Nathalie A. Wall

Humic acids coagulation: influence of divalent cations

The effects of the ionic strength (maintained by LiCl, NaCl or KCl) and Ca2+ and Mg2+ concentration on the coagulation of purified humic acids (HA) was studied. Solutions of known ionic strengths, pcH, Ca2+ and Mg2+ concentrations were prepared with HA and filtered to obtain the fraction with a size smaller than 100 kD. After a 50 day storage, samples of these solutions were filtered again (100 kD) and the total organic C (TOC) of the filtered solutions measured. The HA coagulation increased with salt concentration, with the cationic charge, and for cations of the same charge, with the cationic charge density. The coagulation decreased for pcH values of 4 to 7–8 in the absence of and presence of Mg2+ and Ca2+. In the absence of the divalent cations, the coagulation has a constant value for pcH>8, but, in the presence of Mg2+ and Ca2+, increases at pcH values greater than 9. The coagulation of humic materials occurs whether the samples are exposed to light or kept in the dark, although the coagulation kinetics are slower for the samples kept in the dark. The size distribution of size-fractionated humic solutions changes over time to a size distribution similar to that of the original humic solution before it was size-fractionated. The results are explained by the DLVO theory.

Dissolution of Technetium(IV) Oxide by Natural and Synthetic Organic Ligands Under both Reducing and Oxidizing Conditions

Technetium-99 (Tc) in nuclear waste is a significant environmental concern due to its long half-life and high mobility in the subsurface. Reductive precipitation of technetium(IV) oxides [TcO(2)(s)] is an effective means of immobilizing Tc, thereby impeding its migration in groundwater. However, technetium(IV) oxides are subject to dissolution by oxidants and/or complexing agents. In this study we ascertain the effects of a synthetic organic ligand, ethylenediaminetetraacetate (EDTA), and two natural humic isolates on the dissolution and solubility of technetium(IV) oxides. Pure synthetic technetium(IV) oxide (0.23 mM) was used in batch experiments to determine dissolution kinetics at pH ∼6 under both reducing and oxidizing conditions. All organic ligands were found to enhance the dissolution of technetium(IV) oxides, increasing their solubility from ∼10(-8) M (without ligands) to 4 × 10(-7) M under strictly anoxic conditions. Reduced Tc(IV) was also found to reoxidize rapidly under oxic conditions, with an observed oxidative dissolution rate approximately an order of magnitude higher than that of ligand-promoted dissolution under reducing conditions. Significantly, oxidative dissolution was inhibited by EDTA but enhanced by humic acid compared to experiments without any complexing agents. The redox functional properties of humics, capable of facilitating intramolecular electron transfer, may account for this increased oxidation rate under oxic conditions. Our results highlight the importance of complex interactions for the stability and mobility of Tc and thus for the long-term fate of Tc in contaminated environments.

Interactions of Tc(IV) with Humic Substances

To understand the key processes affecting 99Tc mobility in the subsurface and help with the remediation of contaminated sites, the binding constants of several humic substances (humic and fulvic acids) with Tc(IV) were determined, using a solvent extraction technique. The novelty of this paper lies in the determination of the binding constants of the complexes formed with the individual species TcO(OH)+ and TcO(OH)2(0). Binding constants were found to be 6.8 and between 3.9 and 4.3, for logβ1,-1,1 and logβ1,-2,1, respectively; these values were little modified by a change of ionic strength, in most cases, between 0.1 and 1.0 M, nor were they by the nature and origin of the humic substances. Modeling calculations based on these show TcO(OH)-HA to be the predominant complex in a system containing 20 ppm HA and in the 4-6 pH range, whereas TcO(OH)2(0) and TcO(OH)2-HA are the major species, in the pH 6-8 range.

Complexation of americium with humic, fulvic and citric acids at high ionic strength

Summary The stability constants of the Am3+ complexes of humic, fulvic and citric acids (HA, FA and Cit) were determined as a function of ionic strength (NaCl) using a solvent extraction technique. At a HA degree of ionization of 0.7 for the carboxylate groups, the Am-HA binding constant, logβ1, varies from 8.3 at I = 0.2 m to 7.2 at I = 6.0 m. The logβ1 of Am-Cit varies from 5.9 at I = 0.3 m to 5.10 at I = 5.0 m. Comparison of the binding constants of Am with HA, FA and Cit shows that, at high ionic strength, logβ1(AmCit) ≈ logβ1(AmFA) ≈ logβ1AmHA-1. Comparison of the values of other metals indicate that the logβ1 values for Cit can serve to estimate HA and FA.

Complexation of Tc(IV) with acetate at varying ionic strengths

Abstract In order to accurately model and predict the fate and transport of metals and radionuclides at multiple radio-contaminated sites, there is a need for an understanding on how metals such as technetium interact with their environment. Many contaminated sites are known to contain large amounts of organic ligands that can affect the solubility and mobility of metals. This study focuses on the effect of acetate on the complexation and dissolution of Tc(IV). Studies were performed at pcH 4.5 (±0.3), at which TcOOH+ is the predominant species. The stability constants for the TcOOH-acetate complex were determined at ionic strengths varying from 0.5 to 3.0 M (NaCl), using a solvent extraction method. The stability constants showed the expected increasing trend over the range of ionic strengths, from 2.46±0.03 (I=0.5) to 3.09±0.08 (I=3.0). A stability constant of 2.8±0.16 at zero ionic strength was determined by specific ion interaction theory. Geochemical modeling data suggest that the dissolution of TcO2·1.6H2O is not greatly affected by the presence of acetate, at concentrations equal or less than 20 mM.

Complexation of Tc(IV) with EDTA at varying ionic strength of NaCl

Abstract The stability constant for Tc(IV)/EDTA complexes were determined using a solvent extraction technique at varying ionic strength (NaCl) and the specific ion interaction theory model allowed for calculating stability constants at zero ionic strength. The stability constants at zero ionic strength for the formation of the TcOEDTA2− and TcOHEDTA− complexes are 1020.0 ± 0.4 and 1025.3 ± 0.5, respectively. The modeled Tc(IV) solubility was calculated to be 3.9 × 10−7 M at near-neutral pH and in presence of 2.5 mM EDTA, a result found to be in good agreement with published solubility experimental data. Speciation calculations showed that TcOEDTA2− is the predominant species between pH 4 and 7.5 in presence of 0.171 mM EDTA, while TcO(OH)20 is predominant in basic solution. These studies show that EDTA has a very strong affinity for complexation with Tc(IV) and can increase the environmental mobility of Tc(IV).

Rapid separation of VI/VII technetium oxidation states by solvent extraction with iodonitrotetrazolium chloride

Technetium99 poses a difficult problem at many nuclear waste disposal sites, as there have been multiple incidents of its release to the environment due to large quantities of fission products disposed in storage tanks. Tc is mostly present under two oxidation states, Tc(VII) and Tc(IV) and the separation of Tc(IV) from Tc(VII) is often crucial for laboratory-scale work performed for the study of Tc. This work offers a method for the rapid separation of Tc(IV) from Tc(VII), using a solvent extraction system containing iodonitrotetrazolium chloride and chloroform.

Interactions of Tc(IV) with citrate in NaCl media

Abstract This paper presents the experimental determination of the stability constant for the citrate complexes with TcO(OH)+ and TcO(OH)20 at different ionic strengths (NaCl), using a solvent extraction method. Data show that the stability constants for the formation of TcO(OH)Cit2− and TcO(OH)2Cit3− are 107.5 ± 0.2 and 102.8 ± 0.2, respectively, at zero ionic strength, with an average of 106.5 ± 0.3 and 102.8 ± 0.2, respectively, in the 1.0–3.0 M ionic strength range. PHREEQC calculations based on these stability constants show that the TcO(OH)Cit2− is the predominant species between pH 3 and 8 and that 50 mM citrate leads to a Tc(IV) solubility of 2 × 10−5 M at pH 5.

Various effects of magnetite on international simple glass (ISG) dissolution: implications for the long-term durability of nuclear glasses

Understanding the effect of near-field materials, such as iron corrosion products, on the alteration of vitreous nuclear waste is essential for modeling long-term stability of these waste forms in a geological repository. This work presents experimental results for which monoliths of International Simple Glass—a six oxide borosilicate glass–, with polished and unpolished cut sides, were aged for 70 days under oxic conditions at 90 °C in a solution initially saturated in 29SiO2 at pH 7; then magnetite was added to the leaching environment. Solution and solid analyses were performed to correlate the changes in the surface features and dissolution kinetics. It was found that magnetite primarily influences the mechanically constrained surface of the non-polished sides of the monoliths, with little to no effect on the polished surfaces. This work highlights the importance of the unique chemistry within surface cracks that invokes a drastic change in alteration of glass in environments containing iron corrosion products.Geochemistry: towards durable nuclear glassesImmobilization in glass based hosts is the current geological method for disposal of long-lived radioactive waste from used nuclear fuels. A key factor that has to be understood is the fundamental mechanism that controls the glass dissolution in a geological repository involving complex reactions between glass and iron, and iron corrosion products. The team of Stéphane Gin from DTCD SECM in France and Nathalie Wall from Washington State University in the USA, together with their co-workers, are seeking to decode the alteration of glass waste in the presence of iron corrosion products, specifically magnetite. It is determined that products such as magnetite primarily impact a mechanically constrained surface, particularly in the case of non-polished sides. Conversely, almost no influence can be observed on the polished surface. Such findings have implication for the long term durability of nuclear glasses.

Integrated Computational and Experimental Protocol for Understanding Rh(III) Speciation in Hydrochloric and Nitric Acid Solutions

Platinum group metals (PGMs), including rhodium, generated by the fission of (235)U are present in significant quantities within spent nuclear fuel located on power generation sites in the United States, the amount of which is expected to exceed natural reserves by 2030. Yet, spent fuel raffinates are highly acidic media that may result in complex speciation of the PGM. This work provides an understanding of Rh(III) speciation up to 9 M HCl and HNO3, and utilizes a combination of ultraviolet-visible (UV-vis) and capillary zone electrophoresis data, along with computationally predicted thermochemistry and simulated UV-vis spectra to approximate the relative concentrations of potential species in solution as a function of acid concentration. One Rh(III) species, [Rh(NO3)3], is observed under all conditions in HNO3 and for Rh(III) concentrations smaller than 10(-3) M. In contrast, a variety of chloridated Rh(III) species may exist simultaneously in a HCl medium. The species [RhCl2(H2O)4](+) and [RhCl3(H2O)3] are observed in HCl solutions of concentrations ranging from 0 to 1 M; the species [RhCl4(H2O)2](-), [RhCl5(H2O)](2-), and [Rh2Cl9](3-) are observed between 2 and 9 M HCl.