Hexavalent state

In many reactions, selenium is an oxidant as well as a reductant. Strong oxidants convert selenium dioxide and its derivatives to the hexavalent state. Although hexavalent selenium compounds are oxidants, these are less active and difficult to reduce. Selenium salts resemble the corresponding sulfur and tellurium salts in behavior. 

The roasted pellets or extmdes are ground and leached in water. The hexavalent selenium dissolves as sodium selenate [13410-01 -0] Na2Se04. Sodium teUurate, being highly insoluble in the now very strongly alkaline solution, remains in the residue. The separation between selenium and tellurium is readily achieved, provided all tellurium is oxidized to the hexavalent state. 

Chlorine may initially convert the selenium in solution to the hexavalent state, but as the hydrochloric acidity increases, reduction to the tetravalent state occurs spontaneously. 

Chromium complexes of long-chain fatty acids are exceUent water repeUents which are also used for their food-release properties in certain packaging appHcations. The presence of chromium has raised environmental concerns, despite the fact that the metal is in the trivalent rather than in the highly toxic hexavalent state. This material is available as Qudon (DuPont). 

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 hexavalent state in the aqueous solutions the dioxo ions UO +, Np02+, Pu02+ and AmO + exist. 

The dissolution time for the unreprocessed fuel would be at least 1 million years due to the limited water supply, even if a rapid oxidation of uranium to the hexavalent state and a subse-guent formation of water soluble carbonate complexes are assumed (15). Since the conditions are reducing in the groundwater (see beTow) the dissolution time would probably be several orders of magnitude larger. The unsignificant dissolution of uranium and fission products observed in the Oklo-deposit (16) is an example of a similar extremely slow leaching process in the natural environment. 

For U, which would exist in the hexavalent state (species of 002 + in an aerated system) the distribution coefficients are low. This is expected in a groundwater containing COs -, leading to the formation of soluble anionic complexes. 

The Phillips Cr/silica polymerization catalyst is prepared by impregnating a chromium compound onto a wide pore silica and then calcining in oxygen to activate the catalyst. This leaves the chromium in the hexavalent state, monodispersed on the silica surface. Chromium trioxide (Cr03) has been impregnated mast commonly, but even a trivalent chromium salt can be used since oxidation to Cr(VI) occurs during calcining. 

Fig. 3. As the chromium loading increases, all is stabilized in the hexavalent state until a certain saturation coverage is reached, which depends on calcining temperature. Beyond this limit the excess is converted mainly to Cr203.

Oxygen is adsorbed by the divalent catalyst with a brilliant flash of chemiluminescence, converting the chromium back to its original orange hexavalent state (15,17,41,42). The ease with which this reversal occurs suggests that there is little rearrangement during reduction at 350°C. 

The plutonium is oxidized to Pu(IV) and Pu(VI), while the neptunium ends up in the pentavalent or hexavalent states. Small amounts of plutonium and fission products... 

Following decontamination of the uranium/plutonium from the fission products, the plutonium is separated from the uranium. This is done by reducing the Pu(IV) to nonextractable Pu(III), leaving uranium in the hexavalent state. In the older Purex plants, this was done using Fe2+ while the newer plants add U4+. The plutonium thus ends up in an aqueous phase while the uranium remains in the organic phase. 

The chelation-extraction method determines chromium metal in hexavalent state. In order to determine total chromium, the metal must be oxidized with KMn04 under boiling and the excess KMn04 is destroyed by hydroxylamine hydrochloride prior to chelation and extraction. 

Rai and Serne, 1977 Amacher and Baker, 1982). Both elements can exist in multiple oxidation states in aqueous environments. Chromium can exist in trivalent and hexavelent states, while Pu can occur in trivalent, quadrivalent, pentavalent, and hexavalent states. Additionally, both elements can exist as cationic or anionic species in aqueous systems. Trivalent Cr exists as the cation Cr3+ and its hydrolysis products, or as the anion CrO at very low concentrations. Hexavalent Cr occurs as the dichromate Cr207 or chromate HCrO or CrO anions, depending on pH. Plutonium exists in cationic states such as Pu3+ and Pu02 and anionic forms such as Pu02(C030H ). 

The hexavalent state of uranium ion is the usually encountered ion in the solution chemistry of uranium and its exceptional stability, relative to its other oxidation states as well as to other hexavalent actinide ions, makes studies with this ion much simple. Though chelating acids, in general, extract U(VI) by the equilibrium,... 

The hexavalent state of neptunium and plutonium are identical to that of U(VI), in the sense that both exist as M02+ in aqueous solutions. Whereas U(VI) is quite stable in aqueous solutions Np(VI) and Pu(VI) are unstable, especially when in contact with organic solutions, and need to be stabilized by using holding oxidants. Only a few studies have been reported for these systems. In the extraction with HTTA-B mixtures, the species M02(TTA)2 B was reported6, to be responsible for the observed synergism with both these ions, just like U(VI), and the equilibrium constant data are included in Table 7. It appears that the stabilities of the M02(TTA)2 B adducts of U(VI), Np(VI) and Pu(VI) are of, more or less, similar magnitude. 

The important valence states of chromium are II, III, and VI. Elemental chromium, chromium(O), does not occur naturally. The divalent state (II or chromous) is relatively unstable and is readily oxidized to the trivalent (III or chromic) state. Chromium compounds are stable in the trivalent state and occur in nature in this state in ores, such as ferrochromite (FeCr204). The hexavalent (VI or chromate) is the second most stable state. However, hexavalent chromium rarely occurs naturally, but is produced from anthropogenic sources (EPA 1984a). Chromium in the hexavalent state occurs naturally in the rare mineral crocoite (PbCr04) (Hurlburt 1971). 

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