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 (Tb (3 +), Tm (3 +) and Yb (3 +)) in a saline groundwater environment has been investigat ed in laboratory and in situ experiments. It is shown that the YbEDTA (-) c omplex possesses a lower sorption rate than TbEDTA (-). Based on analysis o f laboratory results and literature describing kinetic studies of complex e xchange reactions, a conceptual decomplexation/sorption model for EDTA comp lexed lanthanides is proposed. The model consists of exchange reactions wit h di-valent ions, Fe (2 +), Mn (2 +) and Ca (2 +), which are both present i n the groundwater and associated to the solid matrix, followed by an, withi n the time frame of the experiments, irreversible sorption of the non-compl exed lanthanide ion. Calculations using the conceptual model has been perfo rmed both for the laboratory experiment results and the in situ experiment results. For the in situ experiment, a computer program was constructed tha t simulates transport in a one-dimensional flow with hydrodynamic dispersio n and kinetic hindered exchange reactions of the lanthanide EDTA complexes with Ca (2 +) Good agreement was obtained between the calculated breakthrou gh curves and those obtained in the in situ experiment, supporting the appl icability of the proposed model and the kinetical data obtained in laborato ry experiments. However, for the long time perspective, there is an indicat ion of an increased mobility of the lanthanide EDTA complexes, which deviat es from the results of the model calculations. A conclusion is that kinetic characteristics of the lanthanide EDTA complexes can be applied for the pr ediction of the transport behaviour, at least in the short-term perspective . (C) 2000 Elsevier Science B.V. All rights reserved.