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Black plate (230,1) . Toxic heavy metal cations, e.g. Pb2 þ , Cd2 þ , Zn2 þ , may also be removed from the environment by cation-exchange into zeolites. . Hþ exchanged zeolites, e.g. ultrastable zeolite Y (USY) are used as solid acid catalysts in the petrochemical industry. Around 300,000 tonnes of synthetic zeolites are manufactured annually for his purpose. . e.g. Ni2 þ -exchanged zeolites have been demonstrated to absorb sulfur compounds (thiophene derivatives and thiols) from model ‘‘diesel’’ in accord with a desire to to reduce transportation-based SO2 emissions, and hence the damaging effects of ‘‘acid rain’’ on rivers and the facades of buildings. . Reduction in NOx emissions from vehicles, using zeolite-loaded ‘‘catalytic converters’’. . Surfactant-modified zeolites have a potential to remove toxic anions, e.g. chromate and organic pollutants, e.g. trichloroethylene, simulta- neously from the environment. It is common that a region is co- contaminated with a mixture of pollutants and such a multifunctional decontamination agent might be very useful. Contaminated ground- waters are a good example. . Anion absorption. Environmental contamination by toxic anions may also be eliminated, by reaction with heavy metal cations previously exchanged into the zeolite, e.g.: Agþ zeolite þ Naþ I ° Naþ zeolite þ AgI ðprecipatedÞ: In this example, a silver exchanged zeolite can be used for removing radioactive iodine (in the form of iodide ions) as insoluble AgI, which is both formed and held within the zeolite matrix. When it is saturated, the zeolite 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 (such as zeolite-A) 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 to make detergents) can be separated from branched alkanes, since the former pass more slowly through a column packed with zeolite 5A in consequence of their preferential penetration of 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. 230 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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