Effects of Calcite Impurities in Bentonite Buffer Systems on Uranium(VI) Solution Speciation and Sorption Behavior

Conference · Tue Aug 14 00:00:00 EDT 2018

OSTI ID:1595751

Tinnacher, Ruth M. ; Pistorino, Jonathan C.; Hall, Nicolas G.; Tournassat, Christophe

In many nuclear waste disposal scenarios, bentonite is the proposed buffer material surrounding nuclear waste containers. Its main mineralogical component, Na-montmorillonite clay, is expected to limit radionuclide mobility to slow diffusion processes due to the clay’s low porosity and permeability, and the high sorption affinity of clay surface complexation and cation exchange sites for many radionuclides. Given its mineral structure, montmorillonite provides two types of porosities that can be accessible to radioactive contaminants: macropores between clay particles and interlayer spaces within clay particles, which become dominant at increasing degrees of bentonite compaction. Despite the fact that bentonite naturally contains a variety of mineral impurities (e.g., quartz, feldspars and calcite), most previous studies have only focused on radionuclide interactions with pure Na-montmorillonite phases. Hence, in this study, we will investigate the influence of calcite impurities on uranium(VI) (U(VI)) solution speciation and sorption behavior in bentonite systems. In the presence of water, calcite impurities are expected to lead to milli-molar concentrations of dissolved calcium over a wide range of pH conditions. Consequently, Ca²⁺ could – at least partially – replace Na⁺ on clay cation exchange sites, with further implications for exchange reactions of cationic U(VI) species. In addition, ternary U(VI)-Ca-carbonate solution species (CaUO₂(CO₃)₃^2- and Ca₂UO₂(CO₃)₃) will largely replace U(VI)-carbonate species (UO₂(CO₃)₃^4- and UO₂(CO₃)₂^2-) at alkaline pH. These larger, but lower-charged U(VI) species, may show different sorption and diffusion characteristics, e.g. if their access to negatively-charged clay interlayer spaces is different from binary U(VI)-carbonate complexes. In this presentation, we will first demonstrate the impacts of calcite impurities on U(VI) solution speciation based on PHREEQC simulations. We will then discuss first results from U(VI) sorption studies in these systems. This research is being performed using funding received from the DOE Office of Nuclear Energy's Nuclear Energy University Program.

• Research Organization: California State University East Bay
• Sponsoring Organization: USDOE Office of Nuclear Energy (NE)
• DOE Contract Number: NE0008683
• OSTI ID: 1595751
• Resource Relation: Conference: Goldschmidt Conference, Boston, Massachusetts, USA, August 12-17 2018
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
54 ENVIRONMENTAL SCIENCES
58 GEOSCIENCES
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY