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  • 1
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 November 2014, Vol.144, pp.258-276
    Description: Ferric oxyhydroxides play an important role in controlling the bioavailability of oxyanions such as arsenate and phosphate in soil. Despite this, little is known about the properties and reactivity of Fe(III)-organic matter phases derived from adsorption (reaction of organic matter (OM) to post-synthesis Fe oxide) coprecipitation (formation of Fe oxides in presence of OM). Coprecipitates and adsorption complexes were synthesized at pH 4 using two natural organic matter (NOM) types extracted from forest floor layers (Oi and Oa horizon) of a Haplic Podzol. Iron(III) coprecipitates were formed at initial molar metal-to-carbon (M/C) ratios of 1.0 and 0.1 and an aluminum (Al)-to-Fe(III) ratio of 0.2. Sample properties were studied by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), N gas adsorption, dynamic light scattering, and electrophoretic mobility measurements. Arsenic [As(V)] adsorption to Fe-OM phases was studied in batch experiments (168 h, pH 4, 100 μM As). The organic carbon (OC) contents of the coprecipitates (82–339 mg g ) were higher than those of adsorption complexes (31 and 36 mg g ), leading to pronounced variations in specific surface area (9–300 m g ), average pore radii (1–9 nm), and total pore volumes (11–374 mm g ) but being independent of the NOM type or the presence of Al. The occlusion of Fe solids by OM (XPS surface concentrations: 60–82 atom% C) caused comparable pH (1.5–2) of adsorption complexes and coprecipitates. The synthesis conditions resulted in different Fe-OM association modes: Fe oxide particles in ‘M/C 0.1’ coprecipitates covered to a larger extent the outermost aggregate surfaces, for some ‘M/C 1.0’ coprecipitates OM effectively enveloped the Fe oxides, while OM in the adsorption complexes primarily covered the outer aggregate surfaces. Despite of their larger OC contents, adsorption of As(V) was fastest to coprecipitates formed at low Fe availability (M/C 0.1) and facilitated by desorption of weakly bonded OC and disaggregation. In contrast, ‘M/C 1.0’ coprecipitates showed a comparable rate of As uptake as the adsorption complexes. While small mesopores (2–10 nm) promoted the fast As uptake particularly to ‘M/C 0.1’ coprecipitates, the presence of micropores (〈2 nm) appeared to impair As desorption. This study shows that the environmental reactivity of poorly crystalline Fe(III) oxides in terrestrial and aquatic systems can largely vary depending on the formation conditions. Carbon-rich Fe phases precipitated at low M/C ratios may play a more important role in oxyanion immobilization and Fe and C cycling than phases formed at higher M/C ratios or respective adsorption complexes.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 2
    Language: English
    In: Soil Biology and Biochemistry, 2011, Vol.43(8), pp.1738-1741
    Description: Understanding the turnover of organic matter (OM) in soils necessitates information on biological stability and ecological functions. For easy characterization of slowly cycling OM, treatments using oxidants such as sodium hypochlorite (NaOCl) have been applied. The rationale for that approach is, however, questionable and concerns exist to which extent abiotic oxidation can mimic biological mineralization. Here we compare biological mineralization of mineral-bound OM to its resistance to chemical oxidation by 6 mass% NaOCl. Water-extractable OM, sorbed to goethite, vermiculite, and pyrophyllite at pH 4.0 and in different background electrolytes (CaCl , NaCl, NaCl–NaH PO ) to favor or exclude certain binding mechanisms, was subsequently subjected to NaOCl treatment (pH 7, either for 18 or 6 × 6 h). Irrespective of mineral surface properties and mechanisms involved in OM sorption, NaOCl removed a constant portion of the sorbed OC. More OC survived when bound to goethite than to vermiculite, thus confirming previous results on the increase of oxidation-resistant OC with increasing Fe and Al (hydr)oxide contents in different soils. Mineralizable OC (within 90 days) was much smaller than the NaOCl-removable OC and both fractions were negatively correlated (  = 0.90 for the 18 h treatment;  = 0.86 for the 6 × 6 h treatment), suggesting that chemically oxidizable OM does not represent the portion of sorbed OM available to biological consumption. ► Organic matter resistant to wet oxidation has been considered a biologically stable fraction. ► The OC fraction in mineral−organic associations oxidizable by sodium hypochlorite was inversely related to the mineralizable OC fraction. ► Organic matter resistant to chemical oxidation cannot be used as proxy for biologically stable OM.
    Keywords: Sodium Hypochlorite ; Naocl ; Wet Oxidation ; Organic Matter ; Stabilization ; Passive Organic Matter Pool ; Stable Organic Matter ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 3
    Language: English
    In: Geochimica et Cosmochimica Acta, Nov 1, 2014, Vol.144, p.258(19)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2014.08.026 Byline: Robert Mikutta, Dennis Lorenz, Georg Guggenberger, Ludwig Haumaier, Anja Freund Abstract: Ferric oxyhydroxides play an important role in controlling the bioavailability of oxyanions such as arsenate and phosphate in soil. Despite this, little is known about the properties and reactivity of Fe(III)-organic matter phases derived from adsorption (reaction of organic matter (OM) to post-synthesis Fe oxide) versus coprecipitation (formation of Fe oxides in presence of OM). Coprecipitates and adsorption complexes were synthesized at pH 4 using two natural organic matter (NOM) types extracted from forest floor layers (Oi and Oa horizon) of a Haplic Podzol. Iron(III) coprecipitates were formed at initial molar metal-to-carbon (M/C) ratios of 1.0 and 0.1 and an aluminum (Al)-to-Fe(III) ratio of 0.2. Sample properties were studied by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), N.sub.2 gas adsorption, dynamic light scattering, and electrophoretic mobility measurements. Arsenic [As(V)] adsorption to Fe-OM phases was studied in batch experiments (168h, pH 4, 100[mu]M As). The organic carbon (OC) contents of the coprecipitates (82-339mgg.sup.-1) were higher than those of adsorption complexes (31 and 36mgg.sup.-1), leading to pronounced variations in specific surface area (9-300m.sup.2 g.sup.-1), average pore radii (1-9nm), and total pore volumes (11-374mm.sup.3 g.sup.-1) but being independent of the NOM type or the presence of Al. The occlusion of Fe solids by OM (XPS surface concentrations: 60-82atom% C) caused comparable pH.sub.PZC (1.5-2) of adsorption complexes and coprecipitates. The synthesis conditions resulted in different Fe-OM association modes: Fe oxide particles in 'M/C 0.1' coprecipitates covered to a larger extent the outermost aggregate surfaces, for some 'M/C 1.0' coprecipitates OM effectively enveloped the Fe oxides, while OM in the adsorption complexes primarily covered the outer aggregate surfaces. Despite of their larger OC contents, adsorption of As(V) was fastest to coprecipitates formed at low Fe availability (M/C 0.1) and facilitated by desorption of weakly bonded OC and disaggregation. In contrast, 'M/C 1.0' coprecipitates showed a comparable rate of As uptake as the adsorption complexes. While small mesopores (2-10nm) promoted the fast As uptake particularly to 'M/C 0.1' coprecipitates, the presence of micropores (〈2nm) appeared to impair As desorption. This study shows that the environmental reactivity of poorly crystalline Fe(III) oxides in terrestrial and aquatic systems can largely vary depending on the formation conditions. Carbon-rich Fe phases precipitated at low M/C ratios may play a more important role in oxyanion immobilization and Fe and C cycling than phases formed at higher M/C ratios or respective adsorption complexes. Article History: Received 17 January 2014; Accepted 21 August 2014 Article Note: (miscellaneous) Associate editor: Jon Chorover
    Keywords: X-Ray Spectroscopy -- Chemical Properties ; Humic Acids -- Chemical Properties ; Oxides -- Chemical Properties ; Phosphates -- Chemical Properties ; Adsorption -- Chemical Properties ; Arsenic -- Chemical Properties
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 4
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(11), pp.3135-3154
    Description: Extracellular polymeric substances (EPS) are continuously produced by bacteria during their growth and metabolism. In soils, EPS are bound to cell surfaces, associated with biofilms, or released into solution where they can react with other solutes and soil particle surfaces. If such reaction results in a decrease in EPS bioaccessibility, it may contribute to stabilization of microbial-derived organic carbon (OC) in soil. Here we examined: (i) the chemical fractionation of EPS produced by a common Gram positive soil bacterial strain ( ) during reaction with dissolved and colloidal Al species and (ii) the resulting stabilization against desorption and microbial decay by the respective coprecipitation (with dissolved Al) and adsorption (with Al(OH) ) processes. Coprecipitates and adsorption complexes obtained following EPS–Al reaction as a function of pH and ionic strength were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The stability of adsorbed and coprecipitated EPS against biodegradation was assessed by mineralization experiments for 1100 h. Up to 60% of the initial 100 mg/L EPS-C was adsorbed at the highest initial molar Al:C ratio (1.86), but this still resulted only in a moderate OC mass fraction in the solid phase (17 mg/g Al(OH) ). In contrast, while coprecipitation by Al was less efficient in removing EPS from solution (maximum values of 33% at molar Al:C ratios of 0.1–0.2), the OC mass fraction in the solid product was substantially larger than that in adsorption complexes. Organic P compounds were preferentially bound during both adsorption and coprecipitation. Data are consistent with strong ligand exchange of EPS phosphoryl groups during adsorption to Al(OH) , whereas for coprecipitation weaker sorption mechanisms are also involved. X-ray photoelectron analyses indicate an intimate mixing of EPS with Al in the coprecipitates, which is not observed in the case of EPS adsorption complexes. The incubation experiments showed that both processes result in overall stabilization of EPS against microbial decay. Stabilization of adsorbed or coprecipitated EPS increased with increasing molar Al:C ratio and biodegradation was correlated with EPS desorption, implying that detachment of EPS from surface sites is a prerequisite for microbial utilization. Results indicate that the mechanisms transferring EPS into Al–organic associations may significantly affect the composition and stability of biomolecular C, N and P in soils. The observed efficient stabilization of EPS might explain the strong microbial character of organic matter in subsoils.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 5
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 March 2018, Vol.224, pp.223-248
    Description: Iron (Fe) oxyhydroxides are important constituents of the soil mineral phase known to stabilize organic matter (OM) under oxic conditions. In an anoxic milieu, however, these Fe-organic associations are exposed to microbial reduction, releasing OM into soil solution. At present, only few studies have addressed the influence of adsorbed natural OM (NOM) on the reductive dissolution of Fe oxyhydroxides. This study therefore examined the impact of both the composition and concentration of adsorbed NOM on microbial Fe reduction with regard to (i) electron shuttling, (ii) complexation of Fe(II,III), (iii) surface site coverage and/or pore blockage, and (iv) aggregation. Adsorption complexes with varying carbon loadings were synthesized using different Fe oxyhydroxides (ferrihydrite, lepidocrocite, goethite, hematite, magnetite) and NOM of different origin (extracellular polymeric substances from OM extracted from soil Oi and Oa horizons). The adsorption complexes were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), N gas adsorption, electrophoretic mobility and particle size measurements, and OM desorption. Incubation experiments under anaerobic conditions were conducted for 16 days comparing two different strains of dissimilatory Fe(III)-reducing bacteria ( , ). Mineral transformation during reduction was assessed via XRD and FTIR. Microbial reduction of the pure Fe oxyhydroxides was controlled by the specific surface area (SSA) and solubility of the minerals. For , the Fe reduction of adsorption complexes strongly correlated with the concentration of potentially usable electron-shuttling molecules for NOM concentrations 〈2 mg C L , whereas for , Fe reduction depended on the particle size and thus aggregation of the adsorption complexes. These diverging results suggest that the influence of NOM on the stability of Fe-organic associations in soils cannot easily be assessed without considering the composition of the microbial soil community.
    Keywords: Microbial Reduction ; Iron Oxyhydroxides ; Natural Organic Matter ; Extracellular Polymeric Substances ; Shewanella Putrefaciens ; Geobacter Metallireducens ; Mineral-Organic Associations ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 6
    In: Global Change Biology, November 2010, Vol.16(11), pp.2990-3003
    Description: Global nitrogen (N) deposition rates in terrestrial environments have quadrupled since preindustrial times, causing structural and functional changes of ecosystems. Different emission reduction policies were therefore devised. The aim of our study was to investigate if, and over what timescale, processes of soil organic matter (OM) transformation respond to a decline in atmospheric N deposition. A N‐saturated spruce forest (current N deposition: 34 kg ha yr; critical N load: 14 kg ha yr), where N deposition has been reduced to 11.5 kg ha yr since 1991, was studied. Besides organic C and organic and inorganic N, noncellulosic carbohydrates, amino sugars and amino acids were determined. A decline in organic N in litter indicated initial effects at plant level. However, there were no changes in biomarkers upon the reduction in N deposition. In addition, inorganic N was not affected by reduced N deposition. The results showed that OM cycling and transformation processes have not responded so far. It was concluded that no direct N deposition effects have occurred due to the large amount of stored organic N, which seems to compensate for the reduction in deposited N. Obviously, the time span of atmospheric N reduction (about 14.5 years) is too short compared with the mean turnover time of litter to cause indirect effects on the composition of organic C and N compounds. It is assumed that ecological processes, such as microbial decomposition or recycling of organic N and C, react slowly, but may start within the next decade with the incorporation of the new litter.
    Keywords: Amino Acid Enantiomers ; Amino Acids ; Amino Sugars ; Biomarker ; N Deposition ; Non‐Cellulosic Carbohydrates ; Soil Organic Nitrogen ; Solling Roof Project
    ISSN: 1354-1013
    E-ISSN: 1365-2486
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  • 7
    Language: English
    In: Chemosphere, January 2015, Vol.119, pp.155-162
    Description: Carbonaceous material from pyrolysis (pyrochars) and hydrothermal carbonization (hydrochars) are applied to soil to improve soil fertility and carbon sequestration. As a positive side effect, the mobility of pesticides and the risk of groundwater contamination can be minimized. However, the impact of various raw materials on the sorption capacity of different pyrochars and hydrochars is poorly understood. Thus, sorption experiments were performed with C-labeled isoproturon (IPU, 0.75 kg ha ) in a loamy sand soil amended with either pyrochar or hydrochar (0.5% and 5% dry weight, respectively). Carbonaceous materials were produced from three different raw materials: corn digestate, miscanthus, woodchips of willow and poplar. After 72 h of incubation, a sequential extraction procedure was conducted to quantify IPU bioavailability, total amount of extractable IPU, and non-extractable pesticide residues (NER). Added char amount, carbonization type, and raw materials had statistically significant effects on the sorption of IPU. The amount of available IPU was reduced by a factor of 10–2283 in treatments with pyrochar and by a factor of 3–13 in hydrochar treatments. The surface area of the charred material was the most predictive variable of IPU sorption to char amended soil. Some physical and chemical char properties tend to correlate with pore water-, methanol- or non-extractable IPU amounts. Due to a low micro-porosity and ash content, high water extractable carbon contents and O-functional groups of hydrochars, the proportion of NER in hydrochar amended soils was considerably lower than in soil amended with pyrochars.
    Keywords: Biochar ; Hydrothermal Carbonization ; Pyrolysis ; Pesticide ; Adsorption ; Bioavailability ; Chemistry ; Ecology
    ISSN: 0045-6535
    E-ISSN: 1879-1298
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  • 8
    Language: English
    In: Chemosphere, Jan, 2015, Vol.119, p.155(8)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.chemosphere.2014.05.059 Byline: Nina Eibisch, Reiner Schroll, Roland Fu[sz], Robert Mikutta, Mirjam Helfrich, Heinz Flessa Abstract: * Chars with high carbonization degree (high C stability) have high sorption capacity. * Impact of raw materials on sorption capacity depends on carbonization conditions. * High bioaccessibility in hydrochar amended soil irrespective of the raw material. * Hydrochars react as H-bond acceptor, pyrochars through hydrophobic and H-bonding. * The micro-porosity determines the amount of non-extractable residues. Article History: Received 10 November 2013; Revised 14 May 2014; Accepted 23 May 2014 Article Note: (miscellaneous) Handling Editor: X. Cao
    Keywords: Raw Materials
    ISSN: 0045-6535
    Source: Cengage Learning, Inc.
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  • 9
    Language: English
    In: Colloids and Surfaces A: Physicochemical and Engineering Aspects, 05 October 2018, Vol.554, pp.156-168
    Description: In order to assess the mobility and function of Fe oxihydroxides in terrestrial and aquatic environments, knowledge of the parameters and conditions determining aggregation and the size of formed aggregates is crucial. Here we study the impact of different organic matter (OM) types on the aggregation of goethite (α-FeOOH) with particular focus on the relevance of surface charge (SC). Synthetic goethite was reacted with galacturonic acid (GA), polygalacturonic acid (PGA), and tannic acid (TA) as model substances as well as with natural dissolved OM (DOM) from a litter (Oi-DOM) and a humified horizon (Oa-DOM). The SC of goethite was adjusted at pH 4 and 6 by the adsorption of organic acids and DOM to equal positive and negative SC as well as point of zero charge ( ). Aggregation was traced by laser light scattering and sedimentation experiments. Aggregation of all goethite-OM associations depended on OM type and could well be explained by SC. Associations of goethite with OM rich in acidic groups (PGA, Oi-DOM, and Oa-DOM) followed the aggregation behavior of pure goethite. Largest aggregates with diameters up to 7 μm formed at , whereas smaller ones (∼0.4 μm) developed at positive or negative SC. Organic substances rich in acidic functional groups interacted strongly with goethite at high additions, thus favoring charge reversal and limiting aggregate growth. For OM with low acidity (TA and GA), adsorption on goethite was incomplete even at high additions. These associations remained close to and, hence, were susceptible to aggregation with maximum diameters at 6 μm. Aggregation was possibly also promoted by the exposure of less polar moieties exposed at the goethite-OM interface. Our data suggest that aggregation in environmental systems such as soils is driven by the nature and acidity of ubiquitous OM, which determines SC by the extent of adsorption to mineral surfaces.
    Keywords: Goethite ; Organic Acids ; Dissolved Organic Matter ; Surface Charge ; Aggregation Kinetics ; Engineering ; Chemistry
    ISSN: 0927-7757
    E-ISSN: 1873-4359
    Source: ScienceDirect Journals (Elsevier)
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  • 10
    In: Global Change Biology, January 2018, Vol.24(1), pp.e183-e189
    Description: Current climate and land‐use changes affect regional and global cycles of silicon (Si), with yet uncertain consequences for ecosystems. The key role of Si in marine ecology by controlling algae growth is well recognized but research on terrestrial ecosystems neglected Si since not considered an essential plant nutrient. However, grasses and various other plants accumulate large amounts of Si, and recently it has been hypothesized that incorporation of Si as a structural plant component may substitute for the energetically more expensive biosynthesis of lignin. Herein, we provide evidence supporting this hypothesis. We demonstrate that in straw of rice () deriving from a large geographic gradient across South‐East Asia, the Si concentrations (ranging from 1.6% to 10.7%) are negatively related to the concentrations of carbon (31.3% to 42.5%) and lignin‐derived phenols (32 to 102 mg/g carbon). Less lignin may explain results of previous studies that Si‐rich straw decomposes faster. Hence, Si seems a significant but hardly recognized factor in organic carbon cycling through grasslands and other ecosystems dominated by Si‐accumulating plants. The key role of silicon in marine ecology by controlling algae growth is well recognized but research on terrestrial ecosystems neglected Si since not considered an essential plant nutrient. However, many plants accumulate large amounts of Si, and recently it has been hypothesized that incorporation of Si as a structural component may substitute for the energetically more expensive biosynthesis of lignin. Herein, we provide evidence supporting this hypothesis. We demonstrate that in rice straw deriving from a large geographic gradient across South‐East Asia, the Si concentrations are negatively related to the concentrations of carbon and lignin‐derived phenols. Our data offer an explanation for previous findings of faster decomposition of Si‐rich rice straw as lignin regulates plant litter decomposition rates. Hence, Si seems a significant but hardly recognized factor in carbon cycling through ecosystems dominated by grass species and/or other Si‐accumulating plants.
    Keywords: Carbon Cycle ; Lignin ; Litter Decomposition ; Rice ; Silicon ; Structural Plant Components
    ISSN: 1354-1013
    E-ISSN: 1365-2486
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