Proton and Trivalent Metal Cation Binding by Dissolved Organic Matter in the Opalinus Clay and the Callovo-Oxfordian Formation

Abstract

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 Callovo-Oxfordian 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, time-resolved 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 microM organic ligands exhibiting a conditional affinity constant of log beta = 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 x 10(-8) M, representing about one-third of total dissolvable europium.