THE COMPARATIVE ION-EXCHANGE CAPACITIES OF NATURAL SEDIMENTARY AND SYNTHETIC ZEOLITES

A large tonnage of natural, sedimentary zeolites is found in the Western United States and world-wide. They offer a substantial potential for a variety of industrial uses including the removal of cations from acid mine drainage and industrial waste waters. There is a wide variation in the cation exchange capacity of zeolites because of the differing nature of various zeolite cage structures, natural structural defects, adsorbed ions, and their associated gangue minerals. In general the effectiveness of several natural and synthetic zeolites has been found to be: Erionite > 13X > Chabazite > Clinoptilolite > > Mordenite congruent to 3A congruent to 4A congruent to 5A where 3A, 4A, 5A and 13X are synthetic zeolites. The approximate order of effectiveness for cation removal from pristine solutions is: Ag+ > Pb+2 > Cd+2 > Zn+2 greater than or equal to CU+2 > > Ni+2 > Hg+2 A pre-exchange process with strong Na+ solutions can remove deleterious exchangeable cations from the natural zeolites and can significantly improve their cation exchange capacity. The uptake of heavy metal ions has been studied using both the batch and column techniques, and the exchange capacities of 'as received', beneficiated and pre-exchanged zeolites have been evaluated. Electron microprobe studies have shown the reason for the relatively poor ion exchange performance of the synthetic zeolites to be due to a diffusion problem. A means of mitigating the problem for the 13X zeolite is given.