The biogeochemical cycle of Rare Earth Elements (REEs) is being affected by their increased use in modern technologies an increased concentrations of certain REEs have been measured in natural waters. Presently there are no regulatory thresholds for REE concentrations and emissions into the environment and knowledge on their environmental behaviour and effects is limited.The fate and behaviour of REEs in the environment is determined by their binding to reactive components such as organic matter, metal oxides, and clays. All of them are considered of key important. However, humic substances (HS, humic and fulvic acids) are the most reactive fractions of natural organic matter, having a high affinity for metal ions. Thus, the use of thermodynamic models which could predict the partitioning and speciation of trace metals in terrestrial and aquatic systems is of key importance. Here, we use the NICA-Donnan (ND) model which is one of the advanced models for ion binding to HS. Nevertheless, ND parameters for REEs binding to HS (except Eu) are still lacking.Here we studied Nd, Gd and Yb binding to a groundwater FA in batch titration experiment in the pH range 3-7. Free metal ion concentrations were determined using the Donnan Membrane Technique, being the first time this technique was used for REEs. Using the experimental data for FA together with published data on REE binding to HA we derived generic ND parameters for all the 14 stable lanthanides. The new model parameters were evaluated with published data.< Réduire