Wednesday, November 7, 2007 - 1:45 PM
287-2
Enhancement of Fe(III) Bioreduction in Soils in the Presence of Added Humic Substances.
Humic substances (HS) have been proposed to facilitate Fe(III) bioreduction in soils and sediments through electron shuttling (ES) and Fe complexation. The ES ability of HS has been suggested to be related to their capacity for transferring electron charge, which is termed their reducing capacity (RC). We found that both HS and mixtures of HS with differing chemical properties (especially carboxylic acid groups and aromatic carbon) but having similar RC exhibited comparable ES capacity and enhanced the Fe(III) bioreduction process by both ES and Fe complexation. Two bioreduction experiments, one utilizing Elliot soil humic acid (ESHA), and another utilizing a mixture of Pahokee Peat humic acid (PPHA) and Suwannee River natural organic matter (SRNOM), were conducted by adding 400 mg L-1 of ESHA or 600 mg L-1 of the mixture (i.e. 240 and 360 mg L-1 of PPHA and SRNOM, respectively) to 20 g L-1 air-dried tropical soil (surface sample of a well-characterized Ultisol from Puerto Rico) and incubating the suspension anaerobically for 31 days. The amounts of HS added were arranged to provide the same RC charge concentration in both experiments (0.225 mM). Production of aqueous Fe(II), determined by UV-Visible Spectrometry, was increased by two- and four fold in the ESHA and PPHA+SRNOM experiments relative to the corresponding controls. The lesser Fe(III) bioreduction enhancement by ESHA as compared to PPHA+SRNOM can be attributed to lesser Fe complexation. The greater effect of Fe complexation for PPHA + SRNOM can be ascribed to the much higher concentration of Fe-complexing carboxylic acid groups in PPHA+SRNOM as compared to that in ESHA. Therefore, although we can relate ES ability to the RC of HS, the intervention of the Fe complexation mechanism can cause differing behavior during Fe(III) bioreduction.
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