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Berlin Brandenburg

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  • 1
    Article
    Article
    Citrate impairs the micropore diffusion of phosphate into pure and C-coated goethite (2006)
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    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
    DOI: 10.1016/j.gca.2005.10.032
    URL: View record in ScienceDirect (Access to full text may be restricted)
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  • 2
    Article
    Article
    Restructuring of polygalacturonate on alumina upon hydration—Effect on phosphate sorption kinetics (2006)
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    Language: English
    In: Geochimica et Cosmochimica Acta, 2006, Vol.70(12), pp.2957-2969
    Description: Hydration of organic coatings in soils is expected to affect the sorption of oxyanions onto hydrous Fe and Al oxides. We hypothesized that the hydration of polygalacturonate (PGA) coatings on alumina (Al O ) increases their permeability for phosphate. Pure and PGA-coated alumina were equilibrated in deionized water for 2 and 170 h at pH 5 and 20 °C before studying (i) their porosity with N gas adsorption and H NMR relaxometry, (ii) structural changes of PGA-coatings with differential scanning calorimetry (DSC), and (iii) the kinetics of phosphate sorption and PGA desorption in batch experiments. Scanning electron micrographs revealed that PGA molecules formed three-dimensional networks with pores ranging in size from 〈10 to several hundred nanometers. Our NMR results showed that the water content of intraparticle alumina pores decreased upon PGA sorption, indicating a displacement of pore water by PGA. The amount of water in interparticle alumina pores increased strongly after PGA addition, however, and was attributed to water in pores of PGA and/or in pores at the PGA-alumina interface. The flexibility of PGA molecules and the fraction of a PGA gel phase increased within one week of hydration, implying restructuring of PGA. Hydration of PGA coatings increased the amount of phosphate defined as instantaneously sorbed by 84%, showing that restructuring of PGA enhanced the accessibility of phosphate to external alumina surfaces. Despite the fact that the efficacy of phosphate to displace PGA was higher after 170 h than after 2 h, a higher phosphate surface loading was required after 170 h to set off PGA desorption. Our findings imply that the number of PGA chain segments directly attached to the alumina surface decreased with time. We conclude that hydration/dehydration of polymeric surface coatings affects the sorption kinetics of oxyanions, and may thus control the sorption and transport of solutes in soils.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
    DOI: 10.1016/j.gca.2006.03.007
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  • 3
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    Article
    Phosphate desorption from goethite in the presence of galacturonate, polygalacturonate, and maize mucigel (Zea mays L.).(Author abstract) (2006)
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    Language: English
    In: Soil Science Society of America Journal, Sept-Oct, 2006, Vol.70(5), p.1731(10)
    Description: Uronates are important constituents of maize mucilage and polyuronates are used as a simplified model of the soil--root interface. We tested whether galacturonate (GA) and polygalacturonate (PGA) impair the diffusion of phosphate (P[O.sub.4]) into and out of pores of a synthetic goethite (147 [m.sup.2] [g.sup.-1]) and whether the effect of maize mucigel (MU) is comparable to PGA. We measured the P[O.sub.4] desorption kinetics of goethites in batch experiments over 2 wk at pH 5. One part of the goethite was equilibrated with organic substances before P[O.sub.4] addition, another part after addition of P[O.sub.4]. Before the desorption experiments, the porosity of our samples was analyzed by [N.sub.2] gas adsorption. In each treatment a rapid initial desorption was followed by a slow desorption reaction, which is assigned to the diffusion of P[O.sub.4] out of mineral pores. No consistent relation between the micro- and mesoporosity and the rate of the slow P[O.sub.4] desorption was observed. Compared with the C-free control, only PGA and MU affected the fraction of P[O.sub.4] mobilized by the fast and slow desorption reaction: when PGA was sorbed to goethite before P[O.sub.4], twice as much P[O.sub.4] was mobilized via the fast reaction than in the treatment where P[O.sub.4] was sorbed before PGA, suggesting a decreased accessibility of goethite pores to P[O.sub.4]. Mucigel, however, showed reversed effects, which is ascribed to its differing chemical composition. In conclusion, PGA seems inappropriate as a model substance for maize MU collected from non-axenic sand cultures. Under the experimental conditions chosen, the efficacy of all organic substances to increase P[O.sub.4] solution concentrations by pore clogging and sorption competition is small.
    Keywords: Phosphates -- Research ; Sorption -- Research
    ISSN: 0361-5995
    E-ISSN: 14350661
    DOI: 10.2136/sssaj2005.0414
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  • 4
    Article
    Article
    Kinetics of phosphate sorption to polygalacturonate-coated goethite.(Author abstract) (2006)
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    Language: English
    In: Soil Science Society of America Journal, March-April, 2006, Vol.70(2), p.541(9)
    Description: Biogenetic polysugars may affect the sorption characteristics of soil mineral particles in the rhizosphere. We hypothesized that polygalacturonate [PGA, ([[C.sub.6][H.sub.7][O.sub.6]).sub.n.sup.-]] coatings on goethite reduce the diffusion of phosphate into the pores of the adsorbent. Goethite was preloaded with PGA (0-10 mg C [g.sup.-1]). The samples were characterized by [N.sub.2] and C[O.sub.2] adsorption, electrophoretic mobility measurements, and scanning electron microscopy/energy dispersive X-ray analysis (SEM-EDX). The phosphate sorption kinetics was studied with batch experiments over 2 wk at pH 5 and an initial phosphate concentration of 250 [micro]M. Pore volume and specific surface area of the goethite samples declined after PGA addition. The PGA coatings reduced the [zeta]-potential of goethite from 42.3 to -39.6 mV at the highest C loading. With increasing PGA-C content and decreasing [zeta]-potential the amount of phosphate sorbed after 2 wk decreased linearly (P 〈 0.001). Sorption of phosphate to pure and PGA-coated goethite showed an initial fast sorption followed by a slow sorption reaction. At the smallest C loading (5.5 mg C [g.sup.-1]) the portion of phosphate retained by the slow reaction was smaller than for the treatment without any PGA, while at higher C loadings the fraction of slowly immobilized phosphate increased. Our results suggest that at low C-loadings PGA impaired the intraparticle diffusion of phosphate. In contrast, the slow step-by-step desorption of PGA (〈52% within 2 wk) or the diffusion of phosphate through PGA coatings or both are rate limiting for the slow phosphate reaction at C loadings 〉 5.5 mg C [g.sup.-1].
    Keywords: Soil Phosphorus -- Research ; Soil Chemistry -- Research ; X-ray Analysis
    ISSN: 0361-5995
    E-ISSN: 14350661
    DOI: 10.2136/sssaj2005.0250
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  • 5
    Article
    Article
    Soil organic matter clogs mineral pores: evidence from [sup.1]H-NMR and [N.sub.2] adsorption.(Author Abstract) (2004)
    Details
    Language: English
    In: Soil Science Society of America Journal, Nov-Dec, 2004, Vol.68(6), p.1853(10)
    Description: Recent [N.sub.2] adsorption studies have suggested a 'pore clogging' effect on mineral soil phases caused by organic matter coatings. For methodological reasons, this pore clogging effect has been studied only after drying. Our hypothesis was that pore clogging is affected by drying of organic coatings. In our study, we used AlOOH, which has been equilibrated with dissolved organic matter (DOM) and polygalacturonic acid [[PGA; [([C.sub.6][H.sub.8][O.sub.6]).sup.n]]. To test our hypothesis, we determined the porosity of moist and freeze-dried AlOOH samples. Freeze-dried samples were analyzed by [N.sub.2] adsorption, moist samples by [sup.1]H-nuclear magnetic resonance (NMR). In addition, the samples were characterized by environmental scanning electron microscopy--energy dispersive x-ray spectroscopy (ESEM-EDX). Both, DOM and PGA significantly reduced specific surface area (SS[A.sub.BET]) of AlOOH by 34 [m.sup.2][g.sup.-1](15%) and 77 [m.sup.2] [g.sup.-1] (36%). The reduction in SS[A.sub.BET] normalized to the amount of C sorbed was 1.0 [m.sup.2] [mg.sup.-1] DOM-C and 5.9 [m.sup.2] [mg.sup.-1] PGA-C. Dissolved OM reduced the pore volume of micro- and small mesopores 〈3 nm whereas PGA also reduced the volume of larger pores. The [sup.1]H-NMR results of moist samples showed that PGA sorption reduced the amount of water in pores 〈4 nm. In addition, the pore size maximum of AlOOH increased by 150%. Polygalacturonic acid coatings created new interparticle pores of about 10- to 70-nm size that are not stable upon freeze-drying. Porosity changes upon DOM-treatment were not commensurable by [sup.1]H-NMR. Our results indicate that clogging of micro- and small mesopores is not an artifact of freeze-drying. Polygalacturonic acid seems not only to cover the mouth of AlOOH-nanometer pores but also to fill them.
    Keywords: Soil Mineralogy -- Research
    ISSN: 0361-5995
    E-ISSN: 14350661
    DOI: 10.2136/sssaj2004.1853
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  • 6
    Article
    Article
    Acid polysaccharide coatings on microporous goethites: controls of slow phosphate sorption.(Author abstract) (2006)
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    Language: English
    In: Soil Science Society of America Journal, Sept-Oct, 2006, Vol.70(5), p.1547(9)
    Description: Organic coatings on Fe oxides can decrease the accessibility of intraparticle pores for oxyanions like phosphate. We hypothesized that the slow sorption of phosphate to goethite coated with polygalacturonate (PGA) is controlled by the accessibility of external goethite surfaces to phosphate rather than by diffusion of phosphate into micropores ([empty set] 〈 2 nm). We studied the phosphate sorption kinetics of pure and PGA-coated goethites that differed in their microporosity ([N.sub.2] at 77 K, 46 vs. 31 [mm.sup.3] [g.sup.-1]). Because drying may affect the structure or surface coverage of PGA, we also tested the effect of freeze-drying on the slow phosphate sorption. The samples were examined by gas adsorption ([N.sub.2], C[O.sub.2]) and electrophoretic mobility measurements. Phosphate sorption and PGA-C desorption were studied in batch experiments for 3 wk at pH 5. In PGA-coated samples, the slow phosphate sorption was independent of micropore volume. Phosphate displaced on average 57% of PGA-C within 3 wk. Similar to phosphate sorption, the PGA-C desorption comprised a rapid initial desorption, which was followed by a slow C desorption. Sorption competition between phosphate and presorbed PGA depended on the 〈10-nm porosity and the C loading of the adsorbent. The efficacy of phosphate to desorb PGA generally increased after freeze-drying. We conclude for PGA-coated goethites that (i) freeze-drying biased the slow phosphate sorption by changing the structure/surface coverage of PGA, and (ii) within the time frame studied, micropores did not limit the rate of the slow phosphate sorption. Rather, the slow, gradual desorption of PGA and/or the diffusion of phosphate through PGA coatings controlled the slow phosphate sorption to PGA-coated goethite.
    Keywords: Mineralogical Research -- Analysis ; Phosphates -- Research ; Sorption -- Research
    ISSN: 0361-5995
    E-ISSN: 14350661
    DOI: 10.2136/sssaj2005.0340
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  • 7
    Article
    Article
    Stabilization of organic matter at micropores (〈2 nm) in acid forest subsoils.(Author abstract) (2006)
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    Language: English
    In: Soil Science Society of America Journal, Nov-Dec, 2006, Vol.70(6), p.2049(8)
    Description: Binding of soil organic matter (OM) at micropore entrances within small mesopores (2-10 nm) has been suggested as a potential mechanism for the stabilization of OM against biodegradation. We hypothesized that the mineral-associated fraction of stable OM [OM resisting treatment with 6% sodium hypochlorite (NaOCl) and subsequently extracted by 10% hydrofluoric acid] is associated with pores 〈10 nm in 12 acid subsoil horizons. To study the coverage of micropores by stable OM sorbed in mesopores, we assumed that most micropores have entrances of mesopore size. We compared the accessibility of C[O.sub.2] at 273 K to micropores after NaOCl treatment with that of [N.sub.2] at 77 K. In contrast to [N.sub.2], diffusion of C[O.sub.2] into micropores is little affected by OM and the difference in both micropore volumes (MIV) is taken as a measure of micropore clogging and thus of the association of stable OM with mesopores. The MIV measured by C[O.sub.2] adsorption was corrected for C[O.sub.2] sorption in OM ([MIV-C[O.sub.2].sup.corr]). In 7 out of 12 samples, the [MIV-C[O.sub.2].sup.corr] equaled the MIV-[N.sub.2], suggesting that micropore entrances are not blocked by stable OM. For four samples the results were ambiguous, whereas in an Eutric Hapludand Bw horizon, a threefold larger [MIV-C[O.sub.2].sup.corr] compared with the MIV-[N.sub.2] indicates that stable OM was associated with pores 〈 10 nm. Based on the findings that (i) mineral MIVs derived from C[O.sub.2] and [N.sub.2] adsorption were similar for most samples after exposure to NaOCl, (ii) the change of MIV-[N.sub.2] on NaOCl treatment was small in all samples, and (iii) no relationship existed of the [MIV-C[O.sub.2].sup.corr] and small mesopore volume with the content of mineral-associated stable organic C (OC), we conclude that the association of OM with micropore entrances in small mesopores does not primarily control the stabilization of OM in these acid subsoils.
    Keywords: Humus -- Research ; Sodium Hypochlorite -- Research ; Forests -- Research ; Forests -- Environmental Aspects
    ISSN: 0361-5995
    E-ISSN: 14350661
    DOI: 10.2136/sssaj2005.0366N
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  • 8
    Article
    Article
    Synthetic coprecipitates of exopolysaccharides and ferrihydrite. Part I: Characterization (2008)
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    Language: English
    In: Geochimica et Cosmochimica Acta, 2008, Vol.72(4), pp.1111-1127
    Description: Iron(III) (hydr)oxides formed at extracellular biosurfaces or in the presence of exopolymeric substances of microbes and plants may significantly differ in their structural and physical properties from their inorganic counterparts. We synthesized ferrihydrite (Fh) in solutions containing acid polysaccharides [polygalacturonic acid (PGA), alginate, xanthan] and compared its properties with that of an abiotic reference by means of X-ray diffraction, transmission electron microscopy, gas adsorption (N , CO ), X-ray absorption spectroscopy, Fe Mössbauer spectroscopy, and electrophoretic mobility measurements. The coprecipitates formed contained up to 37 wt% polymer. Two-line Fh was the dominant mineral phase in all precipitates. The efficacy of polymers to precipitate Fh at neutral pH was higher for polymers with more carboxyl C (PGA ∼ alginate 〉 xanthan). Pure Fh had a specific surface area of 300 m /g; coprecipitation of Fh with polymers reduced the detectable mineral surface area by up to 87%. Likewise, mineral micro- (〈2 nm) and mesoporosity (2–10 nm) decreased by up to 85% with respect to pure Fh, indicative of a strong aggregation of Fh particles by polymers in freeze-dried state. C-1s STXM images showed the embedding of Fh particles in polymer matrices on the micrometer scale. Iron EXAFS spectroscopy revealed no significant changes in the local coordination of Fe(III) between pure Fh and Fh contained in PGA coprecipitates. Fe Mössbauer spectra of coprecipitates confirmed Fh as dominant mineral phase with a slightly reduced particle size and crystallinity of coprecipitate–Fh compared to pure Fh and/or a limited magnetic super-exchange between Fh particles in the coprecipitates due to magnetic dilution by the polysaccharides. The pH of pure Fh in 0.01 M NaClO was 7.1. In contrast, coprecipitates of PGA and alginate had a pH 〈 2. Considering the differences in specific surface area, porosity, and net charge between the coprecipitates and pure Fh, composites of exopolysaccharides and Fe(III) (hydr)oxides are expected to differ in their geochemical reactivity from pure Fe(III) (hydr)oxides, even if the minerals have a similar crystallinity.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
    DOI: 10.1016/j.gca.2007.11.035
    Source: ScienceDirect Journals (Elsevier)
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