Environ. Sci. Technol. 2004, 38, 2044-2051
Evidence for the Interaction of Technetium Colloids with Humic Substances by X-ray Absorption Spectroscopy A . M A E S , * ,† K . G E R A E D T S , † C. BRUGGEMAN,† J. VANCLUYSEN,† A. ROSSBERG,‡ AND C. HENNIG‡ Departement Interfasechemie, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium, and Instut fu ¨ r Radiochemie, Forschungszentrum Rossendorf EV, Rossendorf Inc., Postfach 510119, D-01314 Dresden, Germany
Spectroscopic extended X-ray absorption fine structure (EXAFS) evidence was obtained on the chemical environment of 99Tc(IV) atoms formed upon introduction of TcO4- into four types of laboratory-scale synthetic and natural systems which mimic in situ natural reducing conditions in humicrich geochemical environments: (a) magnetite/pyrite in synthetic groundwater in the absence of humic substances (HSs), (b) magnetite/pyrite in natural Gorleben groundwater in the presence of HSs, (c) Boom clay sediment mixed with synthetic groundwater, and (d) Gorleben sand mixed with natural Gorleben groundwater. The investigated systems obey to pH 8-9 conditions, and all measured samples show similar EXAFS spectra for Tc, which could be fitted by a hydrated TcO2‚xH2O phase. The results are interpreted as follows: upon introduction of high concentrations (millimolar to micromolar) of TcO4to chemically reducing environments, small Tc(IV) oxidic polymers are formed, which either may aggregate into larger units (colloids) and finally precipitate or may interact in their polymeric form with (dissolved and immobile) humic substances. This latter type of interactionsTc(IV) colloid sorption onto HSssdiffers significantly from the generally accepted metal-humate complexation and therefore offers new views on the possible reaction pathways of metals and radionuclides in humic-rich environments.
1. Introduction The redox-sensitive fission product technetium-99 is of great interest in nuclear waste disposal studies because of its potential of contaminating the geosphere due to its very long half-life and high mobility. The purpose of this study is to investigate the nature of Tc species formed upon addition of pertechnetate into (geochemically) reducing conditions in laboratory-scale (synthetic and natural) systems, using X-ray absorption spectroscopy (XAS). These systems are relevant for the geological storage of high-level radioactive waste. Under suitable reducing conditions, e.g., in the presence of a reducing solid phase which can act as an electron donor, pertechnetate (TcO4-) can be reduced to lower oxidation * Corresponding author phone: +3216321598; fax: +3216321998; e-mail:
[email protected]. † Katholieke Universiteit Leuven. ‡ Rossendorf Inc. 2044
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 7, 2004
states, to produce a surface precipitate, TcO2‚xH2O, with a low solubility (1, 2) at neutral to basic conditions. However, when the reduction is performed in the presence of dissolved humic substances, the solubility may be enhanced due to the formation of Tc-HS (humic substance) complexes (3, 4). Using X-ray absorption near-edge spectroscopy (XANES) of batch samples with micromolar Tc content, it was unequivocally demonstrated that Tc(IV) species were formed and were associated with mobile (Gorleben) humic substances (4). The nature of Tc(IV) species produced from the reduction of pertechnetate using various artificial reducing methods (e.g., thermal decomposition of NH4TcO4, electrochemical reduction, ...) is quite diverse and was previously studied by extended X-ray absorption fine structre (EXAFS) analysis (5, 6). Crystalline, hydrated, and polymeric TcO2 species and precipitates have already been identified. However, the detailed chemical mechanisms governing the reduction of Tc(VII) and the formation of Tc(IV) products upon introduction of pertechnetate into geochemically relevant reducing conditions in the presence of humic substances are unknown. The geochemically reducing conditions in our experiments will be mimicked by laboratory-scale “synthetic reducing systems” (containing magnetite and pyrite suspensions in the absence and presence of Gorleben humic subtances) and by laboratory-scale “natural reducing systems” (containing Gorleben sand mixed with Gorleben water and Boom clay mixed with synthetic Boom clay water). All systems provide a reducing atmosphere and contain a high amount of both dissolved and immobile humic substances.
2. Experimental Setup All experiments prior to the XAS measurements were carried out at ambient temperature (22 °C) in two controlledatmosphere gloveboxes flushed with either a mixture of N2 (95%) and H2 (5%) (glovebox 1) or a mixture of N2 (99.6%) and CO2 (0.4%) (glovebox 2). The box atmospheres were circulated over a catalytic converter (Pt) to remove excessive oxygen. The mean concentration of O2 in the gloveboxes throughout the experiments was below 2 ppm. Gorleben groundwater (indicated as GoHy-2227) (GW) containing 165 mg/L organic matter and Gorleben sand (GS) from the potential disposal site at Gorleben (Germany) were used as supplied by Forschungszentrum Karlsruhe (Germany) (7). The water was stored in a closed PP bottle inside glovebox 2 and filtered over a 0.22 µm PVDF filter before use. Synthetic Gorleben groundwater (SGW) was prepared as a mixture of 3.2 × 10-2 M NaCl and 8 × 10-3 M NaHCO3, and was taken as representative for the inorganic composition of the natural Gorleben groundwater (7). The Boom clay sample used originated form the Andra Gallery in the underground research facility (URF) at SCK/CEN (Mol, Belgium). The sample was taken between ring 4 and ring 5 from a vertically downward coring site at 10.03-10.29 m (core 4.6) on May 30, 1996. The sample was packed in PE/AL/PET foil (UCB) to avoid oxidation and was further kept in a stainless steal container at room termperature in glovebox 2. By suspending Boom clay sediment in synthetic Boom clay water (SBCW), part of the Boom clay humic substances are dissolved. The composition of SBCW is given elsewhere (8), and consists mainly of NaHCO3 (10-2 M). Magnetite (Fe3O4) and pyrite (FeS2) were purchased from a local mineral shop. Small-sized fractions (
