Enhancement of Hematite Bioreduction by Natural Organic Matter

Abstract

The effects of natural organic matter (NOM), ferrozine, and AQDS (anthraquinone-2,6-disulfonate) on the reduction of hematite (alpha-Fe2O3) by Shewanella putrefaciens CN32 were studied. It has been proposed that NOM enhances the reduction of Fe(III) by means of electron shuttling or by Fe(II) complexation. Previously both mechanisms were studied separately using "functional analogues" (AQDS for electron shuttling and ferrozine for complexation) and are presently compared with seven different NOMs. AQDS enhanced hematite reduction within the first 24 h of incubation, and this had been ascribed to electron shuttling. Most of the NOMs enhanced hematite reduction after 1 day of incubation indicating that these materials could also serve as electron shuttles. The effect of ferrozine was linear with concentration, and all of the NOMs exhibited this behavior. Fe(II) complexation only enhanced hematite reduction after sufficient Fe(II) had accumulated in the system. Fe(II) complexation appeared to alleviate a suppression of the hematite reduction rate caused by accumulation of Fe(II) in the system. Addition of Fe(II) to the hematite suspension, prior to inoculation with CN32, significantly inhibited hematite reduction and greatly diminished the effects of all of the organic materials, although some enhancement was observed due to addition of anthroquinone-2,6-disulfonate. These results demonstrate that NOM can enhance iron reduction by electron shuttling and by complexation mechanisms.

Fe(III) Oxide Reactivity Toward Biological versus Chemical Reduction

“…This conceptual model is consistent with a recent analysis of the kinetics of hematite reduction by S. putrefaciens strain CN32 (34,35), which showed that initial rates of reduction were under kinetic control (presumably limited by the rate of electron transfer from FeRB cells to the oxide), whereas the long-term extent of reduction was limited by mass transfer of Fe(II) away from oxide/FeRB surfaces. As discussed in Roden and Zachara (7) and reviewed in Roden and Urrutia (17), there is a general relationship between oxide surface area and long-term extent of oxide reduction in closed reaction systems, which results from the function of oxide surfaces as a repository for sorbed and/or surface-precipitated biogenic Fe(II).…”

Developments in Bioremediation of Soils and Sediments Polluted with Metals and Radionuclides. 3. Influence of Chemical Speciation and Bioavailability on Contaminants Immobilization/Mobilization Bio-processes

“…NOM was found to greatly enhance the reduction of a key element in soil and sediment such as Fe(III) metals or Fe(III) oxides by a variety of microorganisms (Lovley 1996;Lovley et al 1998;Nevin & Lovley 2000;Royer et al 2002;Chen et al 2003;Kappler et al 2004). Lovley (1996) postulated that humic substances were likely acting as electron mediators or shuttles between microorganisms and Fe(III) or Fe(III)-oxide minerals.…”

“…Iron-reducing microorganisms such as Shewanella putrefaciens, G. metallireducens, Shewanella alga, and a variety of fermenting bacteria have all been shown to use humic substances as terminal electron acceptors. By incubating NOM with S. putrefaciens, the equivalent Fe(III)-reducing capacity of NOM was reported to range from 0.1 to 0.6 mol/kg (Royer et al 2002;Chen et al 2003). In addition, Chen et al (2003) showed that NOM was able to reduce Fe(III) abiotically.…”

“…On the other hand, the reduced forms of Cr(III) and U(IV) are only sparingly soluble and are strongly retained by soil and sediments (Fendorf 1995;Anderson and Lovley 2002;Rifkin et al 2004). While many studies to date have focused on direct microbial reduction of Cr(VI) and U(VI) (Lovley et al 1991;Shen et al 1996;Chen and Hao, 1998;Abdelouas et al 2000;Fredrickson et al 2000), few studies have examined the effect of NOM on the enhanced microbial reduction of Cr(VI) or U(VI), as has been observed for the reduction of Fe(III) or Fe(III)-oxide minerals (Lovley 1996;Royer et al 2002;Chen et al 2003). Therefore, of particular interest is the possibility that NOM-mediated reduction of Cr(VI) and U(VI) may lead to more rapid immobilization of these metals in soil and thus the remediation of a contaminated site Zhilin et al 2004;Banks et al 2005).…”

Ferrihydrite-associated organic matter (OM) stimulates reduction by <i>Shewanella oneidensis</i> MR-1 and a complex microbial consortia

“…Humic acids should therefore be more effective in electron shuttling than natural soil organic matter. Previous studies have shown that the presence of dissolved humic acids leads to complexation of Fe(II) (Royer et al, 2002), and complexation and dissolution of Fe(III) (Jones et al, 2009), but also potentially enhance Fe(III) reduction via electron shuttling (Hansel et al, 2004;Jiang and Kappler, 2008;Lovley et al, 1996;Roden et al, 2010). Furthermore, the concentrations of dissolved humic acid or the mineral / humic acid ratios have been shown by some studies to increase Fe(III) reduction rates while other studies have not reproduced this result (Amstaetter et al, 2012;Jiang and Kappler, 2008).…”