Marsac, and 3 co-authors, reported in 2010 that the effect of metal loading on 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 pH 3 for REE/C molar ratios ranging from ca 4 x 10-4 to 2.7 x 10-2. Results show that the relative amount of REE bound to HA strongly increases with decreasing REE/C. A middle-REE (MREE) downward concavity is shown by patterns at high metal loading, whereas patterns at low metal loading display a regular increase from La to Lu. Humic Ion Model VI modelling are close to the experimental data variations, provided that:
i) the ΔLK2 parameter (i.e. the Model VI parameter taken into account the presence of strong but low density binding sites) is allowed to increase regularly from La to Lu (from 1.1 to 2.1) and
ii) the published log KMA values (i.e. the REE–HA binding constants specific to Model VI) are slightly modified, in particular with respect to heavy REE. Modelling approach provided evidence that log KdREE patterns with varying REE/C likely arises because REE binding to HA occurs through two types of binding sites in different density:
1. a few strong sites that preferentially complex the heavy REE and thus control the log KdREE atterns at low REE/C; and
2. a larger amount of weaker binding sites that preferentially complex the middle-REE and thus control the log KdREE pattern at high REE/C.
Hence, metal loading exerts a major effect on HA-mediated REE binding, which could explain the diversity of published conditional constants for REE binding with HA. A literature survey suggests that the few strong sites activated at low REE/C could be multidentate carboxylic sites, or perhaps N-, or P-functional groups. Finally, an examination of the literature field data proposed that the described loading effect could account for much of the variation in REE patterns observed in natural organic-rich waters (DOC > 5 mg L-1 and pH ≤ 7).
The authors concluded that the effect of metal loading on the binding of the rare earth elements (REE) to humic acid (HA) was studied by combining ultrafiltration and Inductively Coupled Plasma Mass Spectrometry techniques. REE–HA binding experiments were performed at pH 3 for REE/C ranging from ca 4 x 10-4 to 2.7 x 10-2, using a standard batch equilibration method. Results show that the relative amount of REE bound to HA strongly increases with decreasing REE/C. Moreover, a middle-REE (MREE) downward concavity is displayed by log KdREE patterns at high metal loading, whereas patterns at low metal loading exhibit a regular increase from La to Lu. These results demonstrate that the variability of REE–HA binding patterns published so far is not an artifact resulting from the use of different analytical procedures. The variability of patterns is real, reflecting the presence of two types of sites in HA surface, each occurring in markedly distinct proportions and each having different binding affinities for the REE. The resulting metal loading effect explains the variation of log KdREE patterns with changing REE/C.
Calculations performed using Model VI show that the two types of sites involved in the binding of REE with HA can be defined by the Model VI intrinsic log KMA and ΔLK2 parameters, ΔLK2 parameter being free to vary among the REE series. Optimization of both parameters provides evidence that high-density but weak sites determine the distribution of REE to HA at high REE/C, favoring the complexation of the MREE. By contrast, the few strong sites determine the distribution of REE to HA at low REE/C, favoring the HREE complexation. A literature review suggests that the few strong sites activated at low REE/C could be multidentate carboxylic sites, or perhaps N-, or P-functional groups, whereas the weak sites involved at high metal loading are likely to be monodendate carboxylic sites. Application of Model VI to the experimental data shows that this model closely reproduces the observed variations. Hence, this model is a very powerful tool for studying and taking into account the loading effect that controls REE binding by HA.
Finally, confronting the variability of REE patterns in natural organic-rich waters and their metal loading level, expressed as REE/C, shows that the loading effect discussed here could explain part of the shape variability of REE patterns observed in these waters. Overall, our results suggest that further factors, such as competitive cations and/or colloidal nano-oxides, should be considered to improve the metal loading effect accuracy. Also the variability of humic substances chemical composition should be considered to explain the diversity of the binding sites that may be involved in REE complexation mediated by organic matter. These other factors should be taken into account to model and thus better understand REE cycling and transport in natural systems. Once again, Model VI may provide a reliable and powerful tool for this purpose.
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