"La Roca magica", so printed a Cuban newspaper, in applaud of one of its country's greatest mineral resources - Zeolites. The first zeolite was identified in 1756 (marking this year as its 250th anniversary) by the Swedish mineralogist (Baron) Friedrich Axel Cronstedt, who observed that on heating the stones he had gathered in a blow-pipe flame, they danced about in a froth of hot liquid and steam, appearing as if the stones themselves were boiling. He thus coined the name "zeolite" which from Greek derivation means "stones that boil". The phenomenon he observed provides a vital clue to an essential property of zeolites, which is their ability to absorb a substantial proportion - perhaps half their own volume, depending on the type of zeolite - of water, and indeed of other liquids. Zeolites are aluminosilicates whose essential structure consists of a negatively charged "honeycomb-like" framework, containing (micro)pores of molecular dimensions, normally less than 13 Angstrom units (1.3 nanometres) in diameter. The pores contain sufficient (positively charged) cations to neutralise the framework electric charge, but these are loosely bound and may be exchanged with other cations from solutions placed in contact with the zeolite.
This combination of features confers particular properties upon zeolites and from which unfold a wealth of applications for them. I have already noted the use of natural zeolites (clinoptilolite) in absorbing radioactive caesium (134 and 137) and strontium (90) from the waters used in running nuclear power plants (see my previous posting: "The Stones that Boil - Radioactive Waste Management"). Indeed, it is estimated that the world level industry based on zeolites is worth around $2 trillion (i.e. $2,000 billion) annually. 4 million tonnes of natural zeolites are mined annually, of which 2.5 million tonnes are shipped to China, mainly to make concrete to supply a burgeoning construction industry as the country undergoes an unprecedented phase of industrialisation. There are 48 different types of zeolite known to occur naturally, while another 150 or so have been artificially synthesised. Synthetic zeolites can be engineered with a selectivity to perform specific tasks, and they are in any case of a more homogeneous composition than their naturally occurring counterparts. A good example of a tailored zeolite is H-ZSM-5. It is designated "H" because it is the hydrogen (proton) exchanged form that is referred to, "ZSM" because these are the initial letters of the surnames of the three scientists who created the framework material, and "5" because it was the fifth attempt that worked, attesting to the often serendipitous nature of zeolite synthesis: the previous four batches were presumably consigned to the Mobil waste disposal enterprise.
H-ZSM-5 was introduced by Mobil in 1978 to catalyse the "methanol to gasoline" (MTG) process, in which it cracked methanol (an organic compound with just one carbon atom) into hydrocarbon mixtures (generally of compounds containing 6 to 9 carbon atoms) which are suitable for combustion in internal combustion engines. H-ZSM-5 is also used on a large scale for the selective production of para-xylene, which is oxidised to terephthalic acid and then esterified with glycols to make "polyesters" for the clothing industry.
For environmental applications, it is preferable to use a natural zeolite which can be mined (ideally locally to minimise transportation requirements) and used with the minimum of processing: just crushing the raw material into a powder may be all that is needed. Natural zeolites are produced by the forces of volcanism, and are often associated with mountain ranges, e.g. the Caucasus and the Balkans, while there are also deposits found in the Himalayas and in Switzerland. Essentially, the force of molten magma, which pushes up mountains can escape through a volcanic vent and the ash/glass that is produced may turn (crystallize) into a zeolite by contact and reaction with alkaline lake or ground-waters. I have a nice slide showing a zeolite deposit in New Mexico, which shows a layer of brown tuffacious rock ("tuff") - compressed ash - lying above a layer of pure white zeolite into which it has been converted by contact with alkaline groundwater over a period of up to 50,000 years.
As noted the applications of zeolites are manifest, of which the following list is merely indicative:
*Cation exchange: radioactive decontamination, e.g. removal of Sr and Cs from "dump waters" of nuclear power stations; industrial "water softeners", to prevent lime-scale blocking up cooling pipes in manufacturing facilities; removal of heavy metals from the environment, e.g. lead, zinc, copper, mercury, cadmium.
*Use of zeolites as a "builder" in detergents, to remove and encapsulate Ca2+ and Mg2+ cations which make water "hard", rather than polyphosphates which cause algal bloom in lakes and rivers.
*Anion absorption. Environmental contamination by toxic anions may also be removed, by reaction with heavy metal cations previously exchanged into the zeolite, e.g.:
Ag+ - zeolite + Na+ I- --> Na+ - zeolite + AgI (precipated). In this example, a silver exchanged zeolite can be used for removing radioactive iodine (iodide ions) in the form of insoluble AgI, which is both formed and contained within the zeolite pores. When it is saturated, the material may be removed for disposal.
The principle may be adapted for the management of other toxic anions: e.g. cyanide, arsenic (both arsenite and arsenate), chromate, molybdate and others.
*Molecular sieves: small pore zeolites selectively absorb small polar molecules, e.g. water, and so zeolite "molecular sieves" are highly efficient drying agents for removing traces of water from other solvents.
*Hydrocarbon sieving: linear n-alkanes (needed for detergent manufacture) can be separated from branched alkanes, since the former pass more slowly through a column packed with zeolite 5A in consequence of their preferential absorption within the zeolite pores, which results in a more tortuous passage through the material. Millions of tonnes of n-alkanes are produced annually by this method.
*H+ exchanged zeolites (e.g. H-ZSM-5) are used as solid acid catalysts, e.g. for "cracking" in the petrochemical industry.
*Medical applications: Hemosorb and KwikKlot are commercial products based on zeolites which when applied to wounds (in accidents or surgery) are said to cause an "instant" cessation of bleeding. Also used in kidney dialysis machines, to absorb ammonia from blood and prevent it from building up (a job that healthy kidneys normally do).
*Agriculture: for supplying K+ and NH4+ to plants from soils that have been enriched with zeolites exchanged specifically with these cations. It is suggested that such "zeoponics", as the strategy is called, might be used to grow food on long space missions, e.g. if we ever send "a man to Mars".
*Separation of gases: there are commercial units that can provide oxygen of 95% purity for use in hospitals or for patients e.g. suffering from emphysema and other forms of Obstructive Pulmonary Disease (OPD), by separating it from air. Nitrogen (80% of air) is preferentially absorbed over oxygen because of its much larger molecular electric quadrupole moment, and so enables oxygen to separate from air almost in a state of purity.
*Use in more efficient heating systems. Essentially, the adsorbed water can be driven out of a zeolite by heat, but when the water is readsorbed, heat is given out. The principle can be incorporated into a heat-pump system which uses more of the available energy for actual heating, most of which would otherwise be wasted.
*Desulphurisation of diesel: Ni2+ exchanged zeolites have been demonstrated to absorb sulphur compounds from diesel in accord with an aim to reduce "acid-rain" emissions from transportation.
*Reduction in NOx emissions from vehicles, using zeolite-loaded "catalytic converters".
*Use of natural "tuff" as a light-weight building material, since the rock is soft enough to be cut with a hand saw and durable in fairly dry climates, or it can be fabricated into light-weight cements, e.g. in China which consumes 2.5 million tonnes of zeolite per annum for this purpose.
Zeolites are also effective in remediation strategies, e.g. around 500,000 tonnes of zeolites were used in the clean-up after the nuclear power plant disaster at Chernobyl [see my previous listing: "Chernobyl (26th April 1986)."], which involved monopolising virtually every zeolite production facility in the entire former U.S.S.R. Zeolites were fed to cattle in an effort to keep the radioactive ions out of the milk, and were baked into bread and into biscuits (cookies) for children similarly in an effort to minimise radioactive contamination in humans. Zeolites were also used (albeit on a smaller scale since the problem was far more contained) at "Three Mile Island" a decade or so before Chernobyl. Contaminated, e.g. Brown Field land may be rendered fit for building and even for agriculture by treating the soil with sufficient quantities of zeolites.
Clearly, the uses of zeolites are manifest, and offer unique environmental benefits. I am not a native Spanish speaker, but "La Roca Magica" seems to say it all. I recall that synonyms for "Magica" (the adjective form of "Magico") include "Estupenda" and "Maravillosa". Who could disagree?!
[Based on an invited lecture given by Professor Chris Rhodes at Kingston University, London, on March 15th, 2006].