From experimental variability to the sorption related retention parameters necessary for performance assessment models for nuclear waste disposal systems: The example of Pb adsorption on clay minerals

Abstract

Surface complexation models (SCMs) have been developed in the last decades to describe metal ion sorption to clay minerals and especially to montmorillonite. In principle, these models can provide relevant information about sorption of radionuclides to be used in performance assessment (PA) of radioactive waste disposal systems. However, these SCMs have been developed in parallel with the acquisition of distinct adsorption datasets, which are not always consistent with each other. The objective of this study was to compare new experimental adsorption results with literature data to understand these discrepancies and to propose a SCM approach that could be amenable to determine sorption related retention parameters necessary for PA calculations. This study focused on lead (Pb) adsorption on montmorillonite, illite and in a natural clay (Callovo Oxfordian) as case studies of a strongly sorbing radionuclide that undergoes a range of retention processes depending on the chemical conditions. The experiments showed that many experimental artifacts lead to misinterpretations of the processes underlying the measured retention values. These include Pb precipitation in the presence of carbonate in solution. The determination of SCM parameters to provide sorption related information for PA of clay minerals should rely on preliminary building of an adequate adsorption database, where adequate means that all experimental conditions are met to quantify surface complexation only. *Revised manuscript with no changes marked Click here to view linked References

Identification of montmorillonite particle edge orientations by atomic-force microscopy

“…The Kunipia-P sample used in this study is a purified Na-montmorillonite produced by Kunimine industries Co. Ltd., and which contains nearly 100% montmorillonite (Tachi and Yotsuji, 2014;Orucoglu et al 2018). Kunipia-P montmorillonite was dispersed in deionized water at a solid concentration of 0.01 g L -1 and ultrasonicated for 10 min.…”

Nickel Retention on Callovo-Oxfordian Clay: Applicability of Existing Adsorption Models for Dilute Systems to Real Compact Rock

“…Geologic clay-rich formations, characterized by their low permeability and high metal sorption properties, are studied by several countries as a potential host of radioactive waste repositories. By using model phases and combining molecular and macroscopic approaches, the key retention processes can be identified and parameterized. − Such parameters could then be applied to predict the behavior of metal ions in more complex natural samples, while considering component additivity and simplifications. , From the standpoint of both transport modeling and performance assessment, this approach generally includes kinetically fast and reversible sorption attributed to the clay minerals; metal adsorption on the clay phase is typically represented by ion exchange and surface complexation mechanisms. , Slow kinetic retention processes, such as solid inclusion/diffusion or isomorphic substitution in clay phases and other rock constituent minerals, are usually neglected.…”

Competitive Adsorption Processes at Clay Mineral Surfaces: A Coupled Experimental and Modeling Approach

“… a X – represents a cation exchange site. b Selectivity coefficient values are from Tournasat et al c Selectivity coefficient values are from Orucoglu et al d Selectivity coefficient values are from this study. e Selectivity coefficient values are from Bradbury and Baeyens …”

“…The presence of dissolved inorganic carbon (DIC) concentration of ∼0.25 mmol·L –1 and possible precipitation of hydrocerussite were considered in all samples, in agreement with our previous study on Pb 2+ adsorption on montmorillonite . For the competition experiments with Zn 2+ , possible precipitation of hydrozincite (Zn 5 (OH) 6 (CO3) 2 ) was taken into consideration (its thermodynamic data was taken from the Thermoddem database at ).…”