Abstract
Due to climate change and anthropogenic activities such as agriculture, mining, and urbanization, water contamination has become a very real modern problem. Modern solutions such as activated carbon, reverse osmosis, and ultrafiltration, among others, have been employed in the decontamination of water. These methods are, however, expensive to set up and maintain and therefore have proved a challenge to implement in developing countries. Zeolite materials exhibit excellent structural properties, such as high ion exchange capacity, porosity, and relative surface area, which make them attractive to water decontamination processes. However, conventional zeolites are expensive, and recent research has focused on utilizing low-cost materials such as agro-wastes and clays as raw materials for the synthesis of zeolites. This review aims to discuss the role of low-cost zeolites in their removal of heavy metals and the feasibility of agro-wastes and natural clays in the synthesis of zeolites. Recent research studies based on the synthesis of zeolites from clays and agro-wastes and their application in heavy metal removal have been reviewed and discussed. Agro-wastes such as rice husk ash and sugarcane bagasse ash and layered silicate clays such as kaolinite and smectites are particularly of interest to zeolite synthesis due to their high silica to alumina ratio. Zeolites synthesized through various methods such as hydrothermal, molten salt, and microwave irradiation synthesis have been discussed with their effect on the adsorption of various heavy metals.
1. Introduction
Water utilization for domestic, industrial, or agricultural purposes gives rise to different forms of wastewater [1] in a continuous process due to the never-ending stream of human activities requiring usage of water. According to UN-WWDR [2], over 80% of global wastewater is released to the environment without undergoing proper treatment. This brings about 1.8 billion people at risk of waterborne diseases, owing to contaminated water sources [3]. Dye contaminants resulting from artificial pigments are some of the largest water contaminants in the world. It is estimated that the dye industry discharges approximately 7.5 metric tons to the environment every year [4]. Contaminants such as methylene and celestine blue [5], rhodamine, and crystal violet [6], among others, are known to cause poor light penetration to water bodies, thereby hindering photosynthesis and consequently death to aquatic life. In response to a growing population and economy in most low and lower-middle income countries, exposure to contaminants is only estimated to increase, according to a report by UNEP [7]. With the most agricultural system incorporating modern techniques of farming and especially use of herbicides, they are also becoming a common contaminant in most water systems. Fenuron for instance, a common herbicide used to control growth of weeds, is suspected to be carcinogenic to human beings [8] and highly toxic if ingested in large amounts [9]. It is estimated that only 1% of pesticides and herbicides reach their intended target and the rest finds its way to soil, water systems, and vegetables and other consumable crops [9]. Heavy metals, on the other hand, are recognized as the most poisonous contaminants with adverse effects to the environment and health in general [10].
Wastewater has been classified as domestic, industrial, or storm wastewater [2]. Domestic wastewater arises from domestic water uses in homes or business premises such as cleaning, laundry, and toilet flushing, among others. Domestic wastewater mainly comprises detergent chemicals, fecal matter and urine, and bleach, among other substances [11]. Industrial wastewater arises from industrial processes such as cooling processes or as byproducts of industrial processes. Industrial wastewater can be highly toxic to the environment depending on the involved industrial process [12]. Storm wastewater is generally water due to natural processes such as rainfall and mainly comprises eroded material and other particular matter depending on where the water has runoff [13]. Urban storm water mainly contains heavy metals and other toxic materials due to runoff from open garages and carwashes, especially in many developing countries, where wastewater treatment plans are not available for such small businesses [14]. In cases where there are treatment systems in place inclusive of municipal wastewater treatment systems, major focus is on the removal of biological and solid pollutants through biological treatments in stabilization [15]. However, toxic elements of the wastewater such as heavy metals are left untreated despite the fact that they pose a major health concern [16]. Many methods have been adopted for the removal of heavy metals from wastewater. Use of nanocomposites of multiwalled carbon nanotubes [17, 18], magnetic composites of Fe3O4 [19], and activated carbon [20], among others, have been employed on the small scale. Modern large-scale implementations of water purification systems capable of removing heavy metals is often through reverse osmosis, ultrafiltration, and ion exchange, among others, that are unfortunately not accessible to most low and middle-income countries due to high costs of operation and maintenance [21].
Heavy metals are elements considered to have a relatively high atomic weight or density [22]. These include cadmium, copper, iron, chromium, and lead, among others. Some of these metals in trace amounts are useful in the human body, often taking part in crucial processes such as iron in the formation of hemoglobin [23], copper in the efficient absorption of iron into the body [24], and zinc in the synthesis of proteins in the body [25]. Other heavy metals tend to bioaccumulate in the bloodstream and thus become toxic, often causing adverse effects on the human body. Cadmium, for example, on finding its way to the human body and ultimately to the liver, will cause hepatotoxicity and bioaccumulation in the kidneys in the renal tissues causing nephrotoxicity [26]. In chromium, the oxidized states of Cr (VI) are considered very toxic because of the high solubility in water and mobility. Chromium permissible limits in water are set to 0.1 mg/L according to USEPA [27]. Arsenic is often found in dyes, paints, drugs, and semiconductors, among other sources. It can occur as either arsenite or arsenate in organic or inorganic forms, both of which are harmful to human beings. Chronic toxicity is characterized by keratosis and pigmentation issues [28]. Long-term exposure to arsenic compounds has been associated with neurological issues, diabetes mellitus, and cardiovascular diseases, among others [29]. Arsenic permissible limit is 0.01 mg/L according to WHO in water [30]. Lead contamination occurs from effluents of battery repair or production industries, cosmetic, ammunition, soldering, or old pipes. Children are said to bioabsorb 50% more lead than adults during their development [29]. Due to its ability to be biodeposited into the body, it is reported that 95% of lead once in human body can be deposited as a form of insoluble phosphate in skeletal bones. Acute poisoning of lead has been linked with headaches, loss of appetite, arthritis, and hallucination, among other effects. Chronic effects include mental retardation, psychosis, birth defects, autism, and kidney damage, among others [31]. Allowable limit of lead in drinking water according to WHO is 0.01 mg/L [32]. This article reviews the structure and properties of zeolites derived from agro-based materials and clays, their methods of synthesis, efficiencies for removal of heavy metals in wastewaters, and the challenges or recommendations around the utilization of the derived zeolites.
2. Wastewater Treatment Processes
Wastewater is made up of solids (350–1200 mg/l), dissolved and particulate matter (250–1000 mg/l), microorganisms (up to 109 numbers/ml) and nutrients, heavy metals, and micropollutants [33]. The most problematic pollutants are solids, as they can clog and/or damage the system. The role of wastewater treatment is to convert the influent into a state that can safely be returned back to the water cycle without negatively impacting the environment [1]. To achieve this, physical [9], chemical [34], and biological processes are applied through the stages of treatment discussed. These stages are summarized in Figure 1.
2.1. Preliminary Treatment
This stage involves the removal of grit and large solids that would damage equipment in later stages or be too large to continue the process [36]. At this stage, plastics, pieces or blocks of wood, metals and glass material, as well as surface oil, gravel, and sand due to stormwater are removed using mechanical and physical-chemical methods [35]. The principle of treatment at the preliminary step is based on the particle size of the contaminant; therefore, size exclusion techniques are indispensable. These include use of screens, comminutors, grit-removers, and skimming tanks [37]. Screening devices are made up of wire mesh, parallel bars, grating, perforated plates, etc., to make uniform size openings usable in the removal of floating matter, often referred to as rakings or screenings. It has been found that sanitary sewage (domestic sewage) makes use of an average of 0.0015–0.015 m3/Ml screenings with an average screen size of 25–100 mm [37]. The screenings are disposed off through incineration, burial, digestion, or composting.
2.2. Primary Step
The most prominent method of wastewater treatment at this stage is the physical process, which involves sedimentation by the use of membrane filters that require frequent backwash, which can incur extra costs [38]. However, chemical processes may be employed to induce coagulation and flocculation of suspended materials that will not settle through gravity [39]. By the use of sedimentation and floatation, both inorganic and organic solids are removed from wastewater at this stage. Common contaminants removed at this stage include biochemical oxygen demand, total suspended solids, grease and oils, organic nitrogen, organic phosphorus, and traces of heavy metals. The primary stage manages the removal of 50–70% of the suspended solids and 25–50% of the biochemical oxygen demand [40]. Treatment units that have been applied at this stage include detritus tanks, grit chambers, skimming tanks, screens, and primary sedimentation tanks [41]. The screens at this stage are used in the removal of floating matter with comparatively large particle size. Detritus tanks are used when the flow velocity is low, and they withhold sewage for longer periods (approximately 4 minutes), thereby removing fine settleable particles [41]. In the primary step, chemical precipitation is considered only in special cases where (i) odor becomes a problem; (ii) there is a need to remove phosphorus compounds; (iii) there is industrial waste that is likely to destabilize the biological removal process; and (iv) the strength and flow of the wastewater vary greatly [42].
2.3. Secondary Step
This stage incorporates the use of biological and chemical techniques in the treatment of wastewater from the primary step [43]. Microorganisms are used in the removal of most inorganic and organic contaminants, followed by chemical filtration to remove pathogens [38]. Trickling filters are used to induce the decomposition of organic pollutants in biofilms, a process aided by aerobic or anaerobic microorganisms [38]. About 85% of BOD and TSS are removable at this stage, leaving traces of heavy metals, phosphorus, nitrogen, pathogens, and bacteria for removal at the tertiary stage [44]. Conventional processes applied at this stage include trickling filters, oxidation ponds, activated sludge, and filter pods. Space, initial, and operational costs are factors that determine the choice of an applicable technique in secondary sewage treatment [43]. Often, space is a limiting factor due to the high municipal or local area population and the cost of the land. Therefore, this makes oxidation ponds uneconomical and least adopted in regions where land costs are high [43]. Activated sludge, on the other hand, is widely used since it requires low space, minimizes construction costs, and produces a relatively low odor. This method, however, can be costly to operate owing to energy expenses for the oxygen demand and the air pump operation costs [45]. The trickling filter method offers biological contaminant removal through the salts, rocks, plastics, or stones. These are usually packed in such a way that they allow growth of microorganisms and sewage flow through, in very low velocities, to ensure ample contact with the microorganisms [46]. The filters have relatively low construction cost, provide a cheap system of oxygen delivery, and can exist with nonelectric systems. However, they are temperature-dependent, can be easily congested, and occupy a larger surface area compared to activated sludge types [43].
2.4. Tertiary Step
This stage involves the disinfection of the water to make it reusable again. In this step, chemical processes like chlorination for removal of residual pathogenic bacteria and more specific adsorbents for the removal of toxic elements like dyes and notorious heavy metals are incorporated [47]. The chlorination process, although often applied due to its effectiveness in disinfection, easy to use, and budget friendly, can be hazardous if used in excess [47]. Chlorinated water is freed from harmful microbes, but for disposal into aquatic ecosystems, it needs to be dechlorinated, as chlorine can form potentially carcinogenic compounds such as mutagen X [48] and trihalomethanes [49] when it reacts with some organic materials. Recently, ultraviolet (UV) light [38] and ozone [47]are being employed for disinfection without alteration of the water quality. In UV treatment, the water effluent is passed through ultraviolet radiation, which alters the genetic structure, thus rendering them sterile and unable to infect wildlife or humans. However, the water needs to have undergone thorough pretreatment before this step since residual organic matter can reduce the UV light effectiveness when they shield the microbes [47]. The downside of UV treatment is the relatively high costs of maintenance of the UV lamps. In ozone treatment, microbes are destroyed on contact with ozone and forms no hazardous byproducts [50]. The use of ozone is convenient since it can be produced as needed onsite and needs no storage; equipment, however, can be expensive to maintain [15]. The final stage in the tertiary step must be filtration, which can be achieved using activated carbon fibers, sand or disc, and drum or bag filters [47]. When it is necessary to reduce a contaminant to a specific micron concentration rating, bag filters are used [51]. Drum filters have woven cloth wrapped over a drum, through which the influent gets into the drum and filtrate out the cloth. Backwash removes contaminants and ensures the continuous functioning of the system [52].
Although these three waste water treatment steps are required for any waste water treatment plant, most of the plants in low- and middle-income countries have reported low efficiencies in the removal of heavy metals.
3. Wastewater Treatment Technologies
3.1. Ion Exchange
Ion exchange has seen great usage in the removal of heavy metals in the water purification process, whereby cations are used to exchange for metal ions in water through physical or chemical processes [53]. An electrostatic field often binds exchangeable ions onto functional groups available on a solid matrix that needs to come into contact with mobile contaminated water to remove the metal ions [54]. Varying experimental conditions affect the affinity of the ion exchange for certain species, thereby separating them from the solution [54]. Ion exchangers often make use of natural zeolites and synthetic resins to remove metal ions [53]. The resins (R) work through hydrogen-based ion exchange where the cation exchanger releases its hydrogen ion (H+) into the solution to exchange for an ion (X+) in the solution as follows [54]:
Every ion removed from the solution is replaced by another, thus achieving a state of electroneutrality. Several types of ion exchangers including zeolites structured into ion exchangers have been utilized in the removal of heavy metals. Luca [55] comparatively studied the removal of zinc by zeolite ETS-10-based ion exchanger against municipal zeolite A-based ion exchanger and reported higher removal by ETS-10. A different paper by Kumar [56] studied the adsorption of lead (II) ions on a hybrid ion exchanger and reported maximum adsorption capacity 182.7 mg/g ions at 50°C. The adsorption was reported to follow a pseudo-second-order kinetic model for the adsorbed ion. In a similar paper by Ma et al. [57], upon removal of nickel ions using dual-exchanged (Na+/H+) chelating resin, metal ion removal was reported to be highly dependent on the pH of the solution and the Na+ to H+ ratio. High concentrations of sodium ions led to precipitation of nickel hydroxide, which clogged the ion exchanger. Too much hydrogen ions led to competitive protonation, whose consequence was noted to be reduced nickel ion uptake. The heavy metal ion exchanger method is effective with high metal ion removal efficiency, high selectivity, and rapid adsorption kinetics and is applicable in the whole water treatment range [58]. These advantages have seen its massive utilization in most water purification systems especially in developed countries. The method, however, suffers major challenges such as high operating costs and production of sludge which is expensive to process; resins are easily polluted by organic contaminants, and in the face of heavy pollution, resins spend easily [59].
3.2. Reverse Osmosis
Reverse osmosis (RO) technology makes use of semipermeable membranes (filters) to remove contaminants from water by allowing only water to pass through [60]. With the membrane pore size in the range 0.1–1.0 nm, an applied pressure is necessary to overcome osmotic pressure, thereby requiring high energy to operate [61]. The performance of RO filters is greatly affected by the characteristics of the feed water, such as temperature, salt concentration of the feedwater, and the aforementioned pressure [62]. Generally, a higher feedwater temperature ensures high permeate flow due to increased diffusion rates and lower viscosity associated with higher temperatures. The pressure parameter referred to as net driving pressure (NDP), which is the cumulative sum of all the forces exerted on the RO membrane, is often used to understand the feed pressure [63]. Doubling the NDP hypothetically doubles the flow of the permeate [64]. As water flows through the RO membrane, the TDS concentration on the influent side will always be higher than that on the effluent side and so will the pressure. A gradient of salt concentration parameter, determined as the difference between influent and effluent TDS of an RO, describes salt rate passage in an RO system. The passage is usually independent of the pressure in the system [62].
In recent years, different studies have been conducted to bring solutions for heavy metal removal using reverse osmosis technology. Thaci and Gashi [65] utilized biowaste materials to design water purification systems, utilizing the material as a guide with reverse osmosis technology to remove contaminants such as lead, zinc, cadmium, cobalt, manganese, and nickel. Others have used spiral bound reverse osmosis membrane [66] and antifouling reverse osmosis membrane using crosslinkers [67], among others.
Reverse osmosis method is very effective in water purification system as it only allows water to pass through by blocking all other ions. It requires simple maintenance, and it is preferred because it does not require chemicals to remove ion contaminants [68]. It is, however, prone to clogging, fouling, and scaling and therefore requires frequent filter change and maintenance [69]. This increases maintenance costs. In addition, reverse osmosis is slow in operation and often not self-sustainable [70].
3.3. Electrodialysis
Electrodialysis is often used as a modification of ion exchange membranes to make them more selective in their contaminant removal or to improve their removal capabilities [61]. Ionic and cationic membranes with embedded electrodialysis stacks are sandwiched by two electrodes. With the application of an electric current, anions and cations pass through the electrodialysis membrane and are retained on the ion exchange membranes, thus creating a divided aqueous solution with concentrate (high content of contaminants) and diluent [71]. Since electrodialysis uses membranes, it is applicable in low TSS wastewater with suspended particle diameter not higher than 10 μm [72]. The larger particle size clogs the membranes. Wastewater needs to contain TDS concentrations of not more than 5000 mg/l for effective removal [73]. Other important factors that affect the functioning of an electrodialysis system include the flow rate of wastewater, its temperature, and composition [74]. The applied voltage and the characteristics of the ion exchange membranes also determined to a large extent the efficiency of the system [74].
Electrodialysis systems can produce very high-quality effluents with relatively low energy consumption, achieving 80–95%water recovery rates [75]. An electrodialysis system consumes approximately 0.49kWh/m3 at 1000 mg/l of TDS at 75% recovery and 1.75kWh/m3 at 5000 mg/L of TDS [73]. However, they are selective in their removal and therefore fail to remove colloids and organic matter, often making them uneconomic on their own. Furthermore, electrodialysis systems do not remove neutral toxic components such as bacteria and viruses, requiring posttreatment of the effluent [76]. In addition, electrodialysis operation cost can go above reverse osmosis when influent’s TDS concentration exceeds 12,000 ppm [77].
4. Adsorption
Adsorption process has been utilized in water purification systems extensively, making it one of the most used water purification techniques [78]. The adsorption process occurs physically through electrostatic charges or chemically through formation of bonds on the surface or the pores of porous solids in liquid or gaseous media [79]. In this regard, therefore, adsorption can be described as the process, whereby adhesion of atoms, molecules, or ions occurs in response to excess surface energy due to bond deficiency [80]. To efficiently remove contaminants, an adsorbent needs to possess suitable physicochemical characteristics. These include long service life, high adsorption capacity, high selectivity, and low-cost [81]. Surface functional groups, active surface, pore distribution, and pore diameter are important parameters when characterizing adsorbent materials [81].
4.1. Types of Adsorbents
The most common types of commercial adsorbent currently in use include zeolites-based molecular sieves, polymeric adsorbents, activated alumina, activated carbon, silica gel, and carbon-based molecular sieves [82]. Most of the adsorbents like activated carbon and polymeric adsorbents are usually manufactured, while others such as clay occur naturally. Zeolite adsorbents can be synthesized or occur naturally. The structural properties of the adsorbents determine their applications [53, 81, 83–85]. Table 1 summarizes some applications of common adsorbents.
Adsorbents application and type is largely dependent on their specific characteristics. Synthetic adsorbents are currently most sought after due to their customizability at synthesis and their selectivity in adsorption [92–95]. With the world looking for affordable, renewable, and efficient adsorbent solutions, researchers are looking into utilizing agro-wastes and abundant resources like clay for adsorbent purposes.
4.2. Characteristics of Adsorbents
For a material to be effective as an adsorbent for metal ions, it needs to exhibit some characteristics that fall into either physical or chemical characteristic groupings [85]. Physical characteristics include porosity, surface area, density, and particle size. Porosity refers to the total amount of space void available to the material. The more the pores, the higher the adsorption capacity of the material is [96]. Yakout [97] illustrates the increase in mesoporous pore increase in adsorption studies. A similar study by Tang et al. [98] reported higher Pb2+ and Cd2+ for the activated carbon adsorbent with a higher pore volume, agreeing with this observation. The rate of mass transfer is usually influenced by the external surface area [99]. External mass transfer occurs with the formation of a hydrodynamic layer covering the adsorbent, whereas internal mass transfer occurs through intraparticle diffusion. Efficient adsorption is highly dependent on the internal surface area and is responsible for material pore characterization as microporous, mesoporous, or macroporous. The total volume of pores available to an adsorbent material gives its adsorption capacity [97]. In an adsorptive study by Ouyang et al. [100], the synthesized adsorbent follows pseudo-first-order kinetic model in the adsorption of Pb2+, Cd2+, and Cu2+. This results in the fact that the adsorption process is most likely a physisorption process. This adsorption process follows an external mass transfer phenomenon. In a different study, zeolites synthesized from natural kaolin were used in the adsorption of various metal ions, and the adsorption kinetic study revealed adsorptive energies below 40 kJ/mol, which indicated the physisorption process [101]. Similar findings were reported by Shehata et al. [102].
The chemical characteristics of an adsorbent involve the availability of surface-active functional groups to the material that can interact with contaminants and favorable surface chemistry [103]. Surface chemistry is the determinant of interactions between the adsorbent and the adsorbate, especially adsorption into oxidic sites [96]. In an investigation by Tran et al. [104], studying the effect of modification of the surface chemistry of zeolitic adsorbents on contaminant adsorption, it was reported that the modification of the Na of Y-zeolite improved its cation exchange capacity. Modifying it with surfactants opens the material to a wider range of adsorbates. In a similar study, the authors modified municipal sewage waste incineration fly-ash-based zeolite with Na2PO4 and reported 22 times higher adsorption compared to the unmodified zeolite [105]. Similar studies have reported enhanced metal ion adsorption on the modification of surface chemistry of various zeolite-based adsorbents [105–108].
4.3. Adsorbents in Water Treatment
Activated carbon (AC) has been in use for the adsorption of both inorganic and organic contaminants over the years. The selectivity to inorganic or organic compound adsorption depends on the material of origin and the process of activation [109]. Soft carbonaceous materials generally produce large-pored AC that are often applied in removal of organic compounds [110]. Harder materials produce AC with smaller pores often used in the removal of inorganic contaminants [111]. The process of activation can, however, alter the characteristics of the resultant AC [110]. Activated carbon is prepared from carbonaceous compounds such as wood, and agro-wastes achieve a high surface area and porosity for application in heavy metal adsorption [85]. Activated carbon can be used in various forms ranging granulated form (0.6 – 4 mm granules), powdered form (44 μm), and fibrous structures with low hydrodynamic resistance [112]. Activated carbon has been documented to achieve a high contaminant removal efficiency of more than 94% for contaminant concentration of between 100 and 1600 μg/l with a carbon dose of approximately 1.25 g/l [113]. Activated carbon technology is limited by very high costs of production and use. Researchers circumnavigate this by sourcing cheaper material for the synthesis of activated carbon and/or modifying the surface chemistry of the material [112]. Nejadshafiee and Islami [114] modified activated carbon produced from pistachio shells with Fe3O4 nanoparticles and 1,4-butane sultone and reported high adsorption capacities for Pb2+ (147.05 mg/g), As2+ (151.51 mg/g), and Cd2+ (119.04 mg/g). In a similar study, Kyzas et al. [115] investigated the effect of nanobubbles on the adsorption of Pb2+by activated carbon based on potato peels and reported almost equal adsorption of the metal ion by the modified carbon compared to the unmodified. However, the adsorption duration was boosted by over 300%, which indicated that the nanobubbles had a catalytic effect on the adsorption process.
Carbon nanotubes (CNTs) have one-dimensional tube-like structures resulting from rolled-up graphene sheets. They can be single-walled (SWCNT) or multiwalled (MWCNT) and contain a high surface area for contaminant adsorption. CNTs offer internal sites, interstitial channels, groove sites, and exterior surfaces as active adsorption sites making them excellent adsorbents [96]. Interactive forces due to carbon on the CNT surface cause challenges of aggregation, difficulty in manipulation, and poor dispersibility. Since the active sites on a CNT are found on defected segments of the structure, like pentagons on a body of hexagon structures, their interaction with other compounds in “raw” form is often limited [116]. They are modified by functionalization to enhance their interactions, using covalent (attached to CNTs skeleton) or noncovalent (functional groups coating the walls of CNTs) functionalized [117]. The process of functionalization is subject to the chemical and physical properties of CNT such as the nature of the CNT surface, particle size, and the chemical composition of the material [118]. In a study to compare adsorption performance of oxidized and double-oxidized MWCNTs, Rodríguez and Leiva [119] reported an increase in adsorption of Cu2+ (7.8–14 mg/g), Mn2+ (3.7–6.6 mg/g), and Zn2+ (2.7–4.0 mg/g). The authors reported that the adsorption followed the Langmuir’s adsorption isotherm, which indicated that the adsorption process was a monolayer. The increase in adsorption was an indication of an increment in adsorption sites on the surface of the MWCNT for the double oxidized MWCNT. Other authors have reported similar findings on MWCNT oxidization modifications [120–122]. Polymers such as polydopamine [123] and polyethylenimine [124] have been utilized to functionalize MWCNTs to improve their heavy metal adsorption. High adsorption capacities with polydopamine functionalization have been reported as 318.47 mg/g for Cu2+ and 350.87 mg/g for Pb2+.
Clay and clay-based compounds are other common adsorbents that have been extensively utilized in the removal of heavy metals from wastewater. We have recently published a review article discussing use of clay-based materials in heavy metal removal. The article can be accessed through [125].
4.3.1. Zeolites
Zeolites are crystalline, porous, and hydrated aluminosilicate minerals, characterized by specific molecular pore size [126]. When occurring naturally, they are found in temperatures often below 200°C in basaltic cavities, crystalizing due to hydrothermal alteration or diagenetic processes [126]. Zeolites possess three-dimensional structures that arise from polyhedral [SiO4]4- and [AlO4]5- polyhedral (Figure 2). They are capable of both facile and reversible cation exchange, acting as molecular sieves due to their tetrahedral structure, as shown in Figure 2 [128].
M represents a cation, with valence n, whose purpose is to balance negative charges due to aluminium tetrahedral, x and y are the stoichiometric coefficients of alumina and silica, is the number representing the water molecules. M is often an alkali or an alkali Earth metal.
The crystalline structure gives them fascinating properties, such as shape selectivity, ion exchange, sorption capacity, catalytic activity, and host for advanced materials. The sorptivity of zeolites depends on the opening of the pores and the volume of the voids in the material. The ion exchange of zeolites depends on cation sites nature and how accessible they are for exchange [129].
In heavy metal adsorption studies, natural zeolites such as mordenite, chabazite, and clinoptilolite have been on major focus with performance comparative studies. However, the cation exchange capacity and heavy metal adsorption capacity of natural zeolites are predetermined and therefore limited by natural processes, leaving little room for modification. Synthetic zeolites show flexibility since their topology, pore distribution and size, and cation type in the zeolite framework, and the particle size can be controlled during synthesis [127, 130]. Framework and structure of basic zeolite elements is given in Figure 3.
The arrangement of atoms on the zeolitic molecular structure (Figure 3) gives the lattice a negative charge when Al3+ substitutes Si4+ in an isomorphous fashion [132]. The zeolite solid achieves electroneutrality through counteractions that reside in hydration water in the voids of the zeolite [131].
The framework of a zeolite describes the connectivity of the tetrahedral arranged atoms to achieve the highest form of symmetry. In describing the framework, a 3-letter code is assigned that is in accordance with the International Zeolite Association (IZA), and the codes are usually derived from the zeolite name, that describes the ‘kind of material’ [133]. For example, faujasite has the code FAU, and the MFI code represents zeolite Socony Mobil-Five (or ZSM-5). Common synthetic zeolite types include zeolite A, X, B, Y, and SZM-5, among others. In order to describe a zeolite structure, the framework is described first in terms of the dimensions of the channel system and the opening sizes of the pores. The ring size defining the pore characterizes the pore openings and usually designated the n-ring label, whereby n stands for the number of O- and T-atoms in a ring. Thus, an 8-ring framework has a pore width of about 0.41 nm and is considered microporous, 10-ring with a pore width and considered mesoporous, and 12-ring with 0.74 nm pore width and considered microporous [127]. A few structural features such as channels, cages, sheets, and chains are shared among zeolite frameworks as shown in Figure 4.
(1) Distribution and Occurrence of Natural Zeolites. Zeolites occur within different types of rocks, of differing ages and in diverse geological environments [134]. They are naturally formed as a result of a solid material reacting with its own pore water. Zeolites have been found to be deposited as cavity fillings in altered volcanic rocks [135]. They have also been found in sedimentary rocks of marine origin, formed as a result of the alteration of volcanic glass [126]. Metamorphic rocks have also been reported to contain zeolites formed under extreme geothermal conditions at great depths [131]. The occurrence of some minerals in the zeolite group is summarized in Table 2.
According to Table 2, it can be seen that most rocks in which zeolites occur are generally volcanic and sedimentary rocks. When they occur in volcanic rocks, they occur as cavity fillings often as vapor or fluid deposition, while in sedimentary rocks, they are often alterations of volcanic glass [142].
(2) Properties of Zeolites. Zeolitic minerals exhibit adverse chemical and physical properties depending on their type. In terms of color, they can be transparent or appear in varying colors. In pure form, zeolites will often appear transparent or colorless and brownish reddish to greenish if they contain impurities [143].
Most zeolites exhibit density in the range 2–2.3 g/cm3, although it is not uncommon to observe density values ranging 2.5–2.8 g/cm3 especially those with abundance of Ba atoms, like Brewsterite. Bulk specific gravity has been documented to range between 0.80 and 0.90 g/cm3 [144].
Zeolites exhibit massive ion exchange capacities. The cation exchange capacity (CEC) of zeolite minerals is often in the range 200–300 cmol/kg, although it can reach up to 400 cmol/kg. The value of the CEC is dependent upon factors such as the pH, temperature electrolyte concentration in the solution, structural characteristics of the zeolite, dimensions and the shape of new cation, and the anionic charge density in the framework, among others [145].
Zeolites also exhibit a unique property and adsorption selectivity, which is greatly defined by the diverse ions and molecules with varying sizes trapped within the zeolite pores. This parameter is responsible for the novel molecular filtration capabilities of zeolite, which are dependent on the structure of the zeolitic crystal and the separation capacity that is based on polarity, shape, or size, thus making the zeolite a selective adsorbent [144].
The catalytic properties of zeolites arise in response to the distribution of various acidic regions in the crystalline lattice of the zeolite. In addition to this, the size of both the internal cavities that act as reaction chambers and the surface resources have an effect on the catalytic activity of zeolites. Any zeolite is still able to affect reaction selectivity through molecular traffic control, transition state selectivity, or selectivity of the product or the reaction [146].
Other important properties of zeolites include thermal stability that is about 1000°C, structural stability against acid, alkaline and radioactive environments, refractive indices ranging 1.47–1.52, and water adsorption capacity in the range 45–75 ml/100g [143].
Generally, a zeolitic material will be adverse in color depending on impurity levels and show a density in the range of 2–2.8 g/cm3 with specific gravity values ranging 0.80–0.90 g/cm3. The material will also portray high CEC values of up to 400 cmol/kg and great selectivity in its ion adsorption [147–149].
5. Synthetic Zeolites
Naturally, zeolites have been formed out of the reaction of volcanic ash and basic lake water in a process lasting thousands of years [150]. Synthetic zeolites are fabricated in a simulated hydrothermal process, temperature, and pressure using synthetic silicates or natural materials [151]. Appropriate equipment, energy, and uncontaminated substrates are required to achieve quality synthetic zeolite. In this regard, it is important to note that the synthesis reactions can incur costs that directly influence the price of the product, in which case would be uneconomic. The recent focus in the field of synthetic zeolites is, therefore, reaction cost reduction [152]. This is achievable by using waste or natural raw materials. High silica natural materials such as diatomite, halloysite, clay minerals, pumice, and volcanic glasses have therefore been in the spotlight for zeolite synthesis [153].
Depending on the silica content, synthetic zeolites are distinguished as low silica (with Si/Al ∼ 1–1.5), intermediate silica (with Si/Al = 2.5), and high silica (with Si/Al > 10), a parameter that directly reflects the specific properties of the zeolite [154]. A low Si/Al ratio translates to increased ion exchange capacity for the material and high adsorption capacity towards polar molecules. A high Si/Al ratio translates to high catalytic activity, hydrothermal stability, and increased hydrophobicity [144].
5.1. Synthesis of Zeolites
5.1.1. Factors Affecting Zeolite Synthesis
To achieve high-quality zeolites, years of practical experience have outlined some important factors to consider, namely, the silica to alumina ratio, reactants composition, aging, temperature and time, crystallization, and alkalinity [155]. The composition of the reaction mixture, which incorporates the silica to alumina ratio, the inorganic cations, and OH-ions, requires close attention to achieve highly customized zeolite compounds [156]. Increasing the Si/Al ratio affects the material’s physical properties, while OH− affects the mobility of the silicates from the solid phase to the solution, thus affecting nucleation times [157]. The inorganic cation has the most important role in balancing the charge of the framework and acts as agents directing the structure, thereby directly affecting the product yield and crystal purity. In the synthesis of hierarchical Y zeolites, Feng et al. [158] studied the effect of NH4HF2 etching on the formation of zeolitic crystals with X-ray diffraction (XRD) data showed enhanced peaks for NH4HF2-etched zeolite, which demonstrated the effect of reaction mixture composition on zeolite synthesis.
The nature of reactants and the level of their pretreatment is another important factor. Since the synthesis process involves the utilization of both organic and inorganic precursors, inorganic precursors produce more hydroxylated surfaces, as organic precursors incorporate metal ions into the network [159]. In the synthesis of zeolite W, the researchers used natural zeolite waste. With thermal treatment at 400–900°C, the crystalline zeolite formed remained below 36.5%. However, when they tried alkali fusion treatment at 600°C, a larger amount of Al species was activated and less than 40% of silica species. In this way, more zeolite W crystals were formed [160].
Temperature affects nucleation and rate of crystallization in the synthesis of zeolites. The crystallization rate is usually directly proportional to the temperature change. The nucleation rate, on the other hand, exhibits an inverse proportionality to the temperature change [161]. Zhao et al. [162], in a study to characterize the synthesis of high silica Y zeolites, investigated the effect of precursor pretreatment in the zeolite synthesis with NMR data showing higher zeolite crystal formation for precursors aged at low temperatures.
Reaction time is a factor that is often managed by the crystallization process. The crystallization is adjusted to minimize other phase production, while minimizing the time required to achieve required crystalline phase [163]. In a study investigating the synthesis of zeolite SSZ-13 from coal gangue, the researchers reported gradual increase in synthesis time up to 36 hours [164]. In a similar study, Krachuamram et al. [165] investigated the effect of aging time on crystallites formation and reported an optimum aging time of 3 days for NaX zeolites synthesis.
In summary, the composition of the reaction mixture determines the texture and often the duration of zeolite synthesis. Pretreatment of the precursors can determine the failure or success of zeolite formation. The temperature is often defined by the synthesis method, but plays a role in the nucleation of the zeolite crystals. The reaction time needs to be closely monitored as different zeolites achieve maximum formation within certain time ranges, beyond which crystals degrade.
5.1.2. Methods Applied in the Synthesis of Zeolite
Zeolites are synthesized by heating aluminosilicate raw materials for a period of time that spans hours or days (depending on the type of the raw material and the process conditions) in the presence of high pH solutions [153]. Common methods currently applied in zeolites synthesis include hydrothermal synthesis [166], microwave-assisted synthesis method [167], molten salt [103], activation of alkali fusion activation [168], and synthesis through dialysis [153].
The hydrothermal synthesis method usually simulates the natural conditions through which rocks containing zeolites are formed. Hydrothermal temperatures of 80–350°C are involved in treating aluminosilicate raw materials in an alkaline solution with pH above 8.5 [169]. The reactions that occur (dissolution, gelatinization, condensation, and crystallization) are conducted in an autoclave at elevated pressure [170]. Careful alteration of process parameters gives products with desired properties [153]. Garcia-Villén et al. [166] used the hydrothermal synthesis method to synthesize zeolite using waste from sanitary ware. Researchers used 5M NaOH alkaline solution at temperatures 100, 150, and 200°C for up to 30 days and reported the transformation of quartz and mullite minerals to zeolite. Luo et al. [171] synthesized needle-like zeolites from metakaolin using the same method for utilization in the removal of organic and heavy metal contaminants. Hydrothermal synthesis is a common method in the synthesis of zeolites. It, however, suffers limitations as high cost, especially with the use of high-pressure autoclaves and produces excessive waste, making it environmental unfriendly [172].
Molten salt method for zeolite synthesis was proposed to address the shortcomings of hydrothermal method such as high ratio of solution to solid and low product yields [173]. In the molten salt method, synthesis takes place under molten conditions without the addition of water. The high-silica raw material, a salt, and a convenient alkaline solid are finely ground and melted at convenient temperatures for extended periods of time. They are then cooled, crushed and washed. Common bases used include KOH, NH4F, and NaOH [174, 175]. Common salts used include NaNO3, KNO3, and NH4NO3. This method was often used to synthesize fly ash-based zeolites [176–178]. Since the molten salt method required high temperatures for the synthesis of zeolites, the most serious limitation associated with the method was the high energy consumption rates, often worsened by the duration of synthesis of the material [179]. Recently, authors are using modifications of the method to reduce energy consumption cost through submolten salt (SMS) synthesis. In SMS technology, high-alkaline (above 50%) and high-boiling-point precursors are used to ensure reaction conversion and mass transfer while keeping temperatures low, under ambient pressures [180]. Meng et al. [181] used the SMS method in precursor treatment using 75% alkaline solution and an alkali to ore ratio of 3.5 : 1 at a temperature of 200°C, during the synthesis of zeolite W from potassic rocks. The resulting material achieved an exchange capacity of 156.8mgK+/g and 30.39mgK+/g in KCl and simulated sea water solutions, respectively. Similarly, Krisnandi et al. [182] synthesized ZSM-5 zeolite crystals from natural zeolites using the fragmented SMS method. Natural zeolite was pretreated at 250°C in alkaline solution. The resulting material showed the BET surface area of 262 m2/g, which is typical for a microporous zeolite.
Microwave irradiation is a modern green technique that has been used in the fabrication of zeolites. Different ratios of aluminium to silica sources and structure directors are usually mixed uniformly and irradiated by microwaves in autoclaves [183]. Parameters such as the size and shape of the autoclave, reaction time, and power and temperature of the microwave are adjusted carefully to affect the morphology, as well as the structure, of the zeolite [184]. Majdinasab et al. [185] used the microwave radiation technique in the synthesis of zeolites from pulverized waste glass cullet in comparison with convection heating. The authors reported microwave radiation being more efficient, achieving 60% maximum relative crystallinity of the zeolite. Similarly, Wong et al. [186] successfully synthesized nanocrystalline F-type zeolite using an organic template-free system by using rice husk ash as the silica source under microwave irradiation.
5.1.3. Agro-Waste in Zeolite Synthesis
Rice husk ash (RHA) has been extensively investigated as a potential silica source for the synthesis of zeolites. Many methods have been proposed for the extraction of silica from RHA, including biological treatment, chemical treatment, hydrothermal-baric, and thermal methods at elevated temperatures of 400–700°C [156, 187]. Bohra et al. used RHA alongside aluminium foil as precursors for NaA and NaP zeolite synthesis in two different studies showing a greener approach in the synthesis of zeolites [188]. Most researchers have focused on RHA as an agro-waste source of silica as seen in works of Mallapur et al. [189]; Bohra et al. [190]; Alaba et al. [191]; and Min et al. [156]; among others.
Sugarcane bagasse fly ash (BFA) has also been utilized as a low-cost precursor for zeolite synthesis. BFA is a rich source of alumina, and silica minerals needed to achieve high CEC zeolitic materials. Oliveira et al. [192] synthesized calcined sugarcane BFA and zeolite NaA using the hydrothermal synthesis method for application in the removal of copper ions from wastewater.
Several other agrochemicals that have been utilized in the synthesis of zeolites are summarized in Table 3.
5.1.4. Clays in Zeolite Synthesis
Kaolin-based zeolites have been synthesized in an effort to achieve affordable zeolitic products. Kaolin clay mineral, illustrated in Figure 5, consists of silica tetrahedral joined to alumina octahedral in the ratio of 1 : 1 through shared oxygen atoms. Kaolin-based zeolites will often have Ca, Mg, Fe, Ti, etc., contaminants originating from natural kaolin, which may affect the properties of the final product [199]. Kaolin is often calcined to temperatures ranging from 550 to 950°C to achieve reactive metakaolin for utilization in the synthesis [163].
Fortunately, kaolinite is not the only type of clay mineral utilized in the synthesis of zeolites. Ltaief et al. [200] synthesized faujasite zeolite from Tunisian illitic clay exhibiting hierarchical porosity. Researchers used the molten salt synthesis method using NaOH in muffled furnace. Zeolite crystallization occurred at 60°C in 24 hours. The resultant material was used in adsorption of various heavy metal ions, and removal capacities were reported to be 126 mg/g (Cu2+), 125 mg/g (Co2+), and 98 mg/g (Cr3+). These findings were higher than the commercial faujasite adsorption capacities for the respective metal ions as 120 mg/g, 118 mg/g, and 91 mg/g. This study illustrated a low-cost zeolite material with better efficiency in heavy metal adsorption.
A study by Medina-Rodríguez et al. [201] used Ecuadorian clay to synthesize zeolite X through alkaline fusion and hydrothermal treatment. Sodium aluminate and NaOH were used to fine-tune the properties of the zeolite at synthesis. The resulting zeolite X was reported to have a specific surface area of 376 m2/g, which was 30 times higher compared to clay (12 m2/g). The zeolite was used in the removal of Pb2+ and achieved an adsorption capacity of 24 mg/g, much higher compared to that of clay (13 mg/g).
Moneim and Ahmed [202] synthesized low silica NaX-Faujasite zeolite from different types of Egyptian clays (kaolinite, smectite-kaolinite, and smectite-rich clays) using a combination of hydrothermal synthesis and alkali fusion methods. Crystallization conditions were 100° Cand 48 hours for smectite-kaolinite clay and 72–96 hours for the kaolinite and smectite-rich clays. The authors reported that the smectite-kaolinite clay had better crystallization rates compared to the rest. Zeolite was applied to remove Cr3+, Ni2+, and Mn2+, and the percentage removal was reported to be 100%, 80%, and 75%, respectively.
Gaidoumi et al. [203] reported the successful synthesis of zeolite HS by using natural pyrophyllite clay as raw material. The clay was treated with alkaline, and the resultant material is characterized by X-ray fluorescence and Fourier transform infrared, among other techniques. The authors reported that the resultant zeolite was nearly pure. Aghaei et al. [204] alkali-fused the same clay to synthesize zeolite Y and reported improved acidity and textural properties compared to commercial zeolite Y. Foroughi et al. [205] combined illite, pyrophyllite and kaolinite clays to synthesize zeolite A through fusion technique and reported mesoporous zeolite structure which contained a surface area of about 59.6 m2/g and an average pore size of 8 nm.
Joseph et al. [206] synthesized multiple zeolites using vermiculate-kaolinite clay through the alkaline fusion method. The clay mixture achieved the synthesis of faujasite-based zeolite without pretreatment. On thermal activation, the clay was able to produce gismondine zeolite GIS-NaP1. When it was pretreated by acid leaching, the clay produced mixed-phase quartz and Na-P1 zeolites.
6. Characterization of Zeolites
6.1. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)
Scanning electron microscopy technique is used to analyze surface morphology and topology of zeolitic materials by focusing a beam of low energy electrons on the sample. Interactions of the beam and the material led to emission of electrons and photons that are detected and take part in the formation of SEM image [207]. EDS is useful in the chemical analysis of materials. It is typically equipped with SEM and makes use of X-rays to identify the chemicals present in a sample [208]. Studies in which SEM analysis has been utilized in zeolite characterization are summarized in Table 4.
6.2. Brunauer–Emmett–Teller (BET)Analysis
BET analysis is an important analytical technique in the characterization of zeolites as it gives data regarding adsorption mechanism of the material. By using probing gases, the BET theory seeks to provide information on the specific surface area of multilayer pore systems to understand the adsorptive characteristics of a material [213]. Nitrogen is a common probe gas, although others can be used, as long as they do not chemically react with the surface of the material analyzed [214]. Previous studies whereby BET analysis has been utilized in zeolite characterization are summarized in Table 5.
6.3. X-Ray Diffraction (XRD)
XRD technique is extensively used in the analysis of crystalline compounds, more so in the characterization of zeolite compounds. Data resulting from XRD gives insight into the texture, phase, grain size in average, crystal defects, and strain, among others. This is usually possible through the interference of X-ray beams by lattice planes available in a sample [219]. Crystalline samples are identifiable by referencing a standard database for XRD patterns [220]. Previous studies in which XRD analysis has been used in zeolite characterization are summarized in Table 6.
6.4. Fourier Transform Infrared (FTIR) Spectroscopy
FTIR spectroscopy identifies materials by analyzing the vibrations depicted by chemical bonds within a material’s molecular structure [222]. Different chemical structures absorb infrared radiation at different wavelengths giving rise to a peak and therefore identification [223]. The chemical environment is important for understanding the functional groups in the material [224]. FTIR analysis is necessary to understand how effective the zeolite is in the removal of heavy metals. FTIR spectroscopy is able to analyze mid-IR region between 5000 and 400 cm−1 and near-IR region between 10,000 and 4000 cm−1 [225]. Several studies have utilized FTIR analysis in zeolite characterization with different chemical groups as summarized in Table 7.
6.5. Thermogravimetric Analysis (TGA)
TGA’s sole purpose is identification of a material’s thermal stability and percentage of volatile compounds in its structural composition, usually through monitoring the change in the material’s weight as heating occurs in a predetermined rate [230]. In zeolite studies, TGA is useful in analysis of a zeolite’s amount of pore water. Ezzeddine et al. [231], in a study to utilize zeolite NaX in heavy metal adsorption, used the TGA technique to investigate the water content and absence of the organic template used during synthesis. Authors reported about 26% water loss between 25 and 900°C. Similarly, in a study conducted by Oliveira et al. [192], weight loss was initially observed up to 200°C and attributed to water loss. Additional weight loss was observed between 350 and 580°C and attributed to elimination of carbon and other organic residues. Other works utilizing TGA analysis have been summarized in Table 8.
7. Adsorption Studies
7.1. Adsorption Isotherm Models
Adsorption isotherms show the relationship between the quantity of the solute that has been adsorbed and the solute’s concentration in the liquid phase [131]. Understanding this relationship is important to examine the performance of zeolites in heavy metal adsorption. Different models have been proposed and used over the years to understand this behavior. Some of these methods are discussed.
The Langmuir adsorption isotherm is modeled with the assumption that the maximum adsorption corresponds to saturation of the monolayer adsorbate surface. This implies that all sites of adsorption are identical, and the adsorption energy is constant. The Langmuir isotherm is therefore described by the linear equation [234].where represents the quantity of adsorbed metal ion in (mg/g), as the residual ion concentration at equilibrium, and and , Langmuir’s constants for sorption capacity and energy, respectively. should vary with temperature since it is a representation of the enthalpy of adsorption [131].
The Freundlich isotherm is structured with the assumption that surface adsorption energies are irregular, implying heterogenous nature of adsorption [235]. This model can therefore be described by where represents solute’s adsorption per unit mass of the adsorbent at equilibrium in mg/g, is residual ion concentration in the liquid phase at equilibrium, and is the Freundlich constants determined by the conditions of the experiment [131]. The equation can be rewritten in linear form as
When the value of falls below 1, this is an indication that the adsorption is heterogenous. Values near or equal to 1 indicate a material exhibiting somewhat homogeneous binding sites in relation to its porosity [236]. The utilization of these models in the adsorption analysis of some zeolites has been summarized in Table 9.
Lesser common isotherm models in zeolite metal ion adsorption studies such as Dubinin–Radushkevich (D-R) and Temkin isotherm models exist. D-R isotherm assumes filling of pores in the adsorption process in a multilayer character, involving van der Waals forces [234]. This model is often applied in the expression of Gaussian energy of adsorption distribution upon heterogenous adsorptive surface [243]. It is, however, unrealistic at low pressures since under such conditions, it does not follow Henry’s laws. It is therefore most suitable for intermediate adsorbate concentrations [244]. Its best application in metal ion adsorption is differentiation between physical or chemical adsorption mechanism of adsorbates. D-R is described as follows [30]:where e is Polanyi’s constant, ß is D-R constant, R is the gas constant, T is the absolute temperature in Kelvin, and E is the mean adsorption energy.
Temkin adsorption isotherm considers the indirect effect multiple adsorbents might have on each other (adsorbent/adsorbent interaction), upon the process of adsorption. This isotherm is governed by the assumption that the adsorption heat (ΔHab) of all interacting species decreases with increase in surface coverage. This model is therefore only valid for intermediate adsorbate concentrations [234]. It is given bywhere b (j/mol) and Kt (L/g) represent Temkin constants, calculated from the slope and intercept of the plot of Qe versus ln Ce, respectively [244].
Most zeolite-based adsorption isotherms follow Langmuir model according to Table 9. This translates that the metal ion adsorption is a monolayer adsorption, according to the high R2 values. The few that follow Freundlich isotherm model such as Cu and Cd adsorption by fly-ash synthesized zeolite X [239] imply a multilayer adsorption criterion.
7.2. Adsorption Kinetics
The pseudo-first-order (PFO) model, first described by Lagergren, is described by the linearized equation [245].where is adsorbed adsorbate at time t in mg/g, is adsorption at equilibrium in mg/g, and is rate constant per minute. To determine the value of , a plot of against t is drawn.
The pseudo-second-order (PSO) model has the assumption that solute adsorption is proportional to active sites on the adsorbent. Therefore, the rate of reaction is dependent on solute amount on the surface of the adsorbent. The PSO equation is illustrated as follows:where is a PSO rate constant. Applications of these modes in adsorption studies for synthetic zeolites are summarized in Table 10.
Other lesser common kinetic models in the adsorptive studies of metal ions in zeolite materials include intraparticle diffusion and Elovich models. Intraparticle diffusion model is common for a process whereby there is an increase in the rate of adsorption with increase in temperature, common in chemical adsorption processes. This model is illustrated by where Kp and C are intraparticle diffusion constants. A plot of Q versus √t is used in estimation of the diffusion process. If the plot is observed as a straight line, the adsorption is predicted to involve at least two stages of intraparticle diffusion. This model can also be used to predict the thickness of the interface between the adsorbate and adsorbent [30].
Elovich model, given in equations(12) and (13) [30], is generally applicable to heterogenous-based chemisorption process, and the rate of desorption is negligible. This model is often used in the prediction of surface diffusion and mass and the activation or deactivation energy of an adsorbent [251].
If , thenwhere ß and α are Elovich constants estimated from the plot of Q versus ln (t).
7.3. Thermodynamics of Adsorption
An adsorption process is largely affected by the temperature of the reaction medium. Thermodynamic studies are used focused on studying the behavior of the adsorbent efficiency at different temperatures by analyzing Gibb’s free energy, enthalpy, and entropy of the reaction. The formulae used for this analysis are [53]where is the constant of thermodynamic equilibrium, R is gas constant, T is the absolute temperature, and and are the concentrations of the adsorbate on the adsorbent and residual concentration at equilibrium, respectively [53].
An endothermic adsorption is given by a positive enthalpy value. A negative Gibb’s free energy value implies thermodynamically spontaneous and sustainable at diverse temperatures. A positive entropy value gives an insight that there is randomness at the liquid-solid interface in the adsorption [53]. Thermodynamic studies on various synthetic zeolites are summarized in Table 11.
From Table 11, it can be inferred that the majority of zeolites give a positive Gibb’s free energy value. This implies that, for the most studies summarized, the adsorption of the heavy metals is not thermodynamically spontaneous and therefore not diverse over a large range of temperatures. Zeolite Y, however, synthesized by Liu et al. [255] differed and gave negative Gibbs free energy for both copper (II) and nickel (II) ions adsorption. This implies that the adsorption is feasible over a range of temperatures and therefore sustainable. Zeolites Na-P1 [252], X [253], and Y [256] give positive enthalpy values. This implies endothermic adsorption process (physisorption). The negative enthalpy of 11Å-tobermorite [254] zeolite implies an exothermic adsorption process, which implies a chemisorption process in the metal ions adsorption. Zeolite X and Y give positive entropy values, while 11Å-tobermorite and Na-P1 give negative values for the metal adsorption processes. A positive entropy value implies increased randomness at the solid-liquid interface, and thus, high metal ion adsorption is expected in such systems.
8. Conclusions
Although several methods exist for removal of heavy metals from wastewaters, heavy metals in waters still pose a global health concern. We therefore conclude the following:(1)Most low- and middle-income countries have low uptake of existing methods of heavy metal removal from waste waters such as ion exchange, reverse osmosis, and electrodialysis due to high costs of installation, operation, and maintenance.(2)Low-cost zeolites are efficient heavy metal adsorbents and allow modification to simultaneously remove both organic and inorganic water contaminants.(3)Utilization of agro-wastes in the synthesis of zeolites for the removal of heavy metals is biased towards rice husk ash and bagasse ash. Other kinds of agro-wastes have not been explored.(4)Hydrothermal synthesis of zeolites remains to be the most dominant method of zeolite synthesis even though other greener methods exist.(5)Clay minerals have seen tremendous utilization in the synthesis of zeolites due to high silica to alumina ratios. Kaolin is especially a favorite among researchers, due to the formation of metakaolin, which is highly reactive and rich in aluminosilicates.
9. Areas of Future Research
Based on the reviewed works herein, the authors recommend following potential areas of future research:(1)Cost-benefit analysis of clay and agro-waste zeolite application in heavy metal removal from waste waters(2)Mechanism of removal of heavy metals and other contaminants by zeolites from contaminated water(3)Incorporation of zeolites into other structures to form composite materials for water decontamination(4)Impact of interfering elements on the removal of heavy metals by zeolites(5)Criteria for eligibility of an agro-waste material as a raw material for zeolite synthesis(6)Influence of synthesis methods on the cost, structure, application, and reproducibility of zeolites(7)Influence of raw material source, type, and environment on the structure of synthesized zeolite(8)Acceptability, adoption, and utilization of green chemistry methods of synthesis of zeolites, in developing countries(9)Optimization of thermodynamic models for the analysis of all processes in zeolite heavy metal adsorption
Data Availability
Data cited are included in the cited articles.
Conflicts of Interest
The authors declare no conflicts of interest in the publication of this article.
Acknowledgments
The authors of this work acknowledge every author of the works cited hereof and thank them for their contribution to the compilation of this review. The authors acknowledge Meru University of Science and Technology Library for providing access to scholarly repositories for articles cited in this work.