Sorption of Cs, Ba, Ra, Co, Am, and Eu onto biotite: experiments and modelling
Doktorsavhandling, 2024
A solution to the problem of the highly radioactive waste that is generated by nuclear power is the construction of a final repository. In the Swedish concept for such a repository, the waste is protected by a multi-barrier system, which includes the granitic bedrock as the final barrier. In a worst-case scenario, where the initial barrier system fails, the waste can meet groundwater, and radionuclides may then start to migrate into the pore-system of the bedrock. Migration is highly dependent on the sorption of radionuclides on the minerals that constitutes granitic rock and radionuclide transport models depend on distribution coefficients, Rd (m3/kg) values for each specific radionuclide. However, the values are conditional and cannot easily be predicted if the environmental conditions change.
In this investigation, the aim is to collect data for a predictive Rd model, based on surface complexation modelling of the sorption capacity of the mineral biotite, considered to have a high sorption capacity. A first series of batch sorption experiments were performed with a mixture of 134Cs, 133Ba, 60Co and 152Eu at tracer concentrations of approximately ~10-8 M, with three different ionic strengths of NaClO4, three temperatures, and five different pH values under inert gas conditions ([O₂] <1 ppm) for a duration of up to two months. The results show that the sorption of all four radionuclides was highly dependent on duration of experiment, pH, ionic strength, and temperature. A second series of batch sorption experiments was conducted using 226Ra and 241Am. Strong effects of the ionic strength and pH on the sorption of these two radionuclides were found. The titration experiment was performed with biotite suspensions to determine the pKa values of the biotite mineral. Both sorption and titration data were modelled with a custom-made program package consisting of the PHREEQC geochemical modelling software and PYTHON shell with an error minimization routine. The sorption data for all metals was successfully modelled by considering one (2-pKa) surface complexation site, presumably edge sites on the mineral, in combination with one ion-exchange site presumably on mineral basal plane. The optimized stability constants of Cs, Ba, Co, and Eu were used to evaluate the enthalpy (∆H) and entropy (∆S) of the sorption reactions from van’t Hoff linear plots. For all the surface complexation species, the favorable entropy term was predominant over the unfavorable enthalpy term. On the other hand, for ion-exchange species, enthalpy was found to be favorable and predominant over an unfavorable entropy term.
In this investigation, the aim is to collect data for a predictive Rd model, based on surface complexation modelling of the sorption capacity of the mineral biotite, considered to have a high sorption capacity. A first series of batch sorption experiments were performed with a mixture of 134Cs, 133Ba, 60Co and 152Eu at tracer concentrations of approximately ~10-8 M, with three different ionic strengths of NaClO4, three temperatures, and five different pH values under inert gas conditions ([O₂] <1 ppm) for a duration of up to two months. The results show that the sorption of all four radionuclides was highly dependent on duration of experiment, pH, ionic strength, and temperature. A second series of batch sorption experiments was conducted using 226Ra and 241Am. Strong effects of the ionic strength and pH on the sorption of these two radionuclides were found. The titration experiment was performed with biotite suspensions to determine the pKa values of the biotite mineral. Both sorption and titration data were modelled with a custom-made program package consisting of the PHREEQC geochemical modelling software and PYTHON shell with an error minimization routine. The sorption data for all metals was successfully modelled by considering one (2-pKa) surface complexation site, presumably edge sites on the mineral, in combination with one ion-exchange site presumably on mineral basal plane. The optimized stability constants of Cs, Ba, Co, and Eu were used to evaluate the enthalpy (∆H) and entropy (∆S) of the sorption reactions from van’t Hoff linear plots. For all the surface complexation species, the favorable entropy term was predominant over the unfavorable enthalpy term. On the other hand, for ion-exchange species, enthalpy was found to be favorable and predominant over an unfavorable entropy term.
radionuclide sorption
biotite
and entropy
surface complexation modelling
enthalpy
elevated temperature
Författare
Pawan Kumar
Chalmers, Kemi och kemiteknik, Energi och material
P, Kumar, S. Holgersson, and C. Ekberg, “Cs, Ba, Co, and Eu sorption on biotite at pH 5-9 and varying ionic strength.”
P, Kumar, S. Holgersson, and C. Ekberg, "Effects of pH, ionic strength, and temperature on cesium, barium, cobalt, and europium sorption onto biotite: combined experimental and modelling study."
P, Kumar, S. Holgersson, and C. Ekberg, “Ra and Am sorption on biotite at pH 5-9 and at varying ionic strength.”
Nuclear energy is considered as CO2-neutral, but its primary challenge lies in managing radioactive waste. Even with advanced technologies like the Generation IV reactor cycle, a portion of the waste remains highly radiotoxic and requires secure isolation from human contact and the environment for up to 100,000 years. To address this, the Swedish Nuclear Fuel and Waste Management Company (SKB) developed the KBS-3 method for spent nuclear fuel storage. This concept comprises of an engineered multi-barrier system placed 400-500 m deep in a seismic low activity granitic rock formation characterized by minerals such as plagioclase and K-feldspar, with trace amounts of mica minerals like biotite and chlorite.
In a worst-case scenario where all engineered barriers fail, radioactive waste could come into contact with groundwater, potentially releasing radionuclides into the surrounding host rock. Radionuclides would then migrate with groundwater towards the biosphere, interacting with geological materials, mainly through sorption onto specific minerals. Sorption delays radionuclide migration, and the extent of this delay is quantified using sorption coefficients, known as Rd values. Rd values are critical for predicting potential radioactive releases. However, these values are only applicable under specific chemical conditions. Changes in groundwater chemistry such as pH, ionic strength or temperature due to freshwater influx during glaciations or saline water intrusion from rising sea levels can significantly alter these coefficients. To address this limitation, modeling Rd values under diverse repository conditions is essential, which requires the development of a robust thermodynamic sorption model (TSM) based on experimental data.
In this work batch-sorption experiments were conducted with biotite and radionuclides of the elements Cs, Ba, Ra, Co, Eu, and Am under a wide range of conditions: three background electrolyte concentrations (0.001, 0.01, and 0.1 M NaClO4), five pH levels (5-9), and three temperatures (25, 40 and 60°C). The measured Rd values indicate that changes in all these conditions significantly influence the sorption, where each element shows a distinct behavior.
The sorption data was successfully reproduced using a non-electrostatic TSM incorporating one amphoteric 2-pKa surface complexation site and one ion-exchange site on the surface of biotite. This model was implemented through PHREEQC chemical speciation calculations coupled with PYTHON programming optimization routine. The results were a set of specific surface reactions with associated reactions constants for each radionuclide. Analyses of temperature dependency also show that surface complexation and ion-exchange reactions respond differently to changes in temperature.
In a worst-case scenario where all engineered barriers fail, radioactive waste could come into contact with groundwater, potentially releasing radionuclides into the surrounding host rock. Radionuclides would then migrate with groundwater towards the biosphere, interacting with geological materials, mainly through sorption onto specific minerals. Sorption delays radionuclide migration, and the extent of this delay is quantified using sorption coefficients, known as Rd values. Rd values are critical for predicting potential radioactive releases. However, these values are only applicable under specific chemical conditions. Changes in groundwater chemistry such as pH, ionic strength or temperature due to freshwater influx during glaciations or saline water intrusion from rising sea levels can significantly alter these coefficients. To address this limitation, modeling Rd values under diverse repository conditions is essential, which requires the development of a robust thermodynamic sorption model (TSM) based on experimental data.
In this work batch-sorption experiments were conducted with biotite and radionuclides of the elements Cs, Ba, Ra, Co, Eu, and Am under a wide range of conditions: three background electrolyte concentrations (0.001, 0.01, and 0.1 M NaClO4), five pH levels (5-9), and three temperatures (25, 40 and 60°C). The measured Rd values indicate that changes in all these conditions significantly influence the sorption, where each element shows a distinct behavior.
The sorption data was successfully reproduced using a non-electrostatic TSM incorporating one amphoteric 2-pKa surface complexation site and one ion-exchange site on the surface of biotite. This model was implemented through PHREEQC chemical speciation calculations coupled with PYTHON programming optimization routine. The results were a set of specific surface reactions with associated reactions constants for each radionuclide. Analyses of temperature dependency also show that surface complexation and ion-exchange reactions respond differently to changes in temperature.
Ämneskategorier (SSIF 2011)
Geokemi
ISBN
978-91-8103-144-7
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5602
Utgivare
Chalmers