Environ. Sci. Technol. 2008, 42, 7601–7606
A Spectroscopic Characterization and Quantification of M(III)/Clay Mineral Outer-Sphere Complexes EVA HARTMANN,† BART BAEYENS,§ MICHAEL H. BRADBURY,§ HORST GECKEIS,† AND T H O R S T E N S T U M P F * ,†,‡ Forschungszentrum Karlsruhe, Institut fu ¨ r Nukleare Entsorgung, P.O. Box 3640, D-76021 Karlsruhe, Germany, Ruprecht-Karls-Universita¨t Heidelberg, Physikalisch-Chemisches Institut, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany D-69120, and Paul Scherrer Institut, Laboratory for Waste Management, CH-5232 Villigen PSI, Switzerland
Received April 23, 2008. Revised manuscript received June 30, 2008. Accepted June 30, 2008.
For the long-term safety assessment of deep radioactive waste repositories an understanding of the interactions of actinides with mineral surfaces at a molecular level is necessary. The retention/mobility of the released radionuclides is strongly dependent on sorption/desorption reactions at mineral surfaces. Thus, a quantitative understanding of the uptake mechanisms of actinides on clay minerals will make an important contribution to long-term safety assessments. Using timeresolved laser fluorescence spectroscopy (TRLFS), it was possible to differentiate between nonsorbed aquo ions and outersphere sorbed Cm(III) onto different montmorillonites. In addition, Cm(III)/clay outer-sphere complexation at different ionic strengths using NaCl as the background electrolyte is quantified. Finally, the results are verified by sorption model calculations.
Introduction The behavior of radionuclides in the environment is determined by interface reactions such as adsorption, ion exchange and incorporation processes. In the literature such processes are often described by operational solid-liquid distribution ratios (Rd values). Distribution ratios are defined as the ratio of the quantity of a radionuclide sorbed per unit mass and the equilibrium concentration of the radionuclide. They are macroscopic parameters which are strictly valid only for the mineral and solute combination in the experimental system. For reliable and trustworthy long-term predictions of radionuclide transport behavior interaction mechanisms and processes occurring at the solid-water interface need to be understood at the molecular level. This can only be achieved by the application of spectroscopic methods. Because of the anticipated reducing conditions in radioactive waste repositories in deep geological formations, heavy metal ions are present in low oxidation states. Americium and curium, which contribute significantly to the radio toxicity of high level waste, occur in the trivalent state. * Corresponding author phone: 0049 (0)7247-82 6023; fax: 0049 (0)7247-82 3927; e-mail:
[email protected]. † Forschungszentrum Karlruhe. § Paul Scherrer Institut. ‡ Ruprecht-Karls-University Heidelberg. 10.1021/es801092f CCC: $40.75
Published on Web 09/18/2008
2008 American Chemical Society
Depending on pH even plutonium is expected to exist in the trivalent state in such a strongly reducing environment. Cm(III) has been chosen to represent trivalent actinides in nuclear wastes because its fluorescence spectroscopic sensitivity allows sorbed species to be studied at the molecular level for nanomolar concentrations. Time-resolved laser fluorescence spectroscopy (TRLFS) has proven to be a versatile tool for Cm(III) speciation studies (e.g., refs 1-5) and for sorption studies on various solids (6-15). TRLFS is capable of identifying different species as well as determining their hydration state (16, 17), thus allowing, for example, inner-sphere surface complexes and species incorporated in the crystal lattice of solids to be distinguished from each other. Due to their good sorption characteristics, clay formations are being investigated as potential host rocks for repositories for radioactive waste in several countries such as France, Switzerland, and Belgium. Also, compacted bentonite (70-90 wt.% smectite) is the most favored backfilling material. The reason for this choice is that swelling clay minerals such as smectites have high sorption capacities. Moreover, clay minerals are omnipresent solid phases in the ecosphere. In the case of cation exchange reactions taking place at low pH (
