Comparisons of structural iron reduction in smectites by bacteria and dithionite: II. A variable-temperature Mössbauer spectroscopic study of Garfield nontronite

Abstract

The reduction of structural Fe in smectite may be mediated either abiotically by reaction with chemical reducing agents or biotically by reaction with various bacterial species. The effects of abiotic reduction on clay surface chemistry are much better known than the effects of biotic reduction, and differences between them are still in need of investigation. The purpose of the present study was to compare the effects of dithionite (abiotic) and bacteria (biotic) reduction of structural Fe in nontronite on the clay structure as observed by variabletemperature Mössbauer spectroscopy. Biotic reduction was accomplished by incubating Na-saturated Garfield nontronite (sample API 33a) with Shewanella oneidensis strain MR-1 (Fe II /total Fe achieved was ~17 %). Partial abiotic reduction (Fe II /total Fe ~23 %) was achieved using pH-buffered sodium dithionite. The nontronite was also reduced abiotically to Fe II /total Fe ~96 %. Parallel samples were reoxidized by bubbling O 2 gas through the reduced suspensions at room temperature prior to Mössbauer analysis at 77 and 4 K. At 77 K, the reduction treatments all gave spectra composed of doublets for structural Fe II and Fe III in the nontronite. The spectra for reoxidized samples were largely restored to that of the unaltered sample, except for the sample reduced to 96 %. At 4 K, the spectrum for the 96 % reduced sample was highly complex and clearly reflected magnetic order in the sample. When partially reduced, the spectrum also exhibited magnetic order, but the features were completely different depending on whether reduced biotically or abiotically. The biotically reduced sample appeared to contain distinctly separate domains of Fe II and Fe III within the structure, whereas partial abiotic reduction produced a spectrum representative of Fe II -Fe III pairs as the dominant domain type. The 4 K spectra of the partially reduced, fully reoxidized samples were virtually the same as at 77 K, whereas reoxidation of the 96 % reduced sample produced a spectrum consisting of a magnetically ordered sextet with a minor contribution from a Fe II doublet, indicating significant structural alterations compared to the unaltered sample.

Pertechnetate (TcO4−) reduction by reactive ferrous iron forms in naturally anoxic, redox transition zone sediments from the Hanford Site, USA

“…5b,Fig. EA-5b), with concurrent broadening of the central feature, was consistent with phyllosilicate Fe magnetic ordering (Ribeiro et al, 2009). The rapid and extensive dissolution of Fe(II), Si, and Al from the anoxic sediment in 0.5 M HCl (Table 1; Fig.…”

Low-temperature feldspar and illite formation through bioreduction of Fe(III)-bearing smectite by an alkaliphilic bacterium

“…Nevertheless, the ultimate reduction extent (26.2%) was similar to that achieved by S. putrefaciens CN32 (Jaisi et al, 2005;Zhang et al, 2007a). Incomplete reduction of structural Fe(III) in various clay minerals has been observed for other types of bacteria (Kostka et al, 1996;Dong et al, 2009;Ribeiro et al, 2009;Bishop et al, 2011;Liu et al, 2011Liu et al, , 2012Liu et al, , 2014Dong, 2012). The combined effects of electron transfer pathway, elemental toxicity, and energetics might have been responsible for the incomplete bioreduction of structural Fe(III) in clay minerals.…”

“…Because of the overall negative charge of bacterial surfaces, microorganisms probably preferably attach to clay edges due to electrostatic attraction. Therefore, the electron transfer pathway should have been from the smectite edge to the internal structure of nontronite (Dong et al, 2009;Ribeiro et al, 2009;Stucki, 2011). In doing so, much of the structurallycoordinated Fe(III) in nontronite may not be accessible to iron-reducing microorganisms.…”

Evaluation of redox-active iron sites in smectites using middle and near infrared spectroscopy

“…However, the overall silicate structure was preserved and structural iron dissolution during chemical reduction is usually small (i.e., less than 4% of the total iron (Stucki et al, 1984a;Komadel et al, 1995;Ribeiro et al, 2009). Therefore, structural iron in smectites is considered to represent a renewable source of reduction equivalents for biogeochemical processes and the transformation of contaminants.…”