Advanced Materials for Agriculture, Food, and Environmental Safety

By Ashutosh Tiwari and Mikael Syväjärvi

The book focuses on the role of advanced materials in the food, water and environmental applications.  The monitoring of harmful organisms and toxicants in water, food and beverages is mainly discussed in the respective chapters. The senior contributors write on the following topics:

• Layered double hydroxides and environment
• Corrosion resistance of aluminium alloys of silanes
• New generation material for the removal of arsenic from water
• Prediction and optimization of heavy clay products quality
• Enhancement of physical and mechanical properties of fiber
• Environment friendly acrylates latices
• Nanoparticles for trace analysis of toxins
• Recent development on gold nanomaterial as catalyst 
• Nanosized metal oxide based adsorbents for heavy metal removal
• Phytosynthesized transition metal nanoparticles- novel functional agents for textiles
• Kinetics and equilibrium modeling
• Magnetic nanoparticles for heavy metal removal
• Potential applications of nanoparticles as antipathogens
• Gas barrier properties of biopolymer based nanocomposites: Application in food packing
• Application of zero-valent iron nanoparticles for environmental clean up
• Environmental application of novel TiO2 nanoparticles

• Language: English
• Publisher: Wiley
• Release date: Aug 19, 2014
• ISBN: 9781118773901

Book preview

Preface

The levels of toxic and microbial contamination in food and the environment are influenced by harvesting and slaughtering agro technologies and by the processes applied during the manufacture of food. With current cultivation methods, it is impossible to guarantee the absence of pesticides and pathogenic microorganisms in raw foods of both plant and animal origin. The increasing incidence of widespread foodborne diseases and the resultant socioeconomic impact on the world population have brought food and environmental safety to the forefront of ecological public health concerns. The emerging field of advanced materials based on functional architectures offers potential solutions to some key performance challenges, along with improved sensitivity, longevity, stability, miniaturization and ruggedness, while reducing complexity and production cost.

The overall purpose of this book is to generate new solutions to the technical challenges for easy and rapid detection of food toxicants, microorganisms and environmental pollutants. Since the consumption of food and water is an essential part of the live detection of contaminating organisms, the book is especially focused on monitoring the presence of various toxic molecules within water, food and beverages. Moreover, the development of fundamental methodologies and inventive nanotechnologies is a scientific and technological area affecting many aspects of energy and the environment. These methodologies range from clay materials to aluminium alloys commonly used in various applications—for example, in the aeronautics industry. Nanotechnologies have expanded from semiconductors, photonics and healthcare processes to include environmental technology to reduce pollution. This raises the possibility of using nanotechnology for environmental applications through nanomaterials, processes and tools. In all cases, the fundamental aspects of materials and methods are prerequisites for further use.

In Part 1 of the book, Fundamental Methodologies, the first chapter, Layered Double Hydroxides and the Environment: An Overview, examines layered double hydroxides that can be used for environmental decontamination. Clay materials open up production possibilities since they can be easily synthesized by various cheap and ecofriendly methods. Common decontamination processes are by anion exchange, adsorption and catalytic remediation. Layered double hydroxides not only act to prevent the dispersion of pollutants in effluents or wastewater—for example, by precipitating agents of heavy metal cations—but recently by also targeting molecules and inorganic substances. In the next chapter, Improvement of the Corrosion Resistance of Aluminium Alloys Applying Different Types of Silanes, approaches for protecting aluminium alloys from attack by using different surface treatments are described. The advantages of aluminium alloys are their low density and good mechanical features compared with certain other alloys which experience a high corrosion rate. Unfortunately, corrosion treatments that exhibit good corrosion performance are still commonly used even though they are very toxic, carcinogenic and allergenic. The movement toward a more environmentally-friendly process includes green treatments, as exemplified by a reduced use of chromium solutions in favor of silanes. Silane coatings exhibit advantageous barrier properties due to the dense Si-O-Si network which substantially reduces the penetration of aggressive species to the metallic alloy. The barrier properties then need to be combined with an understanding of defects in the barrier that causes localized corrosion processes. The quality of the protection layer from the treatment is dependent on parameters like treatment time, temperature, and chemistry of silane molecules. Further on, such treated surfaces would ideally also be matched with organic paint—for example, by adhesion properties.

With increasing industrial and agricultural activities, arsenic needs to be considered since it can be mobilized into surface water. In particular, water treatment is an increasingly urgent matter. New Generation Material for the Removal of Arsenic from Water, considers the recovery and sorbent regeneration when a sorbent is exhausted and addresses removal techniques such as solvent extraction and chemical precipitation as synthetic coagulants. The process to restore the sorbent close to its initial state from the metal recovery is a critical step. Subsequently, the arsenic needs to be properly handled and disposed of after recovery. Many metals may be recovered and reused, but this is not as straight forward for arsenic since it has limited markets. The chapter, Prediction and Optimization of Heavy Clay Products Quality, illustrates various process parameters used to design and control brick production such as chemical composition, firing temperature, weight loss, and water absorption. A new technique that enhances the physical and mechanical properties of natural fibers and their composites is described in the chapter, Enhancement of Physical and Mechanical Properties of Sugar Palm Fiber via Vacuum Resin Impregnation. In the chapter, Environmentally-Friendly Acrylates-Based Polymer Lattices, the development of suitable coating by synthetic polymers is discussed. Such polymers are widely used in packaging and construction. Acrylic resins have great durability and weather resistance. The barrier properties are obtained through copolymerization with more hydrophobic monomers. The type of monomer, its sequence length distribution and polymer weight affect the transition temperature and viscoelastic modulus of the polymer which leads to a film with high clarity, good stability and high mechanical strength.

Part 2 of the book, Inventive Nanotechnology, begins with the chapter, Nanoparticles for Trace Analysis of Toxins: Present and Future Scenario, which defines the advances and applications of nanotechnology for removal of water pollutants. These novel nanotechnological approaches make it possible to explore various nanometallic particles for extraction of pollutants and also for nanoremediation with reduced clean-up time without the need for eliminating treatment and disposal of contaminated soil. Highlighted in the next chapter, Recent Developments in Gold Nanomaterial Catalysts for Oxidation Reaction through Green and Sustainable Routes, is the design of reaction specific catalysts in which nanomaterials are used as a key technology for green chemistry. In this oxidation reaction large quantities of agents can be provided—for example, gold nanoparticles as catalyst for oxidation reactions in the gas phase—which is the result of a combination of gold particle size, nature of support material and type of reaction.

Nanosized Metal Oxide-Based Adsorbents for Heavy Metal Removal: A Review, presents an overview of nanosized metal oxides whose high surface area and specific affinity for heavy metal adsorption from the aqueous phase make them attractive for water purification. The methods for fabrication, physicochemical and adsorption properties, as well as application for heavy metal removal from the aqueous phase are described. Other issues requiring further attention are large-size particle aggregation and capacity loss. Highlighted in the chapter, Future Prospects of Phytosynthesized Transition Metal Nanoparticles as Novel Functional Agents for Textiles, are the biological systems used for green synthesis of nanoparticles. The use of plants for making metal nanoparticles is an environmental approach due to their biocompatibility, low toxicity and environmental nature. Plants have shown the most promise as they seem suitable for large-scale biosynthesis compared to microprobes and enzymes. Also, new functionalities upon integration into textile materials may appear. Some of the metals produced are gold, silver and palladium, and other nanoparticles like copper. Zinc and cadmium oxide have gained interest as catalysis, sensors and photonic devices. Since details about plants that produce nanoparticles are not known, this field is of great scientific interest. In the chapter, Functionalized Magnetic Nanoparticles for Heavy Metal Removal from Aqueous Solutions: Kinetics and Equilibrium Modeling, magnetic nanoparticles are placed in the forefront as platforms for detection and separation applications. These particles can be composed of both inorganic and organic components, which position such systems to potentially tune properties of hybrid materials for appropriate functions due to their small size, biocompatibility and superparamagnetic properties.

Pathogens are responsible for diseases which cause the disability and death of millions. The chapter, Potential Application of Nanoparticles as Antipathogens, discusses the interface of microorganisms (pathogens) and nanostructures which can find wide application for use as antipathogens—substances which are used to kill, deactivate and control the pathogens. Nanoparticles such as silver, gold, titanium dioxide and zinc oxide are receiving considerable attention as antimicrobials. In the chapter, Gas Barrier Properties of Biopolymer-Based Nanocomposites: Application in Food Packing, biopolymers and protein-based nanobiocomposites are described in respect to their gas barrier properties, which are promising for lowering oxygen permeability. In food packaging most materials are more or less nondegradable. New biomaterials could be more environmentally friendly and even be both edible and biodegradable. Still, there are issues when performance, processing and cost are combined. In performance, they are water sensitive and have limited mechanical properties with high brittleness. Nanocomposites as packaging materials are still in their infancy. Zero-valent iron, one of the most widely studied nanoparticles, is presented in the chapter, Application of Zero-Valent Iron Nanoparticles for Environmental Clean Up. The attraction of these nanoparticles lies in their combination of nanosize and excellent reducing capability, which results in a powerful remediation tool for reducing toxic and hazardous wastes. The zero-valent iron can be injected into groundwater and aquifers to treat contaminated systems. At the same time it interacts with the contaminant and biotic component of the system. In the chapter, Typical Synthesis and Environmental Application of Novel TiO2 Nanoparticles, titanium dioxide nanoparticles are introduced and reviewed as a player in environmental protection and in the implementation of techniques to remove inorganic or organic pollutants from wastewater. Finally, ZnO nanostructures offer the most promising platform for fabrication of various optoelectronic devices. In the last chapter, Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity, the solution growth processes to fabricate ZnO nanowire films are reviewed. Then, theoretically proposed and experimentally observed defect states and their origin are discussed.

This book is written for readers from diverse backgrounds across various fields such as chemistry, physics, materials science and engineering, medical science, pharmacy, biotechnology, and biomedical engineering. It can be used not only as a textbook for both undergraduate and graduate students, but also as a review and reference book for researchers in the field. We hope that the chapters of this book will provide readers with valuable insight into the major area of nanosafety, green materials and their respective technologies.

Editors

Ashutosh Tiwari, PhD, DSc

Mikael Syväjärvi, PhD

Part 1

FUNDAMENTAL METHODOLOGIES

Chapter 1

Layered Double Hydroxides and the Environment: An Overview

Amita Jaiswal* Ravindra Kumar Gautam and Mahesh Chandra Chattopadhyaya*

Environmental Chemistry Research Laboratory, Department of Chemistry, University of Allahabad, Allahabad, India

*Corresponding author: [e-mail], [e-mail]

Abstract

Due to their versatility, hundreds of millions of tons of clay minerals currently find applications not only in ceramics, building materials, paper coating and fillings, drilling muds, foundry molds, pharmaceuticals, etc., but also as adsorbents, catalysts or catalyst supports, ion exchangers, etc., depending on their specific properties. There are two broad classes of clays: Cationic clays (or clay minerals), widespread in nature, and Anionic clays (or layered double hydroxides), more rare in nature but relatively simple and inexpensive to synthesize on a laboratory or industrial scale. Cationic clays have negatively charged alumina-silicate layers with small cations in the interlayer space to balance the charge, while anionic clays have positively charged brucite type metal hydroxide layers with balancing anions and water molecules located interstitially. The layered double hydroxides (LDHs) belonging to the general class of anionic clay minerals can be of both synthetic and natural origin. Also known as hydrotalcite-like compounds (HTLCs), these materials are interesting because their layer cations can be changed among a wide selection, and the interlayer anion can also be freely chosen. Like cationic clays, they can be pillared and can exchange interlayer species, thus increasing applications and making new routes to synthesize the derivatives.

This chapter deals with the brief history of layered double hydroxides, their structure, properties, synthesis by different methods and characterization, along with their applications mainly in the environmental field.

Keywords: Layered double hydroxides, anionic clays, cationic clays, brucite, interlayer species, heavy metals, dyes, greenhouse gases, surfactants

1.1 Introduction

Layered double hydroxides (LDHs) have been known for a very long time. Around 1842, naturally forming LDHs minerals were discovered in Sweden. Crushing these minerals leads to a white powder similar to talc. These materials were first synthesized by a German scientist, W. Feithnecht (1942), through reaction between dilute solutions of metals with bases, which he named doppelschichtstrukturen or double-sheet structure. The LDHs are also known as hydrotalcite-like compounds (HTLCs). Hydrotalcite (HT) is a hydroxycarbonate of magnesium and aluminium which occurs in nature in foliated and contorted plates or fibrous masses.

During the discovery of hydrotalcite another hydroxycarbonate of magnesium and iron was found, which was called pyroaurite. Pyroaurite was later recognized to be isostructural with hydrotalcite and other minerals containing different elements, all of which were recognized as having similar features. Hydrotalcites have been studied for their use as catalysts and precursors to various other catalysts as early as 1970 [1, 2].

Allman and Taylor studied single crystal X-ray diffraction on mineral samples which revealed the main structural entities of LDHs and disproved Feitknecht’s theory. These studies showed that the two cations were in fact located in a single layer and the interlayers were composed of water and carbonate ions. Although the main entities of LDHs have been elucidated, Evans and Slade [3] have suggested that several intrinsic details still remain to be fully understood. These include the possible stoichiometric range and composition, and the position and arrangement of metals within each cationic layer. Prior to the study by Evans and Slade, Miyata and Okada [4–6] described many structural features of LDHs/HTLCs which have different guest anions.

Layered double hydroxide materials appear in nature and can be readily prepared in the laboratory. In nature they are formed from the weathering of basalts [7, 8] or precipitation [9] in saline water. All natural LDH minerals have a structure similar to hydrotalcite, which has the formula [Mg6Al2 (OH)16] CO3. 4H2O. Unlike clays, however, layered double hydroxides are not discovered in large, commercially exploitable deposits [9]. The LDHs have been prepared using many combinations of divalent and trivalent cations including magnesium, aluminium, zinc, nickel, chromium, iron, copper, indium, gallium and calcium [10–31].

1.2 Structure of Layered Double Hydroxides

Layered double hydroxides (LDHs) are also known as hydrotalcite-like compounds (due to their structural similarities to that mineral) or anionic clays and host-guest layered materials [1, 3, 32–35], which are quite rare in nature. Most LDHs are synthetic phases and their structure resembles the naturally occurring mineral hydrotalcite [Mg6Al2(OH)16] CO3. 4H2O, having the general formula of [M(II)1−x M (III)x (OH)2] (Yn−)x/n. YH2O, where, M(II), M(III) = divalent and trivalent metals respectively, 0.2 < x < 0.33, and Yn− = the exchangeable anions between the layers [10, 36, 37].

The basic layer structure of LDHs is based on brucite [Mg (OH)2], typically associated with small polarizing cations and polarizable anions. It consists of magnesium ions surrounded approximately octahedrally by hydroxide ions. These octahedral units form infinite layers by edge-sharing with the hydroxide ions sitting perpendicular to a plane of the layers. The layers then stack on top of one another to form a three-dimensional structure.

When Mg²+ is replaced by a trivalent cation similar in radius, an overall positive charge results in the hydroxyl sheets and counter balance is provided by carbonate ions which are positioned within the hydroxyl interlayer. In addition to carbonate ions, water molecules are found in the interlayer gallery. The nature of the interlayer anion and the extent of hydration often determine the layer spacing between each brucite-like sheet [38]. The brucite-like sheets may occur in two different symmetries, namely rhombohedral and hexagonal. In nature, the rhombohedral symmetry is widespread. However, in mineral samples, the hexagonal symmetry is seen to favor the interior of the crystallite samples, while the rhombohedral symmetry is found on the exterior. This is a result of cooling during crystallite transformation, in which the extrerior surface of the crystallite cools much quicker than the interior and hence the interior hexagonal form cannot transform due to a higher energy transformation barrier at lower temperature. From these observations, it has been deduced that the hexagonal symmetry is favored by high temperature [1, 4]. Naturally occurring minerals that exhibit a LDH structure include manasseite, pyroaurite, sjogrenite, bar-betonite, takovite, reevesite, desautelsite and stichtite. They differ from one another in the stacking arrangement of the octahedral layers [1, 39].

Conventionally synthesized LDHs are strongly hydrophilic materials, either amorphous or microcrystalline with hexagonal habit, with the dominant faces developed parallel to the metal hydroxide layers. Adjacent layers are tightly bound to each other. Figure 1.1 shows the structure of layered double hydroxides.

Figure 1.1 Structure of layered double hydroxide (LDHs).

One of the advantages of LDHs among layered materials is the great number of possible compositions and metal–anion combinations that can be synthesized. Layered double hydroxides (LDHs) have high charge density. The charge density is dependent on the metal ratio. Since it comprises a divalent and trivalent metal cation, their ratio affects charge density of the layers. A lower divalent/trivalent ratio results in a higher charge density.

1.3 Properties of Layered Double Hydroxides

Layered double hydroxides (LDHs) display unique physical and chemical properties close to those of clay minerals. Some interesting properties of these materials summarized by Del Hoyo [40] are:

High specific surface area (100±300 m²/g)

Memory effect

Anion exchange capacities

Synergistic effects

The LDHs exhibit anion mobility, surface basicity and anion exchangeability due to their positively charged layered structure. The anions and water, which fill the interlayer space, are labile. Therefore a variety of inorganic and organic anions can be intercalated in the interlayer of LDHs through anion exchange reactions [33]. The mixed metal oxides obtained on calcination of LDH usually exhibit properties such as high surface area, surface basicity and formation of homogeneous mixture with small crystallite size when heated to higher temperature [1]. The LDHs as well as the oxides obtained from them exhibit excellent catalytic activity. Structure reconstruction, or so called memory effect, is another important property of LDHs which is unique to this class of layered solids. Structure reconstruction is usually achieved by first decomposing the LDH at suitable high temperature followed by treating the resultant mixed metal oxides with a solution containing a suitable anion [41]. These materials have a high capacity for adsorbing anions as well as cations [38, 42]. Magnetic properties of the LDHs depend on the space between the layers. This space can be adjusted by insertion of organic anions with different chain lengths. This suggests that these hybrid materials would work as tunable magnets [43]. The LDHs intercalated with long-chain surfactant molecules such as dodecyl sulphate have the ability to swell in organic solvents. This property of delamination is exploited in the preparation of monolayers, which are used extensively in the synthesis of nanohybrids and nanocomposites [44].

The interlayer anions present in LDHs can be exchanged by other anions. The order of preference for some common inorganic anions is as follows:

equation

NO3− is an anion which can be easily replaced by a more strongly held one like CO3²−. Therefore when preparing precursor for interaction, nitrate salts are preferred and CO3²− is tried to be kept away. The interaction involves a guest molecule that is introduced into the host without affecting the structure of the host. Upon interaction of the guest molecule, the existing ion is replaced. The interlayer anions weekly bonded to hydroxide layers (e.g., NO3−, Cl−) can be exchanged more easily [45–47].

1.4 Synthesis of Layered Double Hydroxides

Layered double hydroxides (LDHs) can be synthesized with a wide range of compositions and a variety of M(II)/M(III) cation combinations containing different anions in the interlayer spaces. A number of synthetic techniques have been successfully employed in the preparation of LDHs. There are number of methods used to synthesize LDHs which are presented below. [1].

1.4.1 Co-precipitation Method

Co-precipitation remains the most commonly used technique for the synthesis of LDHs. This technique involves the addition of aqueous solutions of M(II), M(III) and the anion that will be incorporated into the LDH structure. This method allows the direct preparation of LDHs for a variety of anions and cations. Therefore, this method is commonly used for the formulation of organic-anion LDHs, which are difficult to obtain by other methods [3]. Following are the three methods of co-precipitation which can be used for the synthesis of LDHs.

1.4.1.1 Co-precipitation by Titration

In this method, a metal salt solution is titrated against a basic solution during which not only solids of LDHs structure occur, but also other solid phases are formed, due to the precipitation of less soluble metal hydroxides prior to the precipitation of the LDHs.

1.4.1.2 Co-precipitation at Low Supersaturation

Co-precipitation at constant pH and at temperature between 60–80°C by the slow addition of mixed solution of M(II) and M(III) salts in the chosen ratio and the base solution. The rate of addition has to be controlled in order to obtain a more homogeneous phase. These conditions are best to obtain higher crystallinity of the material due to the fact that at constant pH the rate of crystal growth is generally lower than the rate of nucleation [3].

This method is most often used, as it is the most effective one. For example, Marchi et al. obtained an amorphous precipitate of Cu/Co/Al hydrotalcites catalysts by co-precipitation at high supersaturation while under the conditions of low supersaturation a crystalline HT was precipitated [48].

1.4.1.3 Co-precipitation at High Supersaturation

In this method, a solution containing a mixture of the metal ion is quickly added to the basic solution and mixed rapidly This results in the formation of LDHs which are generally not very crystalline, but sometimes amorphous product is obtained due to the high number of nuclei formation. The precipitates obtained by this method are washed with distilled water in order to eliminate adsorbed ions and are dried at temperatures up to 120°C[1, 49].

1.4.2 Hydrothermal Synthesis

Hydrothermal preparation methods have been employed in order to control the particle size and distribution. There are two routes to hydrothermal treatment. In the first case, materials are treated at temperatures more than 373K in a pressured autoclave. Here, the LDHs may be synthesized from precursor such as MgO and Al2O3 or from mixtures obtained from the decomposition of the nitrate forms of these precursors [1, 50, 51]. More specifically, the precursors are placed in an autoclave set at temperature above 596K and pressure between 10–130 MPa for a fixed time period. In the other case, LDHs are synthesized at low temperatures and undergo a process of aging. Aging involves refluxing the precipitate at a set temperature for 18 hours.

1.4.3 Urea Hydrolysis Method

Urea is a weak Bronsted base highly soluble in water which has been used for the precipitation of metal ions as hydroxides. The hydrolysis of urea gives a pH of about 9 depending on the temperature of the mixture, which allows its use as precipitating agent in the synthesis of LDHs. The crystallinity degree of LDHs has been observed to depend on the temperature of synthesis and the aging time. Larger particles are formed at low temperatures [52, 53] due to the lower nucleation rate which depends on the decomposition rate of the urea. However, this method is not indicated for the preparation of LDHs with low charge density, but allows the preparation of compounds with high charge density which are difficult to obtain with other procedures [52].

1.4.4 Sol-Gel Method

The sol-gel method involves the formation of a sol by hydrolysis and partial condensation of metallic precursor and followed by the gel formation. As metallic precursor, metallic alkoxides, acetates, acetylacetones and many inorganic salts are used. The properties of the LDH depends on the hydrolysis and condensation rates of the metallic precursors [54], which are modified by different parameters of the reaction like pH, nature and concentration of the precursor, solvent and temperature of the synthesis. The LDHs obtained by this method exhibit specific surface area larger than those obtained by co-precipitation method [55–57], but with controversial results regarding the basicity and the trivalent and divalent metal ion molar ratios [56–58].

1.4.5 Ion-Exchange Method

In this method, there is an exchange of the interlayer anions with anionic guest molecules, i.e., molecules one wishes to introduce into the LDH structure in order to produce the desired LDH–guest compound. The ion exchange in LDHs depends on several factors according to He et al. [59], affinity for the incoming guest anion, the medium in which the exchange occurs, the pH and the chemical composition of the LDH brucite-like layers.

In terms of thermodynamics, ion exchange in LDHs depends mainly on the electrostatic interactions between the positively-charged sheets and the interlamellar anion and, to a lesser extent, on the free energy involved in the changes of hydration [60]. Moreover, the equilibrium constants in the anionic-exchange process depend on the size of the anion. Hence, the anion exchange is favored for anions with high charge density (i.e., high charge and small ionic radius) [61].

The ionic exchange for simple inorganic divalent anions decreases in the order of CO3²− > HPO4²− > SO4²− and for monovalent anions, OH− > F− > Cl− > Br− > NO3− > I−. Synthesis of LDHs using ion exchange methods have been carried out by a number of researchers [5, 39, 62–65].

1.4.6 Rehydration Method

The rehydration method was reported by Miyata, who found that after calcination of the LDHs at 500–800°C, the mixed metal oxide rehydrated and reformed back to a LDH in the presence if anions. This unique memory effect of LDHs provides an effective synthesis route to obtain LDHs with desired inorganic and organic anions, and avoids the incorporation of competing inorganic counter anions [66–73].

1.4.7 Miscellaneous Methods

In addition to the above methods, several other methods which have been applied in the preparation of LDHs are presented below.

1.4.7.1 Salt-Oxide Method

In this method M(III) solution is slowly added into a suspension of M(II) metal oxide or hydroxide. The mixture is heated at room temperature for a few days in order to favor the reaction between the precursors. This method has been used to prepare LDHs having different combinations of divalent and trivalent cations and anions such as ZnCr-Cl, ZnCr-NO3, ZnAl-Cl and ZnAl-NO3.

1.4.7.2 Template Synthesis

In recent years, this method has attracted more attention in the field of Materials Science. Using self-assembled aggregates as a template, inorganic materials can be directed to an ordered structure with specific morphology and size. There have only been a few reports of the synthesis of LDHs using this method [73].

1.4.7.3 Surface Synthesis

When a material is supported on the surface of a support, the resulting composite material is expected that presents improved properties such as mechanical performance, thermal stability and a high degree of dispersion. Some surface synthesis has been reported [74–76] using aqueous ammonia and precipitating agent urea or by impregnation of metallic salts following calcinations and hydrothermal treatment [77–79].

Various methods for the synthesis of LDHs can be used. The choice of the method will depend on the characteristics and application of the required material.

1.5 Characterization of Layered Double Hydroxides

Many physico-chemical techniques have been used to characterize LDHs. Some of the most commonly used techniques are XRD, FTIR and TG/DTA. Some other techniques are more specific for some LDHs such as SEM, DSC, NMR and Raman spectroscopy.

1.5.1 X-ray Diffraction

X-ray diffraction (XRD) is the main analytical technique for characterization of LDHs and crystalline phases. It is also used to determine the interlayer spacing and thickness of a single layer. The technique involves directing X-rays to the crystal. The radiation will be either reflected or diffracted at different angles. This technique is used to study the purity of the LDH intercalated material. The phase purity of the material is determined by the sharpness and/or the broadness of the diffraction peaks. The broader reflections correspond to the amorphous phase, while the sharper reflections correspond to the crystalline phase. From the diffraction data, the d-spacing of the intercalated LDH material can be determined. The reflections with the greatest d-spacing correspond to the d-spacing of the intercalated LDH [80].

1.5.2 Fourier Transform Infrared Spectroscopy

Fourier transform infrared spectroscopy (FTIR) is one of the molecular vibrational spectroscopic techniques used for both qualitative and quantitative analysis [81]. The FTIR analysis is not a diagnostic tool for LDHs, but can be useful to identify the presence of foreign anions in the interlayer between the brucite-like sheets. Besides this, information about the type of bonds formed by the anions and about their orientations can also be obtained. The infrared region is the region found in the wave number range 1.3×10⁴− 3.3×10¹ in the electromagnetic spectrum [82].

The FTIR is used to investigate the structural bonding and chemical properties of compounds [83]. On absorbing radiation by the molecule, the bond can stretch/vibrate or bend [84]. Each molecule absorbs a specific IR radiation depending on the type of functional group present. The resultant peaks in the IR spectra are called the molecular finger print. Therefore, each functional group has its own frequency and this is useful for revealing the presence or absence of these groups from the spectra.

Some specific characteristic peaks of LDHs are: peaks at 3500–4000 cm−1 assigned as –OH absorption and Interlayer CO3²− (LDH-CO3) [85, 86]; peaks at < 1000 cm−1 assigned as the inorganic lattice vibrations of M-O and M-OH modes [86]; peaks at 1560–1400 cm−1 due to intercalated anionic species (e.g., Carboxylate-LDH) [80] and due to strong asymmetric and symmetric stretching bonds, and; peaks around 1360 cm−1 which can be assigned as carbonate impurity and nitrates [86].

1.5.3 Thermogravimetric Analysis and Differential Thermal Analysis

Thermogravimetric analysis (TGA) is used to measure changes in the mass of LDH and estimate the content of water and anion molecule, while diffrential thermal analysis (DTA) can be used to study thermal properties and phase changes that can occur upon heating the LDH/LDH hybrids. Most thermal events correspond to water/anions removal from interlayer spaces.

1.5.4 Other Techniques

Other techniques include scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), X-ray fluorescence spectroscopy (XRF), Raman spectroscopy and nuclear magnetic resonance (NMR).

Scanning electron microscopy is used to study the surface topography of LDH intercalates by which particle morphology, porosity and phase composition within the material can be revealed. Both SEM and TEM provide general pictures of textural and crystal morphologies of LDHs. A hexagonal platelet morphology is usually shown by LDH [36, 87].

Both DSC and TGA are also used to check the chemical and physical properties of a material corresponding to temperature changes. It is used to detect the melting points (enthalpy of melting) and phase transitions of materials. This technique has not been explored very much in the study of LDH intercalates.

X-ray fluorescence spectroscopy is employed for trace element analysis and is also used for the determination of the divalent to trivalent cation ratios.

Raman spectroscopy has been mainly employed to study the structural accommodation of interlayer species and hydrogen bond network in LDH, particularly for oxoanions such as CO3²−, NO3−, SO4²− and CrO4²−, and organic carboxylate anions [22, 88, 89].

Nuclear magnetic resonance spectroscopy is an effective technique for studying the structural environments and dynamic behavior due to its unique ability to simultaneously probe element-specific local structure with high resolution, and to investigate atomic and molecular motion [90–92].

1.6 Applications of Layered Double Hydroxides

Layered double hydroxides (LDHs) represent one of the most technologically promising materials as a consequence of their low cost, easy preparation and the large number of composition variables that can be used. At present, a great deal of applications of LDHs in the field of heterogeneous catalysis, covering a wide range of areas, have made substantial progress, especially in the past decade. There are many possible applications of LDHs, not only as catalysts but also as scavengers of ions in water purification. Their applications in drug delivery systems are also well known. Some possible applications of LDHs are shown in Figure 1.2.

Figure 1.2 Applications of layered double hydroxides in various fields.

1.6.1 Catalytic Applications

Due to their flexible composition, the interest in layered double hydroxide materials has increased over the last decade. They are a relatively easy to synthesize, inexpensive, versatile and potentially recyclable source of a variety of catalyst supports, catalyst precursors or actual catalysts. The use of LDH materials in the field of catalysis is well known and has been reported for basic catalysis (e.g., polymerization of alkene oxide, aldol condensation, transesterification reactions, etc.), reforming of hydrocarbons with H2O, hydrogenation (production of methane, alcohols, paraffins, olefins, etc.), oxidation reactions, support for Zieglar-Natta catalysts during polymerization of olefins, etc. [1, 93]. However, LDHs themselves have not found applications in the field of heterogeneous catalysis,, but when mixed metal oxides derived from LDHs precursors are used. They are found in numerable applications which are listed in Table 1.1.

Table 1.1 Compositions of catalyst precursors (LDHs) and their catalytic applications [1].

Since the catalytic behavior depends primarily on the chosen metals as the catalytic centers [1], metal cations present in the brucite-like layers of the LDHs play an important role for the catalytic properties of the final product. Moreover, the anions present between the layers of the LDHs can have an influence on the catalytic properties. The anions present in the LDHs can also have an effect on other properties such as the crystal size of the catalysts.

1.6.2 Agricultural Applications

Layered double hydroxides possess excellent potential as green carrier for plant nutrients, pesticides and growth regulators and as active principle in animal feeds. However, not much attention has gone into exploring the full potentiality of LDHs. They are one of the idealized inorganic materials for a wide range of agricultural fields not only because their framework can be composed of plant nutrients, but also because their structure offers charming features such as accommodation and controlled release of various active anionic agro-substances, high buffering capacity, high water retention ability and acid neutralizing potential.

Komarneni et al. [94] suggested nitrate-LDH as a potential slow-release fertilizer by synthesizing nitrate-LDH in ambient condition without any considerable contamination of carbonate-LDH. More attention has been given to pesticide formulation that consists mainly of various organic solvents. Lakraimi et al. [95] prepared pesticide-LDH hybrid with 2,4- dichlo-rophenoxyacetate, a broad leaf herbicide, by ion-exchange reaction with a chloride form of ZnAl-LDH for slow release formulation. The acid neutralizing potential and high anion adsorption capacity of LDHs could be explored for animal feeds as a complement to cationic clays, provided their nontoxicity is ensured.

1.6.3 Pharmaceutical Applications

Pharmaceutical applications of LDHs mainly depend on the acid buffering effect and anion exchange property. LDH-derived antacid and antipeptic are representative of their applications in pharmaceutics [96]. Powdered LDHs have been demonstrated to be one important type of green carrier or host for genes and drugs due to the excellent biocompatibility and nontoxicity or low toxicity. MgAl-LDH is used as an important component of drugs, or as nanocarrier for delivery of drugs and genes into cells [97, 98]. Wei et al. [99] demonstrated that LDH was able to be used as an effective nanocarrier by greatly enhancing the thermal- and photo-stabilities for L-Dopa and L-Tyrosine [100]. Choi et al. have made significant investigations on the toxicity of LDH nanoparticles in vitro and in vivo in applications [97]. Xu et al. [101] recently reported an efficient LDH-based delivery for siRNA.

1.6.4 Industrial Applications

Layered double hydroxides have a large anion exchange capacity, making them useful for entrapping pesticides, surfactants, nitrates, chromates, etc., for the treatment of industrial and environmental wastewater.

As has already been pointed out, LDHs are used as catalysts. Their applications are increasing in the industrial production of organic materials and in conversion of natural gas. They also find applications in photochemical reactions. Many authors [102–108] also examined other applications of LDH materials. Some of these applications include the use of hydrotalcite-like compounds as additives in photochemistry, electrochemistry and in functional polymer materials. They have other applications in the pharmaceutical and biochemical industries [102].

1.6.5 Environmental Applications

In the field of environmental technology, much attention has been paid to the potential applications of synthetic materials as sorbents for organic and inorganic pollutants in water. Layered double hydroxides (LDHs) are used for the decontamination of the environment and prevent the dispersion of pollutants in nature. A wide range of contaminants can be removed from industrial effluents or wastewater by anion exchange, adsorption process and catalytic remediation, using LDHs. Recently, many other molecules such as pesticides, toxic organic chemicals, greenhouse gases, heavy metals and some other undesirable inorganic substances have increasingly been targeted for remediation by using LDHs for the protection of earth, health and the environment.

There has been a thorough reinvestigation of LDHs for anion exchange properties due to recent interest in developing the use of anionic clays for environmental remediation. The main characteristics have been studied to clearly characterize the adsorption properties of the materials under vigorous solid/liquid interface conditions. The effect of sorbent composition, surface and bulk adsorption, and concentration of adsorption site have been assessed. The adsorption capacity is deeply affected by the nature of the counteranion of the LDH layer.

In 1983, Miyata [109] first reported the ion exchange isotherm of a series of LDHs leading to the ion selectivity of layered double hydroxides for monovalent and divalent anions, OH− > F− > Cl− > Br− > NO3− > I− and CO3²− > SO4²−. The reconstruction process of the LDHs has been investigated for the removal of environmentally undesirable anion. Parker et al. [110] have pointed out that the high selectivity for carbonate anion prevents LDHs to be used as an anion exchange material unless further treatment is made. They measured anion exchange capacities of hydrotalcite in single and mixed anion solutions by observing the amount of anion adsorbed by freshly prepared hydrotalcite. They have compared the relative preference of anions after 24 h as SO4²− > F− > HPO4²− >Cl− >B(OH)4−>NO3−. This application of layered double hydroxides can be explained by the points shown below.

1.6.5.1 Removal of Heavy Metal Cations

Layered double hydroxides can be used as precipitating agents of heavy metal cations for the decontamination of wastewater. The Mn²+, Fe²+ and Cu²+ cations were removed by synthetic hydrotalcite-like compounds; zac-cagnaite [31] and hydrotalcite thin films were used for the remediation of aqueous wastes containing hazardous metal ions [111].

1.6.5.2 Removal of Nuclear Wastes

Hydrocalumite or ettringite, the calcium form of hydrotalcite, have been evidenced as earlier alteration products of cements or basaltic glasses [112, 113]. They have been studied for their adsorption properties of actinides (Th, U, Np, Pu, Se and Am) [114–116]. The high ability of LDHs and their calcined products to adsorb/exchange anions from solution was also used for the removal of Tc, Re and Mo in their anionic form from radioactive wastewater [117, 118]. Synthetic hydrotalcite has been investigated for the sorption of iodine and iodine-containing anionic species [119–124].

1.6.5.3 Removal of Greenhouse Gases

Layered double hydroxides are efficient scavengers for acid gases recovery from hot gas streams due to their unique and strong basic properties. The recovery of CO2 and SOx from power plant flue gases is considered to be the first step in reducing total carbon and sulfur oxide emissions. Many papers and patents describe the use of calcined LDHs for the adsorption of CO2 [125–128] and sulfur oxide [129–131] gases.

1.6.5.4 Removal or Adsorption of Organic Compounds and Pesticides

Recently, LDHs and their oxides have been investigated as scavengers to treat wastewater containing anionic organic contaminants. The materials are known to function as effective anion adsorbents in the uptake of phenols, terephthalates, anionic surfactants, ionizable pesticides and herbicides, humic and fulvic acids, anionic dyes, and colored organics from aqueous solutions [132–148].

Based on the ability of LDH to intercalate various types of anions in interlayers, organic–inorganic composite materials could be produced by intercalation of organic anions having functional groups in their structures into the interlayer of LDH. The LDHs modified with organic anions are expected to take up nonionic organic compounds from aqueous solution depending on the functional groups in the structure of the intercalated organic anions. For example, Mg–Al LDH intercalated with dodecylsulfate (DS) could adsorb hydrophobic pesticides such as atrazine, linuron and acephate, etc., from aqueous solution. This is due to the modification of the interlayer surface of the LDH from hydrophilic to hydrophobic.

You et al. [149] examined the surfactant-enhanced adsorption of organic compounds by Mg-Al LDHs. Organo-Mg-Al LDHs were prepared by incorporating anionic surfactants, octylsulfate, DS, 4-octylbenzenesul-fonate, and dodecylbenzene sulfonate, into Mg-Al LDH via ion exchange. The anionic surfactants were intercalated into Mg-Al LDH with the surfactants oriented perpendicular in the interlayer. The octylsulfate formed bimolecular films, and other surfactants resulted in monolayer structure. Intercalation of surfactants into Mg-Al LDH decreased the surface area, whereas surfactants dramatically enhanced the LDH affinity for 1,2,4-trichlorobenzene and 1,1,1-trichloroethane in aqueous solutions. Adsorption potential depended on the type of surfactant as well as the configuration of surfactant molecules within Mg-Al LDH interlayers. The adsorption characteristics indicated that the retention of organic compounds by organo-Mg-Al LDHs was due to a partitioning mechanism.

1.6.5.5 Removal of Dye

The discharged effluents of industries contain a large number of dyes, increasing the total COD of wastewaters [150]. Although most of the dyes are nontoxic, many of them are highly toxic, like metals (Cr), with harmful consequences to aquatic life. Moreover, the persistence of color appearance (at concentration > 1mg/L) in treated wastewater prevents their reuse.

The LDHs have high adsorption capacities for dye molecule and can be very competitive with other sorbents. They have been demonstrated in environmental applications due to their high ability to removed color or dye by adsorption reaction [137–145].

1.6.5.6 Removal of Surfactants

Layered double hydroxides have strong surface and interlayer hydrophilic properties due to their high content of hydroxyl groups and water molecules. These properties limit the adsorption and the intercalation of nonionic or hydrophobic contaminants. However, anionic surfactants such as alkyl carboxylate, alkyl sulphate or alkyl sulphonate are easily adsorbed and exchanged in LDH structure [151–153]. Dekany et al. [154, 155] compared the adsorption properties of both cationic surfactant/smectites and anionic surfactant/LDH antitype systems. The higher charge density of the anionic clays explains their higher adsorption capacities in terms of ion exchange capacity percentage. Pavan et al. [156–158] studied the adsorption of dodecylsulphate, octylsulphate, dodecylbenzenesulphonate and octylbenzenesulphonate by Mg-Al-LDH.

1.7 Conclusions

In this chapter we have established that LDHs are an important clay material. They can be easily synthesized in the laboratory by various cheap and eco-friendly methods. The LDHs can be synthesized by varying the composition of precursor according to their applications in various fields.

Layered double hydroxides have great potential for wide use in catalysis in a wide range of applications from refinery to fine chemicals and environment protection, and especially in the development of multifunctional catalysts with tailored properties and catalyst supporters. In this chapter we have examined some cases and discussed recent trends, limits and opportunities offered by this class of materials for application in the environmental and pharmaceutical industries. Attention was focused on the use of layered double hydroxides as an adsorbent for the treatment of water and wastewater from industrial effluents. Since several reviews have already been published, our objective was not to make a systematic review of the topic, but instead to offer a personal view of the more interesting recent direction of research and at the same time, in areas with an increasing number of publications which do not parallel the practical relevance of the topic.

Acknowledgements

The first author is extremely grateful to the University Grant Commission, New Delhi, for the award of Post Doctoral Fellowship for Women (Award Letter No. F. 15/53/12 (SA-II).

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