Redox properties of Boom Clay and their effect on reactions with selenium and uranium

Abstract

Boom Clay (BC) is a potential host rock for geological storage of radioactive waste in the Netherlands and Belgium. The redox properties of the host rock are important in the context of safety assessment as they affect the speciation and thus the mobility of redox sensitive radionuclides. To predict the speciation and retardation of radionuclides in BC requires knowledge on the composition of BC pore water, the presence of reactive solids and it requires an understanding of interactions between BC and radionuclides. BC from two sites in the Netherlands was investigated. Porewater in BC from southwest Netherlands had a higher salinity in comparison to the well‑studied Belgian BC, which influences its redox state. In addition to quantification of the potentially redox‑active mineralogy, the samples were analyzed by mediated electrochemical analyses to quantify its electron donating (EDC) and accepting (EAC) capacities and to assess its reduction potential. A detailed study on mediated electrochemical oxidation and reduction (MEO and MER) of various different standard clay minerals, pyrite in different size fractions, and siderite gave insight in the varying redox‑activity and kinetics of these constituents that are also present in BC. Using the knowledge gained from the mineral standards, MEO on Dutch BC samples revealed that pyrite was the most important contributor to the EDC but that also around 50% of FeII in BC clay minerals was also redox‑active. The relation between these redox properties and their influence on radionuclide speciation was investigated by reacting selenium (Se) and U (uranium), as examples of redox‑active radionuclides, with BC samples. Extensive batch experiments were conducted containing Dutch BC samples, including size-separated fractions and redox‑altered samples, and selenite (SeIV) or uranyl (UVI). In experiments with unaltered BC, aqueous U and Se concentrations decreased by >95% and >99%. For Se, almost all solid-bound SeIV became reduced to Se0 in all size‑fractions. The progress of aqueous Se removal and SeIV reduction could be described by a kinetic model involving reversible adsorption on clay minerals and reduction by pyrite. The results implied that reduction of SeIV to Se0 was not significantly hindered or delayed by SeIV adsorption on clay minerals. Pyrite is likely the most relevant reductant for SeIV in BC although reduction by FeII structurally bound in clay minerals might provide an additional pathway. In experiments with UVI, reduction to UIV accounted only for 64% ‑ 85% of solid-bound U. The solid product, influencing the aqueous concentrations, was ambiguous and is discussed. Redox alterations of BC resulted in mineralogical changes which influenced the reaction kinetics, equilibrium concentrations and solid‑associated products of Se and U. Where Fe‑sequential extractions, X-ray absorption spectroscopy of Fe and X‑ray diffraction could not always detect the often small changes in BC caused by redox alterations, mediated electrochemical analyses could. Mediated electrochemical analyses are most sensitive to redox‑active constituents that have a high reactivity. Although these constituents can represent only minor fractions of the Fe pool and the mineral assemblage in general, they can exert strong influence on the redox speciation of UVI and SeIV.