Preferential extraction of lanthanides over trivalent actinides by monoacidic organophosphates from carboxylic acids and from mixtures of carboxylic and aminopolyacetic acids

Abstract

In the extraction of lanthanides and trivalent actinides by monoacidic organophosphates from mineral acid solutions the distribution coefficients of the two groups of elements overlap, with americium behaving most like cerium or praseodymium. Substitution of carboxylic acids, especially hydroxy acids, for the mineral acids shifts the americium distribution coefficients downward slightly relative to the lanthanides. This effect becomes more pronounced with increasing pH. The presence of strongly-complexing aminopolyacetic acids causes a much larger shift, which increases with the number (at least up to five) of acetic acid groups in the complexing agent. Extraction by di(2-ethylhexyl) phosphoric acid (HDEHP) from lactic acid solutions containing the sodium salt of diethylenetriaminepentaacetic acid (DTPA) provides an effective system for completely separating the two groups of elements. Monoesters of phosphonic acids give similar preferential extraction of the lanthanides. Under conditions optimum for group separation, neodymium is the least extractable of the lanthanides and californium is the most extractable of the transplutonium elements. At a constant pH, extractions of americium, californium, and the middle lanthanides are inversely proportional to the DTPA concentrations from 10 −4 to 10 −1 M, while extractions of the other lanthanides vary so as to reverse the order of extractability of the lower lanthanides. With a constant DTPA concentration, the difference between the extractability of americium and the middle lanthanides increases slowly with pH, while lanthanum and cerium gradually reverse their positions relative to the middle lanthanides. The extracted species contains a metal carboxylate complex.