Reduction Of Hydrous Ferric Oxide Research Articles

Reductive Transformation of Fe(III) (oxyhydr)Oxides by Mesophilic Homoacetogens in the Genus Sporomusa.

Microbial reduction of iron contributes to the dissolution and transformation of iron-containing minerals in nature. Diverse groups of homoacetogenic bacteria (homoacetogens) have been reported to reduce insoluble Fe(III) oxides, such as hydrous ferric oxide (HFO), an Fe(III) mineral commonly found in soils and sediments. Several members of genus Sporomusa reportedly oxidize Fe(0), indicating the presence of an extracellular electron-uptake mechanism. However, the ability of the genus to reduce insoluble Fe(III) oxides is limited, and the underlying reduction mechanism remains to be elucidated. In this study, the HFO reduction ability of three Sporomusa spp. (Sporomusa sp. strain GT1, Sporomusa sphaeroides, and Sporomusa ovata) and a homoacetogen of a different genus (Acetobacterium woodii) were assayed under organotrophic (ethanol) and lithotrophic (H2 + CO2) conditions without a chelator or reducing reagent. All tested homoacetogens showed acetogenic growth and concomitant reduction of HFO under both organotrophic and lithotrophic conditions. Analysis of the growth stoichiometry showed that Fe(III) reduction does not support direct energy conservation, thereby indicating that Fe(III) reduction is a side reaction of acetogenesis to dissipate the excess reducing power. HFO was reduced to a soluble Fe(II) form by microbial activity. In addition, we observed that strain GT1, S. sphaeroides, and S. ovata reduced crystalline Fe(III) oxides, and HFO was reductively transformed into magnetite (Fe3O4) under phosphate-limiting conditions. Separation of HFO by a dialysis membrane still permitted Fe(II) production, although the reduction rate was decreased, suggesting that Fe(III) reduction is at least partially mediated by soluble redox compound(s) secreted from the cells. Finally, culture experiments and comparative genomic analysis suggested that electron transfer by flavins and multiheme c-type cytochrome were not directly correlated with Fe(III) reduction activity. This study reveals the capability of Sporomusa spp. in the reductive transformation of iron mineral and indicates the potential involvement of these organisms in iron and other mineral cycles in nature.

Sources of salinity and arsenic in groundwater in southwest Bangladesh.

BackgroundHigh salinity and arsenic (As) concentrations in groundwater are widespread problems in the tidal deltaplain of southwest Bangladesh. To identify the sources of dissolved salts and As, groundwater samples from the regional shallow Holocene aquifer were collected from tubewells during the dry (May) and wet (October) seasons in 2012–2013. Thirteen drill cores were logged and 27 radiocarbon ages measured on wood fragments to characterize subsurface stratigraphy.ResultsDrill cuttings, exposures in pits and regional studies reveal a >5 m thick surface mud cap overlying a ~30 m thick upper unit of interbedded mud and fine sand layers, and a coarser lower unit up to 60 m thick dominated by clean sands, all with significant horizontal variation in bed continuity and thickness. This thick lower unit accreted at rates of ~2 cm/year through the early Holocene, with local subsidence or compaction rates of 1–3 mm/year. Most tubewells are screened at depths of 15–52 m in sediments deposited 8000–9000 YBP. Compositions of groundwater samples from tubewells show high spatial variability, suggesting limited mixing and low and spatially variable recharge rates and flow velocities. Groundwaters are Na–Cl type and predominantly sulfate-reducing, with specific conductivity (SpC) from 3 to 29 mS/cm, high dissolved organic carbon (DOC) 11–57 mg/L and As 2–258 ug/L, and low sulfur (S) 2–33 mg/L.ConclusionsGroundwater compositions can be explained by burial of tidal channel water and subsequent reaction with dissolved organic matter, resulting in anoxia, hydrous ferric oxide (HFO) reduction, As mobilization, and sulfate (SO4) reduction and removal in the shallow aquifer. Introduction of labile organic carbon in the wet season as rice paddy fertilizer may also cause HFO reduction and As mobilization. Variable modern recharge occurred in areas where the clay cap pinches out or is breached by tidal channels, which would explain previously measured 14C groundwater ages being less than depositional ages. Of samples collected from the shallow aquifer, Bangladesh Government guidelines are exceeded in 46 % for As and 100 % for salinity.

Promotion of Iron Oxide Reduction and Extracellular Electron Transfer in Shewanella oneidensis by DMSO

The dissimilatory metal reducing bacterium Shewanella oneidensis MR-1, known for its capacity of reducing iron and manganese oxides, has great environmental impacts. The iron oxides reducing process is affected by the coexistence of alternative electron acceptors in the environment, while investigation into it is limited so far. In this work, the impact of dimethyl sulphoxide (DMSO), a ubiquitous chemical in marine environment, on the reduction of hydrous ferric oxide (HFO) by S. oneidensis MR-1 was investigated. Results show that DMSO promoted HFO reduction by both wild type and ΔdmsE, but had no effect on the HFO reduction by ΔdmsB, indicating that such a promotion was dependent on the DMSO respiration. With the DMSO dosing, the levels of extracellular flavins and omcA expression were significantly increased in WT and further increased in ΔdmsE. Bioelectrochemical analysis show that DMSO also promoted the extracellular electron transfer of WT and ΔdmsE. These results demonstrate that DMSO could stimulate the HFO reduction through metabolic and genetic regulation in S. oneidensis MR-1, rather than compete for electrons with HFO. This may provide a potential respiratory pathway to enhance the microbial electron flows for environmental and engineering applications.

Groundwater pollution with special focus on arsenic, Dera Ghazi Khan-Pakistan

Thirty-two representative samples of groundwater collected randomly from Dera Ghazi Khan (City)-Pakistan were analyzed for physical and chemical quality parameters with special attention to life threatening parameter arsenic (As). The arsenic (As) concentration in groundwater of study area was found from 1 to 29μgL−1, which lies in the satisfactory range recommended by WHO (i.e. <10μgL−1), for most of the samples examined except for few ones. This mild enrichment of Arsenic in few sites/wells is the result of most probably microbial contamination, metal leaching from the mine wastes and reduction of hydrous ferric oxide (HFO). The arsenic (As) trend with respect to physicochemical water quality parameters was observed during the study. Apart from arsenic (As), physical parameters (pH, EC, TDS, turbidity) and chemical parameters (alkalinity, CO3, HCO3, Cl, F, NO3, SO4, Na, K, total hardness (TH), Ca, Mg and microbial contamination) of potential concern for drinking water were estimated by using analytical techniques. Electrical conductance (EC) and total dissolved solids (TDS) values of groundwater were extremely high from WHO guideline values with no appreciable change in pH values for virtually all samples. Likewise, the chemical quality of groundwater samples also goes beyond the WHO guideline values for parameters such as chlorides, sulfates, sodium, calcium, magnesium and total hardness (TH). During the study it was discovered that the tropical or geographical reason works well behind this undesirable chemical behavior of groundwater.

Fe(III) oxide reduction and carbon tetrachloride dechlorination by a newly isolatedKlebsiella pneumoniaestrain L17

To isolate an iron-reducing bacterium and examine its ability of Fe(III) oxide reduction and dechlorination. A fermentative facultative anaerobe, strain L17 isolated from subterranean sediment, can reduce Fe(III) oxides and carbon tetrachloride (CT). It was identified as Klebsiella pneumoniae by 16S rRNA sequence analysis. Strain L17 can metabolize fermentable substrates such as citrate, glycerol, glucose and sucrose coupled with the reduction of hydrous ferric oxide, goethite, lepidocrocite and hematite. Fe(III) reduction was influenced by crystal structure of Fe(III) oxide, type of fermentable substrate, metabolic status of the strain, and significantly enhanced by addition of anthraquinone-2,6-disulfonate (AQDS). Strain L17 could dechlorinate CT to chloroform, and the rate was accelerated in the presence of Fe(III) oxide and AQDS. Biotic dechlorination by strain L17 and abiotic dechlorination by sorbed Fe(II) were proposed as the two main mechanisms. AQDS might accelerate the dechlorination by transferring electrons from strain L17 to Fe(III) oxide and CT. K. pneumoniae L17 can reduce Fe(III) oxides and CT. The two reductions can occur simultaneously, and be significantly promoted by AQDS. This is the first report of a strain of K. pneumoniae capable of reducing Fe(III) oxides and CT. As a strain of environmental origin, strain L17 may have the potential for bioremediation of chlorinated compound-contaminated sites.

Experimental constraints on Fe isotope fractionation during magnetite and Fe carbonate formation coupled to dissimilatory hydrous ferric oxide reduction

Iron isotope fractionation between aqueous Fe(II) and biogenic magnetite and Fe carbonates produced during reduction of hydrous ferric oxide (HFO) by Shewanella putrefaciens, Shewanella algae, and Geobacter sulfurreducens in laboratory experiments is a function of Fe(III) reduction rates and pathways by which biogenic minerals are formed. High Fe(III) reduction rates produced 56Fe/ 54Fe ratios for Fe(II) aq that are 2–3‰ lower than the HFO substrate, reflecting a kinetic isotope fractionation that was associated with rapid sorption of Fe(II) to HFO. In long-term experiments at low Fe(III) reduction rates, the Fe(II) aq-magnetite fractionation is −1.3‰, and this is interpreted to be the equilibrium fractionation factor at 22°C in the biologic reduction systems studied here. In experiments where Fe carbonate was the major ferrous product of HFO reduction, the estimated equilibrium Fe(II) aq-Fe carbonate fractionations were ca. 0.0‰ for siderite (FeCO 3) and ca. +0.9‰ for Ca-substituted siderite (Ca 0.15Fe 0.85CO 3) at 22°C. Formation of precursor phases such as amorphous nonmagnetic, noncarbonate Fe(II) solids are important in the pathways to formation of biogenic magnetite or siderite, particularly at high Fe(III) reduction rates, and these solids may have 56Fe/ 54Fe ratios that are up to 1‰ lower than Fe(II) aq. Under low Fe(III) reduction rates, where equilibrium is likely to be attained, it appears that both sorbed Fe(II) and amorphous Fe(II)(s) components have isotopic compositions that are similar to those of Fe(II) aq. The relative order of δ 56Fe values for these biogenic minerals and aqueous Fe(II) is: magnetite > siderite ≈ Fe(II) aq > Ca-bearing Fe carbonate, and this is similar to that observed for minerals from natural samples such as Banded Iron Formations (BIFs). Where magnetite from BIFs has δ 56Fe >0‰, the calculated δ 56Fe value for aqueous Fe(II) suggests a source from midocean ridge (MOR) hydrothermal fluids. In contrast, magnetite from BIFs that has δ 56Fe ≤0‰ apparently requires formation from aqueous Fe(II) that had very low δ 56Fe values. Based on this experimental study, formation of low-δ 56Fe Fe(II) aq in nonsulfidic systems seems most likely to have been produced by dissimilatory reduction of ferric oxides by Fe(III)-reducing bacteria.

Arsenic and other drinking water quality issues, Muzaffargarh District, Pakistan

In 49 samples of groundwater, sampled in Muzaffargarh District of south-western Punjab, central Pakistan, concentrations of As exceeded the World Health Organisation provisional guideline value, and United States Environmental Protection Agency (USEPA) Maximum Contaminant Level (MCL), of 10 μg L −1 in 58% of samples and reached up to 906 μg L −1. In this semi-arid region canal irrigation has lead to widespread water-logging, and evaporative concentration of salts has the potential to raise As concentrations in shallow groundwater well above 10 μg L −1. In fact, in rural areas, concentrations stay below 25 μg L −1 because As in the oxic shallow groundwater, and in recharging water, is sorbed to aquifer sediments. In some urban areas, however, shallow groundwater is found to contain elevated levels of As. The spatial distribution of As-rich shallow groundwater indicates either direct contamination with industrial or agricultural chemicals, or some other anthropogenic influence. Geochemical evidence suggests that pollutant organics from unconfined sewage and other sources drives reduction of hydrous ferric oxide (HFO) releasing sorbed As to shallow groundwater. The situation is slightly less clear for seven wells sampled which tap deeper groundwater, all of which were found with >50 μg L −1 As. Here As concentrations seem to increase with depth and differing geochemical signatures are seen, suggesting that As concentrations in older groundwater may be governed by different processes. Other data on parameters of potential concern in drinking water are discussed briefly at the end of the paper.

Electron transfer from Shewanella algae BrY to hydrous ferric oxide is mediated by cell-associated melanin

Shewanella algae BrY uses insoluble mineral oxides as terminal electron acceptors, but the mechanism of electron transfer from cell surface to mineral surface is not well understood. We tested the hypothesis that cell-associated melanin produced by S. algae BrY serves as an electron conduit for bacterial–mineral reduction. Results from Fourier transform infrared spectroscopy and cell surface hydrophobicity assays indicated that extracellular melanin was associated with the cell surface. With H 2 as electron donor, washed cell suspensions of melanin-coated S. algae BrY reduced hydrous ferric oxide (HFO) 10 times faster than cells without melanin. The addition of melanin (20 μg ml −1) to these melanin-free cells increased their HFO reduction rate two-fold. These results suggest that cell-associated melanin acts as an electron conduit for iron mineral reduction by S. algae BrY.

Adsorption and reactivity of hydrous iron oxide nanoparticles on boron-doped diamond

Hydrous iron oxide nanoparticles of 8–16 nm diameter are adsorbed from an aqueous sol onto the surface of polished boron-doped diamond electrodes. Boron-doped diamond acts as an inert substrate with low background currents and a wide potential window, allowing electrochemical and electrocatalytic properties of an electrically non-conducting material such as hydrous iron oxide to be studied. The electrochemical reduction of hydrous ferric oxide in aqueous phosphate buffer at pH 5 at a potential of ca. −0.2 V vs. SCE is accompanied by stripping of the deposit, which allows the determination of the approximate coverage of the oxide on the boron-doped diamond electrode, typically 1.4×10 −9 mol cm −2. Next, hydrous ferric oxide is shown to act as an efficient electrocatalyst for the oxidation of hydroxide to dioxygen. The process is proposed to occur with apparently one electron per hydroxide via a C het, rev E rev C het, irrev -type process, initiated by deprotonation of an active iron oxide surface site. It is also demonstrated that the reaction of hydrogen peroxide on hydrous ferric oxide surfaces (a heterogeneous Fenton-type process) is similarly triggered by the deprotonation of an active surface site.

Immobilization of strontium during iron biomineralization coupled to dissimilatory hydrous ferric oxide reduction

The potential for incorporation of strontium (Sr) into biogenic Fe(II)-bearing minerals formed during microbial reduction of synthetic hydrous ferric oxide (HFO) was investigated in circumneutral bicarbonate-buffered medium containing SrCl2 at concentrations of 10 μM, 100 μM, or 1.0 mM. CaCl2 (10 mM) was added to some experiments to simulate a Ca-rich groundwater. In Ca-free systems, 89 to 100% of total Sr was captured in solid-phase compounds formed during reduction of 30 to 40 mmol Fe(III) L−1 over a 1-month period. A smaller fraction of total Sr (25 to 34%) was incorporated into the solid phase in cultures amended with 10 mM CaCl2. X-ray diffraction identified siderite and ferroan ankerite as major end products of HFO reduction in Ca-free and Ca-amended cultures, respectively. Scanning electron microscopy–energy dispersive x-ray spectroscopy revealed the presence of Sr associated with carbonate phases. Selective extraction of HFO reduction end products indicated that 46 to 100% of the solid-phase Sr was associated with carbonates. The sequestration of Sr into carbonate phases in the Ca-free systems occurred systematically according to a heterogeneous (Doerner-Hoskins) partition coefficient (DD-H) of 1.81 ± 0.15. This DD-H value was 2 to 10 times higher than values determined for incorporation of Sr (10 μM) into FeCO3(s) precipitated abiotically at rates comparable to or greater than rates observed during HFO reduction, and fivefold higher than theoretical partition coefficients for equilibrium Fe(Sr)CO3 solid solution formation. Surface complexation and entrapment of Sr by rapidly growing siderite crystals (and possibly other biogenic Fe(II) solids) provides an explanation for the intensive scavenging of Sr in the Ca-free systems. The results of abiotic siderite precipitation experiments in the presence and absence of excess Ca indicate that substitution of Ca for Sr at foreign element incorporation sites (mass action effect) on growing FeCO3(s) surfaces can account for the inhibition of Sr incorporation into the siderite component of ankerite formed in the Ca-amended HFO reduction experiments. Likewise, substitution of Fe(II) for Sr may explain the absence of major Sr partitioning into the calcite component of ankerite. The findings indicate that under appropriate conditions, sequestration of metals in siderite produced during bacterial Fe(III) oxide reduction may provide a mechanism for retarding the migration of Sr and other divalent metal contaminants in anaerobic, carbonate-rich sedimentary environments.

Formation of Green Rust and Immobilization of Nickel in Response to Bacterial Reduction of Hydrous Ferric Oxide

This investigation documents the formation of Green Rust (GR) and immobilization of Ni 2+ in response to bacterial reduction of hydrous ferric oxide (HFO). In the absence of Ni 2+ , 79% of the total Fe(III) present as HFO was reduced; at 10 -3 and 10 -4 M Ni 2+ , 36% of the total Fe(III) was reduced, whereas 45 to 50% of the total Fe(III) was reduced at 10 -5 M Ni 2+ . The inhibitory effect of 10 -3 and 10 -4 M Ni 2+ on Fe(III)-reduction corresponded to a 50% decrease in number of viable cells relative to the Ni 2+ -free condition, and a 25% decrease at 10 -5 M Ni 2+ . A prominent GR peak at d = 10.9 nm was evident in X-ray diffraction patterns of postreduction residual solids from the cultures. Minor peaks arising for vivianite and magnetite were also present. In samples prepared for scanning electron microscopy, thin hexagonal plates of GR were easily distinguished as a solid phase transformation product of HFO. Small hexagonal sheets and fragments of larger GR plates were also observed in transmission electron microscopy whole mounts together with bacteria that were mineralized by surface precipitates of microcrystalline magnetite. Energy dispersive spectroscopy (EDS) confirmed that GR contained Fe and P, as well as Ni in those samples taken from the Ni 2+ -amended experiments. EDS detected neither P nor Ni in the magnetite precipitates associated with the bacterial cells. Dissolved Ni2 + concentrations decreased in an exponential fashion with respect to time in all experimental systems, corresponding to an overall first-order rate constant k of -0.030 day -1 . At the same time, a strong linear relationship (r 2 = 0.99) between the dissolved and solid phase Ni 2+ /Fe 2+ ratios over the entire period of the Fe(III)reduction experiments provided evidence that the solid-phase partitioning of Ni 2+ in GR extended from equilibrium solid-solution behavior.

Microbially catalyzed nitrate-dependent oxidation of biogenic solid-phase Fe(II) compounds.

The potential for microbially catalyzed NO3(-)-dependent oxidation of solid-phase Fe(II) compounds was examined using a previously described autotrophic, denitrifying, Fe(II)-oxidizing enrichment culture. The following solid-phase Fe(II)-bearing minerals were considered: microbially reduced synthetic goethite, two different end products of microbially hydrous ferric oxide (HFO) reduction (biogenic Fe3O4 and biogenic FeCO3), chemically precipitated FeCO3, and two microbially reduced iron(III) oxide-rich subsoils. The microbially reduced goethite, subsoils, and chemically precipitated FeCO3 were subject to rapid NO3(-)-dependent Fe(II) oxidation. Significant oxidation of biogenic Fe3O4 was observed. Very little biogenic FeCO3 was oxidized. No reduction of NO3- or oxidation of Fe(II) occurred in pasteurized cultures. The molar ratio of NO3- reduced to Fe(II) oxidized in cultures containing chemically precipitated FeCO3, and one of the microbially reduced subsoils approximated the theoretical stoichiometry of 0.2:1. However, molar ratios obtained for oxidation of microbially reduced goethite, the other subsoil, and the HFO reduction end products did not agree with this theoretical value. These discrepancies may be related to heterotrophic NO3- reduction coupled to oxidation of dead Fe(III)-reducing bacterial biomass. Our findings demonstrate that microbally catalyzed NO3(-)-dependent Fe(II) oxidation has the potential to significantly accelerate the oxidation of solid-phase Fe(II) compounds by oxidized N species. This process could have an important influence on the migration of contaminant metals and radionuclides in subsurface environments.

Mineral transformations associated with the microbial reduction of magnetite

Although dissimilatory iron reducing bacteria (DIRB) are capable of reducing a number of metals in oxides and soluble forms, the factors controlling the rate/extent of magnetite reduction and the nature of the mineral products resulting from magnetite reduction are not well understood. This study was carried out to investigate mechanisms and biogeochemical processes occurring during magnetite reduction by the DIRB, Shewanella putrefaciens strains CN32 and MR-1. Reduction experiments were performed with biogenic and synthetic magnetite in well-defined solutions. Biogenic magnetite was generated via microbial reduction of hydrous ferric oxide (HFO). Biogenic magnetite in solutions buffered with either bicarbonate (HCO 3 −) or 1,4-piperazinediethanesulfonic (PIPES), with or without P, was inoculated with strain CN32 and provided with lactate as the electron donor. Synthetic magnetite in a bacteriological growth medium (M1) was inoculated with either aerobically or anaerobically grown cells of strain (CN32 or MR-1). Fe(II) production was determined by HCl extraction of bioreduced samples in comparison to uninoculated controls, and the resulting solids were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, scanning and transmission electron microscopy (SEM and TEM). The extent and rate of biogenic magnetite reduction in the bicarbonate-buffered medium was higher than that in the PIPES-buffered medium, via complexation of bioproduced Fe(II) with HCO 3 − (or PO 4 3−) and formation of siderite (vivianite). S. putrefaciens CN32 reduced more synthetic than biogenic magnetite with differences attributed mainly to medium composition. In the HCO 3 −-buffered solutions, Fe(III) in the biogenic magnetite was reduced to Fe(II), and siderite precipitated. In the PIPES-buffered medium, Fe(III) in biogenic magnetite was also reduced to Fe(II), but no secondary mineral phases were observed. Vivianite formed in those solutions containing P and in all synthetic magnetite treatments where there was sufficient supply of P from the M1 medium. Electron microscopy and Mössbauer spectroscopy results suggest that the reduction process involves dissolution–precipitation mechanisms as opposed to solid state conversion of magnetite to vivianite or siderite. The aqueous medium, pH, strain type, and bacterial growth conditions all affected the extent of magnetite reduction. The ability of DIRB to utilize Fe(III) in crystalline magnetite as an electron acceptor could have significant implications for biogeochemical processes in sediments where Fe(III) in magnetite represents the largest pool of electron acceptor.

Dissimilatory Fe(III) Oxide Reduction by Shewanella alga BrY Requires Adhesion

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used to examine the relationship between adhesion and dissimilatory Fe(III) oxide reduction. Adhesion of Shewanella alga BrY to hydrous ferric oxide (HFO) was correlated with ionic strength and thus was accurately described by the DLVO theory. Reduction of insoluble HFO was also correlated with KCl concentration. In contrast, there was no correlation between soluble Fe(III) reduction and ionic strength. A correlation between HFO reduction rate and adhesion to HFO was observed. These results provide direct evidence that adhesion is requisite for Fe(III) oxide reduction in the absence of soluble electron shuttles.

Biogenic iron mineralization accompanying the dissimilatory reduction of hydrous ferric oxide by a groundwater bacterium

Dissimilatory iron-reducing bacteria (DIRB) couple the oxidation of organic matter or H 2 to the reduction of iron oxides. The factors controlling the rate and extent of these reduction reactions and the resulting solid phases are complex and poorly understood. Batch experiments were conducted with amorphous hydrous ferric oxide (HFO) and the DIRB Shewanella putrefaciens, strain CN32, in well-defined aqueous solutions to investigate the reduction of HFO and formation of biogenic Fe(II) minerals. Lactate-HFO solutions buffered with either bicarbonate or 1,4-piperazinediethanesulfonic acid (PIPES) containing various combinations of phosphate and anthraquinone-2,6-disulfonate (AQDS), were inoculated with S. putrefaciens CN32. AQDS, a humic acid analog that can be reduced to dihydroanthraquinone by CN32, was included because of its ability to function as an electron shuttle during microbial iron reduction and as an indicator of pe. Iron reduction was measured with time, and the resulting solids were analyzed by X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). In HCO 3 − buffered medium with AQDS, HFO was rapidly and extensively reduced, and the resulting solids were dominated by ferrous carbonate (siderite). Ferrous phosphate (vivianite) was also present in HCO 3 − medium containing P, and fine-grained magnetite was present as a minor phase in HCO 3 − medium with or without P. In the PIPES-buffered medium, the rate and extent of reduction was strongly influenced by AQDS and P. With AQDS, HFO was rapidly converted to highly crystalline magnetite whereas in its absence, magnetite mineralization was slower and the final material less crystalline. In PIPES with both P and AQDS, a green rust type compound [Fe (6-x) IIFe x III(OH) 12] x+[(A 2−) x/2 · yH 2O] x− was the dominant solid phase formed; in the absence of AQDS a poorly crystalline product was observed. The measured pe and nature of the solids identified were consistent with thermodynamic considerations. The composition of aqueous media in which microbial iron reduction occurred strongly impacted the rate and extent of iron reduction and the nature of the reduced solids. This, in turn, can provide a feedback control mechanism on microbial metabolism. Hence, in sediments where geochemical conditions promote magnetite formation, two-thirds of the Fe(III) will be sequestered in a form that may not be available for anaerobic bacterial respiration.