What properties should zeolites have?
Zeolite is a porous mineral, chemically classified as a framework aqueous aluminosilicate. The basis of the mineral is 48-84% silica (SiO₂) and 12-20% alumina (Al₂O₃). The mineral contains small amounts of oxides of other metals (potassium, iron, sodium, magnesium, calcium) and water molecules. More than 40 types of natural zeolites are known, differing in the ratio of silicon, aluminum and type of cations. The most significant minerals are mordenite and clinoptilolite. Zeolites are natural (volcanic and sedimentary origin) and synthetic. Natural minerals as minerals began to be mined in the middle of the 3th century. Zeolite deposits are volcanic rocks, shales, and sandstones scattered throughout the world. About XNUMX million tons of natural zeolite are mined annually. The production of synthetic zeolites began relatively recently. Currently, there are more than 150 types of artificial zeolite, differing in pore size, base metal and mineral lattice structure (type A and X).
How the “boiling stone” was discovered
Zeolite was discovered by Swedish chemist Axel Cronstedt in the 18th century. During an experiment on heating stilbite (a type of zeolite), small bubbles began to form on the surface of the mineral, as if boiling. Later, several more minerals were identified, called “zeolites,” which, when lowered into water, emitted gas bubbles for a long time, and when heated, water vapor. Therefore, another name for the mineral is “boiling stone”. But practical use of zeolites was found only two centuries later, when a serious problem of environmental pollution arose. Initially, it was discovered that aluminosilicate minerals can be used to purify water. A deeper study of zeolites began after the accident at the Chernobyl nuclear power plant. Zeolite was used as a means to combat radiation sickness, and was also used to disinfect wastewater in the Pripyat and Dnieper basins.
What is zeolite used for: basic properties of the mineral
Zeolite has high absorption properties. Thanks to its porous structure, the mineral, like a sponge, absorbs and binds heavy metals, radioactive elements, nitrates and nitrites, and petroleum products. In the animal or human body, the mineral absorbs toxins and metabolic products of viruses and pathogenic bacteria. Due to its fine porosity, zeolite is unable to absorb useful substances (proteins, amino acids, vitamins), which distinguishes it from other sorbents. The mineral prevents the entry of toxic substances into plants, reduces the activity of the nitrification process (the transition from the ammonium form of nitrogen to the nitrate form), which significantly reduces the nitrate content in the crop. Zeolite has a high ion exchange ability. Cations of calcium, magnesium, potassium, etc., as a result of ion exchange, are replaced by toxic ions and retained on the crystal lattice of the mineral. Zeolite as a natural catalyst helps accelerate various chemical reactions. The use of zeolite in industry as a catalyst prevents equipment corrosion. The pore sizes of different types of zeolites are strictly individual and vary in the range of 3-10 angstroms. Therefore, each type of zeolite can be used as a molecular sieve to selectively absorb chemicals. The rigid aluminosilicate structure of zeolite prevents the mineral from swelling at high humidity. This property allows the use of zeolite as an anti-caking agent and moisture sorbent.
Areas of application of zeolite
Due to its properties, zeolite is widely used in various fields. The mineral is used in the food and polymer industries, petrochemicals, water treatment, agriculture, construction, metallurgy, oil production and ecology.
Zeolite for plants. Features of the use of zeolite in agriculture
Rising prices for mineral fertilizers, the urgent need to restore soil fertility and minimize the use of highly toxic pesticides determine the high need for the development of new environmentally friendly fertilizers. The unique structure, rich chemical composition, sorption and ion exchange properties of zeolite determine the significant role of the mineral in agriculture. Zeolite is widely used as a fertilizer and agromeliorant. Zeolites have a positive effect on the agrophysical and agrochemical properties of soils, prevent the leaching of applied fertilizers, absorb toxic substances from the soil, and reduce the level of soil acidity. At the same time, the mineral itself acts as a donor of some nutritional elements (calcium, magnesium, sodium, molybdenum, boron, etc.). Thus, the use of zeolite in crop production is a significant factor in increasing the productivity of grown plants.
What are the benefits of zeolite for plants? and how it is used
Zeolite is a mineral high in silicon, which plays an important role in plant life. Zeolite contains on average 25% free silicon in a form accessible to plants. It is known that silicon entering plants optimizes metabolic processes, increases the absorption of nutrients, provides mechanical strength of plants, increases drought and frost resistance, due to the property of silicon to bind free water in plant cells. Thus, zeolite can be used as a fertilizer with a high silicon content. Zeolite maintains an optimal level of soil moisture due to its ability to accumulate moisture and then gradually supply it to plants. Zeolite increases the environmental friendliness of products by preventing the accumulation of nitrates, heavy metals and radionuclides in plants. The use of zeolite with mineral fertilizers prolongs their action, reduces caking, saturates with additional microelements, and keeps plants in the root zone. The use of zeolite promotes the development of the root system, accelerates the growth and development of plants, increases plant resistance to diseases, pests and adverse environmental factors, which leads to an increase in yield and product quality. Zeolite for plants. Methods of application.
- The use of the mineral as a soil-improving component for the preparation of greenhouse substrates.
- Use of zeolite as an independent fertilizer or together with organic and mineral fertilizers.
- The use of zeolite in land reclamation and reclamation, to improve the water-air properties of heavy soils and neutralize soil pesticides and heavy metals.
- Using mineral as an alternative to sand when germinating seeds after sowing. Covering the seeds with a thin layer of zeolite prevents the soil from drying out and serves as a preventive measure against the formation of mold and rot.
- Dusting the roots of young plants, tubers and bulbs when planting with a fine fraction of zeolite promotes more active development of the root system, prevents the development of pathogens and initially provides the plants with nutrients.
- The use of zeolite for sanding the lawn, to improve the air permeability of the turf and prevent stagnation of moisture in the root zone of plants.
- Use as a component of soil for indoor plants.
Which zeolite is better for plants?
For use in plant growing, the best is natural zeolite of volcanic origin. Compared to sedimentary forms, volcanic ones have a number of advantages: firstly, they do not dissolve in water and do not turn into a mushy mass, and secondly, volcanic zeolite contains up to 80% aluminosilicates, while sedimentary zeolite contains only 20%.
Of all types of zeolites, clinoptilolite is the most widely used.
What plants are zeolite suitable for?
Zeolite can be used when growing any indoor or garden plants, added to the soil for growing seedlings, applied under fruit trees and shrubs. Zeolite is great for growing orchids. Their root system develops much more actively in zeolite than in the bark. The use of the mineral when growing cacti makes the soil best suited to the requirements of this group of plants.
Zeolite is even used when growing plants hydroponically. For these purposes, fine and medium fractions of the mineral are used.
Zeolite is not used only when growing epiphytic plants (orchids and bromeliads that are not grown in a substrate or with an open root system).
Instructions on how to use zeolite for growing plants and preparing soil
Zeolite is added to the soil for growing indoor plants and seedlings, applied to the soil during planting or during the growing season. Dust potato tubers, bulbs or the root system of seedlings.
When applied to open ground, zeolite is mixed in the following proportion: 1 part zeolite and 3 parts earth.
Table for preparing soil mixtures for various purposes.
Intended use
Components used (in parts)
Zeolites were discovered more than two hundred years ago by Axel Kronstedt. For a long time they were considered rare minerals, unable to form industrial accumulations. At the same time, it was believed that they had no practical application. However, in the course of research on minerals of the zeolite group, their properties and capabilities were clarified.
General description of the mineral
Most natural zeolites appear due to volcanic activity. During an eruption, magma breaks through and flows to the surface. At the same time, its release is accompanied by gas, dust and a huge amount of ash. If the volcano is located on an island or near the ocean, then lava and ash often fall into the water. When reaching the ocean, hot lava with water and sea salt begin to react together. Over several thousand years, this results in the formation of hard crystalline minerals, later called zeolites.
The name comes from the Greek “zeo” and “lithoz”, which literally means “boiling stone”. It was invented by the Swedish mineralogist Axel Kronstedt. He noticed that during heating the mineral swelled. This was due to the evaporation of water. As a result, the mineral seemed to be boiling due to the fact that it was rapidly losing water.
Water molecules that evaporate during heating are adsorbed in pores and cavities, the size of which is 0,3-1,0 nm. They are found in the crystal structure of zeolites. These cavities appeared due to the structural composition of zeolites. It consists of a framework of linked tetrahedra, having 4 O atoms in the structure, which surround the cation. The most common is silicon. Si-O bonds are found in the three-dimensional structure of silicate tetrahedra. Due to this, open cavities in the form of frames are formed. They are often occupied by water molecules and extra-framework cations. Often they can exchange.
When the mineral is heated or dehydrated, significant volumetric voids are formed, allowing the zeolite to act as a “molecular sieve” or adsorbent. The former ensure the passage of molecules of only a certain size: those larger than or equal to the size of the pore will not be able to pass. When gas or liquid molecules are dehydrated (if the size is small enough), sorption occurs due to the internal structure of the zeolite. In this case, large molecules will be excluded.
Features and applications
For a long time, various researchers and scientists were interested in zeolites. This is due to the flexibility and adaptability of the mineral. During the study, it was found that the material is a good adsorbent, ion exchanger and molecular sieve, which determined its wide range of applications in the future. Zeolites were initially used to separate straight-chain hydrocarbons from branched-chain hydrocarbons. It also began to be used as a chemical sensor to control production processes, monitor the environment and indoor air, control wastewater and automatic exhaust. Zeolites are often used in medical monitoring, air separation, and for the removal of heavy metals. These applications are just the tip of the iceberg.
Zeolites are in demand in environmental, scientific, industrial and everyday applications. At the same time, the methods of application are still being expanded and studied.
Adsorbent
Zeolites began to be used as an adsorbent at the end of the 18th century. Since then, they have been used in a variety of processes to solve a range of environmental problems. Later it was found that zeolite copes with molecules of water, ammonia, hydrogen sulfide, nitrogen and sulfur oxides, as well as carbon dioxide and other compounds.
Industrial wastewater
Over time, the requirements for water quality are becoming higher. Therefore, there was a need to purify liquids coming from various sources. As a result, the use of natural zeolite as an agent for removing contaminants in wastewater has become extremely interesting. A huge amount of research has been carried out.
Wastewater from various industrial processes, such as mining, may have different physicochemical characteristics. They may contain ions of zinc, cobalt, nickel and other heavy metals. They are toxic to all living things even in small quantities. However, their content in wastewater can be extremely high. Such liquid cannot be directed into natural water, as this will spoil the aquatic ecosystem. As a result, there will be health problems for animals, plants and people. Also, they should not be sent to the sewer, as they interfere with the biological process.
City waste water
Purification of wastewater with zeolites allows for more efficient removal of contaminants. The most common household water pollutant is ammonium ions. It appears due to the use of fabric softeners and laundry detergents. They often contain quaternary ammonium salts, dialkyldimetal ammonium chlorides and other chlorides that act as surfactants.
Drinking water
During the research, it was found that in the presence of clinoptilolite (one of the types of zeolite), nitrification of sewage sludge increases. As a result, the mineral began to be used in places where it was necessary to purify waste water for drinking purposes. It is known that when powdered clinoptilolite is added to contaminated water before aeration, an increase in carbon dioxide consumption and sedimentation occurs. This creates a sludge that is easier to dewater and use as fertilizer in the future.
The researchers also found that when sludge is mixed with natural zeolite, pure water with higher quality parameters is obtained:
| Color | on 92% |
| Suspended particles | on 84% |
| Chemical oxygen demand | on 95% |
| Dissolved oxygen | on 950% |
| P2O5 | on 96% |
| NH4 | on 99% |
| SO4 | on 97% |
| DO NOT3 | on 82% |
| DO NOT2 | on 82% |
| Total Cr content | on 90% |
| Mn | on 94% |
| Ni | on 93% |
At the same time, a cohesive sediment of zeo-wastewater is formed, which is odorless. Other studies have used zeolites to reduce heavy metals (lead and chromium) in sewage sludge from municipal wastewater treatment plants. Thus, it was discovered that the indicator can be reduced to 68% if, during purification, sludge is mixed with zeolite in a ratio of 98:2, respectively. It was also discovered that when ultrasonic energy is used, the process of removing heavy metals becomes more effective. According to research, this energy affects not only the structure, but also the physicochemical properties of the mineral, which improves the process of immobilization of heavy metals from wastewater.
It was found that when treating well groundwater with natural zeolite, it is possible to:
- Remove NO3 on 55%.
- Remove Pb by 74%.
- Remove Ag by 79%.
- Improve pH from 9,6 to 7,3.
At the same time, water quality was improved by 93% in color and by 96% in chemical oxygen consumption. In addition, the natural mineral was able to remove 51% of colonial Mycrocystis cyanobacteria, 75% of filamentous cyanobacteria and 92% of Chroococces cyanobacteria.
Zeolites are capable of removing inorganic, organic and organometallic compounds from an aqueous solution. Particles of gases, metals, and radionuclides are also eliminated from the composition. This occurs due to the properties of absorption, adsorption and surface deposition.
Adsorbent
Zeolite is an adsorbent. Thanks to its adsorption properties, precipitation formation is improved. Biological treatment increases biological activity caused by oxygen consumption. This occurs due to the adhesion of bacteria to small grains of zeolite. Bacteria that participate in the formation of sludge are adsorbed on the surface of the mineral, thereby accelerating sedimentation and increasing the activity of sediment formation.
According to research, the adsorption of bacteria directly depends on the pH value. Thus, it was found that the surface charges of zeolites and bacteria can increase if the pH level decreases. As they increase, an increase in electrical repulsion occurs, which reduces adsorption if the pH value increases.
However, if NH adsorption occurs4 + the reverse process occurs. Regardless of the solution concentration, an increase in pH leads to the adsorption of more NH4 + . Rozic’s research has shown that ion exchange is one of the most important mechanisms for removing ammonium contaminants from aqueous solution. So, with complex adsorption and ion exchange during ammonium removal with zeolite, the highest efficiency is achieved at a pH level of 5-7 points. At this stage, the ammonium ion begins to exchange its ions with cations in the zeolite.
Catalyst
The most common use of zeolites is for fluid catalytic cracking. This process is used to crack oil. Here, high-molecular hydrocarbon fractions of crude oil can be converted into high-quality gasoline, olefin gases and other products.
The cracking reaction of petroleum during catalysis by zeolites is based on the transfer of hydrocarbon protons. Based on the research of Farnet and Gorte, transfer acts only as an intermediate stage in the formation of a complex matrix of competitive and sequential reactions, which ultimately allow the creation of a product. Since these reactions focus on the formation of carbocations, they depend on the level of surface acidity and the crystal structure of the zeolites. Due to its property of exchanging protons, the mineral can catalyze these reactions, acting as Bronsted acids (species capable of donating protons).
Selective catalysis
In addition to pore size, the structure of a mineral determines its ability to selectively catalyze reactions depending on its shape. Scientists have divided them into three main types according to the mechanism of action of the reaction:
- Selectivity of reagents whose molecules are too large to penetrate the pores of the mineral. As a result, these reactions are excluded. An example is the dehydration of Ca-A butanols (reactions in which zeolite A is exchanged for calcium. N-butanol is quite easily dehydrated due to its ability to penetrate crystals, while isobutanol cannot be subjected to chemical dehydration, since it is not able to penetrate. This explains why individual zeolites, such as Ca-A and ZSM-5 (which are used by Mobil), are capable of breaking down only straight chain paraffins from a mixture of paraffinic and aromatic hydrocarbons to the product, while cycloparaffins and branched or aromatic hydrocarbons are present. in the mixture will not be affected.
- Product selectivity resulting from the reaction of products formed in pores that are unable to diffuse through the pores of the mineral. This occurs as a result of size or shape incompatibility. At the same time, product molecules, which are sterically less hindered, can diffuse at the microscopic level of the zeolite framework, while more voluminous products are not able to move in the cavities of the mineral. These products may form larger molecules as a result of equilibration. As a result, they accumulate and block the pores, deactivating the catalyst.
- Limited transition state selectivity. This category occurs when some reactions do not occur as a result of an obstacle to the formation of intermediates or conditions during the reaction due to the limited shape or size of the microporous lattice. Intermediate states, which require more space than is available in the intracrystalline void volume, cannot form due to the shape and size of the lattice.
Shape selectivity is directly proportional to pore size. This leads to the fact that the mineral, with a smaller pore size, can suppress reactions that occur with bulky intermediates. For this reason, cracking of butene in small-pore zeolite results in the formation of propene and ethene.
