Mechanisms for accumulation and migration of technetium-99 in saltmarsh sediments

Abstract

This thesis describes the development of analytical methods for both the bulk determination of ⁹⁹Tc, and determination of ⁹⁹Tc in sequential extracts from sediments. These methods have been used to collect data, which, along with trace and major element data have been used to interpret the mechanisms for ⁹⁹Tc input, migration and accumulation in saltmarshes. The inventory of ⁹⁹Tc stored in the Thornflatt Saltmarsh, Esk Estuary has also been determined. The routine determination of ⁹⁹Tc in bulk samples uses ^99mTc as a yield monitor. Samples are ignited stepwise to 550°C and the ⁹⁹Tc is extracted using 8M nitric acid. Many contaminants are precipitated with Fe(OH)₃ and the Tc in the supernant is pre-concentrated and further purified using anion-exchange chromatography. Final separation of Tc from Ru is achieved by extraction of Tc into 5% TnOA in xylene from 2M sulphuric acid. The yield is determined by γ-spectrometric analysis of ^99mTc. Determination of ⁹⁹Tc is made by liquid scintillation counting. Typical recoveries are in the order of 70-95% and the method has a detection limit of 1.7 Bq/kg for a sample size of 10g. Determination of Tc in sequential extracts uses operationally defined procedures to extract: exchangeable Tc, reducible Tc and oxidisable Tc. An initial water wash is used to extract any occluded Tc and a final leach in 8 M nitric acid is used to dissolve any residual Tc. The isolation of ⁹⁹Tc uses TEVA resin for Extracts 1-4 and the decontamination procedure developed for bulk analysis for Extract 5. ^99mTc was used as a yield monitor, and determination of ⁹⁹Tc is by liquid scintillation counting. Limits of detection were dependent on the amount of ^99mTc tracer used but were found to be as low as 2.4 Bq/kg for a sample size of 2g.
A study was made of the mechanisms responsible for the accumulation and migration of Tc in estuarine sediments using sediments collected from saltmarshes at Thornflatt, Carlaverock and the Ribble Estuary. ⁹⁹Tc was present at determinable activities in all the sediment cores taken from these sites.
Good correlations between Tc and CaO as well as CO₃ concentrations and poor correlation between Tc and radionuclides adsorbed to inorganic detritus infer a direct input of 99Tc to marsh sediments. Determination of 99Tc in biota living on the marsh also showed that this was not a significant pathway for input of Tc to the sediments. Sequential extraction data imply sorption to an organic fraction of the sediment.
Stable element and sequential extraction data indicates that Tc is readily oxidised and remobilised before reprecipitation where redox conditions are favourable. Data indicate a reduction potential between those of the Mn^IV to Mn^II reaction and the Fe^III to Fe^II reaction is necessary for re-accumulation to occur, as suggested by published thermodynamic data. Data collected from reducing sediments imply that similar mechanisms are responsible for the accumulation of Mn (e.g. reduction by sulphate reducing bacteria) and the accumulation of Tc.
The inventory of ⁹⁹Tc held within the Thornflatt saltmarsh is proportionally less than that of ¹³⁷Cs or ²⁴¹Am when compared to discharges from Sellafield. However a higher proportion of ⁹⁹Tc is transferred from Seliafield and incorporated into saltmarsh sediments than is suggested by previously published standard distribution coefficient data. Saltmarsh sediments are therefore a more important sink of ⁹⁹Tc than extrapolations made from inventories of other radionuclides would suggest.

More information

Identifiers

Catalogue record

Export record