Lanthanide-humic substances complexation. I. Experimental evidence for a lanthanide contraction effect

The interaction of the lanthanides (Ln) with humic substances (HS) was investigated with a novel chemical speciation tool, Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry (CE-ICP-MS). By using an EDTA-ligand competition method, a bimodal species distribution of LnEDTA and LnHS is attained, separated by CE, and detected online by sector field ICP-MS. We quantified the binding of all 14 rare earth elements (REEs), Sc and Y with Suwannee river fulvic acid, Leonardite coal humic acid, and Elliot soil humic acid under environmental conditions (pH 6-9, 0.001-0.1 mol L-1 NaNO3. 1-1000 nmol L-1 Ln, 10-20 mg L-1 HS). Conditional binding constants for REE-HS interaction (K-c) ranged from 8.9 < log K-c < 16.5 under all experimental conditions, and display a lanthanide contraction effect, Delta K-L(c): a gradual increase in K-c from La to Lu by 2-3 orders of magnitude as a function of decreasing ionic radius. HS polyelectrolyte effects cause Kc to increase with increasing pH and decreasing ionic strength. Delta K-L(c): increases significantly with increasing pH, and likely with decreasing ionic strength. Based on a strong correlation between Delta K-L(c), values and denticity for organic acids, we Suggest that HS form a range of tri- to tetra-dentate complexes under environmental conditions. These results confirm HS to be a strong complexing agent for Ln, and show rigorous experimental evidence for potential REE fractionation by HS complexation. (c) 2006 Published by Elsevier Inc.