Environmental Applications of Natural Zeolitic Materials Based on Their Ion Exchange Properties

Present and potential use of natural zeolites as cation exchangers in environmental protection is reviewed. Siliceous zeolites, such as chabazite, clinoptilolite, mordenite and phillipsite, exhibit good selectivities for cations with low charge density, e.g., Cs+ and NH₄+, and for cations with low hydration energy, such as Pb2+. Zeolitised tuffs, containing the mentioned zeolites, may therefore be utilised for removing the above and other cations from wastewaters before discharge.Continuous processes with fixed beds are usually employed for water purification, such as those in service in the USA and in other countries around the world for ammonium removal from municipal sewage. Direct addition of the ion exchanger is needed when the pollutant, e.g., radionuclide, must be removed from soil and trapped in the zeolite framework, such as in the case of the Chernobyl nuclear accident.Removal of heavy metals from wastewaters is made difficult by the complexity of the effluents to be treated, e.g., by the presence of several cationic pollutants for which zeolite does not exhibit comparably high selectivities.Discontinuous processes (addition of zeolite to the waste solution) are also possible, and recommended in the case of low process efficiency, provided that the zeolitic sludge, resulting from removal of polluting cations, is stabilised-solidified in a cement matrix before disposal or re-use.