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

Mechanisms for accumulation and migration of technetium-99 in saltmarsh sediments
Mechanisms for accumulation and migration of technetium-99 in saltmarsh sediments
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 550C 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 ?-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 lOg. 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. "mTc was used as a
yield monitor, and determination of "Tc 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
MnIV to Mn" reaction and the Fe1" to Fe" 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 24lAm
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.
Wigley, F.
8019bdf9-6dfd-4510-906b-02054025b6db
Wigley, F.
8019bdf9-6dfd-4510-906b-02054025b6db

Wigley, F. (2000) Mechanisms for accumulation and migration of technetium-99 in saltmarsh sediments. University of Southampton, Faculty of Science, School of Ocean and Earth Science, Doctoral Thesis, 213pp.

Record type: Thesis (Doctoral)

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 550C 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 ?-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 lOg. 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. "mTc was used as a
yield monitor, and determination of "Tc 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
MnIV to Mn" reaction and the Fe1" to Fe" 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 24lAm
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.

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Published date: June 2000
Additional Information: Digitized via the E-THOS exercise.
Organisations: University of Southampton

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Local EPrints ID: 42177
URI: https://eprints.soton.ac.uk/id/eprint/42177
PURE UUID: fdf71a9c-47d9-4a4b-8b54-8de72f75a1bb

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Date deposited: 22 Nov 2006
Last modified: 13 Mar 2019 21:12

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Author: F. Wigley

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