title: Effect of metal ions on the indigenous radicals of humic acids: high field electron paramagnetic resonance study.
abstract: The interaction of indigenous radicals of humic acid (HA) with metal cations has been studied using high magnetic field (10.5T-285 GHz) electron paramagnetic resonance (HFEPR) spectroscopy. Strong [HA]-[metal] interaction was observed in the case of heavy metals, Cd(2+), Pb(2+), and Sr(2+), leading to formation of covalent bonds with the radicals of HA. On the contrary, alkaline earth metal ions, such as Mg(2+), generate only electrostatic interaction. The two types of indigenous radicals that exist in all HAs are influenced by the metal cations in a unified manner. This provides evidence that the two types of indigenous radicals in HAs originate from a unique, phenolic, moiety in HA. Mg(2+) ions dramatically changed the pH profile of the two radical types of HA, downshifting their interconversion pK(a) by ca. 3 pH units. This is the first experimental observation of the effect of metals on the H-dissociation of the radical centers in HAs.
authors: Laboratory of Physical Chemistry, Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, 30100 Agrinio, Greece.
authors: Konstantinos C
authors: KC
authors: Christoforidis
authors: Sun
authors: S
authors: Un
authors: Yiannis
authors: Y
authors: Deligiannakis
mesh_terms: D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D005609:Free Radicals / Q000737:chemistry*; D005707:Gallic Acid / Q000737:chemistry; D006812:Humic Substances / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D007477:Ions; D008670:Metals / Q000737:chemistry*; D013696:Temperature
chemical_list: D005609:Free Radicals; D006812:Humic Substances; D007477:Ions; D008670:Metals; D005707:Gallic Acid