Uranium synergistic

The extractant is di(2-ethylhexyl) phosphoric acid (DEHPA) in conjuction with trioctyl phosphine oxide (TOPO). Stripping is by ammonium carbonate, and uranium precipitates as ammonium uranyl tricarbonate. The mixture shows a synergistic effect. The mixture is stable, and extracts uranium in hexavalent state. 

In the addition mechanism, the coordination number of uranium is increased from six to seven, whereas in the substitution mechanism, there is no increase in the coordination number. The absence of any synergistic effect for uranium(IV) and thorium(IV),127 which give coordination-saturated complexes M(HA2)4 with organophosphoric acids in which the metal ions are eight coordinate, is in accord with the addition mechanism. 

In contrast to the chemistry of extraction of uranium(VI) by organophosphorus acids, that of uranium(IV) has not been widely studied. It is known,135 however, that the extraction of U4+ by mono(2-ethylhexyl)phosphoric acid (2-ethylhexyl dihydrogen phosphate), for example, exceeds that of U022+ by a factor of 105. This clearly provides the basis for the strong extraction of uranium(IV) from phosphoric acid solutions by extractants containing monoalkyl phosphates. The existence of a synergistic effect in the extraction of uranium(IV) by mixtures of mono(2-ethylhexyl)phosphoric acid and neutral organophosphorus compounds was also reported recently.136... 

Synergistic extraction, in the system DEHPA and TOPO, is quite interesting for its ability to extract uranium from high concentrations of phosphoric acid (38). This finds application in the recovery of uranium from dilute phosphoric acid medium in... 

Otu, E.O. The synergistic extraction of thorium(lV) and uranium(VI) by 2-ethylhexyl phenylphosphonic acid and micelles of dinonylnaphthalene sulphonic acid, Solvent Extr. Ion Exch. 15 (1997) 1-13. 

Sriram, S. Veeraraghavan, R. Manchanda, V.K. Synergistic extraction of uranium(VI) by alamine 336 in the presence of neutral oxodonors, Radiochim. Acta 84 (1999) 153-157. 

A number of investigations on the synergistic effect of a second extractant, such as amines and oximes, on the extraction of metal carboxylates have been carried out. The utilized synergists include 8-hydroxyquinoline-2-aldoxime for Zr(IV) and Hf(IV) (122,123), various amines (25), Lix 63 (27) and nonylphenol (28) for Cu(II), dialkylphos-phoric acids for Hf(IV) (44), rhodamine B for Be(IE) (102), trioctylphos-phine oxide for U(VI) (69, 77), p-alkylphenol for Cs(I) (1), collidine for Zr(IV) and Sc(IEI) (62), and nonchelating oximes for Ni(II) and Co(II) (103). Mareva et al. (77) have successfully utilized a salicylic acid-trioctylphosphine oxide mixture for the separation of uranium from rare earths, thorium, zirconium, and iron. 

Animal studies designed to examine the combined effects on the kidney of uranium and other heavy metal nephrotoxicants Gead, cadmium) would also be useful to determine whether effects are less than expected on the basis of individual toxicity, additive or synergistic. It is possible at some waste sites that multiple exposures to these metals could occur in humans. 

Ramakumar et al. [33] investigated the transport behavior of uranium across the BLM made from several substituted calixarenes (Figure 31.5) in chloroform and 0.1 M nitric acid as the receiver phase. The presence of TOPO and crown ether (18 crown 6) enhanced the transport rate, which was ascribed to synergistic extraction effect. 

The mechanism of U02 " extraction by monoalkyl phosphoric acid reagents appears to be a more complex process than for their dialkyl counterparts. This results from the polymerization of the monoalkyl phosphate in the organic phase and the hydration of the extracted uranyl species so that variable stoichiometries arise for the extractant/water/UO complex. The extraction of from sulfuric acid by mono-2,6,8-trimethylnonyl phosphoric acid (H2DDP) and mono-n-butyl phosphoric acid (H2MBP) as 0.05 M solutions in benzene was shown to follow equations (61) and (62) when an excess of extractant was present. When an excess of uranium was present, equations (63) and (64) applied where n, x, y and z were variable numbers which depended upon the extent of extractant polymerization and hydration of the extracted species. Synergistic effects may also be found with the monoalkyl phosphoric acid extractants and in one recent example the use of tri-n-octylphosphine ocxide (TOPO) as a synergist with H2MEHP allowed the extraction of U02 from phosphoric acid solutions. The uranium may be returned to the aqueous phase by contact with concentrated acid, which reverses the extraction process by protonation of the phosphate. 

The alkyl phosphoric acids were first recognized as excellent actinide extractants because dibutyl phosphoric acid existed as an impurity in TBP (8). They extract well from unsalted solutions and from systems such as sulfate in which TBP is ineffective. Organo-phosphorus acids are thus suitable for uranium hydr metallurgical applications where ore is leached with sulfuric acid. The Dapex process is an example in which bis(2-ethylhexyl) phosphoric acid (HDEHP) is used alone or in synergistic combination with neutral organophosphorus compounds for uranium and vanadium recovery (9,10). 

There are a number of practical synergistic systems that make use of organophosphorus compounds. A classic example is the combination of HDEHP and TOPO. The TOPO is thought to replace water or HDEHP in the coordination sphere of the metal. Some early work in this area included a study of the extraction of uranium in such systems (82). Commercial processes now exist for the recovery of uranium from wet-process phosphoric acid utilizing synergistic systems (83,84). Descriptive studies of such systems have also been made (85,86). 

Synergistic systems have also been reported in which one of the adducts is in the aqueous phase. The combination of TOPO (in CCI4) and benzoic acid (in aqueous and organic) was shown to extract uranium with a synergistic effect. The organic-phase adduct was reported to be 1102(0104) C COO TOPO. An optimum benzoic acid/TOPO ratio exists, above which compound formation between TOPO and benzoic acid decreases the synergistic effect (91). 

---