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

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Quantification of adsorption processes on clay mineral surfaces is often necessary to predict the extent and the evolution of contaminants’ migration in surficial and underground environments. Many studies have been dedicated to retention measurement as a function of pH and ionic strength in relation with the two main identified adsorption processes for clay minerals, that is, cation exchange on their basal surfaces and surface complexation on their edge surfaces. The latest process has been repeatedly assessed as an effective retention mechanism at circumneutral pH conditions, which often prevail in natural environments. This assessment must however be tempered by the lack of information about competitive processes that can take place with the numerous chemical species present in natural settings, compared to simplified systems investigated in laboratory experiments. In this study, we quantified experimentally the competition between Pb2+, Co2+, Zn2+, and Mg2+ for specific adsorption on montmorillonite edge surfaces. Zn2+ was an effective competitor with Pb2+ and Co2+, and our results showed also unambiguously the influence of Mg2+ concentration levels on the specific adsorption of Pb2+ and Co2+. Because of the high ionic strength used in the experiments, cation exchange with Mg2+ was dismissed as a possible reason for such competition process, leaving specific competitive adsorption on edge surfaces as a unique explanation for our observations. Modeling of Pb2+ adsorption data with a state-of-art electrostatic complexation model for montmorillonite edge surfaces, supported by state-of-the-art, made it possible to distinguish two types of possible competition driving forces: Zn2+ competition for adsorption site occupancy but also detrimental changes in surface electrostatic potential following Mg2+ adsorption on sites neighboring those of Pb2+ adsorption. Mg2+ competition observed in our experiment should apply in most of clayey environments. Consequently, adsorption data obtained on pure clay mineral phases, and the associated models that have been built based on these data without considering the geochemical background in competitive species, may overestimate the retention properties of clay minerals when applied to natural settings.