Zeolites
Zeolites are microporous, tetrahedral, three-dimensional crystalline minerals that are mainly composed of aluminosilicate earth metals. Zeolites are derived from the Greek words "zéō," which means to boil, and "lithos," which means "stone." Their general formula is Mn+1/n(AlO2) − (SiO2)x・yH2O where Mn+ 1/n can be either metal or hydrogen ions. Commonly, two types of zeolites are available: natural and synthetic.
Natural Zeolites: Natural zeolites are found in rocks of volcanic origin, such as igneous, metamorphic, and sedimentary rocks.
Synthetic zeolites: Synthetic zeolites are produced by a hydrothermal process. At present, more than one hundred different types of zeolite structures can be obtained synthetically.
History and origin
At first, natural zeolites were found in vugs in basalt rocks and cavities. Afterward, they were also discovered in sedimentary rocks (19th century). In 1756, Swedish mineralist Alex Fredrik first identified natural zeolite as a mineral after he sampled crystals from a copper mine in Sweden. The golden age period for zeolite development was from 1954 to 1980. However, commercial production and use began in the 1960s in most countries.
Physical properties
Porosity
- Microporous: Zeolites possess a highly porous structure with uniform pores of molecular dimensions. These pores create a vast internal surface area.
- Molecular Sieve Effect: The precise pore size allows zeolites to selectively adsorb molecules based on their size and shape, acting as molecular sieves.
Heat stability
- Robust Framework: Zeolites possess a robust crystalline framework that can withstand high temperatures.
- Thermal Stability: This thermal stability allows them to be used in various high-temperature processes, such as catalysis and drying.
Specific gravity
The specific gravity of zeolites typically ranges from 2.1 to 2.8. However, it can vary slightly depending on the specific type of zeolite and its composition.
Bulk density.
The bulk density of zeolites can vary depending on the specific type and its physical form (powder, granules, etc.). However, a typical range for the bulk density of zeolites is 1600-1800 kg/m³.
Chemical properties
Zeolites are crystalline aluminosilicates known for their unique chemical properties, primarily characterized by a high cation exchange capacity (CEC), thermal stability, and distinct optical and electrical behaviors. Their CEC arises from substituting silicon atoms with aluminum in the framework, creating a net negative charge balanced by exchangeable cations like Na⁺, K⁺, Ca²⁺, and Mg²⁺. This cation exchange is facilitated by the highly porous structure of zeolites, offering a vast surface area for reactions and ion mobility. Zeolites also exhibit negative thermal expansion (NTE), which contracts when heated due to their specific crystal structure. Their electrical conductivity, generally low due to the rigid framework, can be influenced by factors like cation exchange, water content, and temperature. The optical properties of zeolites, including refractive index and dielectric function, are significant in various applications, such as sensors and optical devices.
Thermochemically, zeolites are highly stable and can adsorb various molecules, making them effective in gas separation, catalysis, and water purification. Their thermal stability and high specific heat capacity make them suitable for high-temperature catalytic processes and thermal energy storage. The framework's tetrahedral linkages, with silicon or aluminum at the center surrounded by oxygen atoms, form a three-dimensional network of pores, allowing selective adsorption and catalysis. The internal covalent bonds within tetrahedra and the external linkages between them contribute to zeolites' structural integrity and thermal resistance.
Their thermal and chemical stability and ability to adsorb and release heat make them suitable for high-temperature applications like catalysis and energy storage. In agriculture, these chemical properties are harnessed to enhance soil quality and nutrient management. The high CEC of zeolites enables them to act as slow-release fertilizers, improving the availability of essential nutrients like potassium and ammonium to plants. Furthermore, their ability to adsorb and retain water helps regulate soil moisture, making zeolites a valuable soil amendment to boost crop productivity, especially in arid and nutrient-deficient soils.
Challenges and Future Directions
While zeolites offer numerous advantages, there are still challenges associated with their thermochemical properties:
- Accurate Thermochemical Data: Obtaining accurate thermochemical data for zeolites can be challenging due to their complex structures and variable compositions.
- Predictive Modeling: Developing accurate models to predict zeolites' thermochemical behavior can aid in designing new materials and processes.
- Energy Efficiency: Optimizing the energy efficiency of zeolite-based processes is crucial to reduce their environmental impact.
(a)Topology of faujasite zeolite and location of extra-framework aluminum active sites (S);
(b) rhombohedral faujasite studied ACS.
Potential applications of zeolites in agriculture
Nowadays, zeolites are extensively used for agricultural purposes in some countries. Due to their chemical and physical properties, they can be used as soil conditioners, soil decontaminators, water and nutrient retention agents, eco-safe pesticides, slow-release fertilizers and herbicides, crop improvement agents, wastewater treatment agents, and for the removal of heavy metals from contaminated soil. These minerals are considered soil conditioners to improve soil physical and chemical properties, including infiltration rate, saturated hydraulic conductivity (Ks), water holding capacity (WHC), and cation exchange capacity (CEC). Natural and surface-modified zeolites can efficiently hold water and nutrients, including ammonium (NH₄⁺), nitrate (NO₃⁻), phosphate (PO ₄ ³⁻), potassium (K⁺), and sulfate (SO₄²⁻), in their unique porous structures. Significantly, their application can improve water use efficiency (WUE) and nutrient use efficiency (NUE) in agricultural activities and reduce the potential for surface and groundwater pollution.
In fertilizer use efficiency
The enormous use of chemical fertilizers causes many environmental issues. Even though we applied chemical fertilizers, only a small amount of the nutrients were absorbed by the soil. Excess fertilizers are washed into bodies of water, causing nutrient concentrations to rise, a process known as eutrophication. So, it can be reduced by applying fertilizer mixed with zeolites to increase the nutrient retention capacity of the soils. This method helps slowly release the nutrients for crop uptake.
In soil amendment
Zeolites can be used as a soil amendment to improve the physical, chemical, and biological properties of the soil, such as bulk density, cation exchange capacity, hydraulic conductivity, infiltration rate, water holding capacity of the soil, nutrient retention capacity, reduce the salt concentration in the soil, increase the soil pH, soil organic matter, enzyme activity, and soil microbial biomass.
In the slow release of herbicides
Due to their high cation exchange capacity, many natural and synthetic zeolites are used as effective carriers of herbicides. Herbicides along with zeolites supply the active ingredient of herbicides for a longer duration of efficacy at the required rate for weed control. It will reduce the leaching potential and environmental pollution of herbicides and also reduce the total amount of chemicals required for weed control.
In the removal of heavy metals
Soils are contaminated with heavy metals such as lead, cadmium, zinc, nickel, manganese, chromium, copper, and iron. Due to their high cation exchange capacity, zeolites can remove heavy metals, which are used to treat heavy metal-contaminated soil.
In Water Absorption
Because of their crystalline structure's high porosity, they can retain water molecules in their pores. This increases water use efficiency by increasing the soil water holding capacity, and due to their extraordinarily porous structure, their availability extends longer to crops.
In gas absorption
Natural and synthesized zeolites can be used for gas absorption like methanol (CH3OH), formaldehyde (HCHO), sulfur dioxide (SO2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S), water (H2O), ammonia (NH3), molecular hydrogen (H2), argon (Ar), oxygen (O2), nitrogen (N2), xenon (Xe), helium (He), and krypton (Kr). Zeolites are also used for odor control. Zeolites are sometimes used in intensive animal husbandry to reduce odors in large farms.
In Crop Protection
Zeolites, which are nontoxic by nature, can be used for pest and disease control. They can also be used for storage pest control, such as the confused flour beetle (Triboliumconfusum), the maize weevil (Sitophiluszeamais), the red flour beetle (Triboliumcastaneum), and so on.
In crop photosynthesis enhancement
Foliar application of zeolites on plant leaves will increase the amount of carbon dioxide near the stomata. This leads to an increase in the photosynthetic rate of C3 in the crops. C3 plants like vines, tomato plants, apple trees, and orange trees increase the efficiency of net carbon dioxide (CO2) uptake (velocity of carboxylation) and decrease carbon dioxide loss by the photorespiratory system. This leads to an increase in crop growth, a faster rate of production of the leaf surface, and a decrease in the transpiration rate.
In crop heat stress and sunburn
Zeolite-coated plants significantly increase the leaf reflectiveness of the infrared radiation, and this leads to a reduction in the leaf temperature.
As an animal feed additive
Zeolites can be used as a dietary supplement to aid in disease prevention. Zeolite minerals will increase some mechanisms in the livestock diet.
Conclusion
Even though modern-day farming methods have many advantages, we must also consider their drawbacks, which include the rapid depletion of natural resources, soil fertility losses, groundwater depletion, greenhouse gas emissions, environmental pollution, and so on, all of which are primarily caused by human activity. In many countries, scientists, naturalists, and environmentalists have been working, researching, and fighting to take care of mother nature. So, we can go for alternatives to chemicals, such as the application of natural inputs like zeolites in present-day agriculture. Definitely, zeolites would be a better option for farmers to maintain a healthy environment and for sustainable agriculture.
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