Experimental Validation of a Speciation Model Applied to Organic-Rich Groundwaters
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EXPERIMENTAL VALIDATION OF A SPECIATION MODEL APPLIED TO ORGANIC-RICH GROUNDWATERS
M.B. CRAWFORD, J.J.W. HIGGO AND J. DAVIS Fluid Processes Group, British Geological Survey, Keyworth, Nottingham NG12 5GG.
ABSTRACT Natural organic compounds can play an important role in the transport of radionuclides through the geosphere, but the inclusion of polyelectrolytic fulvic and humic acids into geochemical speciation codes is not straightforward. Schubert ion-exchange experiments have been used to parameterise an equilibrium-based, discrete site-electrostatic model, Model V, which is concerned with predicting trace metal-humic interactions in solution. However, before such a model can be applied to natural groundwaters, its ability to deal with competition between trace cations and anionic ligands has to be tested. PHREEQEV, which incorporates Model V with the inorganic speciation code PHREEQE, has been used to guide experiments based on a modified Hummel-Dreyer method. To improve speed and precision, these experiments used a HPLC method rather than Sephadex gel, and measured the effect of Ca on Co and Ni binding to fulvic acid. The results were compared with the predictions made by PHREEQEV. Although the system was further complicated by the need to add citric acid to prevent binding of metals to the HPLC column, predictions at pH 5 were good for both metals. At pH 7, the Co binding results were not well predicted and the reasons for this are still unclear. Comparisons between the Schubert and Hummel-Dreyer-type experiments show that the two methods are compatible.
INTRODUCTION Natural organic compounds can play an important role in the transport of radionuclides through the geosphere because of their ability to form labile complexes, participate in redox reactions, and to sorb onto solid surfaces. The safety case for any radioactive waste repository is dependent on accurate predictions of radionuclide speciation and this must, therefore, include complexes with organic material. Low molecular weight (< CtO) organic acids can be modelled as single, discrete sites, but about 80% of the dissolved organic carbon in natural waters consists of larger humic and fulvic acids. These molecules have a large number of different sites with a range of thermodynamic properties and the net charge on such humic material varies with solution composition. This means that the ratio of organically-bound to free radionuclide activity varies with respect to parameters such as ionic strength, pH, and ligand concentration and, as such, speciation calculations are not straightforward. Model V is an equilibrium-based, discrete site-electrostatic model that has been developed with the aim of predicting trace metal-humic interactions in solution 1. The applicability of this model to radionuclide speciation in natural groundwaters has been tested experimentally by measuring the binding of Co2+, Ni2+, U0 22+ and Ca 2+ to fulvic acids extracted from three different groundwaters using the Schubert ion-exchange method2 . Intrinsic binding constants wer
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