Actinides thiocyanates

As the anhydrous actinide thiocyanates are not known (only hydrated [M(NCS)4(H20)4], thiocyanate complexes are prepared metathetically. 

Table 21.14 Stability of actinide thiocyanate con lexes in aqueous perchlorate media, at 25°C mainly from refs 77 and 78).

Thousands of compounds of the actinide elements have been prepared, and the properties of some of the important binary compounds are summarized in Table 8 (13,17,18,22). The binary compounds with carbon, boron, nitrogen, siUcon, and sulfur are not included these are of interest, however, because of their stabiUty at high temperatures. A large number of ternary compounds, including numerous oxyhaUdes, and more compHcated compounds have been synthesized and characterized. These include many intermediate (nonstoichiometric) oxides, and besides the nitrates, sulfates, peroxides, and carbonates, compounds such as phosphates, arsenates, cyanides, cyanates, thiocyanates, selenocyanates, sulfites, selenates, selenites, teUurates, tellurites, selenides, and teUurides. 

Thiocyanate. — On the basis of /-orbital hybridization Diamond [351] predicted the formation of stronger actinide complexes with thiocyanate ion than for the rare earths. Subls and Chopfin [352] have studied the ion exchange behaviour of many actinide and rare earth thiocyanate complexes and have shown that europium is eluted much sooner than americium from Dowex-1 with ammonium thiocyanate. The stability constants for the formation of MSCN2+ and M(SCN)2 complexes for Nd3+, Eus+, Pu3+, Am3+, Cm3+, and Cf34 have been measured [353] and are tabulated in Table 25. It is apparent from the table that the formation... 

Table 25. Comparison of the stability constant data of the Eus+ thiocyanate complex with that of Nd3+ and some actinides (p = 1.0 M at 25° C)...

Only a few structures of thiocyanate complexes are known for the lanthanides and actinides, i.e. for Er,341 Th342 and U.343 They all contain terminal N-bonded NCS and have as an interesting aspect a high coordination number, e.g. as in [NEt4]4[Th(NCS)8].342... 

Acoustic emission diazine metal complexes, 80 Acrylonitrile metal complexes, 263 Actinide complexes cupferron, 510 dimethyl sulfoxide IR spectra, 490 phosphines SHAB theory, 1040 phthalocyanines, 864 thiocyanates, 236 Actins, 973 Acylates H3-ligands... 

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

Membrane technology may become essential if zero-discharge mills become a requirement or legislation on water use becomes very restrictive. The type of membrane fractionation required varies according to the use that is to be made of the treated water. This issue is addressed in Chapter 35, which describes the apphcation of membrane processes in the pulp and paper industry for treatment of the effluent generated. Chapter 36 focuses on the apphcation of membrane bioreactors in wastewater treatment. Chapter 37 describes the apphcations of hollow fiber contactors in membrane-assisted solvent extraction for the recovery of metallic pollutants. The apphcations of membrane contactors in the treatment of gaseous waste streams are presented in Chapter 38. Chapter 39 deals with an important development in the strip dispersion technique for actinide recovery/metal separation. Chapter 40 focuses on electrically enhanced membrane separation and catalysis. Chapter 41 contains important case studies on the treatment of effluent in the leather industry. The case studies cover the work carried out at pilot plant level with membrane bioreactors and reverse osmosis. Development in nanofiltration and a case study on the recovery of impurity-free sodium thiocyanate in the acrylic industry are described in Chapter 42. 

The Am thiocyanates have been studied intensively because of the separation of lanthanide and actinide elements in thiocyanate media. Three complexes of general formula Am(SCN) n = 1-3) have been identified from spectroscopic and solvent extraction data. 

For the thiocyanate complexes of lanthanides or actinides, the coordination occurs via the nitrogen atom (electronegativity 3.0). ... 

Another possibility is to apply the double-double effect to the differentiation of outer-from inner-sphere complexes of lanthanides and actinides because both the magnitude and direction of the effect depend on the difference in the electron-donor ability between water in the aquoion and the ligand in the complex. Therefore, a substantial difference between outer- and inner-sphere complexes with respect to the double-double effect should exist. Research in this field, using lanthanide complexes with thiocyanates as an example, is being made in our laboratory. 

Sato, T., Kotani, S., Good, M. L., Extraction of certain anionic actinide complexes from aqneous solntions by long chain alkyl ammo-ninm compounds - 11 Thorium(lV)-thiocyanate system, J. Inorg. Nucl. Chem., 35, (1973), 2547-2554. Cited on pages 373, 374, 544, 565. 

CMP, CMPO system. Horwitz and Kalina (1984), Horwitz et al. (1986) developed neutral bifunctional extractants CMP (RiR2-N,N-di(R ) carbamoylmethyl phosphonate) and CMPO (RiR2-N,N,- di(R )-carbamoylmethyl phosphine oxide, see Fig. 18.20) for the separation of trivalent actinide ions. With thiocyanate as a counter ion in the CMPO system, americium is preferentially extracted and the separation factor S.E (Am /Eu ) is about seven (Muscatello etal. 1982). 

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