Kabul city (the capital of Afghanistan) is one of the fastest-growing cities in the world, with an estimated 4.3 million population in 2020. Kabul city does not have access to any general wastewater treatment system and superficial water and groundwater resources in Kabul city are at risk, due to unsustainable and uncontrolled groundwater abstractions and surface and groundwater polluted biologically and chemically by several types of sources. Hence, most groundwater sources in Kabul are unfortunately contaminated with human coliforms. Thus, the poor condition of groundwater in Kabul city and pollution caused some serious health concerns in the city. According to a report published on the BBC website on 10 Aug 2017, the leading cause of illnesses in Kabul is the use of contaminated water and also the doctors of Antani Hospital as saying that the leading cause of epidemics of more than 70 percent of patients who registered in the hospital is drinking contaminated water[1].

Environmental pollution is one of the most important problems of the modern world[2], this contamination has always been a public concern, particularly water pollution. Metal-contaminated wastewater that originates from anthropogenic activities in numerous sectors, has always been a threat to humans, public health, and the environment[3]. This wastewater commonly includes Ni, Cu, Cd, Cr, and Pb [4] Though heavy metals are normally present in trace amounts, they are considered the most toxic and prevalent components in wastewater sewage. These heavy metals are not absorbed by the body leading to accumulation in soft tissues which is hazardous to human health, consequently, they must be removed from the contaminated streams to meet progressively strict environmental superiority standards [3-5].Removing heavy metals from manufacturing wastewater essential high energy or special operative requirements[6]

There are various treatment processes available for metal-polluted waste streams, such as chemical precipitation, coagulation, solvent extraction, electrolytic processes, oxidation, membrane filtration[7], ion exchange[8], reverse osmosis[6], disambiguation, clotting[9], electrochemical treatment technologies[10], photocatalytic degradation and ads among these techniques, adsorption as a cost-effective technology, offers flexibility in design and operation and, in many cases, it will generate a high-quality treated effluent[7]. Nevertheless, the application of these methods becomes expensive at lower applications but at low concentrations, adsorption, and ion-exchange progressions by zeolites have advantages on removal ability and effectiveness, and cost[8]. 

 This review presents a brief view on the quantification of the benefits of zeolites as effective adsorbents for heavy metal removal in wastewater treatment of Kabul city. The major objectives were: (a) the Benefits of wastewater treatment by using zeolite: and (b) the Environmental effectiveness of wastewater treatment in Kabul city. This review provides a suitable solution for wastewater treatment in Kabul city.

1. Wastewater treatment 

Water is one of the most valuable resources on the planet, and its availability and quality are essential for sustaining life and promoting development. 

Figure1. Indicate the water source and underground water condition in Afghanistan. Ref [11]

However, in many regions of the world, including Afghanistan, water resources are under increasing pressure due to a range of factors, including climate change, population growth, and unsustainable management practices as shown in figure 1.

Some reports provide an in-depth analysis of the current state of water resources in Afghanistan and the potential hazards related to rapid climate warming. The authors explore the various challenges faced by the country and discuss the potential impacts of climate change on water resources. And also suggests adaptation strategies that could help mitigate the risks and ensure the sustainable use of water resources in Afghanistan. By shedding light on the water-related challenges faced by the country and proposing solutions, due the climate change, decreasing of the underground water, and increasing in the population in the capital Kabul city, adoption of the heavy metal water in Afghanistan especially in Kabul is urgent.  A report published in the Washington Post in 2007 shows many children waiting to take drinking water figure 2.

Figure 2. Afghan men and children fill water containers from a tap in Kabul in august. Photo parched from Washington post September 4, 2017.

https://www.washingtonpost.com/world/asia_pacific/in-kabul-access-to-safe-drinking-water-is-a-matter-of-money/2017/08/31/714ea228-8124-11e7-9e7a-20fa8d7a0db6_story.html 

Wastewater is waterborne solids and liquids discharged into sewers that represent the wastes of community life which includes dissolved and suspended organic solids, which are "putrescible" or biologically decomposable [12]. Wastewater is usually composed of cationic pollutants, anionic compounds, organic compounds, and oils that can have toxic effects on the environment[13,14]. Currently, the world is faced with increasing difficulties in high-quality drinking water and for removal of pollutants from municipal, agricultural, and industrial wastewater [15]. The worldwide water withdrawal quantities by the three major sectors (i.e. agriculture, industries, and municipalities) and especially for each continent are displayed in Figure 3. As indicated in the graph, Africa and Asia noticeably discharge the most worldwide agricultural wastewater effluent, which represents America and Europe the least wastewater withdrawn compared to the municipal and industrial wastewater discharge. On the contrary, the industrial sector in Europe signifies the highest discharged wastewater compared to all other continents, according to the water statistics reports, the total amount of water available on earth estimated to be 41,000 km³ that is globally decreases each year. Only 2.5% of the renewable freshwater is appropriate and could be used in the different sectors (irrigation, industrial and domestic). Despite, the total worldwide population requirements of water are annually at an increasing rate (1.14%). Currently, the entire amount of available water does not satisfy the human applications. Henceforward, more efforts are required towards the development of maintainable water treatment technology to solve the existing water situation to introduce a high yield of freshwater [16]

Figure 3. Indicative diagram showing the global quantity of different wastewater types produced yearly at different continents, (results based on estimated quantities from the year 2015).

The adsorption process plays a critical role in water and wastewater treatment technology. Various techniques have been developed to enhance the effectiveness of these processes, typically by chemically modifying the adsorbents. However, these methods often result in additional waste or sewage being introduced into the environment, which is undesirable. To address this issue, one of the effective methods for adsorption of the heavy metals reported by Rajczykowski et all, investigates the effect of a strong magnetic field on the adsorption of heavy metals onto magnetic adsorbents figure 4. The authors synthesized magnetic adsorbents by co-precipitation and characterized them using various techniques. Then conducted batch adsorption experiments with and without the application of a strong magnetic field to evaluate the adsorption capacity of the magnetic adsorbents for lead ions. The results showed that the magnetic field significantly enhanced the adsorption capacity of the magnetic adsorbents for lead ions. This enhancement is due to the magnetic field increasing the collision frequency and kinetic energy of the lead ions with the magnetic adsorbents. The study concludes that the use of a strong magnetic field could potentially improve the efficiency of heavy metal adsorption processes in wastewater treatment.

Figure 4. shows using of the magnet for heavy metal adsorption parched from ref [17]

With the fast development of various industries, a huge quantity of wastewater has been produced from industrial processes and was discharged into soils and water systems wastewater collection, in a proper way, as well as its treatment and disposal, is one of the primary hygienic needs of any society [1] The existence of heavy metals in wastewater due to many manufacturing processes is a legalized problem worldwide, since removing heavy metals from industrial wastewater required high energy or special operational supplies. Thus, several techniques such as adsorption, extraction, disambiguation, clotting, ion exchange, and membrane processes are theoretical for the handling of wastewater pollution. The adsorption method forms an appropriate method for wastewater handling because of its cost-effectiveness and simplicity, among all these methods[6]. In addition, certainly occurring zeolites hold great potential to use as packing material in subsurface reactive barriers intercepting groundwater plumes and for fixed bed reactors designed to remove heavy metals from industrial wastewater[9].

Zeolites were first identified in 1756 and at present, 253 different zeolite framework types have been described in figure 5. They are defined as silica-based crystalline microporous solids in which some of the silicon atoms are replaced by other elements T (either trivalent or tetravalent elements)[10-18]. The term zeolites are derived from the Greek Zeo (boiling) and Lithos (stone), referring to the interesting observation made by Crested of the natural mineral stilbite, the first zeolite discovered by him, which releases steam when subjected to heating in a flame. Today, it is known that the effect occurred due to the desorption of water, which is present inside its channels and cavities [13]  Zeolites are aluminosilicate minerals of alkali or alkaline earth metal which contains crystal water. Zeolites consist of three-dimensional networks of aluminate and silicone dioxide tetrahedral linked by a partnership of all oxygen atoms. The voids make up from 20 to 50 % of the total crystal volume of most zeolites and their exceptional properties, such as adjustable acidity, thermal stability, and diverse pore networks, allow assorted industrial applications, encompassing ion exchange, catalysis, adsorption, and separation processes [6,19]

Figure 5. Different types of Zeolitic framework structures and their pore sizes[13].

Zeolites are porous crystalline alum inosilicate materials [20-22], regarded as one of the best supports due to their unique structure, uniform pores, and channels, high surface area, high adsorption capacity, inexpensive and environmentally benign nature, good thermal and mechanical stability[23]. Zeolites can be obtained naturally from ores or synthesized from chemical reagents via different routes[21,24]. Generally, zeolites are prepared under hydrothermal conditions with temperatures ranging from 80 °C to 190 °C. Currently, zeotype materials, which are composed of elements other than Si and Al, have also been synthesized and synthetic zeolites and zeotype materials with different structures and compositions have been applied in diverse industrial fields as catalysts, adsorbents, and ion exchangers[25]. Zeolites can sort molecules selectively based on a size exclusion process. This is due to a very regular pore structure of molecular dimension [26]

Zeolites have a framework silicate with a three-dimensional cage structure and their structure bears negative charges which can balance by the adsorption of exchangeable cations. Due to the high deposit of zeolite in nature, its low cost, stable structure, even in an acidic environment[9], ^and strong adsorption capacity[27], it has recently drawn attention to use in the treatment of industrial wastewater. According to these attractive properties of zeolite, interest has been increased in the method of heavy metal adsorption from aqueous solution. There are several studies carried out on the effectiveness and the importance of using natural zeolite for the adsorption of heavy metals under different empirical conditions such as pH (The solution pH is one of the crucial parameters in the sorption of heavy metals by zeolites, because of its effect on the charge state of metals and also adsorbent surface. The effect of pH on studied metals uptake by zeolite was evaluated and shown in Figure 6, temperature, granular phase volume, concentration, and induction speed[9]. Table 1 presents the adsorption capacity of various metal ions on different natural zeolites[14].

Zeolites are crystalline materials that are composed of tetrahedral TO₄ (T = Si, Al, among others such as P, Ge, Ti, Fe, and Ga) interconnected by oxygen atoms. The combination of tetrahedral units of silicon (Si⁴⁺) leads to a neutral structure, while the substitution of a Si⁴⁺ atom for an aluminum (Al³⁺) atom causes an unbalanced charge that needs to be compensated by cations, such as Na⁺, Ca²⁺, K⁺, and organic cations such as tetramethylammonium TMA⁺. Zeolites have uni-, bi-, or tridimensional channels, which can be interconnected in various ways to generate solids with higher internal surfaces compared to their external surface. These channels are occupied by water molecules, which are present during the synthesis, and by the hydrated cations, which can be removed using thermal or chemical processes, and in zeolites, intercrystalline voids are maintained and, consequently, their structure. This is different from other hydrated porous materials such as CaSO₄[13].

Figure6. pH affecting on the removal of Cd²⁺, Fe⁺³ and Ni⁺² from the solution (Ci= 2 ppm; size < 0.5 mm; m=0.5g; V=50 ml; t=180 min)[9].

It's noticed from Figure 7 that increasing the external surface area by reducing the adsorbent particle size, results in an increase in the number of available sites for metal uptake, and The smaller particle sizes result in the shortening of the diffusion distance that heavy metals have to travel to get to an adsorption site, hence a faster rate of reaction. It's also observed that Cd⁺² and Fe⁺³ exposed the greatest affinity 80.9 and 79%, while Ni⁺² lowest affinities (65.2 %), and more metals were sorbet, however, by fine-grained zeolite than coarse-grained zeolite[9].

Figure 7: Adsorbent particle size affecting the removal of Cd Fe⁺³ and Ni⁺², from the solution ((Ci= 2 ppm; pH = 6; m=0.5g; V=50 ml; t=180 min)[28].

An especially interesting class of materials for environmental remediation is the surfactant-modified zeolites. These materials, which combine the enhanced cation sorption properties of natural zeolites with the ability to sorb anionic species, non-polar organic species, and pathogens from aqueous streams, are considered for applications as decontamination agents for soils and water basins, backfill and sealing materials in waste repositories and as permeable reactive barriers for the cleaning of waters. The most commonly adjusting agents are quaternary amines (e.g. HDTMA, ODTMA, N-cetyl pyridinium), which form on the zeolite surface a bilayer-like structure altering its surface charge (from negative to positive). The positive surface charge provides sites for the sorption of anions, whereas the organic-rich surface layer provides a partitioning medium for the sorption of non-polar organic compounds. The zeolite’s original cation exchange capacity is also partly retained. The zeolite particles are good carriers of bacteria, which increase the sludge activity in wastewater treatment plants that important drawback of this application is the slow formation of the bacteria layer on the zeolite surface, which does not become immediately effective (it takes almost a week). The variation of zeolites by cation-active polyelectrolytes accelerates the interaction among the bacteria with the zeolite surface further increasing the sludge activity The surfactant-modified zeolites also show the ability to bind pathogens (e.g. Escherichia Coli) from sewage[29].

Figure 8, illustrates the filtration system which is a kind of wastewater treatment using zeolite which proceeds a correct design of the filtration system deep knowledge of the locally available Zeolite medium must be carried when developing applications for Zeolites, it is important to remember that not all of these minerals are the same. Some help to assist plant growth while others make excellent filtration media, but the same Zeolite will not necessarily do both well. There are nearly 50 different types of Zeolites (clinoptilolite, which is the most common natural Zeolite), with varying physical and chemical properties (crystal structure, chemical composition, particle density, cation selectivity, molecular pore size, strength). It is important to know the specific type of Zeolite one is using to assure that it is appropriate for one's needs. Petrographic, mineralogical, and chemical characterization of the Zeolite materials that will be used in this project is a key point to deliver a successful project[30].

Figure8. The Zeolite filtration system

A draft layout of the zeolite filtration unit is reported in Figure 8. The column will be filled with a depth of coarse-grade gravel (8-10 mm diameter), followed by finer-grade gravel (3-5 mm diameter), used to support the Zeolite bed and prevent the Zeolite from being lost from the bottom. A natural Zeolite medium will be loaded into the filter and will be supplied by local Zeolite exploitation mines. The average Zeolite particle size will be greater than 1 mm. The height of the medium will be approximately 1.5 m[30].

Clinoptilolite (CPL), one of the most common natural Zeolites, is a hydrated alumina–silicate, occurring in the Zeolitic volcanic tuffs, and is widespread in many countries in the world. Chitosan (Cs), as one of the most naturally abundant and cheapest biopolymers, has been broadly investigated as an adsorbent for heavy metal removal. Moreover, it is a non-toxic, biodegradable, and biocompatible material being able to adsorb metals that can interact with metal ions of the solution by ion exchange or other complexation reactions due to the presence of amino groups on the polymer matrix⁷. The tests indicated that a metric ton (1000 kg) of St. Cloud- and Brazilian-zeolite could be used to treat 16,000 and 60,000 L of CBNG water, respectively, to lower its sodium adsorption ratio (SAR, mmol½ L-½) from 30 mmol½ L-½ to an acceptable level of 10 mmol½ L-½ which the reduction of sodium in seawater by natural zeolite and chlorine by calcined hydrotalcite was also attempted for agricultural applications[29].

Table 1: Adsorption of heavy metal ions in various natural zeolites

1. Wastewater of Kabul City

Dynamic industrial development is associated with the generation of a significant amount of waste threatening the natural environment. Given the progressive environmental devastation such actions as reasonable management of natural resources, minimization of the industrial waste stream and emissions as well as reducing the amount of solid waste deposited in landfills, become major goals for governments in many countries around the world. Thus, cleaner production arises as an important global concern of the 21st century[31]. Environmental requirements are becoming increasingly important in today's society since there is an increased interest in the industrial use of renewable resources[32]. In concern to worldwide public health, pollution of the environment is one of the major concerns. Recently, environmental pollution is a prodigious hazard to global biodiversity and the human race. The release of industrial effluents into the environment affects the quality of soil, air, and water[33].

The process of rapid urbanization is now universal as more than half of the world's population lives in cities, Kabul city is the capital of Afghanistan, located in the central-east of the country. Drinking water is unfortunately contaminated in Kabul, and most groundwater sources in Kabul are unfortunately contaminated with human coliforms[1]. 

Pit latrines cause health and ecological problems such as microbiological and chemical pollution of groundwater. The survey findings indicate that more than 70% of the households in residential areas use cesspool systems and 27.2 % septic tanks. Cesspools are one of the main sources of groundwater contamination in Kabul city because house owners usually build them inadequately the most often, cesspools are not protected which allows the infiltration of wastewater that causes diseases. According to the Ministry of Rural Rehabilitation and Development (2015), deep wells are polluted biologically and chemically in Kabul city and wastewater is caused by several types of environmental problems in Kabul city such as surface and groundwater pollution, bad odor, flies, mosquitoes and effects on environmental beauty. In Kabul city, 100% of drinking water comes from groundwater sources and wastewater is not reused; that is why groundwater is used for all domestic purposes including agriculture in Kabul city. More than 60 % of the residents still did not know that recycled wastewater could be used for agriculture and other indoor purposes such as toilet flushing[34].

According to the current situation, the norm of water consumption in the cities of Afghanistan is deemed to be 100 liters/per capita/day, and the amount of wastewater production generally takes 60-80% of water consumption. Because the waste of 20-40% of the water is found for irrigation of green areas inside the yard, car washing, etc., this waste is not connected to the municipal sewage system. But in Afghanistan, because the cost of water is higher and on the other hand, the level of society's economy is lower, and water is not used for car wash purposes inside residential houses as well as in-yard sprinklers, etc [1].

The population growth in the urban areas, the oil and goods transport, the releases from vehicle exhausts, the mining and smelting activities, the energy production, and the frequently uncontrolled, use of pesticides resulted in the accumulation of huge amounts of hazardous inorganic and organic pollutants in the environment. Severe environmental contamination has also been observed in cases of nuclear reactor accidents, explosions in nuclear waste storage facilities, and the surroundings of several military and civil fuel reprocessing plants around the world. When the concentration of the pollutants exceeds certain limits and their presence seriously endangers the environment and human health ²⁹. These environmental pollutants have the potential to induce a large range of acute and long-term effects (e.g., endocrine disruption, immunotoxicity, neurological disorders, cancers…) on human health and ecosystems [35]

Table 2: A literature survey

Municipal wastewater has caused all kinds of environmental pollution and diseases in Kabul City. Urban wastewater treatment, in addition, to helping the growth of the economy, makes us a healthy green environment, and also, having a green and healthy environment reduces diseases. Recycling domestic wastewater can find prized and vital solutions to the challenges of safe water resources since wastewater treatment can play a vital role in the economy and can be used as agricultural water. This research finding identified natural zeolites can be treated heavy metals from wastewater of Kabul city and the following are the main points:

1. Adsorption by using Zeolite forms an appropriate method for wastewater handling because of its cost-effectiveness and simplicity, among all introduced methods. Naturally occurring zeolites hold great potential to use as packing material in subsurface reactive barriers intercepting groundwater plumes and for fixed bed reactors designed to remove heavy metals from industrial wastewater.

2. Natural zeolites with the ability to sorb anionic species, non-polar organic species, and pathogens from aqueous streams, are considered for applications as decontamination agents for soils and water basins. Thus, backfill and sealing materials in waste repositories and as permeable reactive barriers for the cleaning of water. These environmental pollutants have the potential to induce a large range of acute and long-term effects on human health and ecosystems.

3.  Zeolites exhibit ion exchange capacity, i.e. they can adsorb specific ions and molecules and Zeolites can sort molecules selectively based on a size exclusion process. 
4. The adsorption efficacy also rises by dropping the element size of the zeolites and the effect of the retention time on the adsorption rate shows that 80% of Cd, Fe, and Ni are adsorbed by the zeolite in the first 120 minutes of zeolite and the distance efficiency was 78.8, 89.1 and 65.5% of the Cd⁺², Fe^{+ 3} and Ni ^{+ 2} ions.

The authors declare that they have no conflict of interest

No funding sources

The study was approved by the  Beijing University of Technology, Beijing 100124, P. R. China.

1. Singh, A. R. N. a. S. K. Modeling of Wastewater Collection Network Using Arc GIS and SewerGEMS in Kabul, Afghanistan. EasyChair preprints 2020, No: 4098.

2. Javanbakht, V.; Ghoreishi, S. M.; Habibi, N.; Javanbakht, M. A novel magnetic chitosan/clinoptilolite/magnetite nanocomposite for highly efficient removal of Pb(II) ions from aqueous solution. Powder Technology 2016, 302, 372-383. DOI: https://doi.org/10.1016/j.powtec.2016.08.069.

3. Chai, W. S.; Cheun, J. Y.; Kumar, P. S.; Mubashir, M.; Majeed, Z.; Banat, F.; Ho, S.-H.; Show, P. L. A review of conventional and novel materials towards heavy metal adsorption in wastewater treatment application. Journal of Cleaner Production 2021, 296, 126589. DOI: https://doi.org/10.1016/j.jclepro.2021.126589.

4. Argun, M. E. Use of clinoptilolite for the removal of nickel ions from water: Kinetics and thermodynamics. Journal of Hazardous Materials 2008, 150 (3), 587-595. DOI: https://doi.org/10.1016/j.jhazmat.2007.05.008.

5. Golomeova, M.; Zendelska, A.; Blažev, K.; Krstev, B.; Golomeov, B. Removal of Heavy Metals from Aqueous Solution using Clinoptilolite and Stilbite. International journal of engineering research and technology 2014, 3.

6. Taamneh, Y.; Sharadqah, S. The removal of heavy metals from aqueous solution using natural Jordanian zeolite. Applied Water Science 2017, 7 (4), 2021-2028. DOI: 10.1007/s13201-016-0382-7.

7. Mthombeni, N. H.; Mbakop, S.; Onyango, M. S. Magnetic Zeolite-Polymer Composite as an Adsorbent for the Remediation of Wastewaters Containing Vanadium.

8. Li, Y.; Bai, P.; Yan, Y.; Yan, W.; Shi, W.; Xu, R. Removal of Zn2+, Pb2+, Cd2+, and Cu2+ from aqueous solution by synthetic clinoptilolite. Microporous and Mesoporous Materials 2019, 273, 203-211. DOI: https://doi.org/10.1016/j.micromeso.2018.07.010.

9. El-Azim, H.; Mourad, F. Removal of Heavy Metals Cd (II), Fe (III) and Ni (II), from Aqueous Solutions by Natural (Clinoptilolite) Zeolites and Application to Industrial Wastewater. Asian Journal of Environment & Ecology 2018, 7, 1-13. DOI: 10.9734/AJEE/2018/41004.

10. Bessa, R.; França, A.; Pereira, A.; Alexandre, N.; Perez Page, M.; Holmes, S.; do Nascimento, R.; Rosa, M.; Anderson, M.; Loiola, A. Hierarchical zeolite based on multiporous zeolite A and bacterial cellulose: An efficient adsorbent of Pb2+. Microporous and Mesoporous Materials 2020, 312. DOI: 10.1016/j.micromeso.2020.110752.

11. Shokory, J. A. N.; Schaefli, B.; Lane, S. N. Water resources of Afghanistan and related hazards under rapid climate warming: a review. Hydrological Sciences Journal 2023, 68 (3), 507-525. DOI: 10.1080/02626667.2022.2159411.

12. Sonune, A.; Ghate, R. Developments in wastewater treatment methods. Desalination 2004, 167, 55-63. DOI: https://doi.org/10.1016/j.desal.2004.06.113.

13. Schwanke, A.; Balzer, R.; Pergher, S. Microporous and Mesoporous Materials from Natural and Inexpensive Sources. 2017.

14. Wang, S.; Peng, Y. Natural zeolites as effective adsorbents in water and wastewater treatment. Chemical Engineering Journal 2010, 156 (1), 11-24. DOI: https://doi.org/10.1016/j.cej.2009.10.029.

15. Kalló, D. n. Applications of Natural Zeolites in Water and Wastewater Treatment. Reviews in Mineralogy and Geochemistry 2001, 45 (1), 519-550. DOI: 10.2138/rmg.2001.45.15 (acccessed 3/25/2023).

16. Tony, M. A. Solar concentration for green environmental remediation opportunity— international review: advances, constraints and their practice in wastewater treatment. International Journal of Environmental Analytical Chemistry 2021, 1-33.

17. Rajczykowski, K.; Loska, K. Stimulation of Heavy Metal Adsorption Process by Using a Strong Magnetic Field. Water, Air, & Soil Pollution 2018, 229 (1), 20. DOI: 10.1007/s11270-017-3672-2.

18. Losada, I. B.; Grobas-Illobre, P.; Misturini, A.; Polaina, J.; Seminovski, Y.; Sastre, G. Separation of an aqueous mixture of 6-kestose/sucrose with zeolites: A molecular dynamics simulation. Microporous and Mesoporous Materials 2021, 319, 111031. DOI: https://doi.org/10.1016/j.micromeso.2021.111031.

19. Tran-Nguyen, P. L.; Ly, K.-P.; Thanh, L. H. V.; Angkawijaya, A. E.; Santoso, S. P.; Tran, N.-P.-D.; Tsai, M.-L.; Ju, Y.-H. Facile synthesis of zeolite NaX using rice husk ash without pretreatment. Journal of the Taiwan Institute of Chemical Engineers 2021, 123, 338-345. DOI: https://doi.org/10.1016/j.jtice.2021.05.009. 

20. Báfero, G. B.; Munsignatti, E. C. O.; Pastore, H. O. Lamellar-zeolitic transformations mediated by diffusionless and recrystallization mechanisms. Microporous and Mesoporous Materials 2021, 323, 111189. DOI: https://doi.org/10.1016/j.micromeso.2021.111189.

21. Báfero, G. B.; Araújo, V. A.; Almeida, R. K. S.; Pastore, H. O. Catalytic performance of ferrierite and omega zeolites obtained through 2D-3D-3D transformation from Na-RUB-18 layered silicate. Microporous and Mesoporous Materials 2020, 302, 110216. DOI: https://doi.org/10.1016/j.micromeso.2020.110216. 

22. Pashkova, V.; Mlekodaj, K.; Klein, P.; Brabec, L.; Zouzelka, R.; Rathousky, J.; Tokarova, V.; Dedecek, J. Mechanochemical Pretreatment for Efficient Solvent-Free Synthesis of SSZ-13 Zeolite. Chemistry – A European Journal 2019, 25 (52), 12068-12073. DOI: https://doi.org/10.1002/chem.201902107. 

23. Eliášová, P.; Opanasenko, M.; Wheatley, P. S.; Shamzhy, M.; Mazur, M.; Nachtigall, P.; Roth, W. J.; Morris, R. E.; Čejka, J. The ADOR mechanism for the synthesis of new zeolites. Chemical Society Reviews 2015, 44 (20), 7177-7206, 10.1039/C5CS00045A. DOI: 10.1039/C5CS00045A.

24. Gao, W.; Amoo, C. C.; Zhang, G.; Javed, M.; Mazonde, B.; Lu, C.; Yang, R.; Xing, C.; Tsubaki, N. Insight into solvent-free synthesis of MOR zeolite and its laboratory scale production. Microporous and Mesoporous Materials 2019, 280, 187-194. DOI: https://doi.org/10.1016/j.micromeso.2019.01.041.

25. Lei, L.; Li, X.; Zhang, X. Ammonium removal from aqueous solutions using microwave-treated natural Chinese zeolite. Separation and Purification Technology 2008, 58 (3), 359-366. DOI: https://doi.org/10.1016/j.seppur.2007.05.008.

26. Ma, Y.-K.; Rigolet, S.; Michelin, L.; Paillaud, J.-L.; Mintova, S.; Khoerunnisa, F.; Daou, T. J.; Ng, E.-P. Facile and fast determination of Si/Al ratio of zeolites using FTIR spectroscopy technique. Microporous and Mesoporous Materials 2021, 311, 110683. DOI: https://doi.org/10.1016/j.micromeso.2020.110683. 

27. Ullah, R.; Sun, J.; Gul, A.; Munir, T.; Wu, X. Evaluations of physico-chemical properties of TiO2/clinoptilolite synthesized via three methods on photocatalytic degradation of crystal violet. Chinese Journal of Chemical Engineering 2021, 33, 181-189. DOI: https://doi.org/10.1016/j.cjche.2020.09.045.

28. Wang, J.; Li, M.; Fu, Y.; Amoo, C. C.; Jiang, Y.; Yang, R.; Sun, X.; Xing, C.; Maturura, E. An ambient pressure method for synthesizing NaY zeolite. Microporous and Mesoporous Materials 2021, 320, 111073. DOI: https://doi.org/10.1016/j.micromeso.2021.111073.

29. Zhu, J.; Liu, Z.; Endo, A.; Yanaba, Y.; Yoshikawa, T.; Wakihara, T.; Okubo, T. Ultrafast, OSDA-free synthesis of mordenite zeolite. CrystEngComm 2017, 19 (4), 632-640, 10.1039/C6CE02237E. DOI: 10.1039/C6CE02237E.

30. Wiśniewska, M.; Fijałkowska, G.; Nosal-Wiercińska, A.; Franus, M.; Panek, R. Adsorption mechanism of poly(vinyl alcohol) on the surfaces of synthetic zeolites: sodalite, Na-P1 and Na-A. Adsorption 2019, 25 (3), 567-574. DOI: 10.1007/s10450-019-00044-2.

31. Zhang, J.; Chu, Y.; Deng, F.; Feng, Z.; Meng, X.; Xiao, F.-S. Evolution of D6R units in the interzeolite transformation from FAU, MFI or *BEA into AEI: transfer or reassembly? Inorganic Chemistry Frontiers 2020, 7 (11), 2204-2211, 10.1039/D0QI00359J. DOI: 10.1039/D0QI00359J.

32. Abd El-Azim, H., & A. Mourad, F. . Removal of Heavy Metals Cd (II), Fe (III) and Ni (II), from Aqueous Solutions by Natural (Clinoptilolite) Zeolites and Application to Industrial Wastewater. Asian Journal of Environment & Ecology 2018, 7 (1), 1-13. DOI: https://doi.org/10.9734/AJEE/2018/41004.

33. Misaelides, P. Application of natural zeolites in environmental remediation: A short review. Microporous and Mesoporous Materials 2011, 144 (1), 15-18. DOI: https://doi.org/10.1016/j.micromeso.2011.03.024.

34. Vacca, S.; Silvano, R.; Virdis, A.; Capra, G. F.; Rosnati, C.; Buondonno, A. Using Zeolite in WWTP and in semi-arid agricultural areas as amendment: a management protocol proposal. 2010; pp 227-241.

35. Bandura, L.; Panek, R.; Madej, J.; Franus, W. Synthesis of zeolite-carbon composites using high-carbon fly ash and their adsorption abilities towards petroleum substances. Fuel 2021, 283, 119173. DOI: https://doi.org/10.1016/j.fuel.2020.119173.

36. Chmielewská, E.; Tylus, W.; Drábik, M.; Majzlan, J.; Kravčak, J.; Williams, C.; Čaplovičová, M.; Čaplovič, Ľ. Structure investigation of nano-FeO(OH) modified clinoptilolite tuff for antimony removal. Microporous and Mesoporous Materials 2017, 248, 222-233. DOI: https://doi.org/10.1016/j.micromeso.2017.04.022.

37. Saravanan, A.; Senthil Kumar, P.; Jeevanantham, S.; Karishma, S.; Tajsabreen, B.; Yaashikaa, P. R.; Reshma, B. Effective water/wastewater treatment methodologies for toxic pollutants removal: Processes and applications towards sustainable development. Chemosphere 2021, 280, 130595. DOI: https://doi.org/10.1016/j.chemosphere.2021.130595.

38. Rahmani, H. Challenges and Resolutions for Sustainable Domestic Wastewater Management in Kabul City, Afghanistan. International Journal of Engineering and Advanced Technology 2020, 9. DOI: 10.35940/ijeat.A1222.109119.