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Berlin Brandenburg

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  • 1
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(18), pp.5122-5139
    Description: ► Hydroxybenzoic acids retard ferrihydrite formation due to Fe(III) complexation. ► Hydroquinone ligands are most effective in retarding ferrihydrite formation. ► Hydroxybenzoic acids decrease the structural long-range order of ferrihydrite. ► Hydroxybenzoic acids increase the structural strain in ferrihydrite. ► The ferrihydrite structure contains 13 ± 3% tetrahedral Fe(III). 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 ≤ 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) 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. , 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.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 2
    Language: English
    In: Geochimica et Cosmochimica Acta, Sept 1, 2014, Vol.140, p.708(12)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2014.05.040 Byline: Christian Mikutta, Christian Schroder, F. Marc Michel Abstract: Amorphous ferric arsenate (AFA, FeAsO.sub.4*xH.sub.2O) is an important As precipitate in a range of oxic As-rich environments, especially acidic sulfide-bearing mine wastes. Its structure has been proposed to consist of small polymers of single corner-sharing FeO.sub.6 octahedra (r.sub.Fe-Fe [approximately equal to]3.6A) to which arsenate is attached as a monodentate binuclear.sup.2C complex ('chain model'). Here, we analyzed the structure of AFA and analogously prepared amorphous ferric phosphates (AFP, FePO.sub.4*xH.sub.2O) by a combination of high-energy total X-ray scattering, Fe K-edge X-ray absorption spectroscopy, and.sup.57Fe Mossbauer spectroscopy. Pair distribution function (PDF) analysis of total X-ray scattering data revealed that the coherently scattering domain size of AFA and AFP is about 8A. The PDFs of AFA lacked Fe-Fe pair correlations at r [approximately equal to]3.6A indicative of single corner-sharing FeO.sub.6 octahedra, which strongly supports a local scorodite (FeAsO.sub.4*2H.sub.2O) structure. Likewise, the PDFs and Fe K-edge extended X-ray absorption fine structure data of AFP were consistent with a local strengite (FePO.sub.4*2H.sub.2O) structure of isolated FeO.sub.6 octahedra being corner-linked to PO.sub.4 tetrahedra (r.sub.Fe-P =3.25(1)A). Mossbauer spectroscopy analyses of AFA and AFP indicated a strong superparamagnetism. While AFA only showed a weak onset of magnetic hyperfine splitting at 5K, magnetic ordering of AFP was completely absent at this temperature. Mossbauer spectroscopy may thus offer a convenient way to identify and quantify AFA and AFP in mineral mixtures containing poorly crystalline Fe(III)-oxyhydroxides. In summary, our results imply a close structural relationship between AFA and AFP and suggest that these amorphous materials serve as templates for the formation of scorodite and strengite (phosphosiderite) in strongly acidic low-temperature environments. Article History: Received 17 January 2014; Accepted 24 May 2014 Article Note: (miscellaneous) Associate editor: Jean Francois Boily
    Keywords: Phosphates -- Analysis ; Spectroscopy -- Analysis ; Iron Compounds -- Analysis
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 3
    Language: English
    In: Geochimica et Cosmochimica Acta, 2010, Vol.74(19), pp.5574-5592
    Description: In oxic environments contaminated with arsenate (As(V)), small polyhydroxycarboxylates such as citrate may impact the structure of precipitating ferrihydrite (Fh) and thus the surface speciation of As(V). In this study, ‘2-line’ Fh was precipitated from ferric nitrate solutions that were neutralized to pH 6.5 in the presence of increasing citrate concentrations and in the absence or presence of As(V). The initial citrate/Fe and As/Fe ratios were 0–50 mol% and 5 mol%, respectively. The reaction products, enriched with up to 0.32 mol citrate per mole Fe, were characterized by X-ray diffraction, transmission electron microscopy, and Fe and As K-edge X-ray absorption spectroscopy. Citrate decreased the particle size of Fh by impairing the polymerization of Fe(O,OH) octahedra via edge and corner linkages. In the presence of citrate and As(V), coordination numbers of Fe decreased by up to 28% relative to pure Fh. Citrate significantly reduced the static disorder of Fe–O bonds, implying a decreased octahedral distortion in Fh. Mean bond distances in Fh were not affected by citrate and remained constant within error at 1.98 Å for Fe–O, 3.03 Å for Fe–Fe1, and 3.45 Å for Fe–Fe2. Likewise, citrate had no effect on the As–Fe (3.31 Å) bond distance in As(V) coprecipitated with Fh. The As K-edge EXAFS data comply with the formation of (i) only monodentate binuclear ( C) As(V) surface complexes and (ii) combinations of C, monodentate mononuclear ( V), and outersphere As(V) surface complexes. Our results suggest that increasing citrate concentrations led to a decreasing V/ C ratio and/or that citrate increasingly impaired the formation of outersphere As(V) complexes. Moreover, citrate stabilized colloidal suspensions of Fh (pH 4.3–6.6, ∼0.45 M) and reduced Fh formation at the expense of soluble Fe(III)-citrate complexes. At initial citrate/Fe ratios ⩾25 mol%, between 8% and 41% of total Fe was bound in Fe(III)-citrate complexes after Fh formation. Polynuclear Fe(III)-citrate species were found to bind As(V) via surface complexes indistinguishable by EXAFS from those of As(V) adsorbed to or coprecipitated with Fh. Our study implies that low molecular weight polyhydroxycarboxylates may enhance the mobility of As(V) in aqueous systems of high ionic strength (e.g., neutralizing acid mine drainage) by colloidal stabilization of suspended Fh particles and the formation of ternary As(V) complexes.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 4
    In: Psychophysiology, September 2013, Vol.50(9), pp.909-919
    Description: The present study investigates the relation of perceived arousal (continuous self‐rating), autonomic nervous system activity (heart rate, heart rate variability) and musical characteristics (sound intensity, musical rhythm) upon listening to a complex musical piece. Twenty amateur musicians listened to two performances of hopin's “Tristesse” with different rhythmic shapes. Besides conventional statistical methods for analyzing psychophysiological reactions (heart rate, respiration rate) and musical variables, semblance analysis was used. Perceived arousal correlated strongly with sound intensity; heart rate showed only a partial response to changes in sound intensity. Larger changes in heart rate were caused by the version with more rhythmic tension. The low‐/high‐frequency ratio of heart rate variability increased—whereas the high frequency component decreased—during music listening. We conclude that autonomic nervous system activity can be modulated not only by sound intensity but also by the interpreter's use of rhythmic tension. Semblance analysis enables us to track the subtle correlations between musical and physiological variables.
    Keywords: Music ; Subjective Arousal ; Sound Intensity ; Heart Rate ; Heart Rate Variability ; Respiration Rate ; Tempo Rubato ; Semblance Analysis
    ISSN: 0048-5772
    E-ISSN: 1469-8986
    E-ISSN: 15405958
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  • 5
    Language: English
    In: Brain Topography, 2012, Vol.25(4), pp.423-430
    Description: Music is capable of inducing emotional arousal. While previous studies used brief musical excerpts to induce one specific emotion, the current study aimed to identify the physiological correlates of continuous changes in subjective emotional states while listening to a complete music piece. A total of 19 participants listened to the first movement of Ludwig van Beethoven’s 5th symphony (duration: ~7.4 min), during which a continuous 76-channel EEG was recorded. In a second session, the subjects evaluated their emotional arousal during the listening. A fast fourier transform was performed and covariance maps of spectral power were computed in association with the subjective arousal ratings. Subjective arousal ratings had good inter-individual correlations. Covariance maps showed a right-frontal suppression of lower alpha-band activity during high arousal. The results indicate that music is a powerful arousal-modulating stimulus. The temporal dynamics of the piece are well suited for sequential analysis, and could be necessary in helping unfold the full emotional power of music.
    Keywords: Arousal ; Emotion ; Music ; EEG ; Frontal alpha-asymmetry
    ISSN: 0896-0267
    E-ISSN: 1573-6792
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  • 6
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 September 2014, Vol.140, pp.708-719
    Description: Amorphous ferric arsenate (AFA, FeAsO · H O) is an important As precipitate in a range of oxic As-rich environments, especially acidic sulfide-bearing mine wastes. Its structure has been proposed to consist of small polymers of single corner-sharing FeO octahedra ( ∼3.6 Å) to which arsenate is attached as a monodentate binuclear C complex (‘chain model’). Here, we analyzed the structure of AFA and analogously prepared amorphous ferric phosphates (AFP, FePO · H O) by a combination of high-energy total X-ray scattering, Fe K-edge X-ray absorption spectroscopy, and Fe Mössbauer spectroscopy. Pair distribution function (PDF) analysis of total X-ray scattering data revealed that the coherently scattering domain size of AFA and AFP is about 8 Å. The PDFs of AFA lacked Fe–Fe pair correlations at ∼3.6 Å indicative of single corner-sharing FeO octahedra, which strongly supports a local scorodite (FeAsO ·2H O) structure. Likewise, the PDFs and Fe K-edge extended X-ray absorption fine structure data of AFP were consistent with a local strengite (FePO ·2H O) structure of isolated FeO octahedra being corner-linked to PO tetrahedra ( = 3.25(1) Å). Mössbauer spectroscopy analyses of AFA and AFP indicated a strong superparamagnetism. While AFA only showed a weak onset of magnetic hyperfine splitting at 5 K, magnetic ordering of AFP was completely absent at this temperature. Mössbauer spectroscopy may thus offer a convenient way to identify and quantify AFA and AFP in mineral mixtures containing poorly crystalline Fe(III)-oxyhydroxides. In summary, our results imply a close structural relationship between AFA and AFP and suggest that these amorphous materials serve as templates for the formation of scorodite and strengite (phosphosiderite) in strongly acidic low-temperature environments.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 7
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 December 2017, Vol.218, pp.237-256
    Description: The formation of realgar (As S ) has recently been identified as a prominent As sequestration pathway in the naturally As-enriched wetland soil at the Mokrsko geochemical anomaly (Czech Republic). Here we used bulk soil and pore water analyses, synchrotron X-ray absorption spectroscopy, S isotopes, and DNA extractions to determine the distribution and speciation of As as a function of soil depth and metabolic properties of microbial communities in wetland soil profiles. Total solid-phase analyses showed that As was strongly correlated with organic matter, caused by a considerable As accumulation (up to 21 g kg ) in an organic-rich soil horizon artificially buried in 1980 at a depth of ∼80 cm. Extended X-ray absorption fine structure spectroscopy revealed that As in the buried organic horizon was predominantly present as realgar occurring as nanocrystallites (50–100 nm) in millimeter-scale deposits associated with particulate organic matter. The realgar was depleted in the S isotope by 9–12.5‰ relative to the aqueous sulfate supplied to the soil, implying its biologically induced formation. Analysis of the microbial communities by 16S rDNA sequencing showed that realgar deposits formed in strictly anaerobic organic-rich domains dominated by sulfate-reducing and fermenting metabolisms. In contrast, realgar deposits were not observed in similar domains with even small contributions of oxidative metabolisms. No association of realgar with specific microbial species was observed. Our investigation shows that strongly reducing microenvironments associated with buried organic matter are significant biogeochemical traps for As, with an estimated As accumulation rate of 61 g As m yr . Nevertheless the production of biologically induced realgar in these microenvironments is too slow to lower As groundwater concentrations at our field site (∼6790 mg L ). Our study demonstrates the intricate link between geochemistry and microbial community dynamics in wetland soils, and provides insights into the conditions necessary to promote As sulfide precipitation in engineered wetlands for the treatment of As-rich waters.
    Keywords: Arsenic Speciation ; Realgar ; Wetland Soil ; X-Ray Absorption Spectroscopy ; Sulfur Isotopes ; Microbial Communities ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 8
    Language: English
    In: Geochimica et Cosmochimica Acta, 2006, Vol.70(3), pp.595-607
    Description: Anions of polycarboxylic low-molecular-weight organic acids (LMWOA) compete with phosphate for sorption sites of hydrous Fe and Al oxides. To test whether the sorption of LMWOA anions decreases the accessibility of micropores (〈2 nm) of goethite (α-FeOOH) for phosphate, we studied the kinetics of citrate-induced changes in microporosity and the phosphate sorption kinetics of synthetic goethite in the presence and absence of citrate in batch systems for 3 weeks (500 μM of each ion, pH 5). We also used C-coated goethite obtained after sorption of dissolved organic matter in order to simulate organic coatings in the soil. We analyzed our samples with N adsorption and electrophoretic mobility measurements. Citrate clogged the micropores of both adsorbents by up to 13% within 1 h of contact. The micropore volume decreased with increasing concentration and residence time of citrate. In the absence of citrate, phosphate diffused into micropores of the pure and C-coated goethite. The C coating (5.6 μmol C m ) did not impair the intraparticle diffusion of phosphate. In the presence of citrate, the diffusion of phosphate into the micropores of both adsorbents was strongly impaired. We attribute this to the micropore clogging and the ligand-induced dissolution of goethite by citrate. While the diffusion limitation of phosphate by citrate was stronger when citrate was added before phosphate to pure goethite, the order of addition of both ions to C-coated goethite had only a minor effect on the intraparticle diffusion of phosphate. Micropore clogging and dissolution of microporous hydrous Fe and Al oxides may be regarded as potential strategies of plants to cope with phosphate deficiency in addition to ligand-exchange.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 9
    Language: English
    In: Geochimica et Cosmochimica Acta, 2008, Vol.72(13), pp.3292-3292
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
    Source: ScienceDirect Journals (Elsevier)
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  • 10
    Language: English
    In: Environmental science & technology, 15 November 2011, Vol.45(22), pp.9550-7
    Description: Formation of ternary complexes between arsenic (As) oxyanions and ferric iron (Fe) complexes of humic substances (HS) is often hypothesized to represent a major mechanism for As-HS interactions under oxic conditions. However, direct evidence for this potentially important binding mechanism is still lacking. To investigate the molecular-scale interaction between arsenate, As(V), and HS in the presence of Fe(III), we reacted fulvic and humic acids with Fe(III) (1 wt %) and equilibrated the Fe(III)-HS complexes formed with As(V) at pH 7 (molar Fe/As ~10). The local (〈5 Å) coordination environments of As and Fe were subsequently studied by means of X-ray absorption spectroscopy. Our results show that 4.5-12.5 μmol As(V)/g HS (25-70% of total As) was associated with Fe(III). At least 70% of this As pool was bound to Fe(III)-HS complexes via inner-sphere complexation. Results obtained from shell fits of As K-edge extended X-ray absorption fine structure (EXAFS) spectra were consistent with a monodentate binuclear ((2)C) and monodentate mononuclear ((1)V) complex stabilized by H-bonds (R(As-Fe) = 3.30 Å). The analysis of Fe K-edge EXAFS spectra revealed that Fe in Fe(III)-HS complexes was predominantly present as oligomeric Fe(III) clusters at neutral pH. Shell-fit results complied with a structural motif in which three corner-sharing Fe(O,OH)(6) octahedra linked by a single μ(3)-O bridge form a planar Fe trimer. In these complexes, the average Fe-C and Fe-Fe bond distances were 2.95 Å and 3.47 Å, respectively. Our study provides the first spectroscopic evidence for ternary complex formation between As(V) and Fe(III)-HS complexes, suggesting that this binding mechanism is of fundamental importance for the cycling of oxyanions such as As(V) in organic-rich, oxic soils and sediments.
    Keywords: Arsenates -- Chemistry ; Ferric Compounds -- Chemistry ; Humic Substances -- Analysis
    ISSN: 0013936X
    E-ISSN: 1520-5851
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