An investigation into technetium binding in sediments |
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| Authors: | Miranda J Keith-Roach Katherine Morris Henning Dahlgaard |
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| Institution: | a Department of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK;b Department of Radiation Research, Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark;c School of the Environment, University of Leeds, Leeds LS2 9JT, UK |
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| Abstract: | Coastal environments with reducing waters and/or sediments represent potential sinks for 99Tc discharged to sea. Here, we have examined estuarine sediments from four geochemically distinct locations that have been exposed to 99Tc discharged from Sellafield. Both the relative uptake and the operationally defined sediment “component” that holds the Tc were investigated in order to establish whether particular biogeochemical processes are predominately responsible for reducing and binding Tc in sediments. Despite the artefacts that can pose problems for sequential extractions, this scheme Int. J. Environ. Anal. Chem. 51 (1993) 187] appears to be robust with regard to Tc. The results show that relative uptake of Tc varied greatly between the sites, with the highest occurring at an almost permanently anoxic fjord (Mariager Fjord, Denmark), followed by a brackish, seasonally eutrophic fjord (Roskilde Fjord, Denmark), then a sub-oxic salt marsh (which receives particle bound radionuclides from a more reducing mud patch; Esk Estuary, UK) and finally sulfidic and iron-reducing muddy sandy sediments (Needle's Eye, Solway Firth, Scotland). High relative uptake at the fjords was explained by the greater mixing of Tc-labelled oxic seawater into the part of the system where reduction of TcO4− was possible. Uptake at Mariager Fjord was higher than at Roskilde Fjord, reflecting the highly reducing geochemical conditions in the water body. Low uptake at Needle's Eye may be related to the speciation of technetium at this site as the sequential extraction data suggest that Tc is associated with the carbonate phase here. Tc carbonates are largely soluble, and this is the first observation of Tc association with carbonate fractions in the natural environment. The other three sites showed that organic matter, in conjunction with reducing conditions, was very important for binding and retaining Tc in sediments. The specific role of sulfides in controlling Tc retention in sediments was unclear because sulfides and organic matter are leached simultaneously in the sequential extraction scheme we applied, but there was evidence that Tc was not associated with acid-volatile iron monosulfides. |
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| Keywords: | Technetium Sequential extraction Anoxic Sediment Sink Regional terms UK Cumbria Esk Estuary UK Solway Firth Needle's Eye Denmark Jutland Mariager Fjord Denmark Sealand Roskilde Fjord |
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