Application of ZnO nanostructures in ceramic and polymeric membranes for water and wastewater technologies: A review

Journal Pre-proofs Review Application of ZnO nanostructures in ceramic and polymeric membranes for water and wastewater technologies: A review Mahdi Sheikh, Mahdieh Pazirofteh, Mostafa Dehghani, Morteza Asghari, Mashallah Rezakazemi, Cesar Valderrama, Jose-Luis Cortina PII: DOI: Reference:

S1385-8947(19)32888-8 https://doi.org/10.1016/j.cej.2019.123475 CEJ 123475

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

7 August 2019 12 October 2019 11 November 2019

Please cite this article as: M. Sheikh, M. Pazirofteh, M. Dehghani, M. Asghari, M. Rezakazemi, C. Valderrama, JL. Cortina, Application of ZnO nanostructures in ceramic and polymeric membranes for water and wastewater technologies: A review, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123475

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Application of ZnO nanostructures in ceramic and polymeric membranes for water and wastewater technologies: A review Mahdi Sheikh1,2#, Mahdieh Pazirofteh3#, Mostafa Dehghani4, Morteza Asghari4, Mashallah Rezakazemi5, Cesar Valderrama1,2,*, Jose-Luis Cortina1,2,6.

1

Chemical Engineering Department, Escola d’Enginyeria de Barcelona Est (EEBE),

Universitat Politècnica de Catalunya (BarcelonaTECH), 08930 Barcelona, Spain. 2

Barcelona Research Center for Multiscale Science and Engineering, Barcelona, Spain.

3

Department of Chemical Engineering, Kermanshah University of Technology,

Kermanshah, Iran. 4

Separation Processes Research Group (SPRG), Department of Engineering, University of

Kashan, Kashan, Iran. 5

Faculty of Chemical and Materials Engineering, Shahrood University of Technology,

Shahrood, Iran. 6

CETaqua, Carretera d'Esplugues, 75, 08940 Cornellà de Llobregat, Spain.

*Correspondence should be addressed to: César Valderrama Departament d’Enginyeria Química, Universitat Politècnica de Catalunya·Barcelona TECH C/ Eduard Maristany, 10-14 (Campus Diagonal-Besòs), 08930 Barcelona, Spain Tel.: 93 4011818 Email: [email protected] # These authors contributed equally to the work.

Abstract

1

Advances in nanotechnology and nanomaterials have led to the development of nanostructured membranes. Zinc oxide (ZnO), a multifunctional nanomaterial, has been postulated as a filler in polymeric and ceramic membranes to improve properties such as roughness, permeability, and fouling resistance. This review is a comprehensive overview of recent progress on the following: i) ZnO nanostructure preparation and characterization, ii) ZnO growth techniques, iii) fabrication of ZnO-based polymeric and ceramic membranes and iv) environmental application studies in water and wastewater technologies using ZnO-embedded polymeric and ceramic membranes. The review also covers approaches to control membrane properties to reduce fouling and biofouling, increase the solvent flux, increase pollutant rejection, and control swelling and deswelling membrane properties. Finally, the main expected applications on membrane-driven processes for environmental applications of water treatment are also analysed.

Keywords: Membrane; Zinc oxide; Water and Wastewater treatment; Nanocomposite

1. Introduction Requirements for clean water have increased with population growth, rapid urbanization, misuse of water resources, and climate disruption creating a global issue of great concern. Of the seven billion people globally, over 15% do not have access to adequate fresh water for a healthy life [1]. Increasing water contamination from the discharge of waterborne pathogens, inorganic pollutants, such as arsenic, selenium, and heavy metals (e.g., cadmium, lead, mercury and chromium), chemicals used in agriculture, human and animal pharmaceutical derivatives, and endocrine disrupters have exacerbated the problem [1].

2

Conventional methods for water treatment trains include filtration, sedimentation, coagulation, disinfection. These processes require large systems, and they are operationally and chemically intensive, which makes them costly ineffective, and time-consuming. Because water and wastewater contain a large number of contaminants, new robust, effective, and low-cost techniques are required to improve the quality of water and wastewater through disinfection and selective removal of specific harmful pollutants [2]. The most relevant solutions under development and implementation are advanced oxidation processes and membrane driven processes. Nanotechnology offers a diversity of promising nanomaterials for being implemented in filtration materials (e.g., membranes) for selective removal of undesired compounds from water and wastewater. These nanomaterials include zinc oxide (ZnO) [3-13], TiO2 [14-19], BiOBr [20, 21], Fe-ZnIn2S4 [22, 23], BiFeO3 [24], Bi3O4Br [24], and Fe2O3 [25, 26]. ZnO nanomaterials can be incorporated in polymeric and ceramic membranes to improve properties such as roughness, permeability, and antifouling resistance [27-34]. They can be used incorporated into the active layer in membranes used in water and wastewater treatment technologies, into fuel cell membranes, into gas separation membranes, or as active component in pollutant removal technologies [28, 35-37]. Significant and relevant research has been performed on industrial applications of membranes for water treatment [38-40], gas separation [41, 42], medical applications [43], chemical sensors [44, 45], and food technology [46, 47]. Water treatment is an important field of activity in a continuous evolution at industrial scale and most efforts have been focused on membrane technologies for water and wastewater applications (e.g., drinking water production, industrial water make-up for processing stages, cooling applications, purification and decontamination of urban and industrial waters). The use of ZnO as a filler to improve

3

membrane properties has been shown to be reliable for water and wastewater purification and decontamination, such as in photocatalytic applications [48]. This use has been also traduced in innovations in the fabrication and analytical tools incorporating ZnO membranes [48]. ZnO nanostructures are materials with great potential, which are used in active layers of polymers and ceramic membranes [49, 50]. ZnO nanomaterials have concerned much attention due to their astonishing characteristics, such as: i) dipole-dipole structure, ii) high active surface area, iii) excellent photocatalysis properties with its wide bandgap (3.37 eV), iv) low toxicity and v) the possibility for getting shaped in different nanoshapes. ZnO semiconductors have excellent photocatalytic activity and degrade pollutants under UV and near-visible light [51]. ZnO with the nanosize structures provide excellent stability, which increases crystallinity (into the polymeric matrix) and due to the fewer defects in comparison with other semiconductors, it can be a perfect candidate for using as a filler in polymeric and ceramic matrices [52]. The ZnO containing surface, which is positively charged, increases the migration of mentioned electrons from inside of nanocrystals onto the surface of ZnO and stops carrier recombination. Hence, not all the semiconductors such as ZnO, without modifier perform well enough under sunlight irradiation, and structural modifications are needed [53]. Then, this nano-oxide can compete the commercialized materials such as TiO2 and can have pollution treatment using the specific properties, which are being introduced earlier. The continued increase in the concentration of organic contaminants in wastewater, as well as the appearance of new hazardous micro-contaminants in water due to the anthropogenic emissions has led to substantial increase of the volume of wastewater with risks concern for humans hand ecosystems health. Fast and highly efficient actions are required to eliminate those new families of hazardous compounds from wastewater. Mimicking nature and taking advantage 4

of millions of years of evolution has been considered as a starting hypothesis in the development of smart and effective material and processes. Hence, as nature purify itself and uses different kinds of natural filters such as leaves, fibres, soil, and human and animals skin, it has been postulated to use mixed matrix composites for purification and filtration [54-57]. Hence, the high demand of a material that can have good separation and treatment properties in water and wastewater applications, as well as working under different scenarios and situations has increased. This has been traduced on the fact that the scientific and industrial community become interested in ZnO/ceramic and polymeric composites that can meet all the criteria needed for the water and wastewater treatment. This review is a comprehensive overview of recent progress on ZnO-embedded membrane technologies. Such an investigation is necessary and should answer questions about recent advances in ZnO-based membranes. Topics include ZnO preparation and characterization, preparation and fabrication of ZnO-based polymeric and ceramic membranes, and technologies to improve their performance in water and wastewater treatment applications. Also discussed are fouling and biofouling potential, permeance flux, pollutant rejection, and swelling and deswelling, as the most significant challenges on water purification with membranes. Filtration, microfiltration, nanofiltration (NF), ultrafiltration (UF), and reverse osmosis (RO), which are of significance when addressing large water treatment capacities, are within the scope of this review.

2. ZnO nanostructures preparation

5

ZnO nanomaterials are commercially available, inexpensive, chemically stable, and non-toxic, and they have been shown to offer a range of radiation absorption and photostability properties. ZnO has different possible morphologies that make it a good candidate for the preparation of membranes. Depending on the pursued applications, their preparation could be achieved by using the following methods and technologies: wet chemical, physical, and electrochemical, among others. The common morphologies are nanorod [44, 58-60], nanotube [61-64], nanowire [65-69], and nanoplate [70, 71]. Selection of the synthesis method, and specially the type of matrix, is linked to the type of target pollutants to be removed or transformed. For instance, to have the specific and tuneable surface wettability of ZnO nanorod, a special two-step preparation is required [72]. Hence, understanding the mechanism of preparing the ZnO before using that in the polymeric or ceramic composite is important and needs to be taken into the account. The following section is thoroughly addressing the most used preparation methods for both ZnO nanomaterials and mixed matrix composites.

2.1.

Wet chemical/solution-based

Wet chemical/solution techniques are promising in their simplicity, fast operation, and costsaving features [73]. In these techniques, the surface of the ZnO material is well-wetted in the preparation stage for the growth of the ZnO nanomaterials (Fig. 1). The most common wet chemical/solution-based techniques include hydrothermal precipitation [74-76], sol-gel [77, 78], microwave [79, 80], emulsion [81, 82], and spray pyrolysis [83] techniques. Table 1 summarizes these techniques, the precursors used, the synthesis conditions, and the nanomaterials shapes and applications. In the case of hydrothermal synthesis, the nanomaterial crystallizes from high-

6

temperature aqueous solutions at high vapour pressures. The sol-gel method, which is a hydrothermal-based method, involves the conversion of monomers into a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of discrete particles.

Fig. 1. Schematic of ZnO nanomaterial growth by wet chemical techniques on the surface of bamboo timber, copyright 2017 [84]. In these methods, the structural and morphological characteristics of the ZnO nanostructures can be controlled by modifying the growth parameters, such as the stoichiometry, chemical nature of the precursor, pH, and temperature [85, 86]. These methods produce two final products, ZnO and amines. These amines are produced from the lateral reaction in the system and ZnO comes from the main reaction. From different surfactant-free hydrothermal processes, controllable ZnO structures and amine precursors (which are the side products of the ZnO production procedure) in flower-like radially-gathered nails, rods, and cylindrical structures can be synthesized. These amine precursors share the common function of secreting a low molecular weight polypeptide hormone, such as secretin, cholecystokinin, and others [87]. The morphology 7

of the ZnO can be altered using different types of amines and it is controlled by the amine concentration, pH, and growth temperature [88]. Nano tetrapod-like, flower-like, and urchin-like ZnO nanostructures form at higher pH values (8) and rod-like structures form at lower values. Alternations in concentration can also change the shape and size of the nanostructures; however, an increase in temperature influences the nanostructure aspect ratio [88]. Precursors of zinc such as mixtures of amines and zinc acetate [75, 89-91] or other types of zinc containing materials ( zinc powder [92], zinc chloride [93], zinc nitrate [76, 94-96], zinc sulfate [79, 97, 98],) have been used. Sedimentation or solid phase formation agents used include NaOH [89, 91, 95], NH3 [94], NH4HCO3 [89, 94], and KOH [99, 100].

2.2.

Physical techniques

Dry physical techniques, unlike wet/chemical techniques, mostly involve no harsh chemical reactions, which increases control and reduces operation costs; although, these techniques usually require high temperature or pressure conditions. The common physical methods for synthesis of ZnO nanostructures by physical techniques are chemical vapour deposition (CVD) [101, 102] and thermal chemical vapour deposition (TCVD) [103]. The difference between CVD and TCVD is that TCVD works in high temperature condition, which improves product quality and efficiency. CVD produces high-quality, high-performance solid materials. During typical CVD, the wafer (substrate) is exposed to one or more volatile precursors, which is due to the high temperature or pressure changes the formation of the materials and place them on the substrate surface to produce the desired deposit. Table 2 summarizes the techniques and conditions

used

for

ZnO

nanostructure

synthesis

using

physical

techniques.

8

Table 1. Wet chemical/solution-based ZnO growth techniques. Methods Sonochemical route

Precursors Synthesis conditions Properties and applications Ref. Zn(NO3)2, NaOH, ethylenediamine room temp Nanoparticle, hexagonal wurtzite structure [95] (EDA), Zn(CH3COO)2, (NH4)2CO3, polyethylene Drying temp: 12 h, 100 °C; zincite structure; spherical particles (diameter ~ 30 nm) [89] glycol (PEG10000), calcination: 3 h, 450 °C application: as a photocatlyst in photocatalytic degradation Zn(NO3)2 calcination: 2 h, 600 °C; Wurtzite structure; particles diameter: 50 nm; [96] application: gas sensor aging: 240 h, 320 °C Zn(NO3)2, NaOH synthesis: 2 h; spherical particles; particle size = 40 nm [94] drying: 2 h, 100 °C Precipitation ZnSO4, NH4HCO3, ethanol drying: overnight, 100 °C; Wurtzite structure; crystallite size: 9‒20 nm; particle size= 12 nm, BET= [98] process calcination: 300‒500 °C 30‒74 m2/g Zn(CH3COO)2, precipitation temp: 85 °C; hexagonal structure, nano rods, flower-like particles details: L = 150 nm, [91] NH3 drying: 10 h, 60 °C D = 200 nm ZnSO4 reaction: 30 min, 60 °C; hexagonal structure, flake-like morphology (Diameter = 0.1‒1 μm, L: 60 [97] NH3 drying: 12 h, 100 °C; nm) NH4HCO3 calcination: 2 h, 400 °C ZnO, NH4HCO3 reaction: ~2 h, 25 °C; hexagonal wurtzite structure (a mineral consisting of zinc sulfide, [74] drying temp: 80 °C; typically occurring as brownish-black pyramidal crystals); flower-like and calcination:1 h, 350 °C rod-like shape (D =15‒25 nm, BET = 50‒70 m2/g) Zn(CH3COO)2, NaOH reaction: 30 min, 75 °C; [75] hexagonal structure; flower shape (L: ˃800 nm); drying: overnight, room temp application: antimicrobial activity Zn(NO3)2, (NH4)2CO3), CH3CH2OH, synthesise: 250°C to 550°C. nanoparticle, wurtzite hexagonal structures, size of particles = 8.34 nm- [76] drying:100 °C, 6 h, 27.58 nm Zn(CH3COO)2·2H2O reaction:2h, room temp, average diameter = 20 nm, [90] drying:12h, calcination: 450 °C application: organic contamination treatment for 3 h Ddirect ZnAC2·2H2O, (NH4)2 CO3, PEG, H2O, calcination:450°C, 3 h, Spherical particles, size =30 nm. [89] precipitation ethanol,1-butanol, o-xylene, toluene aging:85–90°C, 2 h, drying:100 °C, 12h Homogeneous mixed metal sulfate, IPA (isopropyl _ hollow microspheres [104] precipitation alcohol) solvent co-precipitation ZnCl2, Zn(NO3)2·6H2O, precipitation: 1 h, 120°C. nanorods with tapered ends, [105] Zn(CH3COO)2·2H2O and NH3 calcination:3h, 500 application: gas sensors Precipitation in ZnCl2, NH3, CTAB aging: 96 h, ambient temp, [93] zincite structure; particles diameter = 54‒60 nm, BET = ~17 m2/g the presence of calcination: 2 h, 500 °C surfactants Zn(NO3)2, NaOH, SDS, [106] precipitation: 50‒55 min, 101 wurtzite structure, rod-like shape (L = 3.6 μm, D = 400‒500 nm) TEA (triethanolamine) °C nut-like and rice-like shape, size = 1.2‒1.5 μm Zn(CH3COO)2, C2H2O4, reaction temperature: 60 °C; zincite structure; aggregate particles: ~100 nm; rod shape; particles detail ([107] ethanol, methanol drying: 24 h, 80 °C; L: ~500 nm, D: ~100 nm); BET: 53 m2/g; calcination: 500 °C application: decontamination of sarin (neuro-toxic agent)

9

Methods Sol-gel

Hydrothermal

Solvothermal

Microwave technique

Precursors Zn(CH3COO)2, C2H2O4, ethanol

Synthesis conditions Properties and applications reaction: 50 °C, 60 min; hexagonal wurtzite structure; uniform, spherically shaped of particles dried of gel: 80 °C, 20 h; calcined: under flowing air for 4 h at 650 °C reaction: room temp; cylinder-shaped crystallites, D: 25‒30 nm; L: 35‒45 nm drying temp: 60 °C

Ref. [77]

zinc 2-ethylhexanoate, TMAH [78] ((CH3)4NOH), ethanol and 2-propanol Zn(CH3COO)2, diethanolamine, ethanol reaction: room temp; hexagonal wurtzite structure; particles: nanotubes with the size of 70 nm [108] annealed of sol: 2 h, 500 °C ZnCl2, NaOH [109] reaction: 5‒10 h, 100‒220 °C particles morphology: bullet-like (100‒200 nm), rod-like in teflon-lined autoclave (100‒200 nm), sheet (50‒200 nm), polyhedron (200‒400 nm), crushed stone-like (50‒200 nm) Zn(CH3COO)2, NaOH, [110] reaction: 5‒10 h, 100‒200 °C; spherical shape; particles diameter: 55‒110 nm HMTA HMTA concentration: 0‒200 ppm Zn(CH3COO)2, Zn(NO3)2, LiOH, KOH, reaction: 10‒48 h, 120‒250 °C hexagonal (wurtzite) structure, size of microcrystallites: 100 nm‒20 μm [99] NH3 Zn(CH3COO)2, Zn(NO3)2, LiOH, KOH, time of autoclaving: 15 min, particles with irregular ends and holes; aggregates consist of particles of [100] NH3 2‒72 h; 20‒60 nm, BET: 0.49‒6.02 m2/g final pH: 7‒10 trimethylamine N-oxide, 4-picoline N- reaction: 24‒100 h, 180 °C [111] wurtzite structure; particles morphology: nanorods (40‒185 nm), oxide, HCl, toluene, ethylenediamine nanoparticles (24‒60 nm) (EDA), N,N,N’,N’- tetramethylethylenediamine (TMEDA) Zn(CH3COO)2, Zn(NO3)2, reaction: 150‒180 °C; hexagonal (wurtzite) structure, hollow microspheres (2‒5 μm) consisted [112] ethanol, drying: 80 °C in vacuum oven; nano-sized particles and contained channels (10 nm); hollow microspheres imidazolium tetrafluoroborate ionic calcinations: 500 °C consisted of nanorods (~20 nm); flower-like microspheres (2.5 μm) liquid zinc acetylacetonate, methoxy-ethoxy- microwave heating: 800 W, 4 zincite structure; average crystallite size: 9‒31 nm; particles diameter: [113] and n-butoxyethanol, min; 40‒200 nm; BET: 10‒70 m2/g zinc oximate drying: 75 °C in air Zn(NO3)2, H2O, microwave heating: 2 min, 90 hexagonal wurtzite structure, nanorod and nanowire shape (L: ~0.7 μm, [79] HMT (hexamethylenetetramine) °C; drying: 2 h, 60 °C D: ~280 nm); application: electronic and optoelectronic devices ZnSO4·7 H2O, NaOH microwave heating: 15 min at Nanoparticles, Diameter of nanoparticles: average size of 90 nm. [79] ambient temp; drying: 4 h,40°C applications: possess potential biological as efficient antimicrobial agents, drug carriers, bioimaging probes. Zn (NO3)2, NaOH Synthesis:30-100 °C, 5minZnO crystals [80] 30min Zn(NO3)2, NaOH synthesis:25-50 °C,5min-1h ZnO crystals [80] Zn(CH3COO)2·2H2O, (Zn(C2O4)·2HO), drying temp: 50 ◦ nanoflowers, Zn glycerolate nanoplates, application: dye sensitized solar [114] and (bis (2-hydroxy·1-naphthaldehydato) Stirred time: 15 min, cells zinc(II)) glycol solution maximum temp:

10

Methods

Precursors

Synthesis conditions Properties and applications 154 ◦C Zn(NO3)2, reaction: 25 °C, pH~8; drying: grain size: cationic surfactants (40‒50 nm), nonionic surfactants (20‒50 surfactant (ABS, Tween-80 and 40, 24 h, 80 °C; calcination: 2 h, nm), anonic surfactants (~20 nm) 600 ° C21H38BrN Emulsion Zn(C17H33COO)2, NaOH, reaction: 2 h, room temp or 90 °particles morphology: irregular particles aggregates (2‒10 μm); needledecane, ethanol shaped (L: 200‒600 nm, T: 90‒150 nm); nearly spherical and hexagonal (D: 100‒230 nm); spherical and pseudospherical aggregates (D: 150 nm) Zn(CH3COO)2, heptanes, Span-80, reaction: 1 h; hexagonal structure; spherical shape; particles diameter: 0.05‒0.15 μm NH3 aging: 2.5 h; drying: in rotary evaporator; calcination: 2 h, 700‒1000 °C Zn(CH3COO)2, heptanes, reaction: ambient temp; hexagonal structure; particles morphology: solids (164‒955 nm, BET: 8 Span-80, NH3 drying: 24 h, 120 °C m2/g), ellipsoids (459‒2670 nm, BET: 10.6 m2/g), rods (396‒825 nm, BET: 12 m2/g), flakes (220‒712 nm, BET: 20 m2/g); crystallites size: 32‒77 nm; application: photocatalyst Zn(NO3)2, NaOH, heptane, hexanol, reaction: 15 h, 140 °C; drying: hexagonal (wurtzite) structure; particles morphology: needle (L: 150–200 Triton X-100, PEG400 60 °C nm, D: ~55 nm), nanocolumns (L: 80‒100 nm, D: 50-80 nm), spherical (~45 nm) Microemulsion Zn(NO3)2, reaction: 1 h; equivalent spherical diameter: 11.7‒12.9 nm, BET: 82‒91 m2/g; grain oxalic acid, isooctane, benzene, ethanol, calcination: 3 h, 300 °C size: 11‒13 μm diethyl ether, chloroform, acetone, methanol, Aerosol OT Zn(CH3COO)2, Aerosol OT, glycerol, reaction: 24 h, 60‒70 °C; hexagonal wurtzite structure, spherical shape (15‒24 nm), rods shape (L: C20H37NaO7S, drying: 1 h, 100 °C; 66‒72 nm, D: 21‒28 nm) n-heptane, calcination: 3 h, 300‒500 °C NaOH, methanol, chloroform Thermal vapour ZnO, ZnO nanobridge; ZnO nanobridges and nanonails, transport and In2O3, furnace temperature at 1000 °C Applications: optoelectronics (diameters of 50-200 condensation(TV graphite powders TC) nm), ZnO nanonails: furnace temperature 950-970 °C (diameters of 50200) Ultrasound Zn(CH3COO)2 .2H2O, NH3, reaction:5 to 60 min,31-70 hexagonal structure, crystal grains, nanoseeds, nanorods (diameter 50 nm, NaOH °Drying: 24 h, 60 °C length 5–8 μm), nanoflowers (length 1-2 μm), it does not require high temperature and/or highly toxic chemicals Synthesis of tetramethylammonium hydroxide Thermal treatment at 150°C for ZnO nanocrystal, diameter: 2.7 nm Colloidal (N(CH3)4OH·5H2O) dimethylsulfoxide 4h (DMSO), Zn(CH3COO)2·2H2O ethyl acetate, methanol, Tb(CH3COO)3·H2O zinc metal, distilled water, \ drying:320 K Nanoparticles, the average size of all particles is 13.7nm with 5.8nm dispersion. Sodium dodecyl sulfate (C12H25SO4Na) applications: optoelectronic devices and visible–ultraviolet light-emitting

Ref. [96] [115] [116]

[81]

[82] [117]

[118]

[119]

[120] [121]

[122]

11

Methods

Precursors

Hydrolyzation

Zn(NO3)2.6H2O), diethanolamine (NH(CH2CH2OH)2),, CTAB

Self-Assembly spray pyrolysis

Zn(CH3COO)2 Zn(CH3COO)2

zinc acetate Organometallic Zn(c - C6H11)2 synthesis SupercriticalZn(CH3COO)2 water processing Zn(CH3COO)2 Thermal decomposition

Synthesis conditions

Properties and applications Ref. diodes and biological applications reaction:80-100 °C, Nanoparticle, hexagonal wurtzite structures, size in the range of 15-40nm.[123] drying: calcined in the air up to Application: dye sensitized, solar cells. 500 °C at a heating rate of 2 °C /min. reaction:100 °C.2-9h nanorods: two-dimensional hexagonal arrays (diameter: about 80 nm) [124] synthesis temp: 900 °C,2 h, Wurtzite hexagonal structure, size of particles:14.5 nm - 29.5 nm, surface [83] drying: room temp,12 h area:22.88-16.58 reactor temp:400-800 °C particles size: 5 -12 nm, [125] synthesis: room temp particle size:3-6nm, nanorods diameter:3-4 nm, length: up to 120nm [126] reaction:400 °C and 245 atm

nanoparticles: size range of 200-300 nm,

[127]

synthesis:300 °C ,1–12 h

Wurtzite crystal structure, nanowires, diameters: about 40nm, application: ultraviolet emission devices nanoparticles (diameters:100nm, nanowires(diameters:300nm), and nanowalls (thickness diameters:20 nm) particle size: 15 to 25 nm, nanocrystals with a hexagonal wurtzite structure, application: nano scale devices

[128]

Nanobundle, nanospherical, nanocrystal structure, 20 – 30 nm nanotriangle, nano crystal structure, 31 nm Application: nanoscale devise

[131]

A nanowire, regrowth modes of ZnO NWs: axial growth, radial growth, and both directions, length: 1.5 μm, diameter: 300 nm to 900 nm application: gas-sensing or field-emission the average particle size: 17–29 nm, scan rate of 2◦C/min

[132]

Zn(CH3COO)2

synthesis:200–250°C,10 min

(bis(2-hydroxyacetophenato) zinc(II)), Zn(CH3COO)2

stirring temp: 240 °C Reaction temp: 230◦C for 1h, cooling temp: room temp, drying temp: 100 °C Nanobundle: synthesis at 500°Cin the air, cooling at room temp, Dry temp: 50°C, 3 h Nanospherical: cooling at room temp, Dry temp: 50 °C, reaction temp: 170 – 240°C, Reaction temp: 140 – 240°C, cooling temp: 25°C growth temp: 910°C, ramping rate: 50 °C /min, 10-30 min

Nanobundle: (Zn) Nanospherical: (Zn(salen))- oleylamin, Zinc oxalate (Zn(C2O4).2H2O) – oleymine

Chemical vapour ZnO transport and condensation (CVTC) process Alkali Zn(CH3COO)2·2H2O, Zn(NO3)2· 6H2O precipitation

drying temp:100°C for 1h, calcinate temp: 500 °C for 1h

[129] [130]

[133]

12

Table 2. Dry techniques-based for ZnO growth. Methods

CVD

Precursors Zinc acetylacetonate hydrate (Zn(C5H7O2)2.xH2O, fused silica, Si (100), sapphire (110), Zn(C5H7O2)2·xH2O,

Synthesis conditions 130-140 °C, room temp.

Properties and applications nanorods, particles diameters of 45- 90 nm,

Ref. [101]

vaporizing temp in the range 130-140 °C, room nanorods, for practical application to nanoscale optoelectronic devices, [102] temp, nanorods diameter in the range of 60-80 nm Bis (2,4-Pentanedionato) Substrate temp (Ts):100 °C - 300 °C, Crystalline, Distance between the substrate surface and the surface of the source [134] zinc(Zn(C5H7O2)2) material (D): 2.5 mm Metal–organic zinc(II) mesotetrakis (3single-crystalline, nanorods (nanorods diameter: 0.15 μm), nanoparticles of [135] chemical carboxyphenyl) similar thickness (~ 1.8 μm), vapour porphyrin application: solar cells deposition (MOCVD) acetylacetonate of Reaction temp:500°C-600 °C, deposition hexagonal wurtzite structure, whiskers, diameter of 80–100nm. length 0.5–3 [136] zinc(Zn(C5H7O2)2.xH2O, duration about 100 min, vaporizing temp μm. Applications: optic electricity, conductivity, piezoelectricity acetone, alcohol, and in the range of 105–125 C. deionized-water TCVD Zn(s) powder deposition temp:450 °C to 600 °C 450°C - 500 °C (nanoneedle shape, diameters:55 to 80 nm, lengths:(700 nm for [103] 450°C and 500 nm for 500 °C),550°C -600 °C (nanotetrapods shape, diameters:58 to 90 nm, lenghths:400 to 600 nm), application: solar cells, light emitting diodes. PlasmaZn(C2H5)2 Substrate temp:400°C [137] thin films, Zn(C2H5)2 flow rate (mol/min):1.15˟10-5, CO2 flow rate (sccm) enhanced :200 chemical vapor deposition (PECVD) Zn(s) substrate temp:400°C nanorods, diameters nanorods :20-75 nm, lengths :2 μm, chamber pressures in [92] the range of 3 to 20 mTorr, application: constructing two-dimensional photonic crystals Chemical Zn(CH3COO)2 growth temp:300°C ,60 min, Heating rate of the Wurtzite structure; crystallite size 500nm, nanorod flower structures [138] vapour system:5 °C /min condensation(C VC) Physical vapour Zn(s), Au(s), Si (s) Temp of process:525°C,1 h structures crystals such as: nanocandle arrays, wine-bottle-shaped rod arrays [139] deposition (lengths: ~2 μm), nanorivet arrays (lengths:600 nm), periodic diamond-string (PVD) and needle arrays, nanofern and needle arrays, toothshaped belt, spinal-shaped nanostructures and bamboo-shaped nanorod (lengths of nanorods: ~2 μm,diameters range :200-500nm), applications: building various nanodevices, chamber pressure: atmosphere pressure

13

Methods Precursors Thermal Zn(s) powder plasma process Zn(s) or Zn suboxide, graphite Thermal evaporation

Zn(s) Zn(s) Zn(s) powder

Vapour-solid (VS)

Zn(s) powder Zn(s)

Vapour–liquid– Zn(v), solid(VLS) C(s) graphite (carbon source), Si(s), Au(s) as catalyst Zn(s) (CoO)0.1(ZnO)0.9

Synthesis conditions _

Properties and applications Plasma power (kW):6.5, Pressure (Torr):750, Mean crystallite size (nm):26.548.6, BET surface area (m2/g):6.2-17.4, Flow rate of plasma gas (Ar, l/min):15, Flow rate of carrier gas (O2, l/min):1.5, Flow rate of reaction gas (O2, l/min): 0.5-5, Feeding rate of raw material (g/min):0.3 Needle-like growth temp: 800–750 °C, Nanowires (hexagonal wurtzite, diameters normally range from 10 to 200 nm), nanoribbons growth temp: 750–650 °C, nanoribbons (typical widths range of 100 nm to 2μm and width-to-thickness nanowires ratios 10 to 20), and needle-like rods (hexagonal wurtzite structure with lattice grew mainly at the temp range of 650 to 500 °C parameters of a =0.3249 nm and c =0.5206 nm153) growth time for the synthesis of these ZnO single crystalline wurtzite hexagonal nanorods:(average diameters :150–250 nanorods :1–1.5 h, Room temp nm, length: 5–10 μm), chamber pressure: 3 Torr synthesis:600-800°C hexagonal wurtzite structured nanorods (diameters 1–1.5 μm), and nanonails (diameters:1μm) synthesis temp:650 °C crystalline with wurtzite hexagonal, nanowires (diameters and lengths: the range of 20-35 nm) synthesis temp:850°C nanoflower structure hexagonal growth time for the synthesis of these ZnO single crystalline wurtzite hexagonal nanorods: (average diameters :150–250 nanorods :1–1.5 h, Room temp nm, length: 5–10 μm), chamber pressure: 3 Torr

Ref. [85]

molar ratio ZnO: C (2:3), molar ratio (1:1), synthesis temp nanowalls :953–1034 °C and synthesis temp nanocombs: 961–1040 °C

[143]

room temp synthesis temp:100°C

nanowalls, nanocombs (application: nanoscale functional devices such as nanocantilevers, UV nanolaser arrays, optical nanogratings

Wurtzite -type structure, nanorod, diameter: around 300 nm, lengths: 20-35 μm hexagonal zincite structure (Wurtzite), thicknesses 100 nm to 280 nm, Electron concentration (×10^21 cm−3):4.64, Pulsed laser NaCl Synthesis temp:600 °C Needle-like nanorods, tip diameters 20-50 nm, root diameters ~ 60 nm, lengths deposition(PLD in the range 200–800 nm, Application: optoelectronic nanodevices. ) Zn(s), H2O, Drying temp:320 K Nanoparticles, Average size of all particles is 13.7nm with 5.8nm dispersion. Sodium dodecyl sulfate applications: optoelectronic devices and visible–ultraviolet light-emitting (C12H25SO4Na) diodes and biological applications. Atomic layer Zn(C2H5)2,H2O Growth temp:100-130 °C, Deposition temp:90– zinc oxide thin films, (Thickness:90-115 nm) Substrates: Glass, Si (1 0 0) deposition 200 °C, ALD window:100–180 °C (ALD) Zn(s), NaCl temp of the process :20-300 °C, a laser operating polycrystalline with the wurtzite crystal structure for ablation:193 nm, laser pulse energy :100 mJ Laser ablation Zn(s), laser energy:50 mJ, ablation: room temp,3-4h Nanoparticles (size in water and isopropanol as a solvent :14–20 nm), isopropanol, H2O, nanoparticles in the acetone range of 100 nm for ablation in acetone, and platelet-like(diameter:1μm) Zn(s), NaCl typical growth run consisted of 9000 laser shots Application: solar cells with a repetition rate of 10 Hz. highest substrate temperature used was 300°C

[86]

[88] [140] [141] [142] [88]

[144] [145] [146] [122] [147] [148] [149] [148]

14

Methods Vapour-phase transport process

Precursors ZnO(s) In2O3(s) graphite powders

Molecular Zn(s) beam epitaxy (MBE) Anodization Zn(s)sheet with H2O as electrolyte, graphite as a counter electrode Electrospinning Zn(CH3COO)2 PVA Electrophoretic Zn(CH3COO)2·2H2O, deposition(EPD IPA, ) NaOH Ion Zn (s), Mn(s) implantation Template-free Zn(NO3)2.6H2O method Zn(s) RadioZn(s) frequency magnetron sputtering (RF magnetron sputtering) Magnetron Cu(Zn, Sn) (CZT), sputtering and elemental selenium atmosphere Zn(s) powder. RF (radio frequency) plasma

Zn(CH3COO)2

Filtered cathodic vacuum arc (FCVA) Oxidation

Zn(s)

Zn(s)

Synthesis conditions The mixed powders were heated to between 950 and 1000 °C and held for 30 min. temp nanostructures were grown was controlled to be about 820-870°C. synthesis:400 °C

Properties and applications Ref. nanowires are single-crystal In2O3 with 6, 4, and 2 facets, nanorods are single- [150] crystal hexagonal ZnO, nanowires diameter of about 50-500 nm, nanorods diameter of about 20-200 nm, applications: photovoltaics, transparent EMI shielding, supercapacitors, fuel cells, high strength and multifunctional nanocomposites single crystal nanorod with diameters of 15–40 nm, lengths diameters: 1-2 μm, [151]

room temp, reaction time 6-12h, voltages ranging from 1 to 9 V

nanoporous, application: gas sensors

calcination:700 °C for 6h drying:80 °C,

Nanofiber. Specific surface area of the ZnO nanofibers 389.7 m2g-1 as, average [153] diameter: 232 nm. hexagonal wurtzite structure nanocrystallites, particle size of about 4–8nm, [154] application: optoelectronic applications

50 °C - 65 °C.1–5 days.

[152]

upon annealing to 800 °C for 10 - 15 min, under Nano belt, 10 – 40 nm, application: spintronic for reading and writing information high vacuum condition ( 105) calcination: over 500°C,2h Nanoparticles, hexagonal zincite structure. rod-like shapes, length:1.2 μm, diameter:100 nm reaction:300 °C for 35 h hexagonal structure, uniform, nanowires, diameters ranging from 15 to 90 nm synthesis:250°C, RF power:150W, substrate Wurtzite structure nanowall network, the width of the nanowalls:20–45 nm, temp (Ts): 250°C application: fabricating different kinds of nanodevices, gas sensors

[155]

500°C for 30 min

CZTSe film was approximately 2 μm. Application: solar cell applications,

[159]

Zn powder was dropped into a boat heated at 1000 °C RF plasma with a frequency of 13.56MHz was used, RF plasma was produced between stainless steel electrodes with the gap of 2 cm, RF power output:300 W, the tantalum boat was heated to 1000 C. room temp, oxygen pressure of 2 × 10^-4 to 1 × 10^-1

particles of a spherical size of 30nm, RF power output:300 W

[160]

particles of spherical with a particle size of 30 nm, application: ultraviolet (UV) absorption substances and gas sensors

[160]

ZnO thin film structure,

[161]

oxidation:300 ◦C, 5 min,

nanoflakes & nanowires

[162]

[156] [157] [158]

15

According to the data collected in both Tables 1 and 2, it is worth noting that the methods of physical synthesis mostly prepare ZnO materials with tube shapes and lengths up to 10 µm, which are suitable for linkages, sensors, and increasing strength. As for the wet chemical methods are mostly used to prepare nanomaterials with higher surface areas and small sizes, usable for adsorption, photocatalysis, and membrane separation applications. Therefore, in most of the cases, the application describes the method of synthesis and the sensitivity to other materials in the reaction, temperature, and operating conditions. On the other hand, wet chemical methods result in a wide range of novel and usable shapes of ZnO due to the higher influence of the chemical reactions in the procedure, hence, making them better for novel applications and devices.

2.3.

Electrochemical techniques

Electrochemical techniques have recently gained much attention for the synthesis of ZnO nanostructures. Typically, a substrate is pre-treated to produce a seed layer. Common techniques for electrodeposition of ZnO use oxygen or nitrate reduction on the substrate in the presence of Zn(II) salt at neutral pH while increasing the temperature. Increasing the ZnO nanomaterial surface pH by the generation of hydroxyl ions through reduction drives the formation of ZnO nanostructures in electrochemical techniques. The decrease in pH at the surface decreases ZnO solubility on the surface by a factor of 1000. Solubility does not change in the bulk phase, which results in the deposition of ZnO nanostructures on the electrode surface [86-145]. Chemical etching process (Fig. 2) is used for the synthesis of nanorods [163]. Table 3 lists the electrochemical techniques available to prepare ZnO nanodisks/sheets [164, 165] and other shapes of ZnO nanomaterials.

16

Fig. 2. Schematic of the preparation of ZnO using chemical etching process, Copyright [163].

17

Table 3. Electrochemical-based techniques for ZnO growth Methods electrochemical

Constant potential

Chemical etching Cyclic voltammetry Constant current

Precursors nitrate ions, dissolved oxygen Zn(NO3)2 ZnCl2 ZnCl2 Zn(NO3)2 Zn(NO3)2 Zn(NO3)2 Zn(NO3)2 Zn(NO3)2 Zn(NO3)2 ZnCl2 Zn(NO3)2 Zn(NO3)2 Zn(CH3COO)2 Zn(NO3)2 Zn(NO3)2 ZnCl2

Synthesis conditions room temp 70 °C,2h -1 V, 2000 s 1.1V, 1h 30 min, 80°C 1V, 1h -0.9 V, 20 min Sintering at 500°C, 0.5- 1.5h -1.2 V, 30 min -0.6V, 5 min, 65°C -1 to -1.4 V, 5 – 15 min, 70 – 80°C -2.5V, 10 min, 65°C -1.4V, 0°C Calcination -0.8 V, 85°C, 7000s, etching at 85°C, 90 min -1 to 1 V, 2 scans,70°C -0.1 mA, 30 min

Properties and applications hexagonal structure, uniformly large particles 200 nm, diameter ~80 nm nanospheres, diameter in the range of 20–200 nm Nanotube Nanotube Nanorod Nanofiber Nanodisk Nano sheet nanoporous ZnO film nanoporous ZnO film nanostructure ZnO film nanostructure ZnO film Nanobelt Nanofiber Nanotube nanostructure ZnO film ZnO nanopillar array and ZnO nanowire

Ref. [166] [167] [168] [169] [170] [171] [164] [165] [172, 173] [174] [175] [176] [177] [171] [180] [181] [182, 183]

Table 4. Other techniques for the growth of ZnO nanostructures Methods MCP

sol-gel combustion VPD Mechanochemical synthesized by a simple method

Precursors ZnCl2, Na2CO3, NaCl ZnCl2, Na2CO3, NaCl ZnCl2, Na2CO3, NaCl ZnCl2, Na2CO3, NaCl ZnCl2, Na2CO3, NaCl Zn(CH3COO)2·2H2O, citric acid, and H2O Zn(CH3COO)2·2H2O ZnCl2, Na2CO3 and NaCl as diluent ZnCl2, NH3

Synthesis conditions calcination: 2 h, 600 °C Dried temp:100°C for 1 h, reaction temp:450 °C 400 °C 0.5 h 300‒450 °C synthesis:16 h, 80°C, drying:2 h, 600 °C-650 °C 500 °C calcination:600 °C,2 h, heat-treatment temp:400 to 800 °C reaction: room temp, drying:100 °C, calcination:450 °C, 2 h

Properties and applications hexagonal structure; particles diameter: 21‒25 nm hexagonal structure; particles diameter: 18‒35 nm

Ref. [184] [185]

regular shape of particles; diameter ~27 nm, BET: 47 m2/g particles diameter: 27‒56 nm particles diameter: ~51 nm, BET: 23 m2/g Hexagonal nanoplate. Average particle sizes of the ZnO-NPs: 4-9 nm. nanorods, diameters about 50 nm and 200 nm The average crystal size is about 21 nm, particles diameters ˂ 100

[186] [187] [188] [189]

particles in size range of 18 to 31 nm, application: used for the removal of Cd(II) from aqueous solutions

[191]

[190] [184]

18

One of the best techniques used to create a diversity of shapes, sizes, and morphologies of ZnO nanomaterials is an electrochemical method. This method allows for changing the shapes and defining the shapes and sizes only by voltage control. Generally, using a low temperature decreases energy and heat consumption, thus, reducing the costs. On the other hand, other new approaches use high temperatures and long periods of treatment of the nanomaterials, resulting in mostly fine shapes of the ZnO nanostructures.

2.4.

Other techniques

Other techniques for the synthesis of ZnO nanostructures are listed in Table 4. These include mechano-chemical processes (MCP) [203–206], sol-gel combustion [207], and vapour phase deposition (VPD) [208]. MCP, the most used, involves the coupling of chemical and mechanical phenomena on a molecular scale. The mechanisms are often complex and include common photochemical and thermal mechanisms [192].

3. Physical and chemical properties of ZnO nanomaterials ZnO offers superior doping [193], antibacterial [194] photocatalytic, magnetic [43], electrical [43], ferrite [195, 196], optical [43, 197], piezoelectric [197-200], and thermal properties [43]. Table 5 lists the basic physical properties of bulk ZnO [201]. As semiconductors, when the size of a material decreases to the micrometer or nanometre scales, or even smaller, some of its physical properties, such as energy, are altered; this effective parameter is described by the quantum size effect. Quantum confinement is an example of the growth of band-gap energy in quasi-one-dimensional (Q1D) ZnO, which manifests as photoluminescence. ZnO nanomaterial

19

band-gaps also have a quantum size effect [202]. These materials can be prepared and used in different dimensional shapes, such as the 0 and 1, 2, and 3 dimensions, which act differently for different applications. The most important shapes for water and wastewater treatment are the ones with higher dimensions, which give more active surface area and, consequently, more adsorption and removal. These dimensions will be introduced later. X-ray absorption spectroscopy and scanning photoelectron microscopy can show the states of a surface and the size of ZnO nanostructures [203]. The carrier concentration in Q1D systems can be significantly affected by the state of the surface, as shown in studies on nanowire chemical sensing [204-207]. Understanding fundamental physical and chemical properties are vital to the logical design of efficient nanostructures. Understanding of the properties of ZnO nanostructures is critical to improving their potential as building blocks for nanoscale applications. This section includes a critical review of research results on the most important features of ZnO nanostructures.

Table 5. Physical properties of bulk ZnO.

20

Property

Value

Lattice constant (T = 300K) a0

0.32469 nm

c0

0.52069 nm

a0 / c0

1.602

u

0.345

Density

5.606 g/cm3

Melting Point

2248 K

Relative dielectric constant

8.66

Gap Energy

3.37 eV, direct

Intrinsic carrier concentration < 106 cm3 Excitation binding Energy

60 meV

Electron effective mass

0.24 m0

Electron mobility (T = 300)

200 cm2/V.s

Hole effective mass

0.59 m0

Hole mobility (T = 300 K)

5-50 cm2/V.s

Stable phase at 300 K

Wurtzite

Thermal conductivity

0.6, 1-1.2 W.cm-1.o C-1

Linear expansion

a0: 6.5 ×10-6

coefficient(/oC)

c0: 3 ×10-6

Refractive index

2.008, 2.029

Most of the properties in Table 5 are important for applications that are related to separation or removal of contaminants in aqueous solutions. Besides some mechanical

21

properties, which are good for more sensitive applications (e.g., sensors), all of the properties, such as wide bandgap, binding energy, density, electron properties, thermal conductivity, and lattice constants (which are important and necessary for separation and adsorption phenomena in membrane active layers), are the most important properties for separation and treatment applications.

3.1.

Photocatalytic properties

In photocatalysis, an electron-hole couple is created under light force by reduction or oxidation reactions on the surface of the catalyst (Fig. 3). Using a photocatalyst, an organic pollutant can be oxidized directly using a photo-generated dump or mediated by reaction with specific reactive groups, such as the hydroxyl radical (OH.), in solution [208-210]. ZnO offers photocatalytic activity under ultraviolet (UV) light; however, this activity is not constant during the procedure and is dependent on the photo-corrosion [51]. Good stability is provided by ZnO with nanometric dimensions, which improves crystallinity and has few defects [52]. The addition of other components can enhance the photocatalytic property of ZnO prolonging its visible spectrum range [211]. Guo et al. [212] investigated intensively the photocatalytic properties of ZnO, ZnO–TiO2, and TiO2 composites using a low-cost technique at low-temperature. As ZnO photocatalytic properties are known to be affected by the thickness of the coating and the material of the coating, near-visible light for dye degradation was applied. It was found that ZnO had better performance in comparison with TiO2, which is a common material for photocatalytic processes, therefore, ZnO nanostructures, with nanorod and nanotubes shapes, are the most effective

22

candidates for dye removal under photocatalysis in comparison with TiO2, while TiO2 gives better results under UV light. Li et al. [213] also studied the photocatalytic properties of ZnO nanospheres synthesized using an electrochemical technique using polyoxometalates (POMs) at room temperature. POMs have been shown to play a role in the construction of ZnO nanospheres. ZnO photocatalytic properties were successfully corroborated by using the reduction of rhodamine (RhB) as a dye probe under UV light.

Fig. 3. A schematic diagram of the ZnO photocatalytic mechanism.

3.2.

Crystal and surface structure properties of ZnO

23

The polar surface of a ZnO structure (Fig. 4) is a relevant property as reactive nanostructure. ZnO crystals in Wurtzite mineral phase do not have an inversion core. Wurtzite is having a hexagonal structure and in four indices notation and the last digit shows the basal plane's location toward the surface. If Wurtzite crystal nanotubes and nanorods rise along the c-axis, two diverse polar surfaces will be shaped on the opposing sides of the crystal because of the unexpected end of the structure. The negatively and positively charged surfaces [O-(0001) and Zn-(0001)] are the common polar surfaces in ZnO, which result in spontaneous polarization and a normal dipolemoment along the c-axis, as well as a difference in surface energy. Commonly, the polar surfaces must exhibit enormous surface modification to maintain a steady structure; however, ZnO ±(0001) is atomically smooth, flat, and firm, and it does not need to be rebuilt [214-216]. Efforts to recognize the improved steadiness of ZnO ±(0001) polar surfaces are at the forefront of current surface physics [217-220]. Two others are ZnO (0110) and (2110), which are non-polar surfaces with several O and Zn atoms. They have less energy than the (0001) facets. These polar and non-polar properties of ZnO nanomaterials make them more useful for water treatment, specifically when supported on membranes. The polarity of ZnO nanomaterials increases the negative and positive sites in the membrane, therefore, increasing the degradation of dyes under light radiation. On the other hand, based on the polarity of these materials, agglomeration will not be a problem when using lower amounts of these nanomaterials. Therefore, when using a low amount of these materials, the advantages of polarity appear in addition to the economic benefits of using a small amount of an expensive material.

24

Fig. 4. ZnO crystalline cell structure. This image has been provided by the authors using molecular simulation software.

3.3.

Mechanical properties of ZnO

Direct measurement of the mechanical behaviour of nanostructures is more challenging than traditional measurement techniques, which do not apply for bulk materials. ZnO has three advantages that make this nanomaterial superior in different applications. First, it is a semiconductor with a 3.37 eV direct wide band-gap and high stimulation binding energy (60 meV). Second, its wurtzite structure means that ZnO is extremely piezoelectric, which makes it a key property in electromechanically-coupled sensors and transducers. The most important advantage of ZnO is that it is a biocompatible and bio-safe material and can be used for biotreatment applications, such as biofouling control in water and wastewater treatment [221-223]. Q1D ZnO nanostructures are of interest for their unique properties associated with their anisotropic geometry and finite size effects and their role in the elimination of defects from ZnO materials [224]. The lack of proportional centre wurtzite combined with the enormous 25

electromechanical pairing results in robust piezoelectric properties that have prompted the use of ZnO in piezoelectric sensors and mechanical actuators [199, 200, 214]. ZnO features a wide band-gap (3.37 eV), making it a composite semiconductor appropriate for small wavelength optoelectronics. The high excitation binding energy (60 meV) in a crystal of ZnO can produce an efficient excitonic discharge at room temperature. ZnO is also vaporous under visible light and can be made conductive by doping [197, 200, 225].

3.4.

Classification of ZnO nanostructures

The morphology of ZnO nanostructures can be classified as 0D (nanoclusters) [226], 1D (nanotubes, nanowires, nanoparticles, nanorods, and nanobelts) [58, 62, 65, 227, 228], 2D (nanoplates) [70], and 3D (nanotetrapods, nanoflowers) [229, 230]. Other unusual structures, such as nanorings and nanopropellers, can be vaguely classified as 3D nanostructures. Their growth and deposition techniques strongly affect the morphology of ZnO and can be engineered to manufacture novel structures. More specifically, the morphology depends on the increase in temperature, the composition of the source material, and the diffusion rate. A variety of shapes of ZnO nanostructures have been reported. The morphologies currently used in industry (especially for membranes) and applications (especially water and wastewater treatment) are rods [44, 58, 59], needles [231], helixes [232], rings [232], tubes [61, 62], belts [227], wires [65, 66], combs [233], plates/sheets and pellets [70, 71, 234], dandelions [235], snowflakes [236], cones [237], bridges and nails [119], springs [238], propellers [239], hierarchical structures [119], triangles [240], spherical structures and bundles [131, 241, 242], ellipsoidal structures [243], cages [242], pen-like structures [244], flower-like structures [230], whiskers [245, 246], particles [228, 245-247], bows [248], loops [238], tetrapod-like nanorods

26

[229], poly-pods and nanohedgehogs [249], fibres [250], cables [251], pins [252], hollow shells [253], dumbbell-like structures [254], twinning-like structures [255], tower-like structures [256], nut-like structures [257] and nanopores [258]. These morphologies affect the optical, mechanical, and physical properties of ZnO and create the ability for use in multiple applications. Typically, nanostructures range in size from a few hundred nanometres to a few nanometres, in one dimension at least. Nanowires, for example, are on the micron scale, with diameters of several nanometres. This is why they are recognized as 1D nanostructures. ZnO nanostructures have been more useful thus far in the manufacture of membranes as particles, tubes, wires, and rods. Fig. 5 shows transmission electron microscopy (TEM) images of some ZnO morphologies. Some morphologies play important roles in the membrane industry and are common membrane technologies.

27

Fig. 5. Transmission electron microscopy (TEM) images of different ZnO morphologies.(a) nanotriangles, copyright 2009 [240],(b) (needles) an individual needle-shaped ZnO nanorods, copyright 2007 [231], (c) nanoparticle, copyright 2015 [259], (d) nanopellets, copyright 2010 [70] , (e) dandelion, copyright 2005 [235], (f) hexagonal cone-shaped, copyright 2007 [237], (g) nanospherical, copyright 2011 [131], (h) flower made up of nanowires, copyright 2007 [230], (i) nanobundle, copyright 2008 [241], (j) geometry and crystallographic structure of ZnO nanobows [248], copyright 2004. Reproduced with permission from Elsevier Science Ltd.

28

4. ZnO-based membranes 4.1. Porous and dense ZnO thin film Studies show that nanorods and nanowires are the two most practical morphologies to be used for thin-film preparation. Selections of nanorods, nanotubes, or nanowires, which are supported by a substrate, are usually well-known as porous structures. Their exclusive properties and fields of application merit a separate section. Section 4 describes disordered dense ZnO thin films and porous structures. The porous and dense schematics are shown in Fig. 6. It can be observed that by use of these shapes of nanomaterials membranes with high porosity are prepared, and this phenomenon increases the solvent permeability of the membrane while making the membrane more selective due to the presence of nanomaterials, which are highly selective and have good adsorption properties.

Fig. 6. Nanostructured membranes schematics: (a) incorporated dense membrane with nanomaterials, that is, nanoparticles, (b) nanoscale pores porous membrane (c) nanoscale pores-dense membrane on a porous membrane, and (d) incorporated nanomaterials dense membrane, nanoscale pores on a porous membrane. 29

Porous ZnO film synthesis can be achieved by physical and chemical methods. The following chemical synthesis techniques are used to make the foundation of the ZnO nanostructures porous: sol-gel synthesis [127], self-assembled monolayer synthesis [124], and hydrothermal synthesis [132, 162]. Physical synthesis methods are not as effective as chemical methods. Unlike porous films, dense ZnO films can be grown using many Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD) methods. Among them, sputtering [159], Pulsed Laser Deposition (PLD) [145, 146], CVD [102], Molecular Beam Epitaxy (MBE) [151], and Metal-Organic Chemical Vapour Deposition ( MOCVD) [135] can be used to deposit thin ZnO films with diverse crystalline characteristics on different substrates.

4.2. ZnO containing polymeric membranes The use of nanostructures in polymeric membranes for water treatment has increased recently, mainly to increase flux and decrease fouling. Inorganic nanostructures used in polymeric membranes are ZnO, Al2O3, Ag, SiO2, TiO2, and ZrO2(zirconia) [269]. Antibacterial ZnO materials are resistant to UV light, which could improve the antifouling and antibacterial functions of the polymeric membranes. Nano-ZnO is also less expensive than other oxide nanostructures such as Al2O3 and TiO2 [270]. The main phases, filler and polymer, of a Mixed Matrix Membrane (MMM) have a positive mutual influence [271-274]. Membranes of polymeric structures are the most widely option proposed for water and wastewater treatment. The polymers used as supports in ZnO polymeric membranes are polyaminoacid (PAA) [275], chitosan [276, 277], polyvinyl pyrrolidone (PVP) [28], polyvinyl chloride

(PVC)

[278],

polyimide

(PI)

[279],

polyethylene

glycol

(PEG)

[280],

30

polyvinyldifluoride (PVDF) [265, 281], cellulose acetate(CA), polyacrylonitrile (PAN), polypropylene

(PP),

polytetrafluoroethylene(PTFE),

polyvinyl

alcohol

(PVA)

[282],

polyethersulfone (PES) and, polysulfone (PSu) [283-285], [262]. Fig. 7 shows the structures of some of these polymers.

Fig. 7. Chemical structures of polymeric membrane materials commonly used in water and wastewater treatment (polyaminoacid (PAA), chitosan, polyvinyl pyrrolidone (PVP), polyvinyl chloride (PVC), polyimide (PI), polyethylene glycol (PEG), polyvinyldifluoride (PVDF), cellulose acetate (CA), polyacrylonitrile (PAN), polypropylene (PP), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), polyethersulfone (PES) and, polysulfone (PSu).

4.2.1. Preparation of ZnO containing polymeric membranes Preparation procedures of ZnO nanomaterials for improving MMM performance were described in sections 2 and 3. Blended and coated methods are commonly used in the preparation of MMMs. In the blended method, a polymer and a nanomaterial are mixed in a solvent and the

31

solution is used to synthesize MMMs. In the coated method, the polymer and filler solutions are made separately and then the filler is coated onto the casted polymer solution. Wang et al. [277] prepared a ZnO/PVA/chitosan membrane by an electrospinning procedure to produce nanofibers using the blended method with the addition of 20 mg of nanoZnO to a solution containing 6% PVA and 4% chitosan. Chitosan is an antimicrobial polymer that can be more effective when combined with ZnO nanomaterials as a filler, and the mixture can gain the novel properties of both materials at the same time. These membranes have been used for antifouling control applications. On the other hand, when CdTe quantum dots have also been used, the functionality of the membranes in antifouling applications increased. Hairom et al. [28] prepared four types of nano-ZnO/polypiperazine amides by immersing the blended membrane in water overnight for NF of water in RO. ZnO nanomaterials, with sizes of 7–30 nm, were produced, with reduced agglomeration in the membrane. Its application in a membrane bioreactor increased the dye removal efficiency in comparison with the TiO2. Hong et al. [281] prepared a nano-ZnO/PVDF membrane using phase inversion and blending using a doping solution by dissolving the ZnO nano-particles and PVDF in dimethyl-acetamide (DMAc) at 50ºC for 24 h. The casting dope was maintained in the dark for 24 h for removal of air bubbles. By using ZnO in the membrane, studies using bovine serum albumin (BSA) as model foulant compound was successfully removed to produce water with drinking water quality. Due to the high hydrophilicity of ZnO, the hydrophilicity of the membranes increases substantially. Also, due to the agglomeration problem of these nanomaterials, the optimum amount of ZnO loading was 0.05 wt.%, and, after that, the membranes showed weaker purification. Parvizian et al. [278] prepared a nanocomposite ion-exchange membrane with nano-ZnO and PVC using the casting method. ZnO nanostructures acted as semiconductor additives to the membrane. The prepared

32

membranes were used to maintain the pH of the water and the concentration of contaminants in the water solution. Also, by increasing the amount of ZnO, the permeability and selectivity of the membranes increased, which was due to the large surface area and permeable sites of these nanomaterials. A Ag/carbon nanotube (CNT)/polyimide(PI)/ZnO membrane was fabricated by Kou et al. [279] using the coating method, as illustrated in Fig. 8. Ag-modified carbon nanotube materials were added to a polyimide solution and, afterward, using the electrospinning technique, ZnO flower-like nanomaterials were added to the membranes. These 3D nanomaterials have very high active surface areas and permeation sites, which make them the best nominates for wastewater treatment applications. Prepared membranes showed advanced photocatalytic properties and dye removal, as well as great photoelectric activity.

Fig. 8. The preparation process of the Ag–CNT/polyimide(PI) substrate and ZnO [279]. Copyright 2012. Reproduced with permission from Elsevier Science Ltd.

A hybrid ZnO/PES membrane was prepared by phase inversion and blending by Shen et al. [285]. The addition of ZnO nanomaterials decreased the contact angle, therefore increasing the hydrophilicity of the membranes, making them good candidates for water treatment. The 33

solute rejection was maintained, contrary to the increase of water flux increased and better antifouling properties, due to the presence of ZnO nanomaterials. Bai et al. [262] prepared a multifunctional membrane by integrating the benefits of a conformist polymer membrane as the support layer and the hierarchical forest-like TiO2/ZnO nanostructure as a functional layer. Membranes water flux increase, dye rejection increased; and the photocatalytic, antibacterial, and antifouling properties increased. Zhao et al. [286] prepared a nano-ZnO/PVP membrane by coating, using dimethyl-formamide as the solvent. Afterwards, wet phase separation was used to prepare the PES/ZnO membranes. Membranes had great permeability, high hydrophilicity, as well as antifouling properties due to the use of ZnO nanomaterials. Lin et al. [287] prepared a ZnO/PES membrane by non-solvent induced phase separation (NIPS). By using doped silica materials, they increased the wastewater treatment properties, especially dye removal, due to the increased band-gap of the membranes, which increased the photocatalytic properties. The hydrophilicity increased; which is one of the main parameter that controls permeability, and membranes showed higher water flux during the filtration stage. Zhang et al. [288] prepared PVDF/ZnO membranes by immersing the pre-treated PVDF film in the ZnO suspension (coated method) and by blending ZnO nanostructures with PVDF solution and casting the film. The membrane films showed great wastewater removal and also great dye degradation. A multifunctional PAN with a nano-ZnO/Ag membrane was prepared by single-capillary electrospinning by Chen et al. [289]. Table 6 lists the preparation methods for ZnO/polymer membranes that are useful in water and wastewater treatment. The preparation method, type of polymer, use of blending or coating, size of the nanomaterial, and immersion time were recorded.

34

Table 6. Preparation and characterization techniques used for nano-ZnO polymeric membranes. Membrane

Application

Preparation method

PES

Humic acid removal

phase inversion

PVDF

Water treatment

PVDF

Blended

Coated

NM size (nm)

Immersi on time

Ref.

√

-

10-30

24 h

[290]

phase inversion

√

-

50

_

[281]

Ultrafiltration

phase inversion

√

-

_

_

[265]

PVP

Membrane photocatalytic reactor (MPR) for dye treatment

precipitation

√

-

7-30

24 h

[28]

PVC

Water treatment

phase inversion

√

-

20

_

[278]

PI

Water treatment

Electrochemical deposition

-

√

7

_

[279]

PEG

Pervaporation

phase inversion

√

-

-

_

[280]

PPA

Wastewater treatment

precipitation

√

-

50 to 140

_

[259]

CA

Optical, bactericidal and water repellent properties

electrospinning

√

-

-

_

[291]

PVDF

Photo-catalysis and self-cleaning

phase-inversion

√

-

50

_

[265]

PVDF

Water treatment

ALD

-

√

-

_

[292]

PVDF

Oil-water treatment

_

√

-

50

12 h

[293]

PVC

Ultrafiltration

phase inversion

√

-

20-30

_

[294]

PES

Wastewater treatment

phase inversion

√

-

-

3 days

[285]

CA, PVDF

Water treatment

_

-

√

100-300

_

[262]

PVDF

Heavy metal ions removal

Phase inversion

√

-

-

_

[288]

PVDF

Anti-irreversible fouling

wet phase separation

√

-

25

_

[270]

CA

Pervaporation

phase inversion

√

-

-

_

[295]

PES & PVP

Wastewater treatment

_

-

√

180 and 550

_

[286]

35

PSU & PVA

Ultrafiltration

_

√

-

21–23

_

[296]

gelatin biopolymer

antibacterial activity

precipitation

√

-

200-400

_

[297]

PVDF

adsorption of heavy metals

precipitation

√

-

150

20 min

[288]

PES

humic acid removal

phase inversion

√

-

10–30

_

[298]

poly(methyl methacrylate ) (PMMA)

water Treatment

single-step phase separation method

√

-

5

_

[299]

PPA, Polyamide

membrane reactor (MPR)

precipitation

√

-

7 to 30

_

[300]

nafion

water treatment

templating method

√

-

230

24 h

[301]

Nafion ionomer

water treatment

templating method

√

-

230

_

[302]

PES

water treatment

_

-

√

200

_

[303]

PVP

water treatment

phase inversion via immersion precipitation technique

√

-

10-30

1 day

[304]

PES

water treatment

co-precipitation

√

-

35

_

[305]

PES

water treatment

Stöber method

√

-

120

15 min

[287]

PVC

water treatment

-

√

-

-

_

[306]

PAN

water treatment

single capillary electrospinning

-

√

10

1h

[289]

PES

water treatment

non-solvent induced phase separation

√

-

10

15 min

[307]

PEI

water treatment

non-solvent induced phase inversion

-

√

-

_

[308]

chitosan

water treatment

the simple dipping method

√

-

-

2h

[309]

Polysacchari de

water treatment

-

-

√

66-101

_

[310]

36

Table 7. Preparation and characterization techniques used for nano-ZnO ceramic membranes. Nanomaterial

Application

Preparation method

Nanomaterial size(nm)

Ref.

CuO/ZnO/Al2O3

water treatment

on-stream catalytic cracking deposition

-

[311]

ZnO

membrane filtration

CVD

5

[312]

ZnO

water desalination

Phase inversion

-

[313]

ZnO/ anodic aluminum oxide (AAO)

water treatment

Sol-gel

70

[108]

ZnO

wastewater treatment

templating method

8

[302]

SiO2/ZnO

water treatment

slip-casting

12

[314]

37

4.2.2. Comparison of ZnO-coated and blended polymeric membranes The hydrophilicity of ZnO-blended membranes improves with the addition of ZnO nanomaterials. Membranes coated with ZnO nanostructures are improved substantially when compared with blended membranes [43,298] because of water flux increases in the coated membranes more than in the blended membranes. The diameter of the nano-ZnO in the membrane prepared by blending is smaller than in the coating method; thus, filtration of heavy metals and other impurities from water and wastewater improves when using blended ZnO membranes. The preparation method (coated or blended) can change the transport properties of ZnO-polymeric membranes [247,253,258,263,264,267–269,275,279,298–303]. By using the blended method, the nanomaterials will accommodate into the pores of the polymers and, therefore, will show different results due to the preparation difference.

4.2.3. Stability of ZnO containing polymeric membranes The chemical and thermal stability and mechanical strength of ZnO polymeric membranes have been investigated [27, 275, 286, 315]. Zhao et al. [286] added a ZnO nanostructure to improve the thermal stability of a membrane at temperatures below 450°C. The addition of ZnO nanomaterials to PVP membranes, with higher thermal resistance, substantially increased the thermal stability of membranes. Chemical stability was investigated by John et al. [315]. ZnO NP/chitosan nanofilms were prepared by the sol-gel method and dip-coated onto mild steel. Solgel protective coatings showed very good chemical stability for polymer and ZnO. The thermal stability of the membranes synthesized by Zhao et al. [286] increased when nano-ZnO was added

38

as a filler to the PES membrane. Fig. 9 shows that the mass loss suffered by the PES/nano-ZnO membrane at temperatures below 450°C was less than that for a pure PES membrane.

Fig. 9. Thermogravimetric analysis (TGA)of the PES and PES/ZnO (ZnO(I): 0.0445 g of Zinc nitrate.6H2O and ZnO(III): 1.325 g of Zinc nitrate.6H2O used to produce the PES/ZnO membrane) membrane [286]. Copyright 2015, Reproduced with permission from Elsevier Science Ltd. 4.3. ZnO containing ceramic membranes Ceramic membranes, especially Al2O3 and SiO2, have been studied as supports for the preparation of ZnO containing ceramic membranes for water and wastewater treatment [316320]. Ceramic membranes are thermally and chemically more stable than polymer membranes and mechanically stronger. This makes them more attractive for application in membrane processes although, their high cost limits their use at large industrial applications. Preparation mainly occurs using ceramic materials by immersion or soaking them in the nanomaterial solution. Therefore, there is no specific recognized method for preparation, and a large diversity of methods to prepare this kind of membrane have been described. Wang et al. [302] used a film of Nafion membrane soaked in an aqueous solution of zinc(II) nitrate overnight. The film was carefully rinsed and immersed for 24 h. For membrane development, the supports were placed at both ends, trapped by Teflon caps, and set vertically in a Teflon-lined 39

stainless-steel autoclave that was filled with the synthesis solution. The crystallization reaction was carried out at 100°C for 72 h. After gradual, natural cooling to room temperature, the ceramic membranes were immersed in methanol for 1 h and then dried at room temperature for 3 days. The modified two-in-one technique for deposition of ZnO particles was suggested to determine the effect of the pore structure and surface of the support on the development of the Zeolitic Imidazolate Frameworks (ZIF) ceramic membrane on a macroporous coarse tubular support with a pore size of 2–3 µm. Naszalyi et al. [314] prepared a membrane by slip-casting on irregular tubular alumina supports. The samples were prepared by dip-coating on macroporous alumina disks and glass slides to measure photoactivity. All samples were dried at room temperature and stored in the relative humidity for one day. Two consecutive thermal treatments were then carried out; the first at 150°C to complete ZnO condensation and drying and the second at 500°C for mechanical forming of the deposited layers and thermal removal of the organic binders. Yue et al. [108] prepared ZnO using the sol-gel technique. Uniform, well-filled, ordered ZnO nanotubes were prepared in an ultrathin Anodic Aluminium Oxide (AAO) membrane. The use of ultrathin AAO membranes in the sol-gel method may allow for the manufacture of high-quality 1D nanomaterials. The application, nanoparticle type, preparation method, and nanomaterial size of the ceramic membranes are listed in Table 7.

40

5. ZnO-embedded membranes for water and wastewater treatment A large number of studies have examined nano-ZnO membranes for different applications [28, 270, 286, 295, 316, 317, 321]. Industrial, agriculture, urban and environmental pollutants, such as toxic heavy metals, pesticides, fungicides, herbicides, endocrine disruptors, hazardous dyes, and bacterial hazards are requesting for the development of new hygienically friendly purification technologies. The lack of water resources and the need to prevent the spread of pathogen-based diseases by contaminated water has prompted research on advanced technologies where the role of highly chemical resistant membranes with improved fouling resistance and cleaning operation performance is more relevant [322]. The main progress beyond the state of the art on the incorporation of nano-ZnO into polymeric and ceramic membranes and their properties as they relate to fouling and biofouling, swelling and deswelling, elimination of pollutants, and flux of water is reviewed as well.

5.1.

Applications for improving fouling and biofouling control

Nano-ZnO coated and blended membranes are more hydrophilic than unmodified membranes and have better antifouling properties [262, 286]; however, the grade of hydrophilicity of the membrane cannot be compared to the antifouling grade, as pointed out by Zhao et al. [286]. Bai et al. [262] found that a membrane with forest-like hierarchical ZnO nanomaterials used these nanomaterials as the filler in an efficient and well-assembled layer of polymers. This type of membrane showed low membrane fouling; high permeate flux, high photodegradation capacity, and high antibacterial performance. Isawi et al. [313] investigated the efficient photocatalytic removal of Escherichia coli from aqueous solutions using NPs of ZnO under UV light irradiation (Fig. 10). Composite

41

PMAA-g-PA(TFC), PA(TFC), and ZnO NP-modified PMAA-g-PA(TFC) membranes with photocatalytic bactericidal ability were tested by measuring E. coli survival rates under UV light exposure. The antibacterial activity and photocatalytic properties of the PMAA grafting layer improved with the addition of ZnO NPs.

Fig. 10. Thin-film composite membrane with grafting polymerization and of ZnO NPs 2015 [313]. Copyright 2015, Reproduced with permission from Elsevier Science Ltd.

Pure water was tested at room temperature with a PES/nano-ZnO (NP and nanorod type) MMM to cope with fouling [305]. Permeate flux ratios (PFR) (Fig. 11) showed that both types of ZnO nanofillers were effective and decreased fouling events. The atomic force microscopy (AFM) results showed that the surface roughness of these ZnO nanofillers in PES membranes decreased, which improved the antifouling properties as the initial hypothesis according to results reported for RO and NF membranes.

42

Fig. 11. Flux Recovery (FR) and other fouling parameters for nano-ZnO membranes (FR: flux recovery ratio, Ref: reversible fouling ratio, Rirf: irreversible fouling ratio and Rtf: total fouling ratio) [305]. Copyright 2015, Reproduced with permission from Elsevier Science Ltd. Zhao et al. [286] fabricated a nano-ZnO/PES membrane by coating PVP onto nano-ZnO to improve its functionality for water treatments. The results showed that the addition of this nanofiller improved the antifouling properties and compaction resistance over those of PES/PVP membranes with trials using BSA as model foulant. The BSA content decreased quickly during filtration. The hydrophilicity of the surface of the membranes decreased the protein adhesion to the surface, which improved the antifouling properties. A PVDF/nano-ZnO membrane was fabricated for anti-irreversible fouling by Liang et al. [270] and validated at laboratory scale using synthetic wastewater. The PVDF membrane was affected by physically-irreversible fouling and could not recover like the PVDF/ZnO membranes did after rinsing. The nano-ZnO fillers significantly improved the PVDF membrane filtration performance. This was likely due to increased membrane surface hydrophilicity, which increased the antifouling properties of the membrane. Similar results have been reported for an ultrafiltration PVC membrane incorporating ZnO NPs [294]. 43

ZnO nanomaterials, used as fillers in polymeric and ceramic membranes, have demonstrated to improve fouling properties and permeance flux. Several studies have been carried out on the antifouling behaviour of nano-ZnO in membranes, especially in membranes based on PES and PVDF polymers [265, 281, 282, 285, 290, 292, 293, 323, 324].

5.2.

Improving membranes water flux

Blending or coating the ZnO nanostructure onto polymer and ceramic membranes increases the hydrophilicity of the membrane [278, 321]; however, TiO2 metal oxide does not increase the hydrophilicity [14]. The antifouling ability and permeate flux of membranes depend on the surface hydrophilicity, porosity, pore size, thickness of the membranes. This hydrophilicity can be investigated by contact angle characterization, pore size, separation factor, and ultra, micro, or nanofiltration performance [324]. If the amount of ZnO nanomaterials in the membrane increases, the contact angle decreases [290] (Fig. 12) and hydrophilicity and permeate flux increase. It worths to mention that the polymer material is important as some of the polymers are hydrophilic by themselves then, ZnO can improve their hydrophilicity. Shen et al. [285] used PVDF/ZnO membranes with different loading contents of ZnO nanomaterials in water treatments. Contact angle characterization revealed that the contact angles decreased as the ZnO nanomaterial content increased. The hydrophilicity of nano-ZnO improved membrane hydrophilicity, as shown by the decrease in contact angle. Increasing the hydrophilicity also improved the permeate flux of the system. Ahmad et al. [304] measured the contact angles for PES/ZnO membranes with different nano-ZnO contents. All ZnO/PES membranes had lower contact angles than the neat PES membrane. ZnO was added to the PVDF polymeric solution with atomic layer deposition (ALD) to increase the water permeate flux

44

[292]. The SEM and Fourier transform infrared (FTIR) spectroscopy analysis results showed that a thicker hydrophilic ZnO layer emerged on the surface of the membrane and the pore walls by increasing the deposition cycles. The water droplet contact angle decreased for the modified membranes. The hydrophilicity of the membranes and the water permeate flux increased.

Fig. 12. Effect of the addition and the amount of ZnO nanomaterials on the contact angle of water droplets and the hydrophilicity of the membrane. Parvizian et al. [278] found that increasing the ZnO NPs loading ratio to 10% (wt.) increased the permeate water content because hydrophilic ZnO NPs increased the membrane hydrophilicity and diffused higher amounts of water over the membrane structures. Hence, by increasing the amount of ZnO, the permeability and selectivity of the membranes increased, which is due to the large surface area and permeable sites of these nanomaterials. Alhoshan et al. [325] reported that ZnO NPs at 4.01% (w/w) in solution increased the hydrophilicity of CA, the separation factor, and the permeation flux of a methanol/methyl tert-butyl ether system. Hong et al. [281] investigated the effects of nano-ZnO on PVDF microfiltration membranes. They found that an increase in filler in the membrane increased the pure water flux to 452 Lm–2 h–1 and it improved purification. Another study investigated the purification properties of a nano-ZnO/PES UF membrane [286] where the nanofillers were coated with PVP. 45

The water flux of the UF membrane increased by 210% compared with neat and pure PES. They also found that using different dispersion types of ZnO-dimethylformamide (DMF) in the solution changed the water flux values. Liang et al. [270] reported that the water permeability of modified PVDF membranes with nano-ZnO increased significantly over that of pure membranes. This could be the result of the addition of a sufficient amount of nano-ZnO to a sufficient extent to change the microstructure of the PVDF membrane. The hydrophilicity of ZnO nanomaterials remarkably increased the hydrophilicity of the membranes, therefore increasing the water flux and permeability. In a similar study, a PVC/ZnO membrane was fabricated to study the purification efficiency and the influence of metal-organic fillers, such as ZnO, on the pure water flux and fouling of the PVC membranes [294]. The flux of pure water of the modified membranes increased up to 3% (wt) with the addition of nano-ZnO, which was the optimized amount in this work. The 3% (wt) addition of ZnO increased the flux recovery ratio from 69% to 90%, which indicates that the antifouling properties of the PVC/ZnO membranes increased. Leo et al. [326] enhanced the transport properties of a polysulfone (PSF) membrane with the addition of nano-ZnO by blending. The addition of ZnO NPs to the polymer mixture increased the permeate flux by over 100% compared with the pure PSF membrane, due to the enlargement of the pore size or an increase in hydrophilicity of the membrane as postulated hypothesis. Bai et al. [262] examined the advantages of a forest-like TiO2/ZnO layer as a functional layer in polymeric membranes. The hierarchical pores created using this type of nanomaterial improved the water permeance. They also investigated the permeability of the membranes using different operating pressures and found out that by increasing the pressure to 3 bars, the permeability increased up to four times.

46

A complex RO MMM was prepared to improve the flux of water with the addition of aluminium-doped ZnO NPs [327]. Chung et al. [328] synthesized PSF/ZnO and functionalized PSF/ZnO/ethylene glycol (EG) membranes to improve its transport properties. The contact angle, porosity, pore size, and permeability tests (Fig. 13) showed that the water vapour permeability increased with the addition of nano-ZnO, which was further improved with functionalization of the PSF/ZnO membrane with EG. Naszalyi et al. [329] worked on an advanced sol-gel-derived mesoporous SiO2/ZnO coreshell multifunctional ceramic membrane for UF of water. This membrane showed improvements in both photocatalytic properties and porosity (Fig. 14) from SiO2 and ZnO, respectively. The permeability of this kind of membrane improved with the addition of nano-ZnO. Overall, the addition of nan-ZnO as a filler to polymer and ceramic membranes improved the permeance flux and water flux, and the functionalization of nano-ZnO further improved these properties and increased membrane permeability.

47

Fig. 13. Membrane performance for Blank PSF, PSF-ZnO, PSF-ZnO-EG: (i) Contact angle, (ii) Permeability, (iii) Pore size, (iv) Porosity [328]. Copyright 2016, Reproduced with permission from Elsevier Science Ltd.

Fig. 14. SEM images of a) cross-section image SiO2/ZnO membrane and b) SiO2/ZnO thin layer deposited on a glass slide cross-section image [329]. Copyright 2008, Reproduced with permission from Elsevier Science Ltd.

48

5.3.

Antibacterial and photocatalytic properties

Materials containing inorganic, organic, and metallic materials have been used as backups to investigate the photocatalytic properties of the polymer and ceramic membranes. Ceramic and polymeric membranes are extensively used as supports for ZnO photocatalytic membranes [28]. Apart from immobilized ZnO membranes, pure photocatalytic ZnO membranes have been created using ZnO nanowires [261, 262], NPs [226, 266-268, 300, 330, 331], and nanotubes [27]. Wang et al. [299] prepared a polymethyl methacrylate (PMMA) membrane with submicron topography for assessment of biocompatibility and water treatment. The surface charge effects and the combination of NPs were estimated by culturing mammalian cells on the prepared PMMA microstructured surfaces. The results showed that the ZnO NPs efficiently inhibited the growth of L-929 fibroblasts, and the addition of sodium dodecyl-sulphate (SDS) to the PMMA decreased the cell density by 50% over that of pristine PMMA. Wang et al. [301] also investigated the relationship between the nanocrystal photocatalytic properties of ZnO and the oxygen defects in Nafion membranes. Chen et al. [332] prepared a novel type of PVDF/ZnO membranes to be used in an anaerobic membrane bioreactor to investigate the effect of nanoparticle shape and size on the performance of the membrane bioreactor. After six months of operation, the changes in COD levels showed that the best removal efficiency was allocated in the fourth month of the study after an acclimation of the system. Some other factors, such as extracellular polymeric substance (EPS) production, concentration of soluble microbial products (SMP) and colloids, and transmembrane pressure changes, were compared with those obtained with conventional polymeric membranes.

49

Hairom et al. [333] modified a membrane photocatalytic reactor (MPR) with ZnO nanofillers for the degradation of model dyes using NF and UF membranes. Degradation of the model dye by photocatalytic means in the MPR is shown in Fig. 15. The membranes were checked using pure water and saline solutions. The results showed that the permeability of the modified MPR decreased as ZnO increased from 0.1 to 0.5 g/L, but the dye rejection rate also increased. NF also showed less permeance flux than UF membranes in this MPR but had more salt rejection. Hairom et al. [300] investigated the application of ZnO NPs in an MPR to eliminate dye from wastewater using NF and UF membranes. In an MPR, the ZnO NPs were used as a photocatalyst to reduce dyes, which were detached and recovered at the same time [330, 331].

Fig. 15. Schematic of the process of dye degradation by the photocatalytic system in the membrane photocatalytic reactor MPR system [333]. Copyright 2015, Reproduced with permission from Elsevier Science Ltd.

Bojarska et al. [334] modified a PP membrane with nanowire ZnO to remove bacteria and dissolved ions. Comparison of the results of modified and unmodified membranes showed 50

that both the antibacterial and photocatalytic properties of PP membranes were improved in the modified membranes. The use of plasma as the initial modification step improved the deposition of ZnO nanowires on the surface of the membrane and increased its antibacterial properties. A UF membrane [335] and PES and PES–NH2 membranes were assembled using ZnO NPs to improve the antibacterial activity of the membranes, as shown in Fig. 16. In the PES/PES–NH2 membrane, no antibacterial activity was observed. The PES/PES–NH2 membranes modified with at least 0.8% (wt) ZnO NPs showed antibacterial activity. This kind of membranes also showed high water flux and improved the antifouling and biofouling properties without a decrease in the removal ratios of solutes [336].

Fig. 16. Self-assembled PES/PES–NH2 membranes with ZnO NPs [335]. Copyright 2016, Reproduced with permission from Elsevier Science Ltd.

Khan et al. [336] prepared a Cellulose Acetate ( CA )membrane modified with ZnO nanofillers to increase the antibacterial activity. A schematic of the removal of bacteria and impurities from this system is shown in Fig. 17. Increasing the nano-ZnO content in the 51

membrane increased the antibacterial activity of the membrane. In this procedure, the nanomaterials (CA/ZnO and ZnO), pursue the promotion of bactericidal mechanism by the production of very reactive oxygen-based species (e.g. O2–2, OH., H2O2).

Fig. 17. Schematic of integration of cellulose acetate/ZnO membrane on the removal of bacterial contamination on wastewater [336]. Copyright 2015, Reproduced with permission from Elsevier Science Ltd. Malini et al. [337] fabricated a chitosan/nano-ZnO membrane to investigate antibacterial activity using the disk diffusion method. The synthesized chitosan membrane showed very low antibacterial activity, but the antibacterial properties of the chitosan/nano-ZnO membrane were significantly improved due to the presence of nano-ZnO. ZnO NPs were embedded in a sequencing batch reactor (SBR) to eliminate biological impurities in wastewater [310]. ZnO NPs (1 mg/L) significantly affected the settling of the activated sludge. The phosphorus and nitrogen removal efficiencies were also strongly affected by the ZnO NPs. In the activated sludge, the bacterial community was affected by the presence of ZnO NPs. Denaturing gradient gel electrophoresis (DGGE) analysis showed changes in the community structure and a decline in bacterial diversity, indicating that ZnO NPs decreased the ability to handle changes in the influent wastewater conditions. A flat sheet of a PES UF membrane was modified with different nano-ZnO contents to increase rejection of humic acid in 52

another study [304]. Darabi et al. [338] studied the Fe3O4/ZnO nanocomposite for application on forward osmosis systems as draw solution. The antifouling properties of the abovementioned membrane have been investigated as well.

5.4.

Other applications for water and wastewater treatment

5.4.1.

Aerobic activated sludge system

The Membrane Bio-Reactor (MBR) system for removal of NPs performs better than conventional activated sludge (CAS) because it effectively rejects the pollutant and offers increased biological adsorption of high biomass concentrations. Mei et al. [303] presented an MBR system containing nano-ZnO to produce high Soluble Microbial Products (SMPs) to intensify the production of biomass-associated products by enhancing both NP concentration and exposure time. The increase in adsorption was ascribed to the increase in SMP concentration in the presence of ZnO NPs rather than the alternative of fluidity. Filtration testing showed that membrane fouling increased as the NP and SMP concentrations increased, but the removal of bacterial sludge improved with the increase in NPs and SMPs. In another work [310], a SBR was modified with ZnO NPs to investigate bacterial community dynamics and pollutant removal efficiency for activated sludge. The most important conclusion of this work was that activated sludge has poor settling properties, which resulted in sludge loss in the wastewater.

5.4.2.

Removal of metal ions

Pollution caused by heavy metal ions in natural water bodies and wastewater is a critical environmental problem with a harmful effect on living species. Ion-exchange based technologies

53

(e.g., polymeric resins and membranes) can be used to abate this type of contamination effectively. Parvizian et al. [278] worked on the electrochemical characterization of an ionexchange membrane (IEM) modified by nano-ZnO under different electrolyte conditions and using electrically-driven processes, such as Electrodialysis (ED) configurations. The modified IEMs were characterized to improve their performance under different conditions and their performance to extract and concentrate double-charge ions (e.g., most of the toxic heavy metals) from monocharge ions was evaluated. By using this method, membranes were able to buffer the pH of the treated stream, as well as remove metal ions from the water and produce desalinated water. By increasing the amount of ZnO nanomaterials, metal ion removal increased accordingly, and after a certain amount of ZnO loading (10 wt.%), the performance of the membranes decreased. Zhang et al. [288] fabricated PVDF/nano-ZnO hybrid membranes to remove copper ions from an industrial wastewater. Fig. 18 shows the Cu(II) adsorption kinetics on the PVDF/nanoZnO hybrid membrane. Clearly, the adsorption rates improved quickly after 2 h in comparison with PVDF membranes, which indicates that for Cu(II) ions, there were sufficient adsorption sites to be accommodated. The hybrid films had remarkable adsorption ability for Cu(II), and the maximum adsorption capacity of the PVDF/nano-ZnO hybrid membranes was more than nine times that of unspoiled PVDF films. Combining the advantages of both PVDF and ZnO nanomaterials, the PVDF/nano-ZnO membranes could be used in the separation and enrichment of heavy metal ions. MMMs comprising PVC-based and ZnO composites were also prepared [339] to eliminate Pb(II), Hg(II), and Cd(II).

54

Fig. 18. Copper ion adsorption kinetics on the PVDF/nano-ZnO hybrid membrane [288]. Copyright 2014. Reproduced with permission from Elsevier Science Ltd. 5.4.3.

Swelling and deswelling

The amount of water absorbed by a membrane is significant for calculating product material biocompatibility to decide if the membrane can be used for these purposes. A basic relationship exists between polymer network swelling in a solvent and the nature of the polymer and solvent. The membrane swelling capacity influences dimensional stability, electrical resistance, selectivity, and hydraulic permeability. Chaturvedi et al. [340] investigated the swelling and deswelling of a PVA/nano-ZnO membrane and found that as the amount of PVA increased, the swelling ratio decreased. The swelling ratio also declined as the number of freeze-thaw cycles increased. The swelling ratio increased as the concentration of ZnCl2 increased from 0.07 to 0.22 M and then decreased as the concentration increased further from 0.22 to 0.29 M. The amount of water sorption is found to increase from an acidic pH to the neutral range, and a decrease in the swelling ratio was found with an increase in pH into the alkaline range. The swelling ratio increased as the temperature of the swelling medium increased from 15 to 25°C; outside of this range, the swelling ratio

55

decreased. Wahid et al. [341] prepared carboxymethyl chitosan (CMCh)/nano-ZnO hydrogel to investigate the swelling behaviour of the membrane at different pH values. The CMCh/nanoZnO hydrogel showed higher swelling behaviour in solutions with different pH values when compared with neat CMCh. Several studies have used nano-ZnO polymeric and ceramic based membranes for water and wastewater applications. The types of water and wastewater pollutant and water treatment are critical problems to be examined [262, 286, 287, 289, 295, 296, 299-311, 314, 342].

5.4.4. Evaluation of ZnO ceramic and polymeric membranes To emphasise the importance of ZnO in membrane technology and give a comparable idea of usability, functionality, and a thorough evaluation of ZnO ceramic and polymeric membranes, Table 8 summarizes a thorough comparison of ceramic and polymeric membranes with ZnO. Five major indicators have been considered as the key-aspects for the co-evaluation of this material. These key-aspects are: a) technological maturity, b) cost-effectiveness/economies of scale achievement, c) heavier- or lightened- polluted wastewater, d) scaling-up perspectives, e) industrial or large-scale perspectives under specified capacities of wastewater/greywater volumes treated. For the first indicator, technological maturity, it shows that the polymeric composites and membranes are highly developed and can be marked as highly matured technology based on the variety of works have been done in this area and the discussions on these materials that prove them as highly matured technologies [54, 343-354]. For the ZnO/ceramic membrane and membranes, due to the high cost and less application of them, fewer studies have been done on them and they are new composites in comparison with the polymeric materials, then, they can be

56

defined as underdevelopment materials [314, 354-360]. For the cost-effectiveness/economy of scale achievement, ZnO is a cheap material as compared with other types of fillers, in addition, polymeric composites with ZnO are cost-effective due to the low cost of polymers and can be easily used [343-350, 352, 359, 361-365]. While, the ZnO/ceramic composites are expensive and not cost-effective and are being used mainly for special high-added value applications, where polymeric composites due to the less physical and thermal strength and lower separation properties cannot be applied [314, 355-359, 361, 365-368]. The ZnO/polymer composites can be used for lightened polluted wastewater as separation properties of the polymers are not as good as ceramic [343-350, 361, 362, 369] [259, 352, 365, 370, 371], while, the ZnO/ceramic materials can be used for both heavier and lightened polluted wastewater [314, 355-358, 361, 365, 368, 372, 373]. It is possible to scaling up both ceramic and polymeric membranes using ZnO, but, due to the lower price of polymers, they are more commercialized, therefore, they can easily scaled up, as have been observed in the last two decades [352, 359, 365, 370, 374-376]. For the ceramic membranes with ZnO scaling up is being done in a few cases, but needs to have more research focused on that to become more achievable [359, 365, 368, 375]. In the large scale aspect, there are few works that have been done on both of the ZnO/ceramic and polymeric membranes, but, due to the more reliable performance of polymer membranes, these compounds are rapidly attracting large-scale industrial applications. [352, 374, 377, 378], while ceramics may be of interest in industrial applications where polymeric membranes cannot be used [48, 374, 379].

57

Table 8. Comparison of ZnO ceramic and polymeric membranes on water treatment applications. Performance

Polymeric Membrane

Ref.

Ceramic Membrane

Ref.

a) Technological maturity

Highly matured

[54, 343354]

Under development

[314, 354360]

b) Costeffectiveness/economies of scale achievement

Achievable due to the low cost

[343-350, 352, 359, 361-365]

Achievable in special cases due to the higher cost

[314, 355359, 361, 365-368]

c) Heavier- or Lightenedpolluted wastewater

Mostly lightened, in some cases both

[343-350, 361, 362, 369] [259, 352, 365, 370, 371]

Heavier and lightened

[314, 355358, 361, 365, 368, 372, 373]

d) Scaling-up perspectives

Achievable

[352, 359, 365, 370, 374-376]

Achievable for special applications

[359, 365, 368, 375]

e) Industrial or largescale perspectives under specified capacities of wastewater/greywater volumes' treated

Achievable in the near future

[352, 374, 377, 378]

Less achievable

[48, 374, 379]

6. Challenges and future research and development ZnO nanomaterials can be used in polymeric and ceramic membranes to improve properties such as roughness, permeability, and antifouling. Membranes with nano-ZnO as a filler can be prepared by embedding ZnO nanomaterials into different polymer membranes using different methods. The synthesis, morphology, shape, and characterization of ZnO nanostructures and fabrication methods and characterization of the membranes were reviewed. Both ceramic and

58

polymeric membranes have been extensively studied as supports for nano-ZnO membranes in water and wastewater treatment. This review gives some great conclusions and brings new perspectives, such as the fact that the hydrophilicity of blended membranes with a ZnO nanostructure increases. ZnO nanomaterials as fillers in polymeric and ceramic membranes are used in water and wastewater treatment to improve transport, fouling, toxicity, swelling, and removal of metal ions. On the other hands, the biggest challenge in permeability is to synthesis ZnO nanomaterials with higher active surface area and, to do so, ZnO nanomaterials need to be prepared with 3D shapes. The best shape for this purpose is flower-like ZnO, which has nanorod branches. Therefore, one of the biggest challenges for the scientist is to prepare this kind of shape to use their properties for improving both water permeability and pollutants rejection. The addition of nano-ZnO to membranes improves antifouling and bacteria removal properties, and the permeate flux will improve significantly with the addition of this nanomaterial. Improvement in the adsorption of heavy metals results from increasing the amount of nano-ZnO added to the membranes. The effect of the shape of ZnO nanomaterials on membrane properties differs in water and wastewater treatment. The biggest challenge in this area, which needs massive research and studies, is to use biodegradable-based polymeric and ceramic materials and to incorporate ZnO nanomaterials in the membrane active layer instead of using in situ coating methods. In addition, it is necessary to increase the bandgap of these materials, such as quantum dot materials, using different methods, such as the doping method and by functionalizing ZnO nanomaterials with other organic and inorganic functional groups. The electrochemical features of nanocomposite membranes differ according to the changing antibacterial and semiconductor properties of ZnO nanostructures. This kind of IEM is

59

attractive for wastewater treatment. ZnO nanomaterials improve membrane functionality and make the membrane more efficient for water and wastewater treatment. They have been widely used in water and wastewater purification industries in recent decades. For the best results in this type of membrane, porous polymeric fibre should be prepared, and the ZnO nanomaterials should be put into these porous fibres to evaluate the functionality of these membranes in wastewater and water treatment. It can be predicted that the use of membranes embedded with ZnO nanomaterials for water treatment will improve significantly in the future. In addition, since the use of ZnO nanomaterials in these processes has increased in the last decade, it can be concluded that future research and development in this area is bright, even though ZnO is currently not as popular as zeolite, TiO2, CNTs, or graphene.

Acknowledgments This research was supported by the Waste2Product project (ref. CTM2014-57302-R) and by R2MIT project (ref. CTM2017-85346-R) financed by the Spanish Ministry of Economy and Competitiveness (MINECO) and the Catalan Government (ref. 2017-SGR-312).

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Highlights ZnO has been postulated as a filler in polymeric and ceramic membranes Recent progress on ZnO nanostructure preparation and characterization is reported Environmental applications using ZnO-embedded polymeric/ceramic membranes are included Approaches to control membrane properties to reduce fouling and biofouling are also covered

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