Transport modelling of tracers influenced by kinetic hindered sorption — applied to laboratory and in situ studies of lanthanide EDTA complexes

The sorption stability of EDTA complexes of some lanthanides (Tb3+, Tm3+ and Yb3+) in a saline groundwater environment has been investigated in laboratory and in situ experiments. It is shown that the YbEDTA− complex possesses a lower sorption rate than TbEDTA−. Based on analysis of laboratory results and literature describing kinetic studies of complex exchange reactions, a conceptual decomplexation/sorption model for EDTA complexed lanthanides is proposed. The model consists of exchange reactions with di-valent ions, Fe2+, Mn2+ and Ca2+, which are both present in the groundwater and associated to the solid matrix, followed by an, within the time frame of the experiments, irreversible sorption of the non-complexed lanthanide ion. Calculations using the conceptual model has been performed both for the laboratory experiment results and the in situ experiment results. For the in situ experiment, a computer program was constructed that simulates transport in a one-dimensional flow with hydrodynamic dispersion and kinetic hindered exchange reactions of the lanthanide EDTA complexes with Ca2+. Good agreement was obtained between the calculated breakthrough curves and those obtained in the in situ experiment, supporting the applicability of the proposed model and the kinetical data obtained in laboratory experiments. However, for the long time perspective, there is an indication of an increased mobility of the lanthanide EDTA complexes, which deviates from the results of the model calculations. A conclusion is that kinetic characteristics of the lanthanide EDTA complexes can be applied for the prediction of the transport behaviour, at least in the short-term perspective.