Association of desferrioxamine with humic substances and their interaction with cadmium(II) as studied by pyrolysis–gas chromatography–mass spectrometry and nuclear magnetic resonance spectroscopy†
In soils, sediments and aqueous media, the existence of any significant association of organic ligands with humic substances (HS) would complicate metal ion equilibria and kinetics beyond those of existing models that assume competition among non-interacting ligands. Multi-dimensional NMR techniques were applied to obtain kinetic and structural evidence of extensive association of desferrioxamine B (DFOB) with HS. This occurred through diaminopentyl and succinyl protons of DFOB with the aromatic/phenolic and/or saccharidic groups of HS, and exhibited an exchange rate of 15000–25000 s–1. Additionally, the aqueous interaction of HS, DFOB and CdII was probed using pyrolysis–GC–MS analysis directly of microliter volumes. Although pyrolysis–GC–MS data can be difficult to interpret alone, the NMR characterization of ligand–HS association enabled the results to be interpreted. Pyrolysis–GC–MS revealed thermolyzate markers that permitted the quantification of total DFOB and apparent complexes of DFOB·Cd and HS·Cd. The results indicate that the formation of DFOB·HS (or possibly DFOB·Cd·HS) caused significant decreases in the formation of the chelator complex, DFOB·Cd. This decrease did not fit with equilibrium-based concepts since the order of addition strongly influenced all results. Although previously unknown, the results show that such organic ligand–HS interactions do occur, which significantly alter the metal ion chemistry and probably affect bioavailability; the latter is relevant since DFOB is a siderophore synthesized by bacteria to acquire metal ions. Therefore, in order to understand the bioavailability of metal ions in real systems such as the rhizosphere one must consider the interactions of HS with biogenic ligands, for which liquid-state multi-dimensional NMR is a powerful tool. Unlike metal–ligand measurements that are limited to liquid state and/or low paramagnetic samples, the pyrolysis–GC–MS method has the potential to be extended to whole soils and sediments for the analysis of metal ion speciation.