A New Interpretation of Third-Phase Formation in ... - ACS Publications

Chapter 9

A New Interpretation of Third-Phase Formation in the Solvent Extraction of Actinides by TBP Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: June 9, 2006 | doi: 10.1021/bk-2006-0933.ch009

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1,2

R. Chiarizia , M. P. Jensen , M. Borkowski , and K. L. Nash 1,3

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Chemistry Department, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 On leave from INCT, 03-195 Warsaw, Poland. Current address: Los Alamos National Laboratory, Carlsbad Operations, Carlsbad, N M 88200 Current address: Chemistry Department, Washington State University, P.O. Box 644630, Pullman, W A 99164 2

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Third phase formation, i.e., the separation of a second heavy organic phase, is a largely unexplained phenomenon which is observed in solvent extraction of metal species (for example, in the extraction of actinides by tri-n-butyl phosphate, TBP, in aliphatic diluents). Structural approaches for explaining organic phase splitting, based on concepts borrowed from the field of colloid chemistry, have recently appeared in the literature. In this chapter, we summarize the results of our recent small-angle neutron scattering (SANS) investigation of third phase formation in the extraction of uranyl and thorium nitrates by T B P in n-alkanes. A n interpretation of the mechanism of third phase formation, founded on the application of Baxter's "sticky spheres" model, is presented.

© 2006 American Chemical Society

In Separations for the Nuclear Fuel Cycle in the 21st Century; Lumetta, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: June 9, 2006 | doi: 10.1021/bk-2006-0933.ch009

Introduction Studies of solvent extraction of metal species are generally performed at low concentrations of cations and extractant. Under these conditions, it is usually straightforward to determine the stoichiometry of the metal-extractant complexes formed in the organic phase. The complexes identified in this way, however, exist only under conditions of high dilution, where ideal behavior of the solute species can be assumed. It is generally impossible to extrapolate the results obtained at very low metal and extractant concentrations to the conditions usually met in practical applications of solvent extraction, where the extractant concentration is generally high and the organic phase often approaches the saturation limit with respect to the extracted species. Under these more realistic conditions, the discrete metal-extractant entities familiar to coordination chemists can undergo polymerization or association phenomena leading to the formation of much larger species whose behavior can be quite different from that of the complexes found in dilute solutions. Examples of this type of behavior have been reported earlier (1-5). The authors of these studies observed the formation of very large cylindrical aggregates upon loading the organic phase with progressively higher concentrations of certain metal ions. The extractants used in these works, octyl(phenyl)-N,Ndiisobutylcarbamoylmethylphosphine oxide (CMPO) (1,2\ and dialkyl esters of diphosphonic acids (5-5), were Afunctional molecules, containing one P=0 and one C=0, or two P=0 donor groups, respectively. In these systems, polymeric species in the presence of high metal concentrations are not surprising, as metal ions can promote polymerization by bridging functional groups of different extractant molecules. Association phenomena in the organic phase, however, are also expected to take place at high concentrations of the metal-extractant complexes when the extractant is a monofunctional compound, such as, for example, tri-«-butyl phosphate (TBP). In nonpolar diluents, polar species are subjected to attractive van der Waals interactions which can lead to self-assembly of solutes into much larger species. Another phenomenon typically observed in solvent extraction systems involving nonpolar diluents at high loading of the organic phase is that of third phase formation, i.e., the splitting of the organic phase into two layers. The light layer contains most of the diluent and low concentrations of extractant and solutes, while the heavy or "third" phase is a highly concentrated solution of extractant and solutes. In spite of the abundant literature on the subject discussed in a recent review paper (6), a comprehensive description of the energetic and structural aspects of third phase formation is still lacking. Most of the attention in previous works, in fact, has focused on the conditions under which third phase formation is observed or avoided. Preventing third phase formation is of



137 paramount importance in solvent extraction systems of nuclear interest (e.g., TBP-rc-alkane systems) to avoid accidentally assembling a critical mass of fissionable materials in third phases. The physico-chemical origin of the phenomenon of third phase formation is still matter of debate. The traditional explanation is based on simple solubility concepts (7), i.e., to insufficient solubility of the polar metal-ligand complexes in the nonpolar organic phase diluents. More recently, other attempts to interpret third phase formation have been reported. According to Osseo-Asare and his microemulsion model (8), the third phase observed when alkane solutions of T B P are used to extract mineral acids and metal salts from highly acidic aqueous solutions corresponds to the middle phase of a Winsor III system, in which the microemulsion (middle phase) is in equilibrium with both the aqueous and the oil phase. Other authors have semi-quantitatively predicted third phase formation in the extraction of nitric acid by alkyl-substituted malonamides in tf-dodecane by using the Flory-Huggins theory of polymer solutions and the theory of regular solutions (9). Third phase formation can also be considered as the final step of an aggregation process involving the organic phase species. Thiyagarajan et al. were the first to hypothesize that the self-association of the metal-extractant complexes may represent an intermediate stage between a homogeneous solution of monomers or very small particles, and the separation of a second organic phase (1,2). Following this hypothesis, and profiting from our own experience on similar systems (3-5), we decided to revisit from an aggregation standpoint third phase formation in the extraction of actinides by T B P in «-aIkanes. Our initial objective was to relate third phase formation in T B P solvent extraction systems with the formation of large self-assembled species in the organic phase (10-12). In the course of our investigations, however, we realized that a more realistic interpretation of third phase formation could be achieved by using a model in which the major emphasis was placed not on particle growth but, rather, on particle interactions (13-15). Similar conclusions have been independently arrived at by other investigators (16,17). In this chapter we review our most important results obtained in the extraction of actinide nitrates by T B P in H-alkanes and summarize our conclusions on the mechanism and energetics of third phase formation.

Measurements and Calculations Small-angle neutron scattering (SANS) measurements are particularly suitable for the determination of the morphology (shape and size) of the aggregates formed in the organic phase by the extractants used in solvent


138 extraction systems. The SANS technique is based on the different ability of the hydrogen and deuterium atoms to scatter neutrons. If the extractant is dissolved in a deuterated diluent, the scattered neutrons will highlight those volumes of the solution that are occupied by the extractant molecules and their aggregates, thus allowing measurement of their morphology.


Measurements In our investigations, we used deuterated diluents (^-dodecane or ^-octane) for T B P (20% v/v or 0.73 M ) . Aliquots of the T B P solutions were loaded by extracting progressively increasing amounts of U 0 ( N 0 ) or T h ( N 0 ) up to the L O C (limiting organic concentration) condition and beyond. The L O C condition represents the critical metal concentration in the organic phase, i.e., the highest concentration achievable under the specific conditions of the experiment without organic phase splitting. The details of solution preparation and characterization can be found in the original publications (10-15). The SANS measurements were performed at the time-of-flight small-angle neutron diffractometer (SAND) at the Intense Pulsed Neutron Source (IPNS) of Argonne National Laboratory. For each sample the data were collected as scattered intensity, I(Q) (cm' ) vs momentum transfer, Q = (47t/k)sin(0) (A" ), where 0 is half the scattering angle and X is the wavelength of the probing neutrons. 2

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3

4

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Calculations The intensity of the scattered neutrons can be exprressed as (18): I(Q) = N V p

2 p

2

(Ap) P(Q) S(Q) + Ii

(1)

nc

where N is the number of scattering particles per unit volume, V is the particle volume, (Ap) is the contrast factor determined from the scattering length densities of extractant and solvent, P(Q) is the single particle form factor, which describes the angular scattering distribution as a function of particle size and shape, S(Q) is the structure factor which contains information on the interaction between particles, and I is the incoherent scattering background. Our SANS measurements revealed a sharp increase in the scattering intensity at low Q values for solutions containing progressively higher concentrations of extracted metal species. For the interpretation of our SANS measurements we used two different models: 1) the "particle growth model", in which increased scattering intensity was attributed solely to an increase in the p

p

2

inc


139 size of the scattering entities; 2) the "particle interaction model", in which the scattering intensity increased as a consequence of increasingly strong interactions between small particles.

Particle Growth Model


This model implies that interactions between particles are absent or negligible. By setting S(Q) = 1, eq 1 reduces to: 2

2

I(Q) = N V ( A p ) P ( Q ) + I p

p

i n c

(2)

Our SANS data were fitted by using eq 2 and the form factor for an ellipsoid of rotation (11-13). Since at Q=0, P(Q) = 1, after subtraction of the incoherent scattering background from the intensity values, eq 2 becomes: 2

I(0) = N V ( A p ) p

2

(3)

p

The 1(0) values from the ellipsoid of rotation fit were used in eq 3 to calculate the molecular weight of the extractant aggregates, and hence n, the average aggregation number of T B P in the aggregates.

Particle Interaction Model This model implies the ability to mathematically express the structure factor, S(Q) to account for interactions between particles in solution. This can be done by using Baxter's model for hard-spheres with surface adhesion (a.k.a. the "sticky spheres" model) (19). This model allows the calculation of the energy of interaction between small particles assumed as incompressible spheres and provides analytical expressions for the structure factor, S(Q). According to Baxter's model, an approximate value of the potential energy of attraction (which is negative) between two hard spheres, U(r), expressed in k T units (where k = Boltzmann constant), is given by the following equation: B

B

U(r)=

lim ln[12i( "~ S->d d hs

h s

)]

(4)

h s

where d is the diameter of the incompressible (hard) spheres and (5-d ) represents the width of a narrow square attractive well. When the distance between the left edges of two particles is larger than dh but smaller than 5 (i.e., hs

hs

S


140 for d < r < 8), the particles experience attraction. Use of eq 4 requires knowledge of the parameter T . The reciprocal of T , T " , is the "stickiness parameter" and its value is higher when the adhesion between particles is stronger. The limit in eq 4 implies that the calculation of the interparticle attraction potential energy is valid only when the attractive well is extremely narrow, i.e., when its width is within 10% of the particle diameter ((5 - d ) / d

A New Interpretation of Third-Phase Formation in ... - ACS Publications

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