A literature review of actinide-carbonate mineral interactions Page: 7 of 63
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1.0 INTRODUCTION
The geochemistry of actinides is a topic of growing scientific attention as researchers seek more
rigorous and predictive chemical migration models. In order to predict the overall safety of
radioactive waste disposal, it is imperative that radionuclide-mineral-water interactions be
understood within a thermodynamic and kinetic framework. In the past, much research focused on
the generation of empirical parameters such as distribution coefficients and kinetic data that rarely
have predictive value beyond the conditions of the experiment. Future studies should include
thermodynamic and kinetic experiments, as well as observations of the aqueous and solid phases,
to reveal the mechanisms controlling the mobility of radionuclides in the geosphere.
This review summarizes a significant portion of the actinide-carbonate studies that are available
in major geochemical journals. However, it must be noted that few studies have been carried out
within a rigorous thermodynamic or kinetic framework. No attempt is made here to compare the
results of the different studies. The first part of this report reviews sorption and coprecipitation
models commonly applied to mineral-water interactions. The main body summarizes the various
actinide-carbonate studies, concentrating on methodology, models, and results. Following the
actinide reviews are a few select trace element-carbonate studies that may serve as guidelines for
future research in the actinide-carbonate system. Finally, a bibliography of related actinide and
trace element-carbonate studies is provided.
The literature contains a great deal of ambiguity concerning the terms sorption, absorption,
adsorption, precipitation, coprecipitation, and solid solution. Sorption is a broad term that
encompasses three different processes: (1) absorption that involves the diffusion of an aqueous
solute into a porous solid phase; (2) adsorption that involves ions being held at a solid interface by
electrostatic forces (non-specific adsorption or physisorption) and/or the chemical attraction of
coordinatively unsatisfied ions at the surface (specific adsorption or chemisorption); and (3)
surface precipitation, which occurs as a surface phase grows by propagation of a molecular unit
that repeats itself outward into the solution. Surface precipitation may also include the formation of
an ordered solid solution. Coprecipitates or solid solutions may form when a trace ion occupies
structural sites within a mineral's lattice, or more rarely when the ion occupies non-lattice
positions. The term coprecipitation is generally used to describe precipitation of a mixed phase
from solutions supersaturated with respect to both the major and trace solid end-member
components. However, solid solution development is not limited to such conditions; solid
solutions may form by a number of processes, including solid-state diffusion of an adsorbed ion1
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Stout, D. L. & Carroll, S. A. A literature review of actinide-carbonate mineral interactions, report, October 1, 1993; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1276329/m1/7/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.