Kinetic considerations on the antioxidant properties of humic substances: An electrochemical approach

Humic substances (HS) play a key role in a variety of environmental redox processes including the reduction of organic and inorganic pollutants. However, their complex and heterogeneous composition makes the characterization of their redox activity challenging. In this work, electrochemical methods (cyclic and square-wave voltammetry) were used to study the kinetics of the redox reaction between HS and the mediator 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) under pseudo-first order conditions. ABTS was used to mediate the electro transfer from redox active moieties (phenolic and quinonoid) to the glassy carbon working electrode (WE), since the direct analysis of HS is harmed by slow electron transfer kinetics at the WE surface. This study introduces a practical method for determining kinetic constants for the reaction between ABTS and HS by leveraging the catalytic behaviour of the ABTS oxidative peak in the presence of HS, since a clearly defined peak can be measured in the anodic scan at 0.57 V. Both ABTS and HS samples were individually electrochemically characterized over a range of pH and concentration. It was observed that the redox state of HS from different origins could be quantified by using cyclic voltammetry in solutions containing small concentrations of ABTS. Furthermore, second-order homogeneous constants (k'_f) were able to be calculated and were in the range of 0.90-4.03 L s^-1 g^-1. Considering that these differences can be attributed to different HS samples and different HS types (humic and fulvic acids), the obtained results support the relevance of the proposed approach for an accurate assessment of reaction kinetics between HS and radical species.