Influence of Temperature

The e ffect of temperature is typically one of the factors governing the adsorption e fficiency and performance of adsorbents in water purification systems. An increase in temperature with an increase in the adsorption percentage enables the adsorption capacity of clay/LDH-based adsorbents at various temperatures [162]. This is due to the increase in the mobility of contaminants in aqueous solution, which leads to the improvement in the availability of adsorption vacant surface-active sites. Temperature parameter is well-known to have a strong e ffect on di fferent chemical processes. It a ffects the adsorption rate by varying the molecular chain interactions and the solubility of the adsorbate. The e ffect of temperature on the adsorption of Pb(II) on clay/LDH was investigated by Ayawei et al. [162]. It was observed that on increasing the temperature, the percentage removal of metal ions increased (Figure 17). This presented su fficient evidence to conclusively refer to this adsorption process as an endothermic kind of the process.

**Figure 17.** Adsorption capacity and percentage as a function temperature. Reproduced with permission from Reference [162]. Open Access 2015, International Journal of Chemistry.

#### Influence of Sorption Kinetics and Isotherms

The information on adsorption/desorption process and isotherms is one of the key factors required for proper analysis, structure and understanding of the adsorbent–adsorbate system [160]. In recent times, various models have been explored to explicitly explain the adsorption behavior including isotherm models. The latter provide valuable information about adsorption property of a system and the distribution of exchangeable sites on the surface of the adsorbent. In general sorption, diffusion, desorption, sorption isotherms will be discussed in this section [163].

Some of the factors that play a major role in the adsorption/sorption process are explained below. The nanocomposites usually exhibit a very high amount of barrier properties with even a small amount of layered silicate [29]. Some small molecules such as oxygen, carbon dioxide, water and nitrogen permeate through a polymer membrane due to a gas chemical potential gradient through the membrane. The chemical potential difference (Δμ) acts as the driving force for the molecules to permeate from the high chemical potential side to the side of low chemical potential. The mode occurrence of permeant transport in polymers is described using the solution–diffusion model. According to this model, the permeation in polymers consists of three steps as illustrated in Figure 18a (i) sorption of the permeant from the high concentration side onto the membrane/film surface, (ii) diffusion of the permeant along the concentration gradient through the membrane and (iii) desorption through evaporation from the low concentration surface of the membrane. When the permeating molecule interacts with the polymer, then the deviations from a gradient with a straight line can be observed and is known as non-Fickian diffusion explained by the diffusion–relaxation model [164,165].

In this review, we discuss the tortuosity of the diffusive path for nanoplatelets for the gas barrier properties of nanocomposites in Figure 18b. Layered silicate nanoclays such as montmorillonite and kaolinite are the most innovative and promising nanofillers due to their ability to exfoliate to form single nanoplatelets when dispersed in a polymer matrix. The basic theory of the model is that the presence of impermeable clay platelets forces the permeant (gas) molecules to follow a longer diffusion path by traversing around the platelets. Therefore, this is also known as the tortuous path model as illustrated in Figure 18b. The nanoplatelets hinder the diffusion process of small gases through them and form a tortuous path which act as a barrier structure for gases. Tortuous paths explicitly explain the principle of the barrier behavior and its noticeable improvement in nanocomposites, which can be attributed to the impermeability of the layered clay silicate into the polymer matrix. Therefore, the intercalation molecules are placed in a wiggle shape around the nanoparticles in a random way [166,167] as seen in Figure 18b.

**Figure 18.** (**a**) Schematic illustration of the solution diffusion model. Reproduced with permission from Reference [168]. Open Access 2020, MDPI Polymers. (**b**) Schematic illustration of the tortuous path model. Reproduced with permission from Reference [95]. Open Access 2017, RSC Advances.

Follain et al. [169] investigated the water vapor transport properties by pervaporation and sorption measurements and evaluated nanoclay extent of dispersion in polymer matrix. They prepared nanocomposites based on poly(ethylene-co-vinyl acetate) (EVA)/organo-modified Cloisite clays at varying contents by melt blending. Their results showed a noticeable decrease in water permeation flux obtained when nanoplatelets are incorporated into the neat EVA matrix (Figure 19a). This barrier effect is typically attributed to the remarkable increase of the diffusion pathways due to organo-modified Cloisite clay-induced tortuosity effects. Furthermore, the water permeation flux seems to be proportional to the diffusion coefficient, which was found to be reduced due to a plasticization effect of water. The water-induced plasticization effect of sorbed water molecules was highlighted through sorption kinetics, and water barrier behavior was observed.

**Figure 19.** (**a**) Water permeation flux as a function reduced time. (**b**) Water vapor sorption isotherms of the neat EVA matrix and its nanocomposites with an inset of C10A nanoclay isotherm. The dotted lines correspond to the fitting of experimental data with Park's model. Reproduced with permission from Reference [169]. Open Access 2015, The Royal Society of Chemistry.

The water vapor sorption isotherm curves are also known as water mass gain in the equilibrium state versus water activity deduced from water sorption kinetics, reported in Figure 19b. The water sorption capacity of nanoclays is evidently higher than that of the EVA matrix. The incorporation of Cloisite (C10A) into the EVA matrix induced the shift of isotherm curves to lower values at a given activity (Figure 19b), reproducing a reduction in water mass gain. As a result, C10A sorbed higher water mass gain than the matrix, which contributed to increasing sorption capacity of nanocomposites, and this finding is in good agreemen<sup>t</sup> with the reduction of water permeability. The decrease in mass gain can be related to tortuosity e ffects induced by nanoclays in a matrix and which counterbalance the strong water sorption capacity of nanofillers [170]. An increase in nanoclay content, reproduced an increase in water mass gain for nanocomposite samples, which is measured for the whole water activity range due to a more hydrophilic character of nanoclays than the matrix one. This can be attributed to the increase of the polar site number of surfactant-modified nanoclays. The sorption process of the resulting nanocomposites is thus driven by the nanoclay sorption capacity, and the incorporation of nanoclay content into the nonpolar matrix. The nanocomposite with the highest nanoclay loading is categorized by the highest water mass gain. Water molecules are implicitly located in the matrix/nanoclay interfacial regions, and the conclusion is water absorption behavior of nanocomposites obeyed Fick's law. The increased tortuosity within the EVA matrix/nanoclays is therefore in opposite e ffect to the increased water solubility due to the hydrophilic character of nanoclays.

Rajan et al. [171] studied the role of dicarboxylic acid (malic acid (A)) in chitosan (CS)/modified-metal ion decorated bentonite clay (BC) and the defluoridation e fficiency in fluoride contaminated groundwater. The samples were prepared through the solution mixing and their findings revealed the chemical changes of the adsorbent as shown in Figure 20. The bands observed in the range of 3413–3440 cm<sup>−</sup><sup>1</sup> in the spectra of chitosan confirm the presence of O–H bond stretching and N–H bond stretching frequency [172]. The bands at 3046 cm<sup>−</sup><sup>1</sup> and 2900 cm<sup>−</sup><sup>1</sup> related to aliphatic stretching vibrations of –CH, while the bands at 2927 and 2860 cm<sup>−</sup><sup>1</sup> can be attributed to the C–H stretching vibration of the –CH2 groups in chitosan. The aliphatic stretching vibrations of –CH have very low intensity in chitosan [171].

**Figure 20.** FTIR spectrum of (**a**) chitosan (CS), (**b**) aluminum bentonite clay-malic acid chitosan (AlBC-A@CS), (**c**) LaBC-A@CS, (**d**) cerium bentonite clay-malic acid chitosan (CeBC-A@CS) and (**e**) fluoride adsorbed CeBC-A@CS. Reproduced with permission from Reference [151]. Open Access 2020, RSC Advances.

The peak observed at 1596 cm<sup>−</sup><sup>1</sup> is related to N–H bond scission from the primary amine because of free amino groups in the crosslinked chitosan segments [173]. The peak which appeared at 1596 cm<sup>−</sup><sup>1</sup> indicates the aromatic ring stretching vibration. The C=O adsorption peak of secondary hydroxyl groups becomes more pronounced and shifts to 1107 cm<sup>−</sup><sup>1</sup> [174]. It can be seen in Figure 20e that the peak intensity of the hydroxyl group was remarkably reduced because of the fluoride adsorption. Furthermore, the nitrogen adsorption–desorption analysis was done by Rajan and co-authors [170] as evident in Figure 21. The surface area, pore width and pore volume of the synthesized CeBC-A@CS adsorbent was characterized by Brunauer–Emmett–Teller (BET) analysis and reported as 103 <sup>m</sup>2·g<sup>−</sup>1, 12.71 nm and 0.0321 cm3·g<sup>−</sup>1. The synthesized CeBC-A@CS adsorbent possessed high surface area, higher pore width and larger microspore volume, which confirmed that the synthesized adsorbent has maximum adsorption capacity.

**Figure 21.** Nitrogen adsorption–desorption isotherm spectrum of cerium bentonite clay-malic acid chitosan (CeBCA@CS) adsorbent. Reproduced with permission from Reference [151]. Open Access 2020, RSC Advances.

#### **4. Concluding Remarks and Future Prospects**

In summary, polymer/layered clay nanocomposites are the most progressive and alternative procedures for water purification systems. Synthesis of nanomaterials such as nanoclays and/or LDHs play a pivotal role in improving the physicochemical properties of adsorbents for pollutant removal in water. Furthermore, layered double hydroxides (LDHs) materials have attracted considerable attention and are preferred candidates as sorbents in water treatment because of their remarkable ability to eliminate a variety of water contaminants instantaneously. Their unique properties as anion exchangers and their compositional versatility among other factors is an advantage over other types of clays in the field of water treatment.

Various modification techniques were discussed for the preparation of functionalized clay and LDH nanomaterials and showed influence on the dispersion of nanoclay fillers in the polymer matrices. The desired properties for the polymer/clay and LDH nanocomposites are primarily dependent on the type of modifying agents used for functionalization of layered silicates. Solution blending technique and in situ polymerization method seemed to provide good dispersion of clay layers in polymer matrix compared to melt blending technique. This is mainly because of the low viscosity and high agitation power associated with solution blending. However, melt blending is considered as an industrially viable as well as ecofriendly technique and shows high economic potential. The addition of stabilizers and/or compatibilizers during the processing stage is believed to lead to an improved number the properties of polymer/clay and LDH nanocomposites.

Development of an appropriate understanding of the structure, property and formulation relationship in both clay-based nanocomposites and LDH based hybrids needs to be further researched for better output in water purification. LDH-containing hybrids are the new emerging areas of research in water purification processes. Due to their nontoxicity, higher surface area than individual constituents and notable adsorption capacity, these LDH hybrids have attracted a considerable interest in water treatment applications. The polymer/modified-clay nanocomposites with exfoliated morphology disclosed strong reduction in water surface tension and efficiently remove methylene blue (MB) dye and other contaminants from water. The adsorption increased with increasing dye concentration, pH values and increasing temperature due to the increased kinetic energies of the molecules. The morphologies of polymer/LDH nanocomposites exhibited a remarkable high adsorption capacity for the efficient removal of dyes and heavy metal ions from water within a short time interval.

**Author Contributions:** M.M., J.S.S. and M.J.M. conceptualized, co-designed and steered the review as well as co-writing Sections 1, 2, 2.1–2.3 and 3. S.I.M. and K.L. co-wrote Sections 2.4 and 4, while M.M., J.S.S. and M.J.M. compiled the article together. J.S.S. and M.J.M. were responsible for funding acquisition of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Research Foundation (NRF) of South Africa, gran<sup>t</sup> number (s) 114270 and 127278.

**Acknowledgments:** The National Research Foundation (NRF) of South Africa is highly acknowledged for financial support of this research work.

**Conflicts of Interest:** No conflicts of interest declared by the authors.
