A CRITICAL STUDY OF ORGANOCLAYS AND BENTONITES
1.0 Introduction and literature review
Bentonites are clays rich in smectite whose properties such as crystal structure and size, cation exchange capacity (CEC), hydration and swelling, thixotropy, bonding capacity, impermeability, plasticity and tendency to react with organic compounds make them advantageous for a variety of applications. Smectites are 2:1 type of aluminosilicate having crystal lattice that consists of two dimensional layers where central octahedral sheet of alumina is fused to two external silicate layers. Isomorphic substitution within the layers generates negative charges that are counterbalanced by easily replaceable alkali or alkaline earth cations. These cations are defined as exchangeable cations. Forces holding the stacks together are relatively weak and the intercalation of small molecules between the layers is easy. Smectite can be rendered organophilic by exchanging the exchangeable cations with alkylammonium ions. Quaternary ammonium cations of the general form [(CH3)3NR]+ or [(CH3)2NRR0]+,( where R and R0 are hydrocarbon groups), are usually used in the synthesis of organoclays. Depending on the dimensions of R and R0, organoclays display distinct adsorptive properties and abilities. Clay minerals such as smectite and montmorillonite are abundant in nature and known as swelling clays due to their tendency to swell and hydrate in the presence of water. They are used in a wide range of applications including nanocomposites, catalysts, photochemical reaction reagents and adsorbents. One of the foremost industrial applications is as adsorbents for water purification. In this respect, montmorillonite is the most commonly used clay due to its high cation exchange capacity (CEC), swelling properties, high surface areas, and consequential strong adsorption and abs\um or magnesium ion is octahedrally coordinated to six oxygens or hydroxyls. The isomorphous substitution within the layers (e.g. the replacement of Mg2+ or Zn2+ for Al3+ in the octahedral layer, and Al3+ for Si4+ in the tetrahedral sheets) results in a negatively charged surface. The resultant negatively charged clay surface is counterbalanced by exchangeable cations such as Na+ or Ca2+ in the interlayer space. The hydration of the inorganic cations on the exchangeable sites causes clay mineral surfaces to be hydrophilic in nature and these hydrophilic clays are found to be ineffective adsorbents for the removal of organic compounds. Such ineffectiveness have been overcome through ion exchange of inorganic cations with organic cations such as quaternary ammonium cations (QACs), represented as [(CH3)3NR]+, or [(CH3)2NR2]+, where R is a relatively short hydrocarbon group.
The properties of clay minerals are altered upon the formation of organoclays such as those obtained by the intercalation of cationic surfactant molecules into the interlayer space of MMT through an ion exchange process. This changes the surface properties of organoclays to highly hydrophobic and lipophilic and results in an increase in the interlayer or basal spacing, by promoting the generation of new sorption sites in the interlayer of the clay. It is reported that the adsorption capacity of organoclays is improved over and above untreated clays for the removal of various organic contaminants. Besides, organoclays are more cost effective compared with other adsorbents, such as activated carbon and have been shown to be potentially effective for the uptake of water contaminants in aqueous solution.
In recent years organoclays have attracted great interest because of their academic and industrial importance. Organoclay based nanocomposites exhibit remarkable improvements in properties when compared with virgin polymer or conventional micro and macro composites. These improvements include increased strength and heat resistance, decreased gas permeability and flammability and increased biodegradation of biodegradable polymer. Another important application for organoclays is in adsorption such as in pollution prevention and environmental remediation such as treatment of chemical spills, wastewater treatment and harzadous waste landfills and others. The aforementioned applications strongly depend on the structure and properties of the organoclays.
The study of organoclays is a vital subject in current research since various organoclays are widely used as nanocomposite precursors, adsorbents for organic pollutants, rheological control agents and electric materials. The combination of the hydrophobic nature of the surfactant and the layered structure of the silicate layers leads to unique physicochemical properties. In these applications, the behavior and properties of the organoclays strongly depend on the structure and the molecular environment of the organic molecules within the galleries. Organoclays are synthesized by grafting cationic surfactants such as quaternary ammonium compounds into the interlayer.
Organoclays have become an important part of the treatment train to remove creosote and PNAH (poly nuclear aromatic hydro-carbon) from contaminated groundwater at old wood treating facilities and MGP (manufactured gas plant) sites. Organoclays consist of bentonite that is modified with quaternary amines. Bentonite is a volcanic rock whose main constituent is the clay mineral montmorillonite. This gives the bentonite an ion exchange capacity of 70-90 meq/gram. By exchanging the nitrogen end of a quaternary amine onto the surface of the clay platelets, by cation exchange (Exchanging the sodium or calcium ion on the surface for the nitrogen which is positively charged), the bentonite now becomes organically modified and thus organophilic, which also means hydrophobic (Lagaly, 1984). The clay is arranged in a layered structure, platelets stacked on top of each other. When these platelets are placed into water, the amine chains are activated and stand up like dry hair causing pillaring of the platelets, and allowing the end of the amine chains to stand or dangle into the water, reacting with organics that pass by (Mortland et al, 1986). The chains will then dissolve or partition into large organic compounds such as sparingly soluble chlorinated hydrocarbons. Oil is the most prominent of these (Smith et al, 1990). These same compounds, on the other hand, will blind the pores of activated carbon.
The layered expandable clay minerals (e.g., smectite, hydrotalcite) always possess charges on their layer sheet, and these charges will be compensated by counter inorganic ions. Because of the strong hydration of these inorganic ions, the interlayer spaces of the clay minerals are hydrophilic in nature. As a result, the natural clay minerals show rather weak affinity to most of the hydrophobic organic compounds (HOCs), and they are seldom used as sorbents for HOCs (Yariv and Cross, 2001).
Under suitable conditions, the inorganic ions on clay minerals can be replaced by organic ions, and then the interlayer spaces become hydrophobic (Lee et al., 1999; Wang et al., 2004; Volzone et al., 2006; Frost et al., 2008). As a result, the sorption capacity of the modified clay minerals (i.e., organoclays) towards HOC’s can be significantly improved, and the organoclays have found applications in a wide range of organic pollution control fields (Volzone et al., 2006; Frost et al., 2008; Laha et al., 2009). Organoclays can be efficient sorbent for removal of organic pollutants from water (Zhou et al., 2007a,b; Huang et al., 2007; Zhu and Zhu, 2007; Lin and Juang, 2009) and air (Zhu and Su, 2002; Tian et al., 2004; Volzone et al., 2006; Park et al., 2008), and they are also used as landfill liner and sorptive barrier to prevent down gradient pollution of groundwater and aquifer from organic pollutants (Lo, 2001; Lo and Yang, 2001; Brixie and Boyd, 1994). Sorptive mechanisms of organoclays towards HOC’s have been extensively studied in the past decades, and various methods for improving their sorption capacities are proposed accordingly (Sheng et al., 1996; Shen, 2004; Zhu et al., 2007, 2008a,b). For the organoclays modified with organic cations containing long alkyl chains, it is believed that HOCs molecules are incorporated into the alkyl chain formed organic phases (Sheng et al., 1996; Shen, 2004; Zhu et al., 2007). Accordingly, it is suggested that regulating the arrangement model of the alkyl chains of the ions can optimize the sorption capacity of the organoclays (Zhu et al., 2007, 2008a). If the organoclays are synthesized with small organic cations, the HOCs molecules are believed to be primarily adsorbed on the hydrophobic siloxane surface of the organoclays (Shen, 2004; Bartelt-Hunt et al., 2003). Increasing the exposed siloxane surface can improve the sorption capacity of this class of organoclays (Shen, 2004; Zhu et al., 2008b). To reduce the pollution control costs, researchers have also developed some novel processes for the application of organoclays in pollution control (Shen, 2002; Ma and Zhu, 2007; Scurtu et al., 2008). Among these processes, the one-step treatment process is of particular interesting (Shen, 2002; Ma and Zhu, 2007). In this process, the synthesis of organoclays and the application of organoclays in pollution control are combined in one step. As thus, the waste water treatment processes are simplified and the pollution control costs can be greatly reduced (Shen, 2002;Ma and Zhu, 2007).
1.1. What is clay?
Clay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried or fired. Clay deposits are mostly composed of clay minerals (phylloosilicate minerals) and variable amounts of water trapped in the mineral structure by polar attraction. Organic materials which do not impart plasticity may also be a part of clay deposits.
Clays are distinguished from other fine-grained soils by various differences in composition. Silts, which are fine grained soils which do not include clay minerals tend to have large particle sizes than clays but there is some overlap in both particle size and other physical properties, and there are many naturally occurring deposits which include both silts and clays. The distinction between silts and clay varies by discipline. Geologists and soil scientists usually consider the separation to occur at a particle size of 2µm (clays being finer than silts), sedimentologists often use 4-5µm, and colloid chemists use 1um. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, ISO 14688 grades; clay particles as being smaller than 0.063mm and silts one larger.
There are three or four main groups of clays; kaolinite, montmorillonite-smecite, illite and chlorite. Chlorites are not always considered a clay, sometimes being classified as a separate group within the phyllosilicates. There are approximately thirty different types of “pure” clays in these categories but most “natural” clays are mixtures of these different types along with other weathered minerals( Lagaly,1984.).
1.1.1 Clay minerals
Clay minerals likely are the most utilized minerals not just as the soils that grow plants for foods and garment, but a great range of applications, including oil absorbants, iron casting, animal feeds, pottery, china, pharmaceuticals, drilling fluids, waste water treatment, food preparation, paint e.t.c.
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays form flat hexagonal sheets similar to the micas. Clay minerals are common weathering products (including weathering of feldspar) and low temperature hydrothermal alteration products. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone, and siltstone and in fine grained metamorphic slate and phyllite. Clay minerals are usually (but not necessarily) ultrafine-grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so may require special analytical techniques for their identification/study. These include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, and SEM-EDS or automated mineralogy solutions. These methods can be augmented by polarized light microscopy, a traditional technique establishing fundamental occurrences or petrologic relationships.
Clay minerals can be classified as 1:1 or 2:1, this originates from the fact that they are fundamentally built of tetrahedral silicate sheets and octahedral hydroxide sheets, as described in the structure section below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentine. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are talc, vermiculite and montmorillonite.
Clay minerals include the following groups:
- Kaolin group which includes the minerals kaolinite, dickite, halloysite, and nacrite (polymorphs of Al2Si2O5(OH)4). Some sources include the kaolinite-serpentine group due to structural similarities (Bailey 1980).
- Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite and trioctahedral smectites for example saponite. In 2013, analytical tests by the Curiosity rover found results consistent with the presence of smectite clay minerals on the planet Mars.
- Illite group which includes the clay-micas. Illite is the only common mineral.
- Chlorite group includes a wide variety of similar minerals with considerable chemical variation.
- Other 2:1 clay types exist such as sepiolite or attapulgite, clays with long water channels internal to their structure.
Mixed layer clay variations exist for most of the above groups. Ordering is described as random or regular ordering, and is further described by the term reichweite, which is German for range or reach.
1.1.2. Structure of clay minerals
Like all phyllosilicates, clay minerals are characterized by two-dimensional sheets of corner sharing SiO4 tetrahedral and/or AlO4 octahedral. The sheet units have the chemical composition (Al,Si)3O4. Each silica tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedral forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedral “point” in the same direction; i.e. all of the unshared vertices are on the same side of the sheet. In clays, the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminum or magnesium, and coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also forms part of one side of the octahedral sheet, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedral. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorized depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer the clay is known as 1:1 clay. The alternative, known as 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet. Bonding between the tetrahedral and octahedral sheets requires that the tetrahedral sheet becomes corrugated or twisted; causing di-trigonal distortion to the hexagonal array, and the octahedral sheet is flattened. This minimizes the overall bond-valence distortions of the crystallite. Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge, or will have a net negative charge. If the layers are charged this charge is balanced by interlayer cations such as Na+ or K+. In each case the interlayer can also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers.
Fig 1. Structure of montmorillonite ( Papke,Keith1970)
1.1.3 Cation exchange capacity of clay
CEC may be defined as the quality of exchangeable cations expressed in milliequivalent per 100g of ignited weight of clay (Newman 1987).
The cation exchange capacity of clay is a very important tool in the preparation of Organoclay. The cation exchange capacity (CEC) of a clay is a measure of the quantity of negatively charged sites on clay surfaces that can retain positively charged ions (cations) such as calcium (Ca2+), magnesium (Mg2+), and potassium (K+), by electrostatic forces. Cations retained electrostatically are easily exchangeable with cations in the clay solution so a clay with a higher CEC has a greater capacity to maintain adequate quantities of Ca2+, Mg2+ and K+ than a clay with a low CEC. A clay with a higher CEC may not necessarily be more fertile because a clay’s CEC can also be occupied by acid cations such as hydrogen (H+) and aluminum (Al3+). However, when combined with other measures of clay fertility, CEC is a good indicator of clay quality and productivity. Clay CEC is normally expressed in one of two numerically equivalent sets of units: meq/100 g (milliequivalents of charge per 100 g of dry clay) or cmolc/kg (centimoles of charge per kilogram of dry clay). Because of the differing methods to estimate CEC, it is important to know the intended use of the data. For clay classification purposes, a clay’s CEC is often measured at a standard pH value. Examples are the ammonium acetate method of Schollenberger and Dreibelbis (1930) which is buffered at pH 7, and the barium chloride-triethanolamine method of Mehlich (1938) which is buffered at pH 8.2 (Rhoades,1982.) .