Clay Materials for Environmental Remediation

Clays and clay minerals are very important industrial minerals; they have been in use as raw materials for hundreds of industrial applications due to its abundant availability and inexpensive. Clay minerals are usually classified according to its structure and layer type. The classification of Grim becomes the basis for outlining the nomenclature and the differences between the various clay minerals. A simple classification of clay minerals also available in the literatures, in this classification clay mineral divided into four main groups: kaolinite group, illite group, smectite group, and vermiculite. Clay minerals play important role in the environment protection. These minerals have been used in the disposal and storage of hazardous chemicals as well as for remediation of polluted water. The use of clay minerals as the adsorbents for the adsorption of various hazardous substances (heavy metals, dyes, antibiotics, biocide compounds, and other organic chemicals) has been widely studied by a large number of researchers. Some important information about current studies is given and discussed in this chapter.

The chemical nature and pore structure of clay materials generally influence their adsorption capability. In order to increase its adsorption capacity, modification of the pore structure of clay materials has been conducted chemically as well as physically. Inorganic acids, bases, salts, and surfactants have been used for modification of clay minerals. A combination of chemical and physical treatment process has also been employed to modify the surface and structure of clay minerals. The modification of clay minerals using acid can alter the crystalline structure of the clay minerals. The clay minerals modified with quaternary ammonium surfactant have a high affinity towards most of hydrophobic organic compounds. Even though the anionic surfactants are cheaper than other types of surfactants, the anionic surfactants are seldom used for clay modification compared to the cationic and nonionic surfactants. Nonionic surfactant organoclays were more chemically stable than cationic surfactant organoclays. This chapter also describes the use of Gemini surfactants as well as natural surfactant for modification of various clay minerals.

The most important information to understand the adsorption process is the adsorption equilibria. The adsorption equilibria relate the amount of a solute in the surface of adsorbent at equilibrium condition and the equilibrium concentration of the solute in the liquid phase. The adsorption equilibria of a system are described by adsorption isotherm. A number of adsorption equations which originally developed for the gas phase adsorption can be used to represent the liquid phase adsorption equilibria. The theoretical approaches of several adsorption equations are discussed in this chapter. The most widely used isotherm equations for the correlation of adsorption experimental data of hazardous substance—clay mineral systems are the Langmuir and Freundlich equations. Other two and three parameters isotherms such as Dubinin-Radushkevich, Temkin, Toth, Sips, BET, and Redlich-Patterson also have been used to correlate the liquid phase adsorption of various solutes onto clay minerals. The temperature dependent forms of several models and its applicability to describe the adsorption experimental data also given in this chapter. The extension of single component models to multicomponent adsorption system is also discussed. The thermodynamic aspects of the adsorption process also given in this chapter.

The characterization of the solid provides the most valuable information to understand the behavior of the process. Various modern analytical instruments are available for the characterization of clay minerals. Chapter 6 describes several methods which widely used for the characterization of clay minerals for environmental purpose. The methods include nitrogen sorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The analyzing of nitrogen sorption isotherms is used to characterize porous materials, allowing for the determination of specific surface area, pore size distribution as well as the pore volume. The lack of BET method based on nitrogen sorption isotherms is presented in this chapter. The X-ray diffraction provides fingerprint characterization of crystalline materials and the determination of their structure. The scanning electron microscope (SEM) analysis provides surface topography of the clay minerals. The functional groups of the modified clay minerals can be determined by FTIR method.

For the design of the adsorption system, reliable adsorption kinetic data are required. Different concepts are employed to develop a reliable adsorption kinetic model. Most of the available models were developed according to mass transfer principles or the concept of chemical reaction occurring on the surface of a solid. The models based on the surface-reaction kinetic step as controlling the sorption rate have also been proposed, these models include pseudo-first order kinetic (also called as Lagergren model), pseudo-second order kinetic, and Elovich equation. This chapter discusses various aspects of pseudo-first order kinetic and pseudo-second order kinetic in representing the adsorption kinetic data of some hazardous compounds onto clay minerals. The pseudo-first order kinetic is the earliest known equation to describe the rate of sorption in the liquid phase system. The behavior of time constant parameters k ₁ and k ₂ of pseudo-first order kinetics and pseudo-second order kinetics as a function of initial concentration as well as the temperature is discussed in this chapter. One of the kinetic models which also widely used to represent the sorption kinetic data is an intraparticle diffusion model also briefly discussed in this chapter.