Rates and Extent of Reduction of Fe(III) Compounds and O2 by Humic Substances

Humic substances (HS) can be reduced by microorganisms and oxidized by electron acceptors such as Fe(III) or O2. However, redox reactions between HS and highly crystalline Fe(III) minerals and O2 have not yet been quantified. We therefore determined the rates and extent of goethite and hematite reduction by HS in comparison to those of dissolved and poorly crystalline Fe(III) compounds and O2. Although nonreduced HS transferred significant amounts of electrons only to dissolved Fe(III) citrate and ferrihydrite, reduced HS additionally reduced goethite and hematite. The extent of reduction depended on the redox potentials of the Fe(III) compounds. Fewer electrons were transferred from HS to O2 than to Fe(III) despite the more positive redox potential of the O2/H2O redox couple. Reoxidation of reduced HS by O2 took place within minutes and yielded reoxidized HS that were still more reduced than nonreduced HS, indicating that some reduced moieties in HS are protected from reoxidation by O2. Our data suggests (i) reduction of crystalline Fe(III) minerals by reduced HS has to be considered in the environmental electron transfer network, (ii) exposure of reduced HS to O2 does not reoxidize HS completely within short time frames, and therefore, (iii) HS electron shuttling to Fe(III) can occur even in the presence of O2.