Long-term kinetics study and quantitative characterization of the antioxidant capacities of humic and humic-like substances
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Humic substances (HS) play important functions in the environment by radical scavenging in biogeochemical redox reactions, thus influencing behavior of pollutants and preventing damage to cell membranes; this is due to antioxidant properties of HS. Previous studies focused primarily on assessing endpoint antioxidant capacity (AOC) of HS. Our work aimed to estimate long-term kinetics of the antioxidant capacities of humic and humic-like substances under different pH in relation to their specific structural features.
Materials and methods
The 10-h kinetic profiles of four standard HS and two fungi-produced humic-like substances (HLS) were established with Trolox equivalent antioxidant capacity (TEAC) approach using the ABTS decolorization assay. Three pH levels (3.75, 4.25, and 6.80) and a broad range of humic material concentrations (0.5–10 mg L−1) were examined. The data were divided into intervals and fit using exponential functions to evaluate the endpoint AOCs as well as rate constants for the reaction of humic materials with the ABTS radical cation. To further explore the nature of the antioxidant activities of humic materials, the physicochemical features and antioxidant activities of humic compounds were subjected to correlation analysis.
Results and discussion
Our results demonstrated that during the first 40 min, the determined AOCs did not exceed 50 % of the endpoint AOCs for studies of humic materials, indicating that short-term measures of the AOCs of humic materials provide artificially low values due to the presence of slow-acting antioxidant compounds. Due to the instability of ABTS•+ at neutral and alkaline pH values, only the fast antioxidant moieties of humic materials can be assessed with ABTS decolorization approach under these conditions. Our results show that at acidic pH, the antioxidant activity of HLS is mainly related to the presence of nitrogen-containing groups rather than phenols. However, for HS, both nitrogen-containing compounds and phenolic compounds should be considered.
To obtain clearer information concerning the AOC of humic materials, kinetic profiles should first be established, and then endpoint measurements should be taken at a time when the reaction has reached, or at least neared, the endpoint.
KeywordsABTS Antioxidant capacity Humic-like substances Humic substances Kinetics
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