The diffusion coefficients (D) of water and solutes in nanoporousNa-smectite clay barriers have been widely studied because of their importancein high-level radioactive waste (HLRW) management and ...

Molecular Dynamics Simulations of Water and Sodium Diffusion in Smectite Interlayer Nanopores as a Function of Pore Size and Temperature

Ion adsorption and diffusion in smectite: Molecular, pore, and continuum scale views

Diffusion and sorption of Cs+, Na+, I− and HTO in compacted sodium montmorillonite as a function of porewater salinity: Integrated sorption and diffusion model

Diffusion and sorption of Cs + , I - and HTO in samples of the argillaceous Wakkanai Formation from the Horonobe URL, Japan: Clay-based modeling approach

Thermodynamic sorption models (TSMs) offer the potential to improve the incorporation of sorption in environmental modelling of contaminant migration. One specific application is safety cases for radioactive waste repositories, in which radionuclide sorption on mineral surfaces is usually described using distribution coefficients (K"d values). TSMs can be utilised to provide a scientific basis for the range of K"d values included in the repository safety case, and for assessing the response of K"d to changes in chemical conditions. The development of a TSM involves a series of decisions on model features such as numbers and types of surface sites, sorption reactions and electrostatic correction factors. There has been a lack of consensus on the best ways to develop such models, and on the methods of determination of associated parameter values. The present paper therefore presents recommendations on a number of aspects of model development, which are applicable both to radioactive waste disposal and broader environmental applications. The TSM should be calibrated using a comprehensive sorption data set for the contaminant of interest, showing the impact of major geochemical parameters including pH, ionic strength, contaminant concentration, the effect of ligands, and major competing ions. Complex natural materials should be thoroughly characterised in terms of mineralogy, surface area, cation exchange capacity, and presence of impurities. During the application of numerical optimisation programs to simulate sorption data, it is often preferable that the TSM should be fitted to the experimentally determined K"d parameter, rather than to the frequently used percentage sorbed. Two different modelling approaches, the component additivity and generalised composite, can be used for modelling sorption data for complex materials such as soils. Both approaches may be coupled to the same critically reviewed aqueous thermodynamic data sets, and may incorporate the same, or similar, surface reactions and surface species. The quality of the final sorption model can be assessed against the following characteristics: an appropriate level of complexity, documented and traceable decisions, internal consistency, limitations on the number of adjustable parameter values, an adequate fit to a comprehensive calibration data set, and capability of simulating independent data sets. Key recommendations for the process of TSM development include: definition of modelling objectives, identification of major decision points, a clear decision-making rationale with reference to experimental or theoretical evidence, utilisation of a suitable consultative and iterative model development process, testing to the maximum practicable extent, and thorough documentation of key decisions. These recommendations are consistent with international benchmarks for environmental modelling.

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Guidelines for thermodynamic sorption modelling in the context of radioactive waste disposal

Diffusion and sorption of radionuclides in compacted bentonite/montmorillonite are key processes in the safe geological disposal of radioactive waste. In this study, the effects of carbonate and salinity on neptunium(V) diffusion and sorption in compacted sodium montmorillonite were investigated by experimental and modeling approaches. Effective diffusion coefficients (D e ) and distribution coefficients (K d ) of 237 Np(V) in sodium montmorillonite compacted to a dry density of 800 kg m -3 were measured under four chemical conditions with different salinities (0.05/0.5 M NaCl) and carbonate concentrations (0/0.01 M NaHCO 3 ). D e values for carbonate-free conditions were of the order of 10 -10 -10 -11 m 2 s -1 and decreased as salinity increased, and those for carbonate conditions were of the order of 10 -11 -10 -12 m 2 s -1 and showed the opposite dependence. Diffusion-derived K d values for carbonate-free conditions were higher by one order of magnitude than those for carbonate conditions. Diffusion and sorption behaviors were interpreted based on mechanistic models by coupling thermodynamic aqueous speciation, thermodynamic sorption model (TSM) based on ion exchange, and surface complexation reactions, and a diffusion model based on electrical double layer (EDL) theory in homogeneous narrow pores. The model predicted the experimentally observed tendency of D e and K d qualitatively, as a result of the following mechanisms; 1) the dominant aqueous species are NpO 2 + and NpO 2 CO 3 - for carbonate-free and carbonate conditions, respectively, 2) the effects of cation excess and anion exclusion result in opposite tendencies of D e for salinity, 3) higher carbonate in solution inhibits sorption due to the formation of carbonate complexes.

Diffusion and sorption of neptunium(V) in compacted montmorillonite: effects of carbonate and salinity

It is important to understand the coupled processes of sorption and diffusion of radionuclides (RNs) in compacted bentonite, and to develop mechanistic models that can aid in the prediction of the long-term performance of geological disposal systems of radioactive waste. The integrated sorption and diffusion (ISD) model was developed based on the consistent combination of clay–water interaction, sorption and diffusion models. The diffusion model based on the electrical double layer theory describing relative ionic concentrations and viscoelectric effects at the negatively charged clay surface was coupled with porewater chemistry and sorption models. This ISD model was successfully tested for various actinides with a complex chemistry (Np(V), Am(III), U(VI) under conditions where variably charged carbonate complexes are formed) considered in Part 1, by using published diffusion and sorption data (Da, De, Kd) as a function of partial montmorillonite density. Quantitative agreements were observed by consider...

Integrated sorption and diffusion model for bentonite. Part 2: porewater chemistry, sorption and diffusion modeling in compacted systems

In high-level radioactive waste disposal, the alteration of montmorillonite due to the corrosion of carbon steel possibly affects the swelling and self-healing capacity of compacted bentonite used as a buffer material. The nature of the corrosion products in compacted bentonite is also important to evaluate not only the diffusion and sorption behavior of radionuclides but also the chemical composition and redox potential of pore water. In this study, the alteration of montmorillonite in compacted bentonite due to the interaction with carbon steel was analyzed by X-ray diffraction (XRD). The possibility of montmorillonite alteration was also investigated from the cation exchange capacity (CEC) of compacted bentonite and scanning electron microscopy (SEM) observation. The corrosion products distributed in the compacted bentonite were investigated by selective dissolution analysis, which can estimate the crystallinity of Fe-bearing compounds. The valence of Fe in the corrosion products was spectrophotometrically determined. From the XRD analysis, newly formed phyllosilicates resulting from the alteration of montmorillonite could not be identified in compacted bentonite. CEC of compacted bentonite adjacent to the carbon steel, in which high concentration of Fe was extracted, was hardly decreased. No significant differences of clay particles were observed with SEM. Thus, the alteration of montmorillonite was scarcely detected in compacted bentonite in contact with carbon steel for ten years. The selective dissolution and valence analyses suggest that most of the corrosion products of carbon steel existed in Kunipia F, which consists of over 95 wt% montmorillonite, was amorphous, non-crystalline or poorly ordered Fe(OH) 2 . This means that Fe(OH) 2 distributed into compacted bentonite was scarcely crystallized within ten years at 80 o C. From the XRD analysis, small amount of green rust one containing Cl − at the interlayers (GR1(Cl − )) and lepidocrocite were also identified in Kunipia F. Therefore, under this experimental condition, Fe(OH) 2 formed in Kunipia F due to the corrosion of carbon steel was oxidized to GR1(Cl − ) as intermediates, and then GR1(Cl − ) was possibly oxidized to lepidocrocite. On the other hand, GR1(Cl − ) was hardly detected in Kunigel V1, which contains 46–49 wt% montmorillonite, from the XRD analysis.

Investigation of montmorillonite alteration and form of iron corrosion products in compacted bentonite in contact with carbon steel for ten years

To predict the long-term migration of radionuclides (RNs) under variable conditions within the framework of safety analyses for geological disposal, thermodynamic sorption models are very powerful tools. The integrated sorption and diffusion (ISD) model for compacted bentonite was developed to achieve a consistent combination of clay–water interaction, sorption, and diffusion models. The basic premise considered in the ISD model was to consistently use the same simple surface model design and parameters for describing RNs sorption/diffusion as well as clay surface and porewater chemistry. A simple 1-site non-electrostatic surface complexation model in combination with a 1-site ion exchange model was selected to keep sorption model characteristics relatively robust for compacted systems. Fundamental parameters for the proposed model were evaluated from surface titration data for purified montmorillonite. The resulting basic model was then parameterized on the basis of selected published sorption data-sets ...

Integrated sorption and diffusion model for bentonite. Part 1: clay–water interaction and sorption modeling in dispersed systems

Japan Nuclear Cycle Development Institute (JNC) had developed the sorption database (JNC-SDB) which includes distribution coefficient (K{sub d}) data of important radioactive elements for bentonite and rocks in order to define a dataset to evaluate the safety function of retardation by natural barrier and engineered barrier in the H12 report. Then, JNC added to the database the sorption data from 1998 to 2003 collected by literature survey. In this report, Japan Atomic Energy Agency (JAEA) has updated the sorption database: (1) JAEA has widely collected the sorption data in order to extend the sorption database. The JNC-SDB has been added the published data which are not registered in the sorption database so far. (2) For the convenience of users the JNC-SDB was partially improved such as the automatic graph function. (3) Moreover, errors during data input in the part of the JNC-SDB were corrected on the basis of reviewing data in the database according to the guideline; 'evaluating and categorizing the reliability of distribution coefficient values in the sorption database'. In this updated JNC-SDB, 3,205 sorption data for 23 elements, which are important for performance assessment were included. The frequency of K{sub d} for some elements was clearly shown by addition of the sorption data. (author)

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Tadahiro Suyama

Papers

Diffusion and sorption of neptunium(V) in compacted montmorillonite: effects of carbonate and salinity

Integrated sorption and diffusion model for bentonite. Part 2: porewater chemistry, sorption and diffusion modeling in compacted systems

Investigation of montmorillonite alteration and form of iron corrosion products in compacted bentonite in contact with carbon steel for ten years

Integrated sorption and diffusion model for bentonite. Part 1: clay–water interaction and sorption modeling in dispersed systems

An Update of the Sorption Database: Correction and Addition of Published Literature Data