Radio-Labelled Humic Materials in Migration Studies
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RADIO-LABELLED HUMIC MATERIALS IN MIGRATION STUDIES LARS CARLSENa, PIA LASSENa, PETER WARWICKb and AMANDA RANDALLb a National Environmental Research Institute, Department of Environmental Chemistry, Frederiksborgvej 399, DK-4000 Roskilde, Denmark b Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire LEI I 3TU, UK. ABSTRACT Humic- and fulvic acids are able to complex polyvalent metal ions, e.g. radionuclides, leading to soluble complexes of significant strength, thereby decreasing the sorption of these compounds to soils and sediments. The interaction of humic materials with radionuclides may significantly influence the availability and transport of the latter in the environment. Typically, studies along these lines have focussed almost exclusively on the radionuclides, whereas the actual role of the humic material has been elucidated only indirectly. In order directly to study the behaviour of the naturally occurring organic macro-molecules in relation to the environmental fate of radionuclides, radio-labelled humic- and fulvic acids can advantageously be applied. Radio-labels such as 14C and 1251 have successfully been covalently incorporated in humic- and fulvic acids. Labelling of humic substances as well as preliminary migration studies are discussed INTRODUCTION To date, investigations of humic interactions with radionuclides have largely relied on the measurement of the free radionuclide [1]. However, more information would be gained concerning radionuclide-humic acid interactions if the latter could be labelled with a suitable radioactive label. Reactions of radionuclides with humic materials are equilibrium reactions whose stabilty constants may be low enough to allow dissociation of the radionuclide-humic complex when investigating transport processes, solid phase interactions or the biological uptake of the complex. Thus, the use of a double-labelled complex would enable the fate of the free radionuclide, the bound radionuclide and the free humic materials to be more easily followed both qualitatively and quantitatively by measuring the relative levels and type of radiation associated with these different species. For metal-humic investigations, the incorporation of 14C or 1251 into the humic molecule would be suitable radioactive labels. The incorporation of a radioactive label into humic materials must be accomplished by methods which do not significantly change the inherent characteristics of the humic material. An additional requirement for the radiolabel is that its incorporation is irreversible as stability of the labelled material is essential for its use in transport investigations. Stability can be achieved by the covalent bonding of the radiolabel to a suitable site, for example within the aromatic part of the humic molecule. 14 C-labelled humic material has been produced previously by growing plants such as ryegrass in an atmosphere containing 14 CO2 , followed by humification by decomposition in soil [2]. However, this method is very time consuming taki
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