**1. Introduction**

A reliable, affordable, sustainable and easily accessible clean water supply chain for many societies in the entire world is an essential component for healthier life and safe environment. However, due to limited economical resources or lack of infrastructure, millions of poor and vulnerable people including children die annually from diseases caused by an inadequate water supply, poor water quality, sanitation and hygiene. Recently, many countries and communities experienced the global challenge/phenomenon known as "Coronavirus (COVID-19) or COV2 infections", which required a frequent washing of hands with clean water and soap or hand sanitizer to avoid or curb the spread (flatten the curve). These key risk aspects or factors adversely impact on food security, livelihood diversities and learning opportunities for poor and most susceptible households across the world. According to the World Health Organization (WHO), almost 1.7 million people lost their lives because of water pollution, and four billion cases of diverse health issues were reported every year due to water borne diseases [1]. Table 1 represents various types of water contaminants, their sources and negative effects. To improve access to quality and safe drinking water, sanitation, and hygiene (WASH), there must be value-added infrastructure investment in dealing with and managing the freshwater ecosystems and sanitation facilities on a local level in many developing countries. The improved WASH is thus fundamental to poverty reduction, promotion of equality, and support for socioeconomic development under the sustainable development goals (SDGs) [2,3]. The most essential requirements for clean water supply chain is a proper material with high degree of separation capacity, low cost, porosity, and reusability [4–7]. Nanotechnology presents a set of opportunities to develop nanomaterials for e ffective water purification systems. Optimization of the properties like hydrophilicity, hydrophobicity, porosity, mechanical strength and dispersibility [8–10] is the best option to treat wastewater. Due to their high surface area, high chemical reactivity, adsorption capabilities, excellent mechanical strength and cost-e ffectiveness, nanomaterials have a huge potential to e ffectively purify water in numerous ways [8,10–12] by removing various contaminants. This can be done by using di fferent purifiers with di fferent pore sizes such as: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) (Figure 1). However, the main stumbling block associated with addition of 2D nanomaterials is the aggregation or agglomeration that restricts their effective use in many industrial applications. This daunting aggregation or agglomeration challenge of nanomaterials can be minimized by (i) transforming 2D nanomaterials into nanocomposites and (ii) surface modification of 2D nanomaterials, owing to their excellent interfacial interaction between the surface of 2D nanomaterials and polymer matrices. Surface modification of nanomaterials (SMNs), compared to unmodified nanomaterials, has attracted a considerable interest in science communities.



**Figure 1.** Trans-membrane pressure processes for water treatment technologies with different pore sizes. Reproduced with permission from Reference [13]. Copyrights 2018, Elsevier Science Ltd.

Nanocomposites are multi-phasic materials, in which at least one of the phases shows dimensions in the nano range of 10–100 nm [24,25]. Currently, these materials have emerged as alternatives to overcome deficiencies of different engineering materials and are said to be the 21st century materials, due to their design uniqueness and property combinations which are different from conventional composites. Nanocomposite materials can be classified according to their primary phase (matrix) and secondary phase (reinforcing filler) [26,27]. Among different nanocomposites, polymer-based nanocomposites (PNCs) have become a noticeable field of current research interest and innovation development. PNCs have a lot of advantageous multifunctional properties such as film forming ability, dimensional variability, and activated functionalities [8,28]. Generally, the properties of PNCs are strongly related to the type of polymer matrix and the extent of dispersion of nanomaterials incorporated into the polymer matrix, as well as interfacial interactions between the polymer and nanomaterials [29–31]. The improved interfacial interactions of the nanomaterials with pure polymer change the overall morphology leading to synergistic effects in the nanocomposite properties. The accomplished properties are much better than the individual constituents. Finally, the properties of the PNCs are directly dependent on the volume fraction of nanomaterials, aspect ratio, alignment in matrix and other geometrical factors [32,33]. The main challenges in the development of superior PNCs are (i) the selection of appropriate nanomaterials that possess specific interfacial interaction, (ii) compatibility of nanomaterials with polymer matrix and (iii) suitable processing method to evenly disperse and dispense these nanoparticles within a polymer matrix. The impact of polymer nanocomposites (PNCs) in wastewater treatment can be recognized by an uninterrupted rise in publications over the past ten years. This is contrary to the less extensively instigated environmental impacts of nanomaterials in polymer nanocomposites in water purification (Figure 2).

**Figure 2.** Number of publications in polymer nanocomposites (PNCs) and water treatment. Reproduced with permission from Reference [34]. Open Access 2020, MDPI Water.

In this review, we focus on the comparison of 2D nanomaterials such as layered double hydroxides (LDHs) and other nanoclays in polymer-based nanocomposites (PNCs), their preparation methods and multifunctional properties and their use for water purification. The primary goal is to highlight an optimal efficiency of LDHs and other nanoclays adsorption capacities and recent progress of nanomaterials in decontamination ability of various pollutants with selectivity including practical and potential applications.

#### **2. Comparison of Other Nanoclays and LDHs Crystal Structures**

#### *2.1. Other Nanoclays Crystal Structure*

Nanoclays (NCs) are a broad class of naturally occurring inorganic minerals optimized for use in polymer-clay nanocomposites for water purification and environmental protection. NCs are versatile and two-dimensional (2D) building blocks for multifunctional material systems with several property enhancements targeted for many applications. Based on their chemical composition and particle morphology, clay minerals are categorized into many classes such as smectite, chlorite, kaolinite, illite and halloysite. Nanoclays have been studied and developed for various applications [29,31,35] and are abundantly available, very cheap and low environmental impact. Clay minerals are members of the phyllosilicate or sheet clay silicates consisting of hydrated alumina–silicates and can be used as natural nanomaterials or nano-absorbent since the dawn of nanotechnology [36]. Nanoclays are nanoparticles of layered mineral silicates with layered structural units that can lead to the formation complex/multifaceted clay crystallites by stacking these layers [37]. The basic building blocks of clay minerals are tetrahedral silicates and octahedral hydroxide sheets [38]. Octahedral sheets consist of aluminum or magnesium in a six-fold coordination with oxygen from a tetrahedral sheet and with hydroxyl (Figure 3). Tetrahedral sheets consist of silicon–oxygen tetrahedra concomitant to neighboring tetrahedral sharing three corners, while the fourth corner of each tetrahedron sheet is connected to an adjacent octahedral sheet via a covalent bond.

**Figure 3.** The layer phyllosilicate structures: (**a**) Type 1:1, (**b**) Type 2:1, and (**c**) Type 2:1:1. Reproduced with permission from Reference [39]. Open Access 2019, MDPI Animals.

The arrangements of these sheets influence a number of contributing factors in clay silicates. Based on their mineralogical composition, there are nearly thirty different types of nanoclays used in various applications [40,41]. Table 2 depicts three major types of phyllosilicates which are distinguished as 1:1 layer type (T-O), 2:1 layer type (T-O-T) and 2:1:1 layer type (T-O-T:O) common in nanoclay materials. In 1:1 lattice structures (T-O), each tetrahedral is connected to one octahedral sheet, while in 2:1 lattice structures (T-O-T), each octahedral sheet is connected to two tetrahedral sheets, one sheet on each side. Lastly, in 2:1:1 lattice structures (T-O-T:O), each octahedral sheet is adjacent to another octahedral sheet and connected to two tetrahedral sheets [31,42–44].


**Table 2.** Classification of clay minerals and their characteristics.

Halloysite nanoclay is an aluminosilicate nanotube naturally occurring clay material with the average dimensions of 15 nm × 1000 nm [45]. This halloysite nanoclay has (1:1-layer type) and the hollow tube structure is primarily utilized in medical applications, food packaging industry and rheology modification [46]. The most commonly used nanoclay in materials applications is plate-like montmorillonite (MMT) material. This MMT has approximately 1 nm of aluminosilicate layers which are surface coated with metal cations in a multilayer stacks of ~10 μm. Depending on surface modification of the clay layers, MMT can be dispersed in a polymer matrix to form polymer-clay nanocomposites with applications such as, flame-resistance, solidifying agents, water purification and gas permeability modification. MMT clay layers with 2:1 layered silicates of T-O have high cation exchange capacity (CEC) on the siloxane surface that can interact well with different substances like

organic or biological molecules [47,48]. The MMT nanoclay stacks have attracted a lot of interest because of outsized surface area, swelling behavior and high cation exchange capacity [49,50]. Unlike MMTs, halloysite materials are easily dispersed in many polymers showing no exfoliation due to scarcity of OH groups on their surfaces. In addition, these tube-like nanoclays are excellent nanomaterials for numerous chemical molecules [51]. Therefore, the modified clays are used as e ffective reinforcing phase for polymers to improve their mechanical and thermal properties. Nanoclays acting as carriers continuously and constantly released some active molecules such as flame-retardants, antioxidants, anticorrosion and antimicrobial agents [52,53]. In recent years, the research and development of novel polymer/nanoclay composites for water purification has attracted a lot of attention in the field of material chemistry [54]. Rigid nanoclay like layered double hydroxides (LDHs) must be used as an e ffective reinforcing filler to polymer structures and impede the polymer chains free movement adjacent to the filler [29–31,54].

#### *2.2. Layered Double Hydroxides (LDHs) Crystal Structure*

Layered double hydroxides (LDHs) also known as hydrotalcite (HT)-like materials are a class of synthetic two-dimensional (2D) nanostructured anionic clays with a highly tunable brucite [Mg(OH)2]-like layered crystal structure (Figure 4). These inorganic materials contain layers of positively charged metal hydroxides with multivalent anions for neutrality. The LDHs are generally represented by formula

$$\left[\text{M}\_{1-\text{X}}^{2+}\text{M}\_{\text{X}}^{3+}(\text{OH})\_{2}\right]^{\text{X}+}\left[\text{A}^{\text{n}-}\right]\_{\text{X/n}}\text{mH}\_{2}\text{O}\tag{1}$$

In this formula, M2<sup>+</sup> and M3<sup>+</sup> represent the divalent and trivalent layer cations, respectively. An− is the exchangeable anion such as OH<sup>−</sup>, F<sup>−</sup>, NO− 3 , Cl<sup>−</sup>, CO2− 3 and/or SO2− 4 . Reasonably stable LDH phases are often observed only when the value of x varies in the range 0.22–0.33 resulting in M<sup>2</sup>+/M3<sup>+</sup> molar ratios of 2:1 to 4:1 [55–59]. If x is more than 0.33, then an increased number of neighboring M3<sup>+</sup> containing octahedra leads to the formation of M(OH)3. If x is less than 0.2, then an increased number of neighboring M2<sup>+</sup> containing octahedra in the brucite-like sheets resulted in the precipitation of M(OH)2. However, these limits of the value of x must be regarded as the maximum interval, which can be narrower depending on the composition of the LDH.

**Figure 4.** Structure of layered double hydroxide (LDH). Reproduced with permission from Reference [60]. Copyrights 2018, Elsevier Science Ltd.

As result, a large class of isostructural materials, which can be well-thought-out complementary to aluminosilicate clays, with useful physical and chemical properties can be achieved. This can be carried out by changing the nature of the metal cations, the molar ratios of divalent/trivalent cations, and the types of interlayer anions. These compounds are composed of positively charged brucite-type octahedral sheets, interchanging with interlayers containing carbonate anions in the natural mineral or other exchangeable anions in the synthetic hydrotalcite (HT)-like materials, along with water molecules. The hydrogen bonding associated with the interlamellar water molecules serves as a driving force for the stacking of the clay layers (see Figure 4).

#### *2.3. An overview of Preparation Methods of LDHs*

In the last few decades, a number of studies associated with the synthesis of LDH have been reported and some are easy and simple to process for many industrial applications. Various kinds of low cost, environmentally and eco-friendly LDHs can be synthesized by using fundamental methods of choice. These commonly used methods include co-precipitation; ion exchange; reconstruction; sonochemical method; hydrothermal/solvochemical method; sol–gel method; induced hydrolysis method; and urea method [57,59–65].
