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Elevated levels of heavy metal ions and toxic

1.1 INTRODUCTION

The elevated levels of heavy metal ions and toxic organic compounds (Chlorinated and other synthetic organic chemicals) in our environment have been a matter of concern because of their lethal effect both to man and animals. In recent years, several different methods have been adopted for removing these contaminants from water and industrial wastewaters. Various techniques such as precipitation, ion-exchange, conventional coagulation, reverse osmosis and adsorption have been utilised. The selection of method to treat water/wastewater is based on the level of pollutants or contaminants in the wastewater/water, the adsorption capacity of the adsorbent and the efficiency/cost ratio of the adsorbents. Adsorption is one of the most popular and most effective methods for environmental cleanup in terms of removal of heavy metal ions and organic compounds from water, process effluents, wastewater and aqueous solution 1-4. The choice of adsorbent should be such that it's cheap, readily available, non toxic, easily recoverable and/or can be regenerated. Activated carbon is the most widely used adsorbent that has recorded great success because of its high adsorption capacity, surface area, micro-porous structure and special reactivity 5. However its use is limited by its high cost, intra-particle resistant in adsorption processes 6, 7 and the difficulty to regenerate it. Consequent upon these drawbacks, researchers has resorted to alternate adsorbents ranging from common sawdust, various agricultural by-products, clay minerals, zeolite and others 8- 12.

Clays compare favourably with activated carbon in terms of efficiency, cost and availability. They are among the most common minerals on the earth's surface that have been investigated because of their importance and versatility. They find applications in different areas such as in ceramics and pottery, oil refining and decoration, medicine and cosmetics, in construction, agriculture, paper coatings, and fillings, cart litters, as adsorbents, catalysts and catalyst supports, ion-exchangers, polymers, in the production of optical fibres, glass and refractory materials, etc. A further application is the remediation of polluted environments, where toxic metals and harmful organic pollutants from both natural and artificial sources become concentrated 13- 14. The choice of application is depended on their specific properties. In the paint industry, clay acts as a carrier, hence the ease to evenly mix the paint base and colour pigment. Potters and the ceramic industry use this property to produce tiles, plates, bowls, cups etc. Also the surface properties of clay minerals allow them to function as catalysts in many organic reactions 15, 16.

In Pharmacy and cosmetics, clays are adsorbents and recipients for drug delivery 17 and formulations because of their layered charge, high specific surface area, high ion exchange capacity and high adsorptive capacity. Ever since the early 1930's when the investigation of clay began, there has been great expansion of clay minerals investigation. With the development of new research tools for investigating clays, the research has continued and a large quantity of literature abound on clay minerals hence significant advances in the knowledge of all phases of clay mineralogy. The continuing and expanding interest in the investigation of clay minerals is due to their importance, some of which are already mentioned above.

Clays as they occur in nature are rocks that may be consolidated or unconsolidated (insert Ref.18). The term clay can be used in a number of ways. It can refer to; a mixture of minerals generally dominated by clay minerals with clay sized particles less than 2µm. It can refer to a mass of minerals (primarily clay minerals) that develops plasticity when wetted. It can also refer to a large group of extremely fine crystals or particles, often colloidal in size and usually platy in shape. For the reason of clays usually contain more than one mineral and the various clay minerals differing in chemical and physical properties, the term clay is perceived differently by different professionals.

The geologist view clay as raw materials for shale's, the soil scientist, as a dynamic system that supports plant life; the ceramist as a body to be processed in preparation for nitrification; the chemists and technologists, as a catalyst, adsorbent, filler, coater, or source of aluminium, lithium compounds.(INSERT REF 19.). According to Grim, 194820, clays are composed primarily of extremely small crystalline particles of one or more members of a small group of minerals which are called “clay minerals”. These clay minerals are the most important constituents of nearly all clays and to a great extent determine their properties.

Velde, 1995 21 used the term “clay” to apply to both materials having a particle size of less than 2 micrometer and to the family of the minerals that has chemical compositions and common crystal structural characteristics. A clay deposit usually contains non clay like minerals as impurities, although these impurities may actually be essential in determining the unique and specially desired properties of clay.

Broadly defined, Clays may be said to be hydrous aluminosilicates consisting of those minerals that makeup the colloid fraction (<2µm) of soils, sediments, rocks and water. Naturally-occurring clays are complexed materials. They may be composed of mixtures of fine grained clay minerals and clay-sized crystals of other minerals such as quartz, feldspar, carbonates and metal oxides22. The metal oxides/hydroxides well known in soils include those of iron, aluminum, and manganese. The main source of these metal oxides/hydroxides is the chemical weathering of various primary minerals in soil which releases these metals through hydrolytic oxidation or from the deposition of translocated materials 23-25. In the pH range of most soils, the released metals are precipitated as oxides, hydroxides or oxyhydrates 26 and may take the form of discrete particles as well as coatings on clay particle surface27, 28. These metal oxides/hydroxides are commonly called sesquioxides, amorphous or X-ray amorphous constituents. It is known that iron and aluminium oxides and hydroxides may coat some soil clays 29, 30. In natural soils, iron oxides/hydroxides (usually Fe3+ forms) are commonly precipitated or adsorbed to the clay surfaces or admixed as a separate phase 31-34.

Clays are distinctive from other soil types by their plasticity and composition of fine particles which are often colloidal in shape.

1.2 MINERALOGY AND STRUTURE OF CLAY

Claysmay be crystalline or amorphous (amorphous fall into the group of the allophane clays). Most natural clays have a phyllosilicate or sheet structure, whose crystalline structure is based on a combination of coordinated polyhedrons (tetrahedrons and octahedrons), arranged along planes 35. In each tetrahedron, a silicon atom is surrounded by four oxygen atoms to form the tetrahedron (this is the basic unit of the clay structure). The chemical composition of each tetrahedron may be expressed as SiO4 (Fig.2.1a). These tetrahedrons of silica are linked together by covalent bonding through sharing of oxygens to form a hexagonal network of the composition Si-O when repeated indefinitely. The top of the four tetrahedrons all point in one direction and the base are in one plane .

In the tetrahedral structural units, the interlinked oxygens are the basal oxygens, and they are arranged such that they leave a hexagonal- shaped hole or cavity (where an interlayer ion stays if the charge on the oxygen surface is high enough to fix it onto the clay structure) in the network of oxygen atoms. While to the opposite end of the linked tetrahedral sheet lie the apical oxygen atoms and they point away from the interlinked tetrahedral bases. These apical oxygens are shared with other series of cations to form another

polyhedron.

It is also possible for aluminium to be substituted for silicon in the tetrahedral layer thereby resulting into the development of excess negative charge. It may be recalled that Al has a +3 charge while Si has +4. Consequent upon this difference in charge, the negative charge (-2) on the shared oxygen between Al and Si tetrahedral is not satisfied, hence the excess negative charges.

The second basic unit responsible for the pattern of construction of the different clay minerals is aluminium octahedron in which six hydroxyl groups and oxygen atoms are arranged such that each forms the vertices of an octahedron held together by an aluminium atom at the centre [Fig.2.3a]. The octahedral formed are linked together laterally by sharing edges in sheet known as alumina or octahedral sheet. In this sheet, the six hydroxyl groups that form the octahedron are jointly shared by three adjacent octahedra. The cell consists of four aluminium atoms and six hydroxyl groups in such a way that the top and bottom of alumina sheet are hydroxyl surfaces. Arrangement in a hexagonal pattern is also possible .

Aluminium, iron, or magnesium atom can be embedded in the octahedral coordination (and more rarely, chromium, lithium, manganese, or other ions may occupy this position). (Fig.2.3b). When it is aluminium that is present in the octahedral coordination, only two-third of the possible positions are filled to balance the structure, and this is gibbsite structure, and has the formula Al2(OH)6. When magnesium is substituted for aluminium, all the possible positions are filled to balance the structure which is brucite structure, Mg3(OH)6.20

The octahedral sheet is in coordination with oxygen from the tetrahedral sheet. When tetrahedral and octahedral sheets are taken together, a layer is formed. In a clay crystallite, individual layers may be joined to each other by interlayer cations, by van der Waals and by electrostatic forces or by hydrogen bonding.

CLASSIFICATION OF CLAY MINERALS

There are considerable differences of opinions regarding the proper basis for a satisfactory classification of clay minerals. Many different classifications have been suggested by different clay researchers/professionals. However, clay may be classified into a system, groups or types, depending on the concern at the point of classification. Basically, clay minerals are distinguished by; their structure which controls their behaviour; their arrangement of tetrahedral and octahedral sheets; and whether the octahedral sheet is dioctaherda or tioctahedra (Table 2.2).

Furthermore, classification can be according to their polytye or on the basis of their chemical composition. Classification may also be by their swelling property and the way they absorb water or solvent. According to structure, three principal types or groups may be identified, and they are the Kaolinite, the Montmorillonite and the Illite. All these three types principally consist of tiny crystals with platy shapes of layers of molecules which are stacked one on another. All these clay minerals are made up of different combinations of the two sheets - tetrahedral and octahedral sheets (1:1 clay mineral (Kaolinite, halloysite). The three sheet layer structure of silica - alumina - silica trios consisting of the montmorillonite and the illite groups.

THE 1:1 MINERAL (Two layer-group clay minerals)

Different combinations of tetrahedral and octahedral sheets fashions different clay minerals. Basically, all clay minerals are made of two sheets, tetrahedral and octahedral sheets. The combination of one silica sheet with one octahedral sheet forms a basic layer of the two layer- group clay mineral. When there is no absorbed water molecules present inside the structure, the mineral is known as Kaolinite. The formula is [Si4](Al4)O10(OH)8. The basic building blocks are layers of silica tetrahedra (with four apical O) and layers of alumina octahedra (with two apical O shared with silica and four apical OH), in a 1:1 relationship. The unit cell (5.14 Å by 8.93 Å by 7.37 Å) repeats in the x and y directions to form extensive sheets in the xy plane and stacks of sheets in the z direction formed by hydrogen bonds (HO--H) between OH of the alumina of one sheet and O of the silica of the next (Ref.36,37). These sheets (tetrahedral and octahedral) are stacked in sequence to build up the platy crystals and are firmly bound together. The bonding between the layers is by hydrogen bonds and van der waals forces. They are immovable and there is no variation in the distance between them (Fig.2.4). Another mineral in this group, is hydrated halloysite. It is similar to Kaolinite but including four additional water molecules per layer. The structure is [Si4](Al4)O10(OH)8.4H2O. The microscopic crystalline particles are elongated not equidimensional in shape as in Kaolinite. Ross and Kerr (1934) 38, identified two types of halloysites. One that is described as light- coloured, porous and friable, and another that is dense, nonporous and porcelain-like(less hydrous). Usually, the tetrahedral cation sites in the 1:1 layer type are all occupied by Si4+ and the octahedral sites by all Al3+ or Mg2+. Almost that no isomorphous substitution occur in the minerals of this group, such that structural charge in this layer type is almost zero thus no compensating cations in the interlayer space unlike in the three layer clay minerals. However, Kaolinite is the mineral that characterizes most kaolin and are widely distributed in nature. It includes the mineral dickite and nacrite and they come from hydrothermal or pneumatolytic alteration. Pure deposits are mined for porcelain, both bathroom fixtures and fine China cups. It is also used for making paper, paint 39, in beauty therapy and pharmaceutical industries40,41.

Three-layer clay minerals

This group of clay minerals is also called 2:1 clay minerals. This is because of its platy crystal built which are constituted by an octahedral layer been sandwiched between two tetrahedrons thereby giving three layers of silica- alumina- silica per layer. The octahedra unit is coordinated through shared or linking oxygen to two layers of tetrahedrally coordinated ions with hydroxhyls found only in the intermediate layer of the octahedrally coordinated structural unit 42. Ionic substitution can occur in either the tetrahedral or octahedral sheets thereby resulting to the complex chemistry of the various 2:1 clay minerals. Cations that are small enough to enter into tetrahedral coordination with oxygen do so leaving net negative charges on the layer. Cations such as Fe3+ and Al3+ can substitute for Si4+ in the tetrahedral sheet. Likewise, in the octahedral sheet, cations like Mg2+, Fe2+, Fe3+, Li+, Ni2+, Cu2+ and other medium-sized cations can substitute for Al3+. Furthermore, larger cations such as K+, Na+, Cs+, Mg2+ and Ca2+(called interlayer cations) can be located between layers. Mention is made here also that F may sometimes substitute for (OH) in some clay minerals. It is sometimes therefore difficult to distinguish between members of this group, because there are a number of them, all with almost similar crystal structures. Members of the three- layer clay minerals spread into different groups that may be differentiated one from another on the basis of the amount of isomorphous substitution, the type of octahedral layer present, and also on the way the layers are bound together (e.g. into soil particles).

However, three main divisions may be identified in this concern, they are Illite, Smectite and Vermiculite. In Illite, there is isomorphous substitution in the tetrahedral and octahedral sheets and this substitution is larger than in the other two minerals thereby resulting to a higher charge deficiency such that the layers are stacked together by frontal sharing of K+ ions. The K+ bond is very strong that the minerals are stable under normal conditions and do not swell chemically (Fig.2.5). The unhydrated, firmly fixed interlayer ion is always predominantly potassium if not exclusively. (Insert Ref. 43). If the octahedral sheet is entirely by Al3+ coordination (gibbsite), the mineral is called Muscovite and if Mg2+ (brucite) are present, the mineral is Phlogopite.

Vermiculites not only have a smaller isomorphous substitution, but also present a different bonding between layers. There are both di and trioctahedra vermiculites, though of unknown composition. (It is not very easy to ascertain their exact definition). In between the layers is covered by water molecules associated with exchangeable cations that shield the electrical charge of the layers. Most common cations found at the interlayer space are Mg2+ and Ca2+ (Fig.2.6). Substitutions of Al3+ for Si4+ result in charge imbalance(net negative charge) and these may be partially balanced by other substitutions within the unit structure (octahedral substitution). However there is always a residual net-charge per unit cell. Heating vermiculite releases the water from the layers but with heating temperature as high as 500oC and on exposure to moisture at room temperature, the mineral quickly rehydrates. If the mineral is heated up to 700oC there will be no more expansion and rehydration becomes difficult or impossible.(insert ref.44)

The other group of 2:1 minerals is Smectite group, the most common mineral being Montmorillonite with both dioctahedral and trioctahedral sheets. The structure is similar to talc (Mg3Si4O10 (OH)2 . The Montmorillonite group is very sticky, expands when it comes on contact with water and shrinks when dried. It has good plasticity, has high capacity to absorb and hold water and other substances within their layers. This is due to their internal crystal structure. The tetrahedral- octahedral-tetrahedral sheets are only weakly bonded to each other because they are joined by weak van der Waals bond and by outer complexes of cations, surrounded by water molecules(films of bound water of several molecules thick), therefore can easily be separated by water (fig.2.7). It may be because of this exchangeability of cations and water molecules in the interlayer strata that these minerals show great swelling potentials and some of the “clayey” behaviour they show. The cations replacing aluminium in the dioctahedral sheet is mainly magnesium, iron, and zinc. The net negative charge of Montmorillonite is satisfied by cations, which crowd around negatively charged mineral.(Ref)??

Other 2:1 type of clay minerals (layer silicates) are the talc and pyrophyllite group which contain trioctahedra and dioctahedra members respectively. Talc is Mg3Si4O10(OH)2 and pyrophyllite is Al2Si4O10(OH)2 There are no tetrahedral or octahedral substitutions (isomorphous substitutions), no layer charge and no layer materials. All the same, there is often some small amount of substitution in the natural minerals, but this only gives small amount of ionic attraction between layers that supplement van der Waals bonding (which is the main force that holds these layers silicates together).These weak force may be the reason that these materials are soft, has good cleavage, slipperiness and exhibit varying degree of stacking disorder 46.

2:1:1 layer clay minerals

The minerals of this kind of clay results from the additional hydroxide interlayer added to the structure of 2:1 layer or mineral. In 2:1:1 layer silicates with interlayer brucite, each 2:1 part of the structure are separated by a brucite layer. The most important mineral in this group is chlorites. Many substitutions are possible.

Talc + brucite chlorite Mg6Si4O20(OH)8

Al may substitute for Si between Si7Al and Si4Al4. Al may also substitute for Mg between Mg11Al and Mg8Al4. Replacement of Fe2+ for Mg2+ and Fe3+ for Fe2+ may also occur.

The most common 2:1:1 clay mineral is vermiculite, which is a Mg containing or octahedral clay minerals. The primary substitution is of Al for Si in the tetrahedral layer of the talc structural unit (trioctahedra with a Mg or brucite layer). The charge imbalance is compensated by the presence of interlayer cations, mainly magnesium. The Mg occurs in a double sheet of H2O, although not all water sites are occupied. The water molecules form a distorted hexagonal pattern. Each of oxygen is linked to oxygen of the tetrahedral layer by a hydrogen bond. The resulting structure resembles that of chlorites, except that the brucite sheet is only partially filled. This results in a [H2O - Mg -H2O] double sheet. Two third of the available water molecule sites are filled and one third of the cation sites. Swelling in inorganic liquids (e.g. glycerol) occurs with minerals of this group. Expansion with water may also occur 47.

Classification of clay minerals and related phyllosilicates may also be according to whether the octahedral sheet is dioctahedral or trioctahedral. In dioctahedral clays, two out of the three cation positions of the octahedral sheet are occupied. Every third positions vacant, this type is known as gibbsite sheet. There are two types of occupancy of the octahedral sites, one that is with three divalent ions (3R2+) or one having two trivalent ions (2R3+). In trioctahedra there are three ion sites of occupancy.

Classification of clay minerals summarised

Classification of clay minerals based on the distributions of the shape of the clay minerals and the expandable or non expandable character of 2:1 and 1:1 layer:

  1. Amorphous
    • Allophane group
  2. Crystalline
    1. Two-layer type (sheet structures composed of units of one layer of silica tetrahedrons and one layer of alumina octahedrons)
      1. Equidimensional
        • Kaolinite group
        • Kaolinite, nacrite, etc
      2. Elongated
        • Halloysite group
    2. Three-layer types (sheet structures composed of two layers of silica tetrahedrons and one central dioctahedral layer or trioctThahedra layer).
      1. Expanding lattice
        1. Equidimesional
          • Montmorillonite group
          • Montmorillonite, sauconite, etc
          • Vermiculite
        2. Elongate
          • Montmorillonite group
          • Nontronite, saponite, hectorite
      2. Non-expanding lattice
        • Illite group
    3. Regular mixed-layer types(ordered stacking of alternate layers of different types)
      • Chlorite group
    4. Chain structure types (hornblende-like chains of silica tetrahedrons linked together by octahedral groups of oxygen and hydroxyls containing Al and Mg atoms 20.

Table 2.2; Classification of Phyllosilicates with emphasis on clay minerals

Layer Type

Group(X=layer charge)

Subgroup

Species

1:1

Serpentine-kaolinite

(x ~0)

Serpentine (Tr)

Kaolins (Di)

chrysolite, antigorite, lizardite, berthierine

kaolinte, dickite, nacrite, halloysite

Talc-pyrophyllite

(x ~0)

Talc (Tr)

Pyrophyllite(Di)

Talc

Pyrophyllite

Smectite orMontmorillonite

(x ~ 0.2-0.6)

Tr smectite

Di smectite

Saponite, hectorite

Montmorillonite, beidellite, nontronite.

Vermicullite

(x ~ 0.6-0.9)

Tr vermiculites

Di vermicullites

2:1

Illite

(x < 0.9 > 0.6)

Tr illite?

Di illite

Mica

(x ~ 1.0)

Tr micas

Di micas

Biotite, phlogopite

, lepidolite

Muscovite,paragonite

Brittle mica

(x ~ 2.0)

Di brittle micas

Margarite

Chlorite

X varible

Tr, Tr chlorites

Di,

Di chlorites

Di Tri chlorites

Tr Di chlorites

Common names based on Fe2+, Mg2+, Mn2+ ,Ni2+

Donbassite

Sudoite, cookeite (Li)

2:1

Sepiolite-parlygorskite

Inverted ribbons

(with x variables)

2:1:1

Chlorite

X variable

Di chlorite

Fennine, clinochlore, prochlorite

Based on 50, 51, 52. Adopted from 46.

Based on the above classification, clays may be divided into three broad groups. These groups are: anionic clays, cationic clays and layered metal phosphate/phosphonates. Our focus will be mainly on cationic and anionic clays, considering various aspects such as structure, composition, properties, preparation, the modification of these materials( in order to upgrade them technologically) and roles of these parameters in environmental management.

Anionic Clays

This group of clays occur naturally though rare in nature. They are relatively simple and inexpensive to prepare. They have positively charged metal hydroxide layers with balancing anions and water molecules located in the spaces between the layers and are bound to them forming interlayers 53. Anionic clays used industrially are synthesized; they may be described as natural or synthetic lamellar mixed hydroxides having interlayer space containing exchangeable anions. Many names have been used to describe this group of clay type, and the name used depends on the composition and polytypic forms of the minerals. Also widely used is the general terms hydrotalcite-type/hydrotalcite-like compound (HT/HTlc); this term is widely used probably because of the extended research done on them, or layered double hydroxides (LDHs); got from the early work of Feithnecht who called them doppelschichtstrukturen” meaning double sheet structure. These terms are used because of their structural similarities to hydrotalcite, Mg6Al2(OH)16CO3.4H2O. The hydrotalcite structure emanated from the stacking of brucite-like-layers [Mg(OH)2] consisting of positive residual charge resulting from the partial isomorphous substitution of Al3+ cations for Mg2+ cations. This excess positive charge is balanced by interlayer anions which maintain the overall charge neutrality 54, 55. The study of this clay type has gained much attention recently. LDHs have the ability to act as host materials just like various other layered materials (e.g clay minerals, graphite, transition metal phosphates/phosphonates) through the incorporation of guest species into them to generate novel solids with physical and chemical properties that are desirable. These materials were first made artificially in the laboratory in 1942 by Feithnecht, hundred years after Hochstetter reported them in 1842. In the last two decades, research effort on the preparation, characterization and properties has expanded leading to a wide range of potential technological applications, which has been made possible because of their unique intercalation properties 56, 57.

Structure of LDHs

The structure of LDH can be clearly described as a brucite layer consisting of Mg2+ ions that are coordinated octahedrally by hydroxyl groups with the octahedral units sharing edges to form charged neutral sheets 58, 59, (Fig2.9). These sheets are stacked on top of each other, held together by hydrogen bonding 59, 60.

Isomorphous replacement of some of the Mg2+ with trivalent cations like Al3+ bring about the positive charges on the layers making necessary the presence of interlayer charge balancing anions. Molecules of water of crystallization occupy the remaining space66, 67, 68 . Fig.2.9

The general formula to describe the general chemical composition of LDHS can be shown as

[MII 1-x MIII x (OH)2 ]b+ [ An b/n ]. mH2O

Where MII = divalent metal cation (CA2+, Mg2+, Zn2+, Co2+, Ni2+, Cu2+, Mn2+, Fe2+), and also the monovalent cation Li+

MIII = trivalent metal cation (Al3+, Cr3+, Fe3+, Co3+, Mn3+, Ga3+)

A = Interlayer anions of the valence n ( Cl-, NO3-, ClO4-, Br-, F-, OH-, CO32-, SO42-, organic anions etc

b = x or 2x-1 for z = 2 or 1 respectively

x = the metal ratio M3+/M2+ + M3+

m = moles of co-intercalated solvent, generally water of crystallization

The interlayer anions can be displaced by several inorganic and organic anions and the water molecules by other polar molecules 61, 62. Varying the ratio M3+/M2+ + M3+, the anion exchange capacity can be control thereby also controlling the number and arrangement of charge balancing anions in the LDH. The most common anion that is found in LDHs that occur naturally is the carbonates (CO32-), but there is no restriction found as to the nature of the charge balancing anion, therefore in practice, different charge balancing anions can occupy the interlayer region of the LDHs. Possible charge balancing anions are halides, oxo-anions, silicates, polyoxometalate anions, complex anions, iso and heteropolyanion, metallorganic complexes, inorganic and organic anions 59, 64, 65. The only problem with the preparation of compounds is difficulty of avoiding contamination from CO2 when the anion is not carbonates. This is because carbonate anion is so tenaciously incorporated and is held very firmly so because it is the anion most preferred 53, 60, 63 64, that the direct preparation of pure, non carbonate LDHs or hydrotalcites is not very easy to accomplish. However synthesis of a number of compounds of different stoichiometries are possible as long as there is the right environment and the choice of value of x exist in the range of 0.2- 0.33 to obtain pure LDHs. Deviation from this value of x may give the hydroxides or compounds with different structures. Also cations that are too small, such as Be2+, or too large like Cd2+, may gives other types of compounds. 69, 70

Mg2+ ion has ionic radius of 0.65Å, and the trivalent ions that substitute for Mg2+ ion are those ions whose ionic radii are not too different from that of Mg2+. Table 2.3 shows the ionic radii of some bivalent and trivalent cations. When Table 2.3; Ionic radii of some cations, Å.

M(II)

Be

0.30

Mg

0.65

Cu

0.69

Ni

0.72

Co

0.74

Zn

0.74

Fe

0.76

Mn

0.80

Cd

0.97

Ca

0.98

M(III)

Al

0.50

Ga

0.62

Ni

0.62

Co

0.63

Fe

0.64

Mn

0.66

Cr

0.69

V

0.74

Ti

0.76

In

0.81

There are various techniques for synthesizing LDHs, and choice of method is a function of composition required and properties of the compounds. Some accepted methods of synthesising hydrotalcites are coprecipitation, urea hydrolysis, hydrothermal treatment and synthesis, combustion synthesis, sol-gel method, microwave irradiation, steam activation, solvothermal method. Various techniques can be used to characterize the obtained HT/LDHs, some of which are powder X-ray diffraction(XRD), fourier transform infrared spectroscopy(FTIR), infrared spectroscopy(IR), thermal gravimetric analyses(TGA), differential scanning calorimetry (DSC) or differential thermal analyses(DTA), X-ray fluorescence spectrophotometry (XRF),anion exchange capacity(aec) or cation exchange capacity(CEC) measurements and pore size determination.The main industrial application of these clays are as catalysts and catalyst supports, as flame retardant, molecular sieves ion-exchanger in industry. They also find use in medicine (as antacid, antipeptin and stabilizers); and as adsorbents to scavenge heavy metals, stabilize PVC and to treat wastewater and underground water. 58, 64

Cationic Clays

These are layer silicates based on a two dimensional structure of the basic building blocks consisting of tetrahedral and octahedral, majority of their features has been discussed above.(Figs. 2.7, 2.8, Table 2.2). Some examples are the 1:1 minerals (kaolonite and serpentine) with layer thickness of 7.0Å and the 2:1 type with layer thick 10Å Fig 2.4 and 2.7 respectively. Depending on the number of octahedral sites per unit cell occupied and the cations present in the octahedral sheets, clay mineral may be di or trioctahedra. Table 2.2 shows the phyllosilicates, most industrially interesting 2:1 are in bold. Pyrophyllite (di) and talc(tri) are the electroneutral structures found in nature. The remaining mineral resulted from the isomorphous substitution during their formation. The clay lattice becomes negatively charged when a lower valence cation substitutes for higher valence cation. Charge neutrality is maintained by the exchangeable cations. An appropriate choice of exchangeable cations can maintain the hydrophilic- hydrophobic character of the clay mineral surface.71 Modification of this surface is possible but choice of method has to be such that it's inexpensive, less time consuming, not polluting and adhesion between the clay particles and the modifying chemical is strong enough to maintain stability.

Modified clays have found use in applications such as adsorbents of organic pollutants from air, water and soil; rheological control agents, paints, cosmetics, oil well drilling fluids, refractory vanish etc 72, 73, 74.

AIM OF THE RESEARCH

The aim of this research is to develop clay-based materials for environmental management in pollution control and wastewater treatment. Investigating the use of clay based materials as adsorbents for the treatment of industrial effluents both for the removal of metal cations and organic contaminants from wastes. Both natural and synthetic clays will be employed in this application. Our key focus would be to study the different alkylammonium exchange clays to tune the hydrophobic/hydrophilic properties within the interlamellar region of these materials to optimise the adsorption properties for aromatic organic compounds such as toluene and substituted phenols. Comparison with appropriate exchange anionic clay (hydrotalcite-like) materials will be made.

OBJECTIVES OF THE RESEARCH

RESEARCH METHODOLGY

  1. Modification of hydrotalcite-like compound.... with...............................
  2. Physicochemical characterization of both modified and unmodified adsorbents by;
    • Determination of CEC, AEC, SA, Pore volume of adsorbents
    • P.XRD to........
    • FT_IR...........
    • TGA............
    • IR..........
    • XRF.........
  3. Chemical characterization of both modified and unmodified adsorbents by;
    • Adsorption of toluene and substituted phenols
    • Study of the effects of chosen operative variables on adsorption capacities of adsorbents
    • Study kinetics and thermodynamics of adsorption process to ascertain the mechanism of adsorption, the feasibility, industrial applicability of these adsorbents
    • Modelling of data with existing equilibrium and kinetic models
  4. Iv Studying the regeneration of these adsorbents through...........
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