Martin A. Glaus

Trace metal–humate interactions. I. Experimental determination of conditional stability constants

The enhancement of mobility of radionuclides in the geosphere through complexation by humic substances is a source of uncertainty in performance assessment of radioactive waste repositories. Only very few data sets are available which are relevant for performance assessment of an underground repository for radioactive waste. Using the equilibrium dialysis-ligand exchange method developed at the Paul Scherrer Institut, conditional stability constants for the formation of complexes of Aldrich humic acid with Ca2+, NpO2+, Co2+, Ni2+, UO22+ and Eu3+ and complexes of Laurentian soil- and Suwannee River fulvic acid with Co2+, UO22+ and Eu3+ were measured. pH was varied between 5 and 10 and ionic strength between 0.02 and 0.2 M. The data are presented as equilibrium coefficients that are free from any model assumptions. The equilibrium coefficients increased in the order Ca2+≅NpO2+<Co2+< Ni2+<UO22+< Eu3+. The quality of the data is assessed in an extended discussion of statistical and systematical errors, and by a critical ‘rereview’ of the auxiliary stability constants used for the calculation of the equilibrium coefficients. An approximate overall uncertainty of 0.5 log-units is estimated for the stability data reported. The conditional stability constants were found to increase markedly with increasing pH in the case of Co2+, UO22+ and Eu3+. For Ni2+, Ca2+ and NpO2+ this effect was less pronounced. For all metal ions tested, the influence of ionic strength was of less importance, and the conditional stability constants did not show a significant dependence on the type of humic substances investigated.

Review of the kinetics of alkaline degradation of cellulose in view of its relevance for safety assessment of radioactive waste repositories

The degradation of cellulose (a substantial component of low- and intermediate-level radioactive waste) under alkaline conditions occurs via two main processes: a peeling-off reaction and a basecatalyzed cleavage of glycosidic bonds (hydrolysis). Both processes show pseudo-first-order kinetics. At ambient temperature, the peeling-off process is the dominant degradation mechanism, resulting in the formation of mainly isosaccharinic acid. The degradation depends strongly on the degree of polymerization (DP) and on the number of reducing end groups present in cellulose. Beyond pH 12.5, the OH- concentration has only a minor effect on the degradation rate. It was estimated that under repository conditions (alkaline environment, pH 13.3-12.5) about 10% of the cellulosic materials (average DP = 1000-2000) will degrade in the first stage (up to 105 years) by the peeling-off reaction and will cause an ingrowth of isosaccharinic acid in the interstitial cement pore water. In the second stage (105-106 years), alkaline hydrolysis will control the further degradation of the cellulose. The potential role of microorganisms in the degradation of cellulose under alkaline conditions could not be evaluated. Proper assessment of the effect of cellulose degradation on the mobilization of radionuclides basically requires knowing the concentration of isosaccharinic acid in the pore water. This concentration, however, depends on several factors such as the stability of ISA under alkaline conditions, sorption of ISA on cement, formation of sparingly soluble ISA-salts, etc. A discussion of all the relevant processes involved, however, is far beyond the scope of the presented overview.

Trace metal-humate interactions. II. The “conservative roof” model and its application

Abstract The modelling of metal-humate interactions has been a field of active research for more than 3 decades but despite all efforts there is still far from a consensus concerning humic binding models. The authors demonstrate that a synopsis of large sets of reliable experimental data for Ca, Co, Ni, Eu, Am, Cm, Np(V) and U(VI) reveals a consistent picture of the influence of metal concentration, pH and ionic strength on metal-humate interactions. However, this consistent behaviour cannot be interpreted with high numerical accuracy by simple binding models, the need for more adjustable parameters increases proportional to the width of the experimental parameter range to be fitted. This experience triggered the proposal of a pragmatic approach for performance assessment purposes. The “conservative roof” approach does not aim to accurately model all experimental data but rather allows estimates to be made of the maximum effects on metal complexation to be expected from humic substances. A specific “conservative roof” model is applied to situations generally to be expected in deep groundwater and selected cases of interest for planned Swiss repositories of radioactive waste are discussed in detail.

Equilibrium dialysis-ligand exchange: adaptation of the method for determination of conditional stability constants of radionuclide-fulvic acid complexes

Abstract The equilibrium dialysis-ligand exchange technique (EDLE) has been adapted to measure conditional stability constants for the complexation of metal ions with fulvic acid. Since fulvic acid permeates across the membrane during the experiment, the quantities involved have to be determined analytically and taken into account when calculating the conditional stability constants. It is shown here that the larger diffusion rates of fulvic acid compared to those of humic acid do not lead to enhanced errors in the results. Conditional stability constants (cKFA) of Laurentian Soil fulvic acid are determined at various pH values and ionic strengths for the binding of Co2+ and UO2+2. Over the whole pH range, at ionic strength of 0.2 M, log ( c K FA ) averages half a log unit lower than at ionic strength of 0.02 M. A comparison with the corresponding binding constants measured for Aldrich humic acid (cKHA) by the conventional EDLE technique shows that log ( c K FA ) values differ by not more than one order of magnitude from log ( c K HA ) values.

Seeming Steady-State Uphill Diffusion of 22Na+ in Compacted Montmorillonite

Whereas the transport of solutes in nonreactive porous media can mostly be described by diffusion driven by the concentration gradients in the external bulk water phase, the situation for dense clays and clay rocks has been less clear for a long time. The presence of fixed negative surface charges complicates the application of Ficks laws in the case of ionic species. Here we report the seeming uphill diffusion of a (22)Na(+) tracer in compacted sodium montmorillonite, that is, transport directed from a low to a high tracer concentration reservoir. In contrast to the classical through-diffusion technique the present experiments were carried out under the conditions of a gradient in the background electrolyte and using equal initial (22)Na(+) tracer concentrations on both sides of the clay sample. We conclude that the dominant driving force for diffusion is the concentration gradient of exchangeable cations in the nanopores. Commonly used diffusion models, based on concentration gradients in the external bulk water phase, may thus predict incorrect fluxes both in terms of magnitude and direction.

Degradation of Cellulosic Materials Under the Alkaline Conditions of a Cementitious Repository for Low- and Intermediate-Level Radioactive Waste. II. Degradation Kinetics

The degradation of cellulosic materials, differing mainly in the degree of polymerization and the number of reducing end groups, was studied under the alkaline conditions similar to those existing in a cementitious repository for low- and intermediate-level radioactive waste (pH 13.3, T = 25°C). The kinetics of alkaline degradation (peeling-off reaction) were studied and the data analyzed by the model of Haas et al. [13]. The observed kinetic parameters for the propagation reaction and overall stopping reaction were compared with literature data. Although measured under different experimental conditions, literature data and data from this study show a consistent picture. Differences in the extent of degradation observed for the different cellulosic materials could be satisfactorily explained by differences in reducing end group content and, consequently, by differences in the degrees of polymerization. Besides the number of reducing end groups, the degree of amorphousness also plays an important role. The main degradation products formed under the experimental conditions used are α- and β-(gluco)isosaccharinic acid. This is in agreement with many other studies on alkaline degradation of cellulose. The two isomers are formed in roughly equal amounts.

Suitability of various materials for porous filters in diffusion experiments

Abstract The suitability of different porous materials (stainless steel, VYCOR® glass, Al2O3 and PEEK) for use as confining filters in diffusion experiments was evaluated by measuring the effective diffusion coefficients (De) of neutral (HTO) and ionic solutes (Na+, Cs+, Sr2+, Cl–, SeO42−) in the materials in through-diffusion experiments. For stainless steel filters, the De values of the target solutes correlated satisfactorily with their bulk diffusion coefficient in water (Dw); thus, the diffusion process in the stainless steel filters was primarily controlled by the diffusivity of the solvated ions. For the remaining materials, the De and Dw values were also correlated for the target solutes, and the geometric factors were in the sequence: VYCOR® glass < Al2O3 < PEEK. Stainless steel and VYCOR® glass were the most appropriate materials because of their high De values, but a specific interaction of caesium with VYCOR® glass was hypothesised because the De values obtained for this solute were slightly higher than expected.

Long-Term Predictions of the Concentration of a-isosaccharinic Acid in Cement Pore Water

The long-term prediction of the equilibrium concentration of α-isosaccharinic acid (α-ISA) in cement pore water is a crucial step in the assessment of the role of cellulose in the safety of a cementitious repository. The aim of the present contribution is to summarise recent efforts in identifying the most important processes leading to the formation or degradation of α-ISA and in predicting its most likely concentrations in cement pore water. The issues considered are the kinetics involved in the formation of α-ISA, reactions of α-ISA with dissolved or solid compounds that may lead to limitations of its pore water concentrations and the chemical stability of α-ISA in a heterogeneous alkaline environment. Some new results are presented showing that α-ISA is degraded to low-molecular weight organic compounds in the presence of oxygen, whereas such processes occur only to a minor extent under anaerobic conditions. It is concluded that the processes involved in the degradation of cellulose under alkaline conditions are not sufficiently understood to explain fully the observed concentrations of α-ISA in long-term experiments.

Determination of diffusion and sorption parameters of thin confined clay layers by direct fitting of through-diffusion flux

Diffusion in compacted clays is often studied in sandwich-like arrangements where the clay is confined by porous filter plates in order to control its swelling. In some clays (for example, Na-montmorillonite) equilibrated with dilute electrolyte solutions, the fluxes of cationic radiotracers can be quite high due to cation-exchange reactions. Accordingly, the diffusion resistance of clay layers can become comparable with or even smaller than the diffusion resistance of porous filters (such layers are called thin in this study). In view of the typical uncertainties (ca. 20%) of diffusion permeability of porous filters reported in the literature, the diffusion resistance of clay layers cannot be reliably determined from the steady-state diffusion permeability of the filter-clay-filter sandwich in this case. In this study, it is shown that, rather unexpectedly, information on the diffusion permeability of thin clay layers can be obtained from the time dependence of diffusant flux into the outlet compartment because at very short times, there is a characteristic flux delay that does not occur in the limiting case of infinitely large diffusion permeability of clay. The flux behavior at longer times is controlled by the diffusion permeability of the filters, which makes possible its determination directly from through-diffusion data and makes superfluous independent diffusion experiments with filters. This approach has been validated via theoretical interpretation of literature data on the diffusion of (22)Na radiotracers through confined compacted montmorillonite equilibrated with 0.01 M NaClO(4) solution. The filter and clay properties estimated in this way are in good agreement with the literature data.