Types of Ion Exchange Sites in Inorganic Materials and their Origin

For the purposes of this chapter, ion exchange interactions will be defined as those involving the interchange of positively or negatively charged species (atomic or molecular) at an ion exchange site.

There are two types of chemical species which constitute the vast majority of ion exchange sites in inorganic materials:

1. structure-terminating, covalently bonded groups such as -OH

2. charge-compensating groups, electrostatically associated with, and not covalently bonded to, a charged moiety

Type 1 sites, illustrated in Figure 1A, are responsible for the ion exchange properties of materials such as hydrous oxides and single-layer clays. All oxidic materials have these sites to some degree, at the surfaces of particles or crystals or at defect sites within the structure. Ion exchange reactions involving these types of sites may be regarded as chemical reactions, which may display amphoteric nature.

Type 2 sites, illustrated in Figure 1B, are responsible for most of the ion exchange capacity of zeolites, double-layer clays and zirconium phosphates. These sites arise in structures possessing, for instance, charged layers or charged porous frameworks. The exchangeable ions are present to retain overall elec-troneutrality. When materials such as zeolites are concerned, a mixture of Type 1 and Type 2 sites is available, although Type 2 sites will usually greatly outnumber Type 1 sites, and the latter are often ignored. Exchange interactions involving Type 2 sites are physical in nature, as chemical bonds are neither made nor broken.

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