Reactive IMMOBILIZATION OF 99Tc(VII) BY DIFFERENT CRYSTALLINE PHASES IF IRON SULFIDE (FeS2)
Rodriguez, D. M.1), Mayordomo, N.1),Brendler, V.1), Müller, K.1),Schild, D.2), Stumpf, T.1)
1) Institute of Resource Ecology, Helmholtz-Zentrum Dresden - Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany 2) Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen 76344 Germany
99Tc is a fission product with a long half-life of 2.14 × 105 years. Its migration behaviour and bioavailability strongly depend on its speciation in aqueous solution and on its oxidation state. Under aerobic conditions, Tc mainly exists as pertechnetate, TcO4-, which is a highly water-soluble anion that does not significantly sorb on minerals or sediments, i.e. is considered inert and its groundwater migration is favoured. Under reducing conditions, Tc(VII) becomes Tc(IV), whose main species, TcO2, is a solid with a low solubility product and, thus, its mobility decreases. As the presence of reductants like Fe2+ in the near-field of a nuclear waste repository is expected due to canister corrosion, several studies consider 99Tc reductive immobilization by mineral containing reductant moieties, such as magnetite (FeIIFe2IIIO4) or mackinawite (FeS) [1, 2], confirming the 99Tc(VII) reduction and subsequent 99Tc(IV) retention on the mineral surfaces.
Pyrite (cubic FeS2) is a redox sensitive sulfur mineral that has been identified as a good sorbent for Tc(VII) from soil and groundwater in both the absence [3] and presence [4] of humic substances. Under repository conditions, iron sulfide will be formed as both pyrite and marcasite (orthorhombic FeS2) as a result of corrosion processes and microbial action [5]. Moreover, iron sulfides are also accessory minerals in granitic and argillaceous rocks. Therefore, reliable data on 99Tc(VII) retention by both minerals and their mixtures is relevant for the safe disposal of nuclear waste.
We have studied the Tc retention by pure pyrite and by a mixture marcasite-pyrite 60:40 (synthetic FeS2) by a combination of batch experiments and spectroscopy (Raman microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy). We have found that both minerals systems reduce Tc(VII) to Tc(IV). Pyrite removes 100% of Tc from solution within one day at pH ≥ 5.5 whereas in the presence of marcasite the Tc uptake becomes slower (100% reached after 7 days at 6.0 < pH < 9.0) and incomplete (80% at pH 10). The spectroscopic analysis showed that at pH 6.0 the removal mechanism is the same in both systems: inner-sphere complexation between Tc(IV) dimers and hematite (Fe2O3) formed in the surface of the minerals. At pH 10.0, while Tc(IV) is incorporated into magnetite also formed on the surface of pyrite, in the presence of marcasite the formation of sulfate minerals suggests that the redox active species is S2- instead of Fe2+, accounting for the slower and less efficient Tc scavenging.
This work has been developed in the frame of VESPA II project (02E11607B), supported by the German Federal Ministry of Economic Affairs and Energy (BMWi).
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