Stability of 125I and 14C Labelled Boom Clay Organic Matter

A new method to radiolabel natural organic matter by chemical reduction with tritiated sodium borohydride

In this paper, we describe a new method for labeling NOM with the radioisotope tritium (3H) using fulvic acid (FA) as the target NOM fraction. During labeling, FA ketone groups are chemically reduced with tritiated sodium borohydride (NaBH4), while the chemical functionality of the carboxyl and phenol groups is preserved. The labeling procedure was optimized in efficiency experiments that determined the excess concentration of tritiated NaBH4 required for optimum reduction conditions. The chemical characterization of the labeled FA product using FTIR and 1H NMR spectral analysis confirms the proposed reaction mechanism and rules out any significant amounts of impurities or undesirable side reactions. Results from size exclusion chromatography indicate thatthe tritium label is distributed uniformly over the whole molecular size range of FA and that it is stable over time and under various pH conditions. Potential differences in FA sorption behavior onto mineral surfaces due to labeling were excluded based on experimental data. This method produces NOM of high specific activity (e.g., 1.9 mCi mg(-1) FA); this permits the tracing of FA at a detection limit of 0.3 microg L(-1) FA.

Radiolabelling of humic substances with 14C by azo coupling [14C]phenyldiazonium ions

For the first time, natural and synthetic humic substances were radiolabelled by azo coupling [U-(14)C]phenyldiazonium ions onto the aromatic fragments of their macromolecules under mild reaction conditions. The radiolabelling procedure was optimized with respect to pH, reaction temperature and the molar ratio of the humic substance to the labelling compound. The labelled humic substances were purified by precipitation or ultrafiltration. The chemical yields were in the range between 23% and 95%, and the specific radioactivities varied between 68 and 206MBq (14)C per gram of the humic substance, depending on the origin of the humic substance and the purification method. With the (14)C-labelled humic compounds thus obtained, we were able to detect humic substances at concentrations as low as 5microg/L. These radiolabelled compounds can be used in long-term studies because, according to size exclusion chromatography data, there are no signs of their decomposition even after 5 months of storage.

The simultaneous modelling of metal ion and humic substance transport in column experiments

Pulsed column experiments using Co, fulvic acid and porous sediment packing, along with up/down-flooding experiments using Eu, humic acid and intact sandstone blocks have been performed. The elution of metal and humic and their distribution along the sandstone columns have been measured. A mixed equilibrium and kinetic coupled chemical transport model has been used to simulate the results. In both cases, one exchangeable and one non-exchangeable component have been used to simulate the interaction of metal and humic substance. For the pulsed experiments, a simple equilibrium approach was used to model humic sorption, while a two component, kinetic model was required for the sandstone columns.