Antioxidant Properties of Humic Substances

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 (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.

Environmental pH and ionic strength influence the electron-transfer capacity of dissolved organic matter

“…Using electrochemical methods, Aeschbacher et al (2010) determined that the EAC of HA and FA from both aquatic and terrestrial origins was 493-1961 μmol e /(g C). The slightly smaller EAC (199-1210 μmol e /(g C)) of DOM determined in this study can be attributed to the differences in the specific experimental conditions (e.g., pH, E h ) and the DOM source material (Aeschbacher et al 2012). Compared with the results of Aeschbacher et al (2010), considerably lower EDC was obtained in this study.…”

A humic substances product extracted from biochar reduces Arabidopsis root hair density and length under P-sufficient and P-starvation conditions

“…According to those studies, the biochar HSP-like substances are much more abundant in biochars produced at relatively lower temperatures, from cellulosic and manure feedstocks, or in biochars that were pretreated by acidic or alkaline solutions, as compared to untreated biochars produced at higher temperatures or from ligneous feedstocks. Small organic molecules and substances similar to HSP that make up the water soluble fraction of biochar were found to be redox active , much like similar organic materials in natural dissolved organic matter (Fimmen et al 2007;Aeschbacher et al 2012).…”

Ecological Risk Assessment of Arsenic and Metals in Surface Sediments from Estuarine and Coastal Areas of the Southern Bohai Sea, China

“…However, the redox potential (E h ) of humic acid is also highly dependent on pH. A linear decreasing E h with increasing pH was observed for various HAs and other DOM from different sources, 36,52,53 which indicate higher pH is favorable for the reduction by HA. Taken together, the influence of pH on the Au(III) species and E h of HA makes the pH-dependent reduction of Au(III): The reduction rate of Au(III) first increased and then decreased with the increasing of pH.…”