Diorganylarsinic acids: preparation and use as extractants for gold(III) and uranium(VI)

The new compounds dicyclohexylarsinic acid and bis(2-phenylethyl)arsinic acid were prepared by reacting diethylaminodichloroarsine with the appropriate Grignard reagents and oxidizing the hydrolyzed reaction mixtures. The self-association of dicyclohexylarsinic acid (HDCHAA) and its HCl adduct in chloroform were studied by vapor pressure osmometry. HDCHAA undergoes a monomer-dimer association in chloroform with an equilibrium constant of 296 liter/mole at 370C. The behavior of HDCHAA . HCl in chloroform solutions is best described by an indefinite sequential association with an equilibrium constant of 11.5 liter/mole. To explore the potential of diorganylarsinic acids for the extraction of gold(III) and uranium(VI), and to elucidate the influence of the organyl groups on their extractability, aqueous solutions of HAuCl4 or UO2(NO3)2, of widely varying hydrogen ion concentrations, were equilibrated with chloroform solutions of dicyclohexylarsinic acid, dioctylarsinic acid and bis(2-ethylhexyl)arsinic acid. The extraction of gold(III) in the aqueous HCl concentration ranges 4 M < [HCl] < 7 M and 2.7 < pH < 5, at [Cl-] = 1, is best described by the formation of arsinic acid (HA) adducts of the stoichiometry HAuCl4(HA)3(HCl)2 and HAu2Cl7(HA)6, respectively. At pH 1, one equilibration with any one of the arsinic acids transfers about 99.5% of the gold(III) present in aqueous solutions into the chloroform phase. The extraction of UO2+2 from aqueous solutions with pH > 1.5 is quantitative for all three arsinic acids. The structure of the complex is UO2(A)2(HA) (A = R2AsO2-). Uranyl dialkylarsinates were isolated by loading the organic phase with UO2+2 at pH 1. In the pH region 2-4, bis(2-ethylhexyl)arsinic acid extracts gold notably better than dioctylarsinic acid and dicyclohexylarsinic acid. In the acidic region, dicyclohexylarsinic acid is the superior extractant for gold