U(VI) sorption on granite: prediction and experiments

Abstract

Sorption / Predictive modelling / Surface complexation models / Uranium / GraniteSummary. One widely accepted approach -component additivity (CA) -to describe the sorption of contaminants onto complex materials such as rocks or soils is based on the assumption that the surface of a complex mineral assemblage is composed of a mixture of mineral constituents whose surface properties are known from independent studies. An internally consistent SCM (surface complexation model) database can be developed that describes the adsorption reactions of solutes to each phase. Here, the capability of such a methodology was tested, using the code MINTEQA2 including thermodynamic data of the NEA-TDB, and literature data for SCM, namely the DDL model.The sorption characteristics of U(VI) on granite (from Eibenstock, Saxony, Germany, with the main components quartz, albite, orthoclase, and muscovite) was predicted and then compared to batch experiments. Granite plays an important role in the remediation of former uranium ore mining and milling sites, but is also one of the host rocks considered for final disposal of nuclear materials. Safety assessment requires a detailed understanding of this system and its retention potential with regard to hazardous components. Namely the sorption of uranium in this complex rock is not fully understood yet. The experiments thus also provided a better understanding of the far-field behaviour in granitic geological nuclear repositories.The robustness of the prediction was tested by variation of the granite composition and the variation of the specific surface area (SSA) -first all components were predicted with a uniform granite SSA, second with a distinct SSA for each granite component (determined on pure minerals for the same grain size fractions). Changes in compositions yielded only marginal differences in the prediction. Different approaches to SSA showed somewhat larger deviations.In conclusion, the CA methodology is a valid and robust approach to U(VI) sorption onto complex substrates such as granite, provided sufficient information on component phases is available. This should help to minimize experimental efforts and to increase confidence into complex SCM computations.

Mineral–Water Interface Reactions of Actinides

“…In this approach the mineral composition of interest is determined and known relevant surface complexation parameters for the mineral components are used in a predictive way in simulations. Comparisons of the approaches to various systems exist. ,,− The option of using the expected dominant sorbent in a complex sorbent has been successfully applied. ,− This short overview shows that there are various, often equally successful, applications of surface complexation models to handle complex situations.…”

Structures of multinuclear U(vi) species on the hydroxylated α-SiO2(001) surface: insights from DFT calculations

“…Besides U(VI), multinuclear surface species have also been studied as an important topic for the distribution of other actinides in the environment. 37 Knowledge of the adsorption of U(VI) on silica is of significance for the geological disposal of nuclear waste in granite formations, 38 and the understanding of the adsorption of multinuclear U(VI) surface complexes on the a-SiO 2 (001) surface is an important step to build a comprehensive understanding of the behavior of uranyl on the silica surface. Here, we report DFT simulations to reveal the configurations of surface complexes of binuclear and trinuclear U(VI) species at different surface sites.…”

Molecular-Level Speciation of Eu(III) Adsorbed on a Migmatized Gneiss As Determined Using μTRLFS

“…These studies on model systems containing only one mineral phase are essential to understand the interaction with these minerals at a high level of detail, but they cannot accurately describe the behavior of the system as a whole. For instance, studying separate mineral phases cannot reveal how sorption of Eu 3+ on one mineral affects its sorption on the other phases or what effect surface features such as grain boundaries, crystallite orientation, and topography have on the sorption process. ,,, Here, studies of the whole system are required, ideally with spatial resolution to avoid the loss of connection between mineral phase and speciation described above. Several microscopic studies using a variety of techniques are available in the literature.…”