The Ion Exchange Behavior of Oxides

Oxides comprise the most common ion-exchange materials on our planet, with the clay minerals alone, formed by the weathering of rock, having a total mass of around 1025 g. This mass represents almost one-third of the total mass of Earth’s crust and is more than six times the mass of Earth’s oceans. These fine-grained ion exchange materials play a major role in mediating the concentrations of ionic species found in freshwater, groundwater, and our oceans (see Chapter 18). Oxide ion exchangers are also of critical importance in removing contaminants from the environment. Nowhere is this role more apparent than in the removal and sequestration of radioactive elements such as 137Cs, 90Sr, and 99Tc, which are serious hazards present in nuclear wastes. Oxide ion exchangers exhibit several properties that make them materials of choice for treating nuclear wastes, including high selectivity, enhanced stability to radiation damage relative to organic exchangers, and the potential as materials to be condensed further into solid waste after they are loaded with radioactive species. Oxide exchangers are extremely useful for extracting valuable cations from complex fluids, such as the lithium used in our highest energy density batteries. Ion exchange also represents a pathway for creating unique nanomaterials, with applications including battery separators, catalysts, optical materials, magnets, and materials for drug delivery. Oxides materials can exhibit exceptional properties as both cation and anion exchangers for a wide range of separation and water treatment technologies. Although the total ion-exchange capacity of an oxide is important for some applications, such as the deionization of water, separations require the use of oxides and hydroxides having the highest degree of ion-exchange selectivity. For selectivity, oxides must be designed with specific sites that exhibit a much higher affinity for one ion than any other, which requires much more sophistication than just generating a net charge. Here, we describe the key factors that control both the capacity and selectivity of inorganic ion exchangers, including (1) the role of acid–base reactions in controlling surface charge and ion-exchange capacity, (2) the role of local charge distributions in determining ion-exchange selectivity, and (3) the effect of shape and selective solvation on enhancing that selectivity.