The reduction of dissolved iron species by humic acid and subsequent production of reactive oxygen species

Humic acid (HA) is distributed throughout the environment. Many of its properties are known, but its exact structure and function are still in question. To understand the chemistry and biochemical implications of HA on living systems we have focused on understanding radical chemistries of HA in the presence of molecular oxygen and hydrogen peroxide. The work presented here uses EPR spectroscopy to demonstrate that Fe(III) is reduced to Fe(II) by peat derived IIA (PHA), and that Fe(III) is reduced by PHA more efficiently when bound to chelators. However, desferroxamine (DFOM) sequesters Fe(III) from being reduced by PHA and inhibits the Fenton reaction. The reduction of molecular oxygen by Fe(II) gives H2O2 as a significant product. The hydroxyl radical is also produced in this reduction process and is trapped by 5-Diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO). In addition to the formation of H2O2 and OH, another oxidant also appears to be present. Although not isolated, this species displays the characteristics of the ferryl ion. (C) 2002 Elsevier Science Ltd. All rights reserved.