By Wenzhong Zhang
One of the few opportunities that people have to get in contact with ion exchange materials is when they are using water pitchers and water filters. In most cases, when you get curious and try to disassemble a used ion exchange module, you see a wrapped soft membrane or small mysterious and colored beads inside (see picture below). The materials comprising the beads are organic ion exchangers, namely resin. Synthetic organic ion exchangers were first introduced for industrial use during the 1930s. They are made by crosslinked organic polymeric substances and have the ability of exchanging with ions. Because of their quite inexpensive cost and the capability to exchange with a wide range of ions, they are the most suitable candidate for the majority of industrial applications.
Inorganic ion exchange materials have a completely different structural composition to their organic counterparts. The only thing they have in common is that they can also exchange ions. Inorganic ion exchangers are much heavier and much more expensive, but what are the perks of being pure inorganic? Here is a brief summary.
The ion selectivity of inorganic ion exchangers arises from their rigid framework structure and only the ion with matching size could enter the framework. This also gives the scientists the opportunity to tune the structural build blocks of the material and match specific needs. With the organic resins, it is harder to aim for high selectivity because of the soft polymer network.
Endurance of operation condition
Generally, water treatment requires only mild operation conditions: ambient temperature and pressure. Organic materials degrade under high temperatures and shrink under high pressure. More importantly, with higher amount of background salt concentration, resins would fail to work. That gives the opportunity for inorganic ion exchangers. They can endure much worse operation conditions even with high level of radioactivity.
Bacteria cause problems, not only human health problems but also the contamination of ion exchange resins. They can grow happily and properly inside the organic particle. Whereas for inorganic materials, this is not a problem.
For the recovery of rare earth elements from bauxite leachate, ion exchange materials with very high selectivity and acid endurance are sought. Hence, inorganic ion exchange materials should have a spot in the application to this end.