Properties of Zeolites
Adsorption properties:
Under normal conditions, the large cavities and entry channels of zeolites are filled with water molecules forming hydration spheres around the exchangeable cations.
Once the water is removed, usually by heating to 3000 to 4000 C for a few hours molecules having diameters small enough to fit through the entry channels are readily adsorbed on the inner surfaces of the vacant central cavities.
Molecules too large to pass through the entry channels are excluded, giving rise to the well-known “molecular sieving”
The internal surface area available for adsorption ranges up to several hundred square meters per gram, and some zeolites are capable of adsorbing up to about 30 weight percent of a gas, based on the dry weight of the zeolite
In addition to their ability to separate gas molecules on the basis of size and shape, the unusual charge distribution within a dehydrated void volume allows many species with permanent dipole moments to be adsorbed with a selectivity unlike that of almost all other sorbents.
Thus, polar molecules such as water sulfur dioxide, hydrogen sulfide, and carbon dioxide are preferential] y adsorbed by certain zeolites over nonpolar molecules, such as methane, and adsorption p recesses have been developed using natural zeolites by which carbon dioxide and other contaminants can be removed from impure natural gas or methane streams, allowing the gas to be upgraded to high-Btu products.
In addition, the small, but finite, quadripole moment of nitrogen allows it to be adsorbed selectively from air by a dehydrated zeolite, producing oxygen-enriched streams at relatively low cost at room temperature.
Both of the above processes may find application in agricultural technology
Dehydration-rehydration properties:
Because of the uniform nature of the pores of structural cages, crystalline zeolites have fairly narrow pore-size distributions, in contrast to other commercial absorbents, such as activated alumina, carbon, and silica gel.
Adsorption on zeolites is therefore characterized by Langmuir-type isotherms, Here, percent of adsorption capacity is plotted against partial pressure of the adsorbate gas.
Note that almost all of the zeolite’s adsorption capacity for a particular gas (including water) is obtained at very low partial pressures, meaning that although their total adsorption capacity may be somewhat less than those of other absorbents (e.g., silica gel), zeolites are extremely efficient adsorbents even at low partial pressures.
This property has been used in the zeolitic adsorption of traces of water from Freon gas lines of ordinary refrigerators that might otherwise freeze and clog pumps and valves
The extreme nonlinearity of the water adsorption isotherms of zeolites has been exploited recently in the developrnent of solar-energy refrigerators
lon-exchange properties:
The exchangeable cations of a zeolite are also only loosely bonded to the tetrahedral framework and can be removed or exchanged from the framework structure easily by washing with a strong solution of another element.
As such, crystalline zeolites are some of the most effective ion exchangers known to man, with capacities of 3 to 4 meq per gram being common.
This compres with the 0.8 to 1.0 meq per gram cation exchange capacity of bentonite, the only other significant ion-exchanger found in nature Cation-exchange capacity is basically a function of the degree of substitution of aluminum for silicon in the zeolite framework:
the greater the substitution, the greater the charge deficiency of the structure, and the greater the number of alkali or alkaline earth atoms required for electrical neutrality.
In practice however, the cation-exchange capacity is dependent on a number of other factors as well In certain species, cations can be trapped in structural positions that are relatively inaccessible, thereby reducing the effective exchange capacity of that species for that ion, Also, cation sieving may take place if the size of the exchanging cat ion is too large to pass through the entry channels into the central cavities of the structure Cation exchange between a zeolite (Z) and a solution (S) is usually shown by means of an exchange isotherm that plots the fraction of the exchanging ion (X) in the zeolite phase against that in the solution Such is the selectivity of clinoptilolite for cesium or ammonium, for example.
Clinoptilolite will take up these ions readily from solutions even in the presence of high concentrations of competing ions, a facility in their development of an ion-exchange process to remove ammoniacal nitrogen from sewage effluent
Fertilizer and Soil Amendments
Based on their high ion-exchange capacity and water retentivity, natural zeolites have been used extensively in Japan as amendments for sandy soils, and small tonnages have been exported to Taiwan for this purpose (The pronounced selectivity of clinoptilolite for large cations, such as ammonium and potassium, has also been exploited in the preparation of chemical fertilizers that improve the nutrient-retention ability of the soils by promoting a slower release of these elements for uptake by plants, In rice fields, where nitrogen efficiencies of less than 50 percent are not uncommon, Minato reported a 63 percent improvement in the amount of available nitrogen in a highly permeable paddy soil 4 weeks after about 40 tons/acre zeolite had been added along with standard fertilizer
the other hand, noted little change in the vitrification of added ammonia when
clinoptilolite was mixed with a Texas clay soil, although the overall ion-exchange capacity of the soil was increased.
He attributed these conflicting results to the fact that the Japanese soils contained much less clay, thereby accounting for their inherent low ion-exchange capacity and fast-draining properties.
The addition of zeolite, therefore, resulted in a marked improvement in the soil’s ammonium retentivity
These conclusions support those of Hsu, et al who found an increase in the effect of zeolite additions to soil when the clay content of the soil decreased
Although additions of both montmorillonite and mordenite increase the cation-exchange capacity of upland soils, the greater stability of the zeolite to weathering allowed this increase to be retained for a much longer period of time than in the clay-enriched soils
Using clinoptilolite tuff as a soil conditioner the Agricultural Improvement Section of the
Yamagata Prefectural Government, Japan, reported significant increases in the yields of
wheat (13 to 15 percent), eggplant (19 to 55 percent), apples (13 to 38 percent), and carrots (63 percent) when from 4 to 8 tons of zeolite was added per acre
Small, but significant improvements in the dry-weight yields of sorghum in greenhouse experiments using a sandy loam were noted when 0.5 to 3.0 tons of clinoptilolite per acre was added along with normal fertilizer.
However, little improvement was found when raising corn under similar conditions. Hershey, et al. showed that clinoptilolite added to a potting medium for chrysanthemums did not behave like a soluble K source, but was very similar to a slow-release fertilizer, The same fresh-weight yield was achieved with a one-time addition of clinoptilolite as with a daily irrigation of Hoagland’s solution, containing 238 ppm K, for three months (total of 7 g potassium added), with no apparent detrimental effect on the plants)
Both zeolite treatments apparently made considerably more ammonium available to the
plants, especially when clay-poor soils were employed.
The authors suggested that ammonium-exchanged clinoptilolite acted as a slowrelease fertilizer, whereas, natural clinoptilolite acted as a trap for ammonium that was produced by the decomposing urea, and thereby prevented both ammonium and nitrate toxicity by disrupting the bacterial vitrification process
The ammonium selectivity of zeolites was exploited by Varro (85) in the formulation of a fertilizer consisting of a 1:1 mixture of sewage sludge and zeolite, wherein the zeolite apparently controls the release of nitrogen from the organic components of the sludge
pesticides, Fungicides, Herbicides
Similar to their synthetic counterparts, the high adsorption capacities in the dehydrated state and the high ion-exchange capacities of many natural zeolites make them effective carriers of herbicides, fungicides, and pesticides
Clinoptilolite can be an excellent substrate for benzyl phosphorothioate to control stem blasting in rice
Using natural zeolites as a base found that clinoptilolite is more than twice as effective as a carrier of the herbicide benthiocarb in eliminating weeds in paddy fields as other commercial products.
Heavy Metal Traps
Not only do the ion-exchange properties of certain zeolites allow them to be used as carriers of nutrient elements in fertilizers, they can be exploited to trap undesirable metals and prevent their uptake into the food chain.
Pulverized zeolites effectively reduced the transfer of fertilizer-added heavy metals, such as copper cadmium, lead, and zinc, from soils to plants
In view of the attempts being made by sanitary and agricultural engineers to add municipal and industrial sewage sludge to farm and forest soils, natural zeolites may play a major role in this area also.
The nutrient content of such sludges is desirable, but the heavy metals present may accumulate to the point where they become toxic to plant life or to the animals or human beings that may eventual] y eat these plants
Zeolite additives to extract heavy metals may be a key to the safe use of sludge as fertilizer and help extend the life of sludge-disposal sites or of land subjected to the spray-irrigation processes now being developed for the disposal of chlorinated sewage.
the addition of clinoptilolite to soils contaminated with radioactive strontium (Sr) lead to marked decreasing in (sr) in the uptake of strontium by plants, an observation having enormous import in potential treatment of radioactive fallout thntaminat
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