Radical Stabilization in Dissolved Humates

Quinoid entities, in which quinone and hydroquinone groups equilibrate via a semiquinone radical intermediate, are a common structural feature in humic materials. The electron paramagnetic resonance (EPR) signals of these radicals are significantly enhanced in the presence of diamagnetic divalent metal ions such as Mg2+, while monovalent ions do not show the effect. The addition of trivalent ions leads to rapid precipitation, leaving little room for observation. It was noted that the metal ions producing EPR signal enhancement were also underwent effective bridging interactions with humic subunits, forming pseudomicellar structures. Particle growth determined through dynamic light scattering measurements coincided with the onset of EPR signal enhancement, and surface tension measurements further corroborated the coincidence of aggregation. The addition of a chaotrope (urea), which broke up the humic structures, eliminated the EPR signal increases. These observations strongly suggested that bridging interactions by divalent metal ions, and the intramo- lecular aggregation that accompanied it, led to significant stabilization of semiquinone radicals within the humic structure.