Study of Ni(II) sorption on chlorite - a fracture filling mineral in granites
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Study of Ni(II) sorption on chlorite - a fracture filling mineral in granites Å. Gustafsson1,*, M. Molera2 and I. Puigdomenech3 1
Royal Institute of Technology, Inorganic Chemistry, Stockholm, Sweden 2 Royal Institute of Technology, Nuclear Chemistry, Stockholm, Sweden 3 Swedish Nuclear & Fuel Management Co., Stockholm, Sweden
ABSTRACT Chlorite is an Fe(II)-containing phyllosilicate which is often present as a fracture filling mineral in e.g. granitic rocks. It may therefore be significant in influencing redox conditions and sorption processes in granitic groundwaters. The sorption properties of chlorite may therefore be important when modelling the migration of radionuclides under reducing conditions around nuclear waste repositories or in sites contaminated by mining waste. The sorption behaviour of Ni(II) onto a natural chlorite (Karlsborg, Sweden) was investigated using a batch technique. The effects of three different background electrolyte concentrations (0.01 M, 0.1 M and 0.5 M NaClO4), different pH values (ranging from 4 to 11) and different Ni(II) concentrations (10−6 and 10−8 M) were studied under anoxic conditions in a glove-box. Ni(II) solutions were spiked with 63Ni and β-Liquid scintillation counting (LSC) was used to determine the concentration of nickel in the bulk solution, allowing the calculation of solid-water distribution coefficients for the metal ion. The results of the sorption experiments show strong pH dependence at pH > 5, but the sorption is independent of ionic strength. The maximum adsorption is found in the pH range between 7 and 11 with Kd values ≈103 cm3/g. A diffuse double layer model has been used to describe the experimental results. INTRODUCTION The accumulation of ions at mineral surfaces is considered as one of the most important mechanisms for controlling the heavy metal contents in natural waters [1]. The retention of ions on surfaces occurs through different mechanisms, such as surface complexation, surface precipitation and cation exchange [2,3]. Laboratory studies of metal ions at the water-mineral interface are necessary in order to understand the complexity of natural systems [4]. Heavy metal ions originate partly from weathered minerals but mainly from industrial wastes and agriculture. In the area of nuclear power industry a considerable amount of research has been performed regarding the management of the spent nuclear fuel, which contains large quantities of actinides and heavy metals as fission products. In Sweden plans are being made to deposit spent nuclear fuel in a repository located in granitic bedrock ~500 m below ground. The repository concept, KBS-3, consists of several different barriers [5]. Two of the barriers are the backfill and the surrounding bedrock. Chlorite is an accessory mineral that will most probably be present in the backfill and be one of the three dominant iron-containing fracture filling minerals [6], together with pyrite and biotite. Radionuclides eventually released from the fuel matrix in a damaged canister will migrate throug
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