Environ. Sci. Technol. 2008, 42, 5985–5991
Proton and Trivalent Metal Cation Binding by Dissolved Organic Matter in the Opalinus Clay and the Callovo-Oxfordian Formation AMANDINE COURDOUAN,† I S O C H R I S T L , * ,† T H O M A S R A B U N G , ‡ PAUL WERSIN,§ AND RUBEN KRETZSCHMAR† Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Sciences, ETH Zurich, CHN, Zurich CH-8092, Switzerland, Institut fu ¨r Nukleare Entsorgung (INE), Forschungszentrum Karlsruhe, Karlsruhe D-76021, Germany, and National Cooperative for the Disposal of Radioactive Waste (Nagra), Wettingen CH-5430, Switzerland
Received March 13, 2008. Revised manuscript received June 10, 2008. Accepted June 11, 2008.
We investigated the proton and trivalent metal binding of dissolved organic matter (DOM) in in situ pore water and anoxic rock extracts of two potential host rocks for the disposal of radioactive waste, i.e., the Opalinus Clay (OPA) and the CallovoOxfordian formation (COx). The proton, curium, and europium binding properties of the OPA pore water and the extracted DOM of both rocks were studied with acid-base titrations, timeresolved laser fluorescence spectroscopy (TRLFS), and voltammetry, respectively. Protons were mostly buffered by inorganic compounds. DOM contributed to the total proton buffering capacity of the samples only to a small extent. Significant complexation of curium by DOM was observed for OPA pore water by TRFLS in contrast to little complexation by DOM detected in the OPA and COx extracts. The data on europium binding in OPA pore water were described by the presence of 14.3 µM organic ligands exhibiting a conditional affinity constant of log β ) 6.50. Calculations of europium speciation under in situ conditions indicated that carbonates largely controlled the speciation of europium in OPA and COx. In the OPA formation, the presence of DOM may enhance the solubility of europium by 5 × 10-8 M, representing about one-third of total dissolvable europium.
Introduction The Opalinus Clay (OPA) and the Callovo-Oxfordian formation (COx) are currently being evaluated for the storage of high-level radioactive waste in Switzerland and France, respectively (1, 2). As part of the safety assessment, the potential radionuclide transport mechanisms and the retention properties of these rocks have to be investigated. Due to the very low hydraulic conductivities determined for OPA and COx, diffusion dominates the transport of radionuclides * Corresponding author tel: +41 44 633 60 01; fax: +41 44 633 11 18; e-mail:
[email protected]. † ETH Zurich. ‡ Forschungszentrum Karlsruhe. § National Cooperative for the Disposal of Radioactive Waste (Nagra). 10.1021/es8007358 CCC: $40.75
Published on Web 07/11/2008
2008 American Chemical Society
within these formations (1, 2). The diffusion of various ions has been studied for both rocks (3–11). To evaluate the mobility of radionuclides, the retention properties of the clay rocks also have to be considered. Based on transport and sorption data, the migration of selected radionuclides was modeled, considering all major inorganic components of the rocks and the pore waters (12). However, both OPA and COx rocks also contain up to 1% w/w of organic matter. The effect of the organic matter on the transport of radionuclides is still unclear. Most of the organic matter is attached to the mineral phases in both rocks. Only a small fraction is present as dissolved organic matter (DOM) in the pore waters (13, 14). In both formations, the DOM is mainly composed of small compounds (
