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  • Author or Editor: K. Štamberg x
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Summary  

The sorption of Cs(I) and Sr(II) on bentonite and magnetite was experimentally studied and numerically simulated using surface complexation (SCM) and ion-exchange (IExM) models. The empirical system consisted of: (1) synthetic granitic water with a given ionic strength (0.1 or 0.01 NaNO3), (2) radionuclides studied (10-6M CsCl or SrCl2 . 6H2O spiked with 137Cs or 85Sr), and (3) bentonite pre-treated with the aim to remove carbonates, and magnetite as a representative of steel canister corrosion products. The parametric study covered, e.g., the influence of pH, bentonite to magnetite ratio and volume-liquid ratio on the values of selectivity coefficients and K d.

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Abstract  

Transport of 125I, 137Cs+ and 85Sr2+ radionuclides in crushed granitoidic rocks and homogenized soils was studied. Two simple methods for calculation of breakthrough curves in flow column experiments with groundwater as transport medium have been described. The first method, so called non-linear approach, is derived on the assumption of a reversible non-linear sorption isotherm described with Freundlich equation, i.e., with non-constant distribution and retardation coefficients. The second method, so-called linear approach, is applied for reference only, and is based on the assumption of a reversible sorption characterized with linear sorption isotherm, i.e., with constant distribution and retardation coefficients. Both methods model the experimental breakthrough curves with the integrated form of the simple 1-D advection–dispersion equation (ADE) expressed analytically for pulse application of radiotracer to the liquid phase before entering the columns. The integrated form of the ADE equation was modified by the so-called peak position and peak height correction coefficients the advantage of which consists among others in the elimination of the influence of starting concentration. The comparison of both approaches has shown that fitting by means of non-linear approach has given rather reliable values of the transport parameters and calculated dependences, especially in a case of 137Cs system characterized evidently with non-linear type of sorption isotherms. As for 125I, the sorption capacity of all solid samples studied is nearly on the zero level and 125I is practically not retarded, and from this point of view it behaves as non-interacting component. In addition, it was found that the modified ADE gives rather better results than the classical one.

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Abstract  

In this work, the sorption of 137Cs+ dissolved as 137CsNO3 in the solution of 10−6 M CsNO3 in SWG, and its desorption by solution mentioned, were investigated under dynamic conditions in columns with crushed granitic materials of various grain sizes, namely, of pure granite and, of corresponding filling materials. It aims at the quantification of the influence of grain size on the retardation and distribution coefficients of 137Cs, as well as on the other transport parameters (Peclet numbers and dispersion coefficients). For their determination, model based on erfc-function was used, assuming reversible equilibrium linear or non-linear sorption/desorption isotherms. By means of both model approaches, the experimental BTCS were fitted using non-linear regression procedure, in the course of which the parameters mentioned were sought. The obtained results also validated the applicability of the linear equilibrium isotherms of the 137Cs+ sorption/desorption in the studied transport processes and systems. Depending on the grain size, the retardation coefficients were between 40–93 in pure granite and 140–200 in filling materials. These values correspond to distribution coefficients of 11–34 cm3/g and 40–69 cm3/g, respectively. It was found that both retardation and distribution coefficients increase with decreasing grain size.

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Abstract  

Transport and sorption of water-soluble 85Sr2+ and 125I in the columns with beds of crushed crystalline rocks from synthetic groundwater has been studied under dynamic flow conditions. Samples of crystalline rocks: diorite-I, diorite-II, gabbro, granite and tonalite, having the grain size between 0.25 and 0.80 mm, were used. Plastic syringes of 8.8 cm length and 2.1 cm in diameter were applied as columns. The synthetic groundwater was pumped downward through the columns with a seepage velocity of about 0.2 cm/min and the given radioactive nuclide was added into the water stream individually in a form of a short pulse. In case of 85Sr, desorption from diorite-I was also studied using an artificial acid rainfall and then, the longitudinal distribution of the residual 85Sr activity along the bed was measured. Retardation, distribution and hydrodynamic dispersion coefficients were determined by the evaluation of respective breakthrough curves. A corrected integral form of a simple advection–dispersion equation was derived and used for fitting the experimental data. The K d-parameters resulting from dynamic experiments were also compared with the results of static sorption experiments.

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Abstract  

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.

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Abstract  

The amphoteric acid-base behavior of hydrous zirconium oxide (HZO) was investigated by titrating HZO with 0.05M HNO3 and NaOH at constant ionic strength. The sorption of strontium from 0.05M NaNO3 solution was measured as a function of pH. Abrupt increase in sorption was observed at the equilibrium pH of 9. The experimental titration and strontium sorption data on HZO were evaluated using the constant capacitance model (CCM) and diffuse double layer model (DLM). Various model parameters of Surface Complexation Models (SCM) were estimated, numerically, by non-linear regression. Modeling the sorption and speciation of Sr2+ on HZO indicated that the hydrolysis of Sr2+ to lower charged SrOH+ is the pre-requisite for the abrupt sorption behavior at pH 9.

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Study of kinetics, equilibrium and isotope exchange in ion exchange systems. II

Kinetics of isotope exchange in238U(VI)-233U(VI)-strongly acidic cation exchanger systems

Journal of Radioanalytical and Nuclear Chemistry
Authors: J. Cabicar, A. Gosman, J. Plicka, and K. Štamberg

Abstract  

The kinetics of isotope exchange in the238U(VI)-233U(VI)-strongly acidic cation exchanger Ostion KS system was studied in the temperature range 275–307K and for total uranium concentration 2.94·10−4–1.75·10−2 mol·l−1 in UO2(NO3)2 solution. The experimental results were evaluated by means of the “two-film mass-transfer model” and by the use of Fick's diffusion equations which have been proved more suitable for the system studied than McKay's equation. The influence of the temperature was evaluated using the Arrhenius equation. The diffusion character of the process follows also from the value of the activation energy (15.12 kJ·mol−1). In comparison with the UO 2 2+ ↔H+ ion exchange6 the isotope exchange studied is faster and less dependent on temperature (the activation energy is substantially lower).

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Abstract  

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).

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Kinetics, equilibrium and isotope exchange in ion exchange systems

III. Gel kinetics of isotope exchange in23Na-22Na-strongly acidic cation exchanger system

Journal of Radioanalytical and Nuclear Chemistry
Authors: J. Plicka, J. Cabicar, A. Gosman, and K. Štamberg

Abstract  

The kinetics of isotope exchange in the23Na-22Na-strongly acidic cation exchanger system was studied in a batch stirred reactor. Samples of exchangers OSTION KS (containing DVB in range of 1.5–12%) and AMBERLITE IR 120 were used for experimental work. In all cases the concentration of the aqueous solution was 0.2M NaNO3. Fick's equation was used for description of diffusion of ions in the particle. By evaluating the experimental data values were obtained for the self-diffusion coefficients of sodium ions in the exchangers and their dependences on the content of DVB and on the concentration of functional groups in the resin particle.

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Abstract  

The sorption of trace europium, as a trivalent actinide homologue, was studied in the system Gorleben sand - aqueous solution with the aim to elucidate its mechanism. Radiotracer method (152/154Eu) and batch experiments were used. Simultaneously, the distribution of humic substances present in, or added to the system was measured. The evaluation of the sorption was complicated by the adsorption of Eu on the walls of polyethylene vials used for the experiments, which was rather high and had to be taken into consideration. It has been found that Eu sorption on Gorleben sand increases from pH 2 to pH 5-7 and then it decreases. The decrease is due to the complexation of Eu with humic substances leached from Gorleben sand at pH >7. The position of the sorption maximum depends on the composition of the solution and on the liquid-to-solid ratio. It is shifted to lower pH values in the presence of added humic acid (HA), which enhances Eu sorption at low pH values and suppresses it at pH values higher than 5. The regions of the enhancing/suppressing effects coincide with the high/low adsorption of HA on Gorleben sand, respectively. The increasing ionic strength (from 0.01 to 0.1) and europium concentration (3.4.10-8 to 9.3.10-7 mol/l) suppress the relative sorption (expressed in %) at low pH values and enhance it at pH>6-8. Addition of carbonates (5.10-3 mol/l) supports Eu sorption at pH>7.5 so that no decrease with pH is observed till pH 9. Alkaline leaching of the sand significantly changes most of the effects found. These results were qualitatively interpreted and conclusions were drawn on the mechanism of the sorption.

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