Richard A. Royer

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.

Reactions of ferrous iron with hematite

Abstract The adsorption of Fe(II) onto hematite was measured as a function of pH, surface area, and time. The effects of anions (chloride, sulfate, or nitrate) and of Zn(II) were also determined. All experiments were conducted under strict anoxic conditions with 5 or 30 days for equilibration. Results showed that immobilization of Fe(II) on hematite consists of a fast sorption process and one or more slow processes, which probably include both sorption and formation of new phases. Sorption occurred at pH values as low as 4, which has not been reported in existing literature. Some Fe(II) could not be extracted after 20 h with 0.5 N HCl. In the presence of 0.01 M NaCl, all of the added Fe(II) was recovered when pH was below 6, but either 100% or less than 25% of added Fe(II) was recovered when pH was greater than 6. These results are consistent with auto-catalytic formation of magnetite, which was stable relative to hematite for pH above 5.9. However, when sulfate was greater than 1 mM, unextracted Fe(II) was observed at pH above 5 where only approximately 15% of added Fe(II) was recovered by a 0.5 N HCl extraction; these results could not be explained by precipitation of magnetite nor of known sulfate phases. Based on these results, existing models for adsorption of Fe(II) onto ferric oxides (based on experiments of several hours to a day) are not accurate for prediction of environmentally significant Fe(II) reactions with ferric oxides, when much longer times are available for reaction. There was no competition between Zn(II) and Fe(II) for 0.25 mM or less and 90 m 2 l −1 hematite. Zn(II) was completely recovered using 0.5 N HCl for every condition that was tested.

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.

Reaction-based modeling of quinone-mediated bacterial iron(III) reduction

This paper presents and validates a new paradigm for modeling complex biogeochemical systems using a diagonalized reaction-based approach. The bioreduction kinetics of hematite (α-Fe2O3) by the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens strain CN32 in the presence of the soluble electron shuttling compound anthraquinone-2,6-disulfonate (AQDS) is used for presentation/validation purposes. Experiments were conducted under nongrowth conditions with H2 as the electron donor. In the presence of AQDS, both direct biological reduction and indirect chemical reduction of hematite by bioreduced anthrahydroquinone-2,6-disulfonate (AH2DS) can produce Fe(II). Separate experiments were performed to describe the bioreduction of hematite, bioreduction of AQDS, chemical reduction of hematite by AH2DS, Fe(II) sorption to hematite, and Fe(II) biosorption to DMRB. The independently determined rate parameters and equilibrium constants were then used to simulate the parallel kinetic reactions of Fe(II) production in the hematite-with-AQDS experiments. Previously determined rate formulations/parameters for the bioreduction of hematite and Fe(II) sorption to hematite were systematically tested by conducting experiments with different initial conditions. As a result, the rate formulation/parameter for hematite bioreduction was not modified, but the rate parameters for Fe(II) sorption to hematite were modified slightly. The hematite bioreduction rate formulation was first-order with respect to hematite ”free“ surface sites and zero-order with respect to DMRB based on experiments conducted with variable concentrations of hematite and DMRB. The AQDS bioreduction rate formulation was first-order with respect to AQDS and first-order with respect to DMRB based on experiments conducted with variable concentrations of AQDS and DMRB. The chemical reduction of hematite by AH2DS was fast and considered to be an equilibrium reaction. The simulations of hematite-with-AQDS experiments were very sensitive to the equilibrium constant for the hematite-AH2DS reaction. The model simulated the hematite-with-AQDS experiments well if it was assumed that the ferric oxide “surface” phase was more disordered than pure hematite. This is the first reported study where a diagonalized reaction-based model was used to simulate parallel kinetic reactions based on rate formulations/parameters independently obtained from segregated experiments.

Manganese oxide reduction in laboratory microcosms

Manganese biogeochemistry holds special interest for the characterization of passive treatment systems designed to treat acidic mine waters while meeting enforceable effluent discharge limits set for manganese. In the present study, an initial anoxic enrichment culture was developed for use as an inoculum in experimental systems. Standard anoxic microcosms capable of reducing manganese from Mn4+ to Mn2+ were established from the initial enrichment and altered to study the effects of electron acceptor availability and inhibitors on manganese reduction. Manganese reduction was not significantly inhibited in aerobic and nitrate amended microcosms; however, systems amended with metabolic inhibitors (sodium azide or sodium molybdate) exhibited significant inhibition of manganese reduction relative to standard microcosms. The presence of iron was found to influence the partitioning of reduced manganese with adsorption becoming more important with increasing iron to manganese ratios.