**2.2. Type 2:1**

mechanical stability, larger specific surface area, higher charge density, layered structure,

By definition clays are naturally occurring alumino-silicate materials composed mainly of fine grained materials with colloid fraction of soils, rocks, sediments and water [3–5]. Clay minerals are composed of groups of small crystalline particles of one or more members of a group of minerals. These minerals originate from weathering of silicate minerals [6, 7]. Common minerals that constitute clay minerals are kaolinite, illite, mica, vermiculite and montmorillonite or smectite. This chapter presents a review on mineralogical and chemical properties of clay minerals, their surface modification and their application in arsenic and fluoride removal potential from water.

**2. Structure, mineralogical and physicochemical composition of clay**

groups of clay mineral namely 1:1 (kaoline) and 2:1 (smectite and illite) [12, 13].

dickite and hallocite [14]. They consist of single tetrahedral sheet of SiO4

This group of mineral is also called kaolin minerals, which is the basic mineral for kaolinite,

sheet with Al3+ as octahedral cation. Both sheets combine to form a common a unit in such that the tip of silica tetrahedral points toward the octahedral sheet [3]. The layer of the tetrahedral sheet is invented over the octahedral sheet with oxygen atoms and hydroxyls ions present to balance the charges being shared by the silica in the tetrahedral sheet and the aluminum in

and an octahedral

The structure of the clay consists of phyllosilicate sheets that are arranged properly to form structural layers. Individual layer is made by a stack of tetrahedral and octahedral sheets that shape the frame of all the clay mineral [8, 9]. The tetrahedral (T) sheet consist of cations coordinated to four oxygen atoms and linked to adjacent tetrahedral by sharing three corners to form two dimensional hexagonal mesh [10]. The most common tetrahedral cations are Si4+, Al3+ and Fe3+. The second sheet is called octahedral sheet (O), which is comprised of six oxygen atoms which are closely parked together and hydroxyl ions in which cations are arranged to form octahedral coordination and linked to neighboring octahedral by sharing edge. The edge of shared octahedral forms sheet of hexagonal or pseudo hexagonal symmetry and shows different topologies depending on octahedral hydroxyl position [8]. Cations in octahedral sheet are usually Al3+, Fe3+, Mg2+ or Fe2+. When cation with positive valence of three (Al3+ or Fe3+) is present in the octahedral sheet, only two-thirds of the possible positions are filled in order to balance the charges and the mineral is therefore termed dioctahedral. Conversely, when cation with positive charge of two (e.g. Mg2+ and Fe2+) is present, all three positions are filled to balance the structure and the mineral is termed trioctahedral. The phyllosilicate sheets are joined together by sharing the apical oxygen atom or hydroxyls to form hexagonal network with each sheet in a fundamental structure. **Figure 1** depicts structures of octahedron sheets and tetrahedron sheets proposed by Grim [11]. Based on the number and ratio of the sheets in the fundamental structural units, the existing cations substitutions in the octahedrons and tetrahedrons caused for resulting charge of the layers which can be descended into two main

higher cation exchange capacity [2].

46 Current Topics in the Utilization of Clay in Industrial and Medical Applications

**2.1. Type 1:1**

Clay mineral groups such as smectite and vermiculite are part of 2:1 type and it constitutes minerals such as montmorillonite, saponite, nontronite and beidellite. The 2:1 minerals are composed of one octahedral sheet between two tetrahedral sheets. For which the interlayer thickness is 1 nm when the sheet is closed [16]. Generally in this group of clay minerals isomorphic substitutions are observed, for example, possible substitutions of Si4+ in tetrahedron by Al3+ or those of Al3+ in octahedron by Fe2+ [17]. Such substitution leads to permanent negative surface charge at the sheet by the presence of exchangeable cations [18]. The most common exchangeable cations in the interlayers are K+ , Na+ , Ca2+, Mg2+ and H<sup>+</sup> . The 2:1 group minerals have higher charge density, higher surface area and higher swelling capacity [3]. The swelling capacity of these type of clay arise from their structural features that enables water to overcome the electrostatic and van der Waals interactions keeping the layers together and penetrate into surface interlayers leading to hydrolization of Al and Si atoms to aluminol (AlOH) and silanol (SiOH) resulting in expansion. **Figure 3** presents the schematic diagram of 2:1 clay minerals.

Different clay minerals portray different cation capacity depending on the substitution within the lattice structure [11]. Type 1:1 clay minerals such as kaolinite have limited substitution between their lattice structure compared to type 2:1 clay minerals such as smectite, vermiculite and sepiolite and consequently they have lower cation exchange capacity [10]. Several studies have indicated that cation exchange capacity of the clay mineral decreases after modification by inorganic species [2, 20, 21]. This is because modification involves the ion exchange reaction between the exchangeable cations in the clay interlayer and the guest species. Low CEC of modified clays suggests the irreversibility of cationic exchange and thus intercalated metallic polycations are hardly exchanged [21]. **Table 1** summarizes the cation exchange

Mineralogical and Chemical Characteristics of Raw and Modified Clays and Their Application…

One of the essential properties of clay minerals is their larger surface area. This characteristic allows clay minerals to adsorb water and other environmental contaminants [26]. Type 2:1 clay minerals such as smectite and vermiculite possess higher specific surface area as compared type 1:1 clay minerals such as kaolinite and halloysite because of their ability to swell [10]. The total specific surface area of the clay is denoted by the sum of external surface area

Several authors have indicated that total specific area of the clay mineral can be increased through modification to increase their functionality in different areas of application. Hua [27] reported an

Bentonite modified with the combination of Mn oxides and poly(diallyldimethylammonium

**Clay mineral CEC (meq/100 g) Ref** Unmodified smectite 118.5 [2] Fe-exchanged smectite 115.75 [2] Ti-Pillared smectite 105.75 [2] Raw bentonite 265.5 [20] Fe3+ modified bentonite 188.9 [20] Na-montmorillonite 78 [21] Fe(OH)-montmorillonite 12 [21] Al(OH)-montmorillonite 14 [21] Al3+ modified bentonite 183.3 [22] Mukondeni smectite rich clay soils 79.93 [23] Mixed mukondeni clay soils 137.7 [24] Montmorillonite 91.61 [25]

/g after modification with Mn oxides.

http://dx.doi.org/10.5772/intechopen.74474

49

/g. The increase in total specific

and the internal surface area corresponding to the interlayer spaces [8].

increase in surface area of Na-bentonite from 34.1 to 77.2 m2

**Table 1.** Cation exchange capacity of raw and modified clay minerals.

chloride) showed a sharp increase in surface area to 128.9 m2

capacity of different raw and modified clay minerals.

**2.4. Specific surface area**
