Yuanxian Xia

A fluorescence spectroscopic study on the speciation of Cm(III) and Eu(III) in the presence of organic chelates in highly basic solutions

Summary The speciation of Eu(III) and Cm(III) was investigated by time resolved laser fluorescence spectroscopy (TRLFS) over a range of base concentrations from 0.01M NaOH to 7.5M NaOH and in the presence of several organic chelates including EDTA, HEDTA, NTA, and oxalate. The results of this work suggest that both Eu(III) and Cm(III) form strong mixed ligand complexes with organic chelates and the hydroxyl groups(s) in dilute NaOH solutions. However, in concentrated NaOH solutions, Eu(III)-/Cm(III)-containing colloidal nanoparticles are the primary cause for the measured Eu(III)/Cm(III) in the aqueous solutions. Therefore, the interpretation of these data solely in terms of the formation of amphoteric hydroxyl species ( e.g. Eu(OH)4-) would appear to be inappropriate. The organic chelating ligands form strong complexes with surface Cm(III)/Eu(III) sites of the colloidal nanoparticles. For Cm(III), such surface complexes show largely red-shifted fluorescence spectra as compared with the aqueous complexes and unusually short fluorescence lifetimes. The decreased fluorescence lifetimes are likely due to the presence of transition metal impurities in the nanoparticle and reduced inter-nuclear distance between neighboring Cm(III) centers.

The Solubility Product of NaUO2PO4.xH2O Determined in Phosphate and Carbonate Solutions

Abstract The solubility product of NaUO2PO4· x H2O was determined in phosphate containing solutions at low pCH+ values in the absence of carbonate and at higher pCH+ values in the presence of carbonate. NaUO2PO4· x H2O exhibited very low solubilities (∼10−7 M in U) over a broad range of hydrogen ion concentrations, NaNO3 concentrations and in the absence of added carbonate. Time Resolved Laser Fluorescence Spectroscopy (TRLFS) analysis of non-carbonate solutions outside of the acidic region revealed the presence of complex mixtures of aqueous U(VI) hydroxyl or phosphate species and uranium phosphate nanoparticles. The presence of the nanoparticles made it impossible to accurately calculate a solubility product for NaUO2PO4· x H2O in the absence of carbonate and at higher pCH+ values. Therefore in order to increase the concentration of U(VI) in solution and thereby verify the solubility product calculated from the most acidic samples, we systematically introduced know concentrations of carbonate, which resulted in the formation of U(VI) carbonate complexes. Development of an accurate aqueous thermodynamic model for the aqueous U(VI) carbonate complexes then allowed calculation of a solubility product for NaUO2PO4· x H2O in the higher pH samples which was in good agreement with the values for the more acidic samples.

The Aqueous Complexation of Thorium with Citrate under Neutral to Basic Conditions

Summary The aqueous complexation of thorium with citrate was investigated under neutral to basic conditions and over a broad range of ionic strengths. The solubility data for ThO2(am) as a function of citrate concentration indicate the presence of stable species with citrate-to-metal ratios of between two to three. The dependence of the ThO2(am) solubilities on hydrogen ion concentration can also be readily explained by the classical assumption of hydrolysis of the central Th(IV) ion to form mixed thorium-hydroxide-citrate complexes. 13C NMR spectra of the species in solution confirm that the citrate-to-metal ratio of the species in solution is between two and three and show that the citrate attaches to the metal in a bidentate fashion through oxygens on the α-carboxylate and α-alkoxyl groups, rather than through the carboxylate groups. The 13C NMR spectra, as well as a density functional theory (DFT) electronic structure study of the presumptive complexes, suggests that the associated α-hydroxyl proton can be displaced during complex formation. These findings indicate an alternative explanation for the observed changes in solubility as a function of hydrogen ion concentration, the displacement of protons from the citrate alkoxyl groups via metal binding. Removal of protons from the alkoxyl groups or hydrolysis of the central Th(IV) cannot be distinguished by thermodynamic measurements, however the species with the α-hydroxyl proton removed (i.e. ThOH(Cit)25- and Th(Cit)38-) would appear to better represent the microscopic binding. Apparent equilibrium constants for the solution phase reactions of these species and the hydrous thorium oxide have been calculated as a function of ionic strength.

Complexation of Cm(III)/Eu(III) with silicates in basic solutions

Summary The complexation of Cm(III) and Eu(III) with dissolved silica was studied by solubility measurement and time resolved laser fluorescence spectroscopy (TRLFS) in basic solutions over a range of total silica concentrations and ionic strengths (NaNO3). In initial experiments on the solubility of Eu(OH)3 in silicate containing solutions we observed a rapid increase in the soluble Eu(III) concentration under basic conditions. The Eu(III)/Cm(III) silicate complexes that formed caused a significant increase of the hypersensitive 5D0 → 2F2 band around 615 nm relative to the non-hypersensitive 5D0 → 7F1 band at 592 nm for Eu(III). Further studies of Eu(III) and Cm(III) containing solutions showed a shift of the fluorescence spectral maximum from 594 nm to up to 607 nm for Cm(III) along with a significant increase of fluorescence intensities. The fluorescence lifetimes for Eu(III) species increased from 115 μs to 1.8 ms in 3.0 M and 5.0 M NaNO3, corresponding to full coordination, and from 68 μs to 202 μs for Cm(III) in 0.1 M NaNO3, consistent with the removal of 6–7 water molecules upon silicate complexation. The variations of the fluorescence intensity and the concentrations of the monomeric and polymeric silicates suggested that in basic silicate solutions, Eu(III)/Cm(III) mainly complexes with polysilicates.

Thermodynamic model for the solubility of ThO2 (am) in the aqueous Na+ - H+ -OH- -NO3- -H2O-EDTA System

Abstract The solubility of ThO2(am) in the aqueous Na+-H+-OH--NO3--H2O-EDTA system as a function of pCH⁺ (= -log[H+]) and variable NaNO3 (0.5M to 6.0M) has been determined. The experimental observations show that between pCH⁺ values 4.2 to 8.2, a stoichiometric 1:1 Th-EDTA complex forms that completely saturates the added chelate concentration. Th concentrations then decrease linearly with increasing pCH⁺ at pCH⁺>9.0. These changes in solubility are not predicted by currently available thermodynamic models. The ion-interaction model of Pitzer was used to interpret these solubility data. Thermodynamic analysis indicates that the speciation under basic conditions is dominated by monomeric mixed Th-OH-EDTA complexes. X-ray absorption near-edge spectroscopy confirmed the absence of the higher order ( e.g. dimeric) species. The equilibrium constants for the following reactions were determined from analysis of the solubility data: ThO2(am) + EDTA4- + 2H2O ↔ Th(OH)2EDTA2- + 2OH-, log K = -6.0; and ThO2(am) + EDTA4- + 2H2O ↔ Th(OH)3EDTA3- + OH-, log K∼-7.5. Pitzer ion-interaction parameters for the Th(OH)2EDTA2- and the Th(OH)3EDTA3- species were calculated. It was also determined that the solubility method for examining the complexation of tetravalent actinides with EDTA is limited to relatively high aqueous EDTA concentrations, relative to the amount of ThO2(am) precipitate, owing to the adsorption of the EDTA chelator by the solid phase.