Effect of natural organic matter on zinc inhibition of hematite bioreduction by Shewanella putrefaciens CN32.

The effect of zinc on the biological reduction of hematite (alpha-Fe2O3) by the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens CN32 was studied in the presence of four natural organic materials (NOMs). Experiments were performed under non-growth conditions with H2 as the electron donor and zinc inhibition was quantified as the decrease in the 5 d extent of hematite bioreduction as compared to no-zinc controls. Every NOM was shown to significantly increase zinc inhibition during hematite bioreduction. NOMs were shown to alter the distribution of both biogenic Fe(II) and Zn(II) between partitioned (hematite and cell surfaces) and solution phases. To further evaluate the mechanism(s) of NOM-promoted zinc inhibition, similar bioreduction experiments were conducted with nitrate as a soluble electron acceptor, and hematite bioreduction experiments were conducted with manganese which was essentially non-inhibitory in the absence of NOM. The results suggest that Me(II)-NOM complexes may be specifically inhibitory during solid-phase bioreduction via interference of DMRB attachment to hematite through the formation of ternary Me(II)-NOM-hematite complexes.

Inhibition of biological reductive dissolution of hematite by ferrous iron.

Bacterial dissimilatory iron reduction is self-inhibited by the production of ferrous [Fe(II)] iron resulting in diminished iron reduction as Fe(II) accumulates. Experiments were conducted to investigate the mechanisms of Fe(II) inhibition employing the dissimilatory metal-reducing bacterium Shewanella putrefaciens strain CN32 under nongrowth conditions in a system designed to minimize precipitation of ferrous iron minerals. After an initial period (ca. 1 day) of relatively rapid iron reduction, hematite reduction rates were controlled by mass transfer of Fe(II). Experiments in which hematite was equilibrated with Mn(II) prior to inoculation indicated that the observed inhibition was not due to Fe(II) sorption. At longer times, soluble Fe(II) accumulated such that the reaction was slowed due to a decreased thermodynamic driving force. The thermodynamic evaluation also supported the prior conclusion that hydrated hematite surface sites may yield substantially more energy during bioreduction than "bulk" hematite. For well-mixed conditions, the rates of hematite reduction were directly proportional to the biologically available reaction potential.

Sorption kinetics of Fe(II), Zn(II), Co(II), Ni(II), Cd(II), and Fe(II)/Me(II) onto hematite.

The reactions of Fe(II) and other divalent metal ions including Zn, Co, Ni, and Cd on hematite were studied in single and competitive binary systems with high sorbate/sorbent ratios in 10 mM PIPES (pH 6.8) solution under strict anoxic conditions. Adsorbed Me(II) was defined as extractable by 0.5 N HCl within 20 h, and fixed Me(II) was defined as the additional amount that was extracted by 3.0 N HCl within 7 days. Binary systems contained Fe(II) plus a second metal ion. The extent of uptake of divalent metal ions by hematite was in order of Fe> or =Zn>Co> or =Ni>Cd. For all metals tested, there was an instantaneous adsorption followed by a relatively slow stage that continued for the next 1-5 days. This sequence occurred in both single and binary systems, and could have been due to a variety of sorption site types or due to slow conversion from outer- to inner-sphere surface complexes due to increasing surface charge. Sorption competition was observed between Fe(II) and the other metal ions. The displacement of Fe(II) by Me(II) was in order of Ni approximately Zn>Cd, and the displacement of Me(II) by Fe(II) was in order of Cd>Zn approximately Ni>Co. Fixed Fe(II) was in order of Fe+Co (20%)>Fe+Cd (6%)>Fe approximately Zn (4%)>Fe approximately Ni (4%) after 30 days. There was no fixation for the other metals in single or binary systems.

The roles of natural organic matter in chemical and microbial reduction of ferric iron.

Although natural organic matter (NOM) is known to be redox reactive, the roles and effectiveness of specific functional groups of NOM in metal reduction are still a subject of intense investigation. This study entails the investigation of the Fe(III) reduction kinetics and capacity by three fractionated NOM subcomponents in the presence or absence of the dissimilatory metal reducing bacterium Shewanella putrefaciens CN32. Results indicate that NOM was able to reduce Fe(III) abiotically; the reduction was pH-dependent and varied greatly with different fractions of NOM. The polyphenolic-rich NOM-PP fraction exhibited the highest reactivity and oxidation capacity at a low pH (<4) as compared with the carbohydrate-rich NOM-CH fraction and a soil humic acid (soil HA) in reducing Fe(III). However, at a pH>4, soil HA showed a relatively high oxidation capacity, probably resulting from its conformational and solubility changes with an increased solution pH. In the presence of S. putrefaciens CN32, all NOM fractions were found to enhance the microbial reduction of Fe(III) under anaerobic, circumneutral pH conditions. Soil HA was found to be particularly effective in mediating the bioreduction of Fe(III) as compared with the NOM-PP or NOM-CH fractions. NOM-CH was the least effective because it was depleted in both aromatic and polyphenolic organic contents. However, because both soil HA and NOM-PP contain relatively high amounts of aromatic and phenolic compounds, results may indicate that low-molecular-weight polyphenolic organics in NOM-PP were less effective in mediating the bioreduction of Fe(III) at circumneutral pH than the high-molecular-weight polycondensed, conjugated aromatics present in soil HA. These research findings may shed additional light in understanding of the roles and underlying mechanisms of NOM reactions with contaminant metals, radionuclides, and other toxic chemicals in the natural environment.

Enhancement of hematite bioreduction by natural organic matter.

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.

Enhancement of biological reduction of hematite by electron shuttling and Fe(II) complexation.

Natural organic matter (NOM) enhancement of the biological reduction of hematite (alpha-Fe2O3) by the dissimilatory iron-reducing bacterium Shewanella putrefaciens strain CN32 was investigated under nongrowth conditions designed to minimize precipitation of biogenic Fe(II). Hydrogen served as the electron donor. Anthraquinone-2,6-disulfonate (AQDS), methyl viologen, and methylene blue [quinones with an Ew0 (pH 7) of 0.011 V or less], ferrozine [a strong Fe(II) complexing agent], and characterized aquatic NOM (Georgetown NOM or Suwannee River fulvic acid) enhanced bioreduction in 5-day experiments whereas 1,4-benzoquinone (Ew0 value = 0.280 V) did not. A linear relationship existed between total Fe(II) produced and concentrations of ferrozine or NOM but not quinones, except in the case of methylene blue. Such a linear relationship between Fe(II) and methylene blue concentrations could be due to the systems being far undersaturated with respect to methylene blue or the loss of the thermodynamic driving force. A constant concentration of AQDS and variable concentrations of ferrozine produced a linear relationship between total Fe(II) produced and the concentration of ferrozine. Enhancement effects of both AQDS and ferrozine were additive. NOM may serve as both an electron shuttle and an Fe(II) complexant; however, the concentration dependence of hematite reduction with NOM was more similar to ferrozine than quinones. NOM likely enhances hematite reduction initially by electron shuttling and then further by Fe(II) complexation, which prevents Fe(II) sorption to hematite and cell surfaces.