Solvate extraction elements

The solvate extraction mechanism of trivalent 4/and 5/elements can be described by the following equilibrium, where A- symbolizes the anion of the aqueous phase and E is the neutral solvation agent of the organic phase (overbars account for species in the organic phase, where residual aqueous molecules might also hydrate the... 

For a given ionic strength, //, depends on the nature of the coextracted anion A-. To allow the formation and extraction of the neutral complex, the coextracted mineral anion A- has to lose part (or all) of its hydration shell. The smaller the hydration energy of the mineral anion is, the easier is its transfer to the organic phase, and thus the higher is the affinity of the solvation extractant toward trivalent 4/ and 5/ elements (29, 76), as observed in the series chloride < nitrate < perchlorate < pertechnetate, which inversely follows the anion hydration energy order AG/CI ) > AG/NO3) > AG(,(CI04) > AG/TcO/. 

Solvating extractants include the substances such as trialkyl phosphates, ketones and carbitols the most well known example being tributyl phophate which was originally used for the extraction of uranyl nitrate from leach liquors containing nitric acid. A number of other organophosphates have proved of value in the concentration and separation of the lantanide and actinide elements (46). 

Neutral extractants constitute the third important class of solvent extraction reagents for the rare-earth elements. Since they have no charge, neutral extractants only extract neutral complexes or charge-balanced ion pairs. They also tend to extract ion-paired acid molecules such as HNO3. Solvating extractants may be dissolved in an organic diluent, or they may be the organic diluent itself (e.g., diethylether, methylisobutylketone, tributyl-phosphate). Phase transfer is accomplished by solvation of the complex by the extractant, and a typical equilibrium can be written as... 

Solvate extraction, which is also known as adduct extraction, became really important during the Manhattan Project when there was a need to produce uranium, plutonium, and other elements. Thus, tri-n-butyl phosphate was introduced for uranium and plutonium extraction (Flanary 1956). Trioctylphosphine oxide and bis-2-ethylhexyl phosphoric acid were also developed at this time (Mason and Peppard 1963 Siddall 1959 McKay 1956). These reagents are used not only in the nuclear fuel cycle but also for production of lanthanides and many other metals. 

Neutral Extractants. Many neutral organophosphoms extractants are available phosphate esters, phosphonate esters, phosphinate esters, and phosphine oxides. The most popular neutral extractant is tributylphosphate (TBP), which reacts with RE elements according to a solvation mechanism ... 

While the liquid-liquid extraction of inorganic elements as coordination complexes with thiocyanate ions can be traced back to Skey (1867), the extraction from hydrochloric acid into ether of iron(III) (J. W. Rothe, 1892) or gallium (E. H. Swift, 1924) depends on the formation of solvated acido complexes derived from HMC14 extractions of metal complexes from nitric, thio-cyanic, hydrofluoric, hydrochloric and hydrobromic acids were studied exhaustively by Bock and his collaborators (1942—1956).6... 

Accordingly, Du increases with both the concentration of the neutral solvation agent, E, initially present in the organic phase, and that of the mineral anion, A-, initially present in the aqueous phase. Inversely, the back-extraction of the trivalent element is favored by a decrease of the concentration of A- in the aqueous solution. 

If the source of anions A is a mineral acid, a competitive proton-extraction reaction might occur, which will decrease the affinity of the neutral solvation agent for the target trivalent elements ... 

Ensuring high-quality analytical performance in trace analysis, if separation of sample components by extraction is indispensable, requires implementation of the appropriate extraction method and establishment of suitable operational parameters to ensure a high efficiency of extraction. Selection of extraction conditions is crucial for quantitative recovery of analyte, or at least for sufficient effectiveness. If an aqueous solution is one of the extraction phases, problems such as complex-ation, hydrolysis, and solvation can play an important role. Extraction of elements from aqueous to organic phase often requires selection of appropriate ligands and control of pH. 

The structure and behavior of lanthanides and actinides in ILs has recently been summarized in detail by Binnemans and the reader is referred to this excellent review for more details [71]. Among others, it has been pointed out that solvation of the metal ions by the IL components is a major issue in f-element coordination chemistry [71, 79, 218, 219], Understanding the solvation is also a key to understanding f-element extraction processes. Among others, theoretical studies suggest that Ln3+ (Ln = La, Eu, Yb) are surrounded by six PF6 anions in Bmim-PF6 and by 11-13 imidazolium ions in the second ionic sphere. The same authors also suggest that free lanthanide cations, that is, cations without a solvation shell, are poorly soluble in Bmim-PF6 [149, 152, 220-223], They also report that, while [LnCls]3 complexes are unstable in the gas phase, they can be stabilized by solvation in ILs. 

As used in the tri-n-butyl phosphate (TBP), or PUREX, process to recover uranium (U) and plutonium (Pu), the solvent is typically a blend of about 30 vol% TBP and 70 vol% of an aliphatic diluent, say C-12 to C-15 branched paraffins. Spent nuclear fueP is dissolved in nitric acid and then contacted with this TBP solvent. Actinide elements, primarily uranium and plutonium, are coextracted into the organic layer by coordination and solvation with TBP. The extraction of uranium, for example, is thought to occur by the following reaction [14-21] ... 

TBP, but not necessarily with the same solvation number for all elements. Tri-n-octylphosphine oxide (TOPO) and tri-n-butylphosphine oxide (TBPO) extract Pu(IV) and Pu(VI) as the di-solvate.The data of Martin andOckenden is ven in Figs. 9 and 10 for extraction of several elements into 0.1 M TOPO in cyclohexane from HNOg and... 

Used in extraction of lanthanide and actinide elements and in nuclear fuel reprocessing. Plasticiser for cellulose esters, lacquers, plastics and vinyl resins. Used neat or as a soln. in CHCI3, hexane or CCI4 as an extraction solvent for the coordinatively solvated salts (NO3 , Cl ) e.g. U02(N03)2 (TBP)2- Odourless liq. Mod. sol. H2O, misc. org. solvs. df 0.98. Mp < —80°. Bp 289° dec.. 

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