Dyer, 2000, explains that the "first scientific allusion to ion exchange is attributed to two English agricultural chemists in 1850. These were J. T. Way and H. S. Thompson, who independently observed the replacement of calcium in soils by ammonium ions. This discovery was the precursor to the study of inorganic materials capable of 'base' exchange, and in 1858 C. H. Eichorn showed that natural zeolite minerals (chabazite and natrolite) could reversibly exchange cations. The importance of this property in water softening was recognized by H. Gans who, at the turn of the century, patented a series of synthetic amorphous aluminosilicates for this purpose". "A broad definition of ion exchange is that it is the transfer of ions across a boundary; this would then cover movement of ions from one liquid phase to another" . "An ideal ion exchange medium is one that fulfils the following criteria: 1. a regular and reproducible composition and structure; 2. high exchange capacity; 3. a rapid rate of exchange (i.e. an open porous structure); 4. chemical and thermal stability and resistance to 'poisoning' as well as radiation stability when used in the nuclear industry; 5. mechanical strength stability and attrition resistance; 6. consistency in particle size, and compatibility with the demands of the use of large columns in industry" . Moreover, in the case of specific applications, it is relevant "the ability to exchange a specific ion(s) selectively from high concentrations of other ions. This is particularly true for aqueous nuclear waste treatment and in hydrometallurgy" . , studied the "ion-exchange behavior of a natural zeolite from the Zahedan region of Iran toward various heavy-metal cations. The distribution coefficients, equilibrium constant, and Gibbs free energy are calculated from the isotherms data at 298K and 323K as the thermodynamic parameters. ... The ion-exchange experiments of the zeolite are performed to investigate its ability for removing the considered cations from industrial wastewater streams". , note that, "since its first discovery in nature, natrolite has been largely known as a sodium aluminosilicate zeolite, showing very limited preference toward cation exchange". In their article, Lee and coworkers show that "fully K-exchanged natrolite can be prepared from natural Na-natrolite under mild aqueous conditions and used to subsequently produce Rb-and Cs-exchanged natrolites. These cation-exchanged natrolites exhibit successive volume expansions by ca. 10, 15.7, and 18.5% for K-, Rb-, and Cs-forms, respectively, compared to the original Na-natrolite". The volume expansion "occurs by converting the elliptical channels into progressively circular ones. The observed cation-dependent changes in the channel volume and shape thus show the flexibility limits of the natrolite framework and suggest the possible existence of compositionally altered analogues in suitable environments as well as a novel means to tailor the cation selectivity of this class of small pore zeolites toward various industrial and environmental applications" .