The oxidation of carbonate green rust into ferric phases:solid-state reaction or transformation via solution

The oxidation of carbonate green rust, GR(CO₃²⁻), in NaHCO₃ solutions at T = 25°C has been investigated through electrochemical techniques, FTIR, XRD, TEM and SEM. The used GR(CO₃²⁻) samples were made of either suspended solid in solution or a thin electrochemically formed layer on the surface of an iron disc. Depending on experimental conditions, oxidation occurs, with or without major modifications of the GR(CO₃²⁻) structure, suggesting the existence of two pathways: solid-state oxidation (SSO) leading to a ferric oxyhydroxycarbonate as the end product, and a dissolution-oxidation-precipitation (DOP) mechanism leading to ferric oxihydroxides such as lepidocrocite, goethite, or ferrihydrite. A formula was proposed for this ferric oxyhydroxycarbonate, Fe₆^IIIO_(2+x)(OH)_(12-2x)(H₂O)_x(CO₃), assuming that the solid-state oxidation reaction is associated to a deprotonation of the water molecules within the interlayers, or of the hydroxyl groups in the Fe(O,H) octahedra layers. The DOP mechanism involves transformation via solution with the occurrence of soluble ferrous-ferric intermediate species. A discussion about factors influencing the oxidation of carbonate green rust is provided hereafter. The ferric oxyhydroxycarbonate can be reduced back to GR(CO₃²⁻) by a reverse solid-state reduction reaction. The potentiality for a solid-state redox cycling of iron to occur may be considered. The stability of the ferric oxyhydroxycarbonate towards thermodynamically stable ferric phases, such as goethite and hematite, was also studied.     

Structure and stability of the Fe(II)–Fe(III) green rust “fougerite” mineral and its potential for reducing pollutants in soil solutions

Fe(II)–Fe(III) layered double hydroxysalt green rusts, GRs, are very reactive compounds with the general formula, [Fe^II_(1−x) Fe^III_x (OH)₂]^x+·[(x/n) Aⁿ⁻·(m/n) H₂O]^x−, where x is the ratio Fe^III/Fe_tot, and reflects the structure in which brucite-like layers alternate with interlayers of anions Aⁿ⁻ and water molecules. Two types of crystal structure for GRs, GR1 and GR2, represented by the hydroxychloride GR1(Cl⁻) and the hydroxysulphate GR2(SO₄²⁻) are distinguished by X-ray diffraction due to different stacking. By analogy with GR1(Cl⁻) the structure of the fougerite GR mineral, [Fe^II_(1−x) Fe^III_x (OH)₂]^x+·[x OH⁻·(1−x) H₂O]^x-  Fe(OH)_(2+x)·(1−x) H₂O, is proposed displaying interlayers made of OH⁻ ions and water molecules (in situ deprotonation of water molecules is necessary for explaining the flexibility of its composition). The space group of mineral GR1(OH⁻) would be R3̄m, with lattice parameters a≅0.32 and c≅2.25 nm. Stability conditions and the E_h-pH diagram of Fe(OH)_(2+x) (the water molecules are omitted) are determined from hydromorphic soil solution equilibria with GR mineral in Brittany (France). Computed Gibbs free energies of formation from soil solution/mineral equilibrium fit well with a regular solid solution model: μ°[Fe(OH)_(2+x)]=(1−x) μ°[Fe(OH)₂]+x μ°[Fe(OH)₃]+RT [(1−x) ln (1−x)+x ln x]+A₀ x (1−x), where μ°[Fe(OH)₂]=−492.5 kJ mol⁻¹, μ°[Fe(OH)₃]=−641 kJ mol⁻¹ and A₀=−243.9 kJ mol⁻¹ at the average temperature of 9±1°C. The upper limit of occurrence of GR mineral at x=2/3, i.e. Fe₃(OH)₈, is explained by its unstability vs. α-FeOOH and/or magnetite; Fe(OH)₃ is thus a hypothetical compound with a GR structure which cannot be observed. These thermodynamic data and E_h-pH diagrams of Fe(OH)_(2+x) can be used most importantly to predict the possibility that GR minerals reduce some anions in contaminated soils. The cases of NO₃⁻, Se(VI) or Cr(VI) are fully illustrated.     

Biotransformation of two-line silica-ferrihydrite by a dissimilatory Fe(III)-reducing bacterium: formation of carbonate green rust in the presence of phosphate

The reductive biotransformation of two Si-ferrihydrite coprecipitates (1 and 5 mole % Si) by Shewanella putrefaciens, strain CN32, was investigated in 1,4-piperazinediethanesulfonic acid-buffered media (pH ∼7) with lactate as the electron donor. Anthraquinone-2,6-disulfonate, an electron shuttle, was present in the media. Experiments were performed without and with PO₄³⁻ (P) (1 to 20 mmol/L) in media containing 50 mmol/L Fe. Our objectives were to define the combined effects of SiO₄⁴⁻ (Si) and P on the bioreducibility and biomineralization of ferrihydrites under anoxic conditions. Iron reduction was measured as a function of time, solids were characterized by powder X-ray diffraction and Mössbauer spectroscopy, and aqueous solutions were analyzed for Si, P, Cl⁻ and inorganic carbon. Both of the ferrihydrites were rapidly reduced regardless of the Si and P content. Si concentration had no effect on the reduction rate or mineralization products. Magnetite was formed in the absence of P whereas carbonate green rust GR(CO₃²⁻) ([Fe_(6−x)^IIFe^III_x(OH)₁₂]^x+(CO₃²⁻)_0.5x · yH₂O) and vivianite [Fe₃(PO₄)₂ · 8H₂O], were formed when P was present. GR(CO₃²⁻) dominated as a mineral product in samples with <4 mmol/L P. The Fe(II)/Fe(III) ratio of GR(CO₃²⁻) varied with P concentration; the ratio was 2 in 1 mmol/L P and approached 1 with 4- and 10 mmol/L P. Green rust appeared to form by solid-state transformation of ferrihydrite. Media P and Si concentration dictated the mechanism of transformation. In the 1 mole % Si coprecipitate with 1 mmol/L P, an intermediate Fe(II)/Fe(III) phase with structural Fe(II) slowly transformed to GR with time. In contrast, when ferrihydrite contained more Si (5 mole %) and/or contained higher P (4 mmol/L), sorbed Fe(II) and residual ferrihydrite together transformed to GR. Despite similar chemistries, P was shown to have a profound effect on extent of ferrihydrite reduction and biotransformations while that of Si was minimal.     

Redox potential measurements and Mössbauer spectrometry of Fe adsorbed onto Fe (oxyhydr)oxides

The redox properties of Fe^II adsorbed onto a series of Fe^III (oxyhydr)oxides (goethite, lepidocrocite, nano-sized ferric oxide hydrate (nano-FOH), and hydrous ferric oxide (HFO)) have been investigated by rest potential measurements at a platinum electrode, as a function of pH (−log₁₀[H⁺]) and surface coverage. Using the constant capacitance surface complexation model to describe Fe^II adsorption onto these substrates, theoretical values of the suspension redox potential (E_H) have been computed, under the assumption that Fe^II adsorption occurs at crystal growth sites of the substrate surface. Good agreement between calculated and experimental E_H values is observed for nano-FOH and HFO, however the redox potentials measured for lepidocrocite and goethite are significantly more oxidizing than predicted. Mössbauer spectroscopic analysis of ⁵⁷Fe^II adsorbed onto HFO and goethite shows that in both cases the adsorbed ⁵⁷Fe^II is incorporated into the crystal structure of the substrate, in broad agreement with the thermodynamic model, but is almost completely oxidized to ⁵⁷Fe^III. The mechanism by which the adsorbed ⁵⁷Fe^II is oxidized is not resolved in this work, but is thought to be due to electron transfer to the substrate, rather than a net oxidation of the suspension. The disagreement between experimental and calculated rest potential measurements in the goethite and lepidocrocite systems is thought to be due to the poor electrochemical equilibration of these suspensions with the platinum electrode, rather than a failure of the thermodynamic model. The model developed for the redox potential of adsorbed Fe^II allows direct assessment of the reactivity of this species towards oxidized pollutants.     

In situ Mössbauer spectroscopy: Evidence for green rust (fougerite) in a gleysol and its mineralogical transformations with time and depth

A miniaturized Mössbauer spectrometer, adapted to the Earth’s conditions from the instrument developed for Mars space missions, has been used for the first time to study in situ variations with depth and transformations with time of iron minerals in a gleysol. The instrument is set into a PVC tube and can be moved up and down precisely (±1 mm) at the desired depth. Mössbauer spectra were obtained from 15 to 106 cm depth and repeated exactly at the same point at different times to follow mineralogical transformations with time. X-ray diffraction (XRD) and selective extraction techniques were performed on soil samples. The piezometric level of the water table was measured and the composition of the soil solution was monitored in situ and continuously, with a multiparametric and automatic probe. All the Mössbauer spectra obtained are characteristic of Fe(II)-Fe(III) green rust-fougerite, a natural mineral of the meixnerite group, that is, whose structural formula is: [Fe_{1 − x}^II Mg_y Fe_x^III (OH)_2+2y]^x+[xA, mH₂O]^x−, where x is the ratio Fe³⁺/Fe_tot. and A the intercalated anion. The name of fougerite has been formally approved by the Commission on New Minerals and Mineral Names of IMA (number 2003-057), on January 29, 2004. No other iron phases have been found by this way or by XRD. About 90% of total iron is extractible by dithionite-citrate-bicarbonate, and 60% by citratebicarbonate. In the horizons showing oximorphic properties that are in the upper part of the studied soil profile, x ratio in fougerite, deduced from Mössbauer spectra, is approximately 2/3. In the deepest horizons that show reductomorphic properties, x ratio is only 1/3. Fast mineralogical transformations were observed at well-defined points in soil, as evidenced by x ratio variations observed when Mössbauer spectra were acquired at different times at the same depth. Variations of the level of the water table and of pe and pH of the soil solution were simultaneously observed and could explain these mineralogical transformations. A ternary solid solution model previously proposed for OH-fougerite has been extended to chloride, sulphate, and carbonate green rusts to estimate the Gibbs free energies of formation of fougerite, providing for possible anions other than OH⁻ in the interlayer and for Mg substitution. Soil solutions appear as largely oversaturated with respect to OH-fougerite, either oversaturated or undersaturated to “carbonate-fougerite” and “sulphate-fougerite”, and largely undersaturated with respect to “chloro-fougerite”. Fougerite forms most likely from oversaturated solutions by coprecipitation of Fe³⁺ with Fe²⁺ and Mg²⁺. Oxidation and reduction are driven by pH and pe variations, with both long timescale variations and short duration events. Exactly as synthetic green rusts are very reactive compounds in the laboratory, fougerite is thus a very reactive mineral and readily forms, dissolves, or evolves in soils.     

Arsenite sequestration at the surface of nano-Fe(OH)2, ferrous-carbonate hydroxide, and green-rust after bioreduction of arsenic-sorbed lepidocrocite by Shewanella putrefaciens

X-ray Absorption Fine Structure (XAFS) spectroscopy was used in combination with high resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), X-ray energy dispersive spectroscopy (XEDS), X-ray powder diffraction, and Mössbauer spectroscopy to obtain detailed information on arsenic and iron speciation in the products of anaerobic reduction of pure and As(V)- or As(III)-adsorbed lepidocrocite (γ-FeOOH) by Shewanella putrefaciens ATCC 12099. We found that this strain of S. putrefaciens is capable of using Fe(III) in lepidocrocite and As(V) in solution or adsorbed on lepidocrocite surfaces as electron acceptors. Bioreduction of lepidocrocite in the absence of arsenic resulted in the formation of hydroxycarbonate green rust 1 [Fe^II₄Fe^III₂(OH)₁₂CO₃: GR1(CO₃)], which completely converted into ferrous-carbonate hydroxide (Fe^II₂(OH)₂CO₃: FCH) over nine months. This study thus provides the first evidence of bacterial reduction of stoichiometric GR1(CO₃) into FCH. Bioreduction of As(III)-adsorbed lepidocrocite also led to the formation of GR1(CO₃) prior to formation of FCH, but the presence of As(III) slows down this transformation, leading to the co-occurrence of both phases after 22-month of aging. At the end of this experiment, As(III) was found to be adsorbed on the surfaces of GR1(CO₃) and FCH. After five months, bioreduction of As(V)-bearing lepidocrocite led directly to the formation of FCH in association with nanometer-sized particles of a minor As-rich Fe(OH)₂ phase, with no evidence for green rust formation. In this five-month experiment, As(V) was fully converted to As(III), which was dominantly sorbed at the surface of the Fe(OH)₂ nanoparticles as oligomers binding to the edges of Fe(OH)₆ octahedra at the edges of the octahedral layers of Fe(OH)₂. These multinuclear As(III) surface complexes are characterized by As-As pairs at a distance of 3.32 ± 0.02 Å and by As-Fe pairs at a distance of 3.50 ± 0.02 Å and represent a new type of As(III) surface complex. Chemical analyses show that the majority of As(III) produced in the experiments with As present is associated with iron-bearing hydroxycarbonate or hydroxide solids, reinforcing the idea that, at least under some circumstances, bacterial reduction can promote As(III) sequestration instead of mobilizing it into solution.     

Iron(III) accumulations in inland saline waterways,Hunter Valley,Australia: Mineralogy,micromorphology and pore-water geochemistry

Discharge of Fe(II)-rich groundwaters into surface-waters results in the accumulation of Fe(III)-minerals in salinized sand-bed waterways of the Hunter Valley, Australia. The objective of this study was to characterise the mineralogy, micromorphology and pore-water geochemistry of these Fe(III) accumulations. Pore-waters had a circumneutral pH (6.2–7.2), were sub-oxic to oxic (Eh 59–453 mV), and had dissolved Fe(II) concentrations up to 81.6 mg L⁻¹. X-ray diffraction (XRD) on natural and acid-ammonium-oxalate (AAO) extracted samples indicated a dominance of 2-line ferrihydrite in most samples, with lesser amounts of goethite, lepidocrocite, quartz, and alumino-silicate clays. The majority of Fe in the samples was bound in the AAO extractable fraction (Fe^Ox) relative to the Na-dithionite extractable fraction (Fe^Di), with generally high Fe^Ox:Fe^Di ratios (0.52–0.92). The presence of nano-crystalline 2-line ferrihydrite (Fe₅HO₃·4H₂O) with lesser amounts of goethite (α-FeOOH) was confirmed by scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED). In addition, it was found that lepidocrocite (γ-FeOOH), which occurred as nanoparticles as little as ∼5 lattice spacings thick perpendicular to the (0 2 0) lattice plane, was also present in the studied Fe(III) deposits. Overall, the results highlight the complex variability in the crystallinity and particle-size of Fe(III)-minerals which form via oxidation of Fe(II)-rich groundwaters in sand-bed streams. This variability may be attributed to: (1) divergent precipitation conditions influencing the Fe(II) oxidation rate and the associated supply and hydrolysis of the Fe(III) ion, (2) the effect of interfering compounds, and (3) the influence of bacteria, especially Leptothrix ochracea.     

Effects of sediment iron mineral composition on microbially mediated changes in divalent metal speciation: Importance of ferrihydrite

Dissimilatory metal reducing bacteria (DMRB) can influence geochemical processes that affect the speciation and mobility of metallic contaminants within natural environments. Most investigations into the effect of DMRB on sediment geochemistry utilize various synthetic oxides as the Fe^III source (e.g., ferrihydrite, goethite, hematite). These synthetic materials do not represent the mineralogical composition of natural systems, and do not account for the effect of sediment mineral composition on microbially mediated processes. Our experiments with a DMRB (Shewanella putrefaciens 200) and a divalent metal (Zn^II) indicate that, while complexity in sediment mineral composition may not strongly impact the degree of “microbial iron reducibility,” it does alter the geochemical consequences of such microbial activity. The ferrihydrite and clay mineral content are key factors. Microbial reduction of a synthetic blend of goethite and ferrihydrite (VHSA-G) carrying previously adsorbed Zn^II increased both [Zn^II-aq] and the proportion of adsorbed Zn^II that is insoluble in 0.5 M HCl. Microbial reduction of Fe^III in similarly treated iron-bearing clayey sediment (Fe-K-Q) and hematite sand, which contained minimal amounts of ferrihydrite, had no similar effect. Addition of ferrihydrite increased the effect of microbial Fe^III reduction on Zn^II association with a 0.5 M HCl insoluble phase in all sediment treatments, but the effect was inconsequential in the Fe-K-Q. Zinc k-edge X-ray absorption spectroscopy (XAS) data indicate that microbial Fe^III reduction altered Zn^II bonding in fundamentally different ways for VHSA-G and Fe-K-Q. In VHSA-G, ZnO₆ octahedra were present in both sterile and reduced samples; with a slightly increased average Zn-O coordination number and a slightly higher degree of long-range order in the reduced sample. This result may be consistent with enhanced Zn^II substitution within goethite in the microbially reduced sample, though these data do not show the large increase in the degree of Zn-O-metal interactions expected to accompany this change. In Fe-K-Q, microbial Fe^III reduction transforms Zn-O polyhedra from octahedral to tetrahedral coordination and leads to the formation of a ZnCl₂ moiety and an increased degree of multiple scattering. This study indicates that, while many sedimentary iron minerals are easily reduced by DMRB, the effects of microbial Fe^III reduction on trace metal geochemistry are dependent on sediment mineral composition.     

Germanium crystal chemistry in hematite and goethite from the Apex Mine,Utah, and some new data on germanium in aqueous solution and in stottite

At the Apex Mine in southwest Utah, fine-grained hematite contains as much as 1.0 wt. percent Ge, and fine-grained goethite contains as much as 0.7 wt. percent Ge. The mode of Ge incorporation in these minerals was investigated by high-resolution K-edge fluorescence spectroscopy using synchrotron radiation. Analysis of extended fine structure (EXAFS) K-edge data for Ge and Fe shows that Ge substitutes for Fe in the octahedral metal sites of the studied hematite and goethite, with average Ge-ligand bond lengths of 1.88 Å. The solid solution in hematite probably occurs through the coupled substitution 2Fe(III) = Ge(IV) + Fe(II), similar to the coupled substitution 2Fe(III) = Ti(IV) + Fe(II) that occurs in the solid solution series hematite-ilmenite. The solid solution in goethite probably occurs by the loss of an H atom from an OH group, through the coupled substitution Fe(III) + H(I) = Ge(IV). In related experiments, EXAFS data indicate that in a neutral aqueous solution containing 790 ppm Ge, the Ge occurs predominately as Ge(OH)₄, with tetrahedral Ge-OH bond lengths of 1.74 Å. In stottite, FeGe(OH)₆, Fe(II) and Ge(IV) occur in octahedral sites with average Fe-OH and Ge-OH bond lengths of about 2.20 Å and 1.88 Å.     

Copper(II) sorption onto goethite, hematite and lepidocrocite: a surface complexation model based on ab initio molecular geometries and EXAFS spectroscopy

We measured the adsorption of Cu(II) onto goethite (α-FeOOH), hematite (α-Fe₂O₃) and lepidocrocite (γ-FeOOH) from pH 2-7. EXAFS spectra show that Cu(II) adsorbs as (CuO₄H_n)ⁿ⁻⁶ and binuclear (Cu₂O₆H_n)ⁿ⁻⁸ complexes. These form inner-sphere complexes with the iron (hydr)oxide surfaces by corner-sharing with two or three edge-sharing Fe(O,OH)₆ polyhedra. Our interpretation of the EXAFS data is supported by ab initio (density functional theory) geometries of analogue Fe₂(OH)₂(H₂O)₈Cu(OH)₄and Fe₃(OH)₄(H₂O)₁₀Cu₂(OH)₆ clusters. We find no evidence for surface complexes resulting from either monodentate corner-sharing or bidentate edge-sharing between (CuO₄H_n)ⁿ⁻⁶ and Fe(O,OH)₆ polyhedra. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed even though we are supersaturated with respect to CuO and Cu(OH)₂. Having identified the bidentate (FeOH)₂Cu(OH)₂⁰ and tridentate (Fe₃O(OH)₂)Cu₂(OH)₃⁰ surface complexes, we are able to fit the experimental copper(II) adsorption data to the reactions

     

Paleoenvironmental implications of concentration and C/C ratios of Fe(CO3)OH in goethite from a mid-latitude Cenomanian laterite in southwestern Minnesota

A mid-Cretaceous (∼95 Ma) laterite in southwestern Minnesota contains pisolites that consist primarily of gibbsite, quartz, and kaolinite with smaller amounts of goethite (α-FeOOH) and hematite. The presence of minor berthierine (an Fe(II) sheet silicate) suggests that this Cenomanian laterite experienced some degree of low temperature reductive diagenesis during its burial history. The prospects for extracting useful paleoenvironmental information from the pisolitic goethite were explored by studying measured mole fraction (X_m) and δ¹³C_m values of the Fe(CO₃)OH component in solid solution in the goethite using the method of incremental vacuum dehydration-decarbonation.Data arrays that occupy distinctly different domains in plots of δ¹³C_m vs. 1/X_m suggest the existence of two generations of goethite in the pisolites. The apparently younger generation of goethite (“generation 2”) evolves CO₂ at 170 °C, while the older generation (“generation 1”) evolves CO₂ at 220 °C. The distribution of the data suggests that generation 2 goethite is a proxy for mixing of CO₂ from three distinct CO₂ sources in an oxidative environment which post-dated the reductive diagenesis. The small amount of generation 1 goethite seems to have persisted through the reductive diagenesis, and nine of the generation 1 goethite data appear to define a proxy, two-endmember, soil CO₂ mixing line. Such two-component mixing is consistent with expectations for a highly evolved, carbonate-free laterite (i.e., the pre-diagenetic Cenomanian weathering system). The δ¹³C_m values of these nine data points range from −23.1‰ to −13.7‰, whereas X_m values range from 0.0007 to 0.0222. Linear regression of these nine data yields a slope of 0.0064, which corresponds to an ancient tropospheric CO₂ concentration of about 1900 ppmV.Isotopic data from pisolitic kaolinite indicate a paleotemperature of about 24 °C at a paleolatitude of ∼40°N. This is substantially warmer than modern continental temperatures at such latitudes and is consistent with published indications of a generally warmer mid-Cretaceous climate. Moreover, the correspondence of a warmer mid-Cretaceous climate with the inferred, relatively high concentration of Cenomanian tropospheric CO₂ (∼1900 ppmV) is consistent with the idea that variations of atmospheric CO₂ have a relation to climate change. The results of this study emphasize the importance of careful evaluation of incremental dehydration-decarbonation data from natural goethites to assess the possibility that more than one generation of goethite is present in a sample. However, the results also indicate that the carbon isotope information recorded in admixed goethite generations may be sorted out and used in paleoenvironmental interpretations.     

Role of Fe(II) and phosphate in arsenic uptake by coprecipitation

Natural attenuation of arsenic by simple adsorption on oxyhydroxides may be limited due to competing oxyanions, but uptake by coprecipitation may locally sequester arsenic. We have systematically investigated the mechanism and mode (adsorption versus coprecipitation) of arsenic uptake in the presence of carbonate and phosphate, from solutions of inorganic composition similar to many groundwaters. Efficient arsenic removal, >95% As(V) and ∼55% in initial As(III) systems, occurred over 24 h at pHs 5.5-6.5 when Fe(II) and hydroxylapatite (Ca₅(PO₄)₃OH, HAP) “seed” crystals were added to solutions that had been previously reacted with HAP, atmospheric CO_2(g) and O_2(g). Arsenic adsorption was insignificant (<10%) on HAP without Fe(II). Greater uptake in the As(III) system in the presence of Fe(II) was interpreted as due to faster As(III) to As(V) oxidation by molecular oxygen in a putative pathway involving Fe(IV) and As(IV) intermediate species. HAP acts as a pH buffer that allows faster Fe(II) oxidation. Solution analyses coupled with high-resolution transmission electron microscopy (HRTEM), X-ray Energy-Dispersive Spectroscopy (EDS), and X-Ray Absorption Spectroscopy (XAS) indicated the precipitation of sub-spherical particles of an amorphous, chemically-mixed, nanophase, Fe^III[(OH)₃(PO₄)(As^VO₄)]·nH₂O or Fe^III[(OH)₃( PO₄)(As^VO₄)(As^IIIO₃)_minor]·nH₂O, where As^IIIO₃ is a minor component.The mode of As uptake was further investigated in binary coprecipitation (Fe(II) + As(III) or P), and ternary coprecipitation and adsorption experiments (Fe(II) + As(III) + P) at variable As/Fe, P/Fe and As/P/Fe ratios. Foil-like, poorly crystalline, nanoparticles of Fe^III(OH)₃ and sub-spherical, amorphous, chemically-mixed, metastable nanoparticles of Fe^III[(OH)₃, PO₄]·nH₂O coexisted at lower P/Fe ratios than predicted by bulk solubilities of strengite (FePO₄·2H₂O) and goethite (FeOOH). Uptake of As and P in these systems decreased as binary coprecipitation > ternary coprecipitation > ternary adsorption.Significantly, the chemically-mixed, ferric oxyhydroxide-phosphate-arsenate nanophases found here are very similar to those found in the natural environment at slightly acidic to circum-neutral pHs in sub-oxic to oxic systems, such phases may naturally attenuate As mobility in the environment, but it is important to recognize that our system and the natural environment are kinetically evolving, and the ultimate environmental fate of As will depend on the long-term stability and potential phase transformations of these mixed nanophases. Our results also underscore the importance of using sufficiently complex, yet systematically designed, model systems to accurately represent the natural environment.     

Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants

Recent studies show that ferrous iron (Fe^II), which is often abundant in anaerobic soil and groundwater, is capable of abiotically reducing many subsurface contaminants. However, studies also demonstrate that Fe^II redox reactivity in geochemical systems is heavily dependent upon metal speciation. This contribution examines the influence of hydroxamate ligands, including the trihydroxamate siderophore desferrioxamine B (DFOB), on Fe^II reactions with nitroaromatic groundwater contaminants (NACs). Experimental results demonstrate that ring-substituted NACs are reduced to the corresponding aniline products in aqueous solutions containing Fe^II complexes with DFOB and two monohydroxamate ligands (acetohydroxamic acid and salicylhydroxamic acid). Reaction rates are heavily dependent upon solution conditions and the identities of both the Fe^II-complexing hydroxamate ligand and the target NAC. Trends in the observed pseudo-first-order rate constants for reduction of 4-chloronitrobenzene (k_obs, s⁻¹) are quantitatively linked to the formation of Fe^II species with standard one-electron reduction potentials, (Fe^III/Fe^II), below −0.3 V. Linear free energy relationships correlate reaction rates with the (Fe^III/Fe^II) values of different electron-donating Fe^II complexes and with the apparent one-electron reduction potentials of different electron-accepting NACs, (ArNO₂). Experiments describing a redox auto-decomposition mechanism for Fe^II-DFOB complexes that occurs at neutral pH and has implications for the stability of hydroxamate siderophores in anaerobic environments are also presented. Results from this study indicate that hydroxamates and other Fe^III-stabilizing organic ligands can form highly redox-active Fe^II complexes that may contribute to the natural attenuation and remediation of subsurface contaminants.     

Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils

We examined the solubility, mineralogy and geochemical transformations of sedimentary Fe in waterways associated with coastal lowland acid sulfate soils (CLASS). The waterways contained acidic (pH 3.26-3.54), Fe^III-rich (27-138 μM) surface water with low molar Cl:SO₄ ratios (0.086-5.73). The surficial benthic sediments had high concentrations of oxalate-extractable Fe(III) due to schwertmannite precipitation (kinetically favoured by 28-30% of aqueous surface water Fe being present as the Fe^III species). Subsurface sediments contained abundant pore-water HCO₃ (6-20 mM) and were reducing (Eh < −100 mV) with pH 6.0-6.5. The development of reducing conditions caused reductive dissolution of buried schwertmannite and goethite (formed via in situ transformation of schwertmannite). As a consequence, pore-water Fe^II concentrations were high (>2 mM) and were constrained by precipitation-dissolution of siderite. The near-neutral, reducing conditions also promoted SO₄-reduction and the formation of acid-volatile sulfide (AVS). The results show, for the first time for CLASS-associated waterways, that sedimentary AVS consisted mainly of disordered mackinawite. In the presence of abundant pore-water Fe^II, precipitation-dissolution of disordered mackinawite maintained very low (i.e. <0.1 μM) S^−II concentrations. Such low concentrations of S^−II caused slow rates for conversion of disordered mackinawite to pyrite, thereby resulting in relatively low concentrations of pyrite (<300 μmol g⁻¹ as Fe) compared to disordered mackinawite (up to 590 μmol g⁻¹ as Fe). This study shows that interactions between schwertmannite, goethite, siderite, disordered mackinawite and pyrite control the geochemical behaviour of sedimentary Fe in CLASS-associated waterways.     

Controls on Fe reduction and mineral formation by a subsurface bacterium

The reductive dissolution of FeIII (hydr)oxides by dissimilatory iron-reducing bacteria (DIRB) could have a large impact on sediment genesis and Fe transport. If DIRB are able to reduce FeIII in minerals of high structural order to carry out anaerobic respiration, their range could encompass virtually every O₂-free environment containing FeIII and adequate conditions for cell growth. Previous studies have established that Shewanella putrefaciens CN32, a known DIRB, will reduce crystalline Fe oxides when initially grown at high densities in a nutrient-rich broth, conditions that poorly model the environments where CN32 is found. By contrast, we grew CN32 by batch culture solely in a minimal growth medium. The stringent conditions imposed by the growth method better represent the conditions that cells are likely to encounter in their natural habitat. Furthermore, the expression of reductases necessary to carry out dissimilatory Fe reduction depends on the method of growth. It was found that under anaerobic conditions CN32 reduced hydrous ferric oxide (HFO), a poorly crystalline FeIII mineral, and did not reduce suspensions containing 4 mM FeIII in the form of poorly ordered nanometer-sized goethite (α-FeOOH), well-ordered micron-sized goethite, or nanometer-sized hematite (α-Fe₂O₃) crystallites. Transmission electron microscopy (TEM) showed that all minerals but the micron-sized goethite attached extensively to the bacteria and appeared to penetrate the outer cellular membrane. In the treatment with HFO, new FeII and FeIII minerals formed during reduction of HFO-Fe in culture medium containing 4.0 mmol/L P_i (soluble inorganic P), as observed by TEM with energy-dispersive X-ray spectroscopy, selected area electron diffraction, and X-ray diffraction. The minerals included magnetite (Fe₃O₄), goethite, green rust, and vivianite [Fe₃(PO₄)₂ · 8H₂O]. Vivianite appeared to be the stable end product and the mean coherence length was influenced by the rate of FeIII reduction. When P_i was 0.4 mol/L under otherwise identical conditions, goethite was the only mineral observed to form, and less Fe²⁺ was produced overall. Hence, the ability of DIRB to reduce Fe (hydr)oxides may be limited when the bacteria are grown under nutrient-limited conditions, and the minerals that result depend on the vigor of FeIII reduction.     

Reductive transformation of iron and sulfur in schwertmannite-rich accumulations associated with acidified coastal lowlands

We examined the transformations of Fe and S associated with schwertmannite (Fe₈O₈(OH)₆SO₄) reduction in acidified coastal lowlands. This was achieved by conducting a 91 day diffusive-flux column experiment, which involved waterlogging of natural schwertmannite- and organic-rich soil material. This experiment was complemented by short-term batch experiments utilizing synthetic schwertmannite. Waterlogging readily induced bacterial reduction of schwertmannite-derived Fe(III), producing abundant pore-water Fe^II, SO₄ and alkalinity. Production of alkalinity increased pH from pH 3.4 to pH ∼6.5 within the initial 14 days, facilitating the precipitation of siderite (FeCO₃). Interactions between schwertmannite and Fe^II at pH ∼6.5 were found, for the first time, to catalyse the transformation of schwertmannite to goethite (αFeOOH). Thermodynamic calculations indicate that this Fe^II-catalysed transformation shifted the biogeochemical regime from an initial dominance of Fe(III)-reduction to a subsequent co-occurrence of both Fe(III)- and SO₄-reduction. This lead firstly to the formation of elemental S via H₂S oxidation by goethite, and later also to formation of nanoparticulate mackinawite (FeS) via H₂S precipitation with Fe^II. Pyrite (FeS₂) was a quantitatively insignificant product of reductive Fe and S mineralization. This study provides important new insights into Fe and S geochemistry in settings where schwertmannite is subjected to reducing conditions.     

Influence of pH on mineral speciation in a bioreactor simulating acid mine drainage

Mineral precipitates formed under conditions representative of acid mine drainage were prepared by oxidizing 0.1 M FeS0₄ · 7H₂0 solutions at 24°C and pH 2.3, 2.6, 3.0, 3.3 and 3.6 using a bioreactor and a strain ofThiobacillus ferrooxidans. The oxidation of dissolved Fe²⁺ was monitored colorimetrically and was completed within 90 to 120 h at all pHs. Schwertmannite, Fe₈O₈(OH)₆SO₄, was a major component of the precipitates and was the only phase formed at pH 3.0. Jarosite, (H,Na,K)Fe₃(OH)₆(SO₄)₂, increased in abundance with decreasing pH whereas goethite, α-FeOOH, appeared at pH 3.3 and 3.6. A similar relationship between pH and mineralogy has been reported in natural specimens of mine drainage ochres.     

Experimental analysis of arsenic precipitation during microbial sulfate and iron reduction in model aquifer sediment reactors

Microbial SO₄²⁻ reduction limits accumulation of aqueous As in reducing aquifers where the sulfide that is produced forms minerals that sequester As. We examined the potential for As partitioning into As- and Fe-sulfide minerals in anaerobic, semi-continuous flow bioreactors inoculated with 0.5% (g mL⁻¹) fine-grained alluvial aquifer sediment. A fluid residence time of three weeks was maintained over a ca. 300-d incubation period by replacing one-third of the aqueous phase volume of the reactors with fresh medium every seven days. The medium had a composition comparable to natural As-contaminated groundwater with slightly basic pH (7.3) and 7.5 μM aqueous As(V) and also contained 0.8 mM acetate to stimulate microbial activity. Medium was delivered to a reactor system with and without 10 mmol L⁻¹ synthetic goethite (α-FeOOH). In both reactors, influent As(V) was almost completely reduced to As(III). Pure As-sulfide minerals did not form in the Fe-limited reactor. Realgar (As₄S₄) and As₂S₃(am) were undersaturated throughout the experiment. Orpiment (As₂S₃) was saturated while sulfide content was low (∼50 to 150 μM), but precipitation was likely limited by slow kinetics. Reaction-path modeling suggests that, even if these minerals had formed, the dissolved As content of the reactor would have remained at hazardous levels. Mackinawite (Fe_1 + xS; x ? 0.07) formed readily in the Fe-bearing reactor and held dissolved sulfide at levels below saturation for orpiment and realgar. The mackinawite sequestered little As (<0.1 wt.%), however, and aqueous As accumulated to levels above the influent concentration as microbial Fe(III) reduction consumed goethite and mobilized adsorbed As. A relatively small amount of pyrite (FeS₂) and greigite (Fe₃S₄) formed in the Fe-bearing reactor when we injected a polysulfide solution (Na₂S₄) to a final concentration of 0.5 mM after 216, 230, 279, and 286 days. The pyrite, and to a lesser extent the greigite, that formed did sequester As from solution, containing 0.84 and 0.23 wt.% As on average, respectively. Our results suggest that As precipitation during Fe-sulfide formation in nature occurs mainly in conjunction with pyrite formation. Our findings imply that the effectiveness of stimulating microbial SO₄²⁻ reduction to remediate As contamination may be limited by the rate and extent of pyrite formation and the solubility of As-sulfides.     

Surface complexation of arsenic(V) to iron(III) (hydr)oxides: structural mechanism from ab initio molecular geometries and EXAFS spectroscopy

Arsenic(V), as the arsenate (AsO₄)³⁻ ion and its conjugate acids, is strongly sorbed to iron(III) oxides (α-Fe₂O₃), oxide hydroxides (α-,γ-FeOOH) and poorly crystalline ferrihydrite (hydrous ferric oxide). The mechanism by which arsenate complexes with iron oxide hydroxide surfaces is not fully understood. There is clear evidence for inner sphere complexation but the nature of the surface complexes is controversial. Possible surface complexes between AsO₄ tetrahedra and surface FeO₆ polyhedra include bidentate corner-sharing (²C), bidentate edge-sharing (²E) and monodentate corner-sharing (¹V). We predicted the relative energies and geometries of AsO₄-FeOOH surface complexes using density functional theory calculations on analogue Fe₂(OH)₂(H₂O)_nAsO₂(OH)₂³⁺ and Fe₂(OH)₂(H₂O)_nAsO₄⁺ clusters. The bidentate corner-sharing complex is predicted to be substantially (55 kJ/mole) more favored energetically over the hypothetical edge-sharing bidentate complex. The monodentate corner-sharing (¹V) complex is very unstable. We measured EXAFS spectra of 0.3 wt. % (AsO₄)³⁻ sorbed to hematite (α-Fe₂O₃), goethite(α-FeOOH), lepidocrocite(γ-FeOOH) and ferrihydrite and fit the EXAFS directly with multiple scattering. The phase-shift-corrected Fourier transforms of the EXAFS spectra show peaks near 2.85 and 3.26 Å that have been attributed by previous investigators to result from ²E and ²C complexes. However, we show that the peak near 2.85 Å appears to result from As-O-O-As multiple scattering and not from As-Fe backscatter. The observed 3.26 Å As-Fe distance agrees with that predicted for the bidentate corner-sharing surface (²C) complex. We find no evidence for monodentate (¹V) complexes; this agrees with the predicted high energies of such complexes.     

Enthalpy of formation of sulfate green rusts

As a contribution to the systematic study of iron oxide thermodynamics, this work reports enthalpies of formation of green rust, a double layered (Fe^II, Fe^III) hydroxide with the ideal stoichiometry , with sulfate as the anion in the interlayer. Samples were characterized by X-ray powder diffraction, thermogravimetric analysis, infrared spectroscopy, and Mössbauer spectroscopy. Full chemical analysis was performed. Contents of Fe^II, Fe^III, water, and sulfate were obtained. We report standard enthalpies of formation for green rust with different Fe^II/Fe^III ratios. Enthalpies of formation from single cation compounds, namely, Fe(OH)₂, Fe(OH)₃, FeSO₄ and H₂O show reasonable agreement with Gibbs free energies of formation from single cation compounds recalculated from the reported literature values. These values show that green rust has little stabilization over a mechanical mixture of these single cation compounds and there is no thermodynamic preference for any particular Fe^II/Fe^III ratio.