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Antioxidant Properties of Humic Substances

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 el...

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Published in:Environmental science & technology 2012-05, Vol.46 (9), p.4916-4925
Main Authors: Aeschbacher, Michael, Graf, Cornelia, Schwarzenbach, René P, Sander, Michael
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Language:English
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creator Aeschbacher, Michael
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description 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 (E h) 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 E h 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 E h, 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.
doi_str_mv 10.1021/es300039h
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source American Chemical Society:Jisc Collections:American Chemical Society Read & Publish Agreement 2022-2024 (Reading list)
subjects Antioxidants
Antioxidants - analysis
Benzothiazoles
Earth sciences
Earth, ocean, space
Electrons
Exact sciences and technology
Geochemistry
Humic Substances - analysis
Hydrogen-Ion Concentration
Molecules
Oxidation
Oxidation-Reduction
Soil and rock geochemistry
Soils
Sulfonic Acids
Surficial geology
title Antioxidant Properties of Humic Substances
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