A CRITICAL STUDY OF ORGANOCLAYS AND BENTONITES
CHAPTER ONE
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 absum 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.
