Stability constants of tervalent actinide and lanthanide sulfate complexes in aqueous solutions

Abstract

The determination of thermodynamic and struc tural characteristics of metal complexes in solutions is one of the most important problems of chemistry. Sul fate anions are incorporated into atmospheric aero sols, natural waters, and many minerals and ores. Metal sulfate complexes play an important part in nat ural and engineering processes. Despite the fact that the kinetics and equilibrium of formation of metal sul fate complexes have been studied for many years by different experimental and kinetic methods, their thermodynamic properties (stability constants, etc.) are known only for a restricted number of metals [1]. Owing to experimental difficulties in the operation with radioactive compounds, data for actinides are very scanty. There is no common view on the type of complex formed (inner sphere or outer sphere) in the literature. According to [2], the formation of cation complexes in a solution is considered as a multistage process: first, ions approach each other and form sol vent separated ion pairs (SSIP), some of which may form a contact ion pair (CIP) upon the removal of the water molecule that separates the cation and anion. Studies of sulfate complexes with the use of different techniques (conductometric, potentiometric, and spectroscopic methods, dielectric relaxation spectros copy, ultrasonic dispersion, etc.) bring the authors to contradictory conclusions on the dominant type of complex [3][4][5]. The coordination mode of an anion to a cation in solution (monodentate or bidentate) is also little studied. Different methods were noted to reveal a mixture of inner sphere and outer sphere complexes in aqueous lanthanide and actinide sulfate solutions [3], but there exist disagreements on the dominant type. On the basis of dielectric relaxation measure ments and other literature data, Wachter et al. [4] sup pose that only SSIPs are formed in lithium sulfate solutions, and CIPs are not formed at all even at a con centration of 2 mol/L. A critical view on the possibili ties of spectroscopic methods in the measurements of stability constants of complexes is given in [5] with the use of different metal sulfate complexes as an example.The clarification of a reason for a sharp discrep ancy between the stability constants of 1 : 1 carbonate and sulfate complexes of tervalent lanthanides and actinides in aqueous solutions (the stability constants of carbonate complexes are three to four orders of magnitude higher than those of sulfate complexes) remains to be one of the most intriguing unsolved problems. and anions have close values of crystal ionic radii. Both anions are classified as "hard" according to Pearson's terminology, and tervalent lan thanide and actinide cations are also "hard", so inte rionic interactions are dominantly electrostatic. Close values of the radii of anions with the same charge must have led to close values of stability constants of their complexes (for example, from the viewpoint of Bjer rum's ion association theory, association constants must not differ by more than 10%). An "abnorma...

Modeling separation of lanthanides via heterogeneous ligand binding

“…93 Fortunately, whether a certain Ln 3+ binds to a substrate can be determined experimentally. Based on available experimental data, 13,[22][23][24][25][26][27] we assume that sufficient ligand-Ln 3+ binding is achieved in all cases, except for amines discussed in the Results section.…”

“…For ligands in water, we use simple anions and molecules like sulfate, [22][23][24] phosphate, 25 and primary amines, 26,27 which have been demonstrated to bind to Ln 3+ , so there is less concern about desorption during long trajectories. Our work on Ln 3+ -complexes at infinite dilution in water builds on the extensive computational studies of hydration properties of lanthanides.…”

Mixed Solvents

“…39 Authors have calculated relative concentration of different solvate forms of Li + in the mixed solvent ethylenedi-amineDMSO and ethylenediamineDMFA (Figure 8.1.12). 39 Authors have calculated relative concentration of different solvate forms of Li + in the mixed solvent ethylenedi-amineDMSO and ethylenediamineDMFA (Figure 8.1.12).…”