Phosphorus based organic extractants

Table I. Extraction of Uranium and Thorium into Several Neutral Phosphorus-Based Organic Extractants from 2.00 M HNO. Extractant Solutions are 1.00 F in Dodecane...

Some of the advantages of using neutral phosphorus-based organic extractants of the same general type as tributyl phosphate, TBP, designed with specially selected extraction properties in the development of LLE systems for use in reactor fuel reprocessing are ... 

From the knowledge of the extractant characteristics of both neutral and mono-acidic phosphorus-based organic compounds now available, it is possible to tailor-make extractants for a specifically desired separation of two metals. The present study is concerned with neutral mono-nuclear, phosphorus-based extractants for use in affecting the mutual separation of U(VI) and Th(IV). 

Liquid-liquid extraction (LLE) systems using neutral phosphorus-based organic compounds have been the subject of extensive study since Warf (1) first reported the use of tributyl phosphate, TBP, as a useful extractant for cerium(IV), uranyl and thorium nitrates. After more than twenty years, liquid-liquid extraction systems (such as the Purex and Thorex processes) employing TBP dissolved in a suitable diluent versus an aqueous HNO3 phase remain the most widely accepted systems for reactor fuel reprocessing. 

The key intermediate chloromethyloxadiazole 18 was prepared in two steps from inexpensive, commercially available materials as shown in Scheme 5.10. Bishydrazide 19 was prepared in a one-pot procedure by reaction of 35% aqueous hydrazine with ethyl trifluoroacetate in acetonitrile and subsequent addition of chloroacetyl chloride and base. This procedure affords the unsymmetrical bis(hydrazide) 19 in higher than 95% assay yield. While a number of dehydrating agents were found to be effective in the dehydration to prepare 18, phosphorus oxychloride was chosen because of its low cost and relatively benign waste stream. Sub-stoichiometric (0.3 equiv.) amounts were found to be as effective as full equivalents in the reachon when used in conjunction with catalyhc amounts of DMAP as a nucleophilic catalyst. The entire sequence was transformed into a one-pot through process in order to improve efficiency. Following the cyclization, an aqueous work-up was performed and the organic extracts carried directly into the next step. 

The importance of the acidic and steric properties of mono-acidic phosphorus-based extractants in the extraction of metals has been well established. The work of Mason, Peppard, et al. (2, 3) has shown that combined acidic and steric effects which result from altering the structure of the acidic extractants may be varied to give a wide range of extraction constants (Ks) for specific metals. Ks = K[H+]a/F, where Ks is a constant characteristic of the system, K is the distribution ratio, H is the hydrogen ion concentration in the equilibrated aqueous phase, F is the concentration of extractant in the equilibrated organic phase, a and b are the respective hydrogen ion and extractant dependencies. Recent studies, to be published, involving the... 

Two methods, the one based on the alkali lability of RNA and the other on the acid lability of both RNA and DNA, appeared simultaneously in 1945 and have provided the analytical foundation for much of the recent research activity revolving about nucleic acids. The former of these methods is that of Schmidt and Thannhauser (abbreviated here as ST) (91) which depends upon the selective conversion of RNA phosphorus to organic acid-soluble phosphorus (mononucleotides) by alkali after removal of acid-soluble phosphates and of phospholipid phosphorus (16 et ante). The method of Schneider (S) (93) extracts the same lipid-free protein precipitate with hot trichloroacetic or perchloric (95) acid which solubilizes both RNA and DNA the estimation of each in the mixture utilizes specific colorimetric reactions for the (purine) ribose and desoxyribose moieties. These two methods will be discussed in connection with the combined method which follows. 

One of the simplest methods of estimation of PolyPs in extracts is based on the assay of Pi, which is released from the PolyPs by hydrolysis with 1 M HC1 at 90 °C for 10 min. The Pi released under these conditions is defined as labile phosphorus . If the compounds containing organic labile phosphorus (i.e. nucleotide phosphates, sugar phosphates, etc.) were removed from the extracts by adsorption on Norit charcoal, the increase in Pj content after hydrolysis can be attributed to PolyP and pyrophosphate (PPi). Estimation of the PPj content (Mansurova, 1989) before hydrolysis may be needed in some cases for more precise calculations of the PolyP content. Pi may be determined by one of the well-known chemical methods (Fiske and Subarrow, 1925 Weil-Malerbe and Green, 1951). 

Phosphorus is not a TE but a major nutrient element. Nevertheless, fractionation of this element is essential for environmental studies, and hence it seems reasonable to highlight here some relevant SEPs. Four different procedures for the fractionation of P in lake sediment samples have been tested in an interlaboratory study in the framework of the SM T program (Ruban et al., 1999). As a result, a novel scheme based on the Williams protocol (Williams et al., 1976) has been developed aimed at the restoration of lake sediments. The scheme comprises three separate assays (1) sequential extraction of NaOH-extractable (Fe- and Al-bound) and HCl-extractable (Ca-bound) fractions, (2) sequential extraction of inorganic and organic phosphorus and (3) single extraction, after calcination, of concentrated HCl-extractable (total P) fraction (see Table 12.3 for further details). Further discrimination of specific compounds is made feasible by the use of chromatographic and capillary separation techniques as reviewed by Spivakov et al. (1999). 

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