A geochemical model for removal of iron(II)(aq) from mine water discharges

A steady state geochemical model has been developed to assist in understanding surface-catalysed oxidation of aqueous Fe(II) by O₂(aq), which occurs rapidly at circumneutral pH. The model has been applied to assess the possible abiotic removal of Fe(II)(aq) from alkaline ferruginous mine water discharges using engineered reactors with high specific-surface area filter media. The model includes solution and surface speciation equilibrium, oxidation kinetics of dissolved and adsorbed Fe(II) species and mass transfer of O₂(g). Limited field data for such treatment of a mine water discharge were available for model development and assessment of possible parameter values. Model results indicate that an adsorption capacity between 10⁻⁶ and 10⁻⁵ mol l⁻¹ is sufficient for complete removal, by oxidation, of the Fe(II)(aq) load at the discharge. This capacity corresponds approximately to that afforded by surface precipitation of Fe(III) oxide onto plastic trickling filter media typically used for biological treatment of wastewater. Extrapolated literature values for microbial oxidation of Fe(II)(aq) by neutrophilic microbial populations to the simulated reactor conditions suggested that the microbially-mediated rate may be several orders-of-magnitude slower than the surface-catalysed oxidation. Application of the model across a range of mine water discharge qualities shows that high Fe(II)(aq) loadings can be removed if the discharge is sufficiently alkaline. Additional reactor simulations indicate that reactor efficiency decreases dramatically with pH in the near acid region, coinciding with the adsorption edge for Fe²⁺ on Fe oxyhydroxide. Alkaline discharges thus buffer pH within the range where Fe(II)(aq) adsorbs onto the accreting Fe hydroxide mineral surface, and undergoes rapid catalytic oxidation. The results suggest that the proposed treatment technology may be appropriate for highly ferruginous alkaline discharges, typically associated with abandoned deep coal mines.     

Biogeochemistry of Fe(II) oxidation in a photosynthetic microbial mat: Implications for Precambrian Fe(II) oxidation

We studied the role of microbial photosynthesis in the oxidation of Fe(II) to Fe(III) in a high Fe(II) and high Mn(II) hot spring devoid of sulfide and atmospheric oxygen in the source waters. In situ light and dark microelectrode measurements of Fe(II), Mn(II) and O₂ were made in the microbial mat consisting of cyanobacteria and anoxygenic photosynthetic Chloroflexus sp. We show that Fe(II) oxidation occurred when the mat was exposed to varying intensities of sunlight but not near infrared light. We did not observe any Mn(II) oxidation under any light or dark condition over the pH range 5-7. We observed the impact of oxygenic photosynthesis on Fe(II) oxidation, distinct from the influence of atmospheric O₂ and anoxygenic photosynthesis. In situ Fe(II) oxidation rates in the mats and cell suspensions exposed to light are consistent with abiotic oxidation by O₂. The oxidation of Fe(II) to form primary Fe(III) phases contributed to banded iron-formations (BIFs) during the Precambrian. Both oxygenic photosynthesis, which produces O₂ as an oxidizing waste product, and anoxygenic photosynthesis in which Fe(II) is used to fix CO₂ have been proposed as Fe(II) oxidation mechanisms. Although we do not know the specific mechanisms responsible for all Precambrian Fe(II) oxidation, we assessed the relative importance of both mechanisms in this modern hot spring environment. In this environment, cyanobacterial oxygen production accounted for all the observed Fe(II) oxidation. The rate data indicate that a modest population of cyanobacteria could have mediated sufficient Fe(II) oxidation for some BIFs.     

Relative contributions of abiotic and biological factors in Fe(II) oxidation in mine drainage

The oxidation of Fe(II) is apparently the rate-limiting step in passive treatment of coal mine drainage. Little work has been done to determine the kinetics of oxidation in such field systems, and no models of passive treatment systems explicitly consider iron oxidation kinetics. A Stella II^™ model using Fe(II)_init concentration, pH, temperature, Thiobacillus ferrooxidans and O₂ concentration, flow rate, and pond volume is used to predict Fe(II) oxidation rates and concentrations in seventeen ponds under a wide range of conditions (pH 2.8 to 6.8 with Fe(II) concentrations of less than 240 mg L⁻¹) from 6 passive treatment facilities. The oxidation rate is modeled based on the combination of published abiotic and biological laboratory rate laws. Although many other variables have been observed to influence Fe(II) oxidation rates, the 7 variables above allow field systems to be modeled reasonably accurately for conditions in this study.Measured T. ferrooxidans concentrations were approximately 10⁷ times lower than concentrations required in the model to accurately predict field Fe(II) concentrations. This result suggests that either 1) the most probable number enumeration method underestimated the bacterial concentrations, or 2) the biological rate law employed underestimated the influence of bacteria, or both. Due to this discrepancy, bacterial concentrations used in the model for pH values of less than 5 are treated as fit parameters rather than empirically measured values.Predicted Fe(II) concentrations in ponds agree well with measured Fe(II) concentrations, and predicted oxidation rates also agree well with field-measured rates. From pH 2.8 to approximately pH 5, Fe(II) oxidation rates are negatively correlated with pH and catalyzed by T. ferrooxidans. From pH 5 to 6.4, Fe(II) oxidation appears to be primarily abiotic and is positively correlated with pH. Above pH 6.4, oxidation appears to be independent of pH. Above pH 5, treatment efficiency is affected most by changing design parameters in the following order: pH>temperature≈influent Fe(II)>pond volume≈O₂. Little to no increase in Fe(II) oxidation rate occurs due to pH increases above pH 6.4. Failure to consider Fe(II) oxidation rates in treatment system design may result in insufficient Fe removal.     

Experimental evidence for rapid biotic and abiotic reduction of Fe (III) at low temperatures in salt marsh sediments: a possible mechanism for formation of modern sedimentary siderite concretions

Fe (III) reduction is a key component of the global iron cycle, and an important control on carbon mineralization. However, little is known about the relative roles and rates of microbial (biotic) iron reduction, which utilizes organic matter, versus abiotic iron reduction, which occurs without carbon mineralization. This paper reports on the capacity for salt marsh sediments, which typically are rich in iron, to support abiotic reduction of mineral Fe (III) driven by oxidation of sulphide. Sediment was reacted with amorphous FeS under strictly anaerobic conditions at a range of temperatures in biotic and abiotic microcosm experiments. Fe (III) reduction driven by sulphide oxidation occurs abiotically at all temperatures, leading to Fe (II) and elemental sulphur production in all abiotic experiments. In biotic experiments elemental sulphur is also the oxidized sulphur product but higher bicarbonate production leads to FeCO₃ precipitation. Abiotic reduction of Fe (III) occurs at rates that are significant compared with microbial Fe (III) reduction in salt marsh sediments. The solid phases produced by coupled abiotic and biotic reactions, namely elemental sulphur and FeCO₃, are comparable to those seen in nature at Warham, Norfolk, UK. Furthermore, the rates of these processes measured in the microcosm experiments are sufficient to generate siderite concretions on the rapid time scales observed in the field. This work highlights the importance of abiotic Fe (III) reduction alongside heterotrophic reduction, which has implications for iron cycling and carbon mineralization in modern and ancient sediments.     

Reduction of pertechnetate [Tc(VII)] by aqueous Fe(II) and the nature of solid phase redox products

The subsurface behaviour of ⁹⁹Tc, a contaminant resulting from nuclear fuels reprocessing, is dependent on its valence (e.g., IV or VII). Abiotic reduction of soluble Tc(VII) by Fe(II)_(aq) in pH 6-8 solutions was investigated under strictly anoxic conditions using an oxygen trap (<7.5 × 10⁻⁹ atm O₂). The reduction kinetics were strongly pH dependent. Complete and rapid reduction of Tc(VII) to a precipitated Fe/Tc(IV) form was observed when 11 μmol/L of Tc(VII) was reacted with 0.4 mmol/L Fe(II) at pH 7.0 and 8.0, while no significant reduction was observed over 1 month at pH 6.0. Experiments conducted at pH 7.0 with Fe(II)_(aq) = 0.05-0.8 mmol/L further revealed that Tc(VII) reduction was a combination of homogeneous and heterogeneous reaction. Heterogeneous reduction predominated after approximately 0.01 mmol/L of Fe(II) was oxidized. The heterogeneous reaction was more rapid, and was catalyzed by Fe(II) that adsorbed to the Fe/Tc(IV) redox product. Wet chemical and Fe-X-ray absorption near edge spectroscopy measurements (XANES) showed that Fe(II) and Fe(III) were present in the Fe/Tc(IV) redox products after reaction termination. ⁵⁷Fe-Mössbauer, extended X-ray adsorption fine structure (EXAFS), and transmission electron microscopy (TEM) measurements revealed that the Fe/Tc(IV) solid phase was poorly ordered and dominated by Fe(II)-containing ferrihydrite with minor magnetite. Tc(IV) exhibited homogeneous spatial distribution within the precipitates. According to Tc-EXAFS measurements and structural modeling, its molecular environment was consistent with an octahedral Tc(IV) dimer bound in bidentate edge-sharing mode to octahedral Fe(III) associated with surface or vacancy sites in ferrihydrite. The precipitate maintained Tc(IV)_aq concentrations that were slightly below those in equilibrium with amorphous Tc(IV)O₂·nH₂O_(s). The oxidation rate of sorbed Tc(IV) in the Fe/Tc precipitate was considerably slower than Tc(IV)O₂·nH₂O_(s) as a result of its intraparticle/intragrain residence. Precipitates of this nature may form in anoxic sediments or groundwaters, and the intraparticle residence of sorbed/precipitated Tc(IV) may limit ⁹⁹Tc remobilization upon the return of oxidizing conditions.     

The effect of methanogenesis on the geochemistry of low temperature water–Fe0–basalt reactions

Hydrogen gas produced in the subsurface from the hydration of mafic rocks is known to be a major energy source for chemolithotrophic life in extreme environments such as hydrothermal vents. The possibility that in situ anaerobic microorganisms present in the deep subsurface are sustained by low temperature H₂-generating water–rock reactions taking place around them is being investigated. Whether the growth and activity of H₂-utilizing microbes directly influences aqueous geochemistry, rates of mineral dissolution, and the chemical composition of the alteration products is also being quantitatively evaluated.To explore how microorganisms are affected by water–rock reactions, and how their activity may in turn affect reaction progress, laboratory experiments have been conducted to monitor the growth of a methanogenic Archaea in the presence of H_2(g) produced from low temperature water–Fe⁰–basalt reactions. In these systems, the conversion of Fe(II) to Fe(III) and subsequent hydrolysis of water is responsible for the production of H_2(g). To characterize key components of the geochemical system, time series measurements of H₂ and CH₄ gas concentrations, Fe and Si aqueous concentrations, and spatially resolved synchrotron-based analyses of microscale Fe distribution and speciation were conducted. Culture experiments were compared with an abiotic control to document changes in the geochemistry both in the presence and absence of the methanogen.In the control abiotic batch experiment, H₂ was continuously produced, until the headspace became saturated, while in the biotic experiments, microbial consumption of H₂ for methanogenesis draws H₂ down and produces CH₄. Purging the headspace gas reinitiates H₂ and CH₄ production in abiotic and culture experiments, respectively. Mass balance analysis of the amount of CH₄ produced suggests that the total H₂ production in microbial experiments does not exceed the abiotic experiment. Soluble Si concentrations, while buffered to relatively constant values, were higher in culture experiments than the abiotic control.Iron_(aq) concentrations appear to respond to perturbations of H₂ and CH₄ gas concentrations in both culture experiments and the abiotic control. A pulse of Fe preceded the rise in either H₂ or CH₄ production, and as the gas concentrations increased the Fe_(aq) decreased. Iron-bearing mineral assemblages change with increasing reaction time and mineral assemblages vary between culture experiments and the abiotic control. These geochemical trends suggest that there are different reaction paths between the culture experiments and the abiotic control.The hydration of mafic rocks is a common geologic reaction and one that has taken place on Earth for the majority of its history and is postulated to occur on Mars. These reactions are important because of their effect on the rheology and geochemistry of the ocean crust. While most often studied at temperatures of ～250 °C, this work suggests that at lower temperatures microorganisms may have a profound effect on what has long been thought to be solely an abiotic reaction, and may produce diagnostic mineral assemblages that will be preserved in the geological record.     

Kinetics of H2-O2-H2O redox equilibria and formation of metastable H2O2 under low temperature hydrothermal conditions

Hydrothermal experiments were conducted to evaluate the kinetics of H_2(aq) oxidation in the homogeneous H₂-O₂-H₂O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250 °C and 242-497 bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H_2(aq) and O_2(aq), are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high-temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H_2(aq)-O_2(aq) equilibrium become slower with decreasing temperature, allowing excess H_2(aq) to remain in solution. Sluggish reaction rates of H_2(aq) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H_2(aq) and O_2(aq), even at temperatures lower than 40 °C due to H_2(aq) persistence in the seawater/vent fluid mixtures. For these H₂-O₂ disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H_2(aq) oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites.More importantly, when H_2(aq)-O_2(aq) disequilibrium conditions apply, formation of metastable hydrogen peroxide is observed. The yield of H₂O_2(aq) synthesis appears to be enhanced under conditions of elevated H_2(aq)/O_2(aq) molar ratios that correspond to abundant H_2(aq) concentrations. Formation of metastable H₂O₂ is expected to affect the distribution of dissolved organic carbon (DOC) owing to the existence of an additional strong oxidizing agent. Oxidation of magnetite and/or Fe⁺⁺ by hydrogen peroxide could also induce formation of metastable hydroxyl radicals (•OH) through Fenton-type reactions, further broadening the implications of hydrogen peroxide in hydrothermal environments.     

Iron isotope fractionation by Fe(II)-oxidizing photoautotrophic bacteria

Photoautotrophic bacteria that oxidize ferrous iron (Fe[II]) under anaerobic conditions are thought to be ancient in origin, and the ferric (hydr)oxide mineral products of their metabolism are likely to be preserved in ancient rocks. Here, two enrichment cultures of Fe(II)-oxidizing photoautotrophs and a culture of the genus Thiodictyon were studied with respect to their ability to fractionate Fe isotopes. Fe isotope fractionations produced by both the enrichment cultures and the Thiodictyon culture were relatively constant at early stages of the reaction progress, where the ⁵⁶Fe/⁵⁴Fe ratios of poorly crystalline hydrous ferric oxide (HFO) metabolic products were enriched in the heavier isotope relative to aqueous ferrous iron (Fe[II]_aq) by ∼1.5 ± 0.2‰. This fractionation appears to be independent of the rate of photoautotrophic Fe(II)-oxidation, and is comparable to that observed for Fe isotope fractionation by dissimilatory Fe(III)-reducing bacteria. Although there remain a number of uncertainties regarding how the overall measured isotopic fractionation is produced, the most likely mechanisms include (1) an equilibrium effect produced by biological ligands, or (2) a kinetic effect produced by precipitation of HFO overlaid upon equilibrium exchange between Fe(II) and Fe(III) species. The fractionation we observe is similar in direction to that measured for abiotic oxidation of Fe(II)_aq by molecular oxygen. This suggests that the use of Fe isotopes to identify phototrophic Fe(II)-oxidation in the rock record may only be possible during time periods in Earth’s history when independent evidence exists for low ambient oxygen contents.     

Chemical and structural characterization of As immobilization by nanoparticles of mackinawite (FeSm)

The mobility and availability of arsenite, As(III), in anoxic environments is largely controlled by adsorption onto iron sulfides and/or precipitation of arsenic in solid phases. The interaction of As(III) with synthetic mackinawite (FeS_m) in pH 5 and 9 suspensions was investigated using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), STEM elemental mapping, high resolution TEM, and X-ray photoelectron spectroscopy (XPS). At pH 5, arsenic sulfide phases precipitate among the FeS_m particles as discrete particles that are an amorphous hydrous phase of arsenic sulfide. The oxidation state of As in the surface layers of the arsenic sulfide precipitates is ‘realgar-like’ based on XPS results showing that > 75% of the As 3d peak area is due to As with oxidation states between 0 and 2+. Discrete, arsenic sulfide precipitates are absent at pH 9, but elemental mapping in STEM-EDX mode shows that arsenic is uniformly distributed on the FeS_m, suggesting that uptake is caused by the sorption of As(III) oxyanions and/or the precipitation of highly dispersed arsenic sulfides on FeS_m. XPS also revealed that the FeS_m that equilibrated without As(III) has a more oxidized surface composition than the sample at pH 9, as indicated by the higher concentration of O ( three times greater than that at pH 9) and the larger fraction of Fe(III) species making up the total Fe (2p_3/2) peak. These findings provide a better understanding of redox processes and phase transitions upon As(III) adsorption on iron sulfide substrates.     

Reduction of TcO4 by sediment-associated biogenic Fe(II)

The potential for reduction of ⁹⁹TcO₄⁻_(aq) to poorly soluble ⁹⁹TcO₂ · nH₂O_(s) by biogenic sediment-associated Fe(II) was investigated with three Fe(III)-oxide containing subsurface materials and the dissimilatory metal-reducing subsurface bacterium Shewanella putrefaciens CN32. Two of the subsurface materials from the U.S. Department of Energy’s Hanford and Oak Ridge sites contained significant amounts of Mn(III,IV) oxides and net bioreduction of Fe(III) to Fe(II) was not observed until essentially all of the hydroxylamine HCl-extractable Mn was reduced. In anoxic, unreduced sediment or where Mn oxide bioreduction was incomplete, exogenous biogenic TcO₂ · nH₂O_(s) was slowly oxidized over a period of weeks. Subsurface materials that were bioreduced to varying degrees and then pasteurized to eliminate biological activity, reduced TcO₄⁻_(aq) at rates that generally increased with increasing concentrations of 0.5 N HCl-extractable Fe(II). Two of the sediments showed a common relationship between extractable Fe(II) concentration (in mM) and the first-order reduction rate (in h⁻¹), whereas the third demonstrated a markedly different trend. A combination of chemical extractions and ⁵⁷Fe Mössbauer spectroscopy were used to characterize the Fe(III) and Fe(II) phases. There was little evidence of the formation of secondary Fe(II) biominerals as a result of bioreduction, suggesting that the reactive forms of Fe(II) were predominantly surface complexes of different forms. The reduction rates of Tc(VII)O₄⁻ were slowest in the sediment that contained plentiful layer silicates (illite, vermiculite, and smectite), suggesting that Fe(II) sorption complexes on these phases were least reactive toward pertechnetate. These results suggest that the in situ microbial reduction of sediment-associated Fe(III), either naturally or via redox manipulation, may be effective at immobilizing TcO₄⁻_(aq) associated with groundwater contaminant plumes.     

Arsenic repartitioning during biogenic sulfidization and transformation of ferrihydrite

Iron (hydr)oxides are strong sorbents of arsenic (As) that undergo reductive dissolution and transformation upon reaction with dissolved sulfide. Here we examine the transformation and dissolution of As-bearing ferrihydrite and subsequent As repartitioning amongst secondary phases during biotic sulfate reduction. Columns initially containing As(V)-ferrihydrite coated sand, inoculated with the sulfate reducing bacteria Desulfovibrio vulgaris (Hildenborough), were eluted with artificial groundwater containing sulfate and lactate. Rapid and consistent sulfate reduction coupled with lactate oxidation is observed at low As(V) loading (10% of the adsorption maximum). The dominant Fe solid phase transformation products at low As loading include amorphous FeS within the zone of sulfate reduction (near the inlet of the column) and magnetite downstream where Fe(II)_(aq) concentrations increase; As is displaced from the zone of sulfidogenesis and Fe(III)_(s) depletion. At high As(V) loading (50% of the adsorption maximum), sulfate reduction and lactate oxidation are initially slow but gradually increase over time, and all As(V) is reduced to As(III) by the end of experimentation. With the higher As loading, green rust(s), as opposed to magnetite, is a dominant Fe solid phase product. Independent of loading, As is strongly associated with magnetite and residual ferrihydrite, while being excluded from green rust and iron sulfide. Our observations illustrate that sulfidogenesis occurring in proximity with Fe (hydr)oxides induce Fe solid phase transformation and changes in As partitioning; formation of As sulfide minerals, in particular, is inhibited by reactive Fe(III) or Fe(II) either through sulfide oxidation or complexation.     

Hydrous ferric oxide precipitation in the presence of nonmetabolizing bacteria: Constraints on the mechanism of a biotic effect

We have used room temperature and cryogenic ⁵⁷Fe Mössbauer spectroscopy, powder X-ray diffraction (pXRD), mineral magnetometry, and transmission electron microscopy (TEM), to study the synthetic precipitation of hydrous ferric oxides (HFOs) prepared either in the absence (abiotic, a-HFO) or presence (biotic, b-HFO) of nonmetabolizing bacterial cells (Bacillus subtilis or Bacillus licheniformis, ∼10⁸ cells/mL) and under otherwise identical chemical conditions, starting from Fe(II) (10⁻², 10⁻³, or 10⁻⁴ mol/L) under open oxic conditions and at different pH (6-9). We have also performed the first Mössbauer spectroscopy measurements of bacterial cell wall (Bacillus subtilis) surface complexed Fe, where Fe(III) (10^−3.5-10^−4.5 mol/L) was added to a fixed concentration of cells (∼10⁸ cells/mL) under open oxic conditions and at various pH (2.5-4.3). We find that non-metabolic bacterial cell wall surface complexation of Fe is not passive in that it affects Fe speciation in at least two ways: (1) it can reduce Fe(III) to sorbed-Fe²⁺ by a proposed steric and charge transfer effect and (2) it stabilizes Fe(II) as sorbed-Fe²⁺ against ambient oxidation. The cell wall sorption of Fe occurs in a manner that is not compatible with incorporation into the HFO structure (different coordination environment and stabilization of the ferrous state) and the cell wall-sorbed Fe is not chemically bonded to the HFO particle when they coexist (the sorbed Fe is not magnetically polarized by the HFO particle in its magnetically ordered state). This invalidates the concept that sorption is the first step in a heterogeneous nucleation of HFO onto bacterial cell walls. Both the a-HFOs and the b-HFOs are predominantly varieties of ferrihydrite (Fh), often containing admixtures of nanophase lepidocrocite (nLp), yet they show significant abiotic/biotic differences: Biotic Fh has less intraparticle (including surface region) atomic order (Mössbauer quadrupole splitting), smaller primary particle size (magnetometry blocking temperature), weaker Fe to particle bond strength (Mössbauer center shift), and no six-line Fh (6L-Fh) admixture (pXRD, magnetometry). Contrary to current belief, we find that 6L-Fh appears to be precipitated directly, under a-HFO conditions, from either Fe(II) or Fe(III), and depending on Fe concentration and pH, whereas the presence of bacteria disables all such 6L-Fh precipitation and produces two-line Fh (2L-Fh)-like biotic coprecipitates. Given the nature of the differences between a-HFO and b-HFO and their synthesis condition dependences, several biotic precipitation mechanisms (template effect, near-cell environment effect, catalyzed nucleation and/or growth effect, and substrate-based coprecipitation) are ruled out. The prevailing present view of a template or heterogeneous nucleation barrier reduction effect, in particular, is shown not to be the cause of the large observed biotic effects on the resulting HFOs. The only proposed mechanism (relevant to Fh) that is consistent with all our observations is coprecipitation with and possible surface poisoning by ancillary bacteriagenic compounds. That bacterial cell wall functional groups are redox active and the characteristics of biotic (i.e., natural) HFOs compared to those of abiotic (i.e., synthetic) HFOs have several possible biogeochemical implications regarding Fe cycling, in the photic zones of water columns in particular.     

Conditions for biological precipitation of iron by Gallionella ferruginea in a slightly polluted ground water

A sand filter has been built as a pilot plant with the purpose of biological precipitation of Fe from ground water polluted with mainly chlorinated aliphatics. The ground water is pumped directly from a well in a polluted ground water aquifer in Esbjerg, Denmark. The pollution includes trichlorethylene and tetrachlorethylene together with smaller amounts of pesticides. Furthermore the best conditions for Fe precipitating bacteria were not expected to be present because of a relatively high O₂ content, up to 6.7 mg/l, a low Fe content, 0.2 mg/l and a pH of ∼5 in the ground water. Added FeSO₄ increased the Fe content of the ground water to about 4 mg/l. These rather extreme conditions for precipitating Fe were observed over a period of 3 months. The goal of the research was to observe the mechanism of Fe precipitation in a sand filter in the above-mentioned conditions comparative to normal conditions for biotic as well as abiotic Fe mineralization in sand filters of fresh water treatment plants. The Fe precipitating bacterium Gallionella ferrugenia was found to dominate the biotic Fe oxidation/precipitation process despite the extreme conditions. A huge amount of exopolymer from Gallionella was present. The precipitated Fe oxide was determined to be ferrihydrate. The rate of the Fe oxidation/precipitation was found to be about 1000 times faster than formerly found for abiotic physico-chemical oxidation/precipitation processes. The hydrophobic pesticides and some of their degradation products were not adsorbed in the filter. An added hydrophilic pesticide was adsorbed up to 40%. Trichlorethylene was not adsorbed in the filter. The reason for the poor adsorption of the hydrophobic compounds and trichlorethylene is due to the pronounced hydrophilic property of the exopolymers of Gallionella and the precipitated ferrihydrite.     

The influence of sulfides on soluble organic-Fe(III) in anoxic sediment porewaters

Solid and colloidal iron oxides are commonly involved in early diagenesis. More readily available soluble Fe(III) should accelerate the cycling of iron (Fe) and sulfur (S) in sediments. Experiments with synthetic solutions (Taillefert et al. 2000) showed that soluble Fe(III) (i.e., <50 nm diameter) reacts at a mercury voltammetric electrode at circumneutral pH if it is complexed by an organic ligand. The reactivity of soluble organic-Fe(III) with sulfide is greatly increased compared to its solid equivalent (e.g., amorphous hydrous iron oxides or goethite). We report here data from two different creeks of the Hackensack Meadowlands District (New Jersey) collected with solid state Au/Hg voltammetric microelectrodes and other conventional techniques, which confirm the existence of soluble organic-Fe(III) in sediments and its interaction with sulfide. Chemical profiles in these two anoxic sediments show the interaction between iron and sulfur during early diagenesis. Soluble organic-Fe(III) and Fe(II) are dominant in a creek where sulfide is negligible. This dominance suggests that the reductive dissolution of iron oxides goes through the dissolution of solid Fe(III), then reduction to Fe(II), or that soluble organic-Fe(III) is formed by chemical or microbial oxidation of organic-Fe(II) complexes. In a creek sediment where sulfide occurs in significant concentration, the reductive dissolution of Fe(III) is followed by formation of FeS_(aq), which further precipitates. Dissolved sulfide may influence the fate of soluble organic-Fe(III), but the pH may be the key variable behind this process. The high reactivity of soluble organic-Fe(III) and its mobility may result in the shifting of local reactions, at depths where other electron acceptors are used. These data also suggest that estuarine and coastal sediments may not always be at steady state.     

Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite

To better understand reaction pathways of pyrite oxidation and biogeochemical controls on δ¹⁸O and δ³⁴S values of the generated sulfate in acid mine drainage (AMD) and other natural environments, we conducted a series of pyrite oxidation experiments in the laboratory. Our biological and abiotic experiments were conducted under aerobic conditions by using O₂ as an oxidizing agent and under anaerobic conditions by using dissolved Fe(III)_aq as an oxidant with varying δ¹⁸O_H2O values in the presence and absence of Acidithiobacillus ferrooxidans. In addition, aerobic biological experiments were designed as short- and long-term experiments where the final pH was controlled at ∼2.7 and 2.2, respectively. Due to the slower kinetics of abiotic sulfide oxidation, the aerobic abiotic experiments were only conducted as long term with a final pH of ∼2.7. The δ³⁴S_SO4 values from both the biological and abiotic anaerobic experiments indicated a small but significant sulfur isotope fractionation (∼−0.7‰) in contrast to no significant fractionation observed from any of the aerobic experiments. Relative percentages of the incorporation of water-derived oxygen and dissolved oxygen (O₂) to sulfate were estimated, in addition to the oxygen isotope fractionation between sulfate and water, and dissolved oxygen. As expected, during the biological and abiotic anaerobic experiments all of the sulfate oxygen was derived from water. The percentage incorporation of water-derived oxygen into sulfate during the oxidation experiments by O₂ varied with longer incubation and lower pH, but not due to the presence or absence of bacteria. These percentages were estimated as 85%, 92% and 87% from the short-term biological, long-term biological and abiotic control experiments, respectively. An oxygen isotope fractionation effect between sulfate and water (ε¹⁸O_SO4-H2O) of ∼3.5‰ was determined for the anaerobic (biological and abiotic) experiments. This measured value was then used to estimate the oxygen isotope fractionation effects between sulfate and dissolved oxygen in the aerobic experiments which were −10.0‰, −10.8‰, and −9.8‰ for the short-term biological, long-term biological and abiotic control experiments, respectively. Based on the similarity between δ¹⁸O_SO4 values in the biological and abiotic experiments, it is suggested that δ¹⁸O_SO4 values cannot be used to distinguish biological and abiotic mechanisms of pyrite oxidation. The results presented here suggest that Fe(III)_aq is the primary oxidant for pyrite at pH < 3, even in the presence of dissolved oxygen, and that the main oxygen source of sulfate is water-oxygen under both aerobic and anaerobic conditions.     

Effect of bacterial activity on trace metals release from oxidation of sphalerite at low pH (<3) and implications for AMD environment

A series of experiments was conducted to better understand the bacterial influence on the release of trace metals during oxidation of sphalerite mineral and element cycles in acid mine drainage (AMD) systems. Batch experiments were carried out as biotic and abiotic control at pH 3. Acidithiobacillus ferrooxidans, sulfur and Fe(II) oxidizer, was used in the biotic sphalerite experiment. The abiotic control experiment was run without adding the bacteria. The release behavior of six trace metals (As, Cd, Co, Pb, Cu and Mn), Fe and Zn were determined during the period of 54 days. Compared to the abiotic experiments, enhanced oxidation of sphalerite by bacteria produced high sulfate (~2,000 mg/L) and Fe_tot (139 mg/L) along with the low pH (<2.3). Consistent with this, the concentration of trace metals (As, Cd, Co, Pb, Cu and Mn) was significantly higher in the biotic experiments than those in the abiotic experiments. Results indicate that the distributions of Co and Cd in both biotic and abiotic experiments are directly related to the sphalerite dissolution whereas Pb, Cu distribution shows no strong relation to sphalerite dissolution especially in the abiotic experiments. Pb distribution in the solution appears to be controlled by pH-dependent solubility. Approximately 80% of the trace metals were removed from the solution at the end of the biotic experiments along with biologically induced Fe precipitation. Experimental results showed that bacteria play major role not only in the release of trace metal from sphalerite but also in controlling concentration of the metals in the solution by producing Fe-oxyhydroxides. The study suggest that in order to develop an effective rehabilitation strategy for AMD, it is necessary to understand bio/geochemical processes governing mobilization and deposition of trace metals in the environment.     

Impact of natural organic matter on H2O2-mediated oxidation of Fe(II) in a simulated freshwater system

The oxidation of Fe(II) by H₂O₂ has been studied in the presence of Suwannee River fulvic acid, a standard form of natural organic matter, by adding inorganic Fe(II) to solutions containing both H₂O₂ and fulvic acid and monitoring the total Fe(II) concentration using a luminol chemiluminescence method. At pH 8.4 and in the absence of competing metals, Suwannee River fulvic acid significantly retards the rate of Fe(II) oxidation due to gradual formation of a species that is oxidized more slowly than inorganic Fe(II) by both O₂ and H₂O₂. It is suggested that rapid formation of a weak Fe(II)-fulvic acid complex that is not readily oxidized by H₂O₂ is the cause of the reduction in the initial oxidation rate, and that the subsequent further reduction in oxidation rate is a result of the formation of a second type of Fe(II)-fulvic acid complex that is resistant to both O₂ and H₂O₂ oxidation. A kinetic model has been developed that supports this conceptual model. The results demonstrate that, under certain conditions, natural organic matter may stabilize Fe(II) in the presence of elevated H₂O₂ concentrations, significantly increasing the lifetime of ferrous iron and reducing the flux of hydroxyl radicals produced through this oxidation pathway.     

Biomineralization of lepidocrocite and goethite by nitrate-reducing Fe(II)-oxidizing bacteria: Effect of pH, bicarbonate, phosphate, and humic acids

Fe(III) solid phases are the products of Fe(II) oxidation by Fe(II)-oxidizing bacteria, but the Fe(III) phases reported to form within growth experiments are, at times, poorly crystalline and therefore difficult to identify, possibly due to the presence of ligands (e.g., phosphate, carbonate) that complex iron and disrupt iron (hydr)oxide precipitation. The scope of this study was to investigate the influences of geochemical solution conditions (pH, carbonate, phosphate, humic acids) on the Fe(II) oxidation rate and Fe(III) mineralogy. Fe(III) mineral characterization was performed using ⁵⁷Fe-Mössbauer spectroscopy and μ-X-ray diffraction after oxidation of dissolved Fe(II) within Mops-buffered cell suspensions of Acidovorax sp. BoFeN1, a nitrate-reducing, Fe(II)-oxidizing bacterium. Lepidocrocite (γ-FeOOH) (90%), which also forms after chemical oxidation of Fe(II) by dissolved O₂, and goethite (α-FeOOH) (10%) were produced at pH 7.0 in the absence of any strongly complexing ligands. Higher solution pH, increasing concentrations of carbonate species, and increasing concentrations of humic acids promoted goethite formation and caused little or no changes in Fe(II) oxidation rates. Phosphate species resulted in Fe(III) solids unidentifiable to our methods and significantly slowed Fe(II) oxidation rates. Our results suggest that Fe(III) mineralogy formed by bacterial Fe(II) oxidation is strongly influenced by solution chemistry, and the geochemical conditions studied here suggest lepidocrocite and goethite may coexist in aquatic environments where nitrate-reducing, Fe(II)-oxidizing bacteria are active.     

Biogeochemistry of pyrite and iron sulfide oxidation in marine sediments

Pyrite (FeS₂) and iron monosulfide (FeS) play a central role in the sulfur and iron cycles of marine sediments. They may be buried in the sediment or oxidized by O₂ after transport by bioturbation to the sediment surface. FeS₂ and FeS may also be oxidized within the anoxic sediment in which NO₃⁻, Fe(III) oxides, or MnO₂ are available as potential electron acceptors. In chemical experiments, FeS₂ and FeS were oxidized by MnO₂ but not with NO₃⁻ or amorphous Fe(III) oxide (Schippers and Jørgensen, 2001). Here we also show that in experiments with anoxic sediment slurries, a dissolution of tracer-marked ⁵⁵FeS₂ occurred with MnO₂ but not with NO₃⁻ or amorphous Fe(III) oxide as electron acceptor. To study a thermodynamically possible anaerobic microbial FeS₂ and FeS oxidation with NO₃⁻ or amorphous Fe(III) oxide as electron acceptor, more than 300 assays were inoculated with material from several marine sediments and incubated at different temperatures for > 1 yr. Bacteria could not be enriched with FeS₂ as substrate or with FeS and amorphous Fe(III) oxide. With FeS and NO₃⁻, 14 enrichments were obtained. One of these enrichments was further cultivated anaerobically with Fe²⁺ and S⁰ as substrates and NO₃⁻ as electron acceptor, in the presence of ⁵⁵FeS₂, to test for co-oxidation of FeS₂, but an anaerobic microbial dissolution of ⁵⁵FeS₂ could not been detected. FeS₂ and FeS were not oxidized by amorphous Fe(III) oxide in the presence of Fe-complexing organic compounds in a carbonate-buffered solution at pH 8. Despite many different experiments, an anaerobic microbial dissolution of FeS₂ could not be detected; thus, we conclude that this process does not have a significant role in marine sediments. FeS can be oxidized microbially with NO₃⁻ as electron acceptor. O₂ and MnO₂, but not NO₃⁻ or amorphous Fe(III) oxide, are chemical oxidants for both FeS₂ and FeS.     

Microbial reduction of ferrous arsenate: Biogeochemical implications for arsenic mobilization

In reduced aqueous environments, the presence of As in solution is a function of both biotic and abiotic mechanisms. Recent studies have demonstrated a significant release of As(III) through the microbial reduction of dissolved and mineral-bound As(V), which raises health concerns when the greater comparative mobility and toxicity of As(III) is considered. These release mechanisms do not operate in isolation but occur in concert with a number of removal processes, including secondary mineralization and sorption to other natural substrates. Thermodynamic and applied experimental studies have shown that ferrous arsenates, such as symplesite [Fe(II)₃(As(V)O₄)₂·8H₂O], may provide a significant sink for Fe(II) and As(V). In this study, the stability of a representative ferrous arsenate phase in the presence of the arsenate-reducing bacterium Shewanella sp. strain ANA-3 is examined. The reduction of ferrous arsenate by ANA-3 results in the release of aqueous As(III) and, subsequently, the progressive nucleation of a biogenic ferrous arsenite phase proximal to the microbial cells. The valence states of secondary solid-phase products were verified using X-ray absorption spectroscopy (XAS). Electron microscopy reveals that nucleation occurs on cellular exudates which may imply a role of extracellular reduction through c-type cytochromes as investigated in recent literature. These observations provide new insights into the reduction mechanisms of ANA-3 and the biogeochemical cycling of As(III) in natural systems.