Dynamic structure of humic substances: rare earth elements as a fingerprint

Whereas humic substances are known to play a key role in controlling metal speciation and trace element mobility within soils and waters, the understanding of their structure is still unclear and remains a matter of debate. Several models of humic substance structure have been proposed, where humic substances were composed of either: (i) macromolecular polyelectrolytes that can form molecular aggregates or (ii) supramolecular assemblies (molecular aggregates) of small molecules without macromolecular character, joined together by weak attraction forces. This experimental study was designed and dedicated: (i) to follow the size of organic molecules versus ionic strength or pH by the combined means of ultrafiltration and aromaticity data and rare earth element (REE) fingerprinting, and (ii) to investigate the pH and ionic strength effect on the distribution of associated rare earth elements in soil solution. This study supports the presence of supramolecular associations of small molecules and probably the presence of macromolecules in the bulk dissolved organic matter. By contrast to ionic strength, pH appeared to be the major parameter playing on the stability of the humic substance structure. Humic substances displayed dynamic structures, which evolved with regard to pH. Low pH led to a destabilization of the humic substance conformation. This destabilization had an impact on the trace element distribution in soil solution, as assessed by REE data, and conversely, the destabilization degree of humic substances seemed to be influenced by the metal ion charge.