Redox properties of standard humic acids

Humic acid (KA) participates in a variety of electron transfer reactions, b ut its fundamental redox properties, such as formal electrode potential and oxidation capacity, are not well characterized. Specific functionalities i n HA, such as semiquinone free radicals, have been hypothesized to particip ate in redox reactions; however, few supportive data exist. To address thes e 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 degreesC. The formal electrode potential of the three HA samples at p H 5 was 0.778 +/- 0.017 V, decreasing with pH by - 20.0 +/- 3.2 mV per pH u nit. The oxidation capacity of HA (moles of HA charge per unit mass transfe rred 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 complexe d Fe(III) as a mediator was proposed. (C) 2001 Published by Elsevier Scienc e B.V.