A Review of the Role of Natural Clay Minerals as Effective Adsorbents and an Alternative Source of Minerals A Review of the Role of Natural Clay Minerals as Effective Adsorbents and an Alternative Source of Minerals

The minerals with unique properties such as natural clay minerals (NCMs) have promis- ing approach in environmental and industrial sphere. In fact, under some specific conditions the NCMs could be used either as effective adsorbent material or alternative source of minerals. This chapter presents an outline of a general review of factors that affect the application ability of NCMs and a descriptive analysis of NH 4+ and REE adsorption behavior and extraction of rare earth elements (REE) by an ion-exchange with NH 4+ ions onto NCMs. Clays and NCMs both effectively remove various contaminants from aque- ous solution and serve as alternative sources of minerals, as extensively discussed in this chapter. This review compiles thorough literature of current research and highlights the key findings of adsorption (NH 4+ and REE) that use different NCMs as adsorbents or alternative sources of minerals (i.e., REE). The review confirmed that NCMs excellently remove different cations pollutants and have significant potential as alternative source of REE. However, modification and further development of NCMs applications for getting the best adsorption and the best extraction of REE onto NCMs, which would enhance pollution control and leaching system is still needed. transform infrared (FTIR) spectroscopy, X-ray fluorescence (XRF), magic angle spinning (MAS) nuclear magnetic reso-nance (NMR) spectroscopy (MASNMR), X-ray photoelectron spectroscopy (XPS) analyses, N 2 adsorption-desorption isotherms and zeta potential analysis to obtain the mineralogical and physicochemical parameters.


Introduction
Natural clay minerals (NCMs) have gained considerable attention due to their unique properties and their use in huge range of industrial and environmental applications [1,2]. NCMs are unique in the sense that these minerals are studied by, and used in, many disciplines for essential and applied research [3,4]. These minerals are nontoxic to ecosystem and play important role in the development of human civilization. They have been utilized in agricultural applications, engineering and construction applications, environmental remediation, geology, pharmaceuticals, food processing and many other industrial applications [2,5]. The economic benefits look evident due to the fact that NCMs are widespread, and inexpensive compared with other raw materials [6]. For these reasons, NCMs research is being actively pursued by many scientists and in several countries, and the future of clay science seems exciting, and promising.

Structure and composition of the NCMs
NCMs can arguably be considered as phyllosilicate class, containing layered structures of shared octahedral aluminum and tetrahedral silicon sheets, water molecules and hydrated cations that can move in and out of the interlayer spaces [7,8]. Commonly, isomorphous substitution of one cation with another (of similar size but with lesser charge, such as Al 3+ for Si 4+ or Mg 2+ for Al 3+ ) within crystal structures leads to a charge imbalance in silicate NCMs, which accounts for the permanent negative charge on NCMs particles, hence the ability of clays to attract cations to the surface. Amphoteric▬OH groups at the surface/edge of clays (i.e., silanol and aluminol groups) could also contribute to surface charge (pH-dependent reversible charge).
The physical and chemical properties of any particular NCMs are structure and composition dependent. A general review of the structure and composition of the various NCMs are essentially hydrous aluminum silicates that sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces [9]. The atomic structure of NCMs consists of two basic units, an octahedral sheet and a tetrahedral sheet. The octahedral sheet is comprised of closely packed oxygens and hydroxyls in which aluminum, iron, and magnesium atoms are arranged in octahedral coordination ( Figure 1a). The second structural unit is the silica tetrahedral layer in which the silicon atom is equidistant from four oxygens or possibly hydroxyls arranged in a tetrahedron with the silicon atom in the center (Figure 1b). NCMs are usually classified based on their structure and layer type [10]. The classification of Grim becomes the basis for outlining the nomenclature and the differences between the various NCMs [3]. Although, it is not possible to compress the discussion of structure and classification of NCMs in this chapter, a simple classification of NCMs is available in literatures, NCMs can be divided into four main groups: kaolinite group, illite group, smectite group, and vermiculite group.
The structure and composition of the major industrial NCMs are different even though they are each comprised of octahedral and tetrahedral sheets as their basic building blocks. The arrangement and composition of the octahedral and tetrahedral sheets account for most of the differences in their physical and chemical properties. Therefore, for their applications an understanding of the structure, physical and chemical properties attributes of the individual clay minerals is important. The huge variety of physical and chemical properties of NCMs provides unlimited scope for future application, particularly in environment protection or as minerals resources. A general review of the structure and composition of the NCMs are given in this chapter. A more detailed discussions of the structures of the various NCMs were discussed in literature [5,[9][10][11][12][13].

Properties and factors that affect the application ability of natural clay minerals
The use of NCMs for specific applications depends on its type of structure (1.1 or 2.1 layer type) and on its chemical composition [14]. The identity of all the material present in NCMs should be determined in order to evaluate their properties. The most important characteristic of NCMs is the cation exchange capacity (CEC). CEC is to measure the capacity of NCMs to exchange cations from the solution [15], which depends on the volume of the total layer charge. Since the surface layer charge is the function of pH, thus, CEC also changes with pH and regularly CEC is measured at pH 7 [16,17]. The popular metallic cations found in exchange positions in NCMs are Ca 2+ , Mg 2+ , Na + , and K + . The presence of charge in NCMs play important role for cation exchange and the swelling properties of the minerals. The hydrolysis of Si▬OH or Al▬OH bonds along the NCMs lattices supplies the surface charge. Depending on the silica structure and the solution pH, the net surface charge can be either positive or negative. The tetrahedral and octahedral sheets of NCMs usually have a charge. The charge in the NCMs occurs in two forms: structural and surface charge. The structural charge is permanent and present due to ion substitutions, which arises inside the interior of the layers. The surface charges, generally in NCMs depend on the pH value, with 2:1 layer, the surface charge creates on the basal surface of the tetrahedral sheets while the surface charge for layer type 1:1, derives from both of tetrahedral and octahedral sheets. Also, the edges of both 1:1 and 2:1 layer contribute to the surface charge [16]. Several books have explained the details of the structure and properties of NCMs such as Handbook of Clay Science, edited by Bergaya et al. [13].
Furthermore, the presence of amount of exchangeable ions, non-NCMs, soluble salts, and quality of their texture are factors which can affect their properties and applications. The presence of cations in octahedral sheet, and isomorphic substitutions in the octahedral and tetrahedral sheets result in net charge deficits. Varying according to the sheet unit, and ultimately, in different mineral phases giving rise to varied technical behavior. The textural differences between structurally and chemically identical NCMs also affect their adsorptive properties [15,17,18].
The type and amount of non-clay minerals are present with NCMs affect their properties and applications. Non-clay minerals commonly associated with the NCMs include quartz, feldspar, mica, calcite, dolomite, opal C-T, and minor amounts of heavy and trace minerals such as ilmenite, rutile, brookite, anatase, leucoxene, sphene, tourmaline, zircon, kyanite, goethite, hematite, magnetite, garnet, augite, florencite, apatite, andalusite, and barite. Subsequently, when developing applications for NCMs, it is necessary to take these factors into consideration. It is important to know the specific properties of NCMs one is using in order to ensure that it is appropriate for one's needs or to better understand their mechanism behavior during experimental process.

Techniques of NCMs characterization
There is increasing trend in the popularity of productive research in the field of NCMs. Characterization of NCMs is given significance as they are mainly used as cation exchangers, sorbents/hosts, and catalysts. Usually, the characterization of a number of techniques has to be done in order to get comprehensive details of the NCMs.
Moreover, the multitude of techniques also accelerates the process of development of NCMs, particularly as catalysts, as different aspects are discovered. Extensive research in the field of instrumentation has resulted in advanced techniques of analysis that have helped in characterization of molecular sieves in general and NCMs in particular. In common, the characterization of NCMs should provide information about; (i) chemical composition; (ii) structure and morphology; (iii) ability to adsorb and retain molecules, and (iv) ability to chemically convert these molecules.
XRD is generally used to identify the presence and quantitative determination of crystalline NCMs. SEM is useful for morphological analysis and chemical analyses at specific locations. The chemical compositions are determined using XRF. IR and NMR provide insight into acid sites and framework structure. It is not possible to compress the discussion of such techniques and research done on their scope and development. Literatures provide useful insight into various characterization techniques for NCMs [5,13,15].
Here, we compile research data from various publications related to the use of different NCMs-based adsorbents and the leaching technology generations for REE extraction, notably thorough literature of our current researches. In the first part of this chapter, general structure, properties and the factors that affect the application ability of NCMs are discussed. The techniques of NCMs characterization are also summarized. The main goal of this review is to explain why an understanding of the structure and surface properties of the individual NCMs are so important. This chapter provides an elaborate information about the different NCMs as effective adsorbents in environment protection and their importance for the extraction of rare earth elements in ion adsorption clays. It has also adequately summarized the role of factors that affect adsorption (i.e., NH 4 + and REE) and extraction behavior of REE onto NCMs.

Adsorption behavior of contaminants and recovery of rare earth elements adsorbed on natural clay minerals
In environment protection, NCMs have been used in the elimination and storage of hazardous chemicals [14,19,20]. NCMs have the catalytic capability to neutralize certain organic hazardous chemicals [15,20,21]. The ability of NCMs to contain hazardous substances depends on their cation exchange capacity (CEC) while the process of retaining toxic materials mainly occurs by the ion exchange and/or adsorption. Due to their high cation exchange capacity, NCMs are very effective for the adsorption of cations from the solution. Although the NCMs are electronegatively charged [22], these minerals still can adsorb organic and non-ionic substances in significant amounts. The adsorption characteristics are dependent upon the chemical/structural makeup of the adsorbent, the Si/Al ratio, cation type; number and location are particularly influential in adsorption. The adsorption capacity of NCMs can be improved by modification with inorganic salts (NaCl, CaCl 2 , BaCl 2 , NH 4 Cl, AlCl, FeCl 3 ), cationic surfactants, acid, base and organic [23,24]. Consequently, the NCMs become hydrophobic and organophilic, which led to the enhancement of the adsorption of non-ionic and organic compounds [25,26]. Notably, in our brief studies of NH 4 + and REE adsorption, we used nonmodified NCMs, to better understand the natural system reactivity of NCMs, and also the modification of natural minerals at a larger scale may increase the processing cost; thus, reliable water treatment by using non-modified clay minerals is highly desirable. Therefore, utilization of NCMs would solve disposal problem, and also access to inexpensive materials in the wastewater treatment. Moreover, due to low cost of NCMs, there is no need to regenerate them; which provide more advantages in using NCMs as an adsorbent materials.

A brief review of adsorption of NH 4 +
Nitrogen compounds in aqueous environments are commonly found in the form of ammonium ions (NH 4 + ). Important sources of NH 4 + include effluent from the application in agricultural practices and industrial processes resulting in algal bloom in lakes and rivers [27,28]. NH 4 + concentration, in certain surface waters serving as a source of potable water, is much higher than the permissible level, due to large quantities of industrial and municipal wastewater being discharged into existing water resources [29][30][31]. Also, the NH 4 + concentration for most fish species must not exceed 1.5 mg [32,33]. Therefore, complete removal of NH 4 + is required due to its toxicity to the majority of aquatic lives. For these reasons, the prevention of nitrogen pollution with NH 4 + removal from wastewater is of great importance [34][35][36]. Various methods including air stripping, biological methods and activated carbon have been used for NH 4 + removal [37]. However, these techniques are not suitable for use in the removal of low contaminants concentrations, which cause damage both to the environment and life [31]. Additionally, high costs, poor regeneration and uncertainty of outcome are some of the frequently encountered limitations in the application of these methods [36,38,39]. Furthermore, contingency on temperature and climate conditions constitutes another disadvantage in this process.
Compared with the above mentioned methods, high safety, low cost [34,40,41] and relative simplicity of application and operation are some of the attributes that are attracting an increasing focus on the use of adsorption method for environment applications [33,42,43]. Adsorption process is a suitable technique for pollutants removal from wastewater, because of the significant advantages like low-cost, profitability, availability, and effectiveness than other methods. This method is easy to operate and equally effective in the removal of toxic contaminants, even at low concentrations [44].

Investigations of NH 4 + adsorption properties of six natural clay minerals
This research presented six types of NCMs-based adsorbents namely kaolinite, halloysite, montmorillonite, vermiculite, palygorskite, and sepiolite were examined and compared in the same study [44]. The study illustrated that among all the NCMs studied, vermiculite and montmorillonite have the highest ammonium adsorption capacities. The study revealed that the cation exchange is the main mechanism for the NH 4 + adsorption. Negatively charged surface, specific surface area, water absorption process and surface morphology of NCMs might also contribute to the high adsorption capacities. Adsorption kinetics showed that the adsorption behavior followed the pseudo-second-order kinetic model whereas the isotherms fitted the Langmuir model. The insights obtained in this study are useful for applications of NCMs in environmental remediation. The results illustrated that the structure and surface properties of NCMs are the key factors that affect the adsorption capacities for NH 4 + . The study concluded that the NCMs have significant potential as economic, safe and effective adsorbent materials for the NH 4 + adsorption from the aqueous solution at low concentrations.

Adsorption of rare earth elements (REE)
REE group consists of 17 elements and is divided into two categories namely the light rare earths (L-REE) and the heavy rare earths (H-REE). REE have been used widely in metallurgy, chemical engineering, electronics and electrooptics, medicine, biomedicine, for manufacturing of magnetostriction materials and lasers [45][46][47]. Its applications in advanced technologies are increasing [48]. In modern societies, the rare earth elements (REE) are considered because of their unique physical and chemical properties. REE will be of substantial attention for the foreseeable future, with demand likely to grow.
Therefore, due to the increasing use of REE in industries, determination of REE has been of a recent increasing concern. Several analytical techniques were used to determine REE in samples such as inductively coupled plasma-mass spectrometry (ICP-MS) [49], neutron activation (INAA) [50]. Energy dispersive X-ray fluorescence (EDXRF) [51] and inductively coupled plasma optical emission spectroscopy (ICP-OES) [52]. Research should continue to play an important role in the search for rare earth ore deposits and their extraction, ensuring that as little damage is done to the environment as conceivable.
Various techniques have been used for removal of REE ions from aqueous systems such as solid-phase extraction, solvent extraction, ion exchange, ion-selective electrodes [53][54][55][56][57][58][59] and adsorption [60][61][62]. Adsorption method is the best technique because of low cost, simplicity of design and operation. The ion-adsorption type rare earths ore, is mainly located in China and REE in these deposits were released by weathering of REE-rich granites and subsequently adsorbed by NCMs.
Recently, there has been a significant interest in adsorption of rare earth elements with NCMs [1, [63][64][65]. Piasecki and Sverjensky [66] also had studied REE speciation/distribution on wide ranges of pH and ionic strength. They concluded that most of the surface-adsorbed lanthanides occur as simple "clay-REE" or as hydrolyzed "clay-O-REE 2+ " species.
NCMs are electronegative, saturated with cations such as Na + , K + , Ca 2+ , and Mg 2+ are capable of exchanging cations such as REE to the surface. Previous researches have shown that REE contained in NCMs are mainly present as physisorbed ions, which can be easily recovered by a simple ion-exchange procedure [66,67]. It is evident that the adsorption of REE ions on NCMs would have great influence on the mineralization process and the leaching process of the ionadsorption type rare earths ore.

Extraction of REE from ion adsorption clays
The ion-adsorption type rare earths ore are generally formed by weathering of REE rich host rocks (granitic or igneous) and transfer into an aqueous solution which percolates through the weathering body and are adsorbed onto NCMs [68][69][70][71]. This provides evidence that NCMs have the ability of adsorbing lanthanide ions released/solubilized during weathering [72]. However, although, the NCMs deposits containing adsorbed lanthanides which are of substantially lower grade than other types of REE mineral resources, the economic benefits look remarkable, because NCMs are abundant in surface layers in nature, easier ionic exchange of REE, ease of mining and processing [66,73]. It is thus evident that the adsorption of rare earth ions on NCMs would have great influence on the mineralization process and the leaching process of the ion-adsorption type rare earths ore.
During the past 45 years, the leaching technology in REE extraction were investigated [64,74,75]. The extraction of REE has been a long tradition in China using the concept of ion-exchange leaching. In the ion adsorption clays 60-90% of the REE are adsorbed onto NCMs [71] and the adsorbed REE on NCMs could be easily recovered by leaching with monovalent salt solutions.
Recently, the ion-adsorption rare earth ores have the focus of most research endeavors as an alternative source of REE, the results of leaching efficiencies has been reported [66,71,73,75,76]. Based on these findings, (NH 4 ) 2 SO 4 was identified as the best a lixiviant of REE from NCMs.

Investigations of adsorption/extraction behavior for REE onto natural clay minerals
Four natural clay minerals namely kaolinite, montmorillonite, muscovite and illite were systematically investigated and compared for their adsorption/extraction behavior for REE [77]. The study reported that the montmorillonite exhibits highest adsorption and regeneration efficiencies for REE while kaolinite has highest extractions efficiencies for both REE light and heavy in the order of kaolinite > illite > montmorillonite > muscovite. Also study found that the lack extractions of REE from muscovite than other NCMs are believed to presence of iron oxide and biotite mineral (produce iron oxide as a result of its alteration) associated with muscovite. The leaching process of the REE is a kind of the reversibility of the ion-exchange process, it was evident that the cation exchange and negatively charged surfaces are the mechanism for REE adsorption. It was concluded that NH 4 + is lacking as a lixiviant from NCMs since NCMs are associated with iron oxide, particularly, either with NCMs containing iron (i.e., biotite) or minerals which always associate with biotite such as muscovite. The important role of the pH in extraction of REE from NCMs was evidenced, when REE-NCMs come into contact with the NH 4 + solution, the pH is rapidly increased from the initial pH solution for both montmorillonite and muscovite, leading to the decrease of the availability of ion-exchangeable REE with NH 4 + ions. That is one of the factors that influence the reduction in the REE extraction from montmorillonite and muscovite when compared with those of kaolinite and illite. The results illustrated that the structure and surface properties of NCMs are also the key factors that affect the rare earth leaching, consequently identifying the types of NCMs and associated impurities in clay materials is important for getting the best leaching system [77].

Conclusion
NCMs have gained a significant interest among the scientific community, because of their abundance, low cost and their unique properties. In this regard, a systematic comparison study under identical experimental conditions could help to better understand the influence of structure and properties of NCMs on their adsorptive/extraction behaviors towards contaminants and elements in leaching process. Adsorption is a very promising and efficient technology for the removal of hazardous contaminants from water source, thus NCMs have been successfully used as an adsorbent materials and alternative source of minerals. Batch adsorption experiments have demonstrated that the contact time, initial pollutants concentration, adsorbent dosage and solution pH have significant effects on contaminants adsorption/ desorption. Among all the NCMs studied, montmorillonite and vermiculite exhibit the highest adsorption efficiencies towards NH 4 + and REE. The presence of iron oxide with NCMs found to help enhance REE adsorption (REE-Fe-oxides), meanwhile it also influences the extraction of REE because REE-Fe-oxides cannot be easily recovered by monovalent salt solutions using the concept of ion exchange i.e., NH 4 + . The structure and surface properties of NCMs are the key factors that affect the adsorption capacities for contaminants and extraction of minerals (i.e., REE). The review suggests that NCMs can be considered as ideal adsorbents and alternative source of minerals owing to their low cost, abundant, high safety, and good adsorption efficiencies. Identifying the types of NCMs and associated impurities in clay materials is important either for getting the best adsorption or the best leaching system. Thus, when developing applications for NCMs, it is essentially to take these factors into consideration. However, further study is necessary to establish the process parameters to generate better quality of products. Also, modification and thermal treatment of natural clay minerals could provide potential future applications in water treatment.