Effect of loading on the nature of the REE-humate complexes as determined by Yb3+ and Sm3+ LIII-edge EXAFS analysis

- At high and low metal loading, the structure of Sm and Yb complexes with humic acid are determined by EXAFS spectroscopy.
- Model VII predicts that Yb and Sm tend to bind to strong humic N-containing groups at low metal loading.
- At low metal loading the presence of N could not be demonstrated, in contrast with complexes with IDA, NTA or EDTA.
- At high metal loading, carboxylic humic groups are found to be coordinated in bidentate mode.
- The results are discussed in the context of cation competition and supramolecular structure of metal-humate complexes.

Humic substances are major natural complexing agents that strongly influence the cycling of metal cations in the environment. The metal to humic acid (HA) concentration ratio (metal loading) can significantly impact the overall HA complexation properties in natural soils and waters. At low metal loading, cations bind to strong multi-carboxylate or -phenolate sites of HA. In addition, these strong sites might contain soft Lewis bases such as N or S, the contribution of which is still a matter of debate. Based on the possible ability of Rare Earth Elements (REE) to trace HA binding properties, the present study aimed at evaluating the influence of REE-loading on the HA binding mechanisms of two hard Lewis acids, Sm3+ and Yb3+. Extended X-ray Absorption Fine Structure (EXAFS) at the Sm and Yb LIII edge was used to analyze the local coordination of these intermediate and heavy REE ions, respectively, in a low and a high REE-loading HA sample. Data were interpreted by comparison with EXAFS data for a series of aqueous REE complexes with iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA) and a carboxylic-grafted resin. In all these samples, both a Yb-O (~ 2.31 Å) and a Yb-N (~ 2.50 Å) backscattering path could be identified in the first coordination shell of Yb. At low REE loading, the first coordination shell around the Yb bond to HA is described by a single Yb-O backscattering path, i.e. the presence of N could not be evidenced. By contrast, the carboxylic-grafted resin exhibits both a short (2.31 Å) and a long Yb-O average distance (2.44 Å), suggesting the coordination of carboxylate is bidentate. The Fourier transform of the high loading HA samples exhibits a broad first shell peak possibly described by two Yb-O distances (2.29 and 2.43 Å) suggesting two coordination modes of the carboxylic groups. Different Yb-HA complex structures can therefore be observed at different metal loadings. Unfortunately, the Sm EXAFS spectra could only be recorded up to k = 10 Å−1. There was no evidence of either a clear difference between the metal loadings or with Yb-HA complex structures. A comparison between the EXAFS results, PHREEQC-Model VI simulations, and the pattern of the REE complexation constants with simple organic ligands (REE:ligand 1:1 and 1:2 complexes) provides evidence that, at high loading, REE are bound to HA through bi-ligand complexes without any chelation effect in which REE may potentially act as a cation bridge between two organic molecules. At low loading, the fact that N could not be detected in the Yb first shell suggests that, at low loading, Yb binds to O-containing groups of HA rather than N-containing groups. This may call into question the definition of HA strong sites for REE binding such as soft Lewis base-containing sites.