Modeling the iron oxides and oxyhydroxides for the prediction of environmentally sensitive phase transformations

Iron oxides and oxyhydroxides are challenging to model computationally as competing phases may differ in formation energies by only several kJ/mol, they undergo magnetization transitions with temperature, their structures may contain partially occupied sites or long-range ordering of vacancies, and some loose structures require proper description of weak interactions such as hydrogen bonding and dispersive forces. If structures and transformations are to be reliably predicted under different chemical conditions, each of these challenges must be overcome simultaneously, while preserving a high level of numerical accuracy and physical sophistication. Here we present comparative studies of structure, magnetization, and elasticity properties of iron oxides and oxyhydroxides using density functional theory calculations with plane-wave and locally-confined-atomic-orbital basis sets, which are implemented in VASP and SIESTA packages, respectively. We have selected hematite, maghemite, goethite, lepidocrocite, and magnetite as model systems from a total of 13 known iron oxides and oxyhydroxides; and use same convergence criteria and almost equivalent settings in order to make consistent comparisons. Our results show both basis sets can reproduce the energetic stability and magnetic ordering, and are in agreement with experimental observations. There are advantages to choosing one basis set over the other, depending on the intended focus. In our case, we find the method using PW basis set most appropriate, and combine our results to construct the first phase diagram of iron oxides and oxyhydroxides in the space of competing chemical potentials, generated entirely from first principlesComment: 46 pages - Accepted for publication in PRB (19 journal pages), January 201

Structure investigation of nano-FeO(OH) modified clinoptilolite tuff for antimony removal

Biomimetic sol-gel synthesis was used to prepare new FeO(OH) zeolite (clinoptilolite tuff) adsorbent effective for antimony removal. The product was compared with other on the market accessible natural or commercial adsorption materials like granulated ferric hydroxide GEH, powder of zero valent iron (ZVI)- nanofer and the new synthesized oxi(hydr)oxide FeO(OH) and characterized by XRD, XPS, Raman, FT IR, TG, DTA, DTG, TEM and SEM techniques. Based upon the SEM analysis, the oxidized nanofer sample revealed the existence of hematite and goethite and morphology of FeO(OH) dopant confirmed the presence of ferrihydrite, in less extent also magnetite and hematite. Recorded exothermic maxima on DTA curves for powdered FeO(OH) zeolite at 460 °C and for pure component FeO(OH) at 560 °C indicated an 100 °C shifted exothermic effect, which characterized strong chemical interaction of FeO(OH) with zeolite structure. Based upon the XPS analyses, also the difference between Fe species in the raw and FeO(OH) doped zeolite was found in increasing Si/Al ratio, however only at the surface below app. 5 nm, measured as 3.94 for raw and 5.38 for sample treated with alkalic solution. The plotting of adsorption isotherms in the system studied clearly showed the increasing uptake capacity of the adsorbents towards antimony with the increased S(BET) data (GEH ˃FeO(OH)˃FeO(OH) zeolite˃nanofer)

Taking nature into lab: biomineralization by heavy metal-resistant streptomycetes in soil

Biomineralization by heavy metal-resistant streptomycetes was tested to evaluate the potential influence on metal mobilities in soil. Thus, we designed an experiment adopting conditions from classical laboratory methods to natural conditions prevailing in metal-rich soils with media spiked with heavy metals, soil agar, and nutrientenriched or unamended soil incubated with the bacteria. As a result, all strains were able to form struvite minerals (MgNH4PO4 6H2O) on tryptic soy broth (TSB)-media supplemented with AlCl3, MnCl2 and CuSO4, as well as on soil agar. Some strains additionally formed struvite on nutrient-enriched contaminated and control soil, as well as on metal contaminated soil without addition of media components. In contrast, switzerite (Mn3(PO4)2 7H2O) was exclusively formed on minimal media spiked with MnCl2 by four heavy metal-resistant strains, and on nutrient-enriched control soil by one strain. Hydrated nickel hydrogen phosphate was only crystallized on complex media supplemented with NiSO4 by most strains. Thus, mineralization is a dominant property of streptomycetes, with different processes likely to occur under laboratory conditions and sub-natural to natural conditions. This new understanding might have implications for our understanding of biological metal resistance mechanisms. We assume that biogeochemical cycles, nutrient storage and metal resistance might be affected by formation and re-solubilization of minerals like struvite in soil at microscale

Energetics of nanoparticle oxides: interplay between surface energy and polymorphism†

Many oxides tend to form different structures (polymorphs) for small particles. High temperature oxide melt solution calorimetry has been used to measure the enthalpy as a function of polymorphism and surface area for oxides of Al, Ti, and Zr. The results confirm crossovers in polymorph stability at the nanoscale. The energies of internal and external surfaces of zeolitic silicas with open framework structures are an order of magnitude smaller than those of oxides of normal density

The effect of chemical variability and weathering on Raman spectra of enargite and fahlore

Enargite (Cu3AsS4) and tennantite (Cu12As4S13) are typical As-bearing sulfides in intermediate- and high-sulfidation epithermal deposits. Trace and major element variations in enargite and tennantite and their substitution mechanisms are widely described. However, Raman spectra of the minerals with correlative quantitative chemical information are rarely documented, especially for enargite. Therefore, comparative electron and μ-Raman microprobe analyses were performed on enargite and fahlore grains. These spectra can be used in the industrial detection and subsequent removal of As-bearing sulfides prior to ore beneficiation in order to diminish the environmental impact of the metallurgical technologies. A simple Sb5+–As5+ substitution in enargite was confirmed by Raman analyses. Similarly, a complete solid solution series from tetrahedrite to tennantite (i.e., Sb3+–As3+ substitution) can be correlated with a gradual evolution in their Raman spectra. In turn, Te4+ occupies the As3+ and Sb3+ sites in fahlore by the coupled substitution Te4+ + Cu+ → (As, Sb)3+ + (Cu, Fe, Zn)2+. Accordingly, Raman bands of goldfieldite (Te-rich member) are strongly broadened compared with those of tetrahedrite and tennantite. A secondary phase with high porosity and a fibrous or wormlike texture was found in enargite in a weathered sample. The chemical composition, Raman spectrum, and X-ray diffraction signature of the secondary phase resemble tennantite. A gradual transformation of the primary enargite into this secondary phase was visualized by comparative electron and Raman microprobe mapping.</p

Long-range chemical sensitivity in the sulfur K-edge X-ray absorption spectra of substituted thiophenes

© 2014 American Chemical Society. Thiophenes are the simplest aromatic sulfur-containing compounds and are stable and widespread in fossil fuels. Regulation of sulfur levels in fuels and emissions has become and continues to be ever more stringent as part of governments' efforts to address negative environmental impacts of sulfur dioxide. In turn, more effective removal methods are continually being sought. In a chemical sense, thiophenes are somewhat obdurate and hence their removal from fossil fuels poses problems for the industrial chemist. Sulfur K-edge X-ray absorption spectroscopy provides key information on thiophenic components in fuels. Here we present a systematic study of the spectroscopic sensitivity to chemical modifications of the thiophene system. We conclude that while the utility of sulfur K-edge X-ray absorption spectra in understanding the chemical composition of sulfur-containing fossil fuels has already been demonstrated, care must be exercised in interpreting these spectra because the assumption of an invariant spectrum for thiophenic forms may not always be valid

Electron shuttle-mediated microbial Fe(III) reduction under alkaline conditions

Purpose: Extracellular Fe(III) reduction plays an important role in a variety of biogeochemical processes. Several mechanisms for microbial Fe(III) reduction in pH-neutral environments have been proposed, but pathways of microbial Fe(III) reduction within alkaline conditions have not been clearly identified. Alkaline soils are vastly distributed; thus, a better understanding of microbial Fe(III) reduction under alkaline conditions is of significance. The purpose of this study is to explore the dominant mechanism of bacterial iron reduction in alkaline environments. Materials and methods: We used antraquinone-2,6-disulfonate (AQDS) as a representative of quinone moities of humic substances and elemental sulfur and sulfate as sulfur species to investigate the potential role of humic substances and sulfur species in mediating microbial Fe(III) reduction in alkaline environments. We carried out thermodynamic calculations to predict the ability of bacteria to reduce Fe(III) (oxyhydr)oxides under alkaline conditions and the ability of AQDS and sulfur species to serve as electron acceptors for microbial anaerobic respiration in an assumed alkaline soil environments. A series of incubation experiments with two model dissimilatory metal reducing bacteria, Shewanella oneidensis MR-1 and Geobacter sulfurreducens PCA as well as mixed bacteria enriched from a soil were performed to confirm the contribution of AQDS and sulfur species to Fe(III) reduction under alkaline conditions. Results and discussion: Based on thermodynamic calculations, we predicted that, under alkaline conditions, the enzymatic reduction of Fe(III) (oxyhydr)oxides would be thermodynamically feasible but very weak. In our incubation experiments, the reduction of ferrihydrite by anaerobic cultures of Shewanella oneidensis MR-1, Geobacter sulfurreducens PCA or microbes enriched from a soil was significantly increased in the presence of S0 or AQDS. Notably, AQDS contributed more to promoting Fe(III) reduction as a soluble electron shuttle than S0 did under the alkaline conditions probably because of different mechanisms of microbial utilization of AQDS and S0. Conclusions: These results suggest that microbial reduction of Fe(III) (oxyhydr)oxides under alkaline conditions may proceed via a pathway mediated by electron shuttles such as AQDS and S0. Considering the high ability of electron shuttling and vast distribution of humic substances, we suggest that humic substance-mediated Fe(III) reduction may potentially be the dominant mechanism for Fe(III) reduction in alkaline environments

Periodic density functional theory calculations of bulk and the (010) surface of goethite

<p>Abstract</p> <p>Background</p> <p>Goethite is a common and reactive mineral in the environment. The transport of contaminants and anaerobic respiration of microbes are significantly affected by adsorption and reduction reactions involving goethite. An understanding of the mineral-water interface of goethite is critical for determining the molecular-scale mechanisms of adsorption and reduction reactions. In this study, periodic density functional theory (DFT) calculations were performed on the mineral goethite and its (010) surface, using the Vienna <it>Ab Initio </it>Simulation Package (VASP).</p> <p>Results</p> <p>Calculations of the bulk mineral structure accurately reproduced the observed crystal structure and vibrational frequencies, suggesting that this computational methodology was suitable for modeling the goethite-water interface. Energy-minimized structures of bare, hydrated (one H₂O layer) and solvated (three H₂O layers) (010) surfaces were calculated for 1 × 1 and 3 × 3 unit cell slabs. A good correlation between the calculated and observed vibrational frequencies was found for the 1 × 1 solvated surface. However, differences between the 1 × 1 and 3 × 3 slab calculations indicated that larger models may be necessary to simulate the relaxation of water at the interface. Comparison of two hydrated surfaces with molecularly and dissociatively adsorbed H₂O showed a significantly lower potential energy for the former.</p> <p>Conclusion</p> <p>Surface Fe-O and (Fe)O-H bond lengths are reported that may be useful in surface complexation models (SCM) of the goethite (010) surface. These bond lengths were found to change significantly as a function of solvation (i.e., addition of two extra H₂O layers above the surface), indicating that this parameter should be carefully considered in future SCM studies of metal oxide-water interfaces.</p