Experimental Validation of a Speciation Model Applied to Organic-Rich Groundwaters

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.