Modelling of rare earth elements complexation with humic acid
Résumé
The binding of rare earth elements (REE) to humic acid (HA) was studied by combining Ultrafiltration and ICP-MS techniques. REE-HA complexation experiments were performed at various pH conditions (ranging from 2 to 10.5) using a standard batch equilibration method. Results show that the amount of REE bound to HA strongly increase with increasing pH. Moreover, a Middle REE (MREE) downward concavity is evidenced by REE distribution patterns at acidic pH. Modelling of the experimental data using Humic Ion Binding Model VI provided a set of log K$_{MA}$ values (i.e. the REE-HA complexation constants specific to Model VI) for the entire REE series. The log K$_{MA}$ pattern obtained displays a MREE downward concavity. Log K$_{MA}$ values range from 2.42 to 2.79. These binding constants are in good agreement with the few existing datasets quantifying the binding of REE with humic substances except a recently published study which evidence a lanthanide contraction effect (i.e. continuous increase of the constant from La to Lu). The MREE downward concavity displayed by REE- HA complexation pattern determined in this study compares well with results from REE-fulvic acid (FA) and REE- acetic acid complexation studies. This similarity in the REE complexation pattern shapes suggests that carboxylic groups are the main binding sites of REE in HA. This conclusion is further supported by a detailed review of published studies for natural, organic-rich, river- and ground-waters which show no evidence of a lanthanide contraction effect in REE pattern shape. Finally, application of Model VI using the new, experimentally determined log K$_{MA}$ values to World Average River Water confirms earlier suggestions that REE occur predominantly as organic complexes (> 60 %) in the pH range between 5-5.5 and 7-8.5 (i.e. in circumneutral pH waters). The only significant difference as compared to earlier model predictions made using estimated log K$_{MA}$ values is that the experimentally determined log K$_{MA}$ values predict a significantly higher amount of Light REE bound to organic matter under alkaline pH conditions. Taken as a whole, the new experimental results shed additional light on the processes that govern REE pattern shapes in natural, organic-rich waters.