Activated clays are frequently bentonite clays that have been naturally or chemically activated to improve their adsorption properties. Several features, including as abundance, chemical inertness, non-toxicity, and porosity, enable activated clays to be suitable adsorbent materials. Because of their propensity to absorb dangerous chemicals such as dyes, organic compounds, and biocides, activated clays are utilized in environmental protection. Zeolites, on the other hand, are aluminosilicate minerals having an open tetrahedral structure. The open structure of zeolites allows for selective molecular adsorption. Zeolite is used in water treatment and softening. Zeolite materials can be reused. Activated clays are naturally available porous minerals that have been dried to make adsorbent materials (Espantaleon, et al 107). Recently, numerous laboratories and industries worldwide are interested in clay studies. The concern can be justified by clay’s occurrence in nature occupying about forty-two percent of the earth’s surface. Activation of clay is a process used to improve the adsorption characteristics of the clay by chemical or thermal treatment (Mukherjee 2013). When clay is subjected to activation by chemicals, strong acids like sulfuric acid or sodium carbonate are used. The main aim of activating clay is to increase porosity by peripheral dissolution thus increasing the number of active sites. The main demerit of activated clays is its incapability at low temperatures to give up moisture. If the activated clays are subjected to temperatures below fifty degrees Celsius the material starts discharging moisture back into its environment (Espantaleon, et al 107).
In gaseous streams, activated clays are used in the removal of pollutant gasses by adsorption. Industries release toxic gasses into the environment. The abatement of toxic gasses from the environment by thermal oxidation is expensive due to the elevated need for electrical energy (Li, et al 254). Pillared activated clays are used to adsorb gasses because they have an instant hydrophilic-hydrophobic characteristic (Mokaya and Jones 79). Activated clay, therefore can be used to adsorb hydrogen sulfide gas from the environment. The clay acts as an adsorbent and as a binder.
Zeolites are natural minerals mined from specific locations of the world. The minerals are hydrated aluminosilicate composed of interconnected tetrahedral of alumina (AlO4) and silica (SiO4) (Auerbach, Kathleen, and Prabir 2003). Its solid structure is generally a three-dimensional, open crystal structure made of elements such as silicon, oxygen, and aluminum with alkaline earth metals like potassium, magnesium, and sodium.in addition to water trapped between its gaps (Naber, et al 2201). About forty types of Zeolites naturally exist, most of these minerals are formed in both sedimentary and volcanic rocks. The most common types are mordenite, clinoptilolite, and chabazite (Auerbach, Kathleen, and Prabir 2003).
Zeolites are generally stable solids and resistant to numerous environmental conditions. They have high melting point, more than 1000 ℃, thus are resistant to temperatures. Zeolites do not burn, they are resistant to high pressures, and are insoluble in organic solvents and water (Auerbach, Kathleen, and Prabir 2003). Additionally, zeolites do not react with air. Due to their numerous properties zeolites are used as adsorbent materials in gas and vapor streams.
Reasons why Activated Clays are used to Adsorb Organic Substances
The use of activated clay in adsorption of organic substances is important due to many reasons. Activated clays are made of porous pores that allow only specific tiny substances to go through. The pores are very tiny, thus causing unwanted substances to be trapped in between. Activated clay possesses excellent adsorption limit within normal relative humidity and temperature ranges. Furthermore, activated clay in non-toxic and chemically inert making its handling to be safe. It is more economical in relation to other chemically produced adsorbents such as molecular sieves and silica gel. As the activated clay becomes more saturated, its structure is maintained and can effortlessly be reactivated for reuse by heating. Figure 1 represents a sample of activated clay.
Figure 1, Activated clay sample
Uses of Activated Clays in Environmental Protection
Activated clays are useful industrially due to their numerous applications because it's cheap and available in abundance. Activated clay plays a crucial role in environmental protection. For instance, in storage and disposal of hazardous chemicals and polluted water remediation. the utilization of activated clay for adsorbing numerous environmental hazards are as discussed below:
Adsorption of Organic Compounds and Biocides
A biocide is a chemical substance which can hinder metabolism, reduce harm, and get rid of harmful organisms. Since biocides are anticipated to get rid of harmful living organisms, the vast application of their products render significant hazards to human welfare and health and cause extensive consequences on the natural environment (You, Hongting and George 163). The direct discharge and disposal of wastewater with biocides prompt severe and possibly long-term effects on the environment. The use of activated clay has proven to be an effective methodology for eradicating numerous types of hazardous materials from water and gas environment (Snoeyink, 1990). As an adsorbent, activated clay has the capacity to select and retain particular minor elements from liquids or gasses. Physical activation of clays by heating and chemical activation by acids is applied so that the adsorption capacity is increased due to increased number of active sites (Hajjaji and El Arfaoui 419).
Adsorption of dyes
When dyes are released into the environment it causes eco-toxic hazards (Bhattacharyya, Susmita and Gautam 8). The potential hazards due to bio-accumulation of dyes in the environment may harm animals in a food chain. Some of the synthetic dyes are toxic to the environment and humans because of their mutagenic, and carcinogenic properties (Öztürk and Malkoc 108). Due to the dangerous hazards from dyes, activated clay can be used to adsorb these dyes from the environment (Bhattacharyya, Susmita and Gautam 8). The use of activated clay in dyes adsorption is relatively cheaper than other methods. Activated clays depict significant capacities of dye removal (Juang, Wu, and Tseng 526).
The adsorption performance of activated clay hugely depends on the type of dye. Naturally activated clays such as bentonite possess high adsorption ranges for binding cationic (basic) dyes (Espantaleon, et al. 107). Activated bentonite is used to adsorb methyl blue dye from aqueous solutions (Srasra, et al. 415). Factors affecting dye adsorption onto activated clay materials comprise of pH of the solution, the initial concentration of dye, temperature, and the availability of extra impurities (Kurniawan, et al 188). In adsorption systems for liquids, the efficiency of the process is based on the system’s pH, the change in pH causes a difference in adsorbent’s surface charge and the magnitude of ionizing the absorbent molecules (Öztürk and Malkoc 108).
How Zeolite Structure Influences Selective Selection
Zeolites have an open tetrahedral structure (figure 2) that can trap molecules within it (Rožić, Mirela, et al 3678). Zeolites exchange positively charged particles for the metal particles inside them through ion exchange process (Colella, 557)). According to Cronstedt, the zeolite can lose or gain water through reversible hydration (Polat, et al 186). Zeolites structure is composed of numerous openings that allow small molecules to pass through and larger ones are trapped and that is the reason behind it termed as molecular sieves (Csicsery 210). Dissimilar to natural zeolites available randomly in varied sizes, artificial zeolites are produced in uniform and precise sizes (generally from 1µm) to fit specific applications (Bish, and Douglas 2001).
Figure 2: structure of mordenite zeolite (Csicsery 210)
Industrial application of Zeolites
The open-tetrahedral structure of zeolites makes them helpful in a wide range of ways. One of the greatest ordinary uses for zeolites is in water filters and softeners. In ion exchange water treatment, for instance, hard water (rich in calcium and magnesium particles) is channeled through a section loaded with sodium-containing zeolites (Colella, 563). The zeolites trap the calcium and magnesium particles and discharge sodium particles in their place, so the hard water gets to be distinctly softer, however, richer in sodium.
Zeolites are used in water softening or treatment because it acts a mechanical sieve for unwanted molecules (Fraissard, and Ito 354). The most crucial idea with respect to the adsorption of compound molecules by zeolites is molecular sieving. Molecules with a larger diameter are adequately sieved. Zeolites can, therefore, be used to separate particles by size and shape. The affinity of a substance towards the nucleus of zeolites depends on electronic charges. The strong forces of attraction towards the zeolite pores cause high interactions of polar substances like water. Non-polar particles are highly adsorbed because of the polarizing power of electric fields. Therefore, maximum adsorption can be attained by zeolites even in cases where steric interference does not occur (Meier and Olson 89).
Adsorption centered on polarizability, electrostatic fields, and molecular sieving are theoretically reversible and frequently reusable in practice (Georgiev, et al 2009). Due to this, the zeolite can be reused plenty of times, steering between desorption and adsorption. This interprets for the significant economic cost of zeolite in adsorptive uses.
Conclusion
Activated clays are clays whose properties have been improved to increase the adsorptive nature. Commonly activated clays include bentonite. Activated clay can also be reactivated. The presence of pores makes it possible for activated clays to adsorb gaseous and liquids substances. In gaseous processes activated clay adsorbs hydrogen sulfide while in liquid or aqueous systems it adsorbs dyes, organic compounds, and biocides products. The tetrahedral open structure of zeolites is responsible for its selective selection of molecules to be adsorbed (Chen 1996). Natural zeolites act as mechanical sieves with different shapes and sizes while synthetic zeolites are designed with specific size and shapes for particular applications (Bish, and Douglas 2001). There are numerous applications of zeolites in industries (Yilmaz and Ulrich 890). The most vital uses include catalysis, gas separation, and ion exchange such as water softening. Zeolites can be reused and its disposal is easy due to its unreactive nature and resistance to environmental factors like temperature and high pressure (Nordell, 1961).
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