Redox properties of standard humic acids

Abstract

Humic acid (HA) participates in a variety of electron transfer reactions, but its fundamental redox properties, such as formal electrode potential and oxidation capacity, are not well characterized. Specific functionalities in HA, such as semiquinone free radicals, have been hypothesized to participate in redox reactions; however, few supportive data exist. To address these issues, three standard humic acids (Suwannee River, soil, and peat) were titrated at pH 5 and 7 with I 2 as an oxidant under an inert Ar atmosphere at 25°C. The formal electrode potential of the three HA samples at pH 5 was 0.778±0.017 V, decreasing with pH by −20.0±3.2 mV per pH unit. The oxidation capacity of HA (moles of HA charge per unit mass transferred to a strong oxidant) ranged from 1.09 to 6.5 mol c kg −1 at pH 5 and from 3.3 to 11.5 mol c kg −1 at pH 7. The increasing order of oxidation capacity for the humic acids was: soil HA<peat HA<Suwannee River HA. A positive correlation between oxidation capacity and the stable free radical content of HA was found, but the latter property could account for only a tiny fraction of the observed moles of electrons transferred between HA and the I 2 oxidant. Based on these results and those of previous research, a hypothesis for abiotic electron transfer reactions of HA involving complexed Fe(III) as a mediator was proposed.