Zeolite: A Brief Overview
by: Panagiotis Petridis, Agricultural Scientist (PE); Charalambos Karasavvidis, Agricultural Scientist (PE).
Zeolites are clay-silicate materials with a tetrahedral crystal structure featuring cavities, which are occupied by ions and water molecules. These exhibit considerable freedom of movement, a fact which contributes to the occurrence of the phenomena of ion exchange and dehydration in zeolites. Essentially, they are hydrated crystalline aluminosilicate salts of elements from Groups IA and IIA of the periodic table, such as sodium, potassium, magnesium and calcium. The term ‘zeolite’ derives from the property of these minerals to expel the water they contain when heated, giving the impression that they are boiling. The phenomena of dehydration and ion exchange in zeolites are reversible. More than 30 types of zeolites occur in nature, 20 of which are found in sedimentary deposits. The most common zeolites are: clinoptilolite, mordenite, chabazite, analcimite, phillipsite, erionite and eulandite. The best type is that which contains a high proportion of clinoptilolite.
Zeolite deposits are also found in Greece in several areas of Northern Greece (Petrota, Evros) with a very high purity (95 per cent), but unfortunately no licence has yet been granted to exploit the deposit in the Cyclades either. Best of all for agricultural and livestock use are those containing the mineral clinoptilolite. Greek natural zeolite contains 89 per cent clinoptilolite. The use of zeolite is limited and needs to be expanded because, as you have also observed, it offers valuable benefits.
Zeolite
structure The basic structural unit is the tetrahedron [(Si,Al)O4]-4. The tetrahedra [(Si,Al)O4]-4 are linked together (via the shared oxygen atoms at their vertices) and form polyhedra, creating, depending on how they are linked, three-dimensional lattices with channels or cavities filled, on the one hand, with water and, on the other, with cations (Ca, Na, K) in an exchangeable form.
The general chemical formula is: M₂/n O . Al₂O₃ x SiO₂ . y H₂O
where: M = alkaline earth metal, n = cation valence, x = a number from 2 to 10, y = a number from 2 to 7
. The ratio (Si + Al)/O must be equal to 1/2. They belong to the group of tectopyritic minerals, together with quartz and feldspars.
Unlike most tectopyritic minerals, the structure of zeolites is remarkably open, with void volumes reaching 50 per cent of the anhydrous member. They provide space for large cations such as sodium, potassium, barium and calcium, as well as for relatively large molecules and cation groups such as water, ammonia, carbonate ions and nitrate ions.
They are characterised by their ability to reversibly release and absorb water and to exchange their constituent cations. Water is continuously desorbed by heating to approximately 350°C and reabsorbed as the temperature is gradually reduced to room temperature. The large voids in their structure explain the low density of these ores.
Physical and chemical properties of zeolites
: When pure, zeolites are colourless or white. However, they often appear coloured due to the fine dispersion of iron oxides and other impurities. Their density varies from 2 to 2.3 g cm⁻³, except for zeolites rich in Ba (barium), in which the density ranges between 2.5 and 2.8 g cm⁻³. Their relatively low density is due to the presence of water, which fills the channels of the lattice. The refractive index of the various members of the group ranges between 1.47 and 1.52.
The presence of large voids and channels in the zeolite lattice has attracted considerable interest. When the water is removed, these voids can be filled with various gaseous substances, such as ammonia, iodine vapour or even mercury vapour; for example, tosmonite absorbs ethyl alcohol and isopropyl alcohol. This process depends on the specific type of zeolite. In particular, molecules with a diameter greater than that of the channels cannot enter the cavities in the zeolite structure and so cannot be adsorbed. This simple principle underlies the well-known application of many zeolites as ‘molecular sieves’, which are mainly used for the separation of gaseous mixtures. However, the width of the channels is not the only condition for permeability, as the presence of many cations can block the channels, whilst molecular and ionic diffusion are affected by the adsorbed water. In general, cation exchange capacity decreases with water loss. With the exception of analcime and natrolite, in most silicate zeolites, K+ and Na+ tend to be more readily exchangeable than Ca²⁺, as they are monovalent and are therefore held by a weaker electrostatic charge.
In most zeolites, each water molecule corresponds to a number of possible positions within the lattice, and it can move from one to another. Generally, calcium-rich zeolites absorb more water, and in chabazite, eulandite and stilbite, water is retained more readily when their lattice contains Ca²⁺ rather than K⁺.
As regards host rocks, it is believed that zeolites are amongst the most siliceous minerals found in sedimentary rocks, and that zeolite deposits of this type are the largest and most economically significant. They form in environments that vary in terms of age, lithology and deposition, during the process of diagenetic alteration of sedimentary rocks.
Zeolites are characterised by the following properties:
High degree
of hydration Low density and high volume, pores upon hydration
Stability of crystalline structure
High ion-exchange capacity
Uniformity of channels, molecular size
Adsorption capacity (of gases and vapours)
Catalytic
capacity Zeolite – Use in Agriculture
Let us look at the useful applications it may have in agriculture, and indeed in organic farming. Its use in agriculture is very widespread in Japan and Europe, but limited in our country.
Zeolite is applied by incorporating it into the soil in quantities that depend on its particle size distribution (fineness of the material). The finer the material, the smaller the quantity required. In heavy soils, zeolite in granular form (2.5–5 mm) should be preferred, whilst in light soils, it should be used in powder form. It is estimated that 0.5 kg of powdered zeolite covers an area equivalent to that of a football pitch.
Zeolite is usually applied to the soil for three consecutive years and worked into the soil each time. A quantity of 400–500 kilograms per stremma is required, and it must be worked into the soil for its contribution to increased yield to be noticeable. It is applied before the start of the growing season or before planting/sowing. Its beneficial effect is long-lasting (zeolite does not break down like other materials) and you will not need to add any more zeolite for the next 10 years or more.
When applied, it offers a range of valuable benefits, such as:
• It increases nutrient retention capacity (its cation exchange capacity ranges from 2.2 to 4 meq/g, which is double that of bentonite),
• It reduces nutrient losses due to volatilisation (nitrogen losses in the
form of ammonia) or leaching (nitrite and nitrate) and consequently helps to reduce the amount of fertiliser used by 20–25 per cent;
• When used in conjunction with fertilisation, it can help ensure the gradual release of
nutrients, thereby preventing losses whilst also supplying plants with nutrients over a longer period;
• It increases water-holding capacity, particularly in sandy soils (zeolite retains water equivalent to 60 per cent of its weight, which it releases to the plants gradually),
• Neutralises acidic components in the soil (raises the pH),
• Improves soil structure by reducing compaction and increasing aeration,
• It is a 100% natural product suitable for use in organic farming; and finally,
• As a direct result of all the above, it increases and improves plant
yield. Zeolite – Use in Livestock Farming
Farmers who also keep livestock can ‘kill two birds with one stone’. If they use zeolite in animal feed and on stable floors, they will produce manure with exceptional properties. Most of the ammonia that is released into the atmosphere and has a negative impact on animal health will be bound by the zeolite. The ammonia is bound within the material’s micropores, which nitrifying bacteria cannot enter to oxidise it. When this manure is spread on fields, it will provide the beneficial properties of both nitrogen and zeolite. Zeolite in animal feed binds the ammonia produced during digestion, whilst simultaneously releasing potassium, which regulates stomach acidity – a process that facilitates the absorption of minerals and nutrients.
In feeding trials, a 5 per cent improvement in the feed digestion rate was observed. The addition of 2 per cent zeolite to poultry feed resulted in a 40 per cent increase in nitrogen content in the manure. Furthermore, adding 6.25 per cent zeolite to liquid manure reduced ammonia loss by 55 per cent.
Zeolite – Use in water
: The addition of natural zeolite to water results in the binding of inorganic, organic, organometallic compounds, gases, bacteria, cyanobacteria, metals and radionuclides at a rate ranging from 20 per cent to 100 per cent. Zeolite enriches the water with oxygen (oxygen-rich currents) and adjusts the pH of the water towards neutral, raising or lowering it depending on the water’s acidity or alkalinity.
Forms of zeolite for use
It is available on the market in four forms, depending on the size of its grains and for different uses, such as:
1) In powder form for foliar spraying, as well as for medical use.
2) Size 0.01–0.8 millimetres. For enriching feed for farm animals.
3) Size 0.8–2.5 millimetres. For odour control, for bird feed, and for filtering drinking water.
4) Size 2.5–5.0 millimetres. For agricultural use in improving soil fertility, as an additive to chemical fertilisers at a rate of 2–5 per cent (slow-release), in hydroponics, whilst not ruling out the use of the previous grades, which are more potent.
Conclusions regarding Greek zeolite
Greek Natural Zeolite increases agricultural yields by 17–66 per cent, improves the quality of agricultural produce by 4–46 per cent, reduces the use of fertilisers by 56–100 per cent, reduces the use of irrigation water by 33–67 per cent, prevents the leaching and migration of harmful substances from the soil into the aquatic environment, thereby protecting the quality of surface and groundwater.
It is also of very high quality. On average, it contains 89 per cent by weight of HEU-type zeolite, 3 per cent by weight (mica + clay minerals), 5 per cent by weight of feldspars and 3 per cent by weight (quartz + cristobalite). Its mineralogical composition and physicochemical properties make natural zeolite a suitable material for numerous environmental, industrial, agricultural and water-related applications, such as the treatment of municipal sewage and industrial liquid waste, the production of odourless and cohesive zeolite sludge, the treatment of sewage sludge and industrial sludge for safe disposal, the remediation and oxygenation of aquatic ecosystems, capture and removal of cyanobacteria, hexavalent chromium and radionuclides; improvement of constructed wetlands and other water management facilities; improvement of drinking water quality, conversion of manure into odourless fertiliser, animal feed, soil conditioner for agricultural crops, and soil conditioner for acidic and alkaline soils, management of mining waste and the restoration of land to agricultural use, substrate for greenhouses and floriculture, food hygiene and safety, improvement of food flavour and quality, more resilient and healthier turf, dietary supplements, etc.
Bibliography
1. Ch. Karasavvidis, Z. Ioannou, A. Dimirkou, V. Antoniadis, ‘Adsorption of Dyes from Aqueous Solutions onto Modified Zeolites’, Wastewater Purification and Reuse Conference, Heraklion, Crete, 28–30 March 2012
2. G. Gottardi, E. Galli, *Natural Zeolites*, Springer, Berlin, Heidelberg, New York, 1985
3. Misaelides P., Godelitsas A., Charistos V., Ioannou D. & Charistos D. 1994, ‘Heavy Metal Uptake by Zeoliferous Rocks from Metaxades, Thrace, Greece: An exploratory study’. J. Radioanal. Nucl. Chem. Art., 183, 159–166.
4. Valquiria Campos, ‘The Sorption of Toxic Elements onto Natural Zeolite, Synthetic Goethite and Modified Powdered Block Carbon’, Journal of Environmental Earth Science, Vol. 59, Issue 4, 2009