Redox chemistry separating elements

The lanthanide elements are very difficult to separate because of their highly similar chemistry, but the earlier actinide elements have sufficiently different redox chemistry to allow easy chemical separations. This is important in the nuclear power industry, where separations have to be made of the elements produced in fuel rods of nuclear power stations as fission products, and of the products Np and Pu, which arise from the neutron bombardment of the uranium fuel. 

Section 4.3 leads us into the redox chemistry of main group metals or metalloids especially phosphorus. These elements can exist in several oxidation states separated by two-electron steps. Electron transfer to this system might occur in one-electron steps (porphyrin-centred), two-electron steps (metalloid-centred, usually combined with ligand loss or addition) or mixed electron transfer/chemical reaction steps. 

Many of the actinoids are also separated by exploiting their redox behavior. Thorium is exclusively tetravalent and berkelium is chemically similar to cerium, so iodate precipitation of Th and extraction of Bk(IV) with bis(2-ethylhexyl)orthophos-phoric acid (HDEHP) are used to isolated these elements. The differing stabilities of the (III), (IV), (V), and (VI) states of U, Np, and Pu have be exploited in precipitation and solvent extraction separations of these elements from each other and from fission product and other impurities with which they are found. Because of its technical importance, the process chemistry to separate U and Pu in nuclear materials has been highly developed. Extraction of Bk(IV) with HDEHP is used to separate Bk from neighbouring elements. 

Separation of a new element is a key problem. Separations involve methods such as volatilization, electrodeposition, ion-exchange, solvent extraction and precipitation/ adsorption. Separation relies on the unique chemistry of each element although not heavy elements, but useful as an illustration, 643oZn/6429Cu are separated by dissolution in dilute HN03 followed by selective electrodeposition of Cu (a very simple task, as the CuII/0 and ZnII/0 redox potentials differ by 1 V). 

The liquid (sulfur-based compounds) and sohd sulfur cathodes (items 6 and 7) do not develop surface chemistry that can be separated from their main electrochemical redox reactions. Hence, when the reduction of sulfur SO2 or SOCI2 produces insoluble species such as LiCl, LijS, and LijO, they precipitate on the current collector [9]. When formed, LijS can be reoxidized, up to elemental sulfur, via various LLS intermediate compounds [10]. Hence, the current collector, which may be aluminum (Al) plus carbon in the case of sulfur cathodes or carbon in the case of SOCI2 cathodes, does not develop intrinsic surface chemistry beyond the precipitation of the reduction products of the active mass. 

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