Effect of metal ions on the indigenous radicals of humic acids: high field electron paramagnetic resonance study.

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.