Thomas Fanghänel

Aquatic chemistry and solubility phenomena of actinide oxides/hydroxides

The present paper gives an overview of the solubility behavior of actinide oxides/hydroxides, taking into account hydrolysis reactions up to colloid formation. The analogies, systematic trends, and differences in the hydrolysis and solubility constants of actinides in the oxidation states An(III) to An(VI) correlate with the charge and size of the actinide ions. The formation of amorphous and crystalline solids and the discrepancies between the corresponding experimental solubility data may be explained as an effect of particle size. However, using thermodynamic data for the stable crystalline solids, the predicted solubilities are often significantly lower than experimental data (even in long-time experiments), indicating that the solubility is controlled by the surface properties. Typical examples are the known U(VI) solids schoepite and sodium diuranate. The most striking example is provided by the tetravalent actinides. Above the threshold of hydrolysis, the dissolution of microcrystalline or crystalline An(IV) oxides is found to be irreversible. The measured concentrations approach those of the amorphous hydroxides or hydrous oxides.

An Improved Hydrolytically-Stable Bis-Triazinyl-Pyridine (BTP) for Selective Actinide Extraction

Abstract Bis-2,6-(5,6,7,8-tetrahydro-5,9,9-trimethyl-5,8-methano-1,2,4-benzotriazin-3-yl)pyridine (CA-BTP) is presented as a novel optimized extracting agent for the separation of americium(III) and curium(III) from lanthanides(III). CA-BTP is synthesized and studied concerning its extraction properties under conditions relevant to the SANEX process. It is demonstrated that CA-BTP is the first relevant molecule to show both high stability towards highly-acidic process solutions and satisfactory extraction kinetics. Its high solubility in 1-octanol adds to CA-BTPs favorable properties. Furthermore, CA-BTP is synthesized from a commercially available and moderately priced precursor molecule, reducing extracting-agent production costs.

Thermodynamics of trivalent actinides and neodymium in NaCl, MgCl 2 , and CaCl 2 solutions: Solubility, hydrolysis, and ternary Ca-M(III)-OH complexes

Known data on the solubility of Am(OH)3(s) and the hydrolysis of Am(III) and Cm(III), additional information from an extensive solubility study with Nd(OH)3(s) in NaCl, MgCl2, and CaCl2 media of various ionic strengths and spectroscopic (time-resolved laser fluorescence spectroscopy, TRLFS) data for Cm(III) in alkaline CaCl2 solutions are used to evaluate a comprehensive set of standard-state equilibrium constants and ion interaction parameters for the specific ion interaction theory (SIT) and Pitzer equations at 25 °C. The thermodynamic model takes into account the analogous solubility and hydrolysis behavior of trivalent actinides and Nd(III) and covers the entire pH range in dilute to concentrated NaCl, MgCl2, and CaCl2 solutions. In alkali chloride/hydroxide solutions, the formation of the tetrahydroxide complex M(OH)4– requires OH– concentration above 3 mol l–1, whereas in alkaline CaCl2 solutions (at pHc < 12) M(III) complexes with four and six hydroxide ligands are formed. Similar as the recently detected ternary Ca–M(IV)–OH complexes Ca3[Zr(OH)6]4+ and Ca4[Th(OH)8]4+, these complexes are stabilized by the association of Ca2+ ions. The solubility and hydrolysis of Am(III), Cm(III), and Nd(III) in both Ca-free and -containing solutions is consistently described with a model including the ternary Ca–M(III)–OH complexes Ca[M(OH)3]2+, Ca2[M(OH)4]3+, and Ca3[M(OH)6]3+.

Direct nano ESI time-of-flight mass spectrometric investigations on lanthanide BTP complexes in the extraction-relevant diluent 1-octanol

The present work focuses on investigations of a highly selective ligand for Am(III)/Ln(III) separation: bis-triazinyl-pyridine (BTP). By means of nano-electrospray mass spectrometry, complex formation of BTP with selected elements of the lanthanide series is investigated. We show that the diluent drastically influences complex speciation. Measurements obtained in the extraction-relevant diluent 1-octanol show the occurrence of Ln(BTP)i (i = 1–3) species in different relative abundances, depending on the lanthanide used. Here, the relative abundances of the Ln(BTP)3 complexes correlate with the distribution ratios for extraction to the organic phase of the respective lanthanide.

Theoretical investigation of the water/corundum (0001) interface

For the reliable long-term modeling of the actinide migration in geological formations, the adsorption/desorption properties and the reactivity of mineral surfaces must be understood at the molecular level. The adsorption of radioisotopes at mineral surfaces of the aquifer is an important process that leads to the retention of contaminants such as radionuclides. Their transport by the ground water is either retarded or even completely inhibited by the presence of such a surface. Accordingly, this subject is of main importance for the safety assessment of nuclear waste repositories. As part of a joint theoretical/experimental effort, the interaction of water with the corundum (0001) surface is studied using several theoretical methods (Moller-Plesset perturbation theory, coupled cluster singles doubles with triplet corrections, as well as density functional theory). We focus in this study on the determination of the bond lengths and tilt angles of the surface OH species and their respective vibrational frequencies. The theoretical results are confirmed by subsequent simulation of the interface selective nonlinear sum frequency spectra. The excellent agreement of the simulated with the experimental spectra allows an assignment of the observed peaks in the sum frequency spectra of the water/corundum (0001) interface on the basis of our theoretical data. In this theoretical study we are able to give a unique interpretation of the observed sum frequency spectra of the water/corundum (0001) interface.