2.3.9. Urea Method

In general, urea is added to an aqueous solution of preferred M2<sup>+</sup> and M3<sup>+</sup> metal salts and heated under reflux condition for several hours. The precipitate product is collected by filtration, washed thoroughly with deionized water and dried overnight. The rate of urea hydrolysis can possibly increase significantly with an increase in the reaction temperature to 100 ◦C [59]. The urea molecules undergo degradation to form ammonium carbonate, which initiates the precipitation into LDH with CO3<sup>2</sup>− as interlayer anion. This urea method provides high degree of crystallinity and a fine particle size distribution. Urea-based co-precipitation provided the better crystallinity and particle size due to thermal treatment and hydrolysis of urea which is proceeded in a very slow manner [57,59,60].

In comparison to many other nanoclays or layered materials, LDHs have compositional multiplicities in the cationic layers and in the hydrated interlayer of anions for charge balance which lead to some functional diversities. This implies that LDHs among layered materials have the grea<sup>t</sup> advantages and number of possible compositions, metal-anion combinations and morphologies useful for synthesis and processing methods. Apart from that, LDHs can be used in a variety of potential applications due to their anion exchangeability, compositional flexibility, good biocompatibility, low cost, facile synthesis, pH dependent solubility, thermal stability and high chemical versatility [78]. Due to their tunable chemistry and high charge density tailored properties, LDHs have attracted grea<sup>t</sup> attentions in various technologically significant fields and applications such as production of renewable energy [4,21,29], adsorbents [7,41,79], water purification [8–10,80,81], antimicrobial activities [10,24], sensors [29,82–84], flame resistance [48], drug delivery [85,86], cosmetics [87,88] and environmental catalysis [57,89,90]. In our previous work [91], detailed discussion about various applications were made and the current work focuses more on water purification.

#### *2.4. Preparation Methods of Polymer-Clay Nanocomposites (PCNCs) and Surface Modification*

The manufacturing of PCNCs depends mainly on a proper method selection which ensures acceptable level of dispersion of the nanofillers throughout the polymer matrix. Several processing methods were employed in preparing polymer-based clay nanocomposites such as in situ polymerization, the melt blending, and solution blending techniques [92,93] (see Figure 10). In each preparation method, an absolute goal is to achieve a desired uniform dispersion of nanoclays in the pristine polymer matrix. However, there are currently numerous interesting views about the applications and usage of these methods. According to the following studies [18,94,95], melt blending is regarded as a significantly, industrially viable and ecofriendly technique with high economic potential for preparation of polymer–clay nanocomposites. The in situ polymerization method is a commonly used synthesis technique and easy to modify by changing the polymerization conditions [96] and provides uniform dispersion. Both these methods require either a large amount of organic solvent or high viscosity or thermally unstable polymers at high temperatures. In comparison to melt blending, the solution-blending technique often produces pleasing dispersion of clay layers in the polymer matrix [31] due to its low viscosity and high agitation power. Each technique has its own relevant significance and limitations in relation to certain required industrial applications.

**Figure 10.** Illustration of (**a**) in situ polymerization, (**b**) melt intercalation and (**c**) solution intercalation. Reproduced with permission from Reference [97]. Open Access 2014, Royal Society of Chemistry.

#### 2.4.1. In Situ Polymerization Technique

Due to the silicate dispersion deduced information, in situ polymerization is more e ffective in the preparation of composites and can sidestep the harsh thermodynamic requirements related to the polymer intercalation process [18,31,98] (Figure 10a). Furthermore, this polymerization technique (i) tolerates resourceful molecular strategies of the polymer matrix; (ii) it provides an e ffective approach to the synthesis of di fferent polymer/nanoclay composites with prolonged property range and (iii) facilitates the development of the interface between the filler and the polymeric matrices by modification of the matrix composition and structure. Many studies focus on preparing novel polymer/nanoclay composites via the in situ polymerization method and demonstrate the benefits of this method in comparison with other types of synthesis methods [18,31,98,99]. For instance, Ozkose et al. [100] investigated the synthesis of poly(2-ethyl-2-oxazoline)/nanoclay composites for the first time using in-situ polymerization. In their finding, a ring-opening polymerization method was applied, which then initiated the delamination of clay layers in the polymer matrix and led to a composite formation.

## 2.4.2. Melt Blending Technique

Melt blending technique involves direct mixing of layered clay into the molten polymer matrix and can either be immobile or active. In an immobile melt blending (melt annealing), the process is performed under a vacuum at temperatures of approximately 50 ◦C above transition temperatures in the absence of mixing. In an active melt blending, the polymer melting is performed during a melt mixing in the presence of an inert gas [27,101]. As a result, the polymer clay nanocomposites are produced from the enthalpic driving force and influence of the polymer–organoclay interactions. The melt-mixing method (Figure 10b) provides better mixing of the polymer and nanoclay fillers and is well-suited with current industrially and ecofriendly viable processes such as extrusion and injection molding for thermoplastic and elastomeric material manufacturing. The absence of solvents reduces the environmental impact and minimizes potential interactions between the host and polymer solvents, which, in many cases, limits clay dispersion [18,102–104].

### 2.4.3. Solution Blending Technique

Solution-blending is a solvent based process in which the polymer and the prepolymer are soluble, which causes swelling of the clay layers, see Figure 10c. This technique involves thoroughly dispersing the layered silicate within appropriate solvents, which includes polymer/soluble prepolymer. These clay layers are dispersed into the solvent and further mixing with a dissolved polymer would be done to prepare the solution which allows polymer chains to be embedded into the exfoliated clay layers. Upon reaction completion stage, the solvent molecules would have evaporated, trapping the polymer chains intercalated into the gallery of clay interlayers [105,106] and the matrix segments combine with the dispersed clay layers.

The major driving force of intercalation process in solution mixing is the increased total disorder of the system referred to as desorption process of solvent molecules. This entire process normally consists of three stages known as (i) the dispersion of clay in a polymer solution, (ii) well-ordered solvent removal and (iii) lastly composite film casting [102,106,107]. The dispersion of clay in neat polymer necessitates active agitation such as stirring, reflux and shear mixing.

It is well documented that the morphology and dispersion of clay nanoplatelets in polymers is one of the key factors affecting their gas barrier properties [95,108]. One of the most vital challenges in the preparation of polymer/clay nanocomposites with improved barrier performances [95] is to achieve high level of exfoliation and orientation. In general, polymer/clay nanocomposites may result into three possible morphologies referred to as (i) phase-separated, intercalated and exfoliated structures (see Figure 11) [95]. For attainment of phase-separated nanocomposites, clay tactoids are formed throughout the pure polymer matrix, and no separation of clay nanoplatelets occurs. Polymer chains surround clay nanoplatelets but do not penetrate between the clay layers [109] and absence of platelets separation may result in large, micron-sized agglomerates. In intercalated nanocomposties, some of the polymer molecular chains have penetrated the interlayer galleries of the clay tactoids. Due to the penetration of polymer molecular chains, the spacing between individual clay platelets and the overall order of the clay layers is increased and maintained [110]. In exfoliated nanocomposite structures, the clay nanoplatelets are fully separated and dispersed uniformly within the continuous polymer matrix. Exfoliated nanocomposites produce the highest surface area interaction between clay nanoplatelets and neat polymer [111]. After a successful exfoliation, an enhancement in properties can be manifested in barrier properties, as well as improved mechanical properties, decreased solvent uptake, increased thermal stability and flame retardancy [112,113].

However, the main drawback to achieve homogeneous dispersion of most inorganic clays within organic polymers is closely related to the incompatibility between hydrophilic clay and hydrophobic polymer, which often causes agglomeration of clay mineral in the polymer matrix. Thus, surface modification of clay minerals for a good compatibility with the polymer is the most important step to achieve homogeneous dispersion of clay nanoplatelets in polymer matrix [29,31,76,109].

**Figure 11.** The main types of nanocomposites: (**a**) intercalated, (**b**) flocculated and (**c**) exfoliated. Reproduced with permission from Reference [113]. Open Access 2018, IntechOpen.

#### 2.4.4. Surface Modification of Nanoclays and LDHs

Layered silicates including nanoclays and layered double hydroxides (LDHs) can be intercalated with hydrophilic polymers such as thermoplastic, thermosetting and elastomeric polymers. Most commonly used polymers are hydrophobic, while others such as poly(vinyl alcohol) (PVA), poly(ethylene glycol) (PEG), poly(acrylic acid) (PAA), poly(2-oxazoline) (POX), poly(methyl methacrylate) (PMMA), poly (ethylene-co-vinyl acetate) (EVA) are hydrophilic in nature. Despite their various applications, silicate layers also have one primary drawback due to the intrinsic incompatibility of hydrophilic silicate minerals and the hydrophobic polymer matrix. The incorporation of hydrophilic silicate minerals into a hydrophobic polymer causes agglomeration/aggregation, which lead to incompatibility between the components and weak extent of dispersion. Thus, it is indispensable to augmen<sup>t</sup> the degree of dispersion and the compatibility between the polymer matrix and the clay by surface modification [114]. The miscibility between layered silicates and the polymer matrices is enhanced as the clay becomes hydrophobic after surface modification using organic materials. For fabrication of layered silicates with engineering polymers such as thermoplastic or thermosetting, the surfaces of the layered silicate have to be modified by ion-exchange processes using cationic surfactants like quaternary alkylammonium salt, alkylphosphonium-based positively charged species or coupling agents [49]. The surface energy of layered silicates is reduced due to the modification, providing the efficiency and reinforcing characteristics in controlling the stability of the polar polymer matrix [105]. As a result, the interlayer spacing increases to high margins, producing better anchoring of the polymer chains for improvement of the overall properties of the system. The most preferential modification is the addition of coupling agen<sup>t</sup> such as silane, which ensures good compatibility or chemical bonding with polymers, an exchange of the interlayer inorganic cations such as Na<sup>+</sup> with organic ammonium cations. In addition to ionic modifications, covalent and dual modifications (ionic and covalent are possible [115]. Other approaches, such as grafting polymer chains directly onto the surface of a nanoclay or using non-ionic surfactant have also been used [116]. There are two ways of ionic modification, called directly reacting anionic or cationic surfactants with the nanoclay or using ionic liquids. Imidazolium, pyridinium, trihexyltetradecylphosphonium tetrafluoroborate, and trihexyltetradecylphosphonium decanoate salts are commonly used for ionic liquid modification of nanoclays which show better properties [117–122]. The modified clay is commonly referred to as organoclay and the schematic illustration for the modification of clay particles is shown in Figure 12.

Chang et al. [119] prepared and characterized bio-oil phenolic foam (BPF) and surfactant modified bio-oil phenolic foam (MBPF) reinforced with Montmorillonite (MMT) as secondary phase. Their findings showed remarkably enhanced toughness as well as good flame resistance and improved the thermal stability of modified bio-oil phenolic foam (MBPF)-MMT nanocomposite foams compared to unmodified BPF-MMT nanocomposites. Covalently modified clay silicate is often synthesized via a

step-reaction polymerization called condensation polymerization. During this process, the reaction is taking place between the hydroxyl groups from the surface of clays with mono- or tri-alkoxy silanes such as methoxy(dimethyl)octylsilane, tri-alkoxy silanes, trimethoxy(octyl)silane, (3-aminopropyl) triethoxysilane and others. The covalent modification renders the clay surface more hydrophobic [120]. Uwa et al. [121] studied the effect of nanoclay as reinforcing agen<sup>t</sup> on the mechanical properties and thermal conductivity of polypropylene (PP) and maleic-anhydride-grafted-polypropylene (MAPP). The results of PP/MAPP/nanoclay composites exhibited a significant improvement in tensile strength and stiffness with low clay contents. Thermal conductivity analysis revealed that composites with high clay loadings have high resistance to heat. Twofold modifications can be done possibly by first covalently modified clay silicate followed by an ionic modification or vice versa. In comparison to single modifications (either ionic or covalent), dually modified clays show even more improved properties in terms of mechanics, thermal stability, dimensional stability, and viscoelastic characteristics.

**Figure 12.** Schematic representation for the preparation of exfoliated polymer/organomodified clay nanocomposites. Reproduced with permission from Reference [122]. Copyrights 2017, Elsevier Ltd.

Various polymer-based layered silicates nanocomposite systems have been investigated, and their methods, structure and properties are compared and summarized in Table 3. The comparison of polymeric categories such as thermoplastic, elastomeric and thermoset matrices including thermoplastic polyurethane (TPU), polyisoprene (PIP), nacre-thermoset, poly(l-lactic acid) (PLLA), polypropylene (PP), polyamide 11 (PA11), nitrile butadiene rubber (NBR), styrene-butadiene rubber (SBR), Vinyl ester (VE), epoxy (EP), polylactic acid (PLA), Polybutylene terephthalate (PBT), polymethyl methacrylate (PMMA) reinforced with corresponding LDHs and other nanoclays are also included in Table 3 summary. Nevertheless, it is evident in the literature that the polymeric-thermoplastic matrices are utilized more preferentially over the thermosets because of their features such as light weight, can be re-melted/molded, and shaped. Recently, there is a growing demand to safeguard and deal with environmental contaminants and pollutants in preparation of biodegradable matrix/LDHs nanocomposites which are referred to as eco-friendly materials. Layered double hydroxides (LDHs) systems appeared to have better overall properties than most of other nanoclays due to their varied chemical compositions and methods of synthesis. LDHs possess higher layer charge densities and prefer multivalent anions within their interlayer space due to strong electrostatic interactions between the brucite-type sheets and the anions. Therefore, swelling is more difficult in LDHs than for other clay minerals. In short, LDHs containing monovalent anions like nitrate or chloride ions are viewed as good precursors for exchange reactions with charge balance, which lead to some functional diversities.



### *Crystals* **2020**, *10*, 957

#### **3. Properties of Polymer**/**Other Clays and LDHs Nanocomposites**

The intention for the addition of clay minerals to the polymers is to improve the polymer properties and to produce the polymer/clay nanocomposites with desired applications. The key step is to prepare nanocomposites with highly preferred and value-added demand properties, which overcome downsides of polymers while maintaining their intrinsic advantages. Due to the low cost, availability, high aspect ratio as well as desirable nanostructure and interfacial interactions, clays can provide considerable improved properties at very low filler loadings, which help to obtain more useful properties. The nature and properties of constituents as well as preparation methods and conditions affect the final properties of polymer/clay nanocomposites. In this review, various improved properties of polymer/clay nanocomposites as well as the adsorption capacities and removal efficiency of dyes or heavy metal ions in water including morphology are discussed.

#### *3.1. Morphology of Polymer*/*Other Clays and LDHs Nanocomposites*

The morphology of the polymer/layered clay silicate nanocomposites significantly influenced their adsorption capacities and the removal efficiency of dyes or other heavy metal ions in water. The key aspect in nanocomposite structure is the clay–polymer interaction, which affects the dispersion level of clay in polymer matrix. Depending on the dispersion level of layered silicates, the structure can either be separated, intercalated or exfoliated structure [108,132–135]. Surface modification plays also important role in achieving good interaction between polymer and clay which affects the extent of dispersion and improves significantly the adsorption as well as removal of dyes or heavy metal ions from water. Thus, organically modified clay silicates such as montmorillonite (OMMT), kaolinite and LDH are mostly preferred nano reinforcement for proper selection of the functional groups and their abilities of ion-exchange. It is well-known that acid-modified clay resulted in higher rate of dye adsorption, an increased specific surface area and high porosity than in the case of base-modified clay [136].

Highly flame-retardant polymer/deoxyribonucleic acid (DNA)-modified clay nanocomposites were investigated by transmission electron microscopy (TEM) as shown in Figure 13a–f. The dark lines observed in Figure 13a,d are closely related to the layered silicate nanoclays and the light segments are associated to epoxy matrix. Naebe et al. [137] explained that intra-gallery reactions due to interfacial interactions made diffusion of more epoxy monomers within DNA-modified clay possible to enhance clay layers' separation and therefore induced formation of exfoliated structures as seen in Figure 13b. In addition, three individual intercalated ordered structures known as intercalated tactoids could be observed for epoxy nanocomposite at higher contents of clay (Figure 13e).

**Figure 13.** TEM micrographs of epoxy-2.5 wt% of DNA-clay (**<sup>a</sup>**–**<sup>c</sup>**) and epoxy-5 wt% of DNA-clay (**d**–**f**) nanocomposites. Reproduced with permission from Reference [137]. Open Access 2016, Springer Nature.

However, few clay layers possess thin and small tactoids which are uniformly and randomly dispersed in the epoxy resin. This indicates that the DNA-clay modification is an e ffective approach to improve both the exfoliation and dispersion of clay. In addition to achieved dispersion, most of microcracks under an e ffective load are initiated within the intra-layer of semi-stacked clay instead of epoxy-clay interfacial region. This phenomenon verifies that higher contents of clay (5 wt%) resulted in lower reinforcing impact in the overall mechanical properties.

Evidently, low clay content formed an exfoliated structural configuration with individual layers, well-dispersed with more homogeneous distribution (Figure 13c), while high content of clay resulted in flocculated structures. It can be seen in Figure 13f that phase-separated clay tactoid structures are formed and agglomeration at high content prevail over complete delamination of clay layers due to low penetration of epoxy monomers into stacked layers of modified clay [138].

Zubitur et al. [139] studied the poly(lactic acid) (PLA)/modified drug 4-biphenyl acetic acid (Bph)-layered double hydroxide (LDH) nanocomposites. The nanocomposites were prepared by solvent casting with 5 wt% of drug-modified LDH, and the hydrolytic degradation was carried out in a Phosphate-bu ffered saline (PBS) solution at pH 7.2 and 37.8 ◦C. From their XRD results, PLA/LDH-Bph nanocomposites showed no peaks corresponding to LDH-Bph observed and this was attributed to an exfoliation or to the presence of entropic LDH layers. The degree of dispersion of acid/base-modified LDH was found to be good with small tactoids at low magnification, exfoliated layers were observed at higher magnification using TEM images. A number of studies reported the acid modification of clay silicate and layered double hydroxides (LDHs) reinforced with polymer matrices. Table 4 represents the summary of selective studies on acid/base-modification of polymer/clay and LDH nanocomposites for water purification.



#### *3.2. Adsorption of Polymer*/*Nanoclay and LDH Systems*

Various types of polymer/nanoclay composites are currently being explored for the primary usage in water purification and many other applications due to their unique properties, which are different from their counterparts. Polymer/clay nanocomposites used in water purifications technology and their corresponding applications as well as the removal of various heavy metal ions are shown in Figure 14. Adsorption is a removal of soluble material/adsorbed materials called adsorbate (heavy metal ions) present in water and clay nanomaterials (solid adsorbents) are used for the adsorption of contaminants. This process can either be physical (physisorption) or chemical (chemisorption) in nature. Nanoclay minerals have the high specific surface area and high sorption capacity giving high structural and chemical stability towards the adsorption of organic and inorganic contaminants.

**Figure 14.** Role of nanocomposite in water purification and the removal of various heavy metal ions.

Due to its high internal surface area in the range of 500–1500 m<sup>2</sup>/g, the most popular solid adsorbent material is activated carbon which is primarily used in large-industrial scale for water purification systems [41,49]. Thus, adsorption is a surface phenomenon commonly found in nature and plays a key role in water purification technology. Adsorption increases with the increase in the surface area of the adsorbent. This implies that more finely divided or rougher the surface of the adsorbent is, then the greater is the surface area and the adsorption. However, adsorption affinity of the surface area of the adsorbent is independent of the surface area and dependent on the favorable attractive interactions present at the pH value range below 7. When the pH is higher than 7, the electrostatic repulsion forces of the adsorbate-adsorbent are weakened, hence, the reduced adsorption efficiency or removal percentage of contaminants [41,49,131]. The adsorption of contaminants from water mostly depends on hydrophobic interactions between adsorbate and adsorbent, which make grea<sup>t</sup> contributions to the affinity of the organic anion to LDH. Therefore, the hydrophobic interactions interconnect many segments into a cluster in order for each natural organic anion to show a stronger affinity to LDH. The high hydrophilic nature of contaminants reduced significantly the adsorption capacity. The main reason for this is dominant force that decreases the surface tension between the matrix (adsorbent) and the solid adsorbed (adsorbate). The detailed discussion of the effect of pH as one of many factors affecting the adsorption, advantages and disadvantages of various methods of synthesis would also be outlined in the paragraphs or Table 5 below.


**Table 5.** Advantages and disadvantages of different methods of LDHs and clays in water purification.

In recent times, different synthesis and preparation methods of LDHs and other nanoclays have been applied as discussed in Sections 2.3 and 2.4, respectively. These different methods influence the adsorption performance of the nanocomposite and noticeably assist to reduce and remove the high concentration levels of contamination in water [147,148]. Individually, each method has some advantages and disadvantages as represented in Table 5. Amongst all the methods, adsorption is considered the most efficient, economical technique and easy processing to drive for contaminants removal from wastewater. Furthermore, the adsorption is reversible, and adsorbents can be regenerated. In general, there are three types of adsorption known as physisorption (the interaction between adsorbent-adsorbate), chemisorption (adsorbent-solvent) and electrostatic interactions (adsorbate-solvent) [149]. The underpinning principles/mechanisms involved in the adsorption material surface have been studied and reported in relation to factors affecting systems.

#### 3.2.1. Factors Governing the Performance of Clays/LDH Based Adsorbents

In this review, the application of nanomaterials as adsorbents for removal of contaminants such as heavy metal ions and dyes from wastewater has been reviewed. It is important to understand how adsorbents interact with different adsorbates such as heavy metal ions and dyes in the laboratory small scale to determine their potential for application in water purification and their contribution to large scale [148,149]. However, the main challenges for adsorption process are waste products, non-selectivity, instability and low/poor heat transfer leading to long heating and cooling times. To address these challenges, the development and design of suitable and more effective nanoadsorbents with optimum adsorption efficiency for the removal of contaminants in water should be prioritized. In the field of wastewater treatment, different materials prepared through various methods bring about unique functionalities for adsorption efficiency for the removal of contaminants from industrial effluents, surface water, groundwater and tap/drinking water. As stated, many key factors affect the efficiency and performance of LDH/clay adsorbents including pH value, contact time, adsorbent dosage, initial ion concentration, temperature, coexisting ions, and sorption kinetics [148,149]. In this

section, the influence of these factors on adsorption performance and capacity by LDH/clay adsorbents is explained as presented in Table 5.

## Influence of pH Value

The pH of an adsorbate-adsorbent solution is most significant aspect for adsorptive removal of contaminants. This a ffects the type of the surface charge of the adsorbent during water purification by the adsorption technique. It is also the main factor taking care of the type of the surface of the adsorbent, degree of ionization and aqueous adsorbates [149]. The e ffect of solution pH was studied at di fferent pH values from 3.0 to 11.0, and the results are shown in Figure 15a,b. The pH of solution noticeably changed by the addition of diluted HCl/NaOH. The maximum fluoride adsorptions of 98%, 94% and 91%, respectively for cerium bentonite clay-malic acid chitosan (CeBC-A@CS), lanthanum bentonite clay-malic acid chitosan (LaBC-A@CS) and aluminum bentonite clay-malic acid chitosan (AlBC-A@CS) adsorbents was attained at pH of 3.0. It was also reported that the minimum fluoride adsorption for these three di fferent adsorbents was achieved as 43%, 37% and 35%, respectively at pH of 11. At neutral pH of 7.0, the maximum fluoride adsorption was 84%, 82% and 80% for, respectively. The maximum fluoride adsorption capacity can be attributed to the change in the surface charge of the adsorbent. The pH value at point of zero charge (pHzpc) for the adsorbents is 7.1 [150] as represented in Figure 15b. When the pH of solution was less than pHzpc, the fluoride ions moved towards the positively charged surface of the chitosan composites formed by the protonation of OH− and carboxylic acid groups leading to the fluoride adsorption onto the surface [151]. At a pH value above the pHzpc, the fluoride adsorption was exceptionally low. This is probably because the composite surfaces were negatively charged due to deprotonation of the hydroxyl groups, ensuring mutual repulsion forces between the fluoride ions and the composite surfaces [152]. It can be observed that cerium bentonite clay-malic acid chitosan (CeBC-A@CS) adsorbent showed a higher fluoride adsorption capacity than lanthanum bentonite clay-malic acid chitosan (LaBC-A@CS) and aluminum bentonite clay-malic acid chitosan (AlBC-A@CS) adsorbents.

**Figure 15.** (**a**) E ffect of pH and (**b**) pHzpc values of cerium bentonite clay-malic acid chitosan (CeBC-A@CS), lanthanum bentonite clay-malic acid chitosan (LaBC-A@CS) and aluminum bentonite clay-malic acid chitosan (AlBC-A@CS) adsorbents. Reproduced with permission from Reference [151]. Open Access 2020, RSC Advances.

Wei et al. [153] investigated the novel hydrotalcite-like material layered double hydroxide (FeMnMg-LDH) adsorbent synthesized by co-precipitation and its adsorption capacity for the removal of lead ions in water. In order to prevent the precipitation of Pb2+ at the high pH, the experiment pH was set below pH 6. The pH of 6 was maintained to prevent the precipitation of Pb2+, and e ffects of pH on the adsorption are discussed. It was reported that the Pb2+ removal percentage by FeMnMg-LDH adsorbent mostly increased with the increasing pH value. The Pb2+ removal percentage higher than 97% at the pH range 3–6, was achieved. This is an indication of the outstanding e fficiency and performance of FeMnMg-LDH adsorbent in Pb2+ adsorption except when pH is equal to 2. The suspension of the absorbent might take place at extremely low pH resulting into the collapse of the structure of FeMnMg-LDH and therefore reduce Pb2+ adsorption e fficiency and capability. Comparing the bentonite clay and LDH-based adsorbent, it can be concluded that FeMnMg-LDH adsorbent has a higher adsorption capacity of more 97% than others except the cerium bentonite-malic acid chitosan, which appears to have more or less similar adsorption capacity percentage.

## Influence of Contact Time

The contact time significantly a ffects the adsorption process and the economic e fficiency of the process including the adsorption kinetics. Therefore, contact time is profoundly important and dependent factor for performance determination in adsorption process [154]. Figure 16a represents the fluoride adsorption capacity of the three adsorbents (cerium bentonite clay-malic acid chitosan (CeBC-A@CS), lanthanum bentonite clay-malic acid chitosan (LaBC-A@CS) and aluminum bentonite clay-malic acid chitosan (AlBC-A@CS)) at di fferent contact times in the range of 10 to 90 min with neutral pH and initial concentration. The fluoride adsorption capacity of the adsorbents was gradually increased with an increase in contact time. Cerium bentonite clay-malic acid chitosan (CeBC-A@CS) adsorbent obtained the higher fluoride adsorption capacity of 84% than other adsorbents with 62%, 67%, 80%, 82%). Moreover, an equilibrium at 60 min and 45 min was achieved with di fferent adsorbents which suggests the surfaces of the adsorbents CS and BC were completely covered with fluoride ions.

**Figure 16.** (**a**) E ffect of contact time, (**b**) e ffect of dosage, (**c**) e ffect of initial fluoride concentration and (**d**) e ffect of co-ions on the fluoride adsorption of the adsorbents AlBC-A@CS, LaBC-A@CS and CeBC-A@CS at 303 K in neutral pH. Reproduced with permission from Reference [151]. Open Access 2020, RSC Advances.

Jaiswal and Chattopadhyaya [155] studied the effect of contact time on adsorption of Pb(II) on the Co/Bi-LDH synthesized by using co-precipitation method. Their impressive finding was that 90.0% of the adsorptive removal of contaminant, called heavy metal, was accomplished within 120 min of contact time. It was also observed that beyond 120 min, contact time has no effect in heavy metal removal percentage. At the beginning, very high adsorption rates were observed simply because of the larger number of vacancy sites available for the sorption and adsorption equilibria that were then steadily reached [139]. Effect of contact time for bentonite clay and LDH adsorbents using the same co-precipitation method were compared, and it can be concluded that LDH adsorbent seems to have an upper hand in terms of the adsorption capacity percentage for removal of contaminants. The reason for this is that LDH adsorbent has higher surface charge density and more ion exchange binding sites for good adsorption. This could also be attributed to the adsorption on the LDH layers via hydroxide precipitation or metal complexation.
