John R. Bargar

Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting

Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. Here, we identify the origin of a 500-fold OER activity enhancement that can be achieved with mixed (Ni,Fe)oxyhydroxides (Ni(1-x)Fe(x)OOH) over their pure Ni and Fe parent compounds, resulting in one of the most active currently known OER catalysts in alkaline electrolyte. Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe(3+) in Ni(1-x)Fe(x)OOH occupies octahedral sites with unusually short Fe-O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni(1-x)Fe(x)OOH are not active sites for the oxidation of water.

Characterization of the manganese oxide produced by Pseudomonas putida strain MnB1

Manganese oxides form typically in natural aqueous environments via Mn(II) oxidation catalyzed by microorganisms, primarily bacteria, but little is known about the structure of the incipient solid-phase products. The Mn oxide produced by a Pseudomonas species representative of soils and freshwaters was characterized as to composition, average Mn oxidation number, and N2 specific surface area. Electron microscopy, X-ray diffraction, and X-ray absorption near edge structure spectroscopy were applied to complement the physicochemical data with morphological and structural information. A series of synthetic Mn oxides also was analyzed by the same methods to gain better comparative understanding of the structure of the biogenic oxide. The latter was found to be a poorly crystalline layer type Mn(IV) oxide with hexagonal symmetry, significant negative structural charge arising from cation vacancies, and a relatively small number of randomly stacked octahedral sheets per particle. Its properties were comparable to those of δ-MnO2 (vernadite) and a poorly crystalline hexagonal birnessite (“acid birnessite”) synthesized by reduction of permanganate with HCl, but they were very different from those of crystalline triclinic birnessite. Overall, the structure and composition of the Mn oxide produced by P. putida were similar to what has been reported for other freshly precipitated Mn oxides in natural weathering environments, yielding further support to the predominance of biological oxidation as the pathway for Mn oxide formation. Despite variations in the degree of sheet stacking and Mn(III) content, all poorly crystalline oxides studied showed hexagonal symmetry. Thus, there is a need to distinguish layer type Mn oxides with structures similar to those of natural birnessites from the synthetic triclinic variety. We propose designating the unit cell symmetry as an addition to the current nomenclature for these minerals.

Characterization of U(VI)-carbonato ternary complexes on hematite: EXAFS and electrophoretic mobility measurements

We have measured U(VI) adsorption on hematite using EXAFS spectroscopy and electrophoresis under conditions relevant to surface waters and aquifers (0.01 to 10 μM dissolved uranium concentrations, in equilibrium with air, pH 4.5 to 8.5). Both techniques suggest the existence of anionic U(VI)-carbonato ternary complexes. Fits to EXAFS spectra indicate that U(VI) is simultaneously coordinated to surface FeO6 octahedra and carbonate (or bicarbonate) ligands in bidentate fashions, leading to the conclusion that the ternary complexes have an inner-sphere metal bridging (hematite-U(VI)-carbonato) structure. Greater than or equal to 50% of adsorbed U(VI) was comprised of monomeric hematite-U(VI)-carbonato ternary complexes, even at pH 4.5. Multimeric U(VI) species were observed at pH ≥ 6.5 and aqueous U(VI) concentrations approximately an order of magnitude more dilute than the solubility of crystalline β-UO2(OH)2. Based on structural constraints, these complexes were interpreted as dimeric hematite-U(VI)-carbonato ternary complexes. These results suggest that Fe-oxide-U(VI)-carbonato complexes are likely to be important transport-limiting species in oxic aquifers throughout a wide range of pH values.

Surface complexation of Pb(II) at oxide-water interfaces: I. XAFS and bond-valence determination of mononuclear and polynuclear Pb(II) sorption products on aluminum oxides

Abstract Pb(II) sorption on Al2O3 powders was studied as functions of sorption density (from 0.5 to 5.2 μmoles/m2) and [Pb]eq (0.03–1.4 mM) in 0.1 M NaNO3 electrolyte solution using XAFS spectroscopy. At pH 6 and 7, Pb(II) ions were found to be fully hydrolyzed and adsorbed preferentially as mononuclear bidentate complexes to edges of AlO6 octahedra. At higher sorption densities (Γ ≥ 3.4 μmoles · m−2), XAFS results suggest the presence of dimeric Pb(II) surface complexes. A bond-valence model was used in conjunction with these results to constrain the compositions and reaction stoichiometries of adsorption complexes. We conclude that Pb(II) adsorption on alumina is aattributable to complexation by [ Al Al Al  O −1 2 ] and [ AlOH −1 2 ] surface functional groups. Several plausible Pb(II) adsorption reactions are proposed, based on these results, which provide a basis for chemically realistic descriptions of surface complexation of Pb(II) on aluminum oxides.

Surface complexation of Pb(II) at oxide-water interfaces: II. XAFS and bond-valence determination of mononuclear Pb(II) sorption products and surface functional groups on iron oxides

Abstract Pb(II) sorption on goethite and hematite powders was studied at room temperature as a function of pH (6–8), sorption density (2–10 μmoles/m2), and [Pb]eq (0.2 μM – 1.2 mM) in 0.1 M NaNO3 electrolyte using XAFS spectroscopy. Pb(II) ions were found to be hydrolyzed and adsorbed as mononuclear bidentate complexes to edges of FeO6 octahedra on both goethite and hematite under all conditions. Hydrolysis of Pb(II) appears to be a primary source of proton release associated with surface complexation of Pb(II). A bond-valence model was used to relate the relative stabilities of iron-oxide surface functional groups and Pb(II) adsorption complexes to their structures and compositions. This combined approach suggests that Pb(II) adsorption occurs primarily at unprotonated [ Fe Fe Fe  O 1 2 ] sites and at [ Fe  OH 2 +1 2 ] sites. Several adsorption reactions are proposed. Comparison to EXAFS results from Pb(II) adsorption on aluminum oxides suggests that the edge lengths of surface AlO6 or FeO6 octahedra partially determine the reactivities and densities of available surface sites. The results of this study provide a basis for constructing chemically realistic descriptions of Pb(II) surface complexation reactions on Fe (hydr)oxides.

Structural characterization of biogenic Mn oxides produced in seawater by the marine bacillus sp. strain SG-1

Abstract Natural Mn-oxide nanoparticles and grain coatings are ubiquitous in the environment and profoundly impact the water quality and quality of sediments through their ability to degrade and sequester contaminants. These oxides, which are believed to form dominantly via oxidation of Mn2+ by marine and freshwater bacteria, have extremely high sorptive capacities for heavy metals. We have used XANES, EXAFS, and synchrotron (SR)-XRD techniques to study biogenic Mn oxides produced by spores of the marine Bacillus sp. strain SG-1 in seawater as a function of reaction time under in-situ conditions. An EXAFS model was developed to fully account for the structure and features in the data, providing realistic structural information. The first observed biogenic solid-phase Mn-oxide product is a layered phyllomanganate with hexagonal sheet symmetry and an Mn-oxidation state similar to that in δ-MnO2, between 3.7 and 4.0. XRD and SEM-EDS data show the biooxides to have a phyllomanganate 10 Å basal plane spacing and an interlayer containing Ca. With time, a phyllomanganate oxide with pseudo-orthogonal sheet symmetry appears. Fits to these EXAFS spectra suggest the octahedral layers have relatively few Mn octahedral site vacancies in the lattice and the layers bend to accommodate Jahn-Teller distortions creating the change in symmetry. A reaction mechanism is proposed to account for the observed products. The phyllomanganate oxides observed in this study may be the same as the most abundant Mn-oxide phases suspended in the oxic and sub-oxic zones of the oceanic water column that are of global importance in trace metal and nutrient cycling

Biotic and abiotic products of Mn(II) oxidation by spores of the marine Bacillus sp. strain SG-1

Abstract Bacterial Mn(II) oxidization by spores of Bacillus, sp. strain SG-1 has been systematically probed over the time scale 0.22 to 77 days under in-situ conditions and at differing Mn(II) concentrations. Three complementary techniques, K-edge X-ray absorption near-edge spectroscopy (XANES), X-ray emission spectroscopy (XES), and in-situ synchrotron radiation-based X-ray diffraction (SR-XRD), have been utilized to examine time-dependent changes in Mn oxidation state, local-, and long-range structure in amorphous, crystalline, cell-bound, and solute Mn species. The primary solid biogenic product of Mn(II) oxidation is an X-ray amorphous oxide similar to δ-MnO2, which has a Mn oxidation state between 3.7 and 4.0. Reaction of Mn(II) with the primary biogenic oxide results in the production of abiotic secondary products, feitknechtite or a 10 Å Na phyllomanganate. The identity of the secondary product depends upon the Mn(II) concentration as described by thermodynamic relations. A decrease in the dissolved Mn(II) concentration is followed by mineralogic transformation of the secondary products. Thus, Mn(II) appears to act as a reductant toward the biogenic oxide and to control the stability of secondary reaction products. Mineralogic changes similar to these are likely to be commonplace in natural settings where bacterial Mn(II) oxidation is occurring and may liberate sorbed metal ions or alter the rates of important Mn oxide surface-mediated processes such as the degradation of organic molecules. It is plausible that microbes may exploit such mineral transformation reactions to indirectly control specific chemical conditions in the vicinity of the cell.

Non-uraninite Products of Microbial U(VI) Reduction

A promising remediation approach to mitigate subsurface uranium contamination is the stimulation of indigenous bacteria to reduce mobile U(VI) to sparingly soluble U(IV). The product of microbial uranium reduction is often reported as the mineral uraninite. Here, we show that the end products of uranium reduction by several environmentally relevant bacteria (Gram-positive and Gram-negative) and their spores include a variety of U(IV) species other than uraninite. U(IV) products were prepared in chemically variable media and characterized using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) to elucidate the factors favoring/inhibiting uraninite formation and to constrain molecular structure/composition of the non-uraninite reduction products. Molecular complexes of U(IV) were found to be bound to biomass, most likely through P-containing ligands. Minor U(IV)-orthophosphates such as ningyoite [CaU(PO(4))(2)], U(2)O(PO(4))(2), and U(2)(PO(4))(P(3)O(10)) were observed in addition to uraninite. Although factors controlling the predominance of these species are complex, the presence of various solutes was found to generally inhibit uraninite formation. These results suggest a new paradigm for U(IV) in the subsurface, i.e., that non-uraninite U(IV) products may be found more commonly than anticipated. These findings are relevant for bioremediation strategies and underscore the need for characterizing the stability of non-uraninite U(IV) species in natural settings.

Bacteriogenic Manganese Oxides

Microorganisms control the redox cycling of manganese in the natural environment. Although the homogeneous oxidation of Mn(II) to form manganese oxide minerals is slow, solid MnO(2) is the stable form of manganese in the oxygenated portion of the biosphere. Diverse bacteria and fungi have evolved the ability to catalyze this process, producing the manganese oxides found in soils and sediments. Other bacteria have evolved to utilize MnO(2) as a terminal electron acceptor in respiration. This Account summarizes the properties of Mn oxides produced by bacteria (bacteriogenic MnO(2)) and our current thinking about the biochemical mechanisms of bacterial Mn(II) oxidation. According to X-ray absorption spectroscopy and X-ray scattering studies, the MnO(2) produced by bacteria consists of stacked hexagonal sheets of MnO(6) octahedra, but these particles are extremely small and have numerous structural defects, particularly cation vacancies. The defects provide coordination sites for binding exogenous metal ions, which can be adsorbed to a high loading. As a result, bacterial production of MnO(2) influences the bioavailability of these metals in the natural environment. Because of its high surface area and oxidizing power, bacteriogenic MnO(2) efficiently degrades biologically recalcitrant organic molecules to lower-molecular-mass compounds, spurring interest in using these properties in the bioremediation of xenobiotic organic compounds. Finally, bacteriogenic MnO(2) is reduced to soluble Mn(II) rapidly in the presence of exogenous ligands or sunlight. It can therefore help to regulate the bioavailability of Mn(II), which is known to protect organisms from superoxide radicals and is required to assemble the water-splitting complex in photosynthetic organisms. Bioinorganic chemists and microbiologists have long been interested in the biochemical mechanism of Mn(IV) oxide production. The reaction requires a two-electron oxidation of Mn(II), but genetic and biochemical evidence for several bacteria implicate multicopper oxidases (MCOs), which are only known to engage one-electron transfers from substrate to O(2). In experiments with the exosporium of a Mn(II)-oxidizing Bacillus species, we could trap the one-electron oxidation product, Mn(III), as a pyrophosphate complex in an oxygen-dependent reaction inhibited by azide, consistent with MCO catalysis. The Mn(III) pyrophosphate complex can further act as a substrate, reacting in the presence of the exosporium to produce Mn(IV) oxide. Although this process appears to be unprecedented in biology, it is reminiscent of the oxidation of Fe(II) to form Fe(2)O(3) in the ferritin iron storage protein. However, it includes a critical additional step of Mn(III) oxidation or disproportionation. We shall continue to investigate this biochemically unique process with purified enzymes.

In Situ Characterization of Mn(II) Oxidation by Spores of the Marine Bacillus sp. strain SG-1

Abstract Microbial oxidation of Mn(II) and subsequent precipitation of insoluble, reactive Mn(IV) oxides are primary sources of these solid phases in the environment and key controls on Mn cycling in natural waters. We have performed in situ x-ray absorption near-edge structure (XANES) spectroscopic measurements of Mn(II) oxidation by spores of the marine Bacillus sp. strain SG-1 to characterize the intermediates and products of the oxidation reactions. Mn(IV)-oxides resembling δ-MnO2 were observed to form at a rapid rate (within 14 min of reaction onset). Mn(III) intermediates did not occur above detection limit (5 to 10% of total Mn), even though Mn(III)/(II,III) oxides (MnOOH or Mn3O4) should have been more stable than MnO2 under the conditions of the experiments. These results suggest that Mn(IV) is the primary product of bacterial Mn(II) oxidation by Bacillus strain SG-1. Given that SG-1 is a good model for Mn(II)-oxidizing bacteria, these findings help to explain the predominance of Mn(IV)-oxides in aquatic environments.