In recent years, zeolites have been mentioned in the conservation literature, but information on what they are and how they work has been scarce. (A recent paper on passive conservation techniques was given by Yvonne Shashoua of the British Museum at the resins conference in Aberdeen, September 13-14, 1995. She compared the performance of gaseous HCl absorbers and found that the best absorber was zeolite.)
The PQ Corporation's Zeolites and Catalysts Division has given permission to reprint parts of their brochure, "Zeolites and the Environment: The Year 2000," which should fill part of that need. For more information, call the company at 1-800/934-7887 or fax them at 215/293-7510.
The term zeolite was originally coined in the 18th century by a Swedish mineralogist named Cronstedt who observed, upon rapidly heating a natural zeolite, that the stones began to dance about as the water evaporated. Using the Greek words which mean "stone that boils," he called this material zeolite. A commonly used description of a zeolite is a crystalline aluminosilicate with a cage structure. Technically, we speak of a zeolite as a crystalline hydrated aluminosilicate whose framework structure encloses cavities (or pores) occupied by cations and water molecules, both of which have considerable freedom of movement, permitting ion exchange and reversible dehydration. This definition places it in the class of materials known as "molecular sieves." [The pores in dehydrated zeolite are 6 Ångstroms in size, while those of a typical silica gel average about 50 Å, and activated carbon averages 105 Å.]
More than 150 zeolite types have been synthesized and 40 naturally occurring zeolites are known. [The formula of one of the better known zeolites is shown below.] ... Zeolites occur as hydrates, and all members of the family contain at least one silicon atom per aluminum atom.
Na-A Na2O · Al2O3 · 2SiO2 · 4.5 H2O
Zeolites form in nature as a result of the chemical reaction between volcanic glass and saline water. Temperatures favoring the natural reaction range from 27°C to 55°C, and the pH is typically between 9 and 10. Nature requires 50 to 50,000 years to complete the reaction.
Naturally occurring zeolites are rarely phase-pure and are contaminated to varying degrees by other minerals [e.g. Fe++, SO4-, quartz, other zeolites, and amorphous glass]. For this reason, naturally occurring zeolites are excluded from many important commercial applications where uniformity and purity are essential.
Synthetic zeolites hold some key advantages over their natural analogs. The synthetics can, of course, be manufactured in a uniform, phase-pure state. It is also possible to manufacture desirable zeolite structures which do not appear in nature. Zeolite A is a well-known example. Since the principal raw materials used to manufacture zeolites are silica and alumina, which are among the most abundant mineral components on earth, the potential to supply zeolites is virtually unlimited. Finally, zeolite manufacturing processes engineered by man require significantly less time than the 50 to 50,000 years prescribed by nature.
All commercially useful zeolites owe their value to one or more of three properties: adsorption, ion exchange, and catalysis.
The most fundamental consideration regarding the adsorption of chemical species by zeolites is molecular sieving. Species with a kinetic diameter which makes them too large to pass through a zeolite pore are effectively "sieved." This "sieve" effect can be utilized to produce sharp separations of molecules by size and shape.
The particular affinity a species has for an internal zeolite cavity depends on electronic considerations. The strong electrostatic field within a zeolite cavity results in very strong interaction with polar molecules such as water. Non-polar molecules are also strongly adsorbed due to the polarizing power of these electric fields. Thus, excellent separations can be achieved by zeolites even when no steric hindrance occurs.
Adsorption based on molecular sieving, electrostatic fields, and polarizability are always reversible in theory and usually reversible in practice. This allows the zeolite to be reused many times, cycling between adsorption and desorption. This accounts for the considerable economic value of zeolite in adsorptive applications.
Because cations are free to migrate in and out of zeolite structures, zeolites are often used to exchange their cations for those of surrounding fluids. The preference of a given zeolite among available cations can be due to ion sieving or due to a competition between the zeolite phase and aqueous phase for the cations that are present.
... Sodium zeolite A is among the world's most efficient removers of water hardness ions. This is its principal function as a detergent builder.
Catalysis
Zeolites make extremely active catalysts.... Steric phenomena are very important in zeolite catalysis, and a new term, "shape selective catalysis," was coined to describe these effects. Extremely selective reactions can be made to occur over zeolites [when certain products, reactants or transition states are kept from forming within the pores because of size or shape].
Far and away, the largest outlet for zeolite volume is the global laundry detergent market. By the end of 1992 [it was expected that the world would] be consuming detergent zeolite (zeolite A) at the rate of 1.44 million anhydrous metric tons per year.
Unlike phosphates, zeolite A cannot contribute to the eutrophication of lakes, streams or bays....
Other Applications: Refrigeration
The heat of water adsorption for zeolites is high. They also possess high adsorption capacity, undergo reversible adsorption/desorption, and are structurally stable. These properties enable zeolite to be used in solar-powered refrigerators and to store energy during off-peak periods and release it during peak periods. Zeolites can also be used in refrigeration and air cooling systems to reduce water in the air to very low concentrations, allowing very effective evaporative cooling to occur.
Zeolites are found in abundance throughout the world…. However, the value of the synthetic zeolites sold is far higher…. Many of the uses for natural zeolites are environmentally related.
Natural zeolites are being used to treat low and intermediate aqueous waste. Current users are British Nuclear Fuels in Great Britain, West Valley Nuclear and Date Ridge National Laboratory. Natural zeolite has been used in the clean-up at Three Mile Island and Chernobyl.
Certain natural zeolites have a high affinity for ammonium ions and are being used in a tertiary water treatment system at Truckee, California. Municipal effluent is treated by passing it through columns packed with a natural zeolite clinoptilolite to reduce the ammonium ion concentration to less than 2 ppm.
Natural zeolites are uniquely effective in adsorbing ammonia and also adsorb hydrogen sulfide. These properties make natural zeolites ideal for use in pet litter to prevent emanation of irritating odors. For similar reasons, natural zeolites can be used for effective control of irritating gases in horse stalls, barns, kennels, etc.
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