D. Vopálka

Modeling of metal-humate complexation based on the mean molecular weight and charge of humic substances: Application to Eu(III) humate complexes using ion exchange

A general model, the so called Mean Molecular Weight Model (MMWM), of complexation of metal cations (Mez+) with macromolecular polyanions of humic acid (HAp-) is proposed. The model is based on the results of previous studies of the electrophoretic mobility of humate complexes and assumes that the complexation proceeds by consecutive neutralization of the dissociated carboxyl groups of the central polyanion HAp- with Mez+ cations. It reflects the macromolecular character of humic acid, applies molar concentrations of reacting components with equations for stability constants and incorporates also the mean charge of humic macromolecules. The model has been verified with experimental data obtained in the study of complexation of Eu(III) with Aldrich humic acid using ion exchange (Amberlite IR-120), over a broad range of [Eu] to [HA] ratio, at pH 4 and 7.

Kinetics of radioeuropium sorption on Gorleben sand from aqueous solutions and ground water

Kinetics of Eu sorption on sandy sediment from Gorleben, Germany, containing humic substances, was studied by radiotracer method in batch experiments at very low europium concentration (3.4.10-8 mol/l), with the aim to find kinetic parameters suitable for modeling Eu migration in bed of the sediment and to elucidate the mechanism of the sorption. Experiments were evaluated using a new simplified method taking into account simultaneous sorption/desorption of Eu on the walls of sorption vessel. Five kinetic functions were tested, of which that describing diffusion in inert surface layer of sorbent proved most suitable. The effects of pH (3.0-8.8) by addition of Aldrich humic acid (10 mg/l), addition of hydrogencarbonates (5.10-3 mol/l) and preequilibration of the sediment with solution or of Eu with solution were examined. From the results it has been concluded that the kinetics and mechanism of the sorption strongly depend on pH. At pH 4.8 Eu is sorbed mainly as humate complex from the solution of humic acid. The addition of humic acid accelerates the sorption. Carbonate complexes of Eu are the probable forms sorbed at pH 8.8. The presence of humic substances in the slightly alkaline solutions suppresses the rate of the sorption due to slow dissociation of Eu-humate complexes.

Cesium uptake by Ca/Mg bentonite: evaluation of sorption experiments by a multicomponent two-site ion-exchange model

Abstract Cesium sorption on Czech Ca/Mg bentonite was studied using the batch technique for a wide range of both bentonite-to-water ratios (m/V) and initial concentrations of Cs. The experiments revealed non-linear pattern of cesium sorption, which was substantially influenced by the m/V. The results of the exchange reaction between added Cs+ and Mg2+, Ca2+, Na+ and K+ ions present naturally on the bentonite sorption sites enabled to determine the appropriate selectivity coefficients. The capacity of sites with higher selectivity to cesium, which is believed to be associated with the frayed-edge sites, was measured using the AgTU method.

Radiotracer study of the kinetics of complexation and decomplexation of Eu(III) with humic acid using ion exchange

The kinetics of complexation (C) and decomplexation (D) reactions between Eu(III) and Aldrich humic acid (HA) was investigated as a function of pH (pH 4, 5, 6, 7 and 8) in the system Eu(III) - HA - Amberlite IR-120(Na) (I = 0.1). The derivation of the kinetic differential equations was based on the reactions of Eu3+ with, so called, strong (HAS) and weak (HAW) carboxylic groups of HA formulated in accordance with the new complexation model.1 The differential equations determining d[EuaHAS]/dt and d[EubHAW]/dt have the classical form applicable for reversible reactions where the forward reaction is the C-reaction and the reverse one is the D-reaction. Kinetic model used for the evaluation of experimental data includes these differential equations and the film diffusion model of sorption of Eu3+ on Amberlite IR-120(Na).

A simplified approach to modelling underground migration of radionuclides from contaminated river sediments

Abstract Accidental releases of waste water from the first Czechoslovak nuclear power plant, A1, caused contamination of sediments of the Dudvah river, flowing into the Vah river, in Slovakia. Rather high concentrations of 137 Cs and 90 Sr (2150Bq dm −3 and 215Bq dm −3 , respectively) were found in bottom sediments of a former channel of the re-engineered river body at a distance of about 250 m from a village, Siladice, with water-supply wells. In order to assess the possibility of contamination of the wells, underground migration of both radionuclides from the contaminated area was simulated using an original layered convection-diffusion model. K d values determined in laboratory experiments were used. The analysis of the hydrological situation in the area reveals that the critical condition is a dominant horizontal groundwater flow near the water table in the direction from the Vah bank to Siladice, in the periods when the contaminated body lies under the water table. The simulation calculated under conservative conditions showed that the contamination of water-supply wells would not exceed permissible concentration limits.

A simplified approach to evaluation of column experiments as a tool for determination of radionuclide transport parameters in rock-groundwater or soil-groundwater systems

The assessment of the ability of natural barriers to retain radionuclides and retard their transfer in groundwater requires knowledge of important transport parameters, the retardation and dispersion coefficients. The use of dynamic techniques is in this task more effective than that of batch technique, as the conditions of dynamic experiments better simulate the real systems, in which the contaminated groundwater is flowing through the bed of a porous (grained) solid material (crushed rock, soil, or sediment). Two techniques of the contaminant inlet, the pulse injection and step (continuous) inlet are obviously applied. Dynamic column experiments make possible to study the influence of sorption or desorption of studied contaminants on the velocity of their transport through the saturated or unsaturated bed. The transport parameters are determined in the course of evaluation of experimental data, which generally consists of the regression of breakthrough curve by selected analytical solution of the 1-D advection–dispersion equation. With the respect to the kinetics of the contaminant interaction with the surface of the solid phase, there are two basic groups of these solutions: the first responds to the equilibrium dynamics, and the second one to so-called non-equilibrium dynamics. In description of interaction, that implies the mathematical form of the solution of transport equation, it is further possible to specify both the equilibrium isotherm (linear or non-linear) and the type of kinetic equation (e.g., linear driving force model). In this paper, a set of simplified equilibrium dynamic models is presented, that could be recommended for the evaluation of an important range of column experiment in heterogeneous systems accomplished under the equilibrium dynamics conditions.

Modelling of uranium release from waste rock pile

Uranium release/uptake on material of waste rock pile No. 66 at Schlema-Alberoda (Saxony, Germany) was studied by both static (batch) and dynamic (column) experiments with the aim to obtain input data for modelling of uranium migration in the pile. Most of the experiments were carried out by radiotracer method using 233U as the label. An ambiguous influence of humic acid (concentration 10 and 50 mg/L) on the migration of uranium was observed. The column elution experiments were modelled using PHREEQC that enabled to respect the kinetic character of the desorption process.

Carbon Steel Canister Performance Assessment: Iron Transfer Study

Carbon steel has been chosen in the Czech disposal concept of spent fuel disposal in a granite host rock as a reference material for disposal canisters. On the basis of the results of performance assessment studies, it could be decided whether this material is suitable or whether a more corrosion resistant, and also more expensive, material should substitute it. A number of papers have convincingly shown that iron transfer constraints contribute to a significant decrease in corrosion rate, but no study hasso far been devoted to modeling this process. In this paper the effects of initial corrosion rate, corrosion product solubility and porosity and other repository parameters on the transfer of iron to the host rock are modeled using a numerical transport computer code. It was found that the critical parameter for iron transfer is the solubility of corrosion products, considerably affecting the steady state corrosion rate. The initial corrosion rate of carbon steel and the sorption properties of bentonite primarily affect the time needed to achieve a steady state of corrosion. The results of the calculations strongly suggest that the constraints on iron transfer from the canister surface will govern the corrosion rate of carbon steel canisters, whose lifetime, owing to this effect, can stretch to millions of years.

Modelling and simulation of sorption selectivity in the bentonite-HCO3−−CO32−−233U(VI) species system

A surface complexation model, the so-called diffuse double layer model (DLM), was used in the description of the multicomponent system consisting of Czech sodium bentonite SABENYL and synthetic granitic water spiked with233U(VI). The experimental data were evaluated and the characteristic parameters, e.g., the equilibrium constants of all the reactions considered, were obtained and used for the numerical simulation of sorption selectivity of the uranium in relation to the total carbonate concentration. The values of separation factors indicate that the selectivity of uranium sorption in such multicomponent systems can depend not only on the pH and composition of both phases, but also on the phase ratio and starting concentrations of participating components.

Sorption of Sr and Cs onto Czech natural bentonite: experiments and modelling

Within the development of deep geologic repository in the Czech Republic, the local Mg/Ca bentonite has been tested with the aim to assess its sorption qualities with regard to cations of interest for the performance assessment (PA) of the repository. The data of Sr and Cs sorption obtained by batch method were evaluated with the use of ion exchange and surface complexation models. The selectivity coefficients of exchangeable cations were determined, and the selectivity coefficient of Sr was set to be the same as for Ca. The selectivity coefficient of Cs was chosen in accordance with literature and the concentration and stability constant of the surface complex was fitted. One-site ion exchange model for Sr sorption was able to predict the observed Kd dependence on solid-to-liquid ratio. Two-site ion exchange/surface complexation model for Cs sorption simulated the dual sorption mechanism of Cs adequately, thus the predictive ability was lower due to the fact that the protonation of edge sites or competition with Na ion was not taken into account.