Europium sorption experiments with muscovite, orthoclase, and quartz: Modeling of surface complexation and reactive transport

Abstract

For long-term safety analysis of a potential radioactive waste disposal site it is, amongst others, a prerequisite to characterize transport and retardation processes of relevant radionuclides to adequately describe hypothetical release scenarios and to assess the barrier capacity of the contaminant providing rock zone, the geological formation, and the overburden. To simulate migration processes of radionuclides a sound understanding of solid-solution interface reactions is necessary to determine the influence of the geochemical environment on sorption, precipitation, speciation, and dissolution processes. For long-term safety assessments of nuclear waste disposal sites, the behavior of activation and fission products as well as radionuclides from decay chains are of major interest. The trivalent lanthanide europium(III) is a chemical homologue for trivalent actinides such as curium(III) and americium(III). In the field of safety assessments there is still a need for sound data concerning the interaction of minerals with the surrounding solution even for well-known surfaces such as quartz, muscovite, and orthoclase. Up to now only some studies have taken the approach to study the interaction and interrelation of surface charge, surface complexation, and transport processes for trivalent lanthanides and actinides for orthoclase, muscovite, and quartz; and so far only few studies tried to describe all processes under varying geochemical conditions with one set of mineral-specific parameters as it had been done in this study. A vast amount of experiments was carried out and evaluated with mechanistic thermodynamic sorption models; surface complexation parameters of Eu were derived and subsequently used to simulate Eu reactive transport under varying experimental boundary conditions with reactive transport models. It was shown that the approach to simulate different geochemical conditions with one surface complexation parameter set yielded adequate predictions of Eu transport under laboratory and close to nature conditions for quartz systems; for orthoclase Eu transport under the influence of complexing ligands was also satisfyingly represented. This study contributed to fill the gap of sorption and transport data of Eu and, thus, trivalent actinides for ubiquitously present minerals. Expertise in the development of reactive transport models was gained and results offered insight into the transport of trivalent lanthanides and actinides.