Uranyl cation phosphate complexes

Adsorption of cations or anions may be greatly favored or inhibited when they occur as complexes rather than as free (uncomplexed) ions. For example, the hydroxide complexes of uranyl ion (UOl ) are strongly adsorbed by oxide and hydroxide minerals, whereas urany) carbonate complexes are poorly adsorbed by these minerals (Hsi and Langmuir 1985). In fact, carbonate, sulfate, and fluoride complexes of metals are often poorly adsorbed in general, whereas OH and phosphate complexes are usually readily adsorbed, particularly by oxide and hydroxide solids. Metal adsorption is considered at some length in Chap. 10. 

Complicating the development of ISEs for higher actinide ions is their inherent radioactivity. They also have chemistry tiiat often differs from that of the uranyl cation. Actinides from americium to lawrencium display solution-phase chemical features that resemble those of the trivalent lanthanides. Conversely, in certain oxidation states, the early actinides (thorium through neptunium) often mimic transition metals. Also, as mentioned above, many of the actinides can exist in a large number of oxidation states. For instance, in the case of plutonium, four oxidation states can exist simultaneously in aqueous solution. Finally, as true for the lanthanides, complex salts with hydroxide, halogens, perchlorates, sulfates, carbonates, and phosphates are well known for most of the actinides. 

Examples of phosphate stractures containing hydroxonium H3O+ cations are provided by hypo-phosphoric acid dihydrate, tetrametaphosphimic acid (13.23) and hydrogen uranyl phosphate tetra-hydrate, all of which possess complex systems of H bonds in the solid state [28-30]. 

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