Abstract
Adsorption differs from absorption and activated clays are one of the most effective adsorbents in nature. The adsorbing action of activated clay is a selective adsorption process. Due to its effectiveness, activated clay has wide applications such as cleaning up oil spills and adsorbing dyes. Zeolite made up of silicon, aluminum and oxygen is also an adsorber, and it can be synthesized. Synthesis simply increases its pore or cavity sizes and makes it hydrophobic. Zeolites are selective adsorbers because of their unique pore sizes. Zeolites are used in removing heavy metals such as zinc, iron, copper, chromium and so forth from drinking water. Batch or column process could be involved in this adsorption and spent zeolite can be regenerated.
Introduction
Adsorption of ions and/or molecules of gas or liquid is imperative in purification processes. Adsorbents are usually solids whose pore spaces, surface and structures facilitate to adsorb adsorbates and in the process remove them from a given solution or environment. Due to this characteristic property and relevance, adsorbents have been found to be increasingly useful for water purification as well as in removing several substances such as volatile organic substances (VOCs) from the environment. Interestingly, there are lots of known adsorbents in the world today, and these adsorbents may occur in the natural form but can be chemically synthesized, however, activated carbon and zeolites are of special interest to us in this study.
Activated Clay in Adsorption
Unlike ordinary clay, activated clays occur in a limited amount on the earth surface. Nutting (1943) identified that the United States is the largest producer and consumer of activated clay. Adsorbent clays could be naturally active or synthetically activated. Naturally active adsorbent clays are easy to use due to their percolation and granulated nature. Additionally, this kind of adsorbent clay could be reused several times and could also be re-burned. However, activated clays are not quite cohesive; they are incoherent. This kind of adsorbent clay is uneconomical compared to natural clay due to their acid treatment, but they are about four times more powerful than the former. Moreover, unlike natural clays, activated clays cannot be reused (Nutting, 1943).
The activated or active clays are a variety of argillaceous silicates which behave in a similar way under certain conditions (Aqua Technologies, n.d). However, the various forms of active clay have varying compositions and structures. Activated clays are mostly grouped under the name ‘smectite’ which are usually found in the form of hectorite, saponite, and montmorillonite with the montmorillonite which is the most widespread form of smectite. Smectites are generally known as bentonite, a geological term used in identifying them. Adsorbent clays are being activated via the action of an acid. The acid produces materials with good activity, but it involves some environmental and technical disadvantages. For instance, activated clays could swell in the presence of an oxygenated solvent and thus they cannot be used in treating solutions or blends containing water, aldehydes or ketones and so forth.
The montmorillonite group activated clays are of renowned industrial importance. This group is composed of montmorillonite, beidellite or saponite which is simply magnesium aluminosilicates in hydrated form. To distinguish activated clay from other kinds of clays, it is imperative to consider that they contain a higher proportion of silicate (Al2O3 : SiO2= l:2 up to 1:5). Adsorbent (clays) are different clays of the kaolin types (fine white clay), which are mostly composed of kaolinite (Al2O3.2SiO2.2H2O). Moreover, it is difficult to establish the mineralogical characteristics of montmorillonite clay owing to their tiny individual crystals which make them difficult to be identified with a petrographic microscope (Nutting, 1943).
The moisture content most commonly identifies adsorbent clays at different temperatures and in an atmosphere of different humidity. They can also be identified with oil-bleaching tests. However, the moisture tests appear to be less effective because they are not effective enough in excluding the inactive and almost inactivable swelling bentonites. Laboratory tests are the only sufficient ways of identifying montmorillonite clay. However, they could also be distinguished based on their colors. Adsorbent clays have been found to range in color from black to pure white, although on the basis of their color, adsorbent clays may not be clearly distinguished from ceramics and sedimentary clays. Some other tests and physical properties that can facilitate the identification of adsorbent clay include slaking and swelling tests, density, refractive indices, x-ray patterns, clay-water relations and so forth (Guimarães et al. 2007; Nutting, 1943).
The adsorbing action of activated clay
The activated clay has electronegative characteristics on the surface. Hence, it holds potential to adsorb electropositive ions and particles of matter. It is simply a reverse of activated carbon that being electropositive seeks to adsorb negative ions and particles. The adsorbing action of activated clays is mostly confused to be filtering or straining actions but it is not. The decolorizing action of activated clays can be considered a selective adsorption process. This is a similar process used in the dyeing of fabrics. The chemical composition of activated clays shed great light on their adsorbing actions. However, the chemical or mineral composition, molecular structure, or thermal dehydration does not completely describe the adsorbing power of activated clay. For instance, clay that is primarily montmorillonite-beidellite would probably be active or activable, but the swelling bentonites and montmorillonites not derived from volcanic ash are exceptions (Nutting, 1943). The adsorbing action of clay can also be graphically presented as shown in Figure 1.
Figure 1: Graphical presentation for the adsorbing action of activated clay particles.
Source adapted: Guimarães et al. (2007)
Usages
Activated clays possess many interesting applications and usages. Fu et al. (2001) studied color removal by adsorption from water using clay activated by treatment with sulfuric acid. Furthermore, the study conducted by Tsai et al. (2003) indicated that the particle size of activated clay affects the adsorption of paraquat from aqueous solution. Activated clay can also be used in adsorption of dyes from aqueous solutions. Juang (1997) identified that low-cost inorganic acid activated clay can be used for the adsorption of six dyes including two basic, one acid, one dispersed, one direct and one reactive dye from aqueous solution. The study indicated that the activated clay showed high adsorption capacity for the basic dye and low adsorption capacity for the dispersed, direct and reactive dyes. The study generally concluded that activated clays are more effective than other frequently used adsorbents. Ani (2014) conducted an adsorption study in which clay-based and activated carbon adsorbents were used to adsorb dyes. The study involved the removal of basic blue 66 (BB66), acid blue 29 (AB29) and direct 2 (DR2) dyes from wastewater by the adsorption on clay-based and activated carbon adsorbents.
Activated clay can also be used in treating oils, and this is perhaps one of its important usages. Activated clays can adsorb oils and other contaminants or substances present in the soil. Another interesting use of activated clay is in environmental cleanup of oil spills as well as the removal of fats and grease from places like slaughterhouses. Moreover, it is used in decolorizing oils and thereby increasing its sales value and life. Clay is used in processing all lubricating oil because it removes tarry coloring matter as well as other colorless unsaturated compounds that could oxidize. An example of such compound is olefins.
Zeolites
Zeolite can be used as an important adsorbent. It is a rock made up of aluminum, silicon and oxygen and it occurs naturally but can also be artificially synthesized. The rock is said to occur naturally in regions of the world where the volcanic eruption is believed to have occurred in the pre-historic times. Zeolite, discovered by Fredrick Cronstedt, has a natural porosity, and this is because of its crystal structure having significant pore size and cavities (Abbey Newsletter, 1996).
Figure 2: Crystalline structure of under red light. Source adapted: EPA (1998)
The natural zeolite differs considerably form synthetic zeolite in structures as they are limited in their particular pore size and also have an affinity for water (hydrophilic). Like carbon based adsorbent, some synthetic zeolites have an affinity for organics matters with low (or lack) affinity for water (hydrophobic) (EPA, 1998). Thus, these kinds of zeolites can adsorb organic vapors provided that the organic vapors have molecules smaller than their respective pore sizes. Moreover, zeolites are often referred to as molecular sieve due to their uniform pore sizes. These structural features of zeolites are responsible for making it a unique adsorbent and explaining why they are used in adsorbing a wide range of solutes from vapor and gas streams. Moreover, Margeta et al. (2013) also studied structure and diffusion mechanics of zeolite as shown in Figure 3.
Figure 3: Diffusion phenomena on the surface of the zeolite. Source adapted: Margeta et al. (2013)
Zeolite is synthesized in order to define and ensure the desired properties. By synthesizing zeolite, the pore size can be enlarged in order to ensure more effective adsorption, and also synthesis can be used in making hydrophobic zeolite. Furthermore, Arbuznikov et al. (1998) pointed out that the adsorption properties of zeolites are strongly related to the type, number, and location of their accessible cations in which the adsorption takes place.
Why and How Zeolites are used in Water Treatment
Zeolites are used in water treatment process because of their environmental and economic acceptability as well as the efficiency in the process. Natural zeolites are simply hydrated aluminosilicates Na-A Na2O .Al2O3 .2SiO2 . 4.5H2O (Abbey Newsletter, 1996) which has excellent sorption and ion-exchange properties. However, the geological deposits of the zeolite go a long way to determining its effectiveness in various technological processes because it depends on the zeolite's physical and chemical properties.
Moreover, their electronegativity of zeolite adds extra vibes to them because it makes them excellent in removing metal cations from wastewaters (Karmen et al. 2013). Heavy metals such as Zn, Cr, Fe, Pb, Cu, Cd, Mn, and so forth are serious contaminants in drinking water, posing environmental problems. However, natural zeolites can be used in removing these contaminants. Interestingly, various technologies have been developed for removing contaminants or treating water with natural zeolites. These technologies include ion exchange, membrane filtration, coagulation flocculation, chemical precipitation, adsorption, flotation and electrochemical method. Adsorption emerges to be one of the best methods of water treatment with zeolite.
Two procedures can be used in removing contaminants with natural zeolites such as batch and column processes. The batch process is of special interest because of its wide application. In this process, some amount of zeolite is placed in contact with the solution of synthetic or real water at certain times. The quantity, exchange capacity and presence of other cations and anions in the treated water determine the removal capability of metal ions by natural zeolite. Chemically pretreated natural zeolites known as clinoptilolite have a higher removal efficiency of cations in water. Column process involves several aspects that affect the dynamics of the uptake of cations by zeolite and involves factors such as temperature, pH, flow rate, the initial concentration of cation removed by zeolite and so forth (Karmen et al. 2013).
The water purification action of zeolite was also illustrated in a study conducted by Harutyunyan & Pirumyan (2015) which involves the sorption of anionic sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), the cation, from aqueous solution. To achieve the purification, a series of the batch reaction was carried out so as to determine the sorption isotherm of the surfactants to zeolite and the average percentage errors were also computed. Some other factors such as the contact time, adsorbent amount and initial surfactant concentration that influence the capacity of the adsorption were considered. The process showed that CTAB has higher adsorption capacity onto solid over SDS. This process was used in removing surfactant from Nor Kokhb deposit in Noyemberyan, Armenia.
Regenerating Spent Zeolite
The spent adsorbents should be regenerated to make it economical, and the spent zeolite could be easily regenerated. This could be easily done with UV and UV/H2O2 (Singh, 2010). Moreover, zeolite could also be regenerated by using oxidizing phenol in CO2 and H2O under an air flow at 400oC with an increased rate of 2oC/min. This method was used in regenerating zeolite 10 times. Zeolite could also be regenerated through ozonation. In favorable condition, this method could aid in completely mineralizing the organic matter. Moreover, for greater and effectiveness of regeneration on zeolite, the advanced oxidation process can be employed (Singh, 2010).
Conclusion
In conclusion, adsorption is very important in the removal of contaminants including organic particles from substances such as the air and water. Various adsorbents exist, but activated clay and zeolites are of special interest. Activated clay (smectite) is very effective in adsorbing oil as well as dyes and could be used in water purification to remove cations due to their electronegativity. Moreover, the natural zeolite differs considerably form synthetic zeolite in structures. Zeolites are hydrated aluminosilicates having pores and cages. The pore sizes determine the molecular size of particles that can pass through them. Thus, synthesizing zeolites make sense so as to increase their pore sizes and subsequent adsorption efficiency. Zeolites are usually used in water purification process, but they are also used in removing VOCs and so forth. Used zeolites can be regenerated through various methods such as ozonation, the advanced oxidation process and so forth.
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