Rare earth elements fractionation modelling between manganese oxide and humic acid

International audienceSeveral studies demonstrated that rare earth elements (REE) speciation is controlled by organic matter in natural waters. More recently, Davranche et al. [1] provided evidence that manganese oxide (MnO2) could compete with humic acid (HA) for REE binding, according to pH and time reaction. Competitive binding of REE to HA and MnO2 was modelled using two sub-models. Interactions with HA were described using Model VI (Tipping [2]) and adsorption onto MnO2 was described using a surface complexation model (SCM). Model VI parameters for the whole REE series described by Pourret et al. [3] were further refined. SCM parameters necessary to described MnO2 surface reactions were derived from published laboratory data using a linear free energy relationship. Introducing Model VI and SCM into PHREEQC allowed considering cerium redox behaviour. Models coupling was validated by testing laboratory datasets on MnO2 and HA competition for REE binding [1]. Modelling revealed that REE-HA complex was the dominant species whatever the HA/MnO2 ratio. The development of a Ce anomaly appears strongly limited. However, modelling did not reflect the proportion of REE-HA complexes adsorbed onto MnO2. Next step will thus be to introduce ternary surface complexes. This work shed more light on the reliability of speciation modelling to predict REE distribution between HA and MnO2, and will thus become an interesting tool in investigating processes that govern REE speciation in natural waters