Nanostructures in Water Purifications

Abstract

Effective water purification strategies are essential in addressing the serious global concern of scarce water. Nanomaterials have recently garnered considerable interest due to their excellent chemical, mechanical, physical, and biological properties, making them promising candidates for use in water filtration systems. Nanomaterials, with their high hydrophilicity, surface area, and variable surface characteristics, have shown potential in removing water-based contaminants. This review provides an overview of current developments in the development of nano-membrane materials for filtration systems. We discuss various kinds of nanoplatforms, such as polymeric nanocomposites, MXene nanosheets, metal/metal oxide nanoparticles (NPs), carbon nanotubes, metal–organic frameworks, nanofibers, and nanotubes, and their mechanisms of action in removing impurities. Furthermore, we summarize the possibilities and challenges associated with the use of nano-membrane systems, including potential environmental impacts and the need for sustainable and affordable production technologies. Overall, the application of nanomaterials in purifying water shows great potential for providing safe and clean drinking water to people around the world.

1. Introduction

Global warming and water pollution cause difficulties in finding clean water, and this crisis gets worse as the world economy and population grow [1]. Several contaminants caused by industrial or natural activity are detected as impurities in water, rendering it non-potable or unsafe for drinking.

Understanding that water covers 70% of the earth, but only 3% of the water on Earth is drinkable and most of this water (2%) is not accessible as it is locked in ice caps, glaciers or the soil. Over 1.1 billion individuals do not have access to freshwater, and 2.7 billion people globally experience water scarcity for at least 1 month of the year. Poor sanitation affects 2.4 billion people because it puts them at risk for typhoid, water-borne illnesses, and diarrhea. Every year, diarrheal diseases alone claim the lives of 2 million people, predominantly youngsters. Droughts might affect two-thirds of the worldwide people by 2025. Furthermore, environments across the globe will experience increased damage [2]. Thus, development in water filtration systems is critical and urgently needed to maintain the supply of healthy water.

The types of water pollutants can be classified as:

1. Physical pollutants

   Mostly refer to water contamination brought on by rocks and sediments. These contaminants cause liquids’ color, opacity, and texture to alter visibly.

2. Chemical pollutants

   Chemical pollutants can be manmade or natural. Bleach, insecticides, and pipeline corrosion are examples of man-made chemical pollution. Whereas natural chemical pollutants include toxins, nitrogen, and arsenic produced by bacteria.

3. Biological pollutants

   Viruses, bacteria, protozoa, parasites and microorganisms are examples of biological contaminants.

4. Radiological pollutants

   Radiological pollutants can be manmade or natural. They are the least frequent pollutants. However, there is a strong correlation between radioactive substances (e.g. cesuim and arsenic) and major health issues, including cancer. Nearly every ionized molecule can be radioactive, although cesium is the most prevalent element. A water supply can become contaminated by the improper disposal of radioactive waste, damaged reactors, or nuclear fallout. Radiological contaminants are growing in size as a result of heavier reliance on nuclear power.

In another classification as shown in Figure 1, the main sources of water pollution are sewage, industrial waste, global warming, atmospheric deposition, marine dumping, underground storage leakage, radioactive waste and oil pollution. More specifically, wastewater, pigments, heavy metals, radioactive elements, drugs, solvents, and textile dyes are typically the primary pollutants that seriously pollute the world’s largest freshwater resources, having damaging effects on both the health of humans and aquatic species [3]. For examples, the presence of specific metal ions, primarily chlorides, sulphates, and bicarbonates of magnesium and calcium result in the accumulation of lime scale can clog plumbing and encourage galvanic corrosion. Iron is another example, even in minute amounts, as low as 0.3 ppm, iron alters the flavors of foods and beverages, can discolor flooring, sinks, and other fixtures, and can cause pipe blockages [4].

Figure 1.

Sources of water pollution.

2. Water treatment processes

Water treatment processes (WTPs) can be divided into two categories: conventional WTPs and non-conventional WTPs as shown in Table 1. The most fundamental and often used traditional methods in WTP include sedimentation, coagulation, flocculation, and filteration.

Conventional WTPs	Non-conventional WTPs
Coagulation Flocculation Electrocoagulation Filtration Chlorination Fluorination Membrane bioreactors Sand Filtration Disinfection Pulsatube Clarifier Actiflo’ Technology Dissolved Air Floatation (DAF) Sedimentation (Membrane filtration technology) Reverse osmosis Ion exchange Adsorption	Ozone in water treatment Applications of Ultraviolet (UV) Light Biological Activated Carbon (BAC) concept Ultrafiltration (UF) and Microfiltration in Membrane Technology

Table 1.

Comparison of various water treatment processes.

In Australia, coagulation; flocculation; sedimentation; filtration and disinfection as a conential WTPS and they use biological activated carbon treatment as a non-convential WTPS.

2.1 Water quality index

The WQI method employs six criteria to determine the quality of river water: pH, ammonia-nitrogen (AN), suspended solids (SS), and biochemical oxygen demand (BOD) [5]. The Department of Environment Malaysia (DOE) developed the idea of evaluating water quality based on pollutant loading and river water categories under the National Water Quality Standards (NWQS) [5]. Nine variables are combined by the WQI: oxygen that has been broken down, nitrate, pH, temperature, phosphate, fully broken down solids, coliform bacteria, and natural oxygen interest (Body). The Water Quality Index (WQI) utilizes a single worth to represent the overall water quality of a specific source at a given time, based on selected water quality criteria. The ranking is based on the values of Q and W, where Q represents the degree of water quality relative to any given boundary and W refers to the general significance of the individual boundary to the overall quality of water. The five classifications for the positional measures of water quality are as follows [5, 6]:

The WQI is termed,

• extremely bad when it is less than 25.

• awful between 26 and 50.

• moderate between 51 and 70.

• acceptable between 71 and 90.

• great between 91 and 100.

For example, in Australia, the average WQI values for Port Kembla (76.78), Port Jackson (79.07), Port Yamba (77.67), Port Eden (67.42), and Port Newcastle (75.15) were calculated. The water quality ratings for these ports during high and low tides were assessed as good, except for Port Botany and Eden [6].

3. Drawbacks of traditional water purification techniques

Water treatment for consumption and other applications has traditionally been done using conventional methods including chlorination, sedimentation, and membrane processes. Nevertheless, these strategies have several limitations that reduce their potency in tackling the escalating worldwide shortage of drinking water. For example, the application of chlorine as a cleaner may generate hazardous disinfection by - products that may be harmful to people’s health. Chemical precipitation and filtration strategies might also fall short in their capabilities to get removal of some pollutants, like organic toxic elements, which can be dangerous to both the environment and the health of humans. These solutions can also be costly, energy-intensive, and complicated to set up, which makes them less practicable for adoption in remote. Therefore, it is necessary to develop novel and ecologically friendly water purification solutions that can get beyond these limitations and offer everyone a source of pure and secure water. Several techniques are examined for this purpose individually or in combinations as below:

Reverse osmosis: By using this procedure, harmful minerals are removed from the water, making the treated water acidic. Chemicals, chlorine, chloramines, volatile organic compounds, and medicines cannot be removed with this technique. Demands a lot of energy.

Distillation: The majority of toxins are left behind and need a lot of water and energy. Pollutants having boiling points above 100°C are challenging to eliminate.

Ultraviolet/treatment: Expensive approach that is deactivated by water turbidity and cloudiness. Removing heavy metals and other nonliving pollutants is inefficient.

Chemical transformation: Reagents are needed in excess. The finished product can be a poor mixture that cannot be released into the environment. Inactive amid difficult circumstances. This approach is not very discriminating.

Fluctuation and coagulation: To attain the ideal pH, this procedure is difficult and less effective and calls for alkaline additions.

Biological process: Microorganisms are challenging to regulate and sensitive to environmental conditions. The microbial cells are damaged by intermediates. Cost-wise, this is inefficient. It takes time.

Ultrafiltration: Dissolved inorganics cannot be removed using this technique. Demands a lot of energy. Prone to particulate clogging and challenging to clean.

Microfiltration: nitrates, fluoride, metals, salt, volatile organics, color, and other contaminants cannot be removed. It requires routine cleaning. Membrane fouling is inevitable. Less vulnerable to microorganisms, particularly viruses.

Carbon filtration: Nutrients like salt, fluoride, metals, and nitrates cannot be removed. Solids that cannot be dissolved cause clogging. Susceptible to mold. Filters must be changed frequently.

Thus, there was a need to find more advanced techniques to improve the efficiency and decrease the price of filtration systems.

4. Membrane filtration technology

Membrane filtration is a separation method that covers a wide variety of pollutants, from NPs to microparticles, and a wide range of water filtration systems are produced to use a simple and effective way in the filtration of industrial and domestic wastewater. Micro/nanoparticles can be produced using a variety of technologies, such as the Innotech encapsulator, interphase, ionic-gelation methods, vibrational jet-flow, and dripping techniques [7, 8]. As an example of microparticles, Ali et al. (2017), prepared three biopolymeric formulations in the micro size, 2 mm ± 10%, based on chitosan, alginate and carrageenan to chelate heavy metals [9]. The results showed that most of biopolymeric beads can chelate up to 85–100% of cations (500 mg/L).

5. An overview of the nano-membrane systems

Nanomaterials on the other hand can be categorized as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D) based on their dimensions. Quantum dots and nanodispersions are examples of 0D nanomaterials that have all of their dimensions at the nanoscale. Whereas 1D nanomaterials, such as nanotubes and nanowires, have two dimensions at the nanoscale. 2D nanomaterials, such as films with nanosized coatings and nanosheets, are characterized as nanostructures. Whereas mesoporous bulk powders and nanoclusters are a feature of 3D nanomaterials. The design of sustainable and biocompatible membranes and the use of Euglena, nanomaterial-based, carbon- and nanostructure-based membranes for water purification systems are just a few samples of the achievements that have already been produced [10, 11]. This popularization of membrane materials is due mainly to filtration systems’ substantially smaller carbon footprint, cost-effectiveness, and high performance as compared to other methods of purifying water [12].

Today, a wide range of combined mixtures, including volatiles, small particles, and organic solvents, have been effectively removed using membranes with controllable pore sizes based on ultrafiltration, microfiltration, nanofiltration, solvent extraction, gas separation technology, and reverse osmosis. The functional use of membrane separation methods rapidly requires the production of high-performance nanomembranes with excellent stability, high selectivity, permeability, and protective coating properties.

To solve the above problems, nanomaterials of various sizes have been used to develop nano-based membranes. These nanomaterials include polymeric nanocomposites [13], metal/metal oxide NPs [14], polymer/metal/metal oxide NPs [15], zeolites [16], carbon nanotubes [17], graphene/graphene oxide (GO) [18], transition metal dichalcogenide-based nanosheets [19], two-dimensional (2D) MXene nanosheets [20], and metal–organic frameworks [21]. In Figure 2, the utilization of nanomaterials with varied dimensions in the development of nano-adsorbents is presented. With the quick advancement of materials science, several nanostructures, including zwitterionic colloids NPs [22], hyperbranched polymer dendrimer nanostructures [23], biomass-based metallic NPs [24], TiO₂ NPs [25], CoFe₂O₄/FeO/CoFe nanocomposites [26], CuO/CN [27], ZnO nanoneedles-Ti₃C₂ MXene/PVDF [28] and poly (vinyl alcohol) (PVA)-based nanofibers [29], have been employed to develop membrane materials for water purification applications.

Figure 2.

Utilization of nanomaterials with varied dimensions in the development of nano-adsorbents.

5.1 Polymeric nanocomposites

Polymeric nanocomposites are nanosized formed from NPs and polymers. They are employed in water purification to remove pollutants such as microorganisms, organic molecules, and toxic metals. The matrix phase provides durability and mechanical strength, while the nanomaterials offer enhanced reactivity and a large surface area. The adsorption process of polymeric nanocomposites is based on chemical and physical interactions between pollutants and adsorbent. The physical adsorption involves weak intermolecular interactions and Van der Waals forces between pollutants and adsorbent. Chemical adsorption involves the generation of covalent or ionic interactions between pollutants and adsorbent [30].

Two different types of polymers nanobased membranes have been developed:

1. direct coating or self-assembly of polymer nanomaterials into homogeneous nanobased membranes.

2. incorporation of polymer-based nanostructures into a polymer matrix to fabricate mixed matrix polymer nanomaterial-based membranes [31].

5.2 Metal/Metal oxide nanoparticles

Metal/metal oxide NPs are commonly used in water treatment to remove pollutants, including microorganisms, pathogens, toxic metals, and organic compounds. These nanostructures are highly reactive and efficient at removing pollutants due to their unique morphologies. The adsorption processes of metal and metal oxide NPs are based on the chemical/physical interactions between adsorbent and impurities. The surface characteristics of the NPs, such as their structure, morphology, and charge density, play a major role in determining their selectivity and adsorption efficiency. Adsorption mechanisms of these NPs involve both chemical and physical interactions between the adsorbent and adsorbed species. The physical adsorption is based on hydrogen bonding, dipole–dipole attraction, and Van der Waals forces, between pollutants and adsorbents. The high surface area and reactivity of meal-based nanoparticles enhance physical adsorption by improving contact area between adsorbents and adsorbates. Chemical adsorption, on the other hand, involves the formation of covalent or ionic bonds between adsorbents and impurities. This can occur through various mechanisms, including ion exchange and complex formation. Metal and metal oxide NPs can form chemical bonds with a variety of contaminants, including toxic metals and organic compounds. Using advanced oxidation processes, such as photocatalytic activity and Fenton processes, metal and metal oxide NPs can also catalyze the removal of contaminants. Reactive oxygen species, which can oxidize and degrade a wide range of chemicals, are produced during these interactions. In addition, both physical and chemical interactions with impurities play a role in the adsorption mechanism of metal and metal oxide NPs. The adsorption efficiency and specificity of NPs are strongly affected by their surface morphology [32].

5.3 Carbon nanotubes

They form hollow and cylindric structures called carbon nanotubes. They are used in the elimination of pollutants including pathogens, germs, and organic compounds from water. Carbon nanotubes have a broad surface and are reactive, which makes them excellent at removing toxins. The unique electronic properties of CNTs enable them to facilitate the adsorption of polar and ionic pollutants via hydrophobic interactions, hydrogen bonding, π-π interfaces, electrostatic interactions. In contrast, chemical adsorption of CNTs involves the generation of covalent or ionic bonds through various mechanisms, such electrostatic interaction, and hydrogen bonding. Overall, CNTs are well-suited for removing pollutants in water treatment due to their excellent surface area, distinctive electronic properties, and strong adsorption capacity [33].

5.4 MXene nanosheets

MXene nanosheets are effective adsorbents for removing impurities because of their two-dimensional layered structure. The unique surface chemistry and shape of MXene nanosheets form the basis for their adsorption mechanism. Surface functional groups such as -O, -OH, and -F, on surface of MXene nanosheets enable the adsorption of various impurities through hydrogen bonds, electrostatic interactions, and covalent bonds. The high surface area and aspect ratio of the nanosheets create a large number of adsorption sites, enhancing their capacity and selectivity for adsorption [34].

5.5 Metal–organic frameworks

Metal–organic frameworks (MOFs) have an excellent porous structure that makes them promising adsorbents for a variety of pollutants. The adsorption mechanism of MOFs is based on a combination of physical and chemical interactions. The porous structure provides a large surface area and many open metal sites that can interact with the adsorbate. The chemical interactions involve the coordination of metal centers with the adsorbate through various chemical processes such as complexation, ion exchange, electrostatic interaction, pore filling, hydrogen bonding, redox reaction, covalent attachment, encapsulation adsorption, and chelation. The organic ligands in MOFs can also interact with the adsorbate through hydrogen bonding, van der Waals forces, and π-π interactions. Moreover, the structure, size, shape, and functional groups of MOFs can be modified to increase their affinity for specific pollutants, making them an appealing choice for removing a wide range of pollutants [35].

5.6 Nanofibers

Nanofibers are flexible adsorbent materials that find wide applications in water treatment. Although the adsorption mechanism of nanofibers can vary depending on the specific material and impurity, there are general mechanisms that apply to multiple nanofiber-based adsorbents. One of these mechanisms is physical adsorption, where impurities are trapped within the pores and channels of the nanofiber structure through van der Waals forces, reduction–oxidation adsorption, weak intermolecular interactions. Due to their high surface area and porosity, nanofibers provide numerous adsorption sites, making them highly effective. In addition to physical adsorption, some nanofiber-based adsorbents also use chemical adsorption mechanisms. These mechanisms involve the formation of covalent or ionic bonds between impurities and the functional groups on surface of nanofibers or the use of electrostatic interactions to attract and capture charged impurities. Overall, nanofibers’ combination of high surface area, porosity, and chemical reactivity make them highly effective for a wide range of impurities in water [36].

5.7 Nanomembranes

In light of the creation of an effective purification mechanism, membrane systems manufactured with different types of nanostructures inside the polymer matrix typically show a significant enhancement in permeability. Despite the improved purification efficiency of these inorganic NPs-based membrane systems, the majority of inorganic NPs tend to agglomerate and have poor biocompatibility with the biopolymer matrix. A wide range of 2D nanostructured materials, including GO, tungsten disulfide (WS2), molybdenum disulfide (MoS2), 2D oxides, and hexagonal boron nitride, have shown potential as components for innovative separation and purification in the areas of desalination and water treatments, since they possess both the structural performance of coatings and the adaptability of polymeric materials [37]. In brief, using 2D nanosized materials to create these advanced membrane systems represents a potential way to advance water filtration methods. As an example of these membranes is Mxene-based membranes.

5.7.1 MXene- based separation membranes

MXene, or 2D transition metal carbides, has just entered the market as a new 2D material for separation and purification systems. The general formula for MXene, a 2D transition metal carbide and nitride, is Mn + 1AXn (n = 1, 2, and 3). These ceramics were named MAX phases by researchers. d-block transition metal M (V, Cr, Ni, Ti, Sc, etc.). The primary group element is A. Either N or C atoms constitute X. 2D transition metal carbide/nitride (MXene) 2D nanosheets were produced by methodically etching the layers of main group elements from the bulk precursor of the ternary MAX phase [38]. 2D MXene and MXene-composites have been shown to be suitable membrane solutions for oil/water separation [39], sustainable water desalination [40], hydrogen purification [41], and dye/salt separation [42] due to their hydrophilic nature, simple scale-up manufacturing, good biocompatibility, and nontoxicity. Moreover, MXene has considerable benefits for adjustable tuning of the d-spacing of MXene films, simple surface functionalization, and improved adhesion with enabling surfaces due to the chemical functional groups on the surface.

The impacts of several influencing parameters and various synthesis techniques of 2D MXene-nanosheets have been investigated and reported in the literature [43, 44, 45]. With a strong emphasis on filtration mechanisms, the applications of developed membranes for oil/water separation have been widely researched [46]. 2D MXene-nanosheet-based membranes have been reported to exhibit excellent filtration performances in a variety of applications, including the membrane separation of seawater, waste management, and the purification and separation of petroleum and oil products. The membrane stability and regeneration characteristics of the previously employed membranes have also been evaluated [47]. Many difficulties with the development, use, and application of MXene-based membranes have been highlighted, and recommendations for further study have been presented. Uniform 2D MXene-nanosheets-based membranes have been developed because of their varied shapes, adjustable functionality, excellent water solubility, and compatibility with biopolymeric matrices. Furthermore, from 0D to 2D nanostructures can interact with contaminants in a wide range of ways, depending on the kind of pollutants being investigated, including Van der Waals interactions, Lewis’s acid–base, chemical bonding, π–π stacking, hydrophobicity, surface complexation, electrostatic interaction, and hydrogen bonding [48]. Recent studies focused on controlling the hydrophilicity and hydrophobicity, charge property, pore size, surface integrity, and morphology of MXene-based membranes to increase the membrane’s performance.

Table 2 offers a comprehensive list of current examples of adsorption-based nanostructures and their distinctive attributes for efficient pollution removal. These experimental findings, showcasing the diverse array of nanostructures and their applications in combatting industrial and environmental contamination challenges, serve as a general resource for researchers and professionals in the field.

Adsorbent	Adsorbate	Characterization	Adsorption performance	Ref.
Silver NPs combined with yttrium oxide (Ag-Y2O3) nanocomposite	Cu(II) and Cr(VI)	Particle size of 65 ± 3 nm	Adsorption capacity: 815 mg g^−1and 867.85 mg g^−1 for Cu(II) and Cr(VI)	[49]
Bimetallic Fe/Pd NPs	As (III)	Particle sizes ranging from 30 to 60 nm.; 1.176	Adsorption capacity: 1.176 mg g^−1	[50]
Fe3O4 NPs	Indigo carmine	Nanoparticles with diameters from 20 to 30 nm	Adsorption capacity: 11.51 mg g^−1 to 17.45 mg g^−1	[51]
Fe3O4/MgO NPs	Amaranth dye	Nanorods with diameters of 14 nm and length of 22	Adsorption capacity: 17.94 mg g^−1 to 37.98 mg g^−1	[52]
CuO NPs	Methylene blue dye	Average crystalline size of 19.28 nm	Adsorption capacity: 464.24 mg g^−1	[53]
Functionalization of chitosan biopolymer with SiO2 NPs	Acid red 88 dye	Final consistent particle size of 250 μm	Adsorption capacity: 252.4 mg g^−1	[54]
Alginate/poly(acrylamide)/carbon nanotubes	Cationic dyes (methylene blue, MB, and crystal violet, CV)	Total porosity was estimated to be 76.6 ± 0.6%	MB and CV adsorption on the composite increased by 1.5 and 1.6 times.	[55]
Tri-n-Butyl Phosphate functionalized Multi Walled Carbon Nanotubes (MWCNTs-COO-TBP)	Ruthenium	Thickness of entire wall measured to be ~7.5 nm	Adsorption capacity: 151.51 mg g^−1	[56]
Iron nitrate modified carbon nanotube composites	Arsenic	The flocculation of FCNTs has increased, with more and more uniform pores	Uptake: 99.1%	[57]
UiO-66-based MOFs	Congo red (CR), and cationic dyes of methylene blue (MB) and rhodamine B (Rh B)	Angular morphology	Adsorption capacity of CR: 93.05 mg g^−1 and 94.20 mg g^−1	[58]
MXene-based 2D Ti3C2Tx nanosheets	Cadmium (Cd^2+)	Lamella-like structure	Adsorption capacity: 475.6 mg g^−1	[59]
MXene@Ag/cryogel composites	Sulfamethoxazole and mercury	Composite with Ag particle size of 100–200 nm.	Uptake: (98% mercury) and degradation (97% sulfomethoxazole)	[60]

Table 2.

Examples of adsorption-based nanostructures from recent times and their characteristics in the removal of pollutants.

6. Conclusion

Several water treatment methods were discussed in this paper. Sedimentation, filtration, flocculation, and coagulation are some of the most essential and popular classical processes employed in WTP. By weighing the advantages and disadvantages of each technique, it is simple to decide which type of filtration technology is best for which kind of region. The 2D MXene-nanosheet-based membrane has diverse forms, adaptable functionality, good water solubility, and compatibility with biopolymeric matrices, and it has made significant strides, however there are still many obstacles to be overcome before more sophisticated membranes can be created.