Antioxidant Properties
of Humic Substances
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Abstract
Humic substances (HS) are heterogeneous, redox-active
organic macromolecules.
While electron transfer to and from HS under reducing conditions is
well investigated, comparatively little is known on the electron donating
(i.e., antioxidant) properties of HS under oxic conditions. In this
work, the electron donating capacities (EDCs) of terrestrial and aquatic
HS were quantified by mediated electrochemical oxidation over a wide
range of pH values and applied redox potentials (<i>E</i><sub>h</sub>) using 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic
acid) as an electron transfer mediator. Electrochemical oxidation
of three model humic acids (HAs) was largely irreversible, and the
EDCs of these HAs increased with increasing <i>E</i><sub>h</sub> and pH. These results suggest that HS contain a wide variety
of moieties that are oxidized at different potentials and that, upon
oxidation, release protons and undergo irreversible follow-up reactions.
At a given pH and <i>E</i><sub>h</sub>, the EDCs of the
HS correlated well with their titrated phenol contents suggesting
phenolic moieties as major electron donating groups in HS. Comparing
the EDCs of 15 HS with their electron accepting capacities (EACs),
aquatic HS had higher EDCs and lower EACs than terrestrial HS of comparable
aromaticities. These results indicate that oxidative transformation
of HS in the environment results in a depletion of electron donating
phenolic moieties with antioxidant properties relative to the electron
accepting quinone moieties