Anion exchangers:
It may be important to mention here that certain aluminosilicates can also behave as anion exchangers. Within montmorillonite, feldspar and kaolinite of sodalite and camerinite groups the exchange of OH- for Cl-, SO4 2- and PO4 3- has been observed. There are some problems with the use of zeolites as ion exchangers because of some of their properties. The zeolites are soft minerals and thus, are not very abrasive resistant. They have poor mechanical strength and their frameworks are more rigid therefore less open. They swell extremely little and the counter ions in their pores do not move extremely freely. Above all, they suffer partial decomposition through acids and alkalis.
Another lesser known variety of natural ion exchangers is some categories of coals. They hold carboxylic and possibly other weak acid groups. They, therefore, could be used as cation exchangers. Many of these materials swell excessively and are decomposed through alkali. They are, thus, stabilized before use. Soft and hard coals are stabilized through metal ion solutions. Here most lignites and bituminous coals and anthracites could be converted within strong cation exchangers through sulphonation along with fuming strong sulphuric acid. These coals have very limited applications.