the inactivation was effected by holes, hydroxyl radicals and
peroxyl radicals [81]. Huang et al. [82] used graphitic carbon nitride to inactivate E. coli k-12,
and this was carried out in the presence of visible light irradiation. When the photocatalytic
inactivation was carried out for 4 h, there was complete inactivation of the bacteria as
confirmed using a bacteria regrowth test, which showed that there was no bacteria count
after four days of incubation. It was also observed that, when graphitic carbon nitride was
used in the dark, there was no effect on the bacteria. This showed that the inactivation
was affected due to the presence of light. Graphitic carbon nitride nanosheet has also
Appl. Sci. 2021, 11, 1313
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been used as a photocatalyst to eliminate 100% of E. coli within 240 min in the presence of
visible light. The performance of this graphitic carbon nitride nanosheet was better than
that of bulk graphitic carbon nitride because the bulk graphitic carbon nitride could only
eliminate 77.1% of the bacteria under the same conditions [23]. In a similar research, 5 log
and 3 log of the E. coli were killed when nanosheet and bulk graphitic carbon nitride were
respectively used as a photocatalyst in the presence of visible light radiation. When a single
layer of the graphitic carbon nitride was used, 2 × 107 cfu mL−1 was inactivated under the
same conditions [19]. Another metal-free photocatalyst that has been used as an effective
photocatalyst against E. coli is graphene quantum dot. Electrochemical techniques have
been used to produce graphene quantum dots which killed E. coli by damaging the cell
membrane in the presence of light. The killing of the bacteria was confirmed by the use of
the plate count method, where the amount of propidium iodide that was consumed was
used for the estimation. When the experiment was repeated under the same conditions in
the absence of light, no inactivation took place. In addition, the use of ordinary light for
the inactivation was not effective against the bacteria strains [83].
3.6. Inactivation of E. coli by Metal Doped Photocatalysts
Divalent cobalt was used to dope microsphere nanostructure of BiOBrx Cl1-x and the
photocatalyst showed good photocatalytic E. coli inactivation. The good photocatalytic
performance was attributed to the enhanced charge separation and good light absorptivity
as a result of cobalt doping [84]. The doping of divalent cobalt into bismuth was effective
as a result of the abundance of cobalt and the fact that the size of divalent cobalt (0.65 Å)
is comparatively smaller than the size of trivalent bismuth (1.03 Å), which made the
incorporation of divalent cobalt into the lattice structure of BiOBr easy [84,85]. In another
investigation, iron was used to dope polyaniline to form a metal organic framework
which was used as photocatalyst for the inactivation of E. coli. Evidence from electron
paramagnetic resonance, fluorescent test and quenching experiments revealed that h+ , ·OH,
·O2 − and e− are the reacting species that aided the photocatalytic inactivation of bacteria
with hydroxyl radicals being the most predominant [86]. Gold has also been used to dope
nanoparticles of titanium oxide through the seed growth method. The catalytic activity
of titanium oxide improved significantly as a result of the presence of gold as the doping
agent, and it reflected in the inactivation of bacteria better than when pure titanium oxide
was used as photocatalyst under UV light [87]. Silver, another noble metal, was used to
dope oxides of iron to form a composite photocatalyst that was used to inactivate E. coli in
the presence of light [88]. Iron has been used to dope ZnO nanoparticles, and the bandgap
of Fe-doped zinc oxide was found to be lower than the band gap energy of undoped zinc
oxide. This doped photocatalyst was active against E. coli in the presence of sunlight.
3.7. Inactivation of E. coli by Photocatalysts Doped with Non-Metal
Sulphur, nitrogen, phosphorus and carbon have been used to dope series of photocatalysts in order to improve their photocatalytic efficiency [89,90]. For instance, titanium oxide
doped with nitrogen was used as an active photocatalyst against E. coli. The performance
of this photocatalyst increased when carbon, another non-metal, was used to sensitize
the photocatalyst. After 60 min of visible light irradiation, approximately 80% E. coli was
eliminated, and this could be attributed to the lowering of band gap as a result of the
introduction of non-metals [91]. Raisada et al. [92] doped silver vanadate and graphitic
carbon nitride composite with sulphur and phosphorus. The presence of these non-metal
dopants achieved 99% of E. coli inactivation within 1 h in the presence of visible light
radiation.
3.8. Inactivation of E. coli by Heterojunction Systems
One of the most common heterojunction systems is the type II heterojunction system,
which is formed by two semiconductors (SC I and SC II) that possess wide and narrow band
gaps, respectively. The valence band and the conduction band of semiconductor 1 (SC I)
Appl. Sci. 2021, 11, 1313
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are more than the valence band and the conduction band of semiconductor 2 (SC II).
This makes it possible for photo-induced holes to move from the valence band of SC I to
the valence band of SC II, whereas the light-excited electrons will migrate in the reverse
direction (SC I to SC II). The movement in different direction boosts the charge separation,
which culminate in better photocatalytic performance. Another common heterojunction
system is the Z-scheme heterojunction system [93]. The Z-scheme and other heterojunction
schemes are illustrated in Figure 4a–f and it could be with or without semiconductor
at its interface. The semiconductor acts as the electron mediator for charge relay and
recombination. Without the mediator, the photo-excited charge in the valence band of
SC I recombine with the low-lying conduction band of SC II at the interface of SC I and
SC II [94]. The s-scheme has also been reported as another heterojunction system. In the
s-scheme heterojunction system, two n-type semiconductors with different band structures
are combined together. One of the photocatalyst is the oxidation photocatalyst, while
the other one is the reduction photocatalyst [95]. The Schottky scheme has equally been
reported as another way of improving the photocatalytic efficiency of the photocatalysts.
In this heterojunction, a semiconductor makes contact with a metal to generate a potential
difference known as the Schottky barrier. The generation of this potential difference is as a
result of non-similar Fermi levels and work function between the semiconductor and the
metal. The Schottky barrier function elongates the lifespan of the electron by preventing
the recombination of the charge carriers [96,97].
Figure 4. Examples of heterojunction systems. Reproduced from [98], Copyright (2019), with permission from Elsevier.
The combination of nano-Ag2 ZrO3 and graphitic carbon nitride has been used to
fabricate a type II heterojunction system and the resultant photocatalyst was used for
eliminating gram-negative E. coli. Exposure of the E. coli to visible light in the presence of
this heterojunction photocatalysts led to the reduction of the population of E. coli to 3%,
but longer exposure time achieved a reduction of the E. coli to 1.27% [99]. The Z-scheme
heterojunction system made from silver iodide and organic framework having a covalent bond was also investigated for photocatalytic inactivation of E. coli. The inactivation
was affected under visible light irradiation by the hydroxyl radicals, holes and peroxyl
radical as revealed by the scavenger experiment [100]. Complete bacteria disinfection
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(1.5 × 106 CFU mL−1 ) was achieved in 40 min when visible light was used, unlike in
the dark where there was no inactivation of viable cells. After four cycles of using the
photocatalyst, there was still complete inactivation of the bacteria, which shows that the
photocatalyst was reusable [100]. Another Z-scheme heterojunction system that was used
for the inactivation of E. coli under visible light irradiation was made from nanocomposites
of AgI/Bi2 Sn2 O7 . The investigation by electron spin resonance and radical trapping experiments showed that 7.48-log of the bacteria was inactivated via ·OH, ·O2 − , e− and h+ [101].
The Z-scheme heterojunction made from graphitic carbon nitride and nickel phosphide
was also used to inactivate E. coli via the oxidation by holes in the presence of visible light.
The number of E. coli inactivated with this Z-scheme heterojunction system was 10-fold
better than the number inactivated with neat graphitic carbon nitride and graphitic carbon
nitride doped with platinum [102]. Ju et al. [103] fabricated a Z-scheme ternary heterojunction system made from Ag doped AgVO3 and BiVO4 assisted by polyvinylpyrrolidone.
This photocatalyst displayed a good photocatalytic performance for the inactivation of
E. coli through the action of peroxide radicals and holes generated under visible light
irradiation. The Z-scheme heterojunction system of graphene aerogel, bismuth vanadate,
silver chloride and silver which was made by a hydrothermal technique. The formation of
Z-scheme heterojunction boosted both the charge transfer and separation of electrons. This
photocatalyst displayed total inactivation of all the E. coli in the system in less than 25 min
in the presence of visible light irradiation [104].
Finally, one other strategy was adopted to reduce the recombination of holes and
electrons and obtain a better activity during photocatalysis is the formation of the p-n
heterojunction system [105]. E. coli has also been inactivated by the p-n heterojunction
photocatalysts, made from polyaniline and zinc oxide. The improved photocatalytic performance of this photocatalyst was linked to the synergistic effect of the p-n heterojunction as
well as the generation of hydroxyl and peroxyl radicals assisted by relatively large surface
area of the photocatalysts [106]. Examples of other recent studies on the photocatalytic
inactivation of E. coli are shown in Table 1.
Table 1. Recent studies on the photocatalytic inactivation of E. coli.
Photocatalyst
Fabrication Method
MXene/ anatase TiO2
loaded in reactor made
from polyurethane foam
-
Cu2 O on cotton fibers
In-situ synthesis
Metallic silver on Pinewood
biochar support
One-step
carbothermal
reduction
Composites made from
zeolite and titania
Fe3 O4 @SiO2 @ZnOAg3 PO4
Heterostructure of
ZnO/CdS heterojunction
Solid-state dispersion
Light Used
Reactive Species
Ref.
Ultraviolet
e− , HO· and h+
[107]
350 W xenon lamp
·OH, O2· -e− , h+
[108]
Ultraviolet
1O ,
2
·OH, ·O2 – ,e− , h+
[109]
Ultraviolet
O2 −
[110]
Visible light
·OH, O2· -e− , h+
[111]
Visible light
OH˙
[112]
Visible light
O2 , ·OH
[113]
Good performance
Solar light
·OH, h+
[114]
Chemical reduction
Active against E. coli
visible light
O2 − , h+ and OH˙
[115]
Green synthesis
Good E. coli
inactivation
Visible and UV using
metal halide lamp
-
[116]
Hydrothermal
Hydrothermal
Ag/TiO2 /ZnO composite
Hydrothermal
Heterostructure
TiO2 /Ag3 PO4 /graphene
oxide
Ion-exchange and
electrostaticallydriven
assembly
Nanocomposites of
Ag/Cu-cellulose
Reduced graphene oxide
decorated with Ag/ZnO
Performance
3.4 lg order better
than 2.5 lg order
obtained with UV
alone
93.25% E. coli
inactivation
Good E. coli
inactivation
efficiency
40% E. coli reduction
in 1 h
Better performance
than ordinary ZnO
Better performance
than ordinary ZnO
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Table 1. Cont.
Photocatalyst
Fabrication Method
conjugated polymer/ CuO
nanoparticles
-
Nanotube of TiO2/Cux Oy
Anodization method
TiO2 nanowire-sensitized
quantum dots of MnCdS/ZnCuInSe/CuInS2
-
AgBr/AgVO3
Hydrothermal
process
Performance
Better bacteria
passivation when
peroxymonosulphate
was introduced to the
polymer
Inactivation of 97%
E. coli in 1 h
96% E. coli present in
50 cm3 105 colony
forming units ml−1
solution were killed
in 50 min
99.997% bacterial
inactivation
Light Used
Reactive Species
Ref.
UV–vis light
h+ ,
O2· − and H2 O2
[117]
Visible light
e− , h+ , ·OH, and
O2· − radicals
[118]
visible light
·HO2 , ·OH, H2 O2 ,
·O2−
[119]
Visible light
·OH
[71]
4. Inactivation of E. coli by Assisted Photocatalysis
Most studies referred to this method as photoelectrocatalysis. This combined technique is advantageous because it clearly separates the site of oxidation from the site of
reduction, which reduces the crossover of the products [120]. In addition, the measurement
of voltage and current from electrochemical method can serve as parameter for quantifying
the rate of photocatalysis [120]. Pseudomonas aeruginosa and Escherichia coli were removed
from rainwater by combining both photocatalysis and electrochemical methods. This
was achieved by designing a photoelectrochemical reactor, which was placed under the
sun. Evidence from culture-based analysis showed that 5.5-log of E. coli reduction was
achieved [121]. The photoelectrocatalytic system made from nanotubes of titanium oxide
that was doped with the combination of tin oxide, nanoparticles of silver and antimony was
active against E. coli. Over 5-log bacteria elimination was reported when this system was
used for inactivating E. coli within 60 min in the presence of light [122]. Mesones et al. [123]
also reported the use of photoelectrochemical approach to inactivate E. coli, but this was
achieved by using titanium oxide composites as the photocatalyst, while RuOx/Ti was
used as the anode. When only the electrochemical process was used, the active radical for
the inactivation was hydroxyl radical. However, when it was combined with photocatalysis, the active species for the inactivation were chlorine species and hydroxyl radicals [124].
In addition to the use of electrochemical method for assisting photocatalysis, plasmon has
also been used to aid photocatalysis for the inactivation of E. coli [125].
5. Quantitative Estimation of Escherichia coli
One of the most common methods for estimating E. coli is to use Ethidium monoazide
bromide coupled with the quantitative polymerase chain reaction (EMA-qPCR) analysis.
The method is used to validate other methods of E. coli estimation. The pre-treatment carried out on E. coli before carrying out the quantitative polymerase chain reaction involves
the use of Ethidium monoazide bromide. This analysis works based on the membrane
integrity of the bacteria. EMA binds to the DNA of the bacteria with damaged cell membrane and inhibits the bound DNA amplification when subjected to polymerase chain
reaction. The amplification of the DNA would be done for bacteria cells with undamaged
DNA [126]. The use of aerosol generator to estimate the population of E. coli has also
been reported. The suspension of E. coli cells was aerosolized to less than 5 µm to give
bioaerosol containing bacteria. This was followed by a plate counting step and incubation
of nutrient agar at a temperature that is above room temperature for a day [107]. In some
cases, the incubation is assisted with rotary shaker or centrifuge at a particular rotation
per minute before termination of the incubation after some minutes [109]. The counting on
each nutrient agar plate is expressed in colony-forming units (cfu) [107]. Transcriptome
analysis is another method of estimating the amount of E. coli. This method requires the
isolation of RNA of the bacteria before the concentration and purity of the bacteria cells
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would be estimated by a spectrophotometer. The process is immediately followed by the
use of bioanalyzer to determine the integrity of the RNA [56]. The use of fluorescence
detector with 4′ -6-diamidino-2-phenylindole has also been reported [122]. The working
principle of this technique relies on the fact that 4′ -6-diamidino-2-phenylindole stains the
DNA of the bacteria with a damaged cell membrane. Since reactive oxygen species will
damage the cell membrane of E. coli during photocatalysis, in order to achieve accurate
results, the sample containing the bacteria was rinsed several times with saline-phosphate
buffer. The mixture of the stain and the treated sample was then excited and quantified via
the fluorescence detector [122]. Kayani et al. [111] described the disc diffusion method as
an alternative method of quantifying E. coli inactivated by photocatalysis. The prepared
sample was placed in a treated plate, and the plate was then exposed to visible light for
less than 1 h and afterward inoculated for 10 h at 37 ◦ C. An agar well diffusion method
was also reported by Bauer et al. [127]. In this method, four small wells (diameter 0.8 mm)
were dug and the stock solution of the photocatalyst was placed into the dug well, followed by the incubation above room temperature for one day while methanol was used
as control [128]. The disc diffusion method is another method that was used to quantify
bacteria cells, and it involves the growth of E. coli strains on Mueller–Hinton agar for 1 day
at 37 ◦ C, while shaking mildly. The zones without the growth of E. coli are described as
the zone of inhibition, which is usually measured in millimeters [79]. Other methods are
the micro-dilution method and colony-counting method [23,52]. Before the use of colony
counting method, the bacteria cells are usually introduced into the solution of sodium
chloride and spread unto nutrient agar, followed by incubation for a day [59]. Limulus tests
have also been used in determining if the bacteria were inactivated. The test relies on the
determination of the endotoxin cumulative content of E. coli in the samples [129].
6. Photocatalytic Systems Designed for E. coli Inactivation
Different photocatalytic set-ups have been used for the inactivation of E. coli, and
one of them is the liquid-film system. In this set up, light is passed from the top of the
set-up through a transparent pyrex window into the suspension of E. coli. The suspension
is placed on the petri-dish holding a glass coated with the photocatalysts such as titanium
oxide. The liquid-film set-up is different from the membrane separated system, where
the light is passed from the base of the set-up through a normal glass support to the
photocatalyst-coated glass, then to the membrane holding the suspension of E. coli. These
two systems are shown in Figure 5a,b.
Figure 5. (a) Liquid-film photocatalytic system and (b) membrane-separated system. Reprinted
from [59], Copyright (1997), with permission from Elsevier.
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One other design allows the reduction and oxidation to take place simultaneously, and
this makes the investigation of the photocatalytic inactivation to be possible. It is usually
employed where a photocatalyst doped with metal is employed for E. coli inactivation.
The metal dopant act as the reduction site, which boosts the photocatalytic efficiency via
better light-generated charge separation [130]. In this design, the detector that is used is the
carbon microelectrode, which is usually positioned close to the surface of the photocatalysts
as shown in Figure 6a,b, where palladium doped with titanium oxide was used as the
photocatalysts. Materials used to design the carbon microelectrode are copper wire, epoxy
resin, mercury, carbon fiber and pyrex glass tube.
Figure 6. (a) The design of carbon microelectrode, (b) set-up for simultaneous monitoring of oxidation
and reduction during E. coli inactivation. Reprinted from [130]. Copyright (2000), with permission
from Elsevier.
Lu et al. [107] reported the use of photocatalyst-loaded reactor (Figure 7) made from
polyurethane foam and quartz. The polyurethane was used as support that carried the
photocatalyst in order to improve its contact area. The photo-reactor is cylindrical in shape,
it has a double layer and is of considerable size (35 cm in length and 7 cm in diameter).
This reactor has fitted sampler and flow meter to measure the movement of air and aerosol
containing E. coli in the system.
Figure 7. Polyurethane foam design for the inactivation of E. coli. Reprinted from [107], Copyright
(2021), with permission from Elsevier.
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A portable, easy-to-use photocatalytic design, based on the use of photoelectric has
also been reported [131]. It was used to inactivate 5-log of E. coli within 10 s. This reactor
has nanotube photoanode made from titanium oxide photocatalyst, which was formed
into the shape of teacup, and it is powered by small-sized battery that is rechargeable. The
battery is the source of energy for the light emitting diode fitted into the reactor. This novel
photocatalytic design is shown in Figure 8.
Figure 8. Photoelectric cup-like photocatalytic design for E. coli inactivation. Reprinted from [131],
Copyright (2020), with permission from Elsevier.
A relatively new photoreactor set-up, which also utilizes light emitting diode as
a source of illumination was designed with several fixed bed stages (Figure 9). The
photocatalyst used for the design was titanium oxide nanoparticles doped with nitrogen,
and the nanoparticles were rendered immobile on the beads made from glass. Each fixed
bed was made from acrylic sheet with dimensions of 100 mm long, 50 mm wide and 15 mm
high. The fixed bed stage was arranged such that there was gravitational flow from one
stage to another stage and there was continuous flow of water in and out of the reactor.
The suspension of E. coli is usually introduced into the first stage through a peristaltic
pump. The incident photon flux in the set-up is usually measured by potassium ferrioxalate
actinometer [132].
Figure 9. Multiple stage photoreactors for E. coli inactivation. Reprinted from [132], Copyright (2020),
with permission from Elsevier.
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Another design allows both E. coli and the photocatalysts to exist together in the buffer
solution in an electrophotocatalytic system, and platinum wire is used as the electrode
(Figure 10). The system was irradiated by an arc Xenon lamp, and the solution containing
E. coli was continuously stirred for a particular period of time before a little sample of the
solution was taken to determine the inactivation rate. This design requires the screening
out of ultraviolet light (UVC and UVB) achieved by using a band filter. The intensity of
light from the arc Xenon lamp was monitored by using a spectroradiometer [4].
Figure 10. Photoelectrochemical and photocatalytic E. coli inactivation set-up [4]. Open access under
creative common agreement (MDPI).
The quest to solve the problem of mass transfer in the photocatalytic reactors led to
the design of simplified stirred-tank reactor [133]. This reactor was designed by putting
a glass plate coated with the photocatalyst beneath a vessel having a water-jacketed wall
that created a reservoir. Turbulent flow was generated within the reactor through the use
of a stainless steel propeller that was rotated between 0–2500 rpm by a homogenator motor.
The calibration of this homogenator motor was done by using an optical tachometer, and
the mixing was enhanced by a baffle made from stainless steel [134]. This design is shown
in Figure 11.
Figure 11. Stirred tank reactor for E. coli inactivation. Reprinted from [133], Copyright (2020), with
permission from Elsevier.
𝑁 ⁄𝑁 = 𝑒 ((
𝑁 ⁄𝑁 = 𝑒
)
)
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7. Kinetics of the Inactivation of E. coli by Photocatalyst
The inactivation of E. coli depends on the intensity of light, the time of exposure to
this light, and the initial population of the E. coli [135]. As expressed by Chick’s law, there
is a relationship between these factors; and the kinetics of E. coli inactivation usually follow
first-order kinetics. The expression of Chick is presented in Equation (1):
Nt /No = e(−klt)
(1)
where l is the light intensity,
t is the exposure time,
No is the initial population of E. coli,
Nt is the number of E. coli that survived after exposure to light,
K is the first-order rate constant.
Watson also worked along with Chick to form a Chick–Watson Model of inactivation
(Equation (2)):
log Nt/No = −kt
(2)
where k is the inactivation rate at time, t.
Apart from the Chick and Watson models, the Hom model (Equation (3)) is another
model used for the kinetics study of the inactivation of micro-organism, and this model was
later modified to another model known as the modified-Hom model (Equation (4)) [132]:
log Nt/No = −kC n T m
(3)
log Nt/No = −K1 (1 − exp − K2 t)K3
(4)
where C is the load of the catalyst, m and n are empirical parameters, K is the inactivation
rate and k1 , k2 , k3 are constants of the modified-Hom Model.
Finally, the Weibull model is another simple model for investigating the kinetics of
E. coli inactivation either in the presence or absence of heat. When UV light is used for the
treatment, the sigmoidal curve for the inactivation will either be convex or concave, upward
or downward when expressed as a function of UV dose or time of inactivation [136,137]:
αβ log Nt/No = −12.303tαβ
(5)
where α and β are the Weibull parameters, and these parameters are used to estimate the
reliable time(tR ), which is an indication of the percentage of the bacteria that has been
inactivated. The equation relating the reliable time with these parameters is presented in
Equation (6) for a system that utilized UV light [137,138]:
αβt R = α2.3031β
(6)
8. Methods of Detecting E. coli
The detection of E. coli is necessary after the photocatalytic inactivation has been
carried out in order to ascertain the treatment of the water from the E. coli contamination.
One of the methods that has been used to detect the presence of E. coli is polymerase chain
reactions (PCR). Other methods are biosensor-based techniques, immunology-based techniques, Fluorescence in Situ Hybridization (FISH), Pyrosequencing and Oligonucleotide
Microarray. The principle of operations, merits and demerits of these methods are summarized in Table 2.
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Table 2. Methods for detecting E. coli, their principle of operation, merits and demerits.
Detection Method
Principle of Operation
Merit
PCR
Specific target is amplified
exponentially based on
extension, annealing and
denaturalization processes.
Multiplex PCR
Amplification of specific
target
Quantitative real time
PCR
Measurement of amplified
product via rise in
fluorescence
It is possible to detect
multiple targets
simultaneously in a single
reaction; fast.
Its signal strength is not
affected by the presence of
E. coli
Post PCR-analysis not
required;
cross-contamination is
reduced; fast, specific and
sensitive
Oligonucleotide
Microarray
Pyrosequencing
Detect mutation and gene
expression at under
changing condition via
sophisticated genomic
technology
Monitoring of
pyrophostate liberated via
bioluminescence and
reaction of enzyme.
Fast and simultaneous
determination of all genes
in a particular isolate is
possible
Numerous sequences can
be read in one run
Demerit
Ref.
Accurate primer
required to get correct
results.
[139–141]
-
[100,142,143]
One pathogen only can
be detected in a single
reaction
[5,135,144]
Relatively expensive;
lower specificity and
sensitivity
[5,27,101]
It requires
concentrating the water
pathogens before DNA
extraction
Treatment and
concentration are
required; low
sensitivity; results may
be altered by inhibitor
increase during
pre-treatment.
[5,124,145,146]
Fluorescence in Situ
Hybridization (FISH)
It involves the use of
fluorescence dye and rRNA
oligonucleotides
hybridized with the
sample.
Identification and detection
can be achieved in a short
time.
Immunology-Based
Methods
It is based on the
interaction of antigen and
antibody which involve the
use of monoclonal and
polyclonal antibodies.
Multiple pathogens can be
detected at once.
Cross-reactivity of
antigens; low
sensitivity,
pre-treatment required.
[5,140,148,149]
Biosensor Based
Methods
Recognition of the target
analyte by using a
bioreceptor and transducer
Possibility of automation;
short time for analysis;
portable and sample
pre-treatment not required
Its sensitivity is affected
by change in pH,
temperature and mass.
[5,140,150–152]
[5,139,140,147]
PCR is polymerase chain reaction.
9. Conclusions and Recommendations
Contamination of water by pathogens poses a serious challenge to the health and wellbeing of both humans and animals. Hence, their presence in water must be examined often
and the total elimination should be ensured in order to minimize different water-borne
diseases. The total elimination of E. coli from drinking water could be achieved through
the photocatalytic inactivation. New methods of testing and quantifying the E. coli load
in water continues to emerge and studies continue to explore different ways of designing
simple and cheap photocatalytic reactor. A detection method that could combine all the
merits of the present E. coli detectors (fast, sensitive, does not require pretreatment, portable
and can handle multiple targets in a single run) need to be investigated. In addition, a
lot of effort needs to be channeled towards optimizing photocatalytic processes so that
it will be more effective and practicable in the large scale because most of the reported
works are still at the laboratory scale. Furthermore, extended investigations are needed
in order to explore avenues for the immobilization of photocatalysts in order to prevent
leaching into the water. The leaching of the nanoparticles into water could be prevented by
introducing catalyst support. This will also prevent the aggregation and dispersal of the
photocatalyst into the water, which may lead to secondary pollution. Finally, the use of
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type II and Z-scheme heterojunction system for the inactivation of E. coli has been reported
by several studies, but little attention was paid to the use of S-scheme, p-n heterojunction
and Schottky heterojunction systems.
Author Contributions: All the authors contributed equally to the writing of this review article and
therefore consent to the publication. All authors have read and agreed to the published version of
the manuscript.
Funding: This research was funded by the National Research Foundation (NRF), South Africa, Grant
Numbers (UID109333 and UID 116,338). The APC was funded by North-West University, South Africa.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: The authors gratefully acknowledge the North-West University and the National
Research Foundation, South Africa for providing financial assistance.
Conflicts of Interest: The authors declare no conflict of interest.
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