Simultaneous removal of pollutants from water using nanoparticles: A shift from single pollutant control to multiple pollutant control

Science of the Total Environment 656 (2019) 808–833

Contents lists available at ScienceDirect

Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Review

Simultaneous removal of pollutants from water using nanoparticles: A shift from single pollutant control to multiple pollutant control Gloria Ntombenhle Hlongwane a, Patrick Thabang Sekoai b, Meyya Meyyappan c, Kapil Moothi a,⁎ a b c

Department of Chemical Engineering, Faculty of Engineering and the Built Environment, University of Johannesburg, Doornfontein, 2028, Johannesburg, South Africa Hydrogen Infrastructure Centre of Competence, Faculty of Engineering, North-West University, Potchefstroom 2520, South Africa Center for Nanotechnology, NASA Ames Research Center, Moffett Field, California 94035, United States

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Nanotechnology addresses limitations of conventional wastewater treatment. • Nanotechnology research in simultaneous removal of multipollutants is reviewed. • Multipollutant control nanotechnology can possibly streamline treatment processes. • Challenges in multipollutant control using nanotechnology are outlined. • Some required research efforts to overcome issues of scalability are recommended.

a r t i c l e

i n f o

Article history: Received 3 August 2018 Received in revised form 18 October 2018 Accepted 17 November 2018 Available online 22 November 2018 Editor: Ching-Hua Huang Keywords: Water treatment Simultaneous removal Adsorption Disinfection Nanocomposites Nanoparticles

a b s t r a c t The steady increase in population, coupled with the rapid utilization of resources and continuous development of industry and agriculture has led to excess amounts of wastewater with changes in its composition, texture, complexity and toxicity due to the diverse range of pollutants being present in wastewater. The challenges faced by wastewater treatment today are mainly with the complexity of the wastewater as it complicates treatment processes by requiring a combination of technologies, thus resulting in longer treatment times and higher operational costs. Nanotechnology opens up a novel platform that is free from secondary pollution, inexpensive and an effective way to simultaneously remove multiple pollutants from wastewater. Currently, there are a number of studies that have presented a myriad of multi-purpose/multifunctional nanoparticles that simultaneously remove multiple pollutants in water. However, these studies have not been collated to review the direction that nanoparticle assisted wastewater treatment is heading towards. Hence, this critical review explores the feasibility and efficiency of simultaneous removal of co-existing/multiple pollutants in water using nanomaterials. The discussion begins with an introduction of different classes of pollutants and their toxicity followed by an overview and highlights of current research on multipollutant control in water using different nanomaterials as adsorbents, photocatalysts, disinfectants and microbicides. The analysis is concluded with a look at the current attempts being made towards commercialization of multipollutant control/multifunctional nanotechnology inventions. The review presents evidence of simultaneous removal of pathogenic microorganisms, inorganic and organic compound chemical pollutants using nanoparticles. Accordingly, not only is nanotechnology showcased as a promising and an environmentallyfriendly way to solve the limitations of current and conventional centralised water and wastewater treatment facilities but is also presented as a good substitute or supplement in areas without those facilities. © 2018 Elsevier B.V. All rights reserved.

⁎ Corresponding author at: PO Box 17011, Doornfontein, 2028 Johannesburg, South Africa. E-mail address: [email protected] (K. Moothi).

https://doi.org/10.1016/j.scitotenv.2018.11.257 0048-9697/© 2018 Elsevier B.V. All rights reserved.

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Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature survey and selection criteria . . . . . . . . . . . . . . . . . . . . . . Wastewater: pollutants, sources and treatment . . . . . . . . . . . . . . . . . . 3.1. Sources and health effects of pollutants in wastewater . . . . . . . . . . . . 3.2. Wastewater treatment technologies . . . . . . . . . . . . . . . . . . . . 3.3. Co-existence of pollutants. . . . . . . . . . . . . . . . . . . . . . . . . 4. Nanotechnology for simultaneous removal of pollutants from water . . . . . . . . . 4.1. Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Simultaneous adsorption of heavy metal ions . . . . . . . . . . . . 4.1.2. Simultaneous adsorption of polyatomic ions . . . . . . . . . . . . 4.1.3. Simultaneous adsorption of metal ions and polyatomic ions . . . . . 4.1.4. Simultaneous adsorption of organic compounds . . . . . . . . . . 4.1.5. Simultaneous adsorption of heavy metal ions and organic compounds 4.2. Photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Heterogeneous semiconductor photocatalysis . . . . . . . . . . . 4.2.2. Photocatalytic ozonation . . . . . . . . . . . . . . . . . . . . . 4.2.3. Fenton/photo-Fenton reactions . . . . . . . . . . . . . . . . . . 4.2.4. Non-photochemical AOP methods . . . . . . . . . . . . . . . . . 4.3. Disinfection and microbial control . . . . . . . . . . . . . . . . . . . . . 4.4. Integration of nanotechnology techniques . . . . . . . . . . . . . . . . . 4.4.1. Adsorption and photocatalysis . . . . . . . . . . . . . . . . . . 4.4.2. Adsorption, disinfection and microbial control . . . . . . . . . . . 4.4.3. Photocatalysis, disinfection and microbial control . . . . . . . . . . 5. Challenges in multipollutant control . . . . . . . . . . . . . . . . . . . . . . . 5.1. Efficient co-removal of co-existing pollutants . . . . . . . . . . . . . . . . 5.2. Batch-to-batch predictability . . . . . . . . . . . . . . . . . . . . . . . 5.2.1. Initial concentrations of pollutants. . . . . . . . . . . . . . . . . 5.2.2. The pH of wastewater . . . . . . . . . . . . . . . . . . . . . . 5.2.3. Dose of nanoparticle/nanomaterial and contact time . . . . . . . . 5.3. Recovery, management, and disposal of exhausted nanomaterials. . . . . . . 5.4. Regeneration and recyclability. . . . . . . . . . . . . . . . . . . . . . . 5.5. Management and disposal of pollutants after desorption/regeneration. . . . . 5.6. Ecotoxicity of nanomaterials . . . . . . . . . . . . . . . . . . . . . . . 6. Commercialization: rising above the scientific hype . . . . . . . . . . . . . . . . 7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction In recent decades, rapid industrialization and urbanization, advances in agricultural practices and economic development have placed immense strain on water resources and the environment (Alcamo et al., 2017). As the water crisis intensifies and the quality of water deteriorates, there is not only an ever growing global concern over the availability of water for human use but also over its importance for the sustainable development of many countries (WHO, 2015; Alcamo et al., 2017). Water shortages and water pollution are currently the two major water-related problems faced by various countries worldwide (Cosgrove and Rijsberman, 2014). In order to alleviate the water resource crisis, improve the utilization rate of water, the efficiency of water treatment and realize a good social circulation of water the following practices are necessary: • Water conservation and water demand management (reflected mainly in cutting costs, systematic use of water resources, and creating a water-saving society) (Yazdanpanah et al., 2014; Liu et al., 2016; Loucks and Van Beek, 2017); • Pollution-based water management (Wesström et al., 2014; Altenburger et al., 2015); and • The development of non-conventional water resources (Qadir et al., 2007; Tomaszkiewicz et al., 2015). As a result, water-scarce countries have turned to non-conventional water resources which include: rainwater, recycled wastewater (wastewater generated by domestic, commercial and industrial sectors),

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seawater, and brackish groundwater water (Qadir et al., 2007). The amount and rate of urban wastewater production continue to increase exponentially in parallel with rapid urbanization (Lyu et al., 2016; Maryam and Büyükgüngör, 2017). Reclamation and reuse of wastewater has the potential to significantly reduce pollution thus partly alleviating the water resource crisis (Lyu et al., 2016). Proper treatment and purification of water prior to release for consumption is a fundamental step involved in the reclamation and reuse of wastewater (Bora and Dutta, 2014). Unlike when wastewater was comprised of only sewage and agricultural waste (at any given moment), the majority of unconventional water sources worldwide are often contaminated with various different categories of pollutants (Gollavelli et al., 2013; Bora and Dutta, 2014). In addition to generating large amounts of domestic, industrial, agricultural and aquaculture wastewater, the increasing exploitation and utilization of resources by humans, coupled with the growing population and continuous development of industry and agriculture increases the complexity, instability and toxicity of the various types/forms of pollutants present in wastewater (Ota et al., 2013; Li et al., 2016). This co-existence results in combined toxicity and relative mobility thus triggering more severe environmental damage (Yang et al., 2011). The inability to completely purify water is uneconomical as well (Bora and Dutta, 2014). Therefore, efficient treatment and purification of water prior to release using multi-purpose/multifunctional water treatment strategies can efficiently treat various kinds of complex wastewaters and achieve simultaneous removal of various contaminants in water (Gollavelli et al., 2013). Several studies have successfully demonstrated the use of nanoparticles in dechlorination, desalination and removal of contaminants such

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as pathogens, heavy metals, organic and inorganic compounds from water (Kumar et al., 2014; Bora and Dutta, 2014). There have been numerous reviews in the literature exploring and compiling data on nanotechnology for single pollutant control (Bora and Dutta, 2014; Lata and Samadder, 2016; Feng et al., 2018). A majority of the existing review articles not only focus on single pollutant control but the data compiled is often selectively based on a specific group/type of nanoparticles. The following recent literature studies exemplify the aforementioned: • A review of heavy metals removal from wastewater using graphene oxide-based nanomaterials (Lim et al., 2018). • An analysis of properties and efficiency of metal-organic frameworks in the removal of individual heavy metals such a zinc, nickel, mercury, lead, copper, chromium, cadmium and arsenic from ground water has been compiled by Kobielska et al. (2018). • Heavy metal removal from water/wastewater by nanosized metal oxides has been reviewed by Hua et al. (2012).

In other instances, the focus is not only on a selected type/group of nanoparticles but on one application of nanotechnology; e.g. the critical and systematic review of adsorption applications that utilizes carbon nanotubes (CNTs) for heavy metal removal (Ihsanullah et al., 2016). The development of multi-purpose water and wastewater treatment nanotechnologies for simultaneous treatment of multiple co-existing pollutants has already become the focus of recent studies. However, the research on multifunctional treatment of water/wastewater has not been compiled into a comprehensive review. To the best of our knowledge, this article presents for the first time a review of recent research contributions made towards a holistic and simultaneous removal of multiple pollutants from municipal water/wastewater using multipurpose/multifunctional nanotechnology. This provides a comprehensive but yet condensed and concise viewpoint of the status quo as researchers shift from single pollutant control to multiple pollutant control in water and wastewater treatment. The concurrent removal of major co-existing pollutants found in water is critically reviewed based on several nanotechnology techniques used such as adsorption, photocatalysis, disinfection and microbial control. Attempts towards commercialization of multipollutant control/multifunctional nanotechnology inventions are also discussed. The mechanisms between the nanomaterials and pollutants treated as well as any potential interferences in the pollutant removal process are not the scope of this work and shall not be addressed in detail. In general, the potential use of nanoparticles in addressing the difficulty/limitations of current water treatment technologies associated with complete purification of water by simultaneously treating co-existing pollutants in wastewater is emphasised. The multidisciplinary nature of this review provides an entry point even for researchers who have not yet ventured into the interesting use of nanomaterials for multiple pollutant control in water and wastewater treatment. This article should be of interest to environmental scientists, environmental toxicologists, ecologists, chemical/environmental engineers, environmental health scientists and epidemiologists, risk scientists, environmental science managers and administrators.

have been improvements in the application range of pollutant treatment, made possible using nanomaterials (Savage and Diallo, 2005; Qiao et al., 2014). Multi-function and synergism in nanotechnology treatment tools/techniques/agents is achieved through a systematic integration/compounding of multiple raw materials of different functions and/or properties to form multifunctional and synergistic composite materials for water and wastewater treatment (Gehrke et al., 2015). The literature survey criteria for this review article were restricted to studies that used nanocomposites to simultaneously remove pathogens, toxic organic and inorganic compounds from water. 3. Wastewater: pollutants, sources and treatment 3.1. Sources and health effects of pollutants in wastewater Pollutants found in water and wastewater can be broadly classified into three categories: pathogenic microorganisms, toxic organic and inorganic compounds. Table 1 lists the different classes of pollutants found in water, their major sources and effects they have on human health. Inorganic compounds found in water sources are usually heavy metals and polyatomic compounds accounting for 30% of pollutants found in sewage water (Bora and Dutta, 2014). These pollutants are frequently introduced into the environment and water supply through a variety of process activities in industries such as the textile refinery, pulp and steel manufacturing, petroleum and agricultural industries (Inglezakis et al., 2002). Excess bioavailability of heavy metals and polyatomic compounds has been associated with numerous diseases in humans that range from cardiovascular disorders to skeletal malformation (Martin and Griswold, 2009; Saif et al., 2012). The textile, printing, tanning, pharmaceutical and agricultural industries are the major producers of organic compounds that are toxic to the environment (Christin et al., 2004; Anjaneyulu et al., 2005; Jelić et al., 2012). About 70% of the pollutants found in sewage water are organic compounds (Bora and Dutta, 2014). Most organic compounds produced in industries are biodegradable, but their low rate of degradation tends to bio-accumulation (Gu et al., 2016). Although there is no evidence of serious health risks associated with low concentrations of some organic contaminants such as pharmaceuticals, their removal, particularly from potable water, is a preventive measure for possible long-term effects that could result from long-standing exposure (Jelić et al., 2012). Exposure to substantial amounts of organic contaminants such as dyes and herbicides has been implicated to interfere with the metabolism of endocrine hormones, mutagenicity/genotoxicity and cancer (Mathur and Bhatnagar, 2007; Tang et al., 2012). Microbial pollutants found in water and wastewater includes bacteria, viruses and protozoa. These are frequently introduced into water supplies through faecal contamination and soil pollution (Edberg et al., 2000; Ishii et al., 2006; Zvizdić et al., 2005). Microorganisms not only compromise the quality of water but also threaten the general wellbeing of the public and the environment (Leclerc et al., 2002; Zhan et al., 2014). Diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome are some of the waterborne diseases that can result from ingestion of microbial pollutants (Leclerc et al., 2002; Zhan et al., 2014). 3.2. Wastewater treatment technologies

2. Literature survey and selection criteria From here on, multifunctional/multi-purpose water/wastewater treatment shall refer to a shift from single pollutant control to multiple pollutant control in water/wastewater which is in line with global sustainable development strategies and environmental protection policies (Griggs et al., 2013). Weight is placed on agents/nanotechnologies that can treat a variety of wastewaters (e.g. domestic sewage, pharmaceutical wastewater, printing and dyeing sewage and paper plant effluent) through the simultaneous removal of co-existing pollutants (e.g. pathogens, organic and inorganic compounds). In recent years, there

Aesthetic issues are great indicators of possible water contamination. Significant changes in aesthetic parameters of water such as transparency, odour, and turbidity/colour provide clues on possible forms of multiple pollutants present in water. Below are some of the possible forms of pollutants present and how they affect the turbidity/colour of water: • Suspended microscopic organisms and algae: These aquatic microorganisms cause water to be dark green (due to cyanobacteria which are also known as blue-green algae), yellow-brown (due to diatoms),

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Table 1 Overview of pollutants found in water, their major sources and effects they have on human health. Category

Sub-category

Representative pollutants

Major sources

Toxicity

Reference

Inorganic compounds

Heavy metals

Chloride, lead, copper, cadmium, iron, nickel and zinc Arsenate, chromate, nitrate, and sulphate

Mining, chemical refinery, petroleum, textile, steel and agricultural, industries and agricultural industries

Inglezakis, Loizidou and Grigoropoulou, 2002; Martin, and Griswold, 2009; Saif, Kumar and Prasad, 2012

Reactive yellow, reactive blue, methylene blue, safranin-O, methylene blue, orange G, malachite green, and rhodamine blue Herbicides Atrazine, 2,4‑dichlorophenol Pharmaceuticals Ibuprofen Microorganisms Bacteria Escherichia coli and Staphylococcus aureus Viruses Poliovirus-1

Textile, printing, tanning, cosmetic, pharmaceutical and agricultural industries

Cardiovascular disorders, cancer, diabetes, emphysema, hypertension, mellitus, renal damage, and skeletal malformation Mutagenicity/genotoxicity and cancer

Polyatomic compounds Organic compounds

•

•

•

• •

Dyes

Faecal and Soil pollution

or red (due to dinoflagellate and zooplankton) (Brooks et al., 2016; Shin et al., 2017). Suspended minerals: Calcium carbonate and iron are examples of minerals that impart the green and red colour to water/wastewater, respectively (Chaussemier et al., 2015). Toxic heavy metals: The presence of toxic heavy metals also affects the turbidity/colour of water. For example, the reddish colour of acid mine drainage (AMD) is due to iron (Johnson and Hallberg, 2005). Organic substances: Tannin, lignin and humic acids from decaying vegetation give a blue colour to water (Reid and Wood, 1976). Stains: Dyes used in various contemporary industries such as methylene blue give colour to water (Yagub et al., 2014). Detergents: Foaming due to a wide range of detergents causes cloudy/murky water (Goel and Kaur, 2012).

Most of the objectionable odours in water are due to pollutant contamination and can reveal the possible forms of pollutants present. Decaying organic matter, sewage, and oils/petrochemicals are all associated with objectionable odours (Achudume, 2009; Mnaya et al., 2006; Wing et al., 2014). Wastewater generated by domestic, commercial and industrial sectors are currently treated and/or purified to reduce these aesthetic issues using numerous conventional approaches. The main types of conventional wastewater treatment methods are classified as: physical separation, biodegradation, and chemical decomposition methods (Lofrano and Brown, 2010). Physical separation is the simplest treatment method involving the recovery of undissolved suspended pollutants in wastewater via gravity separation and/or centrifugal separation (Lofrano and Brown, 2010). Although the method is simple, physical treatment of wastewater is not absolute, with poor and unstable processing rates making it difficult to meet emission standards. Biodegradation involves the dissolution/conversion of colloidal and suspended organic pollutants in sewage into stable, non-hazardous substances by microbial action (Lofrano and Brown, 2010). Although emission standards are easily achieved, problems associated with biological treatment methods are equipment investment, high operating costs, difficulty in removing heavy metal ions and the inability to operate in winter (Henze et al., 2001; Oller et al., 2011). Chemical decomposition on the other hand, involves the use of water treatment agents like coagulants (Sarkar et al., 2006), flocculants (Lee et al., 2014) and corrosion inhibitors (Weiss et al., 2006) to control the formation of scale and sludge, reduce foam, reduce corrosion of materials in contact with water, remove suspended solids and toxic substances in water, deodorize, soften and stabilize water quality (Tansel, 2008; Hayat et al., 2015; Yu et al., 2017).

Waterborne diseases such as diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome

Christin et al., 2004; Anjaneyulu, Chary and Raj, 2005; Mathur and Bhatnagar, 2007; Jelić, Petrović and Barceló, 2012; Tang et al., 2012; Gu et al., 2016

Edberg et al., 2000; Ishii et al., 2006; Leclerc, Schwartzbrod and Dei-Cas, 2002; Venieri et al., 2014; Zhan et al., 2014; Zvizdić et al., 2005

The most common chemical treatment method used for disinfecting water supplies throughout the world is chlorination (Arnold and Colford Jr, 2007; Sobsey et al., 2008; Sedlak and Von Gunten, 2011). Chlorination has its benefits and drawbacks but the bottom-line is that it is incapable of complete purification of water and has proven to be ineffective in removing contaminants such as heavy metals, and organic and inorganic compounds (Tibbetts, 1995; Sedlak and Von Gunten, 2011). Moreover, chlorination is known to form harmful byproducts when chlorine reacts with dissolved organic and inorganic compounds in water (Gibbons and Laha, 1999; Deborde and Von Gunten, 2008; Gopal et al., 2007) leading to various forms of cancers (Villanueva et al., 2004; Richardson et al., 2007). Overall, the composition and texture of water would be altered by the addition of chemicals and depending on the complexity of sewage/wastewater treatment processes, the chemical treatment cycles may be long, requiring large amounts of chemical agents that result not only in a waste of resources but also incur high accumulative capital and operational costs (Unuabonah et al., 2017). Furthermore, some of the chemical water treatment agents have convoluted preparation processes, poor treatment effects, formation of residues, incapable of being recycled, and cause secondary pollution (Yu et al., 2017). These limitations associated with physical separation, biodegradation, chemical decomposition renders them unconducive to sustainable development. 3.3. Co-existence of pollutants In reality, contaminated water sources do not contain a single pollutant, instead a mixture of pollutants of different classifications often exists (Gollavelli et al., 2013). This co-existence of pollutants in water was well demonstrated in a recent study that investigated and identified the contaminants present in thirty eight streams across the United States of America (Bradley et al., 2017). Thirty four of the thirty eight streams studied were located in urban- and agriculturalinfluenced areas, while the remaining four were located in areas that had a low population density. In this study, the authors identified a mixture of complex pollutants (range of 4 to 161 organic compounds detected per stream) originating from a combination of residential, municipal, agricultural and industrial sources. The detection of fewer pollutants in the four streams located in areas with low population densities was not an unexpected finding, as observed by the authors of the study (Bradley et al., 2017). These results provide corroborating evidence supporting reported speculations that the complexity, instability and toxicity of the various types/forms of pollutants present in wastewater is exacerbated by the increase in the exploitation and utilization of resources by humans for domestic, industrial and/or agricultural use (Ota et al., 2013; Li et al., 2016). The co-existence of pollutants in water sources worldwide is a frequent occurrence (Arjoon et al.,

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Table 2 Overview of nanomaterials used to simultaneously adsorb co-existing pollutants. Nanomaterial

Nanomaterial preparation method

Type of pollutants treated

Name of pollutants

Initial pollutant concentration

pH range

Optimum pH

Nano-alumina modified with 2,4‑dinitrophenylhydrazine

Chemical synthesis

Metal ions

50 mg/L of each pollutant

1.0–5.5

5.0

Graphene oxide-hydrated zirconium oxide nanocomposites Chitosan-methacrylic acid nanoparticles

Hydro-thermal co-precipitation Polymerization

Metal ions

65 mg/L of each pollutant

1.0–11.0

5.0

50 mg/L of each pollutant

3.0–5.0

5.0

Magnetite nanoparticles

Co-precipitation

Metal ions

20 mg/L of each pollutant

Fixed

3.8

Magnesium oxide

Sol-gel method

Metal ions

100 mg/L of each pollutant

2.0–5.0

5.0

Malachite nanoparticles

Chemical synthesis

50 mg/L of each pollutant

4.0–9.0

4.0

Nano-zirconium oxide-crosslinked-nanolayer of carboxymethyl Nanoscale zerovalent iron (nZVI) and Au doped nZVI particles Nickel sulfide nanoparticles loaded on activated carbon Amino-functionalized silica magnetite

Chemical synthesis

Anionic compounds (polyatomic ions) Metal ions

-Pb(II) -Cd(II) -Cr(III) -As(III) -As(V) -Pb(II) -Cd(II) -Ni(II) -Pb (II) -Cr (III) -Pb(II) -Cd(II) -Chromate -Arsenate -Cr (III) -Cr (VI)

1 M of each metal ions

1.0–7.0

7.0 for Cr (III) and 2.0 for Cr (VI) 9.0

1.0–11.0

8.1

Organic compounds

15 mg-N/L for nitrate 40 mg/L of Cd(II) 17.8 mg/L Methylene blue; Safranin-O 5 mg/L 100 mg/L of each pollutant

7.0–9.0

Sonochemical synthesis

1.0–6.0

2 ± 0.5

Thiourea-modified magnetic ion-imprinted chitosan/TiO2 composite Magnetic multi-walled carbon nanotubes (MWCNTs) Magnetic graphene oxide nanocomposite

Hydro-thermal co-precipitation

Metal ion and organic compound

-Cd(II) -Nitrate -Methylene blue -Safranin-O -Reactive black 5 -Sodium dodecylbenzenesulfonate -Cd(II) -2,4‑dichlorophenol

50 mg/L Cd(II); 10 mg/L 2,4‑dichlorophenol

2.0–8.0

6.0–7.0

Chemical synthesis

Metal ion and organic compound Metal ion and organic compound

Calcium alginate encapsulated Ni/Fe nanoparticles beads Defective TiO2−x

Chemical synthesis

Hierarchical vaterite spherulites

Amino-decorated Zr-based magnetic metal-organic frameworks composites Magnetic metal organic frameworks (MOFs) composite

Chemical synthesis Chemical synthesis

Metal ions

Metal ion and polyatomic ion Organic compounds

Metal ion and organic compound Modified Metal ion and solvothermal method organic compound Injection-precipitation Metal ion and organic compound

-Cu (II) -Atrazine -Cd(II) -Methylene blue -Orange G -Cu (II) -Monochlorobenzene -U(VI) -Humic acid -Cd(II) -Congo red

Solvent-assisted ligand exchange

Metal ion and organic compound

-Pb(II) -Methylene blue

Not indicated

2.0–7.0

Could not access

Metal ion and organic compound

-Pb(II) -Malachite green

10 mg/L for Pb(II) and 46,35 mg/L

Not Not indicated indicated

Co-precipitation

2013). Therefore, it is important that practical applications/technologies that solve ‘real life’ problems in the wastewater treatment industry are developed. 4. Nanotechnology for simultaneous removal of pollutants from water Nanotechnology-based systems offer an inexpensive alternative for developing countries where water and wastewater treatment facilities are lacking or non-existent (Qu et al., 2013). These are low-energy systems and/or processes that use nanoparticles to reduce and/or remove contaminants in water (Qu et al., 2013). Nanotechnology-based water and wastewater treatment solutions have gained increasing attention due to specific properties such as large surface area, antimicrobial activity (Durán et al., 2007), photocatalytic activity (Ghosh and Das, 2015), and chemical stability (Y. Zhang et al., 2018, J. Zhang et al., 2018). Moreover, the majority of nanotechnology-based systems can be moulded to suit various aqueous chemistries and has scope for improvement due to the malleability of their properties (Adeleye et al., 2016). Often, nanotechnologies are secondary pollution free, simple to operate, cost

5 mg/L atrazine; (or 30 mg/L 2.0–9.0 Cu (II)) 3.0–10.0 200 mg/L Cd(II); 90 mg/L methylene blue; 60 mg/L orange G 50 mg/L of each pollutant 2.0–5.0

6.0

10 mg/L of each pollutant

2.0–11.0

500 mg/L of Cd(II) and 20 mg/L of Congo red

Fixed

7.0 for U(VI) 2.0 for Humic acid 6.0

4.0

2.0 for orange G; 10.0 for methylene blue 5.0

effective, endeavour to meet emission standards and have low equipment requirements for potential large-scale application (Savage and Diallo, 2005; Qiao et al., 2014). The typical application of nanotechnologies in water and wastewater treatment includes adsorption, photocatalysis, disinfection, separation and sensing (Zhang et al., 2016). However, in this review, special emphasis is given to some of the most promising multi-functional and synergistic water and wastewater treatment tools/techniques/agents that have been used in: • • • •

Adsorption, Photocatalysis, Disinfection and microbial control, and/or Integrated nanotechnology techniques/systems (combination of adsorption, photocatalysis, disinfection and microbial control)

Herein, the feasibility and efficiency of simultaneously removing coexisting/multiple pollutants using nanoparticles are discussed based on the adsorption, photocatalytic and antibacterial activity of the nanoparticle. Also discussed are several disadvantages related to the nanoparticles in terms of ecotoxicology, recyclability and regeneration.

G.N. Hlongwane et al. / Science of the Total Environment 656 (2019) 808–833

Maximum adsorption capacity

Effect of pH

Recovery method

-100.0 mg/g for Cr(III) -83.33 mg/g for Cd(II) -100.0 mg/g for Pb(II) -95.15 mg/g for As(III) -84.89 mg/g for As(V) -11.30 mg/g for Pb(II) -1.84 mg/g for Cd(II) -0.87 mg/g for Ni(II) -3.44 mg/g for Pb (II) -3.19 mg/g for Cr (III) -2614 mg/g for Pb(II) -2294 mg/g for Cd(II) -82.2 mg/g for chromate -57.1 mg/g for arsenate -1120 μmol/g for Cr (III) -680 μmol/g for Cr (VI)

Adsorption increased with increase in pH Adsorption increased with increase in pH Adsorption increased with increase in pH

-188 mg/g for Cd(II) Not indicated -83.33 mg/g for reactive black -62.5 mg/g for sodium dodecylbenzenesulfonate -256.41 mg/g for Cd(II) (98% degradation efficiency for 2,4‑dichlorophenol) -38.91 mg/g for Cu (II) -40.16 mg/g for atrazine -91.29 mg/g for Cd(II) -64.23 mg/g for methylene blue -0.85 mg/g for orange G Not indicated -65 mg/g for U(VI) -142 mg/g for humic acid -984.5 mg/g for Cd(II) -89.0 mg/g for Congo red -102 mg/g for Pb2+ -128 mg/g for methylene blue -219.00 mg/g for Pb2+ -113.67 mg/g for malachite green

Solvent used for regeneration

No. of cycles

References

Centrifugation Elution

Mixture of nitric acid and methanol

3

Afkhami et al., 2010

Not indicated

Nitric acid

5

Centrifugation Elution

Sodium chloride

3

Luo et al., 2013 Heidari et al., 2013

Not investigated

Magnetic field Not indicated

Not indicated

Adsorption increased with increase in pH Adsorption increased with increase in pH Adsorption decreased with increase in pH for Cr (III) and decreased with increase in pH for Cr (VI) Adsorption increased with increase in pH for Cd(II) and Not studied

Centrifugation Elution

Deionised water

Centrifugation pH reversal

Sodium hydroxide

Not indicated

Not indicated

Not indicted

Not Wang et al., indicated 2014 5 Xiong et al., 2015 Not Saikia et al., indicated 2011 Not Mahmoud indicated et al., 2017

Not indicated

Not indicated

Not indicated

Not indicated

Not indicated

Not indicated

Adsorption decreased with increase in pH

Magnetic separation

pH reversal

Sodium hydroxide

Adsorption increased with increase in pH

Not indicated

Elution

Ultrapure water and sodium hydroxide

5

Adsorption increased with increase in pH for Cu(II) and no significant effect for atrazine Adsorption increased with increase in pH for methylene blue and decreased with an increase in pH for orange G Adsorption increased with increase in pH

Magnetic separation Magnetic separation

Elution

Acidic ethanol solution Hydrochloric acid and ethylene glycol Not indicated

4

Not applicable

5

Not indicated

Not Chen et al., indicated 2018 6 Huang et al., 2018

Not indicated

Regeneration method

813

Elution

Elution

Not indicated

Not applicable Simulated Adsorption increased with increase in pH for methylene blue and decreased with an increase in pH for humic acid Not studied Centrifugation Not indicated Adsorption decreased with increase in pH for Pb(II) and decreased with an increase in pH for methylene blue Not studied

4.1. Adsorption Adsorption is one of the most attractive approaches commonly utilized for the removal of toxic inorganic and organic compounds in water due to its versatility for different water systems, high efficiency, simplicity and low cost of operation (Qu et al., 2013). Model adsorbents should meet the following criteria (Wang et al., 2012): • Offer high adsorption capacity, and selectivity to low concentration of pollutants • Desorption of the adsorbed pollutants from the nano-sorbents' surface should be relatively easy • Be infinitely recyclable • Be non-hazardous To date, a variety of nanocomposites such as metal oxides, magnetic metal organic framework composites, and carbon based adsorbents have been explored for the simultaneous removal of multiple coexisting organic and inorganic pollutants. In this section, the effectiveness of some of the adsorbents listed in Table 2 are reviewed based on the criteria mentioned above.

Centrifugation Elution

Filtration

Elution

Nitric acid, methanol and sodium hydroxide Acetone and ethylenediaminetetraacetic acid

Not indicated Not indicated 4

3

3

5

Su et al., 2014 Ghaedi et al., 2014 Araghi and Entezari, 2015 Chen et al., 2012a Tang et al., 2012 Deng et al., 2013 Kuang et al., 2015 Song et al., 2017

Shi et al., 2018

4.1.1. Simultaneous adsorption of heavy metal ions A 2,4‑Dinitrophenylhydrazine modified nano-alumina was one of the first nanocomposite materials to be studied for simultaneous adsorption of heavy metals from water (Afkhami et al., 2010). It was tested for its ability to adsorb six metal cations, Pb(II), Cd(II), Cr(III), Co(II), Ni (II) and Mn(II). However, the highest adsorption capacities were observed for only three of the six metal ions in the mixture, i.e. 100.0 mg/g for Cr(III), 83.3 mg/g for Cd(II) and 100.0 mg/g for Pb(II) ions (Afkhami et al., 2010). A higher maximum adsorption capacity was obtained using magnesium oxide (MgO) nanoparticles: 2294 mg/g for Cd(II) and 2614 mg/g for Pb(II) (Xiong et al., 2015). Furthermore, the MgO nanoparticles exhibited better recyclability and regeneration with the desorption of both Cd(II) and Pb(II) proving difficult only after five washes during elution tests with water while the 2,4‑Dinitrophenylhydrazine modified nano-alumina was reused three times without any significant changes of the adsorption capacities (Afkhami et al., 2010; Xiong et al., 2015). Although the maximum adsorption capacities observed were higher than those found by Afkhami et al. (2010) when 2,4‑Dinitrophenylhydrazine modified nano-alumina was used; using MgO, Xiong et al. (2015) observed competitive adsorption between the Cd(II) and Pb(II) ions and reported the affinity to follow the order of Pb(II) N Cd(II).

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Competitive adsorption between Pb(II) and other metal ions such as Cr(III) ions during their simultaneous removal using magnetite nanoparticles was also reported by Wang et al. (2014). In the presence of Cr(III), the removal of Pb(II) decreased significantly from 80.6% to 41.4%. However, a negligible reduction of Cr(III) to 38.5% from 42.4% was observed in the presence of Pb(II) (Wang et al., 2014). Overall, Pb(II) appears to have a greater affinity to nanoparticles than Cd(II). For instance, an affinity order of Pb(II) N Cd(II) N Ni(II) was reported during the simultaneous elimination of Pb(II), Cd(II), and Ni(II) ions from aqueous solution using recyclable chitosan-methacrylic acid nanoparticles (Heidari et al., 2013). However, when compared to the aforementioned studies, the maximum adsorption capacities reported were low at 11.30 mg/g for Pb(II), 1.84 mg/g for Cd(II), and 0.87 mg/g for Ni (II) (Heidari et al., 2013). In a combined experimental and modelling study, the simultaneous adsorption of Cd(II) with a radionuclide, U(VI), on Sepiolite was investigated (Huang et al., 2015). The study observed a slightly higher Sepiolite maximum sorption capacity of 27.78 and 16.32 mg/g for Cd(II) and U(VI), respectively. As(III) and As(V) is another metal ion, two of whose oxidative states have been simultaneously removed from aqueous solution using nanomaterials (Luo et al., 2013). However, the difference in adsorption capacity between As(III) and As(V) has been reported to be relatively small. Adsorption capacities of 95.15 mg/g for As (III) and 84.89 mg/g for As(V) were observed during their simultaneous removal from drinking water using graphene oxidehydrated zirconium oxide nanoparticles by Luo et al. (2013). The small adsorption capacity difference was attributed to the adsorbent nanoparticles' good anti-interference ability to co-existing anions. In addition to excellent recyclability, the graphene oxidehydrated zirconium oxide nanocomposite lowered the concentration to 0.002 mg/L in the treated tap and well water; a value that is considerably lower than the recommended drinking water Maximum Contaminant Level (MCL) of 10 mg/L (Luo et al., 2013). 4.1.2. Simultaneous adsorption of polyatomic ions Chromate and arsenate are two of the major polyatomic contaminants that often co-exist in potable water bodies. Their simultaneous control has been attempted using malachite nanoparticles (Saikia et al., 2011), which had a maximum adsorption capacity of 82.2 and 57.1 mg/g for chromate and arsenate, respectively. Relative to arsenate adsorption on the malachite nanoparticles surface, the adsorption of chromate was found to be slightly higher through competitive adsorption studies. Due to competitive binding of chromate and arsenate anions on the hydrophilic malachite nanoparticles surface, the overall adsorption efficiencies (%) in the binary mixture were slightly lower (∼70% for chromate and ∼61% for arsenate) than those observed during the single system study (∼75% for chromate and ∼66% for arsenate) (Saikia et al., 2011). 4.1.3. Simultaneous adsorption of metal ions and polyatomic ions Simultaneous treatment of typically co-existing nitrate and cadmium contaminants from groundwater has been investigated using nanoscale zerovalent‑iron (nZVI) (Su et al., 2014). The study had demonstrated that the removal capacity of nZVI had increased to 188 mg/g from 40 mg/g for cadmium in the presence of nitrate. Moreover, the presence of cadmium was found to increase the removal efficiency of nZVI for nitrate from 30% to 100%. However, nitrite accumulation was observed in systems with low concentrations of cadmium. Doping the nZVI with 1 wt% Au reduced the nitrite yield to b3% from 20% (Su et al., 2014). 4.1.4. Simultaneous adsorption of organic compounds Nickel sulphide nanoparticles were one of the first nanoparticles investigated in the simultaneous adsorption of co-existing dyes, methylene blue and safranin-O (Ghaedi et al., 2014). The nickel

sulphide nanoparticles were loaded on activated carbon thus forming a nickel sulphide nanoparticle-loaded activated carbon (NiS-NP-AC) composite (Ghaedi et al., 2014). The predicted removal percentage for each dye was 99.9% at optimum conditions and the adsorption process was reportedly swift at 5.46 min. Ghaedi et al. (2014) reported a competitive adsorption for the NiS-NP-AC active sites between the two dyes particularly in the presence of safraninO. Although it was reported that the nickel sulphide used in this study was produced from a simple precursor using a facile, ‘green’ method, details about the origin of the activated carbon and/or method of its production were not disclosed. If not synthesized from low-cost and natural materials, the high cost of the activated carbon (Crini, 2006) coupled with the reported adsorption competition could potentially limit the large scale production and use of this composite in water and wastewater treatment. With the vast array of nanoparticles being synthesized, efficient and cost effective methods are being explored. For instance, the simultaneous removal of organic pollutants, Reactive black 5 and sodium dodecylbenzenesulfonate from aqueous solutions has been demonstrated using lower cost amino-functionalized silica magnetite nanoparticles (A-S-MNPs) (Araghi and Entezari, 2015). At optimum conditions (pH 2 and 298 K), the observed maximum adsorption capacity for dodecylbenzenesulfonate and Reactive black 5 were 62.5 and 83.33 mg/g, respectively (Araghi and Entezari, 2015). Since the amino groups and silica play important roles in enabling adsorption at a low pH they increase the effectiveness, capacity and selectivity of the adsorbent towards anionic pollutants (Yang and Feng, 2010; Fu et al., 2011). Silica based adsorbents are not only cost-effective but in acidic conditions, silica provides chemical protection against leaching (Jiang et al., 2014). 4.1.5. Simultaneous adsorption of heavy metal ions and organic compounds In the field of integrative treatment of wastewater contaminated by heavy metals and organic compounds such as dyes, artificially synthesized hierarchical vaterite spherulites have exhibited considerable practical effectiveness in the simultaneous removal of heavy metal ion, Cd (II), and dye, Congo red (Chen et al., 2018). Chen et al. (2018) found these scavengers to have a maximum removal capacity of 984.5 and 89.0 mg/g for Cd(II) and Congo red, respectively. Though the vaterite spherulites generally exhibited excellent performance during the simultaneous removal of Cd(II) and Congo red, an inhibition of the removal capacity of vaterite to Cd(II) at Congo red concentration N100 mg/L was found. This could suggest the presence of adsorptive competition between the pollutants (Chen et al., 2018). Defective TiO2−x is another promising nanomaterial that exhibits excellent simultaneous removal of organic pollutants and metal ions. TiO2−x has been shown to have a maximum adsorption capacity of 65 mg/g for U(VI) and 142 mg/g for humic acid at pH = 5.0 (Song et al., 2017). Amino-decorated Zr-based magnetic Metal-Organic Frameworks Composites (Zr-MFCs) are attractive adsorbents for the removal of Pb (II) ions and methylene blue dye from aqueous solution. The reported adsorption capacity of 102 and 128 mg/g for Pb(II) and methylene blue, respectively, is not only high but found to remain unchanged after 6 recycles thus demonstrating superior reusability (Huang et al., 2018). This adsorption capacity is significantly higher than that reported for the simultaneous removal of Cd(II) ions and ionic dyes, methylene blue and orange G using magnetic graphene oxide nanocomposite (Deng et al., 2013). The study, that tested ultra-pure and tap water, reported maximum adsorption capacities of 91.29, 64.23 and 20.85 mg/g for Cd(II), methylene blue and orange G, respectively. Although a synergistic orange G adsorption was obtained in Cd(II)–orange G binary systems, suppression of Cd(II) adsorption was observed in Cd(II)– methylene blue binary systems with an increase in the concentration of methylene blue (Deng et al., 2013). Simultaneous removal of Pb(II) and a different dye, malachite green, has been demonstrated using another magnetic metal organic frameworks (MOF) composite, Cu-

G.N. Hlongwane et al. / Science of the Total Environment 656 (2019) 808–833

MOFs/Fe3O4 adsorbent (Shi et al., 2018). The adsorption capacities of Cu-MOFs/Fe3O4 were found to be 113.67 mg/g for malachite green and 219.00 mg/g for Pb(II) (Shi et al., 2018). Cu-MOFs/Fe3O4 exhibited a higher maximum adsorption capacity for Pb(II) than Zr-MFCs (Huang et al., 2018 and Shi et al., 2018). The organic compounds that have been simultaneously removed together with heavy metal ions are mostly dyes, with a few tests conducted with herbicides as well. Cadmium and a herbicide, 2,4‑dichlorophenol were among the first to be simultaneously treated using a thiourea-modified magnetic ion-imprinted chitosan/TiO 2 (MICT) composite adsorbent. A maximum adsorption capacity of 256.41 mg/g was observed for cadmium, while at the initial concentration of 2,4‑dichlorophenol being 10 mg/L a degradation efficiency of 98% was found and remained fairly constant after 5 cycles (Chen et al., 2012a). Atrazine is another herbicide that was simultaneously treated with Cu (II) using magnetic multi-walled carbon nanotubes (MWCNTs) (Tang et al., 2012). The study showed a maximum adsorption capacity of 40.16 and 38.91 mg/g for atrazine and Cu (II), respectively using magnetic MWCNTs. The results suggested a preferential binding capacity for Cu (II) by the magnetic MWCNTs (Tang et al., 2012). 4.2. Photocatalysis The speed and depth of degradation of pollutants in water can be increased significantly through photocatalysis (Prihod'ko and Soboleva, 2013). Photocatalysis is the use of a light active catalyst or oxidative auxiliary medium to photo-initiate and/or photo-assist the degradation of pollutants (Bora and Dutta, 2014; Oppenländer, 2007). The underlying oxidative reaction mechanism of photo-initiated/photo-assisted degradation of contaminants relies primarily on the formation of highly reactive radicals (Oturan and Aaron, 2014). There are numerous comprehensive reviews on the application of solar energy in water treatment processes, with in-depth explanations of the basic principles, chemistry and mechanisms behind photocatalytic degradation of pollutants in water (Oppenländer, 2007; Oturan and Aaron, 2014; Y. Zhang et al., 2018; Bora and Dutta, 2014; and Wang et al., 2015). Therefore, these concepts will not be replicated in this article. The blanket term advanced oxidation processes (AOPs) is generally used to collectively describe photochemical processes and/or technologies, (Oppenländer, 2007; Horváth et al., 2012). With currents attempts to overcome water scarcity, many nanoparticle-associated AOPs have been investigated for water treatment and purification (Bora and Dutta, 2014; Wang et al., 2015; Baniamerian and Shokrollahzadeh, 2016; Y. Zhang et al., 2018). As a result, a number of nanoparticleassociated AOP based pollutant removal advanced oxidation technologies (AOTs) and/or enhanced oxidation processes (EOPs) have been developed (Oppenländer, 2007). The nanoparticle-associated AOTs and/or EOPs used in wastewater treatment can be separated (depending on the source of the formation of reactive radicals) into the following main processes: • UV wastewater treatment/heterogeneous semiconductor photocatalysis, • Photocatalytic ozonation, • Fenton/Photo-Fenton reactions, and • Non-photochemical AOP methods (these include sulphate based AOTs and electro-Fenton reactions).

Although the attempt of simultaneous photocatalytic degradation of co-existing pollutants using nanomaterials and/or nanocomposites has been reported, it has not been reviewed. Some of the key areas that were found in open literature where photocatalysis has played a crucial role in treating co-existing pollutants in water are compiled and described in Table 3.

815

4.2.1. Heterogeneous semiconductor photocatalysis 4.2.1.1. Simultaneous removal of heavy metal and organic pollutants. Toxic heavy metal ions and organic pollutants are two different types of pollutants that commonly co-exist in wastewater. As photocatalysis seems to be an ecologically viable solution, advances in the field of simultaneous removal of these pollutants from water is necessary. Heterogeneous titanium dioxide (TiO2) photocatalyst/nanoparticles have been investigated for their ability to simultaneously photo-reduce metal ions (copper and silver) and organic dyes (methylene blue) under anoxic conditions and ultra-violet irradiation (Doong et al., 2010). The study reported an almost complete photodegradation of methylene blue by TiO2 nanoparticles within 60 min when the metal ion and methylene blue were treated simultaneously. However, when compared to tests whereby the metal ions were added prior to methylene blue, the rate of degradation decreased by 1.2–1.4 times due to electrons from the TiO2 nanoparticles being competed for by the metal ions and organic dye (Doong et al., 2010). The usefulness of TiO2 is dependent on ultraviolet irradiation, which may limit the deployment of this nanotechnology in countries with inconsistent power supply (Wang et al., 2015). In photocatalytic degradation, the exploration of nanomaterials' response in the visible light range is a current topic of interest. Synchronous removal of heavy metal ion, hexavalent chromium (Cr(VI)) and organic pollutant, phenol, using a gold-loaded graphene oxide/PDPB (polymer poly(diphenylbutadiyne)) composite (Au-GO/ PDPB) under visible radiation has been demonstrated whereby, a phenol degradation rate of 49.4%, and a Cr(VI) reduction rate of 77.4% were reached in approximately 4 h (Liu et al., 2018). 4.2.1.2. Simultaneous removal of multi-organic pollutants. As previously mentioned, different category pollutants often co-exist. However, it is also important to note that different forms or varieties of single/same category pollutants can also co-exist. For example, the textile and agricultural industries often use different types of toxic organic compounds in different forms (Ghasemi et al., 2013). Recently, Silva et al. (2017) demonstrated simultaneous photocatalytic treatment of organic dyes using nanoparticles. Toxic organic compounds, phenol and rhodamine B (RhB) were simultaneously treated using titanate nanotubes-cyanocobalamin materials, showing a reduction of 87% of 20 mg/L of phenol and 94% of 10 mg/L of rhodamine B (RhB) within 90 min of visible light irradiation and 0.2 g/L of the catalyst (Silva et al., 2017). Nevertheless, not all studies that explored photocatalysis as a technique for simultaneous photodegradation of different types of organic pollutants have seen such promising results. For example, the simultaneous photodegradation of methyl orange and bisphenol A (BPA) using a gold/carbon co-doped titanium dioxide photocatalysts showed significantly lower photodegradation rates comparable to monocomponent treatment system (Nyamukamba et al., 2018). After 300 min, only 24% of 10 mg/L methyl orange and 5.38% of 10 mg/L BPA were reportedly degraded when the contaminants were mixed, while the percentage degradation in a mono-component treatment system was found to be 57.16%, and 47.10% for methyl orange and BPA, respectively. Meanwhile, Nyamukamba et al. (2018) observed that on comparing the bi-component and mono-component treatment systems, the photocatalytic degradation of methyl orange was higher than that of BPA. This was attributed to methyl orange selectivity, due to its higher molar absorption coefficient than BPA, higher adsorption of methyl orange on the photocatalyst, and the subsequent reduction of the adsorption sites for BPA. Doping of the TiO2 is not only a modification strategy aimed at enhancing its photocatalytic efficiency, but this is a known method used in photocatalysis to extend the nanomaterial's light response to the visible light region (Wang et al., 2015). A difference in photocatalytic degradation rate between multiple organic contaminants was also reported for a 15% MoO3 loaded SBA-15

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G.N. Hlongwane et al. / Science of the Total Environment 656 (2019) 808–833

Table 3 Overview of simultaneous photocatalytic degradation of pollutants using nanocomposites and/or semiconductors. Type of AOP

Heterogeneous semiconductor photocatalysis Heterogeneous semiconductor photocatalysis Heterogeneous semiconductor photocatalysis Heterogeneous semiconductor photocatalysis Heterogeneous semiconductor photocatalysis Heterogeneous semiconductor photocatalysis

Photocatalyst/nanomaterial

Nanomaterial preparation method

Pollutant types

Name of pollutants

Initial concentration of pollutant

Gold/carbon co-doped titanium dioxide

Chemical synthesis

Organic dyes

Gold-loaded graphene oxide/PDPB (polymer poly(diphenylbutadiyne)) composites Titanate nanotubes-cyanocobalamin

Mechanical agitation and photoreduction method Hydrothermal treatment

Heavy metal ions and organic pollutants Organic pollutants

TiO2–graphene nanocomposites modified with noble metal Pt

Photoreduction and chemical reduction method Photoreduction method

Organic pollutants

Mesoporous silicate SBA-15 doped with 15%MoO3

Sol-gel method

Organic pollutants

Modified fly ash nanoparticles

Not indicated

Heavy metal ion and organic dye

Cobalt ferrite nanoparticles

Microwave irradiation

Organic pollutants

-Phenol -Paracetamol

5 mg/L of each pollutant

FeIIFeIII 2 O4 nanoparticles loaded on powered activated carbon

Chemical co-precipitation

Organic pollutants

-Aniline -Benzotriazole

40 mg/L of aniline and benzotriazole solutions

TiO2 nanopartcles

Not indicated

Organic pollutants

Photocatalytic ozonation

TiO2

Not indicated

Organic pollutants (emerging contaminants)

Photocatalytic ozonation

TiO2 nanoparticles immobilized on ceramic plates

Not indicated

Organic pollutants

Photocatalytic ozonation

Magnetic MWCNTs/TiO2 nanocomposites

Co-precipitation and Sol-gel method

Organic pollutants (emerging contaminants)

Photocatalytic ozonation

TiO2 nanoparticles immobilized onto a montmorillonite support

Hydrothermal synthesis

Organic pollutants

Heterogeneous activation of PS

Nanoscale zero-valent iron supported on bentonite

Liquid-phase reduction method

Heterogeneous activation of PS

Hematite

Sonochemical and calcination

Heavy metal ion and organic pollutant Organic pollutants

-2-chlorophenol -2,4‑dichlorophenoxyacetic acid -Sodium dodecylbenzenesulfonate -Sodium butylnaphthalenesulfonate - Benzyldodecyldimethylammonium bromide -Atenolol (ATL) -Hydrochlorothiazide (HCT) -Ofloxacin (OFX) -Trimethoprim (TMP) -Methyldopa (MDP) -Nalidixic acid (NAD) -famotidine (FAM) - Atenolol (ATL) -Hydrochlorothiazide (HCT) -Ofloxacin (OFX) -Trimethoprim (TMP) -Metronidazole (MET) -Ciprofloxacin (CIP) -Acetaminophen (APAP) -Cr(VI) -Phenol

Electro-Fenton reaction with H2O2

Pd/Fe3O4

Chemical synthesis

Fenton and/or photo-Fenton reaction Fenton and/or photo-Fenton reaction Fenton and/or photo-Fenton reaction Photocatalytic ozonation

Titanium dioxide nanoparticles

photocatalyst (El-Salamony et al., 2017). The multiple organic pollutants studied were polycyclic aromatic hydrocarbons (PAHs) (i.e., anthracene, benzene, naphthalene, and pyrene), phenols (i.e., chlorophenols, hydroxyphenyl, and phenol), and dyes (i.e., methyl red and methyl orange). Although, no significant difference was observed between the dye contaminants, methyl red and methyl orange, the photocatalytic degradation rate order of naphthalene b pyrene b anthracene b benzene within aromatic contaminants, and phenol b chlorophenols ≈ hydroxyphenyl within the phenolic group

Metal ions and organic dye

Heavy metal ion and organic pollutant

-Methyl orange -Bisphenol A

10 mg/L of each pollutant

-Hexavalent chromium (Cr (VI)) -Phenol

10 mg/L of each pollutant

-Phenol -Rhodamine B

20 mg/L of Phenol 10 mg/L of Rhodamine B

-2,4‑dichlorophenoxy acetic acid (2,4-D) -Reactive red 195 (RR195) -Copper -Silver -Methylene blue (MB) -Polycyclic aromatic hydrocarbons (PAHs) -Phenols -Dyes -Methyl-orange -Cadmium

20 mg/L of each pollutant

-2,4-dichlorophenoxyacetic acid (2,4-D) -2‑methyl‑4‑chlorophenoxyacetic acid (MCPA) -Cr(VI) -Humic acid

1 mM of each pollutant

550 mg/L of PAHs, 350 mg/L of phenols, and 100 mg/L of dyes 10−4 M Methyl-orange; 5 × 10−3 M cadmium

0.50 mM of each contaminant

10 mg/L of each pollutant

8 mg/L MAD, 8 mg/L NAD, AND 8 mg/L FAM 10 mg/L of each Emerging contaminants

25 mg/L MET, 5 mg/L CIP, and 5 mg/L APAP 0.38 mM Cr(VI) and 0.11 mM phenol 200 mg/L solution of a mixture of 2,4‑D and MCPA

100 mg/L humic acid and 20 mg/L Cr(VI)

were observed (El-Salamony et al., 2017). Overall, after 120 min under ultraviolet irradiation the degradation rate between the contaminants was found to be 95%, 69%, and 14% thus, ranking in the order of PAHs N phenolics N dyes, respectively. The results suggested that 15% MoO3 loaded SBA-15 catalyst would be efficient for the treatment of PAHs and phenolics containing wastewater produced by the petroleum and petrochemical industry but not efficient for the treatment of dye containing water such as wastewater produced by the textile industry (El-Salamony et al., 2017).

G.N. Hlongwane et al. / Science of the Total Environment 656 (2019) 808–833

Light source

pH range tested

Optimum pH

Range of catalyst weight

Optimum catalyst weight (g/L)

817

Percentage degradation

No. of cycles

24% of MO and 5.38% of BPA in 30 min

Not Nyamukamba indicated et al., 2018

2.0 wt% GO and 1.0 wt% 49.4% of phenol and reduction rate of Au Cr(VI) reached to 77.4% in 4 h

Not Liu et al., 2018 indicated

0.5 wt% gold

References

Visible

Not indicated

Not indicated

0–1 wt% Au

Visible

Not indicated

Not indicated

1–10 wt%

Visible

Not indicated

Not indicated

Not indicated

0.2 g/L

94% of Rhodamine B and 87% of phenol in 90 min

Ultra-violet and visible

Fixed

7.0

Fixed

80 mg/L for RR195 and 200 mg/L for 2,4-D

Not indicated

Ultra-violet

Fixed

7.0

Fixed

1 g/L

Almost 100% of MB in 90 min

Not Doong et al., indicated 2010

Ultra-violet

Not indicated

Not indicated

Fixed

15% Mo (w/w)

95% of aromatic, 69%, phenolic, and 14% of dyes in 120 min

Not El-Salamony indicated et al., 2017

Ultra-violet

Fixed

Fixed

30 wt% fly ash

80% of cadmium and 70% of methyl-orange in 90 min

Ultra-violet

Fixed

8.6 for methyl-orange; 9.9 for cadmium 3.0

Fixed

0.2 g/L

100% degradation of both pollutants

Ultra-violet

Fixed

3 ± 0.2

0.1–0.5 g/L

0.4 g/L

70.4% of aniline and 99.5% of benzotriazole in 120 min

Ultra-violet

Not indicated

Not indicated

0–2.0 g/L

1 g/L

Rate(TOC) = 4.7 ppm ± 1.6 L k/J

Ultra-violet and visible

Fixed

7.0 ± 0.8

Fixed

250 mg/L TiO2 (19 mg/L ozone)

95% removal of emerging contaminants in b30 min.

Ultra-violet and visible

Fixed

7.0

Fixed

0.240 wt%

95.03% MDP, 84.93% NAD, and 99.15% Not Fathinia et al., FAM in 30 min indicated 2015

Ultra-violet and visible

3.0–9.0

7.0

Fixed

200 mg/L

65.7% total organic carbon decrease in 120 min

Ultra-violet A

Not indicated

Not indicated

Fixed

0.04 g/L

64.60% MET, 80.58% CIP, and 50.12% APAP in 15 min

Not used

Fixed

5.0

Fixed

0.50 g/L (based on nZVI)

99.30% of Cr(VI) and 71.50% of phenol in 120 min

4

Diao et al., 2016

Not used

3.0–9.0

6.0 ± 0.2

Hematite (0.1–0.6 g/L) and PS (0.005–0.025 M)

0.2 g/L Hematite and 0.02 PS

68.1% of 2,4-D and 74.5% of MCPA in 120 min

10

Kermani et al., 2018

Not used

1.0–9.0

3.0

3.0–5.0 wt% Pd

5.0 g/L Pd/Fe3O4 (5.0 wt% Pd)

90% of humic acid and 90% of MCPA in 120 min

8

Huang et al., 2017

4.2.2. Photocatalytic ozonation Ozonation is a well-known disinfection technology that has over the years gained notoriety in the field of wastewater treatment (Brienza and Katsoyiannis, 2017). In its basic form, ozonation can be defined as the use of ozone as an oxidation medium to decompose pollutants. The low solubility of ozone and its selectivity towards certain organic compounds under acidic conditions limits the applicability and efficiency of conventional ozonation technologies that use ozone alone (Mehrjouei et al., 2015). Accordingly, the desirability, applicability and

4

Silva et al., 2017

Not Ghasemi indicated et al., 2013

Not Visa and Duta, indicated 2013 Not Rad et al., indicated 2015 5

Ahmadi et al., 2017 Oyama et al., 2011

Not Márquez indicated et al., 2014

5

Yu et al., 2015

Not Hassani et al., indicated 2017

efficiency of ozonation processes in wastewater treatment are often improved by adding light irradiation and/or catalysts to the oxidation medium (Mano et al., 2015; Dong et al., 2015). The combination of photoactivated nanoparticle photocatalysts with ozone (i.e. photocatalytic ozonation) is of interest in this paper (Dong et al., 2009). It is a fairly new topic of research; hence the exact mechanisms implicated in photocatalytic ozonation processes are not well elucidated. However, several proposed mechanisms have been discussed in recently published review articles (Mehrjouei et al., 2015; Xiao

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et al., 2015). Although several studies are available regarding photocatalytic ozonation of the removal of single contaminants, the analysis of mixtures of pollutants using nanoparticle associated photocatalytic ozonation is not frequent. As such, in this section nanoparticles associated with photocatalytic ozonation advances in simultaneous multipollutant control are explored. The most commonly used semiconductor material for photocatalytic ozonation reactions is TiO2 (Moreira et al., 2015; Xiao et al., 2015). Using TiO2 catalysts in photo-ozonation does not only increase the rate of pollutant removal but can also reduce ecotoxicity and enhance the biodegradability of the effluent (Moreira et al., 2015). This was demonstrated by Márquez et al. (2014), in an investigation of TiO2 nanoparticles as a catalyst for the photocatalytic ozonation of a mixture of four pharmaceutical compounds (atenolol, hydrochlorothiazide, ofloxacin and trimethoprim) in a photoreactor tank (gas–liquid separator) under solar radiation. Combining ozone with both solar illumination and TiO2 catalyst was found to degrade the pharmaceutical compound mixture more effectively than when either UV or TiO2 was used alone. In the mineralization study, it was found that the photocatalytic degradation of the pharmaceutical compound mixture (10 mg/L of each pollutant) resulted in ∼95% TOC decrease (40% mineralization) after 30 min, and a further degradation of ∼90% of the phenolic intermediates produced (Márquez et al., 2014). Photocatalytic degradation of 0.50 mM 2,4‑dichlorophenoxyacetic acid (2,4‑D), 0.50 mM bisphenol-A (BPA), 0.50 mM sodium dodecylbenzenesulfonate (DBS), 0.50 mM sodium butylnaphthalenesulfonate (BNS), and 0.50 mM benzyldodecyldimethyl- ammonium bromide (BDDAB) using solar radiation and suspended TiO2 was investigated in a pilot-plant-scale Pyrex glass tubular-type photoreactor (Oyama et al., 2011). In this pilot scale study, mineralization was enhanced and complete mineralization of the various classes was reached within 3 h. This study showcases the feasibility and potential utilization of nanoparticles assisting photocatalytic ozonation in large scale practical applications in wastewater treatment. Nonetheless, the use of TiO2 as suspended powder (Oyama et al., 2011) has the following drawbacks that can impede their practical application in wastewater treatment (Chen et al., 2012b): • • • •

Agglomeration of suspended TiO2 powder at high loadings, Quick e−/h + recombination, Low adsorption ability, and Cumbersome recovery

Therefore to overcome these limitations and improve the photocatalytic activity, the TiO2 is immobilized on appropriate supports (Khataee et al., 2009; Chen et al., 2012b). TiO2 nanoparticles immobilized onto montmorillonite support was used by Hassani et al. (2017) under ultra-violet A irradiation in the presence of ozone to simultaneously degrade a mixture of three pharmaceutical compounds. Metronidazole (MET), acetaminophen (APAP), and ciprofloxacin (CIP) were the three pharmaceuticals studied. In the presence of the TiO2 catalyst (0.04 g/L) and 10 L/h ozone flow rate- 64.60%, 50.12%, 80.58% of 25 mg/L MET, 5 mg/L APAP, and 5 mg/L CIP were simultaneously degraded in 15 min, respectively (Hassani et al., 2017). However, the study did not investigate the effectiveness of the immobilized TiO2 nanoparticles on further degradation of identified intermediate by– products. Using TiO2 nanoparticles immobilized on ceramic plates, higher photocatalytic removal efficiencies of three other pharmaceuticals, i.e. methyldopa (MDP), famotidine (FAM), and nalidixic acid (NAD) were reported (Fathinia et al., 2015). 95.03%, 99.15%, and 84.93% of 8 mg/L MAD, 8 mg/L FAM, 8 mg/L NAD were simultaneously degraded in 30 min (at a 6 L/h ozone flow rate under ultra-violet A irradiation) using TiO2 nanoparticles immobilized on ceramic plates, respectively (Fathinia et al., 2015). The differences in findings observed by Fathinia

et al. (2015) and Hassani et al. (2017) are noteworthy illustrations of how the ozone flow rate/concentration, pollutant concentration, susceptibility of the target pollutant, and contact time can affect the effectiveness of photocatalytic ozonation (Sumegová et al., 2013; Ahmed and Haider, 2018). TiO2 deposited onto magnetic multi-wall carbon nanotubes (MWCNTs) have been used to degrade four pharmaceutical compounds i.e., hydrochlorothiazide (HCT), atenolol (ATL), trimethoprim (TMP), and ofloxacin (OFX) which are frequently found in surface waters and wastewater treatment plant effluents (Yu et al., 2015). It was observed that reasonable removal of the mixture of the four contaminants from the water by ozone alone was possible. However, the mineralization degree achieved through ozone alone was low (44.9%). Accordingly, integrating ozonation with solar photocatalytic assisted by TiO2/MWCNTs increased the mineralization efficiency by almost 21% (Yu et al., 2015). 4.2.3. Fenton/photo-Fenton reactions Photo-Fenton reactions are AOP methods that involve the use of a H2O2 oxidant and ferrous ion catalyst to generate hydroxyl radicals under acidic conditions and light/sunlight irradiation. This described reaction is referred to as a Fenton reaction when performed under no light/sunlight irradiation (Oturan and Aaron, 2014). The use of nanoparticles as heterogeneous Fenton and/or photo-Fenton catalysts has been widely reflected in several reports that study the removal of individual pollutants such 2,4‑dichlorophenol (Guo et al., 2017); Atrazine (Benzaquén et al., 2017); methylene blue (Chen et al., 2016), methylorange (Xu et al., 2018), petachlorophenol (ThanhThuy et al., 2013), rhodamine B (Chen et al., 2013) and tetrabromobisphenol A (An et al., 2013a). Recently, a surge of studies have also explored heterogeneous Fenton and/or photo-Fenton catalytic activity of nanoparticles in the removal of multiple pollutants. However in most of the reports, the ‘simultaneous’ treatment of the multiple pollutants is not clearly reflected. As such, most reports demonstrate the ability of nanoparticles to act as heterogeneous Fenton and/or photo-Fenton catalysts and the nanoparticle(s)' efficiency in the degradation of various kinds of pollutants; but whether the multiple pollutants under investigation are removed simultaneously or treated in mono-components systems is ambivalent. For example, • The photo-Fenton catalytic ability of BiFeO3 nanoparticles to efficiency degrade 30 μmol/L Methyl Violet, 10 μmol/L Rhodamine B, and 3 mmol/L phenol under visible light at 2.21 × 10−2, 5.56 × 10−2 and 2.01 × 10−2 min−1 degradation rates, respectively, has been demonstrated by An et al. (2013b). • The photo-Fenton catalytic ability of a TiO2/Fe2TiO5/Fe2O3 tripleheterojunction nanomaterial to degrade methyl orange and phenol under ultra-violet and visible light irradiation (Deng et al., 2017a, b). Only a few studies were found reporting clear simultaneous removal of pollutants using photo-Fenton systems. In one study a fly ash nanoparticle based visible light active photo-Fenton system was developed for potential simultaneous removal of a heavy metal ion (cadmium) and organic dye (methyl-orange) that often co-exist in textile and dye finishing industries wastewater (Visa and Duta, 2013). At optimum conditions (contact time = 90 min; amount of fly ash = 37.35 wt%; amount of Fe2+/H2O2 = 3 × 10−3 mol L−1 Fe2+/30% H2O2) this fly ash nanoparticle based visible light active photo-Fenton system reached removal efficiencies of up to 88% and 70% for the heavy metal and dye, respectively. FeIIFe2IIIO4 nanoparticles loaded on powered activated carbon (PAC@ II Fe Fe2IIIO4) have also been represented as viable options for simultaneous removal of multiple pollutants from water (Ahmadi et al., 2017). The PAC@FeIIFe2IIIO4 composite was used as a heterogeneous catalyst in a photo-Fenton system under ultra-violet irradiation to treat wastewaters containing mixed aniline and benzotriazole. Excellent

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removal efficiencies of 70.4% for aniline and 99.5% for benzotriazole were not only obtained but the degradation of aniline and benzotriazole in the binary systems was found to be higher than in the monocomponent systems. Moreover, this mesoporous PAC@FeIIFe2IIIO4 composite exhibited great recyclability as it maintained stability for five consecutive cycles of use (Ahmadi et al., 2017). Like most nanomaterial assisted photocatalytic simultaneous degradation of pollutants, the degradation efficiencies of the pollutants are not the same as was reported by Ahmadi et al. (2017) and Visa and Duta (2013). The degradation efficiency of one pollutant is usually higher than the rest. Using photo-Fenton processes assisted by 0.2 g/L of cobalt ferrite nanoparticles -Rad et al. (2015) reported complete (100%) degradation of 5 mg/L phenol and 5 mg/L paracetamol in b200 min under ultraviolet light irradiation (50 mM of H2O2). As can be seen in Table 3, most studies had lower contact times which could explain the incomplete degradation of other pollutants. 4.2.4. Non-photochemical AOP methods The AOTs discussed in Sections 4.2.1, 4.2.1 and 4.2.3 are photochemical processes (Brienza and Katsoyiannis, 2017). Other AOP methods that are not necessarily photo-chemical are making contributions towards the shift from single pollutant control to multiple pollutant control such as electro-Fenton and sulphate radical AOP/advanced oxidation technology based on sulphate radicals. The narrow operating pH range and low solubility of hydroxyl radicals limits their applicability in wastewater treatment (Dewil et al., 2017). Hence, innovations based on sulphate radicals as alternatives to hydroxyl based AOTs are gaining attention (Dewil et al., 2017). Sulphate radicals are generally produced by activation of peroxydisulphate (PDS), persulphate (PS), or peroxymonosulfate (PMS) by transition metal ions, photo radiation, heat, etc. (Deng and Ezyske, 2011). Nonetheless, in recent times heterogeneous activation of PMS/PS/PDS by nanoparticle has sparked scientific interest (Dewil et al., 2017). Nanoparticles are explored due to their low cost, large surface area, strong reductive capacity and high reactivity (Crane and Scott, 2012). As a result, several studies describing nanoparticle heterogeneous activation of PMS/PS/PDS in the degradation of different pollutants exist in literature (Liang et al., 2012; Ji et al., 2013; Jaafarzadeh et al., 2017; Qin et al., 2017; Ghanbari and Moradi, 2017). Heterogeneous activation of PMS/PS/PDS by nanoparticles has already become the focus of recent developments of multi-purpose water and wastewater treatment nanoparticle based AOTs for simultaneous treatment of multiple co-existing pollutants. Nevertheless, heterogeneous activation of PMS/PS/PDS by nanoparticles for multiple pollutant control is not yet widespread. Furthermore, no applications of nanoparticles in the heterogeneous activation of PMS and PDS for simultaneous removal of multiple pollutants from wastewater were found in open literature. Herein, recent advances in heterogeneous activation of PS by nanoparticles for simultaneous removal of multiple pollutants from wastewater are summarized. In recent times the activation of PS for simultaneous treatment of Cr (VI) and phenol polluted water was considered (Diao et al., 2016). As a novel approach, nanoscale zero-valent iron supported on bentonite (BnZVI) was used to activate PS. The PS/B-nZVI displayed enhanced removal rates of 71.50%, and 99.30% compared to B-nZVI (6.50%, and 99.90%) for phenol and Cr(VI), respectively. B-nZV maintained its stability and reactivity for 4 cycles of reuse. The nZVI particles are known to aggregate which is a drawback for water treatment systems (Stefaniuk et al., 2016). By supporting the nZVI particles on bentonite, the reactivity of nZVI particles was increased and aggregation was reduced (Diao et al., 2016). Simultaneous degradation of herbicides, 2‑methyl‑4‑chlorophenoxyacetic acid and 2,4‑dichlorophenoxyacetic acid through activation of PS by hematite nanoparticles was also recently reported (Kermani et al., 2018). Removal rates of 68.1% for 2,4‑dichlorophenoxyacetic acid and 74.5% for 2‑methyl‑4‑chlorophenoxyacetic acid were achieved after 120 min.

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Several nanomaterials such as Cu2O nanocubes (Ai et al., 2008), graphene oxide (Le et al., 2015), nanoscaled BiFeO3 (Luo et al., 2010) and ZnFe2O4 nanoparticles (Wu et al., 2013) have been used to demonstrate single-pollutant control in water. Studies on nanoparticle based electro-Fenton reaction for multiple controls are limited. In one study, the electro-Fenton AOP method assisted by Pd/Fe3O4 nanoparticles was used to remove hexavalent chromium and humic acids from water (Huang et al., 2017). The Pd/Fe3O4 was reused eight times and removed 90% of initial total organic carbon values of 100 mg/L humic acid as well as 90% of the 20 mg/L of Cr(VI) in 120 min. 4.3. Disinfection and microbial control Nanotechnology is opening a new platform to combat and prevent waterborne diseases using nanomaterials. Metallic and metal oxide nanoparticles such as silver and titanium oxide are among the most promising and extensively studied nanoparticles with good antimicrobial properties (Liu et al., 2012). Studies have shown the antimicrobial efficacy of various nanoparticles against bacteria such as E. coli and their antifungal (Sharma and Ghose, 2015) and antiviral activities (Huy et al., 2017). Yet, demonstrations of the antimicrobial efficacy of nanomaterials in simultaneous removal of broad-spectrum microorganisms are scarce. As a result, only one study that used nanoparticles to simultaneously remove bacteria and viruses was found. Unlike chemical disinfection practices such as chlorination, nanotechnology is becoming the preferred purification technique for removal of microorganisms from potable water and wastewater as it does not produce harmful by-products (Zhang et al., 2010). Rapid and simultaneous removal of pathogenic bacteria (E. coli O157:H7) and viruses (Poliovirus-1) from water, has been demonstrated using aminemodified magnetic (Fe3O4–SiO2–NH2) nanoparticles (Zhan et al., 2014). The study reported a 97.4% nonspecific removal efficiency of viruses or E. coli O157:H7 while exhibiting a 92–96.3% efficiency in the detoxification of river samples (Zhan et al., 2014). Due to the existence of co-existing conditions for broad-spectrum microbial growth, the need for convenient and simultaneous removal of bacteria and viruses is critical. Since the shift from single pollutant control to multiple pollutant control is relatively new, the mechanisms involved in the simultaneous removal of broad-spectrum microorganisms using nanoparticles are not clear. Nonetheless, mechanisms hypothesized/reported for single pollutant control using nanoparticles in water could be implicated for multiple pollutant control using nanoparticles. The possible mechanisms include (but are not limited to) the following: • The release of toxic nano ions resulting in disinfection through protein damage and suppression of DNA replication in the microorganisms (Xiu et al., 2012; Prabhu and Poulose, 2012). • The production of reactive oxygen species as a result of a complimentary interaction between the nanoparticles, microorganisms and pollutants (Sirelkhatim et al., 2015; Cai et al., 2017). • Direct adhesion of nanoparticles to microbial surface thus altering/ disrupting the cell membrane and/or cell wall (Li et al., 2008; Hajipour et al., 2012). • Uptake and intracellular accumulation of nanoparticles leading to microbial inactivation (Li et al., 2008; Dimapilis et al., 2017).

4.4. Integration of nanotechnology techniques 4.4.1. Adsorption and photocatalysis In water and wastewater treatment, adsorption and photocatalysis have attracted substantial attention both individually and as an integrative unit (Fosso-Kankeu and Mishra, 2017; Pi et al., 2018). Integrating adsorption and photocatalysis enhances the degradation efficiency of the pollutants treated (Pi et al., 2018). Most studies that used the combination have restricted their study to purification of single pollutants in

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water (Jain et al., 2007; Mu et al., 2017). However, as researchers shift from single pollutant control towards multiple pollutant control, the integrative use of adsorption and photocatalysis is being increasingly adopted in water/wastewater to simultaneously treat: (a) heavy metal and organic pollutants (Benjwal and Kar, 2015), and (b) multi-organic pollutants (Chen et al., 2017). This section reviews the current research progress on simultaneous removal of pollutants using the integration of adsorption and photocatalysis (Table 4). Simultaneous adsorption/degradation of inorganic heavy metal, Pb (II), and organic (methylene blue dye) pollutant from water has been demonstrated by combining the photocatalytic properties of TiO2 with the adsorption properties of AC/CE using titania/activated carbon/carbonized epoxy (TiO2/AC/CE) nanocomposite (Benjwal and Kar, 2015). TiO2/AC/CE adsorbs ~26% of methylene blue while ~2% of methylene blue was adsorbed by pristine TiO2. Overall, the TiO2/AC/CE nanocomposite had higher activity than bare TiO2 nanoparticles and exhibited successful removal of ~97% of Pb(II) ions and ∼90% of methylene blue dye from water (Benjwal and Kar, 2015). Another successful simultaneous removal of metal ions and organic pollutants using photocatalysis and adsorption was demonstrated through a Zn-Fe mixed metal oxide nanocomposite for the removal of a pharmaceutical (ibuprofen) and toxic metal (arsenic) (Di et al., 2017). Upon examination of the mutual effects of the pollutants on removal efficiency under solar irradiation, the study found that Zn-Fe mixed metal oxide nanocomposite had a maximum adsorption capacity of 176.3 mg/g for arsenic while degrading up to 95.7% of ibuprofen in the mono-component system. Although ibuprofen had no significant impact on the removal efficacy of arsenic, the degradation of ibuprofen showed significant decrease relative to arsenic species and their concentration (Di et al., 2017). Nonetheless, the Zn-Fe metal oxide nanocomposite maintained an overall efficiency that was N85% for seven cycles, thus rendering it as a promising nanomaterial for simultaneous removal of coexistent pharmaceutical and toxic metal pollutants from water (Di et al., 2017). The role of adsorption on photocatalytic degradation was evaluated by Chen et al. (2017) using a ZnO/oliganiline nanocomposite for simultaneous degradation of methylene blue, Rhodamine B and Congo red. The ZnO/oliganiline nanocomposite exhibited a higher catalytic degradation rate than ZnO nanoparticles without the added oligoaniline for dye degradation under simultaneous adsorption and photocatalysis mode. This suggests synergism between the adsorbent component and photocatalyst component within the nanocomposites. Moreover, it shows that the adsorbent acts as a supplementary support to the catalyst. However, it is important to note that this role of adsorption on photocatalysis could not be generalized for the ZnO/oligoaniline nanocomposites due to a decrease in the degradation efficiencies for Congo red during simultaneous adsorption and photocatalysis (Chen et al., 2017).

4.4.2. Adsorption, disinfection and microbial control Adsorption and antibacterial activity have been extensively studied for the removal of toxic metals ions (Hua et al., 2012), microorganisms (Amin et al., 2014), and toxic organic compounds from water (Gupta et al., 2013). The removal of organic compounds, microorganisms and toxic heavy metals is usually demonstrated on a single pollutant control standpoint while in reality toxic metals, microorganisms and organic compounds are the types of pollutants that often co-exist in wastewater (Park et al., 2010). Hence, nanomaterials that possess both adsorption capabilities and antimicrobial activity are attractive for use in multipollutant control and treatment of water and wastewater. However, most studies have focused on adsorption and antibacterial activity individually while only a few of the studies have focused on adsorption and antibacterial activity together (Purwajanti et al., 2015). Even fewer studies have explored the collective nature between adsorption and antibacterial activity for the simultaneous removal of organic compounds, microorganisms and toxic heavy metals. One of the promising candidates found in the literature that collectively used adsorption, disinfection and microbial control for simultaneous removal of toxic metal ions and pathogenic microorganism is an inorganic metal oxide, MgO nanoparticle (Cai et al., 2017). The nanosized MgO was found to be highly effective for simultaneous bacterial inactivation and heavy metal removal from aqueous solutions. Unexpectedly, the presence of Cd2+ improved the bacterial inactivation activity of MgO nanoparticles. Cai et al. (2017) attributed this enhanced antibacterial activity to a complimentary interaction between the MgO, bacteria and metal ions. The complementary interaction results in the production of reactive oxygen species followed by membrane destruction that allows for the entry of the metal ions into the bacterial cells thus accelerating bacterial inactivation. Simultaneous treatment of chemical (cationic and anionic dye) and biological (Gram negative and Gram positive bacteria) pollutants from wastewater through adsorption, disinfection and microbial control has also been demonstrated using a clay/alumina/silver composite adsorbent (Yahyaei et al., 2014). The antimicrobial mechanism in silver-mediated nanocomposites has been reported to be a multiple step mechanism that involves the release of silver ions that cause protein damage followed by suppression of DNA replication in the bacteria (Qu et al., 2013). Despite being highly effective in bacterial control, the toxicity of silver nanoparticles to humans and the environment not only limiting but a growing concern (Fewtrell, 2014). Hence Unuabonah et al. (2017) suggested that the authors could have probably done without silver in this nanocomposite since clay minerals are known natural antimicrobials. In another study, Wu et al. (2018) designed and manufactured an inorganic/organic composite nanomaterial of covalently connected magnetic silica (MS) core and polyethylenimine derived quaternary ammonium compound (QAC) corona for the simultaneous removal of

Table 4 Photocatalytic degradation and adsorption of some pollutants/simultaneous adsorption and photocatalysis under different light irradiations. Photo-catalyst

Titania/activated carbon/carbonized epoxy (TiO2/AC/CE) nanocomposite Zn-Fe mixed metal oxides ZnO/oligoaniline nanocomposites

Pollutant Types

Name of pollutants

Solvothermal process

Inorganic (metal ions) and organic pollutants

-Pb(II) -Methylene blue (MB)

Visible

Co-precipitation and calcination

Metal ions and organic pollutants Organic pollutants

-Ibuprofen -Arsenic

Solar

Nanomaterial preparation method

Sonochemical synthesis

-Methylene blue (MB) -Rhodamine B (RhB) -Congo red (CR)

Light source

pH range

Optimum pH

Catalyst weight

Optimum catalyst weight (g/L)

Degradation %

No. of cycles

References

Not Benjwal ∼97% of the Pb(II) ions in 600 min indicated and Kar, 2015 ∼90% of MB dye

Fixed pH

6.5

Not indicated

Not indicated

3.0–9.0

3.0

0.25 to 2.0 g/L

0.5 g/L

95.7% of ibuprofen

6

Di et al., 2017

Fixed weight

20 mg/50 mL

98.2% of MB, 96.7% of RhB and 95.4% of CR degraded in 100 min

7

Chen et al., 2017

Ultra-violet 3.0–9.0 pHMB = 7.2, pHRhB = 7.1, and pHCR = 7.3

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bacteria (Escherichia coli), anionic dye (acid fuchsin), and metal ion (Cu2 + ). The multifunctional QAC-MS composite material was found to have high bactericidal activity towards Escherichia coli and maximum adsorption capacities of 73.0 mg/g for Cu2+ and 653 mg/g for acid fuchsin. Though the multifunctional QAC-MS composite exhibited excellent water purification properties by demonstrating high stability and superior reusability (Wu et al., 2018), more research is required to consolidate its use in large scale procedures. As shown in Table 5, the adsorption capacities of the nanomaterials are rather high except for magnetic graphene (Gollavelli et al., 2013). Although lower maximum adsorption capacity of 4.86 mg/g for Cr(VI), 3.26 mg/g for As(V), and 6.00 mg/g for Pb(II) were observed, the study reported excellent antimicrobial activity (40 μg/L and 100% killing efficacy) towards the bacteria, Escherichia coli (Gollavelli et al., 2013). 4.4.3. Photocatalysis, disinfection and microbial control Several studies have presented nanocomposites with both photocatalytic and microbial properties for the simultaneous treatment of different categories of pollutants. Pant et al. (2013) used TiO2/ZnO decorated carbon nanofiber (CNF) composite to showcase its photocatalytic ability to simultaneously degrade toxic chemical dye, methylene blue and a microbial contaminant, Escherichia coli. The nanocomposite was shown to have excellent antibacterial activity, fast methylene blue degradation and simultaneous adsorption ability under ultra-violet irradiation (Pant et al., 2013). However, the catalyst was reportedly recovered with unchanged adsorption and degradation efficiency for only 2 cycles (Pant et al., 2013). The reduction in methylene blue degradation during the third cycle raises an important issue of a possible limitation of the nanocomposite for large scale commercial use. Furthermore, possible bacterial re-growth has been reported with ultra-violet irradiation (Wang et al., 2015). TiO2 (by itself) has been used for simultaneous photocatalytic degradation of organic pollutants and disinfection of pathogens and has been comprehensively reviewed by Tsydenova et al. (2015). Other nanocomposites studied for simultaneous photocatalytic degradation of organic pollutants and disinfection of microorganism include Er-Al co-doped ZnO photo-anode (J. Zhang et al., 2018); mesoporous Fe3O4@Ag@TiO2 nanocomposite (Tseng et al., 2017);

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macroporous Ag/TiO2 composite (Wang et al., 2017a); and Ag/AgCl– magnetic activated carbon composites (McEvoy and Zhang, 2014). The most notable aspect about the nanocomposites is their design and assembly. For instance, the mesoporous Fe3O4@Ag@TiO2 nanocomposite was designed and assembled such that the Fe3O4 and Ag form the nanocomposite core enclosed within a TiO2, a mesoporous shell. The porous nature of the shell was a strategic implementation to allow the slow release of Ag ions, thus displaying anti-microbial activity over prolonged periods of time (up to 48 h) (Tseng et al., 2017). The same nanocomposite design and assembly is seen for the Ag/TiO2 composite foams where the Ag core is enclosed within the mesoporous TiO2 shell (Wang et al., 2017a). Wang et al. (2017a, b) not only reported a 99% bactericidal efficiency within 3 h, but this assembly of Ag and TiO2 nanoparticles led to stability in its structure. It is also worth noting that researchers are embracing multi-pollutant control by exploring the multi-functionality of nanoparticles during treatment. The economic value of the water treatment process is also increased by concomitantly generating useful products like generating electricity in addition to pollutant degradation and disinfection as demonstrated by J. Zhang et al. (2018). Studies of simultaneous production of energy and multipollutant removal using nanomaterials would fit the scope of this review; however, other photocatalytic studies reporting simultaneous production of energy and removal of multiple pollutants using nanomaterials were not found. Nanomaterials that exhibit both photocatalytic activity and disinfection capabilities have found use in the simultaneous remediation of harmful algal blooms and the toxins (also known as cytotoxins) they produce (Antoniou et al., 2014). The presence of cytoxins in water is considered secondary pollution. In practice, cytotoxins leak into the water during the inactivation and cell destruction of algal cells (Wang et al., 2014). Using TiO2 nanoparticles for simultaneous degradation of a cyanobacteria and algal toxin was demonstrated under ultra-violet light by Pinho et al. (2015). The cyanobacteria and algal toxin treated were Microcystis aeruginosa and microcystin-LR (MC-LR), respectively. A 200 mg/L TiO2 loading was used to obtain complete degradation. Wang et al. (2018) also presented simultaneous inactivation of Microcystis aeruginosa and MC-LR but used a novel poly dimethyl diallyl ammonium chloride (PDDA) modified N,P co-doped TiO2/expanded graphite carbon layer (NPT-EGC) floating photocatalyst under visible

Table 5 Overview of adsorption, disinfection and microbial control. Nanomaterial

Nanomaterial preparation method

Type of pollutants treated

Name of pollutants

Maximum adsorption capacity −73.0 mg/g for Cu2+ −653 mg/g for acid fuchsin −4.86 mg/g for Cr (VI) −3.26 mg/g for As(V) −6.00 mg/g for Pb(II) Not indicated

Sonochemical synthesis

Metal ions, bacteria and organic pollutants

-Cu2+ -Escherichia coli -Acid fuchsin

Magnetic graphene

Microwave irradiation

Metal ions, and bacteria

-Cr(VI) -As(V) -Escherichia coli

MgO nanoparticles

Sol-gel and calcination method Sol-gel and photo-generation method

Bacteria and organic pollutants Bacteria and organic pollutants

-Cd2+ -Escherichia coli

Magnetic silica core and a polymer quaternary ammonium corona nanoparticle

Clay/alumina/silver nanoparticle composites

-Reactive yellow (RY) -Reactive blue (RB) -Escherichia coli -Staphylococcus aureus

−82.08 mg/g for RB −12.87 mg/g for RY

Antimicrobial mechanism

No. of cycles

References

Membrane destruction

5

Wu et al., 2018

–

–

Gollavelli et al., 2013

Reactive oxygen species production, and membrane destruction followed by the entry of metal ions into bacterial cell thus accelerating bacterial inactivation Release of silver ions, protein damage followed by DNA replication suppression

Not Cai et al., indicated 2017 Not Qu et al., indicated 2013 Yahyaei et al., 2014

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light. The TiO2 floating photocatalyst showed a removal efficiency of 88.89% for algae and 83.00% for MC-LR (initial concentrations = 0.4 mg/L) in 9 h. In other works, simultaneous inactivation of Microcystis aeruginosa and MC-LR (initial concentration 0.1 mg/L) was again demonstrated using F-Ce-TiO2/expanded perlite floating photocatalysts (Wang et al., 2017b). A higher removal efficiency of 94.4% for Microcystis aeruginosa and 91.0% for MC-LR in 9 h was observed with this system. The MC-LR initial concentration used by Wang et al. (2017b) was lower than that used by Wang et al. (2018) which could partly explain the high removal efficiencies for MC-LR observed in the former study. The production of reactive oxygen species (Wang et al., 2017b) and direct adhesion of nanoparticles to microbial surface followed by cell lysis (Pinho et al., 2015) were reported as the primary mechanism on cytotoxin disinfection and microbial control by Wang et al. (2017b) and Pinho et al. (2015), respectively. The underlying mechanisms/pathways involved in the degradation of MC-LR using doped TiO2 photocatalysis have been analysed and discussed in-depth in a recent review by Hu et al. (2017). 5. Challenges in multipollutant control Adsorption is one of the most commonly utilized water and wastewater remediation techniques due to its versatility for different water systems, high efficiency, simplicity and low cost of operation (Qu et al., 2013). Photocatalytic water treatment has also been known since the 1980s (Mazzarino, 2001). The efficiency of both techniques to treat various classes of pollutants has been demonstrated in a number of scientific studies (Robert and Malato, 2002; Bahnemann, 2004; Chong et al., 2010; Dong et al., 2015; Pi et al., 2018). The integration of these well-established techniques with nanotechnology was clearly demonstrated in Section 4. The number of new/improved nanotechnology based innovative applications in water and wastewater treatment continues to grow (Cates, 2017). Yet they remain ‘experimental’/‘laboratory-based’ technologies that have rarely demonstrated survival capabilities in large-scale water and wastewater treatment plants despite positive demonstrations of novel nanomaterial features/inventions (Cates, 2017). Therefore, it is no surprise that multiple pollutant control nanotechnologies also face a number of challenges in practical implementation. Challenges in simultaneous removal of co-existing pollutants; batch-to-batch predictability; recovery, management, and disposal of exhausted nanomaterials; regeneration and recyclability; management and disposal of pollutants after desorption/regeneration; and ecotoxicity of nanomaterials are discussed in this section of the paper. 5.1. Efficient co-removal of co-existing pollutants The composition of wastewater produced by various industries is complex due to the presence of co-existing pollutants such as pathogenic microorganisms, toxic organic and inorganic compounds. The biological, physical, and chemical characteristics of these pollutants differ tremendously. This challenges the use of nanoparticles for multiple pollutant control in water and wastewater treatment processes. Nanoparticles do exhibit the ability to remove multiple pollutants. However, the nanoparticles' removal/degradation efficiencies vary from one pollutant to another. The presence of pollutant ‘A’ may affect (i.e., enhance or inhibit) the removal of pollutant ‘B’, while the removal of pollutant ‘A’ is independent to the presence of pollutant ‘B’ as was observed by Cai et al. (2017), Ghaedi et al. (2014), and Wang et al. (2014). In some instances the co-existence may mutually enhance the removal of both pollutants as per Su et al. (2014) observations with nZVI. The removal capacity of nZVI increased for cadmium in the presence of nitrate and the presence of cadmium was also found to increase the removal efficiency of nZVI for nitrate. Overall, as discussed in Sections 4.1 to 4.4, the presence of multiple pollutants resulted in nanoparticles showing

significantly lower adsorption, photodegradation or disinfection rates compared to mono-component treatment systems in some studies (Saikia et al., 2011; Wang et al., 2014; Ghaedi et al., 2014;Xiong et al., 2015; Nyamukamba et al., 2018). The major effect of the co-existence of multiple pollutants was concluded to be competitive interaction/binding on the nanoparticle between the co-existing pollutants. Competitive interaction/binding occurs when several species vie for a single adsorption/oxidation site (Mansouri et al., 2015; Kampalanonwat and Supaphol, 2014; Yu et al., 2016). Nanoparticles would then show higher adsorption, photodegradation or disinfectant rates for the conquistador species. The adsorption/oxidation efficiencies of the species are dependent on the kinetics and thermodynamics of the adsorption, photodegradation or disinfection. The rates of adsorption, photodegradation or disinfection are influenced by the: ○ Binding capacity of pollutant to the nanoparticle: The higher the binding capacity, the greater is the rate of removal in a solution. In a mixed pollutant system, the binding capacity of a given pollutant is reportedly lower than in a mono-component system (Tang et al., 2012; Wang et al., 2014). ○ Binding affinity of the pollutant towards the binding sites of nanoparticles: The greater its binding affinity to the nanoparticles in the presence of competing pollutants, the faster it is degraded. This explains why the presence of one pollutant can inhibit and/or enhance the degradation of the other (Chen et al., 2018; Wang et al., 2014; Xiong et al., 2015). ○ Gibbs free energy: Thermodynamically, the oxidation of a pollutant that generates the least Gibbs free energy will be more favourable than the oxidation of a pollutant that results in higher Gibbs free energy generation under standard conditions. Hence, considerable removal of the pollutant with favoured oxidation will occur before the removal of the pollutant with the least favoured oxidation process under standard conditions (Araghi and Entezari, 2015; Saikia et al., 2011).

These aspects basically highlight that it is difficult to use nanoparticles to treat multiple pollutants in water. Especially in circumstances where one type of nanoparticle is used to treat multiple pollutant of low concentrations. Consequently, to treat multiple pollutants in water under practical real conditions using nanoparticles it is important to: • Understand the underlying mechanisms that govern interactions between co-existing pollutants; and then • Use these mechanisms to guide the development of novel nanocomposite materials that are made up of several nanoparticles which are each selective to a specific kind of pollutant.

5.2. Batch-to-batch predictability An understanding of adsorption, photodegradation, and disinfection processes under practical conditions during simultaneous treatment of pollutants could prove invaluable for the scaling up of these processes (Lyu et al., 2016). This could also enhance the batch-to-batch predictability of the performance of the nanomaterials. Trends observed in practical parameters such as pH, initial concentrations of pollutants, dose of nanoparticles, and contact time by the studies analysed in this paper are outlined here. 5.2.1. Initial concentrations of pollutants Most of the studies reviewed in this paper reported differences in the removal efficiencies of pollutants in the high and low concentration range. As can be seen in Tables 2 and 3, the adsorption studies generally

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Table 6 Overview of biotoxicity and environmental risks of nanotechnology. Scopus bibliographical survey: Number of publications related to nanoparticles- 427,798 Number of nanoparticle-related patents- 236,249 Number of publications related to the toxicity of nanoparticles- 20,035 Number of publications related to the regulation of nanotechnologies- 2433 Types of nanomaterial Fullerenes

Representative nanoparticles Buckminsterfullerenes, carbon nanotubes

Metals

Iron. gold, platinum, silver

Metal oxides

Titanium oxide, zinc oxide, iron oxide

Other nanoparticles

Dendrimers, nanocomposites, polystyrene, quantum dots

Toxicity

References

-Toxic to daphnia specifically Daphnia magna also known as water flea -Toxic to fish such as Danio rerio, Micropterus salmoides, Largemouth bass, and Oncorhynchus mykiss -Toxic to bacteria such as Escherichia coli, Bacillus subtilis, and Shewanella oneidensis (inhibits bacterial growth) -Toxic to human cells lines -Toxic to earthworms (Eisenia foetida) -Toxic to bacterial such as Bacillus subtilis and Escherichia coli (inhibits bacterial growth) -Toxic to fish such as Danio rerio also known as zebra fish -Toxic to mammalian cells: causing brain, liver and stem cells damage -Dermal toxicity: causes skin diseases such as argyrosis in humans -Toxic to algae and plants -Toxic to soil nematodes (Caenorhabdtis elegans) -Toxic to bacterial such as Bacillus subtilis and Escherichia coli (inhibits bacterial growth) -Toxic to earthworms (Eisenia foetida) -Toxic to soil nematodes (Caenorhabdtis elegans) -Inhibits root growth and seed germination (phytotoxic) -Cytotoxic and haemolytic to humans and murine

Cheng et al., 2007; Fortner et al., 2005; Oberdörster, 2004; Roberts et al., 2007; Sayes et al., 2004; Smith et al., 2007; Tang et al., 2007; Petersen et al., 2008

report optimum adsorption at higher concentrations, while photocatalytic processes report optimum degradation at lower concentrations. This effect was also confirmed by Rad et al. (2015) who compared photocatalysis and adsorption for the degradation of phenol and paracetamol in binary systems (Rad et al., 2015). In that study, complete degradation of paracetamol and phenol through a photocatalytic process was found when the initial concentrations of both pollutants were below 100 mg/L; while complete degradation of paracetamol and phenol through an adsorption process was found when the initial concentrations of both pollutants were above 100 mg/L. These results showed that photocatalytic processes may not be effective at degrading high concentration of pollutants. Thus, adsorption processes should be considered as an alternative process for degradation of pollutants present in high concentrations. The study attributed photocatalytic inability to degrade high pollutant concentrations (N200 mg/L) to a limited number of reactive radicals (Rad et al., 2015). Nanoparticles tend to have higher/faster adsorption rates for pollutants with higher concentrations (Chen et al., 2012a; Xiong et al., 2015). At a low concentration, the oxidation reaction rates of pollutants tend to be faster/higher over the surface of catalysts (Lyu et al., 2016). As a result, varying the pollutant concentrations may counteract possible inhibitions between pollutants during their simultaneous degradation in water. An understanding of these phenomena could also prove useful in choosing a removal process that is best suited for the amounts of pollutants present in the wastewater. 5.2.2. The pH of wastewater In reality, the pH of wastewater varies between acidic and alkaline according to the type of pollutants predominantly present in a specific type of wastewater. Therefore, the pH of the wastewater can have an effect on the activity/adsorption capacity of the adsorbents thus either

Braydich-Stolle et al., 2005; Chen and Schluesener, 2008; Griffitt et al., 2007; Hussain et al., 2005; Hussain et al., 2006; Wang et al., 2009; Rana and Kalaichelvan, 2013

Adams et al., 2006; Lin and Xing, 2007; Wang et al., 2009; Khare et al., 2011

Chen et al., 2004; Duncan and Izzo, 2005; Rittner et al., 2002

positively or negatively influencing the adsorption process (Luo et al., 2013). As outlined in Tables 2 and 3, some studies reviewed in this paper reported: • An increase in adsorption capacity of nanoparticles for the co-existing pollutants as the pH increased (Chen et al., 2012a; Araghi and Entezari, 2015; Xiong et al., 2015). • An adsorption rate increase with an increase in pH for one pollutant and a pH variation having no effect on the adsorption capacity of an adsorbent for the co-existing pollutant (Tang et al., 2012). • An increase in adsorption rate with an increase in pH for one pollutant and a decrease in adsorption rate of the co-existing pollutant as the pH increase (Deng et al., 2013).

In general, the nanoparticles had an optimum pH at which the maximum adsorption capacities were reached. The adsorption rates tend to increase or decrease until the optimum pH is reached (Afkhami et al., 2010; Luo et al., 2013). Beyond this threshold adverse effects on the removal efficiencies are reported. Beyond the optimum pH, Chen et al. (2012a) reported precipitation of the nanoparticles leading to a decrease in the removal efficiency of the pollutants. Metal precipitation at greater than the optimum pH that resulted in the deterioration of the adsorbent was also reported by Heidari et al. (2013). These variations in adsorption capacity of nanoparticles with the pH is one the reasons why nanoparticle based adsorbents performed well in controlled laboratory conditions but were difficult to use in real practical conditions to treat multiple pollutant in water. If the maximum adsorption of pollutant ‘A’ on nanoparticles takes place at a pH that is different from that of pollutant ‘B’, different adsorption efficiencies for the pollutants are observed. So when treating multiple pollutants, optimum

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Table 7 Overview of some of the reagents used during regeneration of adsorbents and their ecotoxicity according to the reagents' MSDS sourced from Sigma-Aldrich Corporation. Reagent

Ecotoxicity

Acetone

-Kidney irregularities -Skin dermatitis -Toxic to fish mainly Oncorhynchus mykiss also known as rainbow trout -Toxic to daphnia specifically Daphnia magna also known as water flea -Toxic to fish specifically Lepomis macrochirus also known as Bluegill sunfish -Toxic to daphnia specifically Daphnia magna also known as water flea -Bioaccumulative potential (Bioaccumulation Lepomis macrochirus - 28 days, 80 μg/L) -Toxic to fish specifically Lepomis macrochirus and Oryzias latipes -Toxic to daphnia specifically Daphnia magna also known as water flea -Toxic to Scenedesmus capricornutum (fresh water algae) -Liver irregularities in humans -Corrosive to the respiratory tract -Toxic to fish mainly Oncorhynchus mykiss, Leuciscus idus, and -Pimephales promelas -Toxic to daphnia specifically Daphnia magna also known as water flea -Toxic to fish specifically Lepomis macrochirus also known as Bluegill sunfish -Toxic to Daphnia specifically Daphnia magna also known as water flea -Toxic to fish specifically Lepomis macrochirus also known as Bluegill sunfish -Toxic to daphnia specifically Daphnia magna also known as water flea -Toxic to fish mainly Oncorhynchus mykiss and Gambusia affinis (also known as Mosquito fish) -Toxic to daphnia specifically Daphnia magna also known as water flea

Ethylenediaminetetraacetic acid

Methanol

Nitric acid Ethylene glycol

Hydrochloric acid

Sodium chloride

Sodium hydroxide

conditions for one pollutant may not be necessarily be the optimum conditions for the other co-existing pollutant(s). In real practical conditions, it may be difficult to synchronize the conditions to be optimum for all the pollutants present in water. Maybe the simultaneous removal of pollutants can be designed such that it is sequential, where the degradation/removal of one pollutant results not only in intermediates but the intermediate production also result in low/increases the pH of the solution consequently creating optimum conditions for the removal of the next co-existing pollutant. 5.2.3. Dose of nanoparticle/nanomaterial and contact time The pollutant removal process can be strongly influenced by the dose of the nanomaterial used and contact time with pollutants (Heidari et al., 2013). The removal rate of pollutants generally increases with the nanoparticle dose and contact time up to a threshold. Beyond or below the contact time threshold the removal efficiencies can significantly reduce (Ghasemi et al., 2013; Silva et al., 2017; Deng et al., 2017a, b).

Park, 2013). The powder form is a challenge that has to be faced in order to get nanotechnology inventions to real treatment plants. A considerable number of studies reviewed in this article used centrifugation to recover the nanoparticles after exhaustion (refer to Tables 2, 3, 4 and 5). Separating nanoparticles by centrifugation can be difficult and expensive in real treatment plants (Gulyas, 2014). Immobilization of the nanoparticles on larger and settleable particles such as ceramic plates (Fathinia et al., 2015), montmorillorite (Hassani et al., 2017) and bentonite (Diao et al., 2016) can simplify recovery of exhausted nanoparticles and allow use of the already in-place processes such as sedimentation. Magnetic recovery was also employed by some studies and is promising (Araghi and Entezari, 2015; Tang et al., 2012; Wang et al., 2014). However, its applicability in real treatment plants is currently not feasible (Kudr et al., 2017). The disposal of exhausted nanomaterials (after separation of pollutants from the nanoparticles) is crucial. None of the studies for multiple pollutants control reviewed for this paper addressed this issue. A lack of nanomaterial management and disposal strategies could result in uncontrollable secondary pollution (Lata et al., 2015). The biotoxicity and environmental risks of nanoparticles are summarized in Section 5.6. The recovery, management and disposal of exhausted nanomaterials used in water treatment devices require further exploration. The inability to safely recover nanoparticles from treatment reactors limits the large scale application of multifunctional nanotechnology inventions in water and wastewater treatment. 5.4. Regeneration and recyclability Recyclability and regeneration are important factors that govern the cost-effectiveness of nanomaterials (Qu et al., 2013). Separation of the nanomaterials from pollutants after treatment was achieved by most of the studies through simple elution or pH reversal. Overall, desorption of the pollutants from the surface of the nanoparticles was reportedly easy. As shown in Tables 2, 3, 4 and 5 most of the nanomaterials demonstrated stability and recyclability, with the nanomaterials maintaining almost 100% of their initial properties and adsorption capacity after three to ten regeneration cycles. The authors of most of the studies considered this kind of regeneration high. There is no study that provides guidelines on what can be considered an appropriate number of regeneration cycles. Currently, it appears that any demonstration of recyclability and regeneration would be considered beneficial as it will ultimately result in cost reduction. It should also be noted that all the applications discussed above (Section 4.1.1 to 4.1.5) are laboratory stage research that were not pilot-tested and/or field-tested. Moreover, no economic analysis was performed to render or confirm the recyclability and regeneration. Furthermore, it is noted that none of the studies outlined in Tables 2, 3, 4 and 5 investigated and compared the regeneration and recyclability of the multi-component treatment systems to their mono-component counterparts. Therefore, based on current reports, knowledge regarding any differences that exist in the regeneration and/or recyclability of nanomaterials used for single pollutant and multiple pollutants cannot be conclusively and unbiasedly summarized. Thus, multiple pollutant control studies need to be more comprehensive and evaluate the effects that multiple pollutants (as opposed to a single pollutant) have on regeneration and recyclability of nanomaterials. 5.5. Management and disposal of pollutants after desorption/regeneration

5.3. Recovery, management, and disposal of exhausted nanomaterials The ability to safely and cost-effectively recover (i.e. to separate the nanomaterials from water prior to regeneration/desorption/recycling), manage and dispose exhausted nanomaterials can be used as an indicator for economic feasibility and resourcefulness of nanotechnology based water treatment and purification systems (Lata et al., 2015). Practical studies in the field present nanomaterials with novel qualities and excellent regeneration capacity, but they fail to demonstrate recovery facilities for nanomaterials, particularly those in powder form (Lee and

Management and disposal of pollutants after desorption/regeneration is another issue that requires consideration. No attention was given to the disposal of pollutants in the water systems used for regeneration by any of the studies reviewed. After regeneration/recycling of nanoparticles (i.e., separation of pollutants from the nanomaterials) disposal strategies for pollutants that are not necessarily degraded into less harmful constituents such as in adsorption treatment processes need to be explored. The final pollutant concentration in the water systems used for regeneration may not be safe/suitable for certain end-of-uses such as

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Table 8 Some recent patents of multifunctional nanotechnology inventions for water and wastewater treatment developed worldwide. Country

Patent number

China

CN101497469

China

CN103241793

South Korea

KR20130092106

China

CN104860386

China

CN104973655

China

CN105561952

China

CN106335960

China

CN106423301

China

CN106423300

China

CN106423302

China

CN106587120

Malaysia

MY163255

Type of technology

Inventor(s)

Aluminium/iron/zinc composite for that can urban/sewage water and industrial wastewater treatment Bentonite based multifunctional water treatment agent used for treating refractory wastewater

Ying Fu; Yan Feng; Chunju Zhang; Yuwei Zhang; Zhen Wang Chen Jianjun; Xiang Hai; Tian Shuaihui

Al-Fe/Mg-Si based water treatment agent for simultaneous Park Jung Hwan; Lee Jung Min; elimination of floating colloidal particles, chemical oxygen demand, Kim Jin Woo; Jung Maeng Joon; total phosphorus, total nitrogen, heavy metal, and odour Bae Kwan Ho components in sewage/wastewater Dephosphorization agent simultaneous removal different forms of Gao Xiaolong; Gu Zhaoquan; Duan phosphorus Lianjun; Ji Min; Qiao Yunhong; Fan Jianbing; Dong Junwei Natural zeolite water treatment agent for removal of removal Liu Yanpeng effects on ammonia nitrogen, fluorine, phosphorus, radioactive cesium (137Cs) and strontium (90Sr), organic pollutants and heavy metals in water Multifunctional material PANI-CMC-Fe3O4 for treatment of printing Xie Huifang; Yan Mei; Zhang Qing; and dyeing wastewater Qu Hongxia; Kong Jinming Zhang Bowen Multipurpose polymerized silicate and potassium ferrate based sewage purification agent for removal of ammonia nitrogen, phosphorus, oil contaminants, and heavy metals Fiber/carbon nanotube/Bi2MoO6 three-dimensional recyclable Liu Baojiang; Zhang Jun; Gan Yiming; Zhang Shuai efficient catalytic material for adsorption of heavy metal ions and organic pollutants in water Fiber/carbon nanotube/Bi12TiO2 three-dimensional recycled Liu Baojiang; Zhang Jun; Zhang Shuai; Wang Lili efficient catalytic material fiber/carbon nanotube/Bi12TiO2 three-dimensional recycled efficient catalytic material for removal of bacteria, odour, heavy metal ions and organic pollutants in water Liu Baojiang; Zhang Jun; Gao Pin; Fiber/carbon nanotube/BiPO4 three-dimensional recyclable Liu Lu efficient catalyzing material for removal of bacteria, odour, heavy metal ions and organic pollutants in water Wang Qian; Wang Xiaofei; Shi Laminated composite metal hydroxide and hydrogen peroxide Chunlei water treatment agent for simultaneously and efficiently remove various pollutants Buoyant multifunctional composite material for effective removal Bai Renbi; Han Hui of organic compounds in water and wastewater

drinking water, but recycling pollutants could prove an invaluable disposal method. Pollutants could be selectively recovered from the regeneration water systems and explored for valuable reuse. For instance, pathogenic microorganisms can be harvested and used to synthesize the very nanoparticles used in the wastewater treatment processes. The use of various kinds of microorganisms ranging from bacteria to algae for nanoparticle synthesis is already an increasingly growing research area (Li et al., 2011; Iravani, 2014). Recycling of pollutants (in addition to recycling the nanoparticles) could potentially increase the economic value of nanotechnology based treatment methods. Some of the innovative ways in which the economic value of nanotechnology based water treatment systems can be increased are mentioned in Section 6. 5.6. Ecotoxicity of nanomaterials The field of water and wastewater treatment akin to other scientific and technological fields has been revolutionized by nanotechnology (Ghasemzadeh et al., 2014). As stated in the introduction section, this review sets out to explore the feasibility and efficiency of simultaneous removal of co-existing/multiple pollutants in water using nanomaterials. Thus far, the efficiency of nanotechnologies through various nanoparticles has been sufficiently discussed and/or demonstrated (Section 4.1 to 4.4). However, the long term feasibility of nanotechnology in wastewater treatment cannot be attested to. The application of nanomaterials is not entirely environmentally friendly (Pietroiusti et al., 2018). Nanoparticles have superior qualities/properties compared to microparticles (Crane and Scott, 2012). The long-term stability of

Applicant(s) University of Jinan

Date published 2009-08-05

Hangzhou Yeking 2013-08-14 Environmental Protection Engineering Co. Ltd. C and C CO LTD DR; Park 2014-05-12 Jung Hwan

Gao Xiaolong

2015-08-26

Beijing Guotoushengshi Science and Technology Co. Ltd.

2015-10-14

University of Nanjing Science and Technology Guangxi Yulin Zhengyu Gold And Silver Mineral Tech Dev Co. Ltd. Nat Univ Dong Hwa; Suzhou Kangfu Intelligent Tech Co. Ltd. Nat Univ Dong Hwa; Suzhou Kangfu Intelligent Tech Co Ltd

2016-05-11

Nat Univ Dong Hwa; Suzhou Kangfu Intelligent Tech Co Ltd. Xi'an Tech Univ; Univ Shaanxi Science and Tech National University of Singapore

2017-01-18

2017-02-22

2017-02-22

2017-02-22

2017-04-26

2017-08-30

novel nanomaterials reported in literature has not been tested (Gulyas, 2014). Nanoparticles are believed to be toxic in contrast to their larger counterparts. Nanoparticles used in water treatment might be released into the environment and potentially cause harm to the ecosystem (Pietroiusti et al., 2018). Concerns over the fate, environmental behaviour and toxicity of different classifications of nanomaterials questions the long term feasibility of nanotechnology in wastewater treatment and purification. Therefore, to strategically provide a comprehensive but yet condensed and concise viewpoint of the status quo as researchers shift from single pollutant control to multiple pollutant control using nanoparticles, herein, the biotoxicity and environmental risks of nanomaterials are summarized. The various types of nanomaterials used in water and wastewater treatment can be broadly classified into: fullerenes, metals, metal oxides, and other complex nano-compounds such as nanocomposites, quantum dots, organic polymers, etc. Table 6 lists the different classes of nanomaterials, some of the main representative nanoparticles in each sub-classification, and their biotoxicity/environmental risks. Nanoparticles have been shown to have the potential to induce cardiotoxicity, cytotoxicity, dermal toxicity, genotoxicity, hepatotoxicity, immunotoxicity, and nephrotoxicity in humans (Sahu and Hayes, 2017). Moreover, when released into the environment nanoparticles could have toxic effects to freshwater, aquatic and terrestrial organisms (Baun et al., 2008; Navarro et al., 2008; Salieri et al., 2015; Deng et al., 2017a, b). Buckminsterfullerenes and CNTs are fullerenes with widely studied ecotoxic effects. These have been proven toxic to bacteria, daphnia, earthworms, fish, and human cells lines (Cheng et al., 2007; Fortner et al., 2005; Oberdörster, 2004; Petersen et al., 2008; Roberts et al.,

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2007; Sayes et al., 2004; Smith et al., 2007; Tang et al., 2007). Evidence of biotoxicity associated with low concentrations of some metal nanoparticles such iron, gold, and silver has also been presented. These metal nanoparticles can inhibit bacterial growth; kill certain fish; toxic to terrestrial plants; cause brain, liver and stem cells damage; and/or lead to skin diseases such as argyrosis in humans (Braydich-Stolle et al., 2005; Chen and Schluesener, 2008; Griffitt et al., 2007; Hussain et al., 2005; Hussain et al., 2006; Rana and Kalaichelvan, 2013). In addition to water bodies, most nanoparticles can be transported or transferred to other environments such as soil. In these environments, nanoparticles are not easily degradable, then as a result tend to accumulate (Rana and Kalaichelvan, 2013). The accumulation of some of the most common metal oxide nanoparticles such as titanium oxide, zinc oxide, iron oxide in soil has been associated with phytotoxic effects (inhibition of root growth and seed germination) and toxic effects towards earthworms (Khare et al., 2011; Lee et al., 2010; Lin and Xing, 2007). Other complex nano-compounds have been implicated in cytotoxic and haemolytic effects in humans and murine (Chen et al., 2004; Duncan and Izzo, 2005; Li et al., 2012; Rittner et al., 2002). There is a significant number of articles that comprehensively review the sources, fate, environmental behaviour, and toxicity of different classifications of nanomaterials (Bahadar et al., 2016; Brohi et al., 2017; Buzea et al., 2007; Bystrzejewska-Piotrowska et al., 2009; Fard et al., 2015; Lee et al., 2010; Pietroiusti et al., 2018; Ray et al., 2009; Sahu and Hayes, 2017; Zoroddu et al., 2014). That information can be effectively utilised by various interested parties ranging from environmental toxicologists to material scientists to not only use to critique/assess the safety of nanomaterials but to also guide research ideas/choices when developing nanotechnology based water and wastewater treatments innovations. An exhibition of prudence at the design stage can significantly mitigate potential biological and environmental risks of nanoparticle based technologies. Thus, assuring nanotechnology safety prior and subsequent to its release to the supply chain is critical (Morose, 2010). The methods for the nanomaterial synthesis used by the studies reviewed in this paper (Section 4) have been predominately chemical despite certain studies such as Ghaedi et al. (2014) stating that their nanoparticle preparation method was ‘green’. None of the studies utilized waste to synthesize the nanoparticles either. ‘Green’ nanotechnology/synthesis makes use of environmentally friendly and generally simple low energy input one-pot processes to generate nanomaterials from natural products such as plant extracts (Kharissova et al., 2013). Though nanoparticles have been reported to have adverse effects on both aquatic organisms and human beings, not many of the studies performed placed emphasis on their toxicity (Kumar et al., 2014). This is demonstrated in this paper through a bibliometric survey of literature on the Scopus database summarized on Table 6. In this bibliometric survey we found almost half a million publications related to nanoparticles. Around 4.7% of the publications found made inference to the toxicity of nanoparticles. b1% of publications related to nanoparticles were found to address the regulation of nanoparticles. Table 7 lists the ecotoxicity of some of the reagents that are commonly used during the regeneration of the nanoparticles. The Ecotoxicities are listed according to their Material Safety Data Sheets (MSDS) obtained from Sigma-Aldrich Corporation that cite no available data for toxicity tests such as mobility in soil, bioaccumulative potential, persistence and degradability for most of the reagents. Nonetheless, as shown in Table 7, the reagents are harmful to aquatic organisms and the MSDS have warned against their release into the environment. Although the methods used to synthesize the nanoparticles appear to be non-hazardous, a look at the ecotoxicity of the reagents used during just the regeneration of the nanoparticles is a cause for concern, mainly due to the purely chemical nature of their composition. This calls for a change in the toxicity and safety outlook of nanotechnology applications for water and wastewater treatment. The nanomaterials themselves should not only be non-hazardous but the treatment process in its entirety should be secondary pollution free and use non-hazardous

chemicals. Therefore, as 21st century innovations become dominated by nanotechnology, and the world gears up for the inevitable environmental exposure to nanomaterials (Ray et al., 2009); more prudence within the scientific community is needed with further work focusing on addressing the toxicity of nanoparticles. 6. Commercialization: rising above the scientific hype Commercialisation of academic research is not only a measure of scientific productivity but also presents a pathway to directly contribute to the economy and society (Perkmann et al., 2013). Though nanotechnology currently dominates research studies in water and wastewater treatment, there exists no reliable information about any current commercial multi-pollutant control nanotechnologies. Nevertheless, patents can be used as a direct indicator of the potential commercialisation of technologies (Crespi et al., 2011). Table 8 depicts world-wide patents of multi-purpose/multi-functional water and wastewater treatment nanotechnologies for simultaneous treatment of multiple-control of pollutants. Apart from extracting/drawing correlations between the patents and the research studies in the field of multi-pollutant control in water, the focus was to highlight differences between the proposed research technologies and the patented inventions. It is estimated that the majority of people who do not have access to clean water and basic sanitation live in Africa and Asia (Kumar et al., 2014). As can be observed in Table 8, Asia is making significant contributions in water and wastewater treatment, evident by the number of patents filed by Asian countries. More research and innovation is required from other countries worldwide, particularly African countries, if there is to be any substantial improvement in water treatment technologies. Apart from the general scope of the research, there has been no correlation between the patented technologies and studies published in open literature in terms of the nanomaterials used. This could explain the reason for the advancements made towards multipollutant control systems being at laboratory research stage. Essentially, the number of proposed technologies is high but limited inventions reach the market despite institutional, political, and economic pressures to commercialize potential research developments (Caulfield and Ogbogu, 2015). There exists a wide disconnect between the research directions of publications in the field and real water industry needs (Cates, 2017). Although topical, research published in the field of nanotechnology based water treatment appears to be driven by citation accumulation under the disguise of large-scale prospects over addressing implementation hurdles. At this stage, nanotechnologies are excellent scientific grant/funding magnets but their actual large-scale implementation is not promising. This predicament is exacerbated and/or made acceptable by a surge of publications on materials development that most high-impact journals support over actual process engineering and development (Cates, 2017). The cost associated with recuperating nanoparticles from effluent streams at both laboratory scale and pilot scale has not been thoroughly investigated (Cates, 2017). Over the years, several studies such as Mazzarino (2001); Klamerth et al. (2010); Miranda-García et al. (2011); and Prieto-Rodríguez et al. (2013) have attempted evaluation of nanotechnology based photocatalytic process scaleup. The approach taken by these process scale-up investigations mainly focused on increasing the volume of treated wastewater but not technical specifications such as flow dynamics and materials of construction that influence the large-scale implementation of these processes (Dewil et al., 2017). Nanotechnology based water treatment research needs to go beyond demonstrations and look at possible implementation hurdles and ways to circumvent them. In order to scale up the use of nanoparticles from laboratory processes to industrial-scale applications; it is essential that the overall cost of treatment per unit volume/mass of treated effluent/removed pollutant is estimated (Dewil et al., 2017; Samhaber and Nguyen, 2018).

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Additional aspects that specifically hinder large-scale application of photocatalytic processes in wastewater treatment include the need for a large area of land and inevitable water evaporation in open photocatalytic reactors. These have been highlighted and discussed in greater detail in a review about problems and challenges of solar heterogeneous photocatalytic oxidation processes for water and wastewater treatment (Gulyas, 2014). Braham and Harris (2009) also reviewed in detail all the major design and scale-up considerations required for solar photocatalytic reactors. The quality of scientific research published in open literature is also an important factor. To scale up and/or commercialize scientific research, the economic competitiveness of proposed technologies need to be highlighted in scientific reports. Further scientific demonstrations and modelling studies using real water samples and conditions are also required (Y. Zhang et al., 2018). The reason there has been no correlation between the number of proposed technologies and number of inventions that reach the market could also be due to unrealistic projections of nanotechnology applications in water and wastewater treatment. For example; nanotechnology promises to alleviate issues related to water quality and accessibility in poverty-stricken countries (Adeleye et al., 2016). In reality, the cost-effectiveness of nanotechnology based water treatment applications is contentious. In a recent study performed in Mexico (an emerging nation), a nanotechnologybased water treatment device was found to cost an equivalent of five times the minimum wage (on average) (Olvera et al., 2017). Other issues that hinder commercialization include: • Mass production: Currently, nanoparticle synthesis methods (despite some claiming mass production) are still laboratory-based and unable to deliver large amounts of nanomaterials suitable for industrial-scale use (Prathna et al., 2018). Thus, efforts that focus on designing and developing novel energy efficient nanoparticle synthesis schemes are required (Adeleye et al., 2016). • Quality control and safety: Long term ecotoxicology and environmental safety of nanoparticles is not well understood (Adeleye et al., 2016). The lifecycle of nanomaterials used in water treatment devices need further assessment in more in-depth investigations. • Societal concerns: To ensure public acceptance of water treatment nanotechnologies; the public has to be educated about this exotic technology and given a platform to voice their opinions and concerns (Street et al., 2014). To fully realize societal benefits of nanotechnology-based water treatment inventions, it is critical that the public’s trust is not only gained but also maintained (Duan et al., 2015).

International cooperation; collaborative partnerships between academic researchers, industry and government; and development of start-ups in partnership with experienced business experts could accelerate the commercialization process and should be encouraged. Increasing the economic value of water treatment processes/nanomaterials/ results reviewed in this article could also accelerate their commercialization. The economic value of the water treatment processes associated with nanotechnology can be increased by incorporating side-processes. These processes could include: a) Using nanoparticles to concomitantly treat wastewater and generate renewable energy sources (J. Zhang et al., 2018). b) Using the pollutant-adsorbed nanomaterials from the wastewater treatment process as possible substrates in high-technology applications. For instance: • Dye-adsorbed nanomaterials can be utilised in optoelectronics as dye sensitized solar cells (El-Shishtawy, 2009) and textiles as photochromic dyes (Ayazi-Yazdi et al., 2017). • Other organic compound-adsorbed nanomaterials can be used as substrates in the development of electronic materials such as organic

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light-emitting diodes and organic photovoltaic cells for application in organic electronic devices (Sekine et al., 2014). • Microorganism-adsorbed nanoparticles can be used as sensor elements in bio-chip sensors and/or microelectromechanical systems for biomedical applications (Gurkan et al., 2011; Sah and Baier, 2014). Is the use of nanoparticles in up-scaled processes scientific hype or a prospective technology? It is both – a prospective technology from the lab-scale proof of concept standpoint and scientific hype when it entails meeting the needs of real water and wastewater treatment industry/ market/plants (or when it entails large scale industrial applications/ commercialisation). 7. Conclusion In recent decades efforts have been made towards the remediation of various pollutants in water and wastewater. This paper provides an overview of recent developments in multi-purpose water and wastewater treatment nanotechnologies for simultaneous treatment of multiple coexisting pollutants. Current knowledge about using nanoparticles to remove multiple pollutants from waste/wastewater through adsorption, photocatalysis, disinfection and microbial control is given in a wellstructured presentation based on a gradual increase of the challenging issues that are currently faced in the topic. The scientific data reviewed in this study does suggests that simultaneous removal of microorganisms, inorganic and organic pollutants can be achieved using nanoparticles through adsorption, photocatalysis, and disinfection and/or through the integration of these techniques. These nanotechnologies have an advantage of being free of secondary pollution, simple to operate, cost effective, meeting emission standards and have low equipment requirements for potential large-scale application. However, there are several important issues that need further consideration in research under active investigation and/or future research about the application of nanomaterials in multiple pollutants in water and wastewater treatment plants: • A consistent criteria on what can be considered the best or proper adsorbents, light active catalysts and disinfectants (nanoparticles) for water treatment needs to be defined and ought to consider the toxicity and environmental risks associated with nanoparticles; • A guideline on what can be considered an appropriate number of regeneration cycles needs to be formulated; and the cost associated with recuperating nanoparticles from effluent streams at both laboratory scale and pilot scale needs to be thoroughly investigated; • Recyclability and regeneration is demonstrated by most of the studies, but comparisons between regeneration/recyclability in multi-component and mono-component treatment systems are rarely made. These comparisons are crucial for scaling up the reported studies; • The co-existence of multiple pollutants strongly influences the order of degradation, and removal efficiency thus ultimately resulting in significant batch-to-batch variabilities. Hence, studies that concentrate their efforts into understanding the underlying mechanisms that govern competitive interactions are required along with competitive preventive strategies; • Nanotechnology based adsorption – has been shown to be capable of lowering pollutant concentrations to values (0.002 mg/L) that are considerably lower than the recommended drinking water MCL. However, limited studies have compared pollutant concentrations obtained after treatment to the MCL allowed for the type of water treated, more studies need to address that; • Most of the applications proposed for simultaneous removal of pollutants from water are laboratory stage research that is not pilot-tested and/or field-tested. The research field is saturated with studies on materials development. The field requires more research studies that are steered towards actual process engineering and development; • In order to facilitate large scale application and/or commercialisation

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of nanotechnology for simultaneous multiple pollutant control, the economic competitiveness of proposed technologies need to be highlighted in scientific reports. Further scientific demonstration, modelling and economic analysis studies using real water samples and conditions are required.

Acknowledgements The authors acknowledge financial support from the Department of Science and Technology (DST) and the National Research Foundation (Grant No. 112020). Author contributions All authors were involved in discussing the ideas presented and structuring of the manuscript. The initial manuscript was drafted by G. N. Hlongwane. All the other authors edited revised versions and finalized the manuscript. The final manuscript was read and approved by all the authors. Conflict of interest No conflict of interest is declared by all the authors. References Achudume, A.C., 2009. The effect of petrochemical effluent on the water quality of Ubeji creek in Niger Delta of Nigeria. Bull. Environ. Contam. Toxicol. 83 (3), 410–415. https://doi.org/10.1007/s00128-009-9736-2. Adams, L.K., Lyon, D.Y., Alvarez, P.J., 2006. Comparative eco-toxicity of nanoscale TiO2, SiO2, and ZnO water suspensions. Water Res. 40 (19), 3527–3532. https://doi.org/ 10.1016/j.watres.2006.08.004. Adeleye, A.S., Conway, J.R., Garner, K., Huang, Y., Su, Y., Keller, A.A., 2016. Engineered nanomaterials for water treatment and remediation: costs, benefits, and applicability. Chem. Eng. J. 286, 640–662. https://doi.org/10.1016/j.cej.2015.10.105. Afkhami, A., Saber-Tehrani, M., Bagheri, H., 2010. Simultaneous removal of heavy-metal ions in wastewater samples using nano-alumina modified with 2,4‑dinitrophenylhydrazine. J. Hazard. Mater. 181 (1–3), 836–844. https://doi.org/ 10.1016/j.jhazmat.2010.05.089. Ahmadi, M., Kakavandi, B., Jorfi, S., Azizi, M., 2017. Oxidative degradation of aniline and benzotriazole over PAC@ FeIIFe2IIIO4: a recyclable catalyst in a heterogeneous photo-Fenton-like system. J. Photochem. Photobiol. A. 336, 42–53. https://doi.org/ 10.1016/j.jphotochem.2016.12.014. Ahmed, S.N., Haider, W., 2018. Heterogeneous photocatalysis and its potential applications in water and wastewater treatment: a review. Nanotechnology 29 (34), 342001. https://doi.org/10.1088/1361-6528/aac6ea. Ai, Z., Xiao, H., Mei, T., Liu, J., Zhang, L., Deng, K., Qiu, J., 2008. Electro-Fenton degradation of rhodamine B based on a composite cathode of Cu2O nanocubes and carbon nanotubes. J. Phys. Chem. C 112 (31), 11929–11935. https://doi.org/10.1021/jp803243t. Alcamo, J., Henrichs, T., Rösch, T., 2017. World Water in 2025: Global Modeling and Scenario Analysis for the World Commission on Water for the 21st Century. Report A0002. Centre for Environmental Systems Research, University of Kassel, Kurt Wolters Strasse 3, 34109 Kassel, Germany. Altenburger, R., Ait-Aissa, S., Antczak, P., Backhaus, T., Barceló, D., Seiler, T.B., Brion, F., Busch, W., Chipman, K., de Alda, M.L., de Aragão Umbuzeiro, G., 2015. Future water quality monitoring—adapting tools to deal with mixtures of pollutants in water resource management. Sci. Total Environ. 512, 540–551. https://doi.org/10.1016/j. scitotenv.2014.12.057. Amin, M.T., Alazba, A.A., Manzoor, U., 2014. A review of removal of pollutants from water/ wastewater using different types of nanomaterials. Adv. Mater. Sci. Eng. 2014, 825910. https://doi.org/10.1155/2014/825910. An, J., Zhu, L., Wang, N., Song, Z., Yang, Z., Du, D., Tang, H., 2013a. Photo-Fenton like degradation of tetrabromobisphenol A with grapheneBiFeO3 composite as a catalyst. Chem. Eng. J. 219, 225–237. https://doi.org/10.1016/j.cej.2013.01.013. An, J., Zhu, L., Zhang, Y., Tang, H., 2013b. Efficient visible light photo-Fenton-like degradation of organic pollutants using in situ surface-modified BiFeO3 as a catalyst. J. Environ. Sci. 25 (6), 1213–1225. https://doi.org/10.1016/S1001-0742(12) 60172-7. Anjaneyulu, Y., Chary, N.S., Raj, D.S.S., 2005. Decolourization of industrial effluents– available methods and emerging technologies–a review. Rev. Environ. Sci. Biotechnol. 4 (4), 245–273. https://doi.org/10.1007/s11157-005-1246-z. Antoniou, M.G., De La Cruz, A.A., Pelaez, M.A., Han, C., He, X., Dionysiou, D.D., Song, W., O'Shea, K.E., Ho, L., Newcombe, G., Dixon, M.B., 2014. Practices that prevent the formation of cyanobacterial blooms in water resources and remove cyanotoxins during physical treatment of drinking water. Compr. Water Qual. Purif. 2, 173–195. https:// doi.org/10.1016/B978-0-12-382182-9.00032-3.

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