Acidic organophosphorus extractants

For the case of divalent metal ions (M " ) such as Zn ", Cu ", or Cd whose extraction process with acidic organophosphorus extractants such as DEHPA and DTMPPA does not involve the coextraction of the anion present in the aqueous phase (X ), the extraction reaction could be defined as... 

Preconcentration by an acidic organophosphorus extractant is also effective, though the lanthanides are extracted from dilute acid solutions (10 -10 M) and stripped into acidic (>1M) solutions when HDEHP is employed as the extractant. A representative application of liquid-liquid extraction to rare-earth preconcentration is the 100 to 200-fold concentration of the rare earths in seawater prior to ICP/MS determination using a mixture of HDEHP and mono-(2-ethylhexyl)phosphoric acids (Shabani et al. 1990, Shabani and Masuda 1991). When the same extractant system was adsorbed on a Cig cartridge and used in an extraction chromatographic mode, the rare earths were concentrated 200-1000-fold (Shabani et al. 1992). 

Of the acidic extractants applied to lanthanide/actinide separations, the most common are chelating extractants like j8-diketones, pyrazolones and (to a lesser extent) oxines, and the acidic organophosphorus extractants. Some work has been done evaluating the extraction of the subject metal ions by sulfonic acids, but this class of liquid cation exchangers exhibits little selectivity, and they have not proven particularly useful for lanthanide/actinide separation (Khopkar and Narayanankutty 1968). 

Acidic organophosphorus extractants To avoid the requirement of dilute-acid aqueous solutions with acidic extractants (with attendant pH-control complications), acidic organophosphorus reagents were... 

Kolarik, Z., Grimm, R. Acidic organophosphorus extractants. XXIV. The polymerization behavior of copper(II), cadmium(II), zinc(II), and cobalt(II) complexes of di(2-ethylhexyl) phosphoric acid in fully loaded organic phases. J. Inorg. Nucl. Chem. 1976, 38(9), 1721-1727. 

The lanthanides form many compounds with organic ligands. Some of these compounds are water-soluble, others oil-soluble. Water-soluble compounds have been used extensively for rare-earth separation by ion exchange (qv), for example, complexes form with citric acid, ethylenediaminetetraacetic acid (EDTA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA) (see Chelating AGENTS). The complex formation is pH-dependent. Oil-soluble compounds are used extensively in the industrial separation of rare earths by liquid—liquid extraction. The preferred extractants are carboxylic acids, organophosphorus acids and esters, and tetraalkylammonium salts. 

In Table 1, typical extracting reagents used for separation and enrichment of inorganic elements are summarized. Organophosphorus extractants are often used because of their solubility properties. Di(2-ethylhexyl) phosphoric acid is commonly applied to industrial separations because of its high extractability and high separation factors between many inorganic elements, especially for rare earth elements. Other metal ions are extracted as well as the trivalent metal ions. 

Organophosphorus extractants such as di(2-ethyl-hexyl) phosphoric acid (DEHPA), 2-ethylhexyl-phosphonic acid mono-2-ethylhexyl ester (EHPA), N-benzoyl-A-phenylhydroxylamine (BPHA), and tetra-octylethylenediamine (TOEDA) are often used due to their solubility properties in the stationary organic phase [1-3]. 

Neuman RD, Nai-Fu-Zhou WuJ, Jons MA, Gaonkar AG, General model for aggregation of metal-extractant complexes in acidic organophosphorus solvent extraction systems. Sep. Sci. Technol. 1990 25(13-15), 1655-1674. 

Cortina et al. [17,18] developed a systematic study of the adsorbent extractant reactions of impregnated resins prepared by adsorption of acidic organophosphorus compounds (DEHPA and DTMPA) in Amberlite XAD2. Depending on the chemical nature of the extractant, the adsorbed extractant could be involved in the following chemical reactions ... 

Extraction ofZn(II), Cu(II), and Cd(II) with Impregnated Resins and Levextrel Resins Containing Acidic Organophosphorus Compounds (DEHPA, DTMPPA)... 

Muscatello et al. (117) discuss the use of hollow fiber ILMs containing blfunctlonal organophosphorus extractants to remove americium and plutcnlum from nltrate-nltrlc acid waste streams. A reduction In the actinide concentration In a waste stream would allow disposal of the stream as a low-level waste. Partial neutralization of the nitric acid In the waste stream was necessary to obtain high (>9M9) removal of the Am(III). 

We have Investigated the transfer of americium and, to some extent, plutonium from aqueous nitrate wastes using supported liquid membranes of the blfunctlonal organophosphorus extractants DHDECMP and 0()D(1B)CMP0 (+TBF). The results show good transfer and removal If the nitric acid In the feed Is first neutralized to O.IOM to minimize acid transfer and subsequent back transfer of the metal Ion. 

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