R. Chiarizia

The Kinetics of Metal Solvent Extraction

I. EXTRACTING REAGENTS In the present review, dealing with the kinetics of solvent extraction of metal species, we will consider biphasic systems consisting of: 1. An aqueous electrolyte solution containing a metal cation which, according to the nature and composition of the electrolyte, can exist either as a free (hydrated) ion or as neutral, positively or negatively charged complex species 2. An organic phase which is a solution, in a water-immiscible organic diluent, of an organic molecule called extractant; only organic diluent not exhibiting appreciable extraction properties by themselves will be considered; the extractant will instead be a molecule capable of reacting with the aqueous metal species, transferring them into the organic phase; this reactivity is conferred to the extractant by the presence of a functional group which has binding properties with respect to the metal species; the only purpose of the diluent will then be that of dissolving the extractant and possibly to improve its physica...

Extraction selectivity of organic solutions of a cyclic polyether with respect to the alkali cations

Abstract The extraction of alkali cations (Li+, Na+, K+, Rb+, Cs+) by the neutral cyclic polyether dibenzo-18-crown-6 dissolved in various nitrobenzene-toluene mixtures has been studied. Thermodynamic data have been so obtained regarding the alkali cation-cyclic polyether reaction. The affinity of the cyclic polyether for the alkali cations has been found to vary in the order K+ > Rb+ > Cs+ > Na+ > Li+ for all the diluents compositions. Moreover the independence of the selectivity constant, defined as the equilibrium constant of the reaction: Cs + + MSX ⇌ Cs SX + M + (where M is the alkali cation, S the cyclic polyether, X the picrate ion and the bar indicates organic species) on the diluent composition seem to indicate that the complexes formed with the cyclic polyether are isosteric, i.e. with size, shape and external electronic distribution independent of the chemical nature of the cation.

Interfacial behaviour of dinonylnaphthalenesulfonic acid at the toluene-HClO4 interface

Abstract Interfacial pressure vs concentration curves of dinonylnaphthalenesulfonic acid (HD) at the toluene-HClO 4 water interface have allowed to calculate the c.m.c. of HD in toluene, the free energy of micelle formation, a rough estimate of its cross section at the interface and its degree of polymerization. The last one has been also evaluated through distribution measurements of Eu 3+ . The results obtained with the two techniques have been found in good agreement.

Liquid ion exchange selectivity of dinonylnaphthalenesulfonic acid (HD) with respect to some lanthanide and actinide ions

Abstract The liquid ion exchange selectivity of dinonylnaphthalenesulfonic acid (HD) dissolved in a series of organic diluents covering the dielectric constant range 2·38-34·8 with respect to the cations Ce3+, Eu3+, Gd3+, Tm3+, Am3+, Cm3+, Cf3+ in perchloric solutions has been studied at 25°C. The extent of aggregation of HD in the different diluents and the equilibrium constants of the exchange reactions have been evaluated. As far as the dependence of the ion exchange selectivity on the diluent nature is concerned, a decrease of the selectivity with the increase of the diluent dielectric constant has been evidenced only in the case of M33+H-+ exchange. The reasons of this behaviour are discussed together with literature data concerning other liquid ion exchange systems.

Liquid anion exchange of thiocyanate-nitrate actinide and lanthanide complexes

Abstract The liquid-liquid biphasic chemistry of the system quaternary alkylammonium salt- o -xylene-water-trivalent metal cation (actinide or lanthanide)-thiocyanate-nitrate anions has been studied with the purpose to obtain both information on the aqueous complex formation constants of these elements and parametric equations useful to process chemistry design. The aqueous equilibrium constants between Eu +3 , Ce +3 , Am +3 , Cm +3 , Cf +3 and SCN − and NO 3 − have been obtained at the constant ionic strength of 2·01 M and 25°C, by using a liquid anion exchanger (tricaprylmethylammonium thiocyanate in o -xylene). In the case of the SCN − anion the actinide series shows a tendency to form complexes greater than the lanthanides, while this is approximately the same with the NO 3 − anion. For the actinide elements the tendency to form complexes is greater for SCN − than for NO 3 − . As far as the technological applications are concerned the possibility of performing actinides-lanthanides separation appears promising.

Enthalpy and entropy variations in the liquid cation exchange of some lanthanide ions by dinonylnaphthalenesulfonic acid(HD) and bis(2-ethyl-hexyl)phosphoric acid (HDEHP)

Abstract Free energy, enthalpy and entropy variations concerning the liquid ion exchange reactions of some lanthanides between aqueous perchloric acid solutions and organic solutions of the liquid cation exchangers dinonylnaphthalenesulfonic acid (HD) and bis(2-ethyl-hexyl) phosphoric acid (HDEHP) have been determined. The meaning of the experimentally obtained thermodynamic quantities is discussed on the basis of structural considerations regarding both the aqueous and the organic phase.

The meaning of slope analysis in solvent extraction chemistry: The case of zinc extraction by trilaurylammonium chloride

Abstract By using suitable equations, which are derived by treating the extraction of a metal complex in the general form pMY n + q RY ⇌ (RY) q (MY n ) p (with MYn extractable species, RY extracting agent, (RY) q (MY n ) p complex formed by the extraction reaction, with the bar indicating organic phase species), the physical meaning obtained by the slope analysis of extraction data (logarithmic plot of distribution coefficient vs. extractant concentration) has been discussed. The experimental case of distribution data obtained by extracting ZnCl2 with a benzene solution of trilaurylammonium chloride in a wide range of metal and extractant concentration has been treated.

Mass transfer rate in liquid anion exchange processes—I: Kinetic of the two-phase acid-base reaction in the system trilaurylamine-toluene-HCl-water

Abstract By using a stirred cell with a constant interfacial area the rate of the two phase acid-base reaction between aqueous HCl and trilaurylamine (TLA) dissolved in toluene has been studied. A reaction mechanism, where interfacial chemical reactions are the main rate determining processes, has been hypothized on the basis of both kinetic experiments as well through the knowledge of the interfacial properties of the system. The reactions are the following: H + + ( TLA ) i ⇌( TLAH + ) i slow Cl − + ( TLAH + ) i ⇌( TLAHCl ) i fast ( TLAHCl ) i + TLA ⇌ TLAHCl +(TLA) i slow where i and the bar indicate interfacial and organic phase species respectively. An expression for the overall rate of reaction has been derived and the rate constants evaluated. By means of an approximate model, coupling diffusion and chemical reactions together, it has been shown that a possible diffusional contribution to the overall rate can affect the numerical values of the rate constants while the type of dependence of the rate of extraction on the concentrations is the same. The proposed reaction mechanism has been shown to hold up to equilibrium.

Stability constants of nitrate and chloride complexes of Np(IV), Np(V) and Np(VI) ions

Abstract The formation constants of NpO22+ and Np4+ ions with the Cl− and NO3− ligands have been determined by the TTA extraction method in a 4M constant ionic medium. The complexes between NpO2+ and Cl− and NO3− have been evaluated by studying the potential variations of the Np(VI)/NP(V) couple at a platinum electrode as function of the ligand concentration. The data have been analyzed by using a suitable computer least squares program. The following formation constants have been determined at μ = 4M and T = 25°C: Np4+ +Cl− ⇋NpCl3+ β 1 = 0·77 ± 0·06 Np4+ +2Cl− ⇋NpCl22+ β 0.79 = ± 0·06 Np4+ + 2Cl− ⇋NpCl22+ β 2 = 0·79 ± 0·06 Np4+ + NO3− ⇋ NpNO33+ β 1 = 0·07 ± 0·2 Np4+ + 2NO3− ⇋Np(NO3)22+ β 2 = 0·18 ± 0·05 NpO22+ + Cl− ⇋ NpO2Cl+ β 1 = 0·69 ± 0·05 NpO22+ NO3− ⇋ NpO2NO3+ β 1 = 0·21 ± 0·03 NpO2+ + Cl− ⇋ NpO2Cl β 1 = 0·003 ± 0·003 NpO2+ + Cl− ⇋ NpO2Cl β 1 = 0·003 ± 0·003 NpO2+ + 2Cl− ⇋ NpO2Cl2− β 2 = 0·028 ± 0·003 NpO2+ + NO3− ⇋ NpO2NO3 β 1 = 0·025 ± 0·001 NpO2+ + 2NO3− ⇋ NpO2 (NO3)2− β 2 = 0·043 ± 0·001

Extraction kinetics of iron(III) from aqueous nitrate solutions to toluene solutions of tri-n-butylacetohydroxamic acid

Abstract The forward and reverse extraction rate of Fe 3+ at time zero between aqueous nitrate solutions and toluene solutions of tri- n -butylacetohydroxamic acid, HX, has been studied as function of the composition of the system and the stirring speed of the two phases. Experimental information has been also obtained on the degree of aggregation of HX, its surface active properties, its solubility in the aqueous phase as well as on the equilibrium distribution of Fe(III). Rate equations have been derived. The rate determining step of the extraction reaction has been shown to be the reaction of the free and hydrolyzed iron ions, Fe 3+ (hydrated) and FeOH + (hydrated), with the HX undissociated molecules. The reactions occur simultaneously in the aqueous phase (homogeneous path) and at the interface (heterogeneous path). A correlation between the rate constants and the equilibrium constant of the extraction reaction of Fe(III) has been established.