Actinide complexes phosphates

Inorganic phosphate ligands are important with respect to the behavior of actinides in the environment and as potential waste forms. There have been a number of experimental studies to determine the equilibrium constants in the actinide-phosphoric acid system, but they have been complicated by the formation of relatively insoluble solid phases and the formation of ternary actinide complexes in solution. 

A method is being developed to transform actinide ions in the near surface environment to less soluble, less reactive, thermodynamically stable phosphate minerals phases through application of organophosphorus complexants. These complexants decompose slowly, releasing phosphate to promote the formation of stable phosphate mineral phases, particularly with the more soluble trivalent, pentavalent, and hexavalent actinide ions. The complexant of choice, myo-inositol(hexakisphosphoric acid) or phytic acid, is a natural product widely used as a nutritional supplement. We have determined that phytic acid decomposes slowly in the absence of microbiological effects, that crystalline phosphate minerals are formed as a consequence of its decomposition, and that the formation of actinide (lanthanide) phosphates reduces the solubility of trivalent and hexavalent metal ions under environmental conditions. 

There are no uncertainty estimates in [1951ZEB/ALT]. This review has no access to the piimaiy experimental data, but finds no reason to question the quality of this study and has accordingly selected the equilibrium constants proposed by Zebroski et al. The stabilities of other actinide(IV) phosphates of similar composition are not known, except for the formation of the corresponding Pu(H3P04)" complex, logic (X.IO) has a similar value of (2.4 + 0.3) [2003GUI/FAN]. We note also that the stoichiometries of the selected complexes are the same as those for the uranyl(VI)-phosphate system and there is a reasonable linear correlation between the log,c / ° values (the equilibrium constants for the reactions M + ( )L ML( ), not the reported... 

Actinide ions form complex ions with a large number of organic substances (12). Their extractabiUty by these substances varies from element to element and depends markedly on oxidation state. A number of important separation procedures are based on this property. Solvents that behave in this way are thbutyl phosphate, diethyl ether [60-29-7J, ketones such as diisopropyl ketone [565-80-5] or methyl isobutyl ketone [108-10-17, and several glycol ether type solvents such as diethyl CeUosolve [629-14-1] (ethylene glycol diethyl ether) or dibutyl Carbitol [112-73-2] (diethylene glycol dibutyl ether). 

The solubility of the actinides in the organic phase, the right-hand side of equation 3, is achieved by the weak complexes that tri- -butyl phosphate... 

Sinha (60,61) has suggested that humic and fulvic acids play a major role in mobilising iron and transporting it from the soil to plant roots. At the normal soil pH it is believed that iron bound by the fulvic acid is partially hydroxylated as Fe(OH)2 (62). These complexes interact with phosphate to give an organicmetallic phosphate which may be taken up by plants (60). It has been suggested that the entire humic-iron-phosphate complex is taken up by the roots of plants and not just the iron and phosphate (60, 63). Jorgensen (64) has observed that soil humates suppress the uptake of Pb2+ into plants it is possible that they will also suppress actinide concentration in plants. 

The largest number of automated extraction-chromatographic separations for actinides have used TRU-Resin, and many of these have coupled the column to ICP-MS as an on-line separation (see Table 9.3). TRU-Resin is impregnated with the neutral bifunctional organophosphorus complexant, octyl(phenyl)-A,A-diisobu-tylcarbamoylmethylphosphine oxide (CMPO) in tri-n-butyl phosphate (TBP).26 127 128 The organic stationary phase in this resin binds trivalent, tetravalent, and hexavalent actinide nitrato complexes from nitric acid solutions (see Figure 9.11). The extraction equilibria for representative species are shown in Equations 9.3-9.5, where the bar above a species indicates that it is immobilized on the resin.4... 

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. 

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 ... 

The arrangement of tetrahedra in the structure of (U02)2P60i7 (uranyl ultraphosphate) is perhaps the most complex observed in all actinide polyphosphates [185]. The structure is unusual in that two of the six symmetrically independent phosphate tetrahedra share three vertices each with other phosphate tetrahedra, rather than the two shared vertices found in the remaining phosphate positions (and generally in polyphosphate chain and ring structures). 

Phosphine Chalcogenides as Ligands. - The complexation of lanthanide and actinide ions by phosphine oxide ligands remains an active area, and a theoretical assessment of the coordination of phosphine oxides (and phosphate esters) by trivalent lanthanide ions has appeared." Trivalent lanthanide complexes of the functionalised enol phosphine oxide (265), (and a related phosphonate), have been describedComplexes of thorium(iv) with bis(di-phenylphosphino) ethane dioxide and bis(diphenylphosphinoyl)amide have also been characterised." Calixarene systems which bear phosphine oxide... 

The extraction chromatography of the actinides has been reviewed and the treatment of zirconium phosphate sorbent for use in the separation of actinides has been described. An ion exchange study of the sulphate complexes of plutonium(iii) at 25 °C in IM and 2M perchloric/sulphuric add media found no evidence for the participation of protonated hgands in the complex formation. In the range... 

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