Dissolution of hausmannite (Mn3O4) in the presence of the trihydroxamate siderophore desferrioxamine B

That microbial siderophores may be mediators of Mn(III) biogeochemistry is suggested by recent studies showing that these well known Fe(III)-chelating ligands form very stable Mn(III) aqueous complexes. In this study, we examine the influence of desferrioxamine B (DFOB), a trihydroxamate siderophore, on the dissolution of hausmannite, a mixed valence Mn(II, III) oxide found in soils and freshwater sediments. Batch dissolution experiments were conducted both in the absence (pH 4-9) and in the presence of 100 μM DFOB (pH 5-9). In the absence of the ligand, there is a sharp decrease in the extent of proton-promoted dissolution above pH 5 and no appreciable dissolution above pH 8. The resulting aqueous Mn²⁺ activities were in good agreement with previous studies, indirectly supporting the accepted two-step mechanism involving the formation of manganite and reprecipitation of hausmannite. Desferrioxamine B enhanced hausmannite dissolution over the entire pH range investigated, both via the formation of a Mn(III) complex and through surface-catalyzed reductive dissolution. Above pH 8, non-reductive ligand-promoted dissolution dominated, whereas below pH 8, dissolution was non-stoichiometric with respect to DFOB. Concurrent proton-promoted, ligand-promoted, reductive, and induced dissolution was observed, with Mn release by either reductive or induced dissolution increasing linearly with decreasing pH. The fast kinetics of the DFOB-promoted dissolution of hausmannite, as compared to iron oxides, suggest that the siderophore-promoted dissolution of Mn(III)-bearing minerals may compete with the siderophore-promoted dissolution of Fe(III)-bearing minerals.     

Synthetic coprecipitates of exopolysaccharides and ferrihydrite. Part II: Siderophore-promoted dissolution

Ferrihydrite (Fh) coprecipitated with exopolymers of plants and microbes may differ in its geochemical reactivity from its abiotic counterpart. We synthesized Fh in the presence and absence of acid polysaccharides (polygalacturonic acid (PGA), alginate, xanthan) and characterized the physical and structural properties of the precipitates formed [Mikutta C., Mikutta R., Bonneville S., Wagner F., Voegelin A., Christl I. and Kretzschmar R. (2008) Synthetic coprecipitates of exopolysaccharides and ferrihydrite. Part I: Characterization. Geochim. Cosmochim. Acta]. In this paper, we focus on the reactivity of PGA and alginate coprecipitates and pure Fh, and studied their interaction with the microbial siderophore desferrioxamine B (DFOB) in the presence and absence of low molecular weight organic (LMWO) acid anions (malate, citrate). Batch adsorption and dissolution experiments were performed in the dark at pH 7 in 10 mM NaClO₄ background electrolyte. In the dissolution experiments, different modes of ligand addition were applied (single, simultaneous, stepwise). With an estimated Langmuir sorption maximum of 15 mmol/mol Fe, a PGA coprecipitate with 11% C_org sorbed about four times as much DFOB as pure Fh, and the amount of DFOB sorbed was ∼4-fold larger than estimated from the sum of DFOB sorption to pure Fh and PGA alone. The apparent initial dissolution rates, R_app-initial, and pseudo-first order rate coefficients, k, of the coprecipitates exceeded those of pure Fh by up to two orders of magnitude. Citrate and malate exerted a strong synergistic effect on the DFOB-promoted dissolution of pure Fh, whereas synergistic effects of both anions were absent or negligible for the coprecipitates. R_app-initial of the citrate and DFOB-promoted dissolution of PGA coprecipitates increased with increasing molar C/Fe ratio of the coprecipitates, independent of the charge of the LMWO ligand. Our results indicate that polyuronates stabilize Fh particles sterically and /or electrostatically, thus increasing the mineral surface area accessible to LMWO ligands. In contrast, pure Fh was coagulated at pH 7 (pH_iep of Fh = 7.1), and hence only a small fraction of the Fh surface underwent dissolution. The increase in ligand-accessible surface area of Fh upon coprecipitation with acid polysaccharides seems to primarily control the kinetics of the ligand-promoted dissolution at neutral pH. In pH environments where the solubility of Fe(III) is very low, dissolution rates of Fe(III) (hydr)oxides in such coprecipitates may therefore exceed those of pure minerals by several orders of magnitude, despite a similar crystallinity of the minerals.     

Evidence for ligand hydrolysis and Fe(III) reduction in the dissolution of goethite by desferrioxamine-B

Desferrioxamine-B (DFOB) is a bacterial trihydroxamate siderophore and probably the most studied to date. However, the manner in which DFOB adsorbs at mineral surfaces and promotes dissolution is still under discussion. Here we investigated the adsorption and dissolution reactions in the goethite-DFOB system using both in situ infrared spectroscopic and quantitative analytical methods. Experiments were carried out at a total DFOB concentration of 1 μmol/m², at pH 6, and in the absence of visible light. Our infrared spectroscopic results indicated that the adsorption of DFOB was nearly complete after a 4-h reaction time. In an attempt to determine the coordination mode at the goethite surface, we compared the spectrum of adsorbed DFOB after a 4-h reaction time to the spectra of model aqueous species. However, this approach proved too simplistic in the case of such a complex ligand as DFOB, and we suggest that a more detailed investigation (IR in D₂O, EXAFS of adsorbed model complexes) is needed to elucidate the structure of the adsorbed siderophore. Between a 4-h and 4-day reaction time, we observed the growth of carboxylate stretching bands at 1548 and 1404 cm⁻¹, which are indicators of DFOB hydrolysis. Acetate, a product of DFOB hydrolysis at its terminal hydroxamate group, was quantified by ion chromatography. Its rate of formation was linear and nearly the same as the rate of Fe(III) dissolution. The larger hydrolysis product, a hydroxylamine fragment, was not detected by LC-MS. However, a signal due to the oxidized form of this fragment, a nitroso compound, was found to increase linearly with time, which is an indirect indication for Fe(III) reduction. Based on these findings, we propose that DFOB undergoes metal-enhanced hydrolysis at the mineral surface followed by the reduction of surface Fe(III). While Fe(II) was not detected in solution, this is likely because it remains adsorbed at the goethite surface or becomes buried in the goethite crystal by electron conduction. Taking into account the extent and similarity between the rates of hydrolysis and dissolution, we suggest that a reductive mechanism could play an important part in the dissolution of goethite by DFOB. This possibility has not been considered previously in the absence of light and at circumneutral pH.     

Photodissolution of lepidocrocite (γ-FeOOH) in the presence of desferrioxamine B and aerobactin

Batch adsorption and dissolution experiments with lepidocrocite (γ-FeOOH) and two siderophores, desferrioxamine B (DFOB) and aerobactin, were performed between pH 3 and 8 in the dark and under irradiation with UV-visible light. The increase in surface concentrations of adsorbed DFOB with increasing pH was explained in terms of electrostatic interactions between the protonated and charged terminal amine group of DFOB surface complexes and the charged lepidocrocite surface. The adsorption of aerobactin was consistent with the typical anion-like adsorption behavior of low molecular weight organic acids and indicated that the adsorption properties are strongly determined by the carboxylic acid groups. The adsorption experiments revealed furthermore that the Fe(III)-DFOB solution complex has a very low affinity for the surface, in contrast to Fe(III)-aerobactin solution complexes. In accordance with a surface-controlled mechanism of ligand-promoted dissolution, we found a linear correlation between dissolution rates of lepidocrocite and the surface concentrations of adsorbed DFOB. In the dark, 6- to 8-fold lower dissolution rate coefficients were determined for aerobactin in comparison to DFOB. These results suggested that aerobactin forms surface complexes that are less dissolution-active, characterized by a higher degree of multinuclear surface complexation and/or by less dissolution-active coordination modes of the involved iron-binding groups. For both DFOB and aerobactin, dissolution rate coefficients increased significantly under irradiation with UV-visible light. This increase was interpreted in terms of light-induced reduction of surface Fe(III), primarily by intrinsic photochemical processes of the lepidocrocite bulk phase, based on the observed photoreductive dissolution in the absence of organic ligands between pH 3 and 6. We hypothesize that the α-hydroxycarboxylate group of aerobactin may form a surface complex that additionally promotes photoreductive dissolution by a ligand-to-metal charge-transfer (LMCT) reaction, similar to citrate. However, LMCT reactions involving the α-hydroxycarboxylate group of aerobactin are rather ineffective, based on the comparison of light-induced dissolution rate coefficients determined in the presence of aerobactin and citrate.     

Adsorption of hydroxamate siderophores and EDTA on goethite in the presence of the surfactant sodium dodecyl sulfate

Siderophore-promoted iron acquisition by microorganisms usually occurs in the presence of other organic molecules, including biosurfactants. We have investigated the influence of the anionic surfactant sodium dodecyl sulfate (SDS) on the adsorption of the siderophores DFOB (cationic) and DFOD (neutral) and the ligand EDTA (anionic) onto goethite (α-FeOOH) at pH 6. We also studied the adsorption of the corresponding 1:1 Fe(III)-ligand complexes, which are products of the dissolution process. Adsorption of the two free siderophores increased in a similar fashion with increasing SDS concentration, despite their difference in molecule charge. In contrast, SDS had little effect on the adsorption of EDTA. Adsorption of the Fe-DFOB and Fe-DFOD complexes also increased with increasing SDS concentrations, while adsorption of Fe-EDTA decreased. Our results suggest that hydrophobic interactions between adsorbed surfactants and siderophores are more important than electrostatic interactions. However, for strongly hydrophilic molecules, such as EDTA and its iron complex, the influence of SDS on their adsorption seems to depend on their tendency to form inner-sphere or outer-sphere surface complexes. Our results demonstrate that surfactants have a strong influence on the adsorption of siderophores to Fe oxides, which has important implications for siderophore-promoted dissolution of iron oxides and biological iron acquisition.     

Effect of hydroxamate siderophores on Fe release and Pb(II) adsorption by goethite

Hydroxamate siderophores are biologically-synthesized, Fe(III)-specific ligands which are common in soil environments. In this paper, we report an investigation of their adsorption by the iron oxyhydroxide, goethite; their influence on goethite dissolution kinetics; and their ability to affect Pb(II) adsorption by the goethite surface. The siderophores used were desferrioxamine B (DFO-B), a fungal siderophore, and desferrioxamine D₁, an acetyl derivative of DFO-B (DFO-D1). Siderophore adsorption isotherms yielded maximum surface concentrations of 1.5 (DFO-B) or 3.5 (DFO-D1) μmol/g at pH 6.6, whereas adsorption envelopes showed either cation-like (DFO-B) or ligand-like (DFO-D1) behavior. Above pH 8, the adsorbed concentrations of both siderophores were similar. The dissolution rate of goethite in the presence of 240 μM DFO-B or DFO-D1 was 0.02 or 0.17 μmol/g hr, respectively. Comparison of these results with related literature data on the reactions between goethite and acetohydroxamic acid, a monohydroxamate ligand, suggested that the three hydroxamate groups in DFO-D1 coordinate to Fe(III) surface sites relatively independently. The results also demonstrated a significant depleting effect of 240 μM DFO-B or DFO-D1 on Pb(II) adsorption by goethite at pH > 6.5, but there was no effect of adsorbed Pb(II) on the goethite dissolution rate.     

Temperature dependence of goethite dissolution promoted by trihydroxamate siderophores

This article reports an investigation of the temperature dependence of goethite dissolution kinetics in the presence of desferrioxamine B (DFO-B), a trihydroxamate siderophore, and its acetyl derivative, desferrioxamine D1 (DFO-D1). At 25 and 40°C, DFO-D1 dissolved goethite at twice the rate of DFO-B, whereas at 55°C, the behavior of the two ligands was almost the same. Increasing the temperature from 25 to 55°C caused little or no significant change in DFO-B or DFO-D1 adsorption by goethite. A pseudo-first-order rate coefficient for dissolution, calculated as the ratio of the mass-normalized dissolution rate coefficient to the surface excess of siderophore, was approximately the same at 25 and 40°C for both siderophores. At 55°C, however, this rate coefficient for DFO-D1 was about half that for DFO-B. Analysis of the temperature dependence of the mass-normalized dissolution rate coefficient via the Arrhenius equation led to an apparent activation energy that was larger for DFO-B than for DFO-D1, but much smaller than that reported for the proton-promoted dissolution of goethite. A compensation law was found to relate the pre-exponential factor to the apparent activation energy in the Arrhenius equation, in agreement with what has been noted for the proton-promoted dissolution of oxide minerals and for the complexation of Fe³⁺ by DFO-B and simple hydroxamate ligands in aqueous solution. Analysis of these results suggested that the siderophores adsorb on goethite with a only single hydroxamate group in bidentate ligation with an Fe(III) center.     

Fe isotopic fractionation during mineral dissolution with and without bacteria

Fe released into solution is isotopically lighter (enriched in the lighter isotope) than hornblende starting material when dissolution occurs in the presence of the siderophore desferrioxamine mesylate (DFAM). In contrast, Fe released from goethite dissolving in the presence of DFAM is isotopically unchanged. Furthermore, Δ⁵⁶Fe_{solution-hornblende} for Fe released to solution in the presence of ligands varies with the affinity of the ligand for Fe. The extent of isotopic fractionation of Fe released from hornblende also increases when experiments are agitated continuously. The Fe isotope fractionation observed during hornblende dissolution with organic ligands is attributed predominantly to retention of ⁵⁶Fe in an altered surface layer, while the lack of isotopic fractionation during goethite dissolution in DFAM is consistent with the lack of an altered layer. When a siderophore-producing soil bacterium is added to the system (without added organic ligands), Fe released to solution from both hornblende and goethite differs isotopically from Fe in the bulk mineral: Δ⁵⁶Fe_{solution-starting material} = −0.56 ± 0.19 (hornblende) and −1.44 ± 0.16 (goethite). Increased isotopic fractionation is attributed in this case to the fact that as bacterial respiration depletes the system in oxygen and aqueous Fe is reduced, equilibration between aqueous ferrous and ferric iron creates a pool of isotopically heavy ferric iron that is assimilated by bacterial cells. Adsorption of isotopically heavy ferrous iron (Fe(II) enriched in the heavier isotope) or precipitation of isotopically heavy Fe minerals may also contribute to observed fractionations.To test whether these Fe isotope signatures are recorded in natural systems, we also investigated extractions of samples of soils from which the bacteria were isolated. These extractions show variability in the isotopic signatures of exchangeable Fe and Fe oxyhydroxide fractions from one soil sample to another, but exchangeable Fe is observed to be lighter than Fe in soil Fe oxyhydroxides and hornblende. This observation is consistent with isotopically light Fe-organic complexes in soil pore water derived from the Fe-silicate starting materials in the presence of growing microorganisms, as documented in experiments reported here. The contributions from phenomena including organic ligand-promoted nonstoichiometric dissolution of Fe silicates, uptake of ferric iron by organisms, adsorption of isotopically heavy ferrous iron, and precipitation of iron minerals should create complex isotopic signatures in soils. Better understanding of these processes and the timescales over which they contribute to fractionation is needed.     

Sorption and precipitation of Co(II) in Hanford sediments and alkaline aluminate solutions

Sorption and precipitation of Co(II) in simplified model systems related to the Hanford site high-level nuclear waste tank leakage were investigated through solution studies, geochemical modeling, and X-ray absorption fine structure (XAFS) spectroscopy. Studies of Co(II) sorption to pristine Hanford sediments (ERDF and Sub), which consist predominantly of quartz, plagioclase, and alkali feldspar, show an adsorption edge centered at pH ≈ 8.0 for both sediments studied, with sorption >99% above pH ≈ 9.0. Aqueous SiO₂ resulting from dissolution of the sediments increased in concentration with increasing pH, though the systems remained undersaturated with respect to quartz. XAFS studies of Co(II) sorption to both sediment samples reveal the oxidation of Co(II) to Co(III), likely by dissolved O₂, although this oxidation was incomplete in the Sub sediment samples. The authors propose that Fe(II) species, either in aqueous solution or at mineral surfaces, partially inhibited Co(II) oxidation in the Sub sediment samples, as these sediments contain significantly higher quantities of Fe(II)-bearing minerals which likely partially dissolved under the high-pH solution conditions. In alkaline solutions, Al precipitated as bayerite, gibbsite, or a mixture of the two at pH > 7; an amorphous gel formed at pH values less than 7. Aqueous Co concentrations were well below the solubility of known Co-bearing phases at low pH, suggesting that Co was removed from solution through an adsorption mechanism. At higher pH values, Co concentrations closely matched the solubility of a Co-bearing hydrotalcite-like solid. XAFS spectra of Co(II) sorbed to Al-hydroxide precipitates are similar to previously reported spectra for such hydrotalcite-like phases. The precipitation processes observed in this study can significantly reduce the environmental hazard posed by ⁶⁰Co in the environment.     

Mineralogical and morphological constraints on the reduction of Fe(III) minerals by Geobacter sulfurreducens

The biologically-mediated reduction of synthetic samples of the Fe(III)-bearing minerals hematite, goethite, lepidocrocite, feroxhyte, ford ferrihydrite, akaganeite and schwertmannite by Geobacter sulfurreducens has been investigated using microbiological techniques in conjunction with X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). This combination of approaches offers unique insights into the influence of subtle variations in the crystallinity of a given mineral on biogeochemical processes, and has highlighted the importance of (oxyhydr)oxide crystallite morphology in determining the changes occurring in a given mineral phase. Problems arising from normalising the biological Fe(III) reduction rates relative to the specific surface areas of the starting materials are also highlighted. These problems are caused primarily by particle aggregation, and compounded when using spectrophotometric assays to monitor reduction. For example, the initial rates of Fe(III) reduction observed for two synthetic feroxyhytes with different crystallinities (as shown by XRD and TEM studies) but almost identical surface areas, differ substantially. Both microbiological and high-resolution TEM studies show that hematite and goethite are susceptible to limited amounts of Fe(III) reduction, as evidenced by the accumulation of Fe(II) during incubation with G. sulfurreducens and the growth of nodular structures on crystalline goethite laths during incubation. Lepidocrocite and akaganeite readily transform into mixtures of magnetite and goethite, and XRD data indicate that the proportion of magnetite increases within the transformation products as the crystallinity of the starting material decreases. The presence of anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle increases both the initial rate and longer term extent of biological Fe(III) reduction for all of the synthetic minerals examined. High-resolution XPS indicates subtle but measurable differences in the Fe(III):Fe(II) ratios at the mineral surfaces following extended incubation. For example, for a poorly crystalline schwertmannite, deconvolution of the Fe2p_3/2 peak suggests that the Fe(III):Fe(II) ratio of the near-surface regions varies from 1.0 in the starting material to 0.9 following 144 h of incubation with G.sulfurreducens, and to 0.75 following the same incubation period in the presence of 10 μM AQDS. These results have important implications for the biogeochemical cycling of iron.     

Mechanisms of siderophore sorption to smectite and siderophore-enhanced release of structural Fe

Sorption of the trihydroxamate siderophores desferrioxamine-B and -D (DFOB and DFOD, respectively) and of the monohydroxamate ligand acetohydroxamic acid (aHA) to smectite were examined in batch sorption studies (pH 5.5, 0.1 M ionic strength) coupled with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Both DFOB and DFOD, which have similar molecular structures but different charge properties (cationic versus neutral, respectively) showed a high affinity for smectite. In contrast, the smaller aHA molecule did not sorb appreciably. XRD analysis indicated that DFOB and DFOD each absorbed in the interlamellar region of the clay to give d-spacings of 13.4-13.7 Å at equilibrium solution concentrations <250 μM. FTIR spectra of sorbed DFOB and DFOD indicated that the conformation of each species was distinct from its conformation in the crystalline or dissolved states. At elevated initial solution concentrations of 500-1500 μM, DFOB formed a bilayer in the clay interlayer. Changes in the FTIR spectra of the DFOB-loaded clay samples at these higher surface loadings were consistent with the presence of a metal-siderophore complex in the interlayer. DFOB and DFOD both enhanced Fe and Al release from smectite, but aHA did not. Possible dissolution mechanisms are discussed in light of the FTIR and batch dissolution results.     

Sorption of ferric iron from ferrioxamine B to synthetic and biogenic layer type manganese oxides

Siderophores are biogenic chelating agents produced in terrestrial and marine environments that increase the bioavailability of ferric iron. Recent work has suggested that both aqueous and solid-phase Mn(III) may affect siderophore-mediated iron transport, but scant information appears to be available about the potential roles of layer type manganese oxides, which are relatively abundant in soils and the oligotrophic marine water column. To probe the effects of layer type manganese oxides on the stability of aqueous Fe-siderophore complexes, we studied the sorption of ferrioxamine B [Fe(III)HDFOB⁺, an Fe(III) chelate of the trihydroxamate siderophore desferrioxamine B (DFOB)] to two synthetic birnessites [layer type Mn(III,IV) oxides] and a biogenic birnessite produced by Pseudomonas putida GB-1. We found that all of these predominantly Mn(IV) oxides greatly reduced the aqueous concentration of Fe(III)HDFOB⁺ at pH 8. Analysis of Fe K-edge EXAFS spectra indicated that a dominant fraction of Fe(III) associated with the Mn(IV) oxides is not complexed by DFOB as in solution, but instead Fe(III) is specifically adsorbed to the mineral structure at multiple sites, thus indicating that the Mn(IV) oxides displaced Fe(III) from the siderophore complex. These results indicate that layer type manganese oxides, including biogenic minerals, may sequester iron from soluble ferric complexes. We conclude that the sorption of iron-siderophore complexes may play a significant role in the bioavailability and biogeochemical cycling of iron in marine and terrestrial environments.     

Contrasting effects of Al substitution on microbial reduction of Fe(III) (hydr)oxides

Aluminum, one of the most abundant elements in soils and sediments, is commonly found co-precipitated with Fe in natural Fe(III) (hydr)oxides; yet, little is known about how Al substitution impacts bacterial Fe(III) reduction. Accordingly, we investigated the reduction of Al substituted (0-13 mol% Al) goethite, lepidocrocite, and ferrihydrite by the model dissimilatory Fe(III)-reducing bacterium (DIRB), Shewanella putrefaciens CN32. Here we reveal that the impact of Al on microbial reduction varies with Fe(III) (hydr)oxide type. No significant difference in Fe(III) reduction was observed for either goethite or lepidocrocite as a function of Al substitution. In contrast, Fe(III) reduction rates significantly decreased with increasing Al substitution of ferrihydrite, with reduction rates of 13% Al-ferrihydrite more than 50% lower than pure ferrihydrite. Although Al substitution changed the minerals’ surface area, particle size, structural disorder, and abiotic dissolution rates, we did not observe a direct correlation between any of these physiochemical properties and the trends in bacterial Fe(III) reduction. Based on projected Al-dependent Fe(III) reduction rates, reduction rates of ferrihydrite fall below those of lepidocrocite and goethite at substitution levels equal to or greater than 18 mol% Al. Given the prevalence of Al substitution in natural Fe(III) (hydr)oxides, our results bring into question the conventional assumptions about Fe (hydr)oxide bioavailability and suggest a more prominent role of natural lepidocrocite and goethite phases in impacting DIRB activity in soils and sediments.     

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.     

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.     

Dissolution mechanisms of goethite in the presence of siderophores and organic acids

In dynamic natural systems such as soils and surface waters, transient biogeochemical processes can induce strong chemical non-steady-state conditions. In this paper, we investigate the effects of non-steady-state conditions on ligand-controlled iron oxide dissolution. The rates of goethite dissolution at pH 6 in the presence of low molecular weight organic acids (oxalate, citrate or malonate) were observed. Non-steady-state conditions were induced by rapid additions of fungal, bacterial or plant siderophores. In the presence of the low molecular weight organic acids, dissolved iron concentrations are below detection limit as predicted by equilibrium solubility calculations. The rapid addition of the siderophores triggered reproducible, fast dissolution of kinetically labile iron from the iron oxide surface. The same effect was observed upon rapid additions of high citrate concentrations to goethite-oxalate suspensions. The concentration of the labile iron pool at the mineral surface was a function of the surface concentration of the low molecular weight organic acids and of the reaction time before addition of the siderophores. Isotopic exchange with ⁵⁹Fe independently confirmed the existence of the labile iron pool before addition of the siderophore. A dissolution mechanism was elucidated that is consistent with these observations and with accepted models of ligand-controlled dissolution. We conclude that the fast dissolution reaction observed here is an important process in biological iron acquisition and that it is based on a general geochemical mechanism.     

Siderophore adsorption to and dissolution of kaolinite at pH 3 to 7 and 22°C

Siderophores are Fe(III)-specific ligands produced by many aerobic microorganisms under conditions of iron stress. This study examined adsorption of the commercial trihydroxamate siderophore, desferrioxamine B (DFO-B), to an iron-containing kaolinite (0.1 bulk wt.% Fe) and examined DFO-B effects on initial kaolinite dissolution and iron release rates. Adsorption experiments were conducted at pH 3 to 8 in 0.01-M NaClO₄ in the dark and at 22°C; batch initial dissolution experiments were conducted to 96 h.The adsorption envelope (i.e., adsorption as a function of pH) of DFO-B on kaolinite was consistent with cation-like behavior, with adsorption increasing above kaolinite’s pH_pznpc of 4.9. DFO-B enhanced aluminum release from kaolinite at pH 3 to 7, relative to HNO₃, which is consistent with the ligand’s high binding affinity for Al. Correlation between DFO-B adsorption and aluminum release suggested a surface-controlled, ligand-promoted dissolution mechanism. DFO-B had no effect relative to HNO₃ on silicon release at pH 3 and 5, but moderately enhanced silicon release at pH 7. DFO-B enhanced iron release from kaolinite, with dissolved iron concentrations in the 10-ppb range at 96-h reaction time. These results show that kaolinite may serve as a source of iron to aerobic microorganisms in iron-limited conditions and that siderophores may affect kaolinite dissolution and iron content.     

中非赞比亚成矿带谦比希铜钴矿床钴的赋存状态与成矿规律

谦比希铜钴矿床是中非赞比亚成矿带重要的超大型铜钴矿床之一,由主矿体、西矿体和东南矿体3部分组成,赋矿层位主要为下罗安亚群敏多拉组和基特韦组,岩性包括泥质板岩、泥质石英岩、板岩、石英砂岩等.现有资料对于矿床中钴的赋存状态和含钴矿物成因的研究较为薄弱,这也直接制约了对钴矿的成因认识及今后找矿方向.本次研究聚焦谦比希矿床中的...     

Schwertmannite transformation to goethite via the Fe(II) pathway: Reaction rates and implications for iron-sulfide formation

Schwertmannite (Fe₈O₈(OH)₆SO₄) is a common Fe(III)-oxyhydroxysulfate mineral in acid-sulfate systems, where its formation and fate strongly influence water quality. The present study examines transformation of schwertmannite to goethite (FeOOH), as catalyzed by interactions with Fe(II) in anoxic aquatic environments. This study also evaluates the role of the Fe(II) pathway in influencing the formation of iron-sulfide minerals in such environments. At pH > 5, the rates of Fe(II)-catalyzed schwertmannite transformation were several orders of magnitude faster than transformation in the absence of Fe(II). Complete transformation of schwertmannite occurred within only 3-5 h at pH > 6 and Fe(II)_(aq) ? 5 mmol L⁻¹. Model calculations indicate that the Fe(II)-catalyzed transformation of schwertmannite to goethite greatly decreases the reactivity of the Fe(III) pool, thereby favoring SO₄-reduction and facilitating the formation of iron-sulfide minerals (particularly mackinawite, tetragonal FeS). Examination of in situ sediment geochemistry in an acid-sulfate system revealed that the rapid Fe(II)-catalyzed transformation was consistent with an abrupt shift from an acidic Fe(III)-reducing regime with abundant schwertmannite near the sediment surface, to a near-neutral mackinawite-forming regime where goethite was dominant. This study demonstrates that the Fe(II) pathway exerts a major influence on schwertmannite transformation and iron-sulfide formation in anoxic acid-sulfate systems. These findings have important implications for understanding acidity dynamics and trace element mobility in such systems.     

Stability of YREE complexes with the trihydroxamate siderophore desferrioxamine B at seawater ionic strength

Organic complexation of yttrium and the rare earth elements (YREEs), although generally believed to be important, is an understudied aspect of YREE solution speciation in the open ocean. We report the first series of stability constants for complexes of YREEs (except Ce and Pm) with the trihydroxamate siderophore desferrioxamine B (DFOB), representing a class of small organic ligands that have an extraordinary selectivity for Fe(III) and are found in surface seawater at low-picomolar concentrations. Constants were measured by potentiometric titration of DFOB (pH 3-10) in the presence of single YREEs, in simple media at seawater ionic strength (NaClO₄ or NaCl, I = 0.7 M). Under these circumstances, the terminal amine of DFOB does not deprotonate. The four acid dissociation constants of the siderophore were determined separately by potentiometric titration of DFOB alone.Values for the bidentate (log β₁), tetradentate (log β₂), and hexadentate (log β₃) complexes of La-Lu range from 4.88 to 6.53, 7.70 to 11.27, and 10.09 to 15.19, respectively, while Y falls between Gd and Tb in each case. Linear free-energy relations of the three stability constants with the first YREE hydrolysis constant, log , yield regression coefficients of >0.97. On the other hand, plots of the constants vs. the radius of the inner hydration sphere display an increasing deviation from linearity for the lightest REEs (La > Pr > Nd). This may signify steric constraints in DFOB folding around bulkier cations, a larger mismatch in coordination number, or a substantial degree of covalence in the YREE-hydroxamate bond.Complexes of the YREEs with DFOB are many orders of magnitude more stable than those with carbonate, the dominant inorganic YREE ligand in seawater. Speciation modeling with MINEQL indicates that, for an average seawater composition, the hexadentate complex could constitute as much as 28% of dissolved Lu at free DFOB concentrations as low as 10⁻¹³ M. Such conditions might occur when DFOB or other siderophores are present in excess over metals for which they have high affinity, like Fe(III) and Co(III), for example during plankton blooms. Even if it turns out that trihydroxamate siderophores are not the dominant organic YREE ligand in seawater, our results establish a benchmark for producing effects on YREE solution speciation comparable to that of DFOB: the free concentration of any weaker organic ligand L must exceed that of DFOB by a factor β₃/_Lβ₁, assuming its first order complex is formed in greatest abundance.