Georges Ona-Nguema

Green Rust Formation during Fe(II) Oxidation by the Nitrate-Reducing Acidovorax sp Strain BoFeN1

Green rust (GR) as highly reactive iron mineral potentially plays a key role for the fate of (in)organic contaminants, such as chromium or arsenic, and nitroaromatic compounds functioning both as sorbent and reductant. GR forms as corrosion product of steel but is also naturally present in hydromorphic soils and sediments forming as metastable intermediate during microbial Fe(III) reduction. Although already suggested to form during microbial Fe(II) oxidation, clear evidence for GR formation during microbial Fe(II) oxidation was lacking. In the present study, powder XRD, synchrotron-based XAS, Mössbauer spectroscopy, and TEM demonstrated unambiguously the formation of GR as an intermediate product during Fe(II) oxidation by the nitrate-reducing Fe(II)-oxidizer Acidovorax sp. strain BoFeN1. The spatial distribution and Fe redox-state of the precipitates associated with the cells were visualized by STXM. It showed the presence of extracellular Fe(III), which can be explained by Fe(III) export from the cells or extracellular Fe(II) oxidation by an oxidant diffusing from the cells. Moreover, GR can be oxidized by nitrate/nitrite and is known as a catalyst for oxidation of dissolved Fe(II) by nitrite/nitrate and may thus contribute to the production of extracellular Fe(III). As a result, strain BoFeN1 may contribute to Fe(II) oxidation and nitrate reduction both by an direct enzymatic pathway and an indirect GR-mediated process.

XANES Evidence for Rapid Arsenic(III) Oxidation at Magnetite and Ferrihydrite Surfaces by Dissolved O2 via Fe2+-Mediated Reactions

To reduce the adverse effects of arsenic on humans, various technologies are used to remove arsenic from groundwater, most relying on As adsorption on Fe-(oxyhydr)oxides and concomitant oxidation of As(III) by dissolved O(2). This reaction can be catalyzed by microbial activity or by strongly oxidizing radical species known to form upon oxidation of Fe(II) by dissolved O(2). Such catalyzed oxidation reactions have been invoked to explain the enhanced kinetics of As(III) oxidation in aerated water, in the presence of zerovalent iron or dissolved Fe(II). In the present study, we used arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy to investigate the role of Fe(II) in the oxidation of As(III) at the surface of magnetite and ferrihydrite under oxygenated conditions. Our results show rapid oxidation of As(III) to As(V) upon sorption onto magnetite under oxic conditions at neutral pH. Moreover, under similar oxic conditions, As(III) oxidized upon sorption onto ferrihydrite only after addition of Fe(II)(aq) within the investigated time frame of 24 h. These results confirm that Fe(II) is able to catalyze As(III) oxidation in the presence of dissolved O(2) and suggest that oxidation of As(III) upon sorption on magnetite under oxic conditions can be explained by an Fe(2+)-mediated Fenton-like reactions. Thus, the present study shows that magnetite might be an efficient alternative to the current use of oxidants and Fe(II) to remove As from aerated water. In addition, this study emphasizes that special care is needed to preserve arsenic oxidation state during laboratory sorption experiments as well as in collecting As-bearing samples from natural environments.

Evidence for Different Surface Speciation of Arsenite and Arsenate on Green Rust: An EXAFS and XANES Study

The knowledge of arsenic speciation at the surface of green rusts (GRs), [Fe(II)((1-x))Fe(III)(x) (OH)(2)](x+) (CO(3), Cl, SO(4))(x-), is environmentally relevant because arsenic sorption onto GRs could contribute to arsenic retention in anoxic environments (hydromorphic soils, marine sediments, etc.). The nature of arsenic adsorption complexes on hydroxychloride green rust 1 (GR1Cl) at near-neutral pH under anoxic conditions was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy at the As K-edge. Sorption data indicate that As(V) sorbs more efficiently than As(III) at the studied As loadings (0.27 micromol m(-2) and 2.7 micromol m(-2)). EXAFS results indicate that arsenite [As(III)] and arsenate [As(V)] form inner-sphere complexes on the surface of GR1Cl at arsenic surface coverages of 0.27 and 2.70 micromol m(-2), with distinct types of As(III) and As(V) sorption complexes, which change in relative concentration as a function of arsenic loading. For As(V), the EXAFS-derived As-Fe distances (3.34 +/- 0.02 and 3.49 +/- 0.02 A) suggest the presence of binuclear bidentate double-corner complexes ((2)C) and monodentate mononuclear corner-sharing complexes ((1)V). For As(III), EXAFS-derived As-As distance (3.32 +/- 0.02 A) and As-Fe distances (3.49 +/- 0.02 and 4.72 +/- 0.02 A) are consistent with the presence of dimers of As(III) pyramids binding to the edges of the GR1Cl layers by corner sharing with FeO(6) octahedra. However, (2)C and (1)V As(III) complexes cannot be excluded. These results improve our knowledge of the mode of As(V) and As(III) inner-sphere adsorption on green rusts, which will help to constrain sorption modeling of arsenic in soils, sediments, and aquifers.

EXAFS and HRTEM evidence for As(III)-containing surface precipitates on nanocrystalline magnetite: implications for As sequestration.

Arsenic sorption onto iron oxide spinels such as magnetite could contribute to immobilization of arsenite (AsO3(3-)), the reduced, highly toxic form of arsenic in contaminated anoxic groundwaters, as well as to putative remediation processes. Nanocrystalline magnetite (<20 nm) is known to exhibit higher efficiency for arsenite sorption than larger particles, sorbing as much as approximately 20 micromol/m2 of arsenite. To improve our understanding of this process, we investigated the molecular level structure of As(III)-containing sorption products on two types of fine-grained magnetite: (1) a biogenic one with an average particle diameter of 34 nm produced by reduction of lepidocrocite (gamma-FeOOH) by Shewanella putrefaciens and (2) a synthetic, abiotic, nanocrystalline magnetite with an average particle diameter of 11 nm. Results from extended X-ray absorption spectroscopy (EXAFS) for both types of magnetite with As(III) surface coverages of up to 5 micromol/m2 indicate that As(III) forms dominantly inner-sphere, tridentate, hexanuclear, corner-sharing surface complexes (3C) in which AsO3 pyramids occupy vacant tetrahedral sites on octahedrally terminated {111} surfaces of magnetite. Formation of this type of surface complex results in a decrease in dissolved As(III) concentration below the maximum concentration level recommended by the World Health Organization (10 microg/L), which corresponds to As(III) surface coverages of 0.16 and 0.19 micromol/m2 in our experiments. In addition, high-resolution transmission electron microscopy (HRTEM) coupled with energy dispersive X-ray spectroscopy (EDXS) analyses revealed the occurrence of an amorphous As(III)-rich surface precipitate forming at As(III) surface coverages as low as 1.61 micromol/m2. This phase hosts the majority of adsorbed arsenite at surface coverages exceeding the theoretical maximum site density of vacant tetrahedral sites on the magnetite {111} surface (3.2 sites/nm2 or 5.3 micromol/m2). This finding helps to explain the exceptional As(III) sorption capacity of nanocrystalline magnetite particles (>10 micromol/m2). However, the higher solubility of the amorphous surface precipitate compared to the 3C surface complexes causes a dramatic increase of dissolved As concentration for coverages above 1.9 micromol/m2.

Distinctive Arsenic(V) Trapping Modes by Magnetite Nanoparticles Induced by Different Sorption Processes

Arsenic sorption onto iron oxide spinels such as magnetite may contribute to arsenic immobilization at redox fronts in soils, sediments, and aquifers, as well as in putative remediation and water treatment technologies. We have investigated As(V) speciation resulting from different sorption processes on magnetite nanoparticles, including both adsorption and precipitation, using X-ray absorption fine structure (XAFS) spectroscopy and transmission electron microscopy (TEM). XAFS results suggest that AsO(4) tetrahedra form predominantly inner-sphere bidentate corner-sharing ((2)C) complexes and outer-sphere complexes on magnetite in the adsorption experiments. In the precipitation experiments, an increasing fraction of AsO(4) tetrahedra appears to be incorporated in clusters having a magnetite-like local structure with increasing As loading, the remaining fraction of As being adsorbed at the surface of magnetite particles. In the sample with the highest As loading (15.7 μmol/m(2)) XAFS data indicate that As(V) is fully incorporated in such clusters. Such processes help to explain the significantly higher arsenic uptake in precipitation samples compared to those generated in adsorption experiments. In addition, for the precipitation samples, TEM observations indicate the formation of amorphous coatings and small (~3 nm) nanoparticles associated with larger (~20-40 nm) magnetite nanoparticles, which are absent in the adsorption samples. These results suggest that As(V) could form complexes at the surfaces of the small nanoparticles and could be progressively incorporated in their structure with increasing As loading. These results provide some of the fundamental knowledge about As(V)-magnetite interactions that is essential for developing effective water treatment technologies for arsenic.

Biogenic vs. abiogenic magnetite nanoparticles: A XMCD study

Abstract X-ray magnetic circular dichroism (XMCD) experiments were carried out to compare the Fe2+/Fe3+ ratio in nanomagnetite chemically produced from lepidocrocite and nanomagnetite biogenically produced by the Fe-reducing bacterium Shewanella putrefaciens. Together with TEM imaging, these measurements showed that the biotic magnetite nanoparticles were of excellent quality, with small size dispersion and high crystallinity. From the XMCD measurements, it could be shown that the biogenic nanomagnetite contained a higher amount of Fe2+ than the abiogenic nanomagnetite.

Formation of Hydroxysulphate Green Rust 2 as a Single Iron(II-III) Mineral in Microbial Culture

Although GR2(SO4 2-) can be easily formed by abiotic synthesis, the biotic formation of hydroxysulphate as a single iron(II-III) mineral in microbial culture and its characterization was not achieved. This study was carried out to investigate the sole formation of GR2(SO4 2-) during the reduction of γ-FeOOH by a dissimilatory iron-respiring bacterium, Shewanella putrefaciens CIP 8040T. Reduction experiments were performed in a non-buffered medium devoid of organic compounds, with 25 mM of sulphate and with a range of lepidocrocite concentrations with H2 as the electron donor under nongrowth conditions. The resulting biogenic solids, after iron-respiring activity, were characterized by X-ray diffraction (XRD), transmission Mössbauer spectroscopy (TMS) and electron microscopy (SEM and TEM). The sulphate has been identified as the intercalated anion by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). In addition, the structure of this sulphate anion was discussed. Our experimental study demonstrated that, under H2 atmosphere, the biogenic solid was a GR2(SO4 2-), as the sole iron(II-III) bearing mineral, whatever the initial lepidocrocite concentration. The crystals of the biotically formed GR2(SO4 2-) are significantly larger than those observed for GR2(SO4 2-) obtained through abiotic preparation, < 15 μ m diameter as against 0.5–4 μm, respectively.

Arsenic scavenging by aluminum-substituted ferrihydrites in a circumneutral pH river impacted by acid mine drainage.

Ferrihydrite (Fh) is a nanocrystalline ferric oxyhydroxide involved in the retention of pollutants in natural systems and in water-treatment processes. The status and properties of major chemical impurities in natural Fh is however still scarcely documented. Here we investigated the structure of aluminum-rich Fh, and their role in arsenic scavenging in river-bed sediments from a circumneutral river (pH 6-7) impacted by an arsenic-rich acid mine drainage (AMD). Extended X-ray absorption fine structure (EXAFS) spectroscopy at the Fe K-edge shows that Fh is the predominant mineral phase forming after neutralization of the AMD, in association with minor amount of schwertmannite transported from the AMD. TEM-EDXS elemental mapping and SEM-EDXS analyses combined with EXAFS analysis indicates that Al(3+) substitutes for Fe(3+) ions into the Fh structure in the natural sediment samples, with local aluminum concentration within the 25-30 ± 10 mol %Al range. Synthetic aluminous Fh prepared in the present study are found to be less Al-substituted (14-20 ± 5 mol %Al). Finally, EXAFS analysis at the arsenic K-edge indicates that As(V) form similar inner-sphere surface complexes on the natural and synthetic Al-substituted Fh studied. Our results provide direct evidence for the scavenging of arsenic by natural Al-Fh, which emphasize the possible implication of such material for scavenging pollutants in natural or engineered systems.

Competitive Formation of Hydroxycarbonate Green Rust 1 versus Hydroxysulphate Green Rust 2 in Shewanella putrefaciens Cultures

The formation of hydroxysulphate green rust 2, a Fe(II-III) compound commonly found during corrosion processes of iron-based materials in seawater, has not yet been reported in bacterial cultures. Here we used Shewanella putrefaciens, a dissimilatory iron-reducing bacterium to anaerobically catalyze the transformation of a ferric oxyhydroxide, lepidocrocite (γ-FeOOH), into Fe(II) in the presence of various sulphate concentrations. Biotransformation assays of γ-FeOOH were performed with formate as the electron donor under a variety of concentrations. The results showed that the competitive formation of hydroxycarbonate green rust 1 (GR1(CO3 2−)) and hydroxysulphate green rust 2 (GR2(SO4 2 −)) depended upon the relative ratio (R) of bicarbonate and sulphate concentrations. When R ≥ 0.17, GR1(CO3 2 −) only was formed whereas when R < 0.17, a mixture of GR2(SO4 2 −) and GR1(CO3 2 −) was obtained. These results demonstrated that the hydroxysulphate GR2 can originate from the microbial reduction of γ-FeOOH and confirmed the preference for carbonate over sulphate during green rust precipitation. The solid phases were characterized by X-ray diffraction, transmission Mössbauer spectroscopy and scanning electron microscopy. Diffuse reflectance infrared Fourier transform spectroscopy confirmed the presence of intercalated carbonate and sulphate in green rusts structure. This study sheds light on the influence of dissimilatory iron-reducing bacteria on microbiologically influenced corrosion.

Arsenic(III) and Arsenic(V) Speciation during Transformation of Lepidocrocite to Magnetite

Bioreduction of As(V) and As-bearing iron oxides is considered to be one of the key processes leading to arsenic pollution in groundwaters in South and Southeast Asia. Recent laboratory studies with simple aqueous media showed that secondary Fe(II)-bearing phases (e.g., magnetite and green rust), which commonly precipitate during bioreduction of iron oxides, captured arsenic species. The aim of the present study was to follow arsenic speciation during the abiotic Fe(II)-induced transformation of As(III)- and As(V)-doped lepidocrocite to magnetite, and to evaluate the influence of arsenic on the transformation kinetics and pathway. We found green rust formation is an intermediate phase in the transformation. Both As(III) and As(V) slowed the transformation, with the effect being greater for As(III) than for As(V). Prior to the formation of magnetite, As(III) adsorbed on both lepidocrocite and green rust, whereas As(V) associated exclusively with green rust, When magnetite precipitated, As(III) formed surface complexes on magnetite nanoparticles and As(V) is thought to have been incorporated into the magnetite structure. These processes dramatically lowered the availability of As in the anoxic systems studied. These results provide insights into the behavior of arsenic during magnetite precipitation in reducing environments. We also found that As(V) removal from solution was higher than As(III) removal following magnetite formation, which suggests that conversion of As(III) to As(V) is preferred when using As-magnetite precipitation to treat As-contaminated groundwaters.