Nuclear Site Remediation - American Chemical Society

Chapter 5

Downloaded by PENNSYLVANIA STATE UNIV on July 28, 2013 | http://pubs.acs.org Publication Date: November 29, 2000 | doi: 10.1021/bk-2001-0778.ch005

The Aqueous Complexation of Eu(III) with Organic Chelates at High-Base Concentration: Molecular and Thermodynamic Modeling Results Andrew R. Felmy, Zheming Wang, David A. Dixon, Alan G. Joly, James R. Rustad, and Marvin J . Mason Pacific Northwest National Laboratory, 3350 Q Street, Richland, WA 99352

The solubility of Eu(OH) (c) was investigated over a broad range of NaOH concentrations extending to 7.5 molal in the presence and absence of added E D T A , and N T A . Thermodynamic analysis of the experimental data indicates that formation of mixed metal-chelate-hydroxide complexes is occurring in solution. These mixed metal-chelate-hydroxide complexes increase the effectiveness of the chelates in solubilizing Eu(III) under high base conditions. The number of hydroxide ions bound to the metal-ligand complex varies with the chelate type. The structural and energetic reasons for this variability in solution coorelate with density functional theory (DFT) calculations for gas phase La(III) clusters. In the case of E D T A solution complexes form with only one E D T A and one hydroxyl ions associated with the metal-chelate complex (i.e. Eu(OH)EDTA2-) whereas in the case of N T A there are either one or two N T A molecules and at most only one hydroxyl associated with the metal-chelate complex (e.g. E u O H N T A or EuOH(NTA)24-). The first determination of the stability constants for two of these species (Eu(OH)EDTA2and EuOH(NTA)24-) as well as for the Eu(OH)4- species are proposed. A n aqueous thermodynamic model is presented which describes all of the available thermodynamic data for these chemical systems, to high ionic strength, and correlates with the DFT calculations. The implications of these results in regard to processing Department of Energy (DOE) high level wastes is also discussed. 3

-

© 2001 American Chemical Society In Nuclear Site Remediation; Eller, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

64

Introduction It has long been known that the organic chelates ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid (NTA) can form strong aqueous complexes with a variety of metal ions, including the rareearths and trivalent actinides [Am(III), Cm(III), and Pu(III)] " . However, in the case of E D T A , the ionic size of the trivalent actinides and rare-earth ions makes it impossible for only simple sexidentate complexes to form and such solution species must involve additional coordinated water molecules . These coordinated water molecules are then subject to displacement by other ligands in solution and this has led to the identification of several different types of mixed ligand complexes* " * including complexes where hydroxide ion has substituted for coordinated water under high pH conditions . Similar mixed ligand complexes have also been identified for other metal ions including Cu(II), Ni(II), Co(II), and Al(III) " . In addition, the possibility exists, based upon analogy with Fe(III) complexes that bridging dimeric complexes involving H E D T A , and possibly E D T A , can also form at high p H . Formation of such mixed ligand complexes can have significant implications for the chemical speciation of trivalent actinides and rare-earths under basic conditions. Such highly basic condition are precisely the conditions encountered in high-level-waste (HLW) storage tanks at Hanford and other DOE sites which also can contain significant concentrations of chelating agents including E D T A and N T A * . The presence of these chelating agents along with trivalent actinides (Am(III), Cm(III)) can result in solubilizing the actinides from the sludge and creating difficulties in subsequent actinide/radionuclide separation processes. As a result, developing a complete understanding of the effects of organic chelating agents on the solubility and aqueous speciation (including the identification of mixed chelate hydroxide complexes) of the trivalent actinides in H L W solutions is a key factor in developing effective waste processing strategies. (l

3)


(4,5)

6

8

(1,3,9)

(,0

l2)

(,3,l4)

(15)

(l6

l8)

(l9)

In this paper a combined experimental and molecular/thermodynamic modeling approach is used to study the hydrolysis and complexation of Eu(III) [a trivalent actinide analog] in NaOH solutions in both the presence and absence of the organic chelating agents E D T A and N T A . Eu(III) was selected for study since 1) it present in significant concentration in high level nuclear waste storage tanks at D O E sites which contain high base concentrations, 2) it is an excellent analog for trivalent actinides (e.g. Am(III), Cm(III), and Pu(III) which can also be present in tank solutions, 3) lanthanide species can be treated by high level molecular modeling simulations, 4) only limited experimental data on the hydrolysis and chelate complexation of lanthanides or trivalent actinides are available at high base (>lm) concentration, and 5) Eu(III) has excellent fluorescence properties for determining the molecular structure of the aqueous complexes. This combined approach consists of DFT calculations on gas phase

In Nuclear Site Remediation; Eller, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

65

clusters to help define the structure and relative stability of metal-chelatehydroxide clusters, 2) use of Time Resolved Laser Luminescence Spectroscopy (TRLFS) to help determine the species present in solution, and 3) thermodynamic measurements and analysis of Eu(III)-NaOH-chelate solutions. The experimental thermodynamic data were obtained by solubility methods owing to the relatively low solubility of Eu(OH) (c) under high base conditions. The solubility studies were conducted over a broad range of NaOH concentrations in both the presence and absence of added E D T A and N T A . Downloaded by PENNSYLVANIA STATE UNIV on July 28, 2013 | http://pubs.acs.org Publication Date: November 29, 2000 | doi: 10.1021/bk-2001-0778.ch005

3

Experimental Procedures (20)

Eu(OH) (c) was prepared using the same procedure outlined by Rao et a l . for the preparation of crystalline Nd(OH) (c). The resulting material was examined by X-Ray Diffraction ( X R D ) and found to be fully crystalline Eu(OH) (c). This material was used in all subsequent studies. The solubility studies using Eu(OH) (c) were conducted in a controlled atmosphere chamber filled with nitrogen gas to prevent the contact of the solutions with atmospheric C 0 . A n upper limit of 0.01 M in chelate concentration was selected since it is the approximate upper limit present in tank waste and at such chelate concentrations it is still possible to maintain a sufficient mass of solid Eu(OH) (c) in each tube. The NaOH concentrations ranged from 0.01 M to 7.5M. Suspensions were sampled at various times to determine, or establish, equilibrium conditions. Sampling consisted of p H measurements, at NaOH concentrations lower than 0.1M, followed by centrifugation at 2000 g for 7 to 10 minutes. Aliquots of the supernate were filtered though Amicon-type F-25 Centriflo membrane cones with an approximate pore size of 0.0018 μ. Eu concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS). Total organic carbon, used to estimate dissolved chelate concentrations, was determined on selected samples using a Shimadzu TOC 5000A analyzer with a 680°C furnace. Selected samples of the solid precipitates covering the entire range of NaOH concentration were analyzed by X-Ray diffraction analysis (XRD) after the end of the equilibration period. Fully crystalline Eu(OH) (c) was the only phase identified in the appropriate experiments. A l l studies were conducted at room temperature (22-23°C). The D F band at -617 nm. To record the fluorescence decay curve, the sample was excited at the peak maxima of the D -> F selective spectra and detected at the emission maxima at -617 nm. The 3

3

3

3

2

3

3

5

7

0

0

5

7

0

2

5

0

7

0


66

data analysis was done with WaveMetrix.

commercial software

IGOR

licensed by

Molecular Modeling Approach We have previously shown that the geometries and frequencies of transition metal and actinide compounds can be predicted reliably at the local density functional theory level " . Thus we have used this approach to predict the structures of complexes of a Ln(III) metal with a variety of organic ligands of interest to this study. We initially attempted calculations on the Eu(III) complex with eight water molecules. Eu(III) nominally has the electronic structure f which places 6 electrons in the 4f orbital. This makes convergence of the wavefiinction extremely difficult and we were unable to get any of our calculations (using N W C h e m ) to converge even when starting from good estimates of the structure. We thus decided to use La(III) to model the Eu(III) species as the atomic radii for the ions are similar (1.28 Â vs. 0.98 Â ) and the fact that the L a has no f electrons which minimizes convergence problems. A l l calculations were done with the density functional theory program DGauss " on SGI computer systems. The calculations were performed using a pseudopotential for the L n core electrons and the remaining electrons are treated with a polarized valence double zeta basis set (2s/lp/2d). The remaining atoms (C,N,0,H) were treated with the DZVP2 basis set . A l l calculations were done at the local level with the potential fit of Vosko, Wilk and Nusair . Geometries were optimized by using analytic gradients.


(21

26)

(27-28)

( 2 9 )

(30

32)

(33,34)

(35)

(36)

Thermodynamic Model The aqueous thermodynamic model used in this study to interpret the solubility data is the ion-interaction model of Pitzer . This model emphasizes a detailed description of the specific ion interactions in the solution. A detailed description of the exact form is given elsewhere . The Pitzer thermodynamic model was used because it is applicable from zero to high concentration, and our solubility data extend to high ionic strength (I ~ 6m). In these calculations the stability constants for E D T A , and N T A complexes with Eu(III) recommended by Martell and Smith , see Table 1, are included in all calculations. (37,38)

(39_4,)

(,)



3

3+

4

18.0 -25.6

3+

2

Eu + 40H' = Eu(OH) " Eu(OH) (c) = E u + 30H'

2

4

2

3

6.84 3.95

3

EuNTA(aq) + OH'= EuOHNTA' Eu(NTA) + OH' = EuOH(NTA) '

3

11.32 9.32

3+

3

Eu + NTA '= EuNTA EuNTA(aq) + NTA '= Eu(NTA) '

3+

15.45 4.03

2

Eu + HEDTA '= EuHEDTA' EuHEDTA + OH" = Eu(OH)HEDTA"

4

17.29 4.87

3+

LogK

Eu + EDTA ' = EuEDTA' EuEDTA" + OH" = EuOHEDTA '

Reaction

(1)

This Study

This Study

(1)

(I)

(1)

(1)

(1)

This Study

(1)

Reference

Table 1. Equilibrium constants for Eu(III) aqueous complexes and solid phases used in this study. AH values from Martell and Smith (1995) are for ionic strengths of zero or 0.1.


68 Results and Discussion


In this section the experimental and modeling results for the chemical systems with and without added chelators are presented. A section comparing the stability constants calculated here with the molecular modeling results for gas phase clusters is also included.

EDTA The solubility data for Eu(OH) (c) in the presence and absence of different concentrations of E D T A , Figure 1, shows an extremely large increase in solubility as a result of E D T A complexation. In fact, this increase in solubility is far more than would be predicted by currently available thermodynamic models for this chemical system. As an example, the calculated solubility of Eu(OH) (c) in these solutions using all of the stability constants recommended by Martell and Smith , Table 1, are several orders of magnitude lower than the experimental data. In these calculations the ion-interaction parameters for N a EDTA * were taken from the recent work of Pokrovsky et al. . Clearly, there is a need to improve the thermodynamic models for this chemical system. These data also show a couple of other interesting features. First, the solubility data at different equilibration times are similar, indicating that equilibrium, or at least steady-state, concentrations were reached fairly rapidly (i.e.

Nuclear Site Remediation - American Chemical Society

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