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Chemistry, structure, properties and application of molecular sieves

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  • Save American Journal of Materials Science 2017, 7(5): 196-221 DOI: 10.5923/j.materials.20170705.12 A Review of the Chemistry, Structure, Properties and Applications of Zeolites Mohau Moshoeshoe1,*, Misael Silas Nadiye-Tabbiruka2, Veronica Obuseng2 1Department of Chemistry, University of Lesotho, Mbabane, Swaziland 2Department of Chemistry, University of Botswana, Gaborone, Botswana Abstract This review article describes the structure of zeolites starting from atomic level to the complex networks of channels and voids that permeate the material. The chemistry within the material is outlined relating it to its crystal structure. The resulting properties are described relating them to the crystal structure and the chemistry. Finally, the applications of the material are summarised relating them to the material crystal structure, chemistry and properties. Keywords Zeolites, Zeolite crystal structure, Zeolite applications, Zeolite properties 1. General Introduction 1.1. Background 1.1.1. Introduction Most natural zeolites are formed as a result of volcanic activity. When volcanoes erupt, magma (molten rock within the earth) breaks through the earth’s crust and flows out in form of lava accompanied by gases, dust and thick ash. Volcanoes normally occur where tectonic plates are diverging or converging. In cases where such locations are on an island or near an ocean, the ejected lava and ash often flow into the sea. Upon reaching the sea, the hot lava, water and the salt from the sea undergo reactions which, over the course of thousands of years, have led to the production of crystalline solids known as zeolites [1-3]. The word zeolite is formed from two Greek words “zeo” = boil & “lithos” = stone; to mean boiling stones [4]. It was given to this type of substances in 1756, by a Swedish mineralogist named Axel Fredrik Cronstedt, who discovered them and their trait of intumescence. He observed that upon heating this mineral steam was released, as water evaporated and the zeolite seemed to be boiling because of the rapid water loss [3-6]. The water molecules that are lost on heating have been adsorbed in the pores and cavities (of dimensions ranging from 0.3 nm to 1.0 nm) present in the zeolites’ crystalline structure [7]. These cavities result from the structural composition of zeolites, which is characterized by a framework of linked tetrahedra, each consisting of four O * Corresponding author: (Mohau Moshoeshoe) Published online at Copyright © 2017 Scientific & Academic Publishing. All Rights Reserved atoms surrounding a cation – usually Si. These Si-O bonds are arranged in a three-dimensional structure of silicate tetrahedra, leading to the presence of open cavities in the form of channels and cages, which are usually occupied by H2O molecules and extra-framework cations that are commonly exchangeable. [4, 8, 9]. Heating or dehydrating zeolites results in high void volumes, which impart to the zeolite the so called ‘‘molecular sieve’’ and adsorbent properties. Molecular sieves only allow molecules of a certain size (equal to, or less than the pore size) to pass through entry channels, whereas molecules larger than the pore size are excluded. Once dehydrated, gas or liquid molecules that are small enough to pass through the channel openings may be sorbed to the inner zeolite structure, whereas larger molecules are excluded [5]. 1.1.2. Chemical Composition All zeolites are composed of an elementary structure of an aluminosilicate framework which comprises of a tetrahedral arrangement of silicon cations (Si4+) and aluminium cations (Al3+) that are surrounded by four oxygen anions (O2-). Each oxygen ion within the Si-O and Al-O bonds connects two cations and is shared between two tetrahedrons (as shown in figure 1.1), thus yielding a macromolecular three-dimensional framework of SiO2 and AlO2 tetrahedral building blocks. In this arrangement of atoms, each tetrahedron consists of four O atoms surrounding a Si or Al cation, resulting in a three-dimensional structure of silicate tetrahedra with a Si:O ratio of 1:2 [10]. Some Si4+ ions are substituted by Al3+ ions, resulting in a net negative charge in the tectosilicate framework (Figures 1.2 and 1.3). This charge arises from the difference in formal valency between the (AlO4)5- and (SiO4)4tetrahedrons and is normally located on one of the oxygen American Journal of Materials Science 2017, 7(5): 196-221 197 anions connected to an aluminium cation. The resulting negative sites are balanced by counterions which are usually alkaline or alkaline earth metals, such as Na+, K+ or Ca2+ in most cases. Li+, Mg2+, Sr2+ and Ba2+ are also found in some zeolites [10]. These ions are found on the external surface of zeolite, bound with the aluminosilicate structure by weaker electrostatic bonds [11, 12]. 4- 5- O O O O Si Al O O O O Figure 1.1. Tetrahedral arrangement of the SiO4 and AlO4 molecules forming unit blocks of a zeolite O O O Al O O O Si O H Figure 1.2. Tetrahedral arrangement of the Si-O and Al-O bonds forming a unit block of a zeolite Figure 1.3. A two-dimensional representation of the framework structure of zeolites [9]. Men+ signify extraframework cations These mobile non-framework counterions – which are commonly exchangeable – are situated in cavities which result from the 3-dimenisonal Si-O/Al-O bond tetrahedra framework. H2O molecules are also found in these cavities (which, when aligned, become channels) and are the reason why zeolites can be hydrated at low temperatures. The general chemical formula for natural zeolites is [(Li, Na, K)a(Mg, Ca, Sr, Ba)d(Al(a+2d)Sin-(a+2d)O2n]·mH2O [4, 8, 9, 10, 13, 14]. 1.1.3. Formation of Zeolites Many zeolites occur naturally as minerals, and are extensively mined in many parts of the world. Others are synthetic, and are made commercially for specific uses, or produced by research scientists trying to understand more about their chemistry. Almost all natural zeolites occur in cavities of volcanic lava flows [15] and therefore have to be mined. Murata et al., [16] have suggested that zeolites are formed through diagenetic rock-water reactions – reactions through which the change of sediments or existing sedimentary rocks results in a different sedimentary rock during and after rock formation. These reactions also result in the production of other secondary minerals. This metamorphosis of volcanic debris leads to production of zeolites in layers or “zeolite zones” called facies [17]. The type of the zeolite formed is determined by various factors. Chief among these is the form of volcanic matter which reaches the water – whether lava or ash. The type of water involved (whether marine, fresh water lakes, ground waters or saline shallow lakes), together with the alkalinity of the water and the type and concentration of ions it contains also play a crucial part [18]. Zeolites are formed in the hardened lava either during diagenesis resulting from active geothermal systems in areas of high heat flow, during burial metamorphism of the lava pile or during hydrothermal alteration of continental basalts [19]. Zeolites form in these locations as a result of very low grade metamorphism [20, 21]. While others are formed in metamorphic regions, others form under subtle amounts of heat and pressure – conditions which can just barely be called metamorphic. Pe-piper [22] has suggested that mordenite – a zeolite named after the small community of Morden, in Canada, where it was first found – is formed through hydrothermal circulation of alkaline lake waters instead of metamorphic processes as was previously believed. Furthermore, Hay [23] has shown that zeolite formation may occur under any or a combination of the following conditions: saline, alkaline lakes or soils; deep-sea sediments; low-temperature open hydrologic systems; burial diagenesis; and hydrothermal-geothermal systems. Saline, alkaline lakes cause volcanic ash layers to alter rapidly to zeolites, resulting in the formation of relatively pure deposits. Most of the zeolites formed during diagenetic processes in sedimentary rocks can be grouped into several types of geological environments or hydrological systems such as hydrologically open systems, hydrologically closed systems, soil and surficial deposits, deep marine sediments and marine sediments from arc-source terrains [19]. Natural zeolite ores are found in many parts of the world among rocks near active or extinct volcanoes. Most of the world’s supply is obtained in Asia, Australia and Europe; with the United States contributing about one percent [24]. Zeolites are used in a variety of applications worldwide due to their unique porous properties. Their applications comprise different areas of industry inclusive of technology and environmental remediation such as pollution control and disposal of hazardous materials. This review intends to look at the role of zeolites in 198 Mohau Moshoeshoe et al.: A Review of the Chemistry, Structure, Properties and Applications of Zeolites modern industrial applications. These roles will be classified as (a) industrial applications (water purification); (b) medicinal applications; and (c) catalysis. 2. Crystal Structure 2.1. Structures of Zeolites Zeolites are not an easily definable family of crystalline solids and are similarly not simple to categorise [25]. In 1997, the subcommittee on zeolites of the International Mineralogical Association, Commission on New Minerals and Mineral Names agreed that any substance with a topologically equivalent structure, also possessing essential zeolitic characteristics (i.e. a framework structure with cavities occupied by ions and water molecules which have considerable freedom of movement, permitting ion exchange, molecular “sieving”, absorption, diffusion, dehydration, reversible dehydration and catalysis) be classified as a zeolite irrespective of its Si and Al content in tetrahedral sites [4, 26]. Subsequently, a revised definition of a zeolite was proposed as follows: A zeolite mineral is a crystalline substance with a structure characterized by a framework of linked tetrahedra, each consisting of four O atoms surrounding a cation. This framework contains open cavities in the form of channels and cages. These are usually occupied by H2O molecules and extra-framework cations that are commonly exchangeable. The channels are large enough to allow the passage of guest species. In the hydrated phases, dehydration occurs at temperatures mostly below about 400 °C and is largely reversible. The framework may be interrupted by (OH, F) groups; these occupy a tetrahedron apex that is not shared with adjacent tetrahedra. [4]. 2.1.1. Primary and Secondary Building Units The crystal structures of zeolites are normally categorized into primary building units (PBUs) and secondary building units (SBUs). The PBUs are the (SiO4)4+ and (AlO4)5+ tetrahedra. These combine by sharing oxygens with adjacent tetrahedra to form a spacial arrangement of simple geometric forms – the SBUs. The SBUs come in a variety of forms – some being single rings, double rings, polyhedra or even more complex units which are linked together in a variety of ways to produce a unique system of channels and cages. A zeolite’s unit cell always contains an integral number of SBUs. At present, 23 different types of SBUs are known to exist. These are shown in Figure 2.1 [25]. Figure 2.1. Secondary Building Units and their Symbols. Number in parentheses indicates frequency of occurrence American Journal of Materials Science 2017, 7(5): 196-221 199 Figures 2.2 and 2.3 show how PBUs join together to form SBUs and different forms of SBUs, respectively. An example of how SBUs are linked together to produce a unique zeolite structure is shown in Figure 2.4 and Figure 2.5 [27, 28]. Figure 2.2. A Combination of (SiO4)4+ PBUs to form SBUs, alignment of which results in a cage Figure 2.5. Structures of three different zeolites and their micropore systems [5] Secondary building units are non-chiral and may contain up to 16 T-atoms. Since a unit cell always contains an integral number of them, they are derived assuming that the entire framework is made up of one type of SBU only. Figure 2.3. A chemical model of a complex zeolite structure. The differently sized holes represent channels and cages. Image courtesy of Geoffrey Price, University of Tulsa [29] Figure 2.4. Different orientations of Secondary building units (SBUs). The intersections of lines represent the centres of tetrahedral cations (Al or Si). a) Single 4- and 6- rings; b) Double 4- and 6- rings; c) Truncated cubo-octahedrons composed of 4- and 6- rings; d) Four truncated cubo-octahedrons linked together by four double 4- rings This special arrangement of SBUs contributes to the crystalline structure, hence the type and morphology of different species of zeolites (Fig 2.5 & 2.6). 2.1.2. Composite Building Units In addition to the PBUs and SBUs described above, zeolites may contain other components such as double rings, cancrinite cages and alpha cavities. These are called composite building units CBUs. They appear in several different framework structures, and can be useful in identifying relationships between framework types. Unlike SBUs, CBUs are not necessarily achiral, and cannot be used to build the entire framework [25]. The most common example of CBUs is the rings, whose sizes are decided by the number of tetrahedrons in a ring. A ring is therefore characterised by, and named after the number of tetrahedrons it consists of. A ring consisting of n tetrahedrons is called an n-ring. Zeolite rings frequently comprise of 4, 5, 6, 8, 10, or 12 tetrahedrons. Frameworks with rings of 14, 18, and 20 tetrahedrons have been reported [30-34] while 3-, 7-, or 9-ring frameworks are rare [35-37]. As shown in Figure 2.6, the size of the rings in the framework determines the pore size in different zeolites. The entrance to the pore is known as a window [38]. Rings may be joined to form more complex CBU structures such as prisms and cages. Cages are defined as polyhedrons whose largest rings are not big enough to allow the passage of molecules larger than water [39, 38]. They can be viewed as channels of limited or fixed length. They are however bigger in diameter than normal channels and are only accessible via the channels themselves. Cages are usually formed at the crossing of two channel systems. These 200 Mohau Moshoeshoe et al.: A Review of the Chemistry, Structure, Properties and Applications of Zeolites typically trap those molecules which are bigger than the channel systems, molecules which may have been formed within the cages or during synthesis. Figure 2.6. Composition and size of rings in the zeolitic framework. The pore size of a specific zeolite is determined by the ring type [39] Together, these rings, channels and cages form composite building units of zeolites. Examples of composite building units are given in Figure 2.7 and Appendix 1. According to Bedioui [40], the structure of zeolites may be summarized as follows: the PBU (SiO4)4- or (AlO4)5tetrahedra (Fig. 2.8 a) are connected through their corners of shared oxygen atoms to form a wide range of small SBUs (Fig. 2.8 b). These are interconnected to form a wide range of polyhedra – the CBUs (Fig. 2.8 c), which in turn connect to form the infinitely extended frameworks of the various specific zeolite crystal structures. This summary is depicted in Figures 2.8 (a-d) and 2.9. The corners of the polyhedra represent Si or Al atoms and the connecting lines represent the shared oxygen atoms [5]. Different types of zeolites (e.g. clinoptilolite, mordenite, erionite etc.) naturally occur with dissimilar morphology. Clinoptilolite and heulandites are isostructural and both generally occur as plates or laths with tabular form. Mordenite usually occurs as fine fibers or as thin laths and needles. Chabazite has a cube-like appearance due to its rhombohedal SBUs (Figure 2.10). Figure 2.7. Examples of CBUs. Each unit is identified with a lower-case three-character code in italics and the number of T-atoms in the unit. Framework types containing the unit are listed below each unit American Journal of Materials Science 2017, 7(5): 196-221 201 Figure 2.8. Development of a zeolite from (a) the framework PBU – the (SiO4)4- or (AlO4)5- – through (b) the SBUs and (c) CBUs to (d) a zeolite structure Figure 2.9. Structures of four selected zeolites and their micropore systems and dimensions [42] Because of the crystalline structure consisting of a three-dimensional tetrahedral network of silicon and aluminium linked together by common oxygen atoms, zeolites are naturally porous. These pores are a product of interlinked cages, resulting from the tetrahedral structural arrangement of atoms and they are normally of sizes less than 2nm, hence zeolites belong to a group of microporous materials [43]. According to the IUPAC definition [44], microporous materials are those materials having pore sizes less than 2nm. Materials with pore sizes between 2nm and 50nm are classified as mesoporous, while those with pore diameters greater than 50 nm belong to the macroporous category. 202 Mohau Moshoeshoe et al.: A Review of the Chemistry, Structure, Properties and Applications of Zeolites Figure 2.10. Scanning electron micrographs of common natural zeolites. a) Clinoptilolite (Cp) and modernite (Md); b) Chabazite (Ch) and smectite (Sm); c) Erionite; d) Phistlistine and e) Analzime (An) and Chabazite (Ch) [40] 3. Properties and Applications 3.1. Applications of Zeolites Over the years zeolites have attracted a great deal of attention among researchers and scientists due to their flexibility and adaptability. Following their discovery in 1756 by Axel Fredrik Cronstedt, zeolites were found to be good adsorbents, ion exchangers and molecular sieves. The molecular sieve properties of zeolites in particular are extensively employed in industry. Zeolites have been used in the separation of straight-chain hydrocarbons from branched-chain hydrocarbons [27], chemical sensors [45] in industrial process control, environmental and indoor air-quality monitoring, effluent and auto-exhaust control, medical monitoring [5], air separation [46] and removal of heavy metals [47] to mention but a few. Zeolites continue to find various applications in solving environmental, scientific, industrial and day to day problems. Their usefulness and their applications in chemistry (and day-to-day life) is addressed in this section. 3.2. Purification of Water The earliest use of zeolites was in their application as adsorbents in 1777 by Fontana and Scheele. Since then, their adsorbance properties have enabled them to be applied in a variety of processes used to solve environmental issues. Zeolites were subsequently found to be good adsorbents for molecules such as H2O, NH3, H2S, NO, NO2, SO2, and CO2 to mention but a few [48]. 3.2.1. Treatment of Industrial Wastewater Increasing demands for high quality drinking water has led to a worldwide need to purify water from various sources including natural, industrial, agricultural and municipal waste waters. Consequently, the use of natural zeolites as agents in the removal of wastewater contaminants has gathered tremendous interest culminating in extensive studies. Wastewater streams resulting from industrial processes (such as mining and manufacturing) have different physical-chemical characteristics. They may contain ions of metals like Sb, Cr, Cu, Pb, Zn, Co, and Ni, together with waste liquids which are generated by metal finishing or the mineral processing industries [49, 50]. These metals, which are toxic even at trace levels, may exist in these waters at very high concentrations. Such waters should not be American Journal of Materials Science 2017, 7(5): 196-221 203 discharged directly into natural waters as they pose a great risk for the aquatic ecosystem, resulting in several types of health problems into animals, plants and human beings. Furthermore, these waters should not be discharged directly into the sewerage system as they interfere with the biological wastewater treatment processes. Additionally, these biological processes may not be capable of fully removing toxic metals from these waters [51-53]. 3.2.2. Treatment of Municipal Wastewater Treatment of municipal wastewaters by the use of zeolites is aimed at enhancing the efficiency of the pollutant reduction process. One of the significant contaminants of greywater (wastewater originated from kitchen, bathroom and laundry in households) is the ammonium ion (NH4+). Sources of ammonium in household water include ammonium salts, which function as acidity regulators, thickeners and stabilisers in kitchen detergents. Bathroom ammonia is mainly from urine, whereas laundry wastewater contains ammonium ion from the use of fabric softeners and laundry disinfectant agents. These normally contain quartenary ammonium salts, dialkyldimethylammonium chlorides, distearyldimethylammonium chloride and/or alkyldimethylbenzylammonium chlorides, which function as cationic surfactants [54, 55]. 3.2.3. Treatment of Drinking Water After it was discovered that the presence of clinoptilolite can enhance nitrification of sewage sludge, natural zeolites have been used in various places for the treatment of municipal wastewater for drinking purposes. The addition of powdered clinoptilolite to sewage before aeration has been reported to lead to increases in O2 consumption and sedimentation. This results in a sludge that can be more easily dewatered and, hence, used as a fertilizer [56]. Mixing sludge with natural zeolites for the purpose of treatment has been reported to lead to production of clear water with improved quality parameters such as color (by 92%), suspended particles (by 94%), chemical oxygen demand (by 95%), dissolved oxygen (by 950%), P2O5 (by 96%), NH4 (by 99%), SO4 (by 97%), NO3 (by 92%), NO2 (by 82%), total Cr (by 90%), Mn (by 94%) and Ni contents (by 93%). Additionally, production of an odourless and cohesive zeo-sewage sludge was reported [57]. In other studies, zeolites have been used to reduce levels of heavy metals – such as lead and chromium – which are found in sewage sludge from municipal wastewater treatment plants. Kosobucki et al., (2008) [58] have demonstrated that through the use of zeolites, up to 68% of heavy metals can be removed from sludge by addition of zeolites to sludge at a ratio of 2:98 (zeolite:sludge). Application of ultrasonic energy appeared to improve the process of heavy metal removal from sludge. According to Kosobucki and co-workers [58], the ultrasonic energy serves to influence both the structure and physicochemical properties of the zeolite, hence leading to improvements in the immobilization process of heavy metals from the sewage sludge. Treatment of well groundwater with natural zeolites was shown to remove 55 % of NO3, 74% of Pb, 79% of Ag and improved pH from 9.6 to 7.3. Quality parameters were improved by 93% for the color and 96% for the chemical oxygen demand. In addition, the zeolite removed 51% of colonial Mycrocystis cyanobacteria, 75% of Filamentous cyanobacteria as well as 92% Chroococcus cyanobacteria from their culture. The natural zeolites’ ability to remove inorganic, organic, and organometallic compounds, as well as gas species, metals and radionuclides from their aqueous solutions can be attributed to absorption (mainly ion exchange), adsorption and surface precipitation processes [57]. 3.2.4. Properties of Zeolites Enabling Water Purification Cation Exchange Capacity Most of the methods in which natural zeolites are used, for the purification of water are based on their cation-exchange behaviour. In this process, the exchangeable zeolite surface ions are replaced at the sites by ions from the solution. The dissolved cations are removed from water by being exchanged with the cations on a zeolite's extra-framework exchange sites. According to Kalló [59], many natural zeolites (e.g. clinoptilotite, mordenite, philtipsite, chabasite) are selective on several toxic metals which are often present in industrial waters (e.g. Cu2+, Ag+, Zn2+ Cd2+, Hg2+, Pb2+, Cr3+, Mo2+, Mn2+, Co2+ and Ni2+). In addition to these metals, these zeolites are highly selective on the NH4+ ion and exchange it preferably even in the presence of competing cations. These ions are removed from the water and replaced with biologically acceptable cations such as Na+, K+, Mg2+, Ca2+ or H+ from the zeolite exchange sites. Smical (2011) [28] has shown that the ion exchange process may be presented by the following equation: Reaction 3.1 Where: zA and zB represent the A & B exchangeable ion charges and the coefficients (z) and (aq) refer to zeolite and to aqueous solution, respectively. The reaction proceeds until equilibrium is attained. The point of equilibrium is defined largely by the theoretical cation exchange capacity (TCEC) of the zeolite, which in turn, is given by the sum of free extra-framework cations of the zeolite. This number of free extra-framework cations of the zeolite is directly related to the amount of aluminum present in the framework and hence the amount of Al3+ that replaces Si4+ in the structure. Cation exchange capacity is usually expressed in milliequivalents (meq) metal per 100g of zeolite. Since the exchangeable cations of natural zeolites are usually represented by: Na+, K+, Mg2+, and Ca2+, for a particular natural zeolite, TCEC = Σ (Na, K, Mg, Ca) as is shown on Table 3.1 [60-63]. 204 Mohau Moshoeshoe et al.: A Review of the Chemistry, Structure, Properties and Applications of Zeolites Zeolite Name Analcime Chabazite Clinoptilolite Heulandite Mordenite Philipsite Laumontite Natrolite Erionite Faujasite Ferrierite FTC ANA CHA HEU HEU MOR PHI LAU NAT ERI FAU FER Table 3.1. Cation exchange capacity (CEC) of zeolites [60, 64] Formula Si/Al Ratio Main cation Na16(Al16Si32O96) •16H2O Ca2(Al4Si8O24) •12H2O (Na,K)6(Si30Al6O72) •20H2O Ca4(Si28Al8O72) •24H2O Na2KCa2(Al8Si40O96) •28H2O K2(Ca0.5Na)4(Al6Si10O32) •12H2O Ca4(Al8Si16O48) •16H2O Na16(Al16Si24O80)•16H2O (Na2K2Ca)2(Al4Si14O36)•15H2O (Na2,Ca,Mg)3.5(Al7Si17O48) •32(H2O) (Na,K)2Mg(Si,Al)18O36(OH) •9H2O 1.5 – 2.8 1.4 – 4.0 4.0 – 5.7 4.0 – 6.2 4.0 – 5.7 1.1 – 3.3 1.9 – 2.4 1.2 – 1.7 2.6 – 3.8 2.1 – 2.8 4.9 – 5.7 Na Na, K, Ca Na, K, Ca Na, K, Ca, Sr Na, K, Ca Na, K, Ca Na, K, Mg Na Na, K, Ca Na, K, Mg Ca CEC (meq/g) 3.6 – 5.3 2.5 – 4.7 2.0 – 2.6 2.2 – 2.5 2.0 – 2.4 2.9 – 5.6 3.8 – 4.3 2.9 – 3.2 2.7 – 3.4 3.0 – 3.4 2.1 – 2.3 The total CEC given in the table below differs from the ideal cationic exchange capacity, which is defined based on the chemical formula of a pure zeolite. According to Perego et al., [64] there are several factors which determine the actual / operating cation exchange. These include (a) the zeolite’s framework structure; (b) the zeolite’s framework electrostatic field strength; (c) the nature of the cation and its charge density; (d) the composition of the contacting solution; (e) the pH of the contacting solution (pH is a measure of protons, which are also cations and are exchangeable); (f) the composition and work-up of the raw mineral used as a zeolite and; (g) the process apparatus and operating conditions (e.g. continuous stirred tank vs. fixed bed column). Due to these, literature result comparison is extremely difficult and often contradictory. This fact is also supported by a report by Castaldi et al., [43], who have shown that the sorption capacity of a particular zeolite is not fully given by the total cation exchange capacity due to the fact that the specific crystal structure and the distribution and accessibility of the exchange sites for cations play the greatest roles in determining the extent at which cations in the zeolite will be exchanged. Compared to other ion exchange materials such as organic resins, the use of natural zeolites is expedient in that they provide low-cost treatment, exhibit excellent selectivity at low temperatures, release non-toxic exchangeable cations (e.g. K+, Na+, Ca2+ and Mg2+) to the environment, provide simple operation as well as easy maintenance of the full-scale applications, are compact in size and can be used in relatively little space [12, 65, 66]. Natural zeolites are however rarely obtained in pure form. They are normally contaminated to varying degrees with other minerals such as quartz, amorphous glass and other feldspars [67, 68]. Consequently, natural zeolites are excluded from many important commercial applications which require uniformity and purity [69]. Adsorption Kinetics A series of studies undertaken by Widiastuti et al., [12] have shown that the efficiency of a zeolite to remove ammonia from greywater is influenced by several parameters, among which are initial ammonium concentration, contact time and pH. Adsorption kinetics can be used to investigate the process of adsorption of contaminants onto the adsorbents. This helps to clarify the mechanism of adsorption (which depends on the physical and/or chemical characteristics of the adsorbent as well as the mass transport process). Five different kinetic models (pseudo first order, pseudo second order, Bangham, intra-particle diffusion and Elovich models) have been used to try and elucidate the mechanism of ammonium adsorption onto zeolite particles [6, 12, 70]. The pseudo first order is determined by equation 3.1 below: (3.1) where qt (mg/g) is the amount of adsorbate adsorbed at time t (min), qe (mg/g) is the equilibrium adsorption capacity, and kf (min−1) is the rate constant of pseudo first order model (min−1). The pseudo second order is determined by equation 3.2 below: (3.2) Where all the terms retain their definition as per Eq 3.1 and ks is the rate constant of pseudo second order model (in g•(mg/min)-1). The Bangham model is represented by the following equation (Eq 3.3): American Journal of Materials Science 2017, 7(5): 196-221 205 (3.3) where Co is the initial concentration of adsorbate in solution (mg/L), V is the volume of solution (mL), m is the mass of adsorbent per liter of solution (g/L), qt (mg/g) is the amount of adsorbate retained at time t, and α (<1) and ko are constants. The intra-particle diffusion model is given by Eq 3.4: (3.4) where kid is the intra-particle diffusion rate constant. If adsorption follows the intra-particle diffusion mechanism, then plot of qt versus t1/2 will be a straight line with a slope = kid and intercept = C. The values of the intercepts give an indication about the thickness of the double layer – the larger the intercept the greater is the double layer effect. This effect can be minimised by working in salt solution. The Elovich equation is given by (3.5) Integrating Eq 3.5 and applying the initial conditions qt=0 at t=0 and qt=qt at t=t, yields the Elovich model (Eq 3.6) (3.6) where α is the initial adsorption rate (mg/(g min)) and the parameter β is related to the extent of surface coverage (g/mg) and activation energy. Figure 3.1 shows that the pseudo second order fits the kinetics data best for 5 and 50 mg/L ammonium concentrations with an R2 value of 0.99. With this understanding of the reaction pathway and mechanism acquired, it is possible to predict the rate at which pollutants are removed from aqueous solutions, thus enabling the design of appropriate sorption treatment plants. The formula for a pseudo second order reaction rate is given by equation 3.2 above. Integrating this for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt, Eq 3.2 may be rearranged as shown by Eq 3.7: (3.7) A plot of t/qt against t is linear with slope = 1/qe and intercept = 1/ksqe2, where qe is the equilibrium adsorption capacity and ks is the pseudo second order constant. The expression ksqe2 may be used to define the initial sorption rate, h (mg/g min) as t→0. Consequently, h, qe and ks can be determined experimentally from the slope and intercept [72-74]. Figure 3.1. (A) Pseudo first order, (B) Pseudo second order, (C) Bangham, and (D) Elovich kinetic plots for ammonia removal by the zeolite at initial concentrations 5 mg/L and 50 mg/L, contact time 8 h, temperature 25°C and ratio of solid/liquid 1 g/100 mL [12]

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