Rare Earth Elements Complexation with Humic Acid - 2007

Rare Earth Deposits & Mining

Pourret, and 3 co-authors, reported in 2007 the the binding of rare earth elements (REE) to humic acid (HA) was studied by combining Ultrafiltration and Inductively Coupled Plasma Mass Spectrometry 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 KMA 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 but quite different from 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 suggests that carboxylic groups are the main binding sites of REE in HA.

The authors concluded that the above  is further illustrated by a detailed review of published studies for natural, organic-rich, river water and groundwater which show no evidence of a lanthanide contraction effect in REE pattern. 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 (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.