Influence of Biogenic Fe(II) on the Extent of Microbial Reduction of Fe(III) in Clay Minerals Nontronite, Illite, and Chlorite
Microbial reduction of Fe(III) in clay minerals is an important process that affects properties of clay-rich materials and iron biogeochemical cycling in natural environments. Microbial reduction often ceases before all Fe(III) in clay minerals is exhausted. The factors causing the cessation are, however, not well understood. The objective of this study was to assess the role of biogenic Fe(II) in microbial reduction of Fe(III) in various clay minerals. Bioreduction experiments were performed in a batch system, where lactate was used as the sole electron donor, Fe(III) in clay minerals as the sole electron acceptor, and Shewanella putrefaciens CN32 as the mediator with and without an electron shuttle AQDS. Our results showed that bioreduction activity ceased within two weeks with variable extents of bioreduction of structural Fe(III) in clay minerals. When fresh CN32 cells were added to the old cultures (6 months), bioreduction resumed and extents increased. This result indicated that the previous cessation of Fe(III) bioreduction was not necessarily due to the exhaustion of bioavailable Fe(III) in the mineral structure, and suggested that the changes of cell physiology or solution chemistry, such as Fe(II) production during microbial reduction, affected the extent of bioreduction. To investigate the effect of Fe(II) production on Fe(III) bioreduction, a typical bioreduction process (consisting of lactate, clay, cells and AQDS) was separated into two steps: 1. AQDS was reduced by cells in the absence of clay but in the presence of variable Fe(II) concentrations; 2. reduction of Fe(III) in clays by biogenic AH2DS in the absence of cells. The inhibitory effect of Fe(II) on CN32 activity was confirmed. TEM analysis revealed a thick electron dense halo surrounding the cell surfaces that most likely resulted from Fe(II) sorption/precipitation. Such electron dense materials might have blocked or interfered electron transfers on cell surfaces. The inhibitory effect of Fe(II) was also observed in AH2DS reduction of clay Fe(III). The reduction extent consistently decreased with an increasing concentration of presorbed Fe(II) (onto clay surfaces) at the start of reduction experiments. The relative reduction extent (i.e., reduction extent after normalization to the reduction extent when spiked Fe(II) was zero) was similar for all clay minerals studied and showed a systematic decrease with increasing clay-sorbed Fe(II) concentration. These results suggest a similar inhibitory effect of clay-sorbed Fe(II) on the reduction extent for different clay minerals. An equilibrium thermodynamic model was established with independently estimated parameters to evaluate whether the cessation of Fe(III) reduction by AH2DS was due to the exhaustion of reaction free energy. Model-calculated reduction extents were, however, over 50% higher than experimentally measured, indicating that other factors, such as blockage of the electron transfer chain and mineralogy, restricted the reduction extent. This study also revealed that the relative reducibility of Fe(III) in different clay was as follows: nontronite > chlorite > illite. This order is qualitatively consistent with the differences in crystal chemistry of these minerals.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 909233
- Report Number(s):
- PNNL-SA-49307; GCACAK; 4691; KP1302000; TRN: US200722%%1046
- Journal Information:
- Geochimica et Cosmochimica Acta, 71(5):1145-1158, Vol. 71, Issue 5; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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