Applications of Layered Double Hydroxide: Comparison
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Layered double hydroxides (LDHs), a type of synthetic clay with assorted potential applications, are deliberated upon in view of their specific properties, such as adsorbent-specific behavior, biocompatibility, fire-retardant capacity, and catalytic and anion exchange properties, among others. LDHs are materials with two-dimensional morphology, high porosity, and exceptionally tunable and exchangeable anionic particles with sensible interlayer spaces. The remarkable feature of LDHs is their flexibility in maintaining the interlayer spaces endowing them with the capacity to accommodate a variety of ionic species, suitable for many applications.

 

  • Layered double hydroxides, Sensing application, Pollutants removal, capacitors

1. Layered Double Hydroxides (LDHs) as Catalysts

LDHs prepared by conventional methods have attracted considerable interest in the fields of solid-phase catalysis as a support media. In earlier studies, LDHs were reported to be effective supports for Ziegler catalysts during olefin polymerization viz. LDHs with a series of possible combinations of metal ions mainly of Ni-Al, Mg-Zn-Al, Mg-Mn-Al, Mg-Co-Mn, Cu-Al, Co-Cr, Mg-Al-Cr and Mg-Al, calcined at 200 °C to 450 °C. Among these, Mg-Mn-Al–CO3-LDH heated at 473 K showed the highest catalytic activity of polyethylene production [69,70][1][2]. In another report, integrated vanadium oxide catalysts supported by calcined Mg-Al-LDHs were used for oxidative dehydrogenation of butane [71][3]. Similarly, vanadium oxide(V2O5)-impregnated Mg/Al hydrotalcite have been used for the synthesis of isobutyraldehyde from methanol and n-propanol in the vapor phase [72][4]. It has been demonstrated that calcined as well as uncalcined LDHs may be used as supports for noble metal catalysts [73][5]. With enhanced attention towards environmental and economic concerns, heterogeneous solid-base catalysts such as calcined LDHs have gain importance among scientists due to their ease of separation, recyclability, simple handling, and low cost [74][6]. Specifically, mixed oxide LDHs, due to their excellent ability to provide Bronsted base sites, have received attention, and are apt to replace homogeneous base catalysts, as they are more recyclable and environmentally benign. These are used for numerous organic reactions, such as various condensation reactions (Knoevenagel, Aldol, and Claisen–Schmidt) and addition reactions (Michael additions and Henry reactions), among others. Rock-salt-type LDHs possess additional weightage due to the availability of both acid and base sites, the strength, and relative amounts of which depend on the molar ratio of cations and calcination temperature. Concise reviews on the development of LDHs as precursors of multifunctional catalysts and catalytic materials has also been presented in the literature [75,76,77][7][8][9]. Copper (Cu)-containing LDH materials accompanying N-arylation have shown good yield at 100 °C to 160 °C. Likhar and coworkers examined the N-arylation of benzylamine and cycloalkyl amines with some chlorobenzenes substituted with an electron-withdrawing group over catalyst Cu-Al-LDH-K2CO3. Normally, the reaction yield is good to excellent (45–93%) at 100–160 °C for 8–16 h. They also noted that the presence of the electron-withdrawing group in chlorobenzene was a key for this reaction to take place [78][10]. It has been reported that secondary and tertiary amines can be oxidized with various oxidizing reagents over the catalyst with specifically increased alkalinity: intercalating OBu–anion in Mg-Al-LDH converts it from a weak base to stronger [79][11].

2. Photocatalysis

The unique characteristic of LDHs and similarity to semiconductors [80][12] has attracted the attention of many researchers. Improved performance in photocatalytic activity was demonstrated by Fu et al. in doping Zn-Cr-LDHs with terbium cations [81][13], wherein double the catalytic activity was observed on doping with an optimum composition of 0.5%. Gomes Silva et al. carried out studies on photocatalytic water splitting using visible light irradiation with a series of Zn-to-metal atomic ratios at different metallic ratios of Ti, Ce, and Cr to generate mixed metallic LDHs [82][14]. Zn-Cr LDH showed high quantum yields of 60.9% at 410 nm and 12.2% at 570 nm, with higher oxygen generation capacity than tungstate. The conversion of carbon dioxide to alcohol via a photochemical pathway is one of the most pursued topics in modern chemistry. It not only facilitates the removal of the potent greenhouse gas but also provides an alternative pathway to convert chemically less reactive CO2 to liquid fuel. Iguchi et al. observed that the photoreduction of CO2 into methanol was achieved by trimetallic carbonate of ZnCuGa-CO3 LDHs with a promising result of >97% [83][15].

3. LDHs for Water Treatment and Environmental Remediation

LDH materials have acquired extraordinary interest in numerous potential applications, such as water treatment [84][16], drug delivery [85][17], catalysis [86][18]. Their application in water treatment as adsorbents has great potential because of their minimal expense, high surface area, profoundly tunable inside engineering [87][19], nontoxicity [88][20], and interchangeable anions [89][21]. Recently, the modification of LDH hybrids has gained interest in wastewater treatment. It originates from the hybridization of LDH with other materials, such as graphene (G), carbon nanofibers (CNFs), and carbon nanotubes (CNTs) [90][22] for the availability of freshwater without harmful substances and microorganisms [91][23]. Fast industrial growth and urbanization lead to water pollution due to organic dyes used in paints, plastic, and textiles, and it is increasing day by day along with other organic and inorganic pollutants. The low biodegradability, profound shading, and complex sweet-smelling development of colors make the dye-containing industrial waste a harmful, aesthetic contaminant dangerous to aquatic life [92,93][24][25]. Other potent nonbiodegradable pollutants are heavy metals that are harmful at even low concentrations and contain zinc (Zn), lead (Pb), copper (Cu), cadmium (Cd), chromium (Cr), mercury (Hg), nickel (Ni), arsenic (As), and thallium (Tl). These contaminants are present in the effluent of oil refining, coal mining, metal plating, agricultural assets [94,95,96,97][26][27][28][29]. Some inorganic anions and oxyanions, such as arsenite, arsenate, selenite, selenate, chromate, phosphate, and nitrate, as well as monoatomic anions such as fluoride, chloride, bromide, and iodide also exist in water, causing water pollution [97,98][29][30]. In water and wastewater, a few oxyanions, along with such constituents as cations and humic substances conceivably impact the science of LDHs and the speciation of oxyanions in the fluid framework. Subsequently, a comprehensive adsorptive execution of LDHs in the multi-oxyanion framework comprising different oxyanions and lattices of interest has been identified. Lazaridis et al. studied the sorptive flotation framework for promising arsenate and chromate evacuation and successful particle separation [99[31][32],100], but satisfactory results were obtained by Gilman et al. via the “porous pot” technique for removal of arsenic (As) and illustrated that LDH might be an attractive adsorbent for wastewater treatment [98][30]. LDHs may be used in powder and granular forms, in a scattered powder structure, or in a fixed-bed granular structure. Granular LDHs have been found superior to powdered LDHs because they retain their intrinsic sorption properties with sufficient mechanical strength and toughness. Gillman et al. found that granular LDHs can reduce high concentrations of oxyanions [101][33]. After the sorption process, desorption is required to check recyclability of LDHs. Kuzawa et al. proposed a plan where the phosphate PO43− that is sorbed could be recovered as calcium phosphate by adding CaCl2 as eluent [102][34]. Wang et al. showed that Cr (VI) could be recuperated from Cr(VI)-stacked Li–Al LDHs by resuspending them in steaming hot water to upgrade synchronous Li+ deintercalation from Li–Al LDH structures in fluid arrangement and Cr(VI) desorption [103][35]. Murayama et al. investigated column operations for the removal of low-concentration harmful species. Pelletized granular LDH and 5% polyvinyl alcohol solution can remove low concentrations of harmful anionic species of As(III), As(V), Se(IV), and Cr(VI) from wastewater. Among the LDHs examined, Mg-Al-NO3 LDH showed the ideal immigration of anionic species, and it showed brilliant evacuation of anions of As(V), Se(IV), and Cr(VI) at very low anionic concentration [104][36]. Recently, Sari et al. applied Mn-Fe LDHs as adsorbent material for the removal of arsenic from synthetic acid mine drainage (AMD) containing many heavy metals, such as Zn, Pb, Ni, Co, and Cd [105][37]. Kameda et al. reported the use of acid media and higher HNO3 uptake capacity for calcined LDHs, in which the acid media helps to increase the sorption process and proton utilization throughout reconstruction reactions [106][38]. On the other hand, Socias-Viciana et al. also observed that in neutral solutions, uptake capacity of nitrate can be improved above 99% by enhancing calcination and sorption temperature [107][39]. Significant improvement in CO32− removal capacity of Mg-Al-CO3 LDHs on calcination and replacement of carbonate by NO3 as Mg-Al-NO3 was reported by Goh et al. with an additional increase in uptake capacity to 170 mg/g, associated with a surface area increment due to a decrease in the average particle size of nitrate-intercalated LDHs to 122 nm [108][40]. Yoshida et al. proposed a mechanism of higher AsO43− (arsenate)uptake capacity and affinity for Mg-Fe LDHs in comparison to that of Mg-Al LDHs on the basis of formation of inner-sphere complexes by Mg-Fe LDHs, as arsenate anions once adsorbed were not easily desorbed from Fe3+-containing LDHs. By similar mechanisms, chromate uptake was also explained: pH of the solution was the main factor in determining the effectiveness of the different removal mechanisms [109][41]. Goswamee et al. demonstrated the sorption of dichromate at low pH by calcined and uncalcined Ni-Al-CO3, Mg-Al-CO3, and Zn-Cr-CO3 LDHs [110][42]. Koilraj and Kannan explained the surface precipitation mechanism that increased phosphate sorption with decreasing pH and sorbent crystallinity [111][43].
Hydrotalcite, a natural layered mineral, has been used for adsorptive removal of anions [112,113][44][45] as well as cations such as Cu2+, Cd2+ and Pb2+ using Zn-Al-EDTA hydrotalcite by chelation [114][46]. Dyes released in the atmosphere leave genuine negative effects on the ecosystem and furthermore on human well-being. Consequently, these should be expelled from water bodies. LDHs have also been utilized for colored wastewater treatment. Zang et al. demonstrated a mesoporous magnetic NiFe2O4–Zn Cu Cr-LDH composite and its potential for pragmatic adsorption of Congo red from wastewater under various conditions by utilizing single-factor tests, which offered the ideal adsorption conditions. Under these conditions, the adsorbent displayed high evacuation effectiveness over 97% with an exceptionally wide introductory Congo red focus of 150–450 mg/L [115][47]. Recently, a superabsorbent nanocomposite, Fe3O4/PEG-Mg-Al-LDH, was reported for ultrahigh efficiency in the removal of organic dyes [116][48].
Due to their exceptional physical and electronic properties, reasonable cost, high adaptability, and convenience, polymer chemistry and polymer-based materials are perceived as key segments in numerous significant businesses, such as automobiles, electronics, and aviation. One serious issue with numerous polymers is that they are profoundly combustible and can produce a lot of toxic smoke during ignition, which represents an incredible danger to human security and altogether confines their applications in numerous areas [117,118,119][49][50][51]. To resolve this issue, viable strategies are expected to add some nanosized fire-resistant filler into the polymer matrices. Layered double hydroxides have potential applications as flame-retardant polymer composites, and there has been rapid development in the research field for introducing the combination of fire-resistant polymer LDH nanocomposites. However, the mechanism behind the flame-retardant behavior of LDH is still not completely understood. Briefly, it may be summarized that during thermal decomposition, LDHs may lose the interlayer water molecules and disintegration of the intercalated anions as carbonates (CO3−2), along with metal hydroxide complexes occurring. Theis results in generation of water and fire extinguisher gases such as CO2, which may decrease the fuel available for combustion and bring down the heat discharge to stop the burning and ultimately stop the combustion. When not enough fuel remains to propagate the reaction further, this would initiate the formation of an expanded carbonaceous coating or char over the polymer surface, which hinders contact with the air to decrease heat release during the combustion and reduce smoke production as well [120,121][52][53]. A summary of the flame-retardant mechanism of LDHs may be attributed to the combination of the following three functions:
(1)
The endothermic decomposition of LDH works as a heat sink.
(2)
Decomposition of LDH leads to formation of mixed metal oxides, which act as an insulating film on the surface.
(3)
Generation of bound water and carbon dioxide thereby diluting the flammable gases.
The flame-retardant property of LDHs can be improved by intercalating suitable anions, such as borate or phosphate, into the interlamellar region of LDH [122][54]. LDHs have been melt-blended into polypropylene (PP) with intumescing fire retardant (IFR).
The advanced flame-retardant behavior and smoke-suppression properties of LDHs are derived from their distinctive layered structure with exchangeable anions. LDHs may be customized by intercalating organic anions into the interlamellar spaces. These organically modified LDHs can be utilized in the form of nanofillers for the synthesis of polymer–LDH nanocomposites [124,125][55][56]. Many LDH nanocomposites have been reported for flame-retardant applications to polymers such as polypropylene (PP) [126][57], polyethylene(PE), polymethyl methacrylate(PMMA) [127[58][59],128], acrylonitrile–butadiene–styrene (ABS), polystyrene (PS) [129][60], polyvinyl chloride (PVC), polylactic acid (PLA), polyamide 6 (PA 6), [130][61], ethylene–propylene–diene terpolymer (EPDM) [131][62] and polyethylene–vinyl acetate (EVA), [132][63]. On the other hand, most of these polymers are very combustible and can discharge a lot of smoke on burning. Thus, LDHs have been explored as fire retardants to diminish the flammability of these polymers.
Various polymers, including PMMA, PP, PVC, PE, EVA, PS, PLA, ABS, UP, PA 6, EVA [132][63] and EPDM, have been examined as fire-resistant polymer–LDH nanocomposites [133][64]. It has been mentioned that LDHs are proficient for all the previously mentioned polymers. To acquire the best fire-resistant property, various polymers require various sorts of LDHs, and among a variety of divalent cations, Zn and Mg appear more suited for flame-retardant execution over others. There is still no systematic understanding of the methods by which the varieties of divalent and trivalent metals are valuable. This should be identified, along with the thermal stability and distribution of LDHs.

4. LDHs for Removal of Greenhouse Gases

Numerous articles and patents have illustrated the utilization of calcined LDHs for the effective adsorption of polluting gases, such as carbon dioxide and sulfur oxide [134][65]. Calcined LDHs (hydrotalcite) show solid fundamental properties that make them proficient scavengers for acidic gas recuperation from hot gas streams. The recuperation of CO2 and SO2 from power-plant-produced gases is considered the initial phase in decreasing absolute carbon and sulfur oxide emissions. [Ca6Fe2(OH)16](CO3).xH2O, [Mg6Fe(OH)16)(CO3)].xH2O and [Ca2Al(OH)2](NO3).xH2O are found to be effective sorbents for mitigating SO2 from tube gas and maintaining frosty sides of coal-burning power plants. Graphene oxide-incorporated layered double hydroxides (GO-LDHs) were identified for adsorbing carbon dioxide with increased efficiency. The adsorption efficiency of LDH has been enhanced by more than 60% just by increasing 7 wt% GO concentration [135][66]. Additional emphasis ought to be placed on preparing more combinations of LDHs to mitigate pollution problems.

5. LDHs for Removal of Pesticides and Related Persistent Organic Pollutants (POP)

The US Environmental Protection Agency (EPA) has classified phenols as carcinogenic pollutants due to their high toxicity. In many developing countries, phenols are still playing an important role in disinfectant production, plastic production, and pesticides, presenting many health risks and environmental pollution. Phenols are fairly soluble in water in the form of phenolates, their pKa being quite variable according to their substituent and thus leaving them in water as persistent organic pollutants. Ulibarri and coworkers studied the sorption of trinitrophenol and trichlorophenol (2, 4, 6-trinitrophenol and 2, 4, 5-tricholorophenol) by LDHs (calcined and uncalcined), at every pH value, trinitrophenol (TNP) was exchanged more easily than trichlorophenol (TCP). TNP displaced CO3−2 anions from the particle surface of uncalcined LDHs in neutral as well as in alkaline media [136,137,138][67][68][69]. The higher uptake capacity of 4-nitrophenol than phenol was studied by Chen et al. by reconstruction of calcined Mg-Al LDHs and interactions between the layers and the -NO2 group of 4-nitro phenol, which was removed by intercalation between the layers of LDHs while phenol slowly adhered at the sorbent surface [139][70]. The sorption of 2, 4-dinitrophenol and 2-methyl-4, 6-dinitrophenol by calcined and uncalcined Mg-Al LDHs was also studied by Chaara et al. [140][71]. El Shafei et al. reported that the compensating anion has a noticeable influence on the adsorption properties of LDHs toward 4-chlorophenol. At low equilibrium concentration, adsorption of 4-chlorophenol at pH above 10 occurred at the ends of the layered structure accompanied by Cl/OH substitute with an increasing order in the interlayer spaces due to substitution [141][72]. Other than phenols, contamination of soils and groundwater by pesticides in modern agriculture systems is a matter of big concern. Pesticide molecules with ionizable functional groups such as -OH, -COOH, -SO3H produce highly soluble anionic species in water by acidic dissociation. Anionic clays with hydrophilic and positive characters on their surface are found to be effective sorbents for anionic and highly polar organic pesticides. Besse et al. studied the adsorption of pesticides belonging to the phenoxyacetic acids family of pollutants by Mg-Al LDHs, and the adsorption capacity was found to be increased with increased layer charge density [142][73]. Adsorption on LDHs is proposed by an anion exchange mechanism via two steps: anion exchange at the surface followed by an interlayer anion exchange process. The adsorption capacity has been found to be dependent on the nature of the starting anions, mostly following the affinity order NO−3 < Cl−1 < CO3−2, as proposed by Miyata [143][74].

6. Source of Nutrient Storage for Plants

The use of fertilizers, especially nitrogen (N)- and phosphorus (P)-containing ones, is inevitable in obtaining high agricultural yields. It is not an easy task to maintain the proportion of these elements, and it is necessary to introduce a more efficient and sustainable way to carry out the requirement. The intercalation of N and P within LDH came as an interesting option to optimize N and P supply to plants and some reports in this domain suggested LDH as a slow-release source of these nutrients. Mg-Al-Cl-LDH has been shown high potential to be used as a nutrient exchanger [144][75]. Berbler et al. (2014) [145][76] synthesized and characterized Mg-Al-NO3− LDH and deployed it for sustainable release of nitrates (NO3−) in the soil; nitrate release was monitored for acidic soil and basic one at different pH and temperatures. The release of NO3− intercalated with LDH showed that slow releasing process worked better for basic soil media as in the acidic soil the sustainable process worked for 16 days (15 °C), whereas in basic soil at the same temperature, this process could be observed for maximum 20 days. The results for the different pH and temperature conditions encouraged the use of these materials as sources for the slow release of NO3− in the soil. A new class of inorganic fertilizers may be explored by introducing least harmful anionic clays.

7. LDHs as Adsorbents for Anionic Pollutants

The multifunctionality of LDHs permits them to adsorb both cationic and anionic contaminants. The most common anionic species are utilized in agriculture sectors viz. nitrate, phosphates, and other agrochemicals. These pollutants can be removed by anionic clays by means of the adsorption processes and then slowly released back to the soil for plant growth or pest control, in a process of recycling or reuse. Terry (2009) [146][77] studied a clay mineral with a structure identical to LDH [Mg2Al(OH)6]2CO3.3H2O, as low-cost anion exchangers to remove nitrate and phosphate from the solution. The residual concentration of measured anions in the solution is found to be lower than the levels recommended by the Environmental Protection Agency (EPA) for drinking water. Experiments were also conducted to study the mutual effect of one anion on the removal of another. It was concluded that nitrate (NO3−) did not affect phosphate (PO4−2) removal and vice versa even in a wide range of concentrations. Li et al. (2005) [147][78] prepared MgAl-LDH intercalated with number of anions viz. nitrate (NO3−), carbonate (CO2−3) and chloride (Cl−). The adsorption capacities of materials were evaluated for glyphosate removal. The adsorption experiments examined that glyphosate removal by MgAl-LDH occurred in two ways: adsorption on outer surface as preliminary step and later on via interlamellar anion exchange. Glyphosate at low concentrations is singly adsorbed on the LDH outer surface, while with increasing concentrations the interlamellar anion exchange occurred. The glyphosate adsorption capacity of MgAl-LDH is reported to be increased with increasing lamellar charge density (Mg2+/Al3+ molar ratio) in LDH which resulted in increased electrostatic attraction also. The influence of interlamellar anions on the amount of agrochemicals retained by MgAl-LDH decreases in the order of Cl− > NO3− > CO32− anions as exchangeable ions.

8. LDHs for Biomedical Applications

In the biomedical field, several inorganic materials have been investigated, such as silicon oxide, calcium phosphate, gold, iron oxide, and layered double hydroxides (LDHs) to examine their efficacy in target drug delivery. These inorganic materials show efficient drug delivery through sufficient availability, easy surface functionality, good biocompatibility, the potential for target delivery, and controlled release from inorganic nanomaterials. LDHs have garnered the attention of researchers due to their nontoxicity. In vivo and in vitro biocompatibility is utilized for gene delivery, drug delivery, bioimaging, and biosensing areas [148,149,150][79][80][81]. LDHs can participate by exchanging anions with nucleic acids (DNA, RNA), drugs, enzymes., and the specific abilities of layered double hydroxides in executing the task of targeted drug delivery to the location in a controlled and sustained way at a particular pH make them important for drug delivery applications. A suspension test has been conducted by researchers to examine the drug release abilities of LDH materials in a simulated intestinal fluid buffer at pH 7–8 and to deliver RNA and DNA to mammalian cells in vivo by incorporating them with LDH only or LDH with a drug for treating diseases [150,151][81][82]. The drug–LDH hybrids are utilized as superior anticancer drug delivery systems [152][83] without any side effects that are clearly revealing the increasing importance of LDH in biomedical applications.

9. LDH as Biosensors

The nontoxicity, biocompatibility and excellent biocatalytic properties make LDHs efficient biosensors. Urea biosensors work on the idea of immobilization of urease enzyme into oppositely charged clays [153][84]. More typical enzymes include the fabrication of oxidoreductase enzymes/LDH amperometric biosensors such as trans-ketolase, acetylcholinesterase, horseradish peroxidase, and glucose oxidase [154][85]. Unfortunately, enzyme-based biosensors do not show versatility due to their low stability and potential to be affected by temperature, pH, and ionic strength [155][86]. Hence, most researchers in recent times have focused on the development of enzyme-free biosensors based on the functionalization of electrodes using nanomaterials that provide them with high sensitivity [156][87].

10. LDH as Supercapacitors

LDH composites based on nickel (Ni) and combined with carbon-based nanomaterials have been used as electrodes for supercapacitors [157,158][88][89]. Ni-Al LDH nanosheets grown in situ on carbon nanotubes (CNT) exhibited promising capacitive performance. The introduction of CNTs assisted in better performance that hindered the restacking of LDHs during synthesis and provided a surface for conduction. In another example, honeycomb-like cobalt-based LDHs were deposited in situ on multilayer graphene for an energy storage device that showed a high capacitance of 883.5 Fg-1 [159][90]. Therefore, LDH composites open the pathway for utilizing hybrid supercapacitors at a comparatively low cost.

11. Applications of LDHs in Display and Sensing

LDH-based photo functional materials have been obtained through controlled variations of the guest species. The fluorescence performance and stability of the guest species have been effectively enhanced by the introduction of LDHs. For displays and polarized emission, some intercalated LDH composites with a specifically tailored arrangement have been reported in the literature [160,161][91][92]. Multifunctional materials have been constructed by immobilization of QDs on the surface of LDH by self-assembly that respond to changes in pH, temperature, pressure, and light in the field of sensors. Multicolored luminescence materials have been obtained by introducing a diverse variety of chromophores (organic dyes, polymers, and quantum dots) in LDHs [162][93]. LDHs have also been designed to detect heavy-metal ions (HMIs), biomolecules, and chemosensors for environmental pollutants [163][94]. LDHs have unlimited application with ease of tunability.
 
 
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