Abstract
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.
Highlights
The transformation of natural organic matter leads to the formation of humic substances
Particle growth determined through dynamic light scattering measurements coincided with the onset of electron paramagnetic resonance (EPR) signal enhancement, and surface tension measurements further corroborated the coincidence of aggregation
Before The EPR spectra of humic acid (HA) obtained in this study displayed a narrow line at an average value of g = 2.00361 (Figure 1), which is consistent with the characteristic sharp resonance produced by the semiquinone free radical [3,5,15]
Summary
The transformation of natural organic matter leads to the formation of humic substances. Under aerobic conditions of composting or humification in terrestrial ecosystems the main components of these materials are fulvic acid (FA) and humic acid (HA) They are a complex, operationally defined suite of substances built around a highly aromatic backbone and containing an abundance of oxygenated functionalities, including carboxylic, phenolic, alcoholic and carbonyl groups [1]. Structural changes during ageing of HA lead to an increase in the number of polyphenolic and quinoid units The latter generally exist as equilibrated quinone/ Hydroquinone structures encompassing a radical semiquinone intermediate that can be investigated by electron paramagnetic resonance spectroscopy (EPR) [2]. Upon the addition of cations, especially polyvalent ones, HA folds and shortens, forming compact structures with relatively hydrophobic interiors and hydrophilic surfaces This it thought to be due to a combination of charge neutralization and functional group bridging [10].
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