Actinide complexes hydroxides

Salts of 0x0 anions, snch as nitrate, snlfate, perchlorate, iodate, hydroxide, carbonate, phosphate, oxalate, and so on, are important for the separation and reprocessing of technologically important actinides, while hydroxide, carbonate, and phosphate ions are important for the chemical behavior of the actinides in the enviromnent. The general trends of complexes formed in aqneons solntions are as follows ... 

Mono(organo)nickel compounds, via oxidative addition, 8, 44 Monoorganotin hydroxides, preparation, 3, 850 Monoorganotin oxides, preparation, 3, 850 Mono(pentamethylcyclopentadienyl) actinide(IV) compounds, reactions, 4, 207 Mono(pentamethylcyclopentadienyl) lanthanide(III) compounds, synthesis and characteristics, 4, 66 Mono(pentamethylcyclopentadienyl) uranium(IV) sulfido complex, synthesis, 4, 207-208 Mono(phenoxy-aldehyde) trichlorides, with Zr(IV),... 

Hydrolysis reactions are common to all actinide ions in nearneutral solutions, and take place either in parallel with or predominantly over other complexation reactions. In connection with the migration studies of actinide ions in natural waters, attention recently has been focused on hydrolysis reactions of actinides since these reactions are important in determining the solubility of the actinide hydroxide or oxide. Although numerous studies have been made (1-4) to determine stability constants of various hydrolysis products, much of the necessary data are still lacking. The acquisition of these data and further improvement or verification of the existing data is desirable. 

Addition of ammonium carbonate to a solution containing an actinide(III), (IV),(V) or (VI) ion gives the following results. Only actinide(VI) ions form soluble carbonato complex ions. Actinide(III) and (IV) ions precipitate as their hydroxides or basic carbonates, and actinide(V) ion precipitates as a double carbonate. Therefore, in dilute ammonium carbonate medium, U(VI) ion can be separated primarily from Np(V), Pu(IV), Am(III) and Cm(III) ions. Further addition of ammonium carbonate leads to complex ion formation and the dissolution of actinide(IV) precipitates. However, most of the actinide(III) and (V) ions remain as precipitates under this condition. Crystalline precipitates of actinide(IV) and (VI) carbonato complex anions are formed by addition of hexamminecobalt(III), hexaureachromium(III) or hexa-mminechromium(III) salt to the ammonium carbonate solution containing actinide(IV) and (VI) ions. 

Actinide(V) and (VI) ions form soluble complex ions with peroxide ion in slightly alkaline medium, whereas actinide(III) and (IV) ions precipitate as hydroxides. Actinide(VI) ions in slightly alkaline hydrogen peroxide solution precipitate upon addition of cobalt(III) complex salts. Figure 7 shows the precipitation behavior of U(VI) peroxo complex ion with the following kinds of cobalt(III) complex salts ... 

The solution reactions of Cm3+ closely resemble those of the lanthanide and actinide +3 ions, and the fluoride, oxalate, phosphate, iodate, and hydroxide are insoluble. Complexes appear to be weaker than those of preceding elements. 

Hydroxides. The hydrolysis and carbonate complexation of the actinides has been recently reviewed." Plutonium(III) hydrolysis is not well known because Pu is readily oxidized to Pu in aqueous solutions, particularly at near-neutral and basic pH. The first hydrolysis product, Pu(OH) ", has been identified in acid solution up to pH 3 (where it is about 70% formed) before oxidation to Pu prevents further study." The first hydrolysis product of Np has been similarly studied." The hydroxide solids, Pu(0H)3 xH20 and Np(0H)3 xH20, are prepared by precipitation and presumed to be isostuctural with Am(OH)3. 

Hydroxides. The hydrolysis of Np has been studied more than that of any other pentavalent actinide because it is the most stable oxidation state for Np and it is an actinide ion of significant concern for environmental migration. Pentavalent uranium disproportionates in aqueous solution at pH values where hydrolysis would occur. Hydrolysis products for Pa, Pu, and Am are very similar to, but much less stable than those of Np, so only Np hydroxides will be described in detail. Neptunyl hydrolyzes at about pH 9, to form the stepwise products, Np02(0H) and Np02(0H)2 ", which have been identified by optical absorbance and Raman spectroscopy. " In addition to the hydroxide these complexes likely have two or three inner-sphere waters in the equatorial plane and pentagonal bipyramidal coordination geometry. 

The chemistry of actinide ions is generally determined by their oxidation states. The trivalent, tetravalent and hexavalent oxidation states are strongly complexed by numerous naturally occurring ligands (carbonates, humates, hydroxide) and man-made complexants (like EDTA), moderately complexed by sulfate and fluoride, and weakly complexed by chloride (7). Under environmental conditions, most uncomplexed metal ions are sorbed on surfaces (2), but the formation of soluble complexes can impede this process. With the exception of thorium, which exists exclusively in the tetravalent oxidation state under relevant conditions, the dominant solution phase species for the early actinides are the pentavalent and hexavalent oxidation states. The transplutonium actinides exist only in the trivalent state under environmentally relevant conditions. 

---