EXAFS study of the nature of zinc complexation sites in humic substances as a function of Zn concentration

Humic substances are known to play a key role in the immobilisation of metals in soils. The main complexing ligands that are supposed to interact with metals are carboxylic and phenolic. The complexation mechanism of zinc by two natural humic acids (T and F) was studied by EXAFS spectroscopy at the Zn-K edge over a wide range of zinc concentration. The ligation mechanism of Zn was found to vary with the metal concentration and the nature of the humic acid. As Zn concentration is raised, the amplitude of EXAFS spectra decreases, which is interpreted as an increased distribution of Ligand sites. At low Zn concentration (300-5000 mg/Kg), zinc forms inner sphere complexes in octahedral coordination for T humic acid, and in octahedral and tetrahedral coordinations for F humic acid. At intermediate Zn concentration (32 g/Kg), a large number of different sites are observed and only octahedral coordination is detected. At high Zn concentration (500 g/Kg), most Zn forms outer sphere complexes. The difference of complexation mechanisms between humic acids F and T can be related to differences in their structure as indicated by C-13-NMR spectroscopy.