Ferrous phosphate surface precipitates resulting from the reduction of intragrain 6-line ferrihydrite by Shewanella oneidensis MR-1

The reductive biotransformation of 6-line ferrihydrite located within porous silica (intragrain ferrihydrite) by Shewanella oneidensis MR-1 was investigated and compared to the behavior of 6-line ferrihydrite in suspension (free ferrihydrite). The effect of buffer type (PIPES and NaHCO₃), phosphate (P), and an electron shuttle (AQDS) on the extent of reduction and formation of Fe(II) secondary phases was investigated under anoxic conditions. Electron microscopy and micro X-ray diffraction were applied to evaluate the morphology and mineralogy of the biogenic precipitates and to study the distribution of microorganisms on the surface of porous silica after bioreduction. Kinetic reduction experiments with free and intragrain ferrihydrite revealed contrasting behavior with respect to the buffer and presence of P. The overall amount of intragrain ferrihydrite reduction was less than that of free ferrihydrite [at 5 mmol L⁻¹ Fe(III)_T]. Reductive mineralization was not observed in the intragrain ferrihydrite incubations without P, and all biogenic Fe(II) concentrated in the aqueous phase. Irrespective of buffer and AQDS addition, rosettes of Fe(II) phosphate of approximate 20-30 μm size were observed on porous silica when P was present. The rosettes grew not only on the silica surface but also within it, forming a coherent spherical structure. These precipitates were well colonized by microorganisms and contained extracellular materials at the end of incubation. Microbial extracellular polymeric substances may have adsorbed Fe(II) promoting Fe(II) phosphate nucleation with subsequent crystal growth proceeding in different directions from a common center.     

The effect of silica and natural organic matter on the Fe(II)-catalysed transformation and reactivity of Fe(III) minerals

The Fe(II)-catalysed transformation of synthetic schwertmannite, ferrihydrite, jarosite and lepidocrocite to more stable, crystalline Fe(III) oxyhydroxides is prevented by high, natural concentrations of Si and natural organic matter (NOM). Adsorption isotherms demonstrate that Si adsorbs to the iron minerals investigated and that increasing amounts of adsorbed Si results in a decrease in isotope exchange between aqueous Fe(II) and the Fe(III) mineral. This suggests that the adsorption of Si inhibits the direct adsorption of Fe(II) onto the mineral surface, providing an explanation for the inhibitory effect of Si on the Fe(II)-catalysed transformation of Fe(III) minerals. During the synthesis of lepidocrocite and ferrihydrite, the presence of equimolar concentrations of Si and Fe resulted in the formation of 2-line ferrihydrite containing co-precipitated Si in both cases. Isotope exchange experiments conducted with this freeze-dried Si co-precipitated ferrihydrite species (Si-ferrihydrite) demonstrated that the rate and extent of isotope exchange between aqueous Fe(II) and solid ⁵⁵Fe(III) was very similar to that of 2-line ferrihydrite formed in the absence of Si and which had not been allowed to dry. In contrast to un-dried ferrihydrite formed in the absence of Si, Si-ferrihydrite did not transform into a more crystalline Fe(III) mineral phase over the 7-day period of investigation. Reductive dissolution studies using ascorbic acid demonstrated that both dried Si-ferrihydrite and un-dried 2-line ferrihydrite were very reactive, suggesting these species may be major contributors to the rapid release of dissolved iron following flooding and the onset of conditions conducive to reductive dissolution in acid sulphate soil environments.     

Fractionation of Cu and Zn isotopes during adsorption onto amorphous Fe(III) oxyhydroxide: Experimental mixing of acid rock drainage and ambient river water

Fractionation of Cu and Zn isotopes during adsorption onto amorphous ferric oxyhydroxide is examined in experimental mixtures of metal-rich acid rock drainage and relatively pure river water and during batch adsorption experiments using synthetic ferrihydrite. A diverse set of Cu- and Zn-bearing solutions was examined, including natural waters, complex synthetic acid rock drainage, and simple NaNO₃ electrolyte. Metal adsorption data are combined with isotopic measurements of dissolved Cu (⁶⁵Cu/⁶³Cu) and Zn (⁶⁶Zn/⁶⁴Zn) in each of the experiments. Fractionation of Cu and Zn isotopes occurs during adsorption of the metal onto amorphous ferric oxyhydroxide. The adsorption data are modeled successfully using the diffuse double layer model in PHREEQC. The isotopic data are best described by a closed system, equilibrium exchange model. The fractionation factors (α_soln-solid) are 0.99927 ± 0.00008 for Cu and 0.99948 ± 0.00004 for Zn or, alternately, the separation factors (Δ_soln-solid) are −0.73 ± 0.08‰ for Cu and −0.52 ± 0.04‰ for Zn. These factors indicate that the heavier isotope preferentially adsorbs onto the oxyhydroxide surface, which is consistent with shorter metal-oxygen bonds and lower coordination number for the metal at the surface relative to the aqueous ion. Fractionation of Cu isotopes also is greater than that for Zn isotopes. Limited isotopic data for adsorption of Cu, Fe(II), and Zn onto amorphous ferric oxyhydroxide suggest that isotopic fractionation is related to the intrinsic equilibrium constants that define aqueous metal interactions with oxyhydroxide surface sites. Greater isotopic fractionation occurs with stronger metal binding by the oxyhydroxide with Cu > Zn > Fe(II).     

Effect of adsorbed and substituted Al on Fe(II)-induced mineralization pathways of ferrihydrite

The poorly crystalline Fe(III) hydroxide ferrihydrite is considered one of the most important sinks for (in)organic contaminants and nutrients within soils, sediments, and waters. The ripening of ferrihydrite to more stable and hence less reactive phases such as goethite is catalyzed by surface reaction with aqueous Fe(II). While ferrihydrite within most natural environments contains high concentrations of adsorbed or co-precipitated cations (particularly Al), little is known regarding the impact of these cations on Fe(II)-induced transformation of ferrihydrite to secondary phases. Accordingly, we explored the extent, rates, and pathways of Fe(II)-induced secondary mineralization of Al-ferrihydrites by reacting aqueous Fe(II) (0.2 and 2.0 mM) with 2-line ferrihydrite containing a range of Al levels substituted within (6-24 mol% Al) or adsorbed on the surface (0.1-27% Γ_max). Here, we show that regardless of the Fe(II) concentration, Al substituted within or adsorbed on ferrihydrite results in diminished secondary mineralization and preservation of ferrihydrite. In contrast to pure ferrihydrite, the concentration of Fe(II) may not in fact influence the mineralization products of Al-compromised ferrihydrites. Furthermore, the secondary mineral profiles upon Fe(II) reaction with ferrihydrite are not only a function of Al concentration but also the mode of Al incorporation. While Al substitution impedes lepidocrocite formation and magnetite nucleation, Al adsorption completely inhibits goethite formation and appears to have a lesser impact on magnetite nucleation. When normalized to total Al content associated with ferrihydrite, Al adsorption results in greater degree of ferrihydrite preservation relative to Al substitution. These findings provide insight into mechanisms that may be responsible for ferrihydrite preservation and low levels of secondary magnetite typically found in sedimentary environments. Considering the preponderance of cation substitution within and adsorption on ferrihydrite in soils and sediments, the reactivity of natural (compromised) ferrihydrites and the subsequent impact on mineral evolution needs to be more fully explored.     

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.     

The mineralogic transformation of ferrihydrite induced by heterogeneous reaction with bioreduced anthraquinone disulfonate (AQDS) and the role of phosphate

Bioreduced anthraquinone-2,6-disulfonate (AH₂DS; dihydro-anthraquinone) was reacted with a 2-line, Si-substituted ferrihydrite under anoxic conditions at neutral pH in PIPES buffer. Phosphate (P) and bicarbonate (C); common adsorptive oxyanions and media/buffer components known to effect ferrihydrite mineralization; and Fe(II)_aq (as a catalytic mineralization agent) were used in comparative experiments. Heterogeneous AH₂DS oxidation coupled with Fe(III) reduction occurred within 0.13-1 day, with mineralogic transformation occurring thereafter. The product suite included lepidocrocite, goethite, and/or magnetite, with proportions varing with reductant:oxidant ratio (r:o) and the presence of P or C. Lepidocrocite was the primary product at low r:o in the absence of P or C, with evidence for multiple formation pathways. Phosphate inhibited reductive recrystallization, while C promoted goethite formation. Stoichiometric magnetite was the sole product at higher r:o in the absence and presence of P. Lepidocrocite was the primary mineralization product in the Fe(II)_aq system, with magnetite observed at near equal amounts when Fe(II) was high [Fe(II)/Fe(III)] = 0.5 and P was absent. P had a greater effect on reductive mineralization in the Fe(II)_aq system, while AQDS was more effective than Fe(II)_aq in promoting magnetite formation. The mineral products of the direct AH₂DS-driven reductive reaction are different from those observed in AH₂DS-ferrihydite systems with metal reducing bacteria, particularly in presence of P.     

Ferrihydrite dissolution by pyridine-2,6-bis(monothiocarboxylic acid) and hydrolysis products

Pyridine-2,6-bis(monothiocarboxylate) (pdtc), a metabolic product of microorganisms, including Pseudomonas putida and Pseudomonas stutzeri was investigated for its ability of dissolve Fe(III)(hydr)oxides at pH 7.5. Concentration dependent dissolution of ferrihydrite under anaerobic environment showed saturation of the dissolution rate at the higher concentration of pdtc. The surface controlled ferrihydrite dissolution rate was determined to be 1.2 × 10⁻⁶ mol m⁻² h⁻¹. Anaerobic dissolution of ferrihydrite by pyridine-2,6-dicarboxylic acid or dipicolinic acid (dpa), a hydrolysis product of pdtc, was investigated to study the mechanism(s) involved in the pdtc facilitated ferrihydrite dissolution. These studies suggest that pdtc dissolved ferrihydrite using a reduction step, where dpa chelates the Fe reduced by a second hydrolysis product, H₂S. Dpa facilitated dissolution of ferrihydrite showed very small increase in the Fe dissolution when the concentration of external reductant, ascorbate, was doubled, suggesting the surface dynamics being dominated by the interactions between dpa and ferrihydrite. Greater than stoichiometric amounts of Fe were mobilized during dpa dissolution of ferrihydrite assisted by ascorbate and cysteine. This is attributed to the catalytic dissolution of Fe(III)(hydr)oxides by the in situ generated Fe(II) in the presence of a complex former, dpa.     

Sequestration of arsenate from aqueous solution using 2-line ferrihydrite: equilibria,kinetics, and X-ray absorption spectroscopic analysis

Arsenic(V), as the arsenate (AsO₄ ^3?) ion and its conjugate acids, has a strong affinity on Fe, Mn, and Al (oxyhydr)oxides and clay minerals. Removal of arsenate from aqueous solution by poorly crystalline ferrihydrite (hydrous ferric oxide) via a combination of macroscopic (equilibria and kinetics of sorption) and X-ray absorption spectroscopic studies was investigated. The removal of arsenate significantly decreased with increasing pH and sorption maxima of approximately 1.994 mmol/g (0.192 mol_As/mol_Fe) were achieved at pH 2.0. The Langmuir isotherm is most appropriate for arsenate sorption over the wide range of pH, indicating that arsenate sorption preferentially takes place at relatively homogenous and monolayer sites rather than heterogeneous and multilayer surfaces. The kinetic study demonstrated that arsenate sorption onto 2-line ferrihydrite is considerably fast, and sorption equilibrium was achieved within the reaction time of 2 h. X-ray absorption near-edge structure spectroscopy indicates no change in oxidation state of arsenate following interaction with the ferrihydrite surfaces. Extended X-ray absorption fine structure spectroscopy supports the efficient removal of arsenate by the 2-line ferrihydrite through the formation of highly stable inner-sphere surface complexes, such as bidentate binuclear corner-sharing (²C) and bidentate mononuclear edge-sharing (²E) complexes.     

Structural constraints of ferric (hydr)oxides on dissimilatory iron reduction and the fate of Fe(II)

Due to the strong reducing capacity of ferrous Fe, the fate of Fe(II) following dissimilatory iron reduction will have a profound bearing on biogeochemical cycles. We have previously observed the rapid and near complete conversion of 2-line ferrihydrite to goethite (minor phase) and magnetite (major phase) under advective flow in an organic carbon-rich artificial groundwater medium. Yet, in many mineralogically mature environments, well-ordered iron (hydr)oxide phases dominate and may therefore control the extent and rate of Fe(III) reduction. Accordingly, here we compare the reducing capacity and Fe(II) sequestration mechanisms of goethite and hematite to 2-line ferrihydrite under advective flow within a medium mimicking that of natural groundwater supplemented with organic carbon. Introduction of dissolved organic carbon upon flow initiation results in the onset of dissimilatory iron reduction of all three Fe phases (2-line ferrihydrite, goethite, and hematite). While the initial surface area normalized rates are similar (∼10⁻¹¹ mol Fe(II) m⁻² g⁻¹), the total amount of Fe(III) reduced over time along with the mechanisms and extent of Fe(II) sequestration differ among the three iron (hydr)oxide substrates. Following 16 d of reaction, the amount of Fe(III) reduced within the ferrihydrite, goethite, and hematite columns is 25, 5, and 1%, respectively. While 83% of the Fe(II) produced in the ferrihydrite system is retained within the solid-phase, merely 17% is retained within both the goethite and hematite columns. Magnetite precipitation is responsible for the majority of Fe(II) sequestration within ferrihydrite, yet magnetite was not detected in either the goethite or hematite systems. Instead, Fe(II) may be sequestered as localized spinel-like (magnetite) domains within surface hydrated layers (ca. 1 nm thick) on goethite and hematite or by electron delocalization within the bulk phase. The decreased solubility of goethite and hematite relative to ferrihydrite, resulting in lower Fe(III)_aq and bacterially-generated Fe(II)_aq concentrations, may hinder magnetite precipitation beyond mere surface reorganization into nanometer-sized, spinel-like domains. Nevertheless, following an initial, more rapid reduction period, the three Fe (hydr)oxides support similar aqueous ferrous iron concentrations, bacterial populations, and microbial Fe(III) reduction rates. A decline in microbial reduction rates and further Fe(II) retention in the solid-phase correlates with the initial degree of phase disorder (high energy sites). As such, sustained microbial reduction of 2-line ferrihydrite, goethite, and hematite appears to be controlled, in large part, by changes in surface reactivity (energy), which is influenced by microbial reduction and secondary Fe(II) sequestration processes regardless of structural order (crystallinity) and surface area.     

X-ray absorption spectroscopy study on the effect of hydroxybenzoic acids on the formation and structure of ferrihydrite

Organic ligands are known to interfere with the polymerization of Fe(III), but the extent of interference has not been systematically studied as a function of structural ligand properties. This study examines how the number and position of phenol groups in hydroxybenzoic acids affect both ferrihydrite formation and its local (<5 Å) Fe coordination. To this end, acid Fe(III) nitrate solutions were neutralized up to pH 6.0 in the presence of 4-hydroxybenzoic acid (4HB), 2,4-dihydroxybenzoic acid (2,4DHB), and the hydroquinone 3,4-dihydroxybenzoic acid (3,4DHB). The initial molar ligand/Fe ratios ranged from 0 to 0.6. The precipitates were dialyzed, lyophilized, and subsequently studied by X-ray absorption spectroscopy and synchrotron X-ray diffraction. The solids contained up to 32 wt.% organic C (4HB ∼ 2,4DHB < 3,4DHB). Only precipitates formed in 3,4DHB solutions comprised considerable amounts of Fe(II) (Fe(II)/Fe_tot ≤ 6 mol%), implying the abiotic mineralization of the catechol-group bearing ligand during Fe(III) hydrolysis under oxic conditions. Hydroxybenzoic acids decreased ferrihydrite formation in the order 4HB ∼ 2,4DHB ? 3,4DHB, which documents that phenol group position rather than the number of phenol groups controls the ligand’s interaction with Fe(III). The coordination numbers of edge- and double corner-sharing Fe in the precipitates decreased by up to 100%. Linear combination fitting (LCF) of Fe K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra revealed that this decrease was due to increasing amounts of organic Fe(III) complexes in the precipitates. Although EXAFS derived coordination numbers of Fe in ferrihydrite remained constant within error, all organic ligands decreased the coherently scattering domain (CSD) size of ferrihydrite as indicated by synchrotron X-ray diffraction analysis (4HB < 2,4DHB ? 3,4DHB). With decreasing particle size of ferrihydrite its Fe(O,OH)₆ octahedra became progressively distorted as evidenced by an increasing loss of centrosymmetry of the Fe sites. Pre-edge peak analysis of the Fe K-edge XANES spectra in conjunction with LCF results implied that ferrihydrite contains on an average 13 ± 3% tetrahedral Fe(III), which is in very good agreement with the revised single-phase structural model of ferrihydrite (Michel, F. M., Barron, V., Torrent, J., Morales, M. P. et al. (2010) Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism. Proc. Natl. Acad. Sci. USA107, 2787-2792). The results suggest that hydroxybenzoic acid moieties of natural organic matter (NOM) effectively suppress ferrihydrite precipitation as they kinetically control the availability of inorganic Fe(III) species for nucleation and/or polymerization reactions. As a consequence, NOM can trigger the formation of small ferrihydrite nanoparticles with increased structural strain. These factors may eventually enhance the biogeochemical reactivity of ferrihydrite formed in NOM-rich environments. This study highlights the role of hydroquinone structures of NOM for Fe complexation, polymerization, and redox speciation.     

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.     

Synthesis and phase transformations involving scorodite, ferric arsenate and arsenical ferrihydrite: Implications for arsenic mobility

Scorodite, ferric arsenate and arsenical ferrihydrite are important arsenic carriers occurring in a wide range of environments and are also common precipitates used by metallurgical industries to control arsenic in effluents. Solubility and stability of these compounds are controversial because of the complexities in their identification and characterization in heterogeneous media. To provide insights into the formation of scorodite, ferric arsenate and ferrihydrite, series of synthesis experiments were carried out at 70 °C and pH 1, 2, 3 and 4.5 from 0.2 M Fe(SO₄)_1.5 solutions also containing 0.02-0.2 M Na₂HAsO₄. The precipitates were characterized by transmission electron microscopy, X-ray diffraction and X-ray absorption fine structure techniques. Ferric arsenate, characterized by two broad diffuse peaks on the XRD pattern and having the structural formula of FeAsO₄·4-7H₂O, is a precursor to scorodite formation. As defined by As XAFS and Fe XAFS, the local structure of ferric arsenate is profoundly different than that of scorodite. It is postulated that the ferric arsenate structure is made of single chains of corner-sharing Fe(O,OH)₆ octahedra with bridging arsenate tetrahedra alternating along the chains. Scorodite was precipitated from solutions with Fe/As molar ratios of 1 over the pH range of 1-4.5. The pH strongly controls the kinetics of scorodite formation and its transformation from ferric arsenate. The scorodite crystallite size increased from 7 to 33 nm by ripening and aggregation. Precipitates, resulting from continuous synthesis at pH 4.5 from solutions having Fe/As molar ratios ranging from 1 to 4 and resembling the compounds referred to as ferric arsenate, arsenical ferrihydrite and As-rich hydrous ferric oxide in the literature, represent variable mixtures of ferric arsenate and ferrihydrite. When the Fe/As ratio increases, the proportion of ferrihydrite increases at the expense of ferric arsenate. Arsenate adsorption appears to retard ferrihydrite growth in the precipitates with molar Fe/As ratios of 1-4, whereas increased reaction gradually transforms two-line ferrihydrite to six-line ferrihydrite at Fe/As ratios of 5 and greater.     

Reductive dissolution of arsenical ferrihydrite by bacteria

Mining and metallurgical processing of gold and base metal ores can lead to the release of arsenic into the aqueous environment as a result of the weathering and leaching of As-bearing minerals during processing and following disposal. Arsenic in process solutions and mine drainage can be effectively stabilized through the precipitation of ferrihydrite. However, under anaerobic conditions imposed by burial and waste cover systems, ferrihydrite is susceptible to microbial reduction. This research, stimulated by the paucity of information and limited understanding of the microbial reduction of arsenical ferrihydrite, was conducted on synthetic adsorbed and co-precipitated arsenical 6-line ferrihydrite (Fe/As molar ratio of 10/1) using Shewanella sp. ANA-3 and Shewanella putrefaciens CN32 in a chemically defined medium containing 0.045 mM phosphate concentration. Both bacteria were equally effective in their reducing abilities around pH 7, resulting in initial rates of formation of dissolved As(III) of 0.10 μM/h for the adsorbed, and 0.08 μM/h for the co-precipitated arsenical 6-line ferrihydrite samples. The solid phases in the post-reduction samples were characterized by powder X-ray diffraction (XRD), micro-XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron microprobe and X-ray absorption spectroscopy (XAS) techniques. The results indicate the formation of secondary phases such as a biogenic Fe(II)–As(III) compound, akaganeite, goethite, hematite and possibly magnetite during bacterial reduction experiments. Holes and bacterial imprints measuring about 1–2 μm were observed on the surfaces of the secondary phases formed after 1200 h of reduction. This study demonstrates the influence of Fe and As reducing bacteria on the release of significant concentrations of more mobile and toxic As(III) species from arsenical 6-line ferrihydrite, more readily from the adsorbed than from the co-precipitated ferrihydrite.     

Secondary mineralization pathways induced by dissimilatory iron reduction of ferrihydrite under advective flow

Iron (hydr)oxides not only serve as potent sorbents and repositories for nutrients and contaminants but also provide a terminal electron acceptor for microbial respiration. The microbial reduction of Fe (hydr)oxides and the subsequent secondary solid-phase transformations will, therefore, have a profound influence on the biogeochemical cycling of Fe as well as associated metals. Here we elucidate the pathways and mechanisms of secondary mineralization during dissimilatory iron reduction by a common iron-reducing bacterium, Shewanella putrefaciens (strain CN32), of 2-line ferrihydrite under advective flow conditions. Secondary mineralization of ferrihydrite occurs via a coupled, biotic-abiotic pathway primarily resulting in the production of magnetite and goethite with minor amounts of green rust. Operating mineralization pathways are driven by competing abiotic reactions of bacterially generated ferrous iron with the ferrihydrite surface. Subsequent to the initial sorption of ferrous iron on ferrihydrite, goethite (via dissolution/reprecipitation) and/or magnetite (via solid-state conversion) precipitation ensues resulting in the spatial coupling of both goethite and magnetite with the ferrihydrite surface. The distribution of goethite and magnetite within the column is dictated, in large part, by flow-induced ferrous Fe profiles. While goethite precipitation occurs over a large Fe(II) concentration range, magnetite accumulation is only observed at concentrations exceeding 0.3 mmol/L (equivalent to 0.5 mmol Fe[II]/g ferrihydrite) following 16 d of reaction. Consequently, transport-regulated ferrous Fe profiles result in a progression of magnetite levels downgradient within the column. Declining microbial reduction over time results in lower Fe(II) concentrations and a subsequent shift in magnetite precipitation mechanisms from nucleation to crystal growth. While the initial precipitation rate of goethite exceeds that of magnetite, continued growth is inhibited by magnetite formation, potentially a result of lower Fe(III) activity. Conversely, the presence of lower initial Fe(II) concentrations followed by higher concentrations promotes goethite accumulation and inhibits magnetite precipitation even when Fe(II) concentrations later increase, thus revealing the importance of both the rate of Fe(II) generation and flow-induced Fe(II) profiles. As such, the operating secondary mineralization pathways following reductive dissolution of ferrihydrite at a given pH are governed principally by flow-regulated Fe(II) concentration, which drives mineral precipitation kinetics and selection of competing mineral pathways.     

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.     

Phosphate bonding configuration on ferrihydrite based on molecular orbital calculations and XANES fingerprinting

Sorption of phosphate by Fe(III)- and Al(III)-(hydr)oxide minerals regulates the mobility of this potential water pollutant in the environment. The objective of this research was to determine the molecular configuration of phosphate bound on ferrihydrite at pH 6 by interpreting P K-edge XANES spectra in terms of bonding mode. XANES and UV-visible absorption spectra for aqueous Fe(III)-PO₄ solutions (Fe/P molar ratio = 0-2.0) provided experimental trends for energies of P(3p)-O(2p) and Fe(3d)-O(2p) antibonding molecular orbitals. Molecular orbitals for Fe(III)-PO₄ or Al(III)-PO₄ complexes in idealized monodentate or bidentate bonding mode were generated by conceptual bonding arguments, and Extended-Hückel molecular orbital computations were used to understand and assign XANES spectral features to bound electronic states. The strong white line at the absorption edge in P K-edge XANES spectra for Fe-PO₄ or Al-PO₄ systems is attributable to an electronic transition from a P 1s atomic orbital into P(3p)-O(2p) or P(3p)-O(2p)-Al(3p) antibonding molecular orbitals, respectively. For Fe-PO₄ systems, a XANES peak at 2-5 eV below the edge was assigned to a P 1s electron transition into Fe(4p)-O(2p) antibonding molecular orbitals. Similarly, a shoulder on the low-energy side of the white line for variscite corresponds to a transition into Al(3p)-O(2p) orbitals. In monodentate-bonded phosphate, Fe-O bonding is optimized and P-O bonding is weakened, and the converse is true of bidentate-bonded phosphate. These differences explained an inverse correlation between energies of P(3p)-O(2p) and Fe(3d)-O(2p) antibonding molecular orbitals consistent with a monodentate-to-bidentate transition in aqueous Fe(III)-PO₄ solutions. The intensity of the XANES pre-edge feature in Fe(III)-bonded systems increased with increasing number of Fe(III)-O-P bonds. Based on the similarity of intensity and splitting of the pre-edge feature for phosphate sorbed on ferrihydrite at 750 mmol/kg at pH 6 and aqueous Fe-PO₄ solutions containing predominantly bidentate complexes, XANES results indicated that phosphate adsorbed on ferrihydrite was predominantly a bidentate-binuclear surface complex.     

Influence of uranium (VI) on the metabolic activity of stable multispecies biofilms studied by oxygen microsensors and fluorescence microscopy

The effect of uranium added in ecologically relevant concentrations (1 × 10⁻⁵ and 1 × 10⁻⁶ M) to stable multispecies biofilms was studied by electrochemical oxygen microsensors with tip diameters of 10 μm and by confocal laser fluorescence microscopy (CLSM). The microsensor profile measurements in the stable multispecies biofilms exposed to uranium showed that the oxygen concentration decreased faster with increasing biofilm depth compared to the uranium free biofilms. In the uranium containing biofilms, the oxygen consumption, calculated from the steady-state microprofiles, showed high consumption rates of up to 61.7 nmol cm⁻³ s⁻¹ in the top layer (0-70 μm) and much lower consumption rates in the lower zone of the biofilms. Staining experiments with 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) and 4,6-diamidino-2-phenylindole (DAPI) confirmed the high respiratory activities of the bacteria in the upper layer. Analysis of the amplified 16S rRNA gene fragments showed that the addition of uranium in ecologically relevant concentrations did not change the bacterial diversity in the stable multispecies biofilms and is therefore not responsible for the different oxygen profiles in the biofilms. The fast decrease in the oxygen concentrations in the biofilm profiles showed that the bacteria in the top region of the biofilms, i.e., the metabolically most active biofilm zone, battle the toxic effects of aqueous uranium with an increased respiratory activity. This increased respiratory activity results in O₂ depleted zones closer to the biofilm/air interface which may trigger uranium redox processes, since suitable redox partners, e.g., extracellular polymeric substance (EPS) and other organics (e.g., metabolites), are sufficiently available in the biofilm porewaters. Such redox reactions may lead to precipitation of uranium (IV) solids and consequently to a removal of uranium from the aqueous phase.     

Lead coprecipitation with iron oxyhydroxide nano-particles

Pb²⁺ and Fe³⁺ coprecipitation was studied with sorption edge measurements, desorption experiments, sorbent aging, High Resolution Transmission and Analytical Electron Microscopy (HR TEM-AEM), and geochemical modeling. Companion adsorption experiments were also conducted for comparison. The macroscopic chemical and near atomic scale HRTEM data supplemented our molecule scale analysis with EXAFS (Kelly et al., 2008). Coprecipitation of Pb²⁺ with ferric oxyhydroxides occurred at ∼pH 4 and is more efficient than adsorption in removing Pb²⁺ from aqueous solutions at similar sorbate/sorbent ratios and pH. X-ray Diffraction (XRD) shows peaks of lepidocrocite and two additional broad peaks similar to fine particles of 2-line ferrihydrite (2LFh). HRTEM of the Pb-Fe coprecipitates shows a mixture of 2-6 nm diameter spheres and 8-20 by 200-300 nm needles, both uniformly distributed with Pb²⁺. Geochemical modeling shows that surface complexation models fit the experimental data of low Pb:Fe ratios when a high site density is used. Desorption experiments show that more Pb²⁺ was released from loaded sorbents collected from adsorption experiments than from Pb to Fe coprecipitates at dilute EDTA concentrations. Desorbed Pb²⁺ versus dissolved Fe³⁺ data show a linear relationship for coprecipitation (CPT) desorption experiments but a parabolic relationship for adsorption (ADS) experiments.Based on these results, we hypothesize that Pb²⁺ was first adsorbed onto the nanometer-sized, metastable, iron oxyhydroxide polymers of 2LFh with domain size of 2-3 nm. As these nano-particles assembled into larger particles, some Pb²⁺ was trapped in the iron oxyhydroxide structure and re-arranged to form solid solutions. Therefore, the CPT contact method produced more efficient removal of Pb²⁺ than the adsorption contact method, and Pb²⁺ bound in CPT solids represent a more stable sequestration of Pb²⁺ in the environment than Pb²⁺ adsorbed on iron oxyhydroxide surfaces.     

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.     

Iron-oxide crystallinity increases during soil redox oscillations

An Inceptisol A-horizon from Hawaii was subjected to a series of reduction-oxidation cycles—14 d cycle length over a 56 d duration—across the “soil-Fe” [Fe(OH)₃.Fe²⁺_(aq), log K^o = 15.74] equilibrium in triplicate redox-stat reactors. Each reducing event simulated the flush of organic C and diminished O₂ that accompanies a rainfall-induced leaching of bioavailable reductants from the forest floor into mineral soil. The soil contained considerable amounts of short-range ordered (SRO) minerals (e.g., nano-goethite and allophane) and organic matter (11% org-C). Room temperature and cryogenic ⁵⁷Fe Mössbauer spectroscopy showed that the iron-bearing minerals were dominated by nano- to micro-scale goethite, and that ferrihydrite was not present. Over the four full cycles, fluctuations in E_h (from 200 to 700 mV) and pFe²⁺ (from 2.5 to 5.5) were inversely correlated with those of pH (5.5 to 4). Here, we focus on the solubility dynamics of the framework elements (Si, Fe, Ti, and Al) that constitute 35% of the oxygen-free soil dry mass. Intra-cycle oscillations in dissolved (<3 kDa) metals peaked during the reduction half-cycles. Similar intra-cycle oscillations were observed in the HCl and acid ammonium oxalate (AAO) extractable pools. The cumulative response of soil solids during multiple redox oscillations included: (1) a decrease in most HCl and AAO extractable metals and (2) a transformation of SRO Fe (as nano-goethite) to micro-crystalline goethite and micro-crystalline hematite. This may be the first direct demonstration that Fe oxide crystallinity increases during redox oscillations—an a priori unexpected result.