Environmental Technology & Innovation 24 (2021) 102032

Contents lists available at ScienceDirect

Environmental Technology & Innovation

journal homepage: www.elsevier.com/locate/eti

A review of nanotechnological applications to detect and

control surface water pollution
∗
Nur Hanis Hayati Hairom a,b , Chin Fhong Soon a , ,
Radin Maya Saphira Radin Mohamed c , Marlia Morsin a , Nurfarina Zainal d ,
Nafarizal Nayan a , Che Zalina Zulkifli e , Nor Hazlyna Harun f
a
Microelectronics and Nanotechnology–Shamsuddin Research Centre (MiNT-SRC), Institute for Integrated Engineering, Universiti
Tun Hussein Onn Malaysia, 86400, Parit Raja, Batu Pahat, Johor, Malaysia
b
Faculty of Engineering Technology, Universiti Tun Hussein Onn Malaysia, Hab Pendidikan Tinggi Pagoh, KM 1, Jalan
Panchor, 84600, Muar, Johor, Malaysia
c
Micropollutant Research Centre (MPRC), Faculty of Civil Engineering & Built Environment, Universiti Tun Hussein Onn
Malaysia, Parit Raja, Batu Pahat, 86400, Malaysia
d
Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia, 86400, Parit Raja, Batu
Pahat, Johor, Malaysia
e
Department of Computing, Faculty of Arts, Computing and Creative Industry, Universiti Pendidikan Sultan Idris, Tanjung
Malim 35900, Perak, Malaysia
f
Data Science Research Lab, School Of Computing, College Arts & Sciences, Universiti Utara Malaysia, Sintok, 06010 Bukit Kayu
Hitam, Kedah, Malaysia

article info a b s t r a c t

Article history: Surface water is extremely susceptible to pollution stemming from human activities,
Received 11 July 2021 such as the expansion of urban and suburban areas, industries, cities, and agriculture.
Received in revised form 14 October 2021 In fact, sources of surface water have become the most common discharge sites for
Accepted 15 October 2021
wastewater, which may contain microorganisms, pharmaceutical waste, heavy metals,
Available online 19 October 2021
and harmful pollutants. As a reference standard for clean water, the water quality stan-
Keywords: dards and index of Malaysia were used. In comparison with conventional wastewater
Surface water treatment methods, new nanomaterial-based methods for water filtration and purifica-
Nanotechnology tion are drawing attention as more efficient methods for water pollution detection and
Pollution detection treatment. This prompts the use of nanotechnology applications to control surface water
Wastewater treatment
pollution and quality, as surface water is the main source of water consumption for hu-
Green environment
mans, animals, and plants. This paper reviewed the application of nanotechnology for the
detection and treatment of surface water pollution to ensure the sustainability of a green
environment. This paper also highlighted the application of nanotechnology, namely,
nanofiltration membranes, photocatalysis, plasma discharge, and nano-adsorbents, in
wastewater treatment, as well as the application of nano-sensors for monitoring surface
water quality. The integration of nano-adsorbents in the conventional technology may
increase treatment efficiency because nano-adsorbents have demonstrated remarkable
performance in the removal of contaminants in wastewater. The hurdles, challenges, and
outlook of nanotechnology for wastewater treatment were addressed in this review.
The insights presented in this paper may provide opportunities and directions to
expand studies pertaining to the applications of nanotechnology for future surface water
treatment.
© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC
BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Contents

1. Introduction............................................................................................................................................................................................... 2

∗ Corresponding author.
E-mail address: soon@uthm.edu.my (C.F. Soon).

https://doi.org/10.1016/j.eti.2021.102032
2352-1864/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.
org/licenses/by-nc-nd/4.0/).
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

2. Review methodology................................................................................................................................................................................ 3
3. Surface water quality requirements for green environment .............................................................................................................. 3
3.1. Water quality standards.............................................................................................................................................................. 5
3.1.1. Industry effluents and wastewater ............................................................................................................................ 6
3.2. Conventional methods of water treatment .............................................................................................................................. 7
3.2.1. Coagulation and flocculation in water treatment .................................................................................................... 7
3.2.2. Removal of ammoniacal nitrogen (NH + 4 -N) for water treatment using biological processes .......................... 8
4. Prospective nanotechnology in surface water treatment .................................................................................................................... 10
4.1. Nanofiltration membrane in surface water treatment............................................................................................................ 10
4.2. Photocatalysis process in surface water treatment ................................................................................................................. 11
4.3. Plasma discharge in surface water treatment.......................................................................................................................... 13
4.4. Nano-adsorbents for wastewater remediation......................................................................................................................... 16
5. Nano-sensors for surface water quality monitoring ........................................................................................................................... 17
6. Challenges and future outlook................................................................................................................................................................ 18
7. Conclusions................................................................................................................................................................................................ 19
CRediT authorship contribution statement ........................................................................................................................................... 20
Declaration of competing interest.......................................................................................................................................................... 20
Acknowledgements .................................................................................................................................................................................. 20
References ................................................................................................................................................................................................. 20

1. Introduction

Surface water, which includes lakes, dams, canals, rivers, and streams, is a vital resource providing three-fourths
of water to agriculture and industries and one-third of drinking water to domestic households (Sasakova et al., 2018;
Jonnalagadda and Mhere, 2001). Despite its utmost importance in supporting life, surface water is extremely susceptible
to pollution stemming from uncontrolled activities of agriculture and industrialisation. According to the United Na-
tions World Water Development Report 2015 (WWAP, 2015), 90% of sewage in developing countries is dumped untreated
directly into water bodies. Additionally, industries disposed approximately 300 to 400 megatons of contaminated waste
in water bodies yearly (Boretti and Rosa, 2019). Nitrate used by agriculture is the most common chemical pollutant deter-
mined in groundwater aquifers (Javier et al., 2018). Industrial and domestic wastewater discharges have led to increased
freshwater pollution and clean water resource depletion (Andersson et al., 2016). Owing to its easy accessibility, surface
water sources, such as rivers, are the most common discharge sites for wastewater, which may contain microorganisms,
heavy metals, and hazardous chemicals (Chowdhary et al., 2018; Kulkarni, 2020).
Once micropollutants enter into surface water, it is difficult and costly to remove these contaminants via conventional
water remediation systems, and hence, the micropollutants in micro- or nano-gram per litres continue to accrue in the
surface water. Due to the unsupervised release of industrial effluents into surface water, heavy metals have been detected
in fish from rivers, which hold potential risks to human health (Jia et al., 2017; Maurya et al., 2019). Underlying water
pollution problems and poor water management might progress to even more serious issues, such as massive disease
outbreaks (Singh et al., 2013; Nava et al., 2017), due to the uncontrollable proliferation of bacteria, viruses, and fungi
in water sources. Unfortunately, conventional water remediation methods have less effectiveness in removing a broad
spectrum of micropollutants in surface water.
Recently, the application of nanotechnology in water treatment has gained wide attention and is being actively
investigated due to its remarkable properties (Di Natale et al., 2020; Elboughdiri, 2020; Janani et al., 2021; Sajjadi et al.,
2021). In the field of water treatment, nanotechnology can be classified into three main applications: (1) to restore
(remediate) and purify contaminated water, (2) to detect pollution, and (3) to prevent pollution (Yunus et al., 2012).
This has led to the demand for nano-sensors with high sensitivity for the detection of micro-, nano-, and molecular
pollutants. To the best of our knowledge, previous review articles focused much on the fundamental studies of methods
using nanotechnology in addressing the removal or monitoring of water pollutants in wastewater. There is a gap of a
missing review associating laboratory nanotechnological solutions with water treatment systems’ implementation. Hence,
in this review, we revealed the requirement of clean water standards, the up-to-date nanotechnology for water pollution
remediation and detection, and the feasibility of nanotechnology in water treatment systems.
First, this review paper provided water quality standards as a guide for water treatment research. The guide to
the chemical contents of industrial wastewater was also included in this review. Conventional methods used for
water treatment include coagulation/flocculation, aeration screening, filtration, straining, sedimentation, disinfection, and
fluoridation (Ministry of Health, 2014). Different issues arising from conventional methods were emphasised, such as
secondary waste production, biological methods’ requirements, and environmental parameters’ influences. Synthesising
membrane for nanofiltration is an area with the highest research interest. The characteristics of nano-filter membranes,
the nanofiltration process, and the limitations of nano-filter membranes for large-scale water filtration were discussed.
This review also examined the photocatalysis method, which is an advanced oxidation process in treating xenobiotic
organic compounds and emerging pollutants. The low pollutant-removal rate of the photocatalysis method as a trade-off to
its effectiveness in degrading organic pollutants was discussed (Molla et al., 2020). The design parameters influencing the
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N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

efficiency of photocatalysis activities were disclosed in this review. In addition, the plasma discharge system, which applies
high energy in discharging reactive species to remove toxic organic compounds in raw water, was studied (Van Nguyen
et al., 2019). This technique offers simplicity and high efficiency in removing toxic organic compounds and microbes from
raw water. This review also addressed the use of nano-adsorbents, which is a more recent research technique applied for
water remediation. The remarkable efficiency of nano-adsorbents in removing pollutants is attributed to several properties
of nanomaterials, such as large surface area, reactivity, high adsorption capability, and simplicity for application (Janani
et al., 2021). Plausibly, nano-adsorbents exhibit high effectiveness in removing organic compounds from wastewater,
such as pigments (Andrade et al., 2019). However, some practical issues need to be addressed before nano-adsorbents
experimented at the lab scale can be applied for large-scale water treatment. This is because wastewater discarded into
surface water is far more complex and there is also a debate on whether nanomaterials applied for wastewater treatment
are recoverable and safe to the environment. These issues were extensively discussed in this review.
Different types of nano-sensors have been applied for detecting pesticides, surfactants, dyes, phenolic compounds,
and new emerging pharmaceutical contaminants in water (Dasgupta et al., 2017; Sai et al., 2019; Steffens et al.,
2017; Vikesland, 2018; Wei et al., 2015; Willner and Vikesland, 2018). In nano-sensor applications, nanomaterials offer
various notable properties, such as high surface area, fluorophore radiation that enables visual detection, antimicrobial
activities, tunable pore size, and reactive surface chemistry. The specificity, sensitivity, and detection limit of nano-
sensors in complex wastewater are intensive research areas. The opportunity to design nano-sensors for sensing emerging
contaminants is of high priority and demand in addressing recent water pollution problems (Gomes et al., 2020). Towards
the end of this review paper, insights into future research priorities and challenges were suggested.
The aim of this review paper was to provide insights for readers on existing standards in determining surface water
quality in a green environment, the types of contaminants in surface water and sources, the overview on conventional
and nanotechnology wastewater treatments, water pollution detection, and the future prospect of wastewater treatment
technology. The current review paper aimed to guide readers in identifying upcoming research gaps for the development
of a more efficient, sustainable, and feasible technology for wastewater treatment. The objectives of this review were
to (1) elucidate surface water quality requirements, (2) review conventional water treatment methods, and (3) present
the application and the research gap of nanotechnology for surface water pollution treatment and detection. The
knowledge gap between the nanotechnology for water treatment research in the laboratory and the upscaling challenges
of nanotechnology for implementation in large-scale water treatment was highlighted.

2. Review methodology

The literature review of this paper was conducted via ScienceDirect using keywords such as ‘‘membrane’’, ‘‘photo-
catalysis’’, ‘‘plasma’’, ‘‘nano-adsorbent’’, and ‘‘wastewater treatment’’ to search for articles published from January 2015
to January 2022. The research result for the number of published articles is as summarised in Fig. 1. This exercise provided
an indication of the most recent trend of research and new technology to justify the proposition of this review article.
The literature review result was massive but after reviewing and shortlisting suitable articles, the overall section of the
review was organised and analysed with appropriate examples.
Fig. 1 indicates that the number of published articles associated with water treatment technology increased propor-
tionally over the past six years. In fact, the number of publications on wastewater treatment nanotechnology are so
overwhelming that upcoming new articles are already available for year 2022. The number of published articles related
to membrane and wastewater treatment is enormous and continued to increase. A similar rising trend in the number of
publications was observed for plasma and photocatalysis wastewater treatment technology. The number of publications on
plasma and photocatalysis wastewater treatment technology is approximately 30% fewer than the number of publications
on membrane wastewater treatment technology. Amid the search for new technology in solving global wastewater
pollution problems, nano-adsorbent technology in wastewater treatment is indicated with increasing scientific interest.
The research in this area is still at the fundamental research level and hence offers many new research opportunities.

3. Surface water quality requirements for green environment

In general, there are two types of water sources, which are surface water and groundwater, as illustrated in Fig. 2
(Encyclopaedia Britannica, 2020). Groundwater refers to the bulk of water below the water table (the underground
boundary that divides saturated and unsaturated ground levels). This type of water is confined by subsurface layers of
rocks and is contained in and filtered by the soil, resulting in less contaminated water. One of its main uses is as a source
of mineral water for consumers (Olivier and Xu, 2019).
As the name implies, surface water is water found in bodies such as rivers, oceans, lakes, ponds, streams, and other
surface cavities (EAP Task Force, 2008). The source of surface water is mostly rainfall and the mixture of groundwater
and water run-off. In detail, there are three types of surface water: perennial (Davis et al., 2017), ephemeral (Flinchum
et al., 2020), and man-made (Donchyts et al., 2016). The permanent type of surface water that continues year-round is
called perennial. Ephemeral or semi-permanent surface water types include small water sources, such as water holes,
small creeks, and lagoons. These types of surface water exist only during certain periods throughout the year. Artificial
structures, such as canals, wells, reservoirs, dams, and constructed wetlands, are types of man-made surface water. Surface
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N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

Fig. 1. Number of published research articles on wastewater treatment using nanotechnology.

Source: ScienceDirect.com, 2015 to 2022.

Fig. 2. Illustration of surface water and groundwater.

Source: Readapted from Encyclopaedia Britannica
(2020).

water is mainly used as drinking water and for crop irrigation. The industrial sector is the main consumer of surface water,
for example, in the thermoelectric-power industry for cooling electricity-generating equipment (Zheng et al., 2020). More
importantly, surface water is needed not only by humans but also by other living beings, such as animals and plants.
Due to surface water’s geographical position, the exposure of pollution into surface water is very high. Hence, an urgent
need to control the quality of surface water is required to improve the health of the environment and sustain ecological
stability. The quality control of surface water requires two main steps: (1) pollution prevention at the water source and (2)
precautionary regulations. For the first step, the awareness to sustain a green environment is important to produce better-
quality environment and ecosystem, for instance, clean air, preserved natural resources, and a nontoxic environment.
Many actions need to be taken to protect the health of the environment either from human activities or industrialisation.
Industries should invest and implement green technologies by utilising green and renewable energy resources, such as
solar, wind, hydro, and biomass. The harvest of these kinds of resources will conserve energy and reduce dependency
on finite natural sources, such as oil, natural gases, and coal, as well as curbing climate change (Owusu and Asumadu-
Sarkodie, 2016; Figueres et al., 2017). As earth’s citizens, people need to adopt a green life with healthy practices, for
instance, planting trees, conserving water, eliminating food wastes, and many other ways to be environmentally friendly.
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These changes can help reduce pollution, which threatens the environment, as well as helping to protect natural resources
and reducing the triggers towards climate change. For the second step, authorities set regulations, such as licencing of
wastewater discharges. Compliance with the rules becomes a key element for successful policies to prevent, control, and
reduce toxic and hazardous water pollutants from ordinary point sources into aquatic ecosystems, as well as pollutants
from non-point sources, such as agricultural (Sasakova et al., 2018) and industrial sectors (Sankhla et al., 2019).
Therefore, many countries have comprehensive water laws and regulations to control water quality by setting limits
on hazardous substances, toxic compounds, and other variables that can affect the odour and colour of water. Regulations
have been revised from time to time to fulfil the need and the growth of the population and industries. For instance,
the US government via the United States Environmental Protection Agency (EPA) has established the Surface Water
Treatment Rules (SWTRs) (Ahmad, 2017) as the basis for water laws, while the Eastern Europe, Caucasus, and Central Asia
(EECCA) countries have reformed their surface water quality regulations (EAP Task Force, 2008) to protect water resources
from degradation, to maintain and enhance water quality, and to ensure the sustainability of water use. Underdeveloped
countries, such as Nigeria, also have their own water laws and issued specific decrees to protect, restore, and preserve
the ecosystem. Nigeria issued a directive to protect, rehabilitate, and conserve the ecological system of the Nigerian
environment via the Federal Environmental Protection Agency (FEPA) (Eneh, 2011).

3.1. Water quality standards

Surface water pollution is often associated with diffuse sources that leach into surface water and groundwater by
rain, flushes, or leakages. Diffuse sources are mainly from industrial effluents, urbanisation, religious and social practices,
agricultural runoff (chemical fertiliser and pesticides), and accidents (oil leakage and/or spills, burning of fossil fuels,
nuclear fallout, etc.). Apart from human activities, natural causes can also contribute to water pollution, such as algal
blooms, animal waste, volcano eruptions, and silt from storms and floods. These pollutants affect surface water bodies.
Water pollution can cause the destruction of biodiversity, including the natural ecosystem of aquatic life, human lives, and
the environment. For example, the increase of algal blooms in lakes and the dumping of effluents, such as oil, into lakes,
rivers, or oceans can cause the food chain to be contaminated not only for aquatic or marine life but also for humans and
living organisms. Normally, healthy water bodies and a healthy environment can be identified, wherein the surroundings
have numerous habitats including flora and fauna.
The quality of water bodies can be identified using three main parameters, which cover physical, chemical, and
biological characteristics. In the case of physical characteristics, changes in polluted water bodies can be observed
from the condition of the water bodies themselves. Changes in the physical characteristics of water bodies are visible
through features such as turbidity, colour, temperature, taste, and odour. For example, water with turbidity will show
brown/cloudiness or other colours instead of its true transparent colour. High turbidity in water will cause decreased pH
value and amount of oxygen, where the photosynthetic reaction is prevented from occurring due to the limited penetration
of light through the water. Turbidity also results in acidic water, which is harmful to the aquatic and plant (flora and
fauna) ecosystem. Environmentalists use a turbidity meter, such as Jackson turbidimeter or photometer, to measure the
percentage of light to examine the quality of water bodies (Peavy et al., 1985; Weiner and Matthews, 2003).
The apparent colour of water bodies is partly due to suspended matters or compounds; originally, pure water is
colourless. Once water bodies are exposed to or encounter organic wastes, such as leaves, the water’s colour will change.
The water will collect the tannin in the leaves, thus becoming yellowish-brown in hue. Industrial effluents will add more
colouration to the water bodies, for example, reddish, brownish, or blackish colour, due to the chemical elements in the
wastewater. The mixing of organic compounds and industrial effluents with chlorine has been suspected to be cancer-
causing agents (carcinogens). Environmentalists use chromatography and quantitative tests to measure and analyse the
taste and odour of water. The features of turbidity and colour of water bodies have an impact on the taste and odour of
the water itself. For example, polluted water bodies that contained sulphur, algae, and oily substances may contribute
to smelly odour, such as that of a rotten egg, and bitter taste (Peavy et al., 1985; Weiner and Matthews, 2003). The
temperature of water bodies is usually affected by the surrounding atmosphere, such as weather, and by the seasons,
namely, winter, spring, summer, and fall. The temperature of polluted water bodies becomes a critical issue when chemical
substances from industry effluents react with natural water and vaporised, which causes harm to humans and aquatic
life.
The reaction between water bodies and chemical parameters, such as total dissolved solids, organics, and metals, causes
deficiency in water quality, which results in the decrease of oxygen and the increase of nitrogen in water bodies. Total
dissolved solids refer to the suspended remaining materials after the filtering process, and the chemical solvents may
be in solid, liquid, or gas form. Most dissolved substances are undesired in water due to toxicity and carcinogenicity
issues. This chemical reaction will cause low conductivity of water bodies, as well as increased water temperature, and
hence will greatly impact aquatic plants and animals (flora and fauna). The Wheatstone bridge principle has been used to
measure and analyse water bodies’ conductivity. Industrial effluents are responsible for the issue of decreased oxygen
in water, hence disturbing the ecological system, for example, the actions of microorganisms on organic substances.
Microorganisms in water bodies naturally dissolve organic components for decomposition via the oxidation or reduction
process. The dominant undertaking is certainly the oxidation process, which occurs when oxygen is available in the
water, and hence microbial organic decomposition produces stable and acceptable compounds. However, in the absence of
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oxygen, industrial effluents cause the decomposition process to produce unstable and undesirable compounds. Thus, this
affects nature’s ecological system. Moreover, excessive amounts of metals in water are commonly identified as hazardous,
and some metals are toxic. Either one of these conditions is harmful to the ecosystem. For example, uncontrolled amounts
of metals in water bodies can cause cardiac and kidney problems to human health, and one indicator of such water is the
bitter taste of water. Measurements, such as atomic absorption spectrophotometry, have been used to examine metals in
water bodies (Peavy et al., 1985; Weiner and Matthews, 2003).
Pertaining to the biological aspect of water bodies, a major problem is the existence of pathogens in water. Waterborne
pathogens, such as viruses, bacteria, parasitic worms, and protozoa, can live in polluted lakes, rivers, and oceans. These
pathogens come from transported water, for example, from industrial effluents and nature, and they usually require a
host for growth and reproduction. Waterborne pathogens can maintain their infection capabilities for a long time (Peavy
et al., 1985; Weiner and Matthews, 2003). The existence of these pathogens can pose risks to the health of the aquatic
ecosystem and to human health. These biological factors can infect humans with diseases from the water, for example,
cholera, which can cause death. All these effects will also cause the destruction of nature’s biodiversity, thus affecting the
mortality of humans and any living organisms.
Human activities in satisfying the needs of modern industrial society have significant impacts on the ecological system,
producing air and water pollution. To sustain a green environment, reducing or eliminating water pollution is a must. Thus,
great efforts have been undertaken by countries around the world to sustain the environment by having national policies
or standards and practices to control and improve water security and ecosystem sustainability.
National policies and regional standards are important in helping to identify pollution control and evaluation of water
quality. Each country in the world has its regional water quality standards in determining the Water Quality Index
(WQI). Generally, water standards and regulations are led by the Department of Environment (DOE) of the government.
Assessments on WQI values can determine the health of a water body before the water can be consumed as drinking water
and used for other purposes, such as irrigation. There are six main parameters used to assess the WQI, namely, ammoniacal
nitrogen, chemical oxygen demand (COD), pH (alkaline or acid value), total suspended solids (TSS), dissolved oxygen (DO),
and biochemical oxygen demand (BOD). These parameters are considered to assess the WQI associated with the chemical,
physical, and biological conditions of the water. Each of these parameters has a significant impact on each other, thus
playing a crucial role in determining the quality of water. The values of DO, COD, BOD, and ammoniacal nitrogen (values
of nitrogen and phosphorus) reflect the chemical concentrations in the water, whereas pH values, turbidity, temperature,
and total suspended solids (TSS) refer to physical changes in polluted water.
Faecal coliform microorganisms, such as Escherichia coli (E. coli), Giardia, Campylobacter, Salmonella, and Cryptosporidium
(Maharjan et al., 2020), can contribute to changes in the biological condition of water bodies. The amount and occurrence
of chemical, physical, and biological forms will influence water quality. Thus, the WQI is an important factor and acts
as an indicator to identify water quality. The standard formula to calculate the WQI is shown in Eq. (1) (Department of
Environment, 2016).

WQI = (0.22 ∗ SIDO) + (0.19 ∗ SIBOD) + (0.16 ∗ SICOD) + (0.15 ∗ SIAN) + (0.16 ∗ SISS) + (0.12 ∗ SIpH) (1)

where SIDO is Subindex DO (% saturation), SIBOD is Subindex BOD, SIAN is Subindex Ammoniacal Nitrogen, SISS is
Subindex Suspended Solids, and SIpH is Subindex pH. The range of the WQI is between zero and 100 (0 ≤ WQI ≤
100) (Department of Environment, 2016). The result obtained from WQI measurements hence leads to water quality
classification. In Malaysia, the WQI is divided into five classes (I, II, III, IV, and V) and the values identify the quality
of the water body, either clean, slightly polluted, and polluted (Department of Environment, 2016). All the parameters
mentioned above are measured in milligram per litre (mg/l), except for pH values. All the physical, chemical, and biological
parameters contribute to the amount of dissolved oxygen in the water.

3.1.1. Industry effluents and wastewater

Every day, communities produce waste discharges either in solid, liquid, or air form, depending on daily needs.
Wastewater is defined as liquid that carries out wastes, which can be from households, industries, groundwater (under-
ground water), surface water, and stormwater (rain or ice/snowmelt). Industry effluents refer to liquid wastes or sewage
releases from various types of industrial activities. There are positive and negative impacts of untreated wastewater on
the ecological system and the environment. The positive aspect is that untreated wastewater may contain nutrients that
are useful for the growth of flora and fauna. However, on the negative side, untreated wastewater may contain toxic
compounds and is potentially carcinogenic. This also applies to untreated industrial effluents. To protect ecological habitats
and for health concerns, the management of wastewater and industry effluents becomes necessary in every country in
the world. The standards of each country’s industrial effluents and wastewater may vary depending on the country.
In the case of effluents and wastewater from industries in Malaysia, each industry must follow the rules and meet
the standards that have been established by the Malaysian government and enforced by the DOE. Prior to disposing
industrial effluents or mixed effluents (wastewater and industrial effluents) into the soil and into inland water sources
(rivers or reservoirs), each industry must obey and meet the DOE’s standard specifications, which can be referred to as
the Environmental Quality Regulation Act 2009 PU (A) 434/2009 (Environmental Quality Act 1974, 2009). The reports
show acceptable chemical concentration values, which are measured in mg/l, of the wastewater that are allowed to be
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N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

Fig. 3. Sizes of particles in water.

discharged either near or into the upstream (catchment) or downstream (any inland) of water areas. The discharging into
upstream water follows Standard A, whereas the discharging into downstream water follows Standard B.
It is also highlighted that, if using Standard A, the total concentration of disposed metals must not be greater than 0.5
mg/l. This is because a higher concentration of metals could affect the public supply of water for human consumption,
such as potable water. This standard is applied in all states in Malaysia, which are Johor, Pahang, Kelantan, Perlis, Perak,
Penang, Selangor, Sarawak, Labuan, Sabah, Terengganu, Negeri Sembilan, and Melaka. In addition, for Standard B, the
metal concentration must not be greater than 3.0 mg/l, and it must be a total of 1.0 mg/l if in soluble form. Standard B is
also applied to phenol and free chlorine, which may exist in industrial effluents. In this case, the concentration of phenol
must not be greater than 0.2 mg/l, while for free chlorine, the concentration must not be greater than 1.0 mg/l.

3.2. Conventional methods of water treatment

Various physiochemical conventional methods are used for water treatment to remove pollutants. These methods
can be stand-alone processes or in combination with other processes to treat polluted water depending on the class
of water quality. Among conventional methods used for water treatment are coagulation/flocculation, aeration screening,
filtration, straining, sedimentation, disinfection, and fluoridation (Ministry of Health, 2014). Special treatment processes
include ion exchange, pre-disinfection, electrodialysis, activated carbon adsorption, pre-sedimentation, reverse osmosis,
and chlorination/fluoridation (Ministry of Health, 2014). Routine tests required for process control in treatment plants
are the jar test and measurements of turbidity, residual chlorine, colour, pH, residual chlorine, residual aluminium, and
fluoride. Table 1 shows the system involved in conventional methods for surface water treatment. Two examples of
conventional methods are coagulation/flocculation and the biological process. In these two methods, the pollutants are
organic or inorganic materials, metals, and ammoniacal nitrogen in water.

3.2.1. Coagulation and flocculation in water treatment

In the conventional method of coagulation/flocculation, raw water will normally be screened before the coagulation
and flocculation processes of water treatment are applied. Following the screening process, a chemical coagulant, such
as metallic salt (aluminium sulphate or ferric chloride), will be added to the process. The function of the metallic salt
in the coagulant process is to separate the compounds or particles in water by rapidly mixing with the metallic salt and
water. Compounds or particles in water have various features of shape, size, charge, and density. For example, viruses and
bacteria have different sizes, which are 0.01–0.1 µm and 0.1–10 µm, respectively, as shown in Fig. 3. Normally, chemical
coagulants, such as colour-producing organic substances and clay, are added to the water to neutralise negative charges
and non-settable solids. After the charges of the particles are neutralised, small colloidal particles with sizes ranging from
0.001–10 µm (suspended particles) are formed. This rapid-mix coagulation process takes a short time of approximately
10–180 s (Yousaf et al., 2019). At this stage, the sedimentation process is obviously not efficient enough to remove the
small colloids. Therefore, flocculation will occur prior to the sedimentation process. This produces microflocs with slightly
larger sizes in the range of 1–100 µm as visible suspended particles. The agglomeration of the microflocs relies on the
turbulence of the propeller in the flocculation process to promote collisions between the microflocs. The completion time
for flocculation takes longer than that of coagulation due to the slow mixing process. It takes about 15–20 min or more
to complete the process, after which larger visible flocs are produced, called pin flocs.
7
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

Table 1
Conventional methods for surface water treatment.
Method Influential parameters Advantages Disadvantages Reference
Coagulation/flocculation • Concentrations and • Effective in removing • Use of conventional Chekli et al. (2015),
properties of coagulants non-soluble compounds coagulants, such as Al- Hussain et al. (2019),
and flocculants and microorganisms and Fe-based salts (e.g., Jassim et al. (2020)
• pH (protozoa, bacteria, and alum, ferric chloride,
• Temperature viruses) polyaluminium chloride,
• A cost-effective and polyferric sulphate), at a
energy-saving treatment high dose will produce
and is easy to operate substantial amount of
sludge, requiring further
treatment
• Unable to effectively
remove natural organic
matters (NOMs) and
disinfection by-products
(DBPs)
• Seasonal changes in
surface water turbidity
complicate the selection
of appropriate coagulants
for treatment

Biological sand filtration • Temperature • A low-cost, simple, • Slow filtration rate Jenkins et al. (2012),
• Filtration rate and effective method • Sensitive to algae in Maurya et al. (2020)
• Particle size of • Uses biological process incoming water
medium to treat water without • Less effective in cold
• Bed depth the need for chemicals climate, as biological
or electricity activity declines
• Easy to construct and • Susceptible to clogging,
requires minimum which requires cleaning
maintenance • Sensitive to extreme
• Long lifespan (>10 turbidity for chemically
years) soft raw water
• Highly effective in
removing
microorganisms,
turbidity, and heavy
metals from raw water
• High reliability and
easy installation in
urban and rural areas

(continued on next page)

3.2.2. Removal of ammoniacal nitrogen (NH + 4 -N) for water treatment using biological processes
The biological treatment of water for the removal of pollutants can be done either in aerobic or anaerobic conditions.
Bacteria, fungi, and algae are usually present in water. They use pollutants as sources of nutrients for growth, producing
harmful substances. The overgrowth of algal blooms, for example, causes increased ammonium, leading to water toxicity.
Ammoniacal nitrogen refers to the amount of ammonium in water, such as industrial wastewater (Purwono et al.,
2017) and water from climate change (Mohamed, 2018). Excessive levels of nitrogen concentration will deteriorate
the quality of water, hence affecting the health of humans and living organisms, as well as affecting local economies
(Mohamed, 2018). For example, nitrogen compounds are nutrients that stimulate the activity of algal blooms. Ammoniacal
nitrogen is toxic to aquatic life, and it will cause the reduction in the amount of oxygen in water (Purwono et al., 2017;
Mohamed, 2018). The routine conventional method in the removal of nitrogen in water is usually carried out using
two bioprocesses, namely, autotrophic nitrification and heterotrophic denitrification. In the nitrification–denitrification
process, ammonium is oxidised into nitrite or nitrate under aerobic conditions and then further reduced into nitrogen
gas or molecular nitrogen under anaerobic conditions (Wang et al., 2021; Thakur and Medhi, 2019). In the case of
treating ammonium-rich and nitrogen-rich wastewater, this technology comes to its limits when treating wastewater
with a low C/N ratio (Wang et al., 2021, 2019). Thus, alternative bioprocesses are used for nitrogen removal, such as
anaerobic ammonium oxidation (anammox) and partial nitrification (PN), which have high efficiency and low consumption
of autotrophic biological nitrogen (Wang et al., 2019). Another process is used for treating wastewater that contains
high ammonium-nitrogen contents or a low C/N ratio (less than 7.0), namely, the Completely Autotrophic Nitrogen
Removal Over Nitrite (CANON) start-up process, which is more economical and environment-friendly in comparison with
conventional nitrification–denitrification processes (Wang et al., 2021).
However, conventional methods are not totally efficient and have several limitations to meet the criterion of environ-
mental discharges. For example, in the gravity flow system, the efficiency of the small parallel channel (furrow irrigation)
8
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

Table 1 (continued).
Method Influential parameters Advantages Disadvantages Reference
Disinfection • Water pH • Able to eliminate or • Not suitable to treat Palansooriya et al.
• Water temperature inactivate numerous raw water with high (2020), Sharma and
• Concentration of pathogenic levels of suspended Bhattacharya (2017)
disinfectant agents microorganisms solids, turbidity, colour,
• Contact time between • It does not affect or soluble organic
bacteria and agent minerals in water matters
• Able to degrade • Operated using
several organic electricity
contaminants • Chemical agents for
• A simple method and disinfection can
a less fastidious and decompose
technically workable spontaneously, which
system may reduce disinfection
• Availability of cheap ability
chemicals • Possible water
discolouration from
potassium permanganate
(KMnO4 ) or iodine, or
taste and odour issues
from excessive chlorine
• Biocidal efficacy of
disinfectants is
complicated as
numerous variables need
to be controlled
• Generation of
disinfection by-products
(e.g., haloacetic acids,
trihalomethanes,
bromate and chlorite)
from reaction of
disinfectants with
natural organic matters,
bromine, and iodine,
which are harmful to
human health

Dissolved air flotation • Bubble size and • Able to operate at high • Not suitable for raw Edzwald (2006)
density surface loadings water with high-density
• Bubble suspension • Require only small and solids or turbidities
concentrations (mass, shallow plants (>100 NTU)
number, and volume) • High solid • Energy-consuming, as
• Separation distance concentration of sludge it uses recycle-water
and rise velocities produced without pumping and air
needing thickening compressing
before dewatering • Needs to be protected
• Efficient in removing from freezing and rain
low density particles and to avid floated solids
flocs from settling
• Suitable to treat raw
water with low to
moderate turbidity and
water that contains
algae and natural colour
• Short start-up time
• Not sensitive to
temperature

(continued on next page)

is generally less than 65%. The discharge of wastewater can easily be exposed to the surroundings, thus risking human
health. Moreover, furrow irrigation cannot be applied on steep lands (Liew et al., 2014). Coagulation and flocculation may
produce secondary wastes from the agglomeration of microflocs into visible colloids, or pin flocs; hence, this potentially
can produce other hazardous substances. In terms of biological and ecological methods, a longer time is needed to treat
polluted water. For example, the microbial method needs time for the microbes to grow. Environmental factors, such
as temperature and changes of season, might affect the biological methods, which need extended time. One option to
minimise this problem is by integrating nanotechnology into conventional methods for surface water treatment and
9
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

Table 1 (continued).
Method Influential parameters Advantages Disadvantages Reference
Activated carbon • Adsorbent pore size • Activated charcoal is • Frequent filter changes Jjagwe et al. (2021), Liu
adsorption • Contaminant effective in removing • Generation of carbon et al. (2020), Nageeb
concentration carbon-based impurities, fines (2013), Sharma and
• Solution pH chlorine, odour, and • A large number of Bhattacharya (2017)
• Contact time colour variables related to
• Adsorbent dosage • Low-cost adsorbents adsorption are involved
• Solution temperature are available (e.g., rice
• Agitation speed husk, sewage sludge,
cedar sawdust)
• Cost-effective
• Long lifespan
• High adsorption
performance and
usability

Ion exchange • pH • Simple with low • Resins are not able to Esmaeili and Foroutan
• Temperature operating cost remove organic (2015), Ince and
• Exposure time • Suitable to separate compounds or biological Kaplan Ince (2020),
• Ion charges contaminants (cations contaminants Sharma and
and anions) from dilute • Resins need to be Bhattacharya (2017)
waste solutions and sanitised or regenerated
water purification regularly, as bacterial
• Efficient in removing colonies may multiply
even traces of impurities on the resin’s surface
from solutions and contaminate
• High treatment drinking water
capacity and fast kinetics • Low resin lifespan if
• Simple regeneration treating highly polluted
process water
• Resins are easily
maintained with long
lifespan

quality control. The combination of advanced nanotechnology with conventional methods may offer a new possible way
of controlling the destruction of the natural ecosystem by treating surface water.

4. Prospective nanotechnology in surface water treatment

Over the years, many studies have been conducted on surface water treatment. Most of them emphasised the removal
of undesirable harmful constituents from water. Nowadays, the rapidly advancing field of nanotechnology offers possible
solutions to be integrated into conventional methods for the treatment and quality control of surface water. The basic
principle of nanotechnology is using materials, devices, or systems at the nanometre scale (1–100 nm) to remove
contamination from surface water.
Polluted surface water being neglected for several years due to increased population and industrial development can
cause irreversible environmental pollution. Nano-sensors may be incorporated into the water quality monitoring system
by decision-makers on various water management issues. This incorporation includes, but is not restricted to, categorising
compliance to regulatory surface water quality requirements, recognising non-regulatory surface water quality for critical
consumers (for example, industries requiring certain chemical processes) and other significant areas throughout the
system, establishing water quality modelling, flushing, and executing a contamination warning system (CWS).
This review discussed four methods, namely, using nanofiltration membranes, photocatalysis process, plasma dis-
charge, and nano-adsorbents, of which some are environmentally friendly and involve fewer amounts of chemicals
compared with other methods using nanotechnology.

4.1. Nanofiltration membrane in surface water treatment

Membrane technology has been considered as an alternative in surface water treatment. It has gained great research
interest owing to its outstanding potential to solve environmental issues, such as contaminant removal, water reuse,
and recycling valuable components from waste streams. This technology also functions as an effective filtration process
to allow a few molecules to pass through while retaining others (Verma et al., 2012). Membranes refer to selectively
permeable barriers to ions and organic molecules (Candela, 2018). A membrane’s pore size indicates the mean pore size
on the membrane’s surface, and it also indicates the size of particles to be repelled by the membrane (Anovitz and Cole,
2015). In general, there are four main types of separation membranes, namely, microfiltration (MF), ultrafiltration (UF),
10
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

nanofiltration (NF), and reverse osmosis (RO) membranes. Each type of membrane has a specific range of pore sizes. For
example, the pore sizes on MF membranes range from 50 to 500 nm, which are the largest pore sizes among the four main
membrane types. For UF membranes, pore sizes range from 2 nm to 50 nm. NF membranes have pore sizes of ≤2 nm,
which are much smaller than those of MF and UF membranes. The main advantage of an NF membrane is that it can
be used to filter particles such as salts, synthetic dyes, and sugars. It is generally confined to specialised uses. For an RO
membrane, its pore sizes range from 0.3 nm to 0.6 nm, which makes it the best separation membrane available. Owing to
its much smaller pore sizes, an RO membrane requires a higher pressure than that of MF membranes. It is more suitable
to be used in the filtration of salt or metallic ions from water (Promo, 2009).
The nanofiltration (NF) membrane separation process is considered an effective method for surface water treatment
for removing dissolved organic carbon (DOC), hardness (for example, Ca2þand Mg2þ: >80% removal), and a number of
organic micropollutants (Zhang et al., 2006). Table 2 shows the results of surface water treatment via various nanofiltration
membranes. The results confirmed that these NF membranes can be sufficient for large-scale surface water treatment
owing to their capability to overcome issues regarding surface water quality. Nevertheless, this is only a vague conclusion
and more supporting data are needed. In addition, feasibility studies related to NF membrane technology for large-scale
surface water treatment have not been well reported, which may be due to:

(a) various drainage sources into the water body without consistent data being recorded or absence of recording
devices, and
(b) high costs for implementation and maintenance.

4.2. Photocatalysis process in surface water treatment

Effluent discharges from wastewater, sewage, and greywater undergo compulsory phases of water treatment processes
before being released into surface water bodies. Commonly, untreated wastewater and greywater contain various organic
pollutants, such as xenobiotic organic compounds (XOCs), emerging pollutants, and micropollutants (Lai et al., 2016).
These constituents can be more prevalent in urban rivers and are potentially adverse even at trace level and difficult to
remove via conventional treatment due to the strong chemical bonding of the organic pollutants (Ding et al., 2015; Moreira
et al., 2016; Wang et al., 2020). These constituents, despite their adverse effects, are not yet included in the Malaysian
Department of Environmental (DOE) drinking water standards (Najah et al., 2021). These pollutants usually come from the
pharmaceutical industry, agricultural industry, textile industry, and domestic activities (Ceretta et al., 2020; Jahdi et al.,
2020; Ragam et al., 2020; Tang et al., 2020; Wu et al., 2020). Photocatalysis is one of the advanced oxidation processes
(AOPs) suitable to address problems in water treatment. This method has been widely studied for the degradation of
various organic pollutants in wastewater and greywater (Rueda-Marquez et al., 2020).
In photocatalysis, the structure of organic pollutants is broken and eliminated, leading to the mineralisation of CO2
and H2 O. This phenomenon is represented by the decrease of total organic carbon (TOC) (Ji et al., 2013). The principle
behind photocatalysis involves the illumination of photons to produce • OH in water. Photons have energy equal to or
greater than that of their bandgap. Thus, when illuminated with photons, the generation of e− /h+ pairs forms reactive
species, such as hydroxyl radicals (• OH) and superoxide radical anions (Ismail et al., 2016). • OH is a very powerful oxidant,
which can degrade stable organic pollutants with low selectivity of attack (Wang et al., 2020; Islam et al., 2020). The
efficiency of the photocatalytic degradation of organic pollutants depends on several factors, which influence the kinetics
of photocatalysis. The related factors are the initial concentration of the reactant, light source, type and mass of the
photocatalyst, pH, and irradiation time (Malato et al., 2009). The versatility of photocatalysis is also enhanced by its
different possible means for • OH production, thus enabling better compliance with specific treatment requirements. The
process of photocatalysis, which involves the reaction between the photocatalyst and the pollutants via the illumination of
a light source, is illustrated in Fig. 4. The main reaction is activated on the photocatalyst’s surface by a specific wavelength
of the light source, such as direct sunlight radiation, visible-light irradiation, solar radiation, or ultraviolet (UV) radiation
(Borges et al., 2016).
Photocatalyst selection depends on the capability of the photocatalyst material to generate the charge carriers on the
conduction and valence bands, which are electrons and holes, when photoexcited by a light source (Zhao et al., 2020).
The formation of electrons and holes on both bands causes the recombination of electron–hole pairs and influences the
performance of photocatalysis. The recombination of electron–hole pairs is related to the selection of the photocatalyst
material (Moreira et al., 2016; Hayati et al., 2020; Kurniawan et al., 2020; Orooji et al., 2020). Kurniawan et al. (2020)
reported that a low recombination rate of electron–hole pairs can improve photocatalysis performance. A photocatalyst
will absorb photons impacting electrons in the occupied outer orbital of the atom’s valence band, which elevates electrons
to the unoccupied conduction band, leading to the excited state of the conduction band’s electrons and the positive valence
band’s holes (Rehman et al., 2009).
Photocatalysts such as titanium dioxide (TiO2 ) and zinc oxide (ZnO) are the most common photocatalysts applied in
photocatalytic degradation due to their effectiveness in degrading stable organic pollutants (Lai et al., 2016; Zhao et al.,
2014). However, the application of TiO2 or ZnO alone is insufficient to decrease electron–hole recombination. To enhance
photocatalysis performance, TiO2 and ZnO are usually modified using various techniques. Table 3 shows the application
of the photocatalysis process of organic pollutant removal from wastewater and greywater. The removal of pollutants
by using the photocatalysis process depends on the photocatalyst and the chemical bonding of the pollutants. Stable
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N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

Table 2
Surface water treatment via various nanofiltration membranes.
NF membrane Pore size Contact Molecular Parameters Surface water Result Ref.
◦
type (nm) angle ( ) weight value
cut-off
(MWCO)
Dow Filmtec 0.68 44.7 ± 1.9 200–400 pH 7.10 6.35 Foureaux et al.
NF 90 Conductivity (µS/cm) 161 21.40 (2019)
Turbidity (NTU) 56.5 0.06
TSS (mg/L) 20 <0.001
Apparent colour (mg Pt-Co/L) 196 <5
UV Absorbance (254 nm) 0.349 0.010
TOC (mg/L) 2.53 0.53
Ca (mg/L) 12.38 <2.50
Mg (mg/L) <1.25 <1.25
Na (mg/L) 6.40 <1.00
K (mg/L) 2.70 <2.50
Dow Filmtec 0.84 32.6 ± 1.3 200–400 pH 7.10 6.57 Foureaux et al.
NF 270 Conductivity (µS/cm) 161 33.6 (2019)
Turbidity (NTU) 56.5 0.08
TSS (mg/L) 20 <0.001
Apparent colour (mg Pt-Co/L) 196 <5
UV Absorbance (254 nm) 0.349 0.009
TOC (mg/L) 2.53 0.91
Ca (mg/L) 12.38 <2.50
Mg (mg/L) <1.25 <1.25
Na (mg/L) 6.40 <1.00
K (mg/L) 2.70 <2.50
Koch 0.82 26.9 ± 0.4 200 pH 7.10 6.41 Foureaux et al.
Membrane MPF Conductivity (µS/cm) 161 35.60 (2019)
34 Turbidity (NTU) 56.5 0.07
TSS (mg/L) 20 <0.001
Apparent colour (mg Pt-Co/L) 196 <5
UV Absorbance (254 nm) 0.349 0.018
TOC (mg/L) 2.53 1.00
Ca (mg/L) 12.38 <2.50
Mg (mg/L) <1.25 <1.25
Na (mg/L) 6.40 <1.00
K (mg/L) 2.70 <2.50
GE Osmonics 0.76 40.6 ± 5.2 150–300 pH 7.10 6.37 Foureaux et al.
DK Conductivity (µS/cm) 161 17.89 (2019)
Turbidity (NTU) 56.5 0.08
TSS (mg/L) 20 <0.001
Apparent colour (mg Pt-Co/L) 196 <5
UV Absorbance (254 nm) 0.349 0.002
TOC (mg/L) 2.53 0.86
Ca (mg/L) 12.38 <2.50
Mg (mg/L) <1.25 <1.25
Na (mg/L) K (mg/L) 6.40 <1.00
2.70 <2.50
(continued on next page)

pollutants require a photocatalyst with high photocatalytic activity, as well as a significant light irradiation reaction,
to break the chemical bonds of the pollutants. As shown in Table 3, the performance of photocatalytic degradation by
using only TiO2 as the photocatalyst has different efficiencies against different pollutants; for instance, the removal of
tetracycline was only 25% with 150 min of photocatalysis, and the removal of metalaxyl was 99.9% with 30 min of
photocatalysis (Wu et al., 2020; Islam et al., 2020). Also, the efficiency of photocatalysis varies when using different
types of photocatalysts. In addition, the performance of the photocatalysis process is influenced by irradiation time; for
example, the application of TiO2 photocatalyst alone reached 83% removal of diazinon with 50 h of photocatalysis (Molla
et al., 2020). Prolonged irradiation time slightly enhances the removal rate of pollutants.
To enhance the performance of photocatalysis, many researchers have combined the photocatalysis process with other
methods of treatment to improve the removal rate of the targeted pollutant. The combinations of the photocatalysis
process with biochar, biological treatment, and ultrasound are presented in Table 3. The performance of photocatalysis
combined with ultrasound reached up to 99.9% removal of sulfadiazine with 80 min of irradiation time (Kurniawan et al.,
2020). The combination of photocatalysis and biological treatment removed 95.7% of Azo dye direct black 22 with 60 min
of photocatalysis (Ceretta et al., 2020). The combination of photocatalysis with other methods in treating water can
improve the removal rate depending on the stability of the targeted pollutant’s chemical structure.
12
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

Table 2 (continued).
NF membrane Pore size Contact Molecular Parameters Surface water Result Ref.
◦
type (nm) angle ( ) weight value
cut-off
(MWCO)
GE Osmonics 0.47 62.2 ± 4.2 150–300 pH 7.10 6.33 Foureaux et al.
Duracid Conductivity (µS/cm) 161 14.70 (2019)
Turbidity (NTU) 56.5 0.08
TSS (mg/L) 20 <0.001
Apparent colour (mg Pt-Co/L) 196 <5
UV Absorbance (254 nm) 0.349 0.010
TOC (mg/L) 2.53 0.47
Ca (mg/L) 12.38 <2.50
Mg (mg/L) <1.25 <1.25
Na (mg/L) 6.40 <1.00
K (mg/L) 2.70 <2.50
NF270 NA 27–55 300 NaCl NA 40% rejection Sari and
MgSO4 > 97% rejection Chellam (2017)
Ca+2 91.8% rejection
Na+ 60% rejection
Mg+2 94.2% rejection
2
SO−
4 99.6% rejection
NF90 NA 54–63 200 NaCl NA 85% rejection Sari and
MgSO4 > 97% rejection Chellam (2017)
Ca+2 99.8% rejection
Na+ 97.4% rejection
Mg+2 99.9% rejection
2
SO−
4 99.9% rejection
HFW 1000 NF NA NA NA DOC (mgC L−1 ) 8.7 >90% rejection Köhler et al.
membrane UV Absorbance (254 nm) 0.28 cm−1 96% rejection (2016)
module
(0.20 m (800)
in diameter
and 1.54 m
(60.500) long)

Fig. 4. Illustration of photocatalysis process for degradation of organic pollutants.

The photocatalysis process for removing organic pollutants before discharging wastewater, greywater, and sewerage
into water bodies could secure the quality of surface water and it also could prevent organic pollutants from accumulating
in surface water. Previous studies have reported that using only conventional water treatment methods is insufficient
in removing stable organic pollutants due to the pollutants’ strong chemical bonding (Yang et al., 2017; Zhang et al.,
2009). This has led to the application of the photocatalysis process for organic pollutant removal before the wastewater
is discharged.

4.3. Plasma discharge in surface water treatment

Plasma methods have been deemed as an alternative to conventional water treatment methods by effectively
combining ultraviolet radiation, active chemicals, and high electric fields. Generally, plasma can be defined as partially
13
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

Table 3
Applications of photocatalysis process in water.
No. Water Photocatalysis Photocatalyst Light Source Target Pollutant Efficiency Ref.
Matrix Technique
1 Wastewater Photocatalytic TiO2 nanodiamond UV light Bisphenol A (BPA) 99.9% (100 Hunge et al.
degradation irradiation min) (2021)
2 Medical Photocatalytic TiO2 Visible light Tetracycline (TC) 25.1% (150 Wu et al.
wastewater degradation min) (2020)
3 Textile Biological (bio) ZnO/polypyrrole UV irradiation Azo dye direct 95.7% (96 h Ceretta
wastewater and photocatalysis (125 W) black 22 - bio; 60 et al. (2020)
degradation (PD) min - PD)
4 Textile Photocatalytic TiO2 UV irradiation Rhodamine B (RB) RB = 93.8% Kiwaan
wastewater degradation (280 nm) Acid Red 57 AR57 = et al. (2020)
(AR57) 90.7%
(190 min)
5 Textile Photocatalytic Graphene oxide/TiO2 Sunlight Salicylic acid 57% Hunge et al.
wastewater degradation illumination (2020)
6 Medical Photocatalytic Red mud Visible light Tetracycline (TC) 88.4% (80 W. (Shi
wastewater degradation min) et al., 2020)
7 Medical Heterojunction Z-scheme Visible light Sulfamethazine 91.4% (60 Wen et al.
wastewater photocatalysis Agl/Bi4 V2 O1 1 min) (2020)
8 Agricultural Heterojunction ZCS/SIS core–shell Visible light Rhodamine B (RB) RB = 97% Tang et al.
wastewater photocatalysis nanorod and (60 min) (2020)
imidacloprid (IM) IM = 55%
(240 min)
9 Wastewater Photocatalytic F-Pd co-doped TiO2 Solar simulator/ Sulfamethoxazole 98.9% (70 Jahdi et al.
degradation nanocomposite direct sunlight (SMX) min) (2020)
irradiation
10 Medical Photocat- Zn Co-layered double UV light Gemifloxacin 92% (Gholami
wastewater alytic/biochar hydroxide (GMF) et al., 2020)
(Zn–Co–LDH)
11 Medical Ultrasound- MgO/carbon UV irradiation Sulfadiazine (SDZ) 99.9% (80 Hayati et al.
wastewater assisted nanotubes (150 W) min) (2020)
photocatalytic
degradation
12 Medical Z-scheme CO3 O4 /TiO2 Visible light Enrofloxacin 95.6% (100 Wang et al.
wastewater heterojunction (500 W xenon min) (2020)
photocatalysis lamp)
13 Textile Photocatalytic TiO2 /Graphene oxide UV–Vis irradiation Methylene Blue 99% (4 h) Kurniawan
wastewater degradation et al. (2020)
14 Industrial Photocatalytic sGO-Ag2 VO4 /Ag Direct sunlight Dye 99.95% (60 Ragam et al.
wastewater degradation irradiation min) (2020)
15 Agricultural Photocatalytic TiO2 Direct sunlight Diazinon 83% (50 h) Molla et al.
wastewater oxidation irradiation (2020)
16 Agricultural Photocatalytic TiO2 Solar irradiation Metalaxyl 99.9% (30 Islam et al.
wastewater degradation min) (2020)
17 Bathroom Photocatalytic Zinc oxide Direct sunlight Basic Red 51 89.01% Yashni et al.
greywater degradation nanoparticles (ZnO irradiation (BR51) (5.5 h) (2020)
NPs)
18 Domestic Photocatalytic Nitrogen-doped TiO2 Solar radiation Phosphate (P) P = 55% Priyanka
greywater degradation (NP-TiO2 ) Benzophenone BP = 98.5% et al. (2020)
(BP) (6 h)
19 Wastewater Photocatalytic Zn-doped Cu2 O Visible light Ciprofloxacin 94.6% (240 Yu et al.
degradation particles (500 W metal min) (2019)
halide lamp)
20 River water Photodegradation NR UV light Fluoroquinolone NR Barra Carac-
process ciprofloxacin ciolo et al.
(2018)
(continued on next page)

ionised gas, comprising electrons, free radicals, ions, and neutrals, typically produced by various electricity discharges at
adjacent temperatures (Jiang et al., 2014; Conrads and Schmidt, 2000). The energy applied causes the ionisation of the
gas (Chen et al., 2014). In addition, plasma is a source of high electric fields charged with oxidising species (radicals and
molecules), energy particles, reactive species (hydrogen atoms, nitrogen atoms, and aqueous electrons) (Jiang et al., 2014)
(Fig. 5), electrohydraulic cavitation, ultrasound, UV lights, and shock waves (Ajo et al., 2017; Annemie et al., 2002; Locke
et al., 2006).
In the field of water treatment using nanotechnology, plasma discharge plays an important role. It offers process
simplicity, is effective in removing toxic organic compounds in raw water and wastewater, and is economical and
environmentally friendly (Locke et al., 2006; Shi et al., 2009). Yang et al. (2011) described that plasma discharge initiated
14
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

Table 3 (continued).
No. Water Photocatalysis Photocatalyst Light Source Target Pollutant Efficiency Ref.
Matrix Technique
21 Industrial Photosonochemical NR Ultrasound/UV (15 2-phenoxyethanol 76% (2 h) Boutamine
wastewater process mvcm et al. (2018)
−2
/253.7 nm)
22 Wastewater Fenton reaction NR UV light 4- FR = 66.01% Ji et al.
(FR) and irradiation (300 W methylbenylidene PF = 96.71% (2017)
photo-fenton (PF) mercury lamp) camphor (4-MBC) (90 min)
23 Urban Photocatalytic TiO2 -coated glass UV irradiation Carbamazepine, NR Moreira
wastewater ozonation Raschig ring (LED) isoproturon, et al. (2016)
treatment clarithromycin,
plant norfluoxetine,
fluoxetine, 1
7-alpha-
ethinylestradiol,
17-beta-estradiol.
24 Textile Photocatalytic TiO2 and ZnO Solar UV radiation Methylene Blue 98% (270 Chekir et al.
wastewater degradation min) (2016)

* NR means No report.

Fig. 5. Reactive species generated by plasma discharge systems in gas and liquid.

between two electrodes causes the medium between the two electrodes to ionise and then produce a plasma channel.
Plasma discharge produces ultraviolet radiation, which converts adjacent water molecules into active radical species,
owing to the high energy levels produced by the discharge. Through contact with active radicals, microorganisms can
be effectively deactivated, and organic contaminants can be oxidised. Furthermore, plasma discharge in water generates
various active species as by-products. This is influenced by several parameters, namely, rise time, applied voltage, pulse
duration, polarity, total energy, and water electrical conductivity. Among these active species, the most important species
are atomic oxygen, hydroxyl radical, ozone, and hydrogen peroxide for the sterilisation and removal of undesired organic
compounds in water.
There are several plasma discharge methods based on direct high-voltage electrical discharge in water (electrohydraulic
discharge) or in the gas phase overhead the water (non-thermal plasma), such as dielectric barrier discharge, pulsed corona
discharge, gliding arc discharge, and contact glow discharge (Zeghioud et al., 2020). For corona discharge, a small curvature
of an electrical arc is discharged between electrodes generating voltage of 50 to 100 kV and current of 10−6 to 10−4 A
(Cheng et al., 2012), whereas a plasma arc is generated between two metal electrodes/dielectric materials made of glass,
quartz, ceramic, pyrex, and polytetrafluoroethylene for dielectric barrier discharge (Manoj Kumar Reddy et al., 2013; Reddy
et al., 2013). On the other hand, an electric arc is generated between an electrode and an electrolyte solution in the glow
15
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

Table 4
Applications of plasma discharge for water treatment.
Types of plasma discharge Type of water Parameter Performance Ref.
Cold plasma Surface water Applied voltage range: •Colour: <5.0 TCU Van Nguyen et al. (2019)
15–18 kV •Turbidity: 1.9 ± 0.7 NTU
Water flow rate range: 2–3 •pH: 7.2 ± 0.1
L/min •Ammonia: 0.4 ± 0.5 mg/L
• Total iron: 0.3 ± 0.2 mg/L
• Total coliform: 10.0 ± 4.0
MPN/100 mL
• E. coli: 5.0 ± 3.0
MPN/100 mL

Continuous flow pulse River and lake Power: 578.67 W • Emerging contaminants Singh et al. (2019)
corona discharge reactor water Flow rate: 10 mL/min (ECs) were completely
removed from lake water
• 91%–100% ECs degraded
in river water
• After treatment, toxicities
of ECs were completely
eliminated

Dielectric barrier discharge Domestic wastewater Voltage: 14.6 kHz • Alkalinity: improved by Tanakaran and Matra (2019)
10.48%
• Dissolved oxygen:
improved by 10.09%
• Conductivity: improved
by 17.79%
• BOD: enhanced by 7.5
times
• COD: enhanced by 37.5%

Dielectric barrier discharge Surface water Voltage = 3.5 kV • TSS: reduced to 30.12% Quyen et al. (2017)
Frequency = 5.5 kHz • COD: reduced to 33%
Atmospheric pressure • pH: 5
• Eliminated microorganism
• Decomposed 33% of
organic compounds after
120 min

discharge, and the solution or wastewater may act as another electrode (Van de Moortel et al., 2017; Gudmundsson and
Hecimovic, 2017). Gliding arc discharge is an electrical discharge between two different electrodes. When high electrical
power is introduced, larger short-term active species are formed (Brisset et al., 2016). When water is in contact with
the plasma, the resulting advanced oxidation breaks down and eliminates the ionic bonds and covalent linking of organic
substances, inorganic substances, and bacteria (Quyen et al., 2017). As a result, water quality is improved. The applications
of plasma discharge in water treatment are indicated in Table 4. However, while the setup cost of the plasma discharge
system may be economical, but it is worth noting that the plasma system treatment requires high electrical energy, which
is a high-cost process.

4.4. Nano-adsorbents for wastewater remediation

Nano-adsorbents are nanomaterials modified or functionalised to adsorb hazardous chemicals and heavy metal con-
taminants in water. Examples of nanomaterials applied as nano-adsorbents are carbon-based materials (Nasrollahzadeh
et al., 2021a), oxide nanoparticles (Nasrollahzadeh et al., 2021a,b), metal-based nanoparticles (Khodadadi et al., 2017),
silica nano-adsorbents (Di Natale et al., 2020), polymer-based nano-adsorbents (Sajjadi et al., 2021; Nasrollahzadeh et al.,
2021c), nanocomposites (Naghdi et al., 2018), and zeolites (Elboughdiri, 2020). This approach attracts scientific research
attention for water remediation due to several attractive properties of nanomaterials, such as large surface area, reactivity,
high adsorption capability, and easy distribution in water (Janani et al., 2021). For the removal of water dyes, a nano-
adsorbent and a reducing agent are mixed into the polluted water and followed by stirring, which initiates the reduction
and catalytic processes (Sajjadi et al., 2019; Tajbakhsh et al., 2016). Subsequently, the chemical composition is reduced
into a less toxic compound. In addition to the treatment of dye in wastewater, nano-adsorbents are applied in the form of
nanoparticles or a thin film for the removal of metal ions or herbicides in solutions (Nasrollahzadeh et al., 2018; Nezafat
et al., 2021; Tajbakhsh et al., 2016). A recent review paper has discussed different types of adsorbents applied for water
remediation and their adsorbent efficiency with different parameters (Janani et al., 2021). This review comprehensively
reported the success of various nano-adsorbents for the removal of heavy metals either in a single solution or quaternary
16
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

solutions. Multi-walled carbon nanotubes (MWCNTs) have been indicated with the ability to remove Pb2+ and Ni2+ up
to 93% and 86%, respectively (Farghali et al., 2017). Magnetically activated graphene oxides successfully removed 71%
of atrazine at 45 ◦ C (Andrade et al., 2019). These oxides and metal-based nano-adsorbents after a laser ablation process
showed even higher effectiveness in cleaning up dye-polluted water via the photocatalytic reaction (Jaleh et al., 2021).
However, many nano-adsorbents are not ready for practical water remediation due to the high production cost, difficulty
in the recovery of the spent adsorbent, the threat to aquatic life, and less applicability at the industrial scale (Janani et al.,
2021). It is worth pointing out in this review that the optimum absorption capacity of most nano-adsorbents designed for
metal ions or dye removal were performed under pH 2–7 using a small experimental volume, a single waste solution, and
controlled temperature. However, water treatment plants usually release treated water having pH of 7 or 8, (Department
of Environment, 2016).
Moreover, lab-scale wastewater treatment research may need to consider if the nano-adsorbent produced is sufficient
for the application in water treatment plants, which typically treat 30 million gallons of wastewater per day (Weiner and
Matthews, 2003).

5. Nano-sensors for surface water quality monitoring

The purpose of a surface water monitoring system is to characterise water and detect changes in the physical, chemical,
and biological characteristics of water over time. The system allows the identification of specific events or new and
emerging problems. The data collected is useful for the design of specific pollution detection systems or mitigation
programmes. Compliance to mitigate the pollution level detected usually refers to the national WQI requirements. Early
detection of alleged pollution due to human, agricultural, or industrial activities would alert the authority to deploy
effective pollution control measures or emergency responses to the severe pollution detected.
Water authority departments perform water monitoring scheduled at specific locations. The parameters of surface wa-
ter are sampled and tested on-site regularly every week or every month using specific protocols and portable equipment.
Some critical areas at sources of drinking water are installed with a long-term water quality monitoring system. Water
monitoring includes monitoring for (1) physical, (2) organic, and (3) biological contaminants (Su et al., 2020). For physical
monitoring, parameters of interest are pH, temperature, dissolved oxygen, nutrients (nitrate, phosphorus, ammonia, etc.),
turbidity, total suspended solids, total dissolved solids, temperature, total alkalinity, and conductivity. Pollutants can be
oil, metals, and pesticides. Changes in these parameters influence the ecological system and threaten the sources of
drinking water. In addition to physical parameters, numerous organic compounds, such as pesticides, surfactants, dyes,
phenolic compounds, and new emerging pharmaceutical contaminants, have been found in rivers and are currently gaining
the attention of water authorities (Ben et al., 2019). Among organic compounds, organochlorinated pesticides (OCPs),
brominated flame retardants (BFRs), polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) bind
strongly to sediments that these compounds are transferable to aquatic life, hence endangering food safety (Justino et al.,
2016). Biological contaminants refer to viruses, bacteria, protozoans, and parasites that could enter water. The concern is
that these living organisms may be pathogenic and cause diseases in aquatic life. Pathogens that could cause waterborne
diseases include Giardia, Cryptosporidium, Vibrio, E. coli, Salmonella, Shigella, Legionella, hepatitis A virus, poliovirus, Vibrio
cholerae, enterovirus, norovirus, and rotavirus (Ojha, 2020). Another issue is the presence of cyanobacteria in freshwater,
water reservoirs, irrigation water, and water in recreation pools. These bacteria produce cyanotoxins and microcystins
that are potent to humans. Samples extracted from water might contain small molecules and organisms that are beyond
the detection limit of current sensor systems.
For detailed analyses of water content, water authorities perform traditional analytical techniques in the laboratory to
determine the chemicals in the water via liquid chromatography, atomic absorption spectroscopy, gas chromatography-
mass spectroscopy (GCMS), immunoassay, quantitative polymerase chain reaction, and electrochemical techniques
(Vikesland, 2018; Kor and Zarei, 2014). These techniques are highly sensitive but involve a lengthy process of sample
pre-treatment and arduous analysis (Kempińska and Kot-Wasik, 2018).
Nano-sensors are gaining substantial interest in environmental monitoring due to their sensing ability at low de-
tection limits, selectivity, and high sensitivity (Mistewicz et al., 2017; Ramezani et al., 2020). These sensors are de-
signed using nanomaterials with nanostructures that provide high surface-area-to-volume and binding affinity to target
molecules. Nanostructures include nanorice, nanorods, nanowires, nanoparticles, nanopillars, nanoflowers, and quantum
dots (Ruedas-Rama et al., 2011; Steffens et al., 2017; Liu et al., 2011; Tsvetkov et al., 2013). All the unique features
at the nanometric scale are functionalised with recognition elements, which include aptamers, enzymes, proteins, and
antibodies, to enable the detection of pharmaceutical contaminants, pesticides, herbicides, metals, and ions present in
water samples at an ultra-low range (Vikesland, 2018; Steffens et al., 2017; Willner and Vikesland, 2018; Sai et al., 2019;
Dasgupta et al., 2017; Wei et al., 2015). In comparison with conventional water quality sensors, nano-sensors offer higher
sensitivity in sensing pollutants and in early disease detection.
Nano-sensors for water quality monitoring are classified based on the sensors’ detection modes, which are optical,
electrochemical, magnetic, and mechanical. Optical-based Fe3 O4 @Ag nano-sensors seem to have the sensitivity to detect
mercury ions (Hg2+ ) at 10 pM via surface-enhanced Raman resonance scattering (SERRS). An Ag/Hg amalgam is formed
when there is a redox reaction occurring between nano-silver and Hg2+ at the sensor’s surface. SERRS’ signal intensity
increases with the increase in Hg2+ concentration. However, other metallic divalent cations may also conjugate with
17
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

the electrode via the redox reaction (Song et al., 2017). Another chemical nano-sensor developed for the detection
of mercury ions is the nano-sensor with thymine-decorated gold nanoparticles based on fibre-optic surface plasmon
resonance (SPR) excitation (Yuan et al., 2021). The immobilised thymine binds specifically with mercury ions and thus
shifts the SPR wavelength. This fibre-optic sensor indicates a high selectivity to mercury ions than other heavy metal ions
in environmental water and is potentially useful for on-site mercury ion testing. The coupling of localised surface plasmon
resonance and metallic nanoparticles forms an optical transducer that is potentially sensitive in detecting contaminants
based on the shift of the emission peak’s excitation wavelength. The absorption of molecules on gold or silver nanoparticles
shifts the absorption peak and thus many analytes are detectable based on colour spectrum changes. Carbon dots have
been demonstrated with sensitivity to the presence of heavy metals in environmental water (Zhang et al., 2020). The
detection is based on radiometric changes of fluorescence or photoluminescence of carbon dots bound with different
concentrations of heavy metal ions, such as chromium, lead, cadmium, mercury, and arsenate. Although the detection
limit of carbon-dot sensing platforms for metallic ions is down to a few micromolar in environmental water, the detection
limit is considerably high compared with those of SERRS and localised SPR techniques (Wei et al., 2015).
Electrochemical nano-sensors detect electrical changes in the sensor material, in which electrons are donated or
removed from the sensing material, such as nanotubes, nanocages, and nanowires. By treating the nanomaterial with dif-
ferent functional materials, the nano-sensor can be functionalised with chemical receptors for specific molecule detection,
for example, with biomarkers that recognise new emerging pharmaceutical contaminants. Aptamer-based toxin biosensors
on a glassy carbon electrode coated with cobalt (II) salicylaldiimine metallodendrimer and silver nanoparticles (AgNPs)
are able to detect microcystin–leucine–arginine (LR) in the nanomolar range (Bilibana et al., 2016). Aptamer as an artificial
oligonucleotide receptor target can bind with microcystin-LR. The binding of microcystin-LR to the aptamer sensor has
been characterised by a three-electrode voltammetry measurement system. For sensors that involve coating with peptide
molecules, the repeatability of the sensors for sensing is influenced by preparation procedures, pH, temperature, and
thickness of the thin film, which are required to be well controlled. The preparation of aptamer sensors and their
readiness to be used on-site are important issues to be considered. In addition to sensing cyanotoxins, electrochemical
sensors are also applied to sense chemical compounds in water. Ruthenium bipyridine–graphene oxide ([Ru(bpy)3 ]2+ -
GO) electroactive nanocomposite electrodes have demonstrated good selectivity and repeatability in sensing arsenite and
arsenate in water at a detection limit of 20 nM (Gumpu et al., 2018). However, the sensing of arsenite was conducted
with a controlled concentration at a specific pH, in which the environmental water samples needed to be pre-treated.
Magnetic nanoparticles (MNPs) have been broadly applied as bacterial and chemical separation agents and revealed
as effective agents for the decontamination of diverse environmental substances (Shrivas et al., 2019). The formation of
MNP aggregates on the surface of the bacteria enables bacteria separation using a magnetic field (Xu et al., 2019). Upon
surface modification, MNPs could be conjugated with bacteria, leading to the detection of various types of bacteria using
optical techniques, such as SERRS, SPR, fluorescence quantification, and colourimetry (Yuan et al., 2018). Additionally,
MNPs have been applied to immobilise organophosphorus pesticides via the adsorption of pesticides on alkaloid-
functionalised graphene-oxide-based silica coated with MNPs. The pesticides captured can be separated and analysed
using GCMS, as reported in Wanjeri et al. (2018). However, the analytical methods presented are not suitable for on-site
testing and remain as bench-top equipment. In addition to bacteria and pesticides detection, manganese-based magnetic
nanoparticles coated with gold and conjugated with cysteine have been used for lead trapping and detection (Kong et al.,
2013). The composite system, when chelated with lead, would indicate an electrical current increase in voltammetric
measurement (Kong et al., 2013). It is important to note that such a detection system may not be specific to lead because
thiolate ligands of cysteines could also bind strongly to cadmium, copper, iron, and zinc ions (Niroshini et al., 2003; Pace
and Weerapana, 2014).
Based on the electrochemical, optical, and magnetic methods discussed, there are vast opportunities to design new
nano-sensors for the detection of a cocktail of emerging contaminants (ECs) that could affect the microbiome of water,
which include antibiotics, perfluorinated compounds, pesticides, hormones, anti-inflammatory drugs, and UV filters
(Gomes et al., 2020). Little is known whether these ECs influence microorganisms in water. Nano-sensors for the accurate
measurement of ECs in surface water are very much needed. To the best of our knowledge, the research in discovering
nanomaterials as multi-functional materials in pollutant detection and treatment has been rarely explored.

6. Challenges and future outlook

The key objectives of membrane application in water treatment are highly dependent on promoting its sustainability,
cost-effectiveness, and environmental-friendliness (ElSherbiny and Panglisch, 2021). Membrane fouling is still the main
concern that hinders the widespread application of membrane separation processes in water treatment, including surface
water. Among the most important foulants present in surface water are natural organic matters, comprising heterogeneous
mixtures with various molecular weights and functional groups (phenolic, hydroxyl, and carbonyl groups and carboxylic
acid), as well as vegetative debris and algae (Zularisam et al., 2010). To overcome the performance limitation and enhance
the efficiency of membranes for water treatment, ElSherbiny and Panglisch (2021) suggested several approaches, namely
the optimisation of membrane operating conditions, modification of intrinsic membrane characteristics, system design
related to membrane modules’ arrangement, hybridisation with new treatment technologies, and membrane distillation.
According to Hidalgo and Murcia (2021), recent studies are looking into the potential of modifying membranes with new
18
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

materials, such as chitosan, nanoparticles, and organic compounds. Other areas of interest to study are ion-exchange resin
membranes, liquid membranes, and the economic significance in predicting membrane changes.
Photocatalysis applications in water treatment are inevitably challenging in terms of catalyst agglomeration, particu-
larly for TiO2 , which greatly affects particle size preservation, surface area reduction, and reusable lifespan (Chong et al.,
2010). This prompts the selection and exploration of other types of metal oxide sulphides as the semiconductor for
photocatalysis. The performance of a photocatalyst depends on the properties of the contaminant being treated, which
may require design and material testing for specific applications (Loeb et al., 2019). Another main challenge in photo-
catalysis is the techniques used for catalyst synthesis. For instance, the co-precipitation method has limitations in terms
of being easily contaminated and having a complex purification process (Mashuri et al., 2020). Thus, Mashuri et al. (2020)
suggested potential improvements in synthesis techniques concerning factors that should be emphasised and properly
controlled, namely, semiconductor bandgap, type of carrier transport, material crystallinity, surface area, and photocatalyst
stability. Recently, studies have also focused on photocatalyst immobilisation with polymeric membranes, instead of
ceramic membranes, for water treatment, which demonstrates better performance than a suspended photocatalyst in
preventing suspension residual in the treated water after filtration (Zakria et al., 2021). Although agglomeration could
occur on membrane surfaces depending on the types of photocatalyst used, deposition techniques of photocatalyst could
be used to improve uniform photocatalyst distribution and photodegradation performance. Therefore, parameters such as
stirring condition, concentration, pressure, flow rate, temperature and others should be optimised to ensure uniform and
strong photocatalyst attachment on the membrane and high photocatalytic efficiency (Zakria et al., 2021). The challenges
identified for photocatalysis technology should provide a favourable future outlook on expanding its potential in water
treatment.
The application of plasma discharge for water purification has exhibited promising results in small-scale experiments,
but it may encounter challenges in scaling up the system in terms of throughput and treatment volume (Foster,
2017). Other shortcomings of plasma discharge are the possibility of generating highly toxic degradation by-products,
decreased efficiency when the concentrations of organic pollutants and mineral salts increase, and expensive and high
gas consumption for large-scale applications in some plasma-based methods. According to Zeghioud et al. (2020), plasma
technology may undergo long-term reliability tests on its performance durability, normal working life, and operational
safety to improve its commercial viability for full-scale water and wastewater treatment. The combination of plasma
technology with other processes appears to be practical and effective and has great potential to remove aqueous
contaminants on laboratory and pilot scales. This provides more opportunities to expand relevant studies on the feasibility
of plasma discharge for surface water treatment on a wide scale.
Producing eco-friendly and bio-based nano-adsorbents while maintaining all the excellent characteristics of nano-
materials could be the future direction of research (Nasrollahzadeh et al., 2021a, 2018). The nano-adsorbents designed
should be able to perform over a wide temperature range instead of at a specific temperature. The time-dependent
properties, water/chemical resistance, and degradability of polymeric-based nano-adsorbents should be prioritised. This
is because the elevated temperature requirement incurs a heating cost in the wastewater treatment process. Any future
nano-adsorbents being investigated is recommended to be eco-friendly, robust, and tested in real wastewater samples,
which is far more complex than simulated solutions. The application of suspended nano-adsorbents in wastewater
treatment is controversial due to the harm to aquatic life. Till date, there is scarce research that indicated the method of
recollection or recovery of nano-adsorbents after the wastewater is treated. The recyclability of nano-adsorbents claimed
(Sajjadi et al., 2021) using high-speed centrifugation may not be practically viable for large-scale water processing. To
overcome this issue, there has been a suggestion to anneal nano-adsorbents on a substrate, but this inevitably reduces
the reactivity and the surface of the nano-adsorbent (Jaleh et al., 2021). Magnetic nano-adsorbents incorporated with a
green photocatalytic agent may be a more scalable method for industrial processes. The utilisation of nano-adsorbents has
demonstrated the adsorption of limited metal ions (Janani et al., 2021). Hence, more investigations should be undertaken
to synthesise nanoparticles with the adsorption of more types of metal ions. Above all, the cytotoxicity and potential risks
of nano-adsorbents to aquatic life and the ecosystem should be carefully investigated.

7. Conclusions

Different types of nanotechnology applications for surface water pollution and quality control have been widely
discussed and reviewed. Surface water is prone to pollution due to heavy human activities. Owing to easy accessibil-
ity, surface water sources, such as rivers, are the most common discharge sites for wastewater, which may contain
microorganisms, heavy metals, and harmful pollutants. Over time, these clean water resources will deplete and jeopardise
humans, as well as animals and plants. Conventional methods have focused on the removal of organic or inorganic
pollutants via flocculation, coagulation, and biological techniques. However, conventional methods are not totally efficient,
with several limitations for achieving the criterion of environment-quality discharges. Therefore, various methods using
nanotechnology, such as nanofiltration membranes, photocatalysts, plasma discharge, nano-sensors, and nano-adsorbents,
present great prospects for surface water treatment and quality control. With the advent of nano-adsorbents, the existing
water treatment technology may be coupled with nano-adsorbents, which has shown high efficiency and selectivity in
the removal of organic and inorganic pollutants. Despite the promising and optimistic application of nanotechnology for
surface water treatment, there are some challenges. These challenges could provide opportunities and directions to expand
research on the application of nanotechnology in surface water treatment based on future outlook.
19
N.H.H. Hairom, C.F. Soon, R.M.S.R. Mohamed et al. Environmental Technology & Innovation 24 (2021) 102032

CRediT authorship contribution statement

Nur Hanis Hayati Hairom: Writing – original draft, Resources, Visualisation, Project administration. Chin Fhong Soon:
Conceptualisation, Writing – original draft, Writing – review & editing, Supervision, Formal analysis, Validation, Funding
acquisition. Radin Maya Saphira Radin Mohamed: Writing – original draft, Resources, Supervision. Marlia Morsin:
Writing – original draft, Resources, Formal analysis. Nurfarina Zainal: Writing – original draft, Resources, Formal analysis.
Nafarizal Nayan: Writing – original draft, Resources, Formal analysis. Che Zalina Zulkifli: Conceptualisation, Supervision.
Nor Hazlyna Harun: Conceptualisation, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have
appeared to influence the work reported in this paper.

Acknowledgements

This research is funded by the Ministry of Higher Education Malaysia under the Research Excellence Consortium Grant
Scheme (KKP) or KPM-Special Grant RMK-10 (JPT(BPKI)1000/016/018/25(54) or KKP Vot. No. K343).

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