1,303 research outputs found
Modelling Cu(II) adsorption to ferrihydrite and ferrihydrite–bacteria composites: Deviation from additive adsorption in the composite sorption system
No full textBacterially associated iron (hydr)oxides are widespread in natural environments and are potent scavengers of dissolved metal ions. However, it is unclear whether metal sorption on these composites adheres to the additivity principle, and thus whether metal concentrations in environments where these composites comprise a significant proportion of the reactive iron phases can be modelled assuming component additivity. Here we address this issue for Cu adsorption on ferrihydrite–Bacillus subtilis composites. We precipitated pure ferrihydrite and ferrihydrite composites with different ferrihydrite:bacteria mass ratios, and measured Cu adsorption as a function of pH, Cu adsorbed concentration and composite mass ratio. We develop a molecular-level surface complexation model for Cu adsorption on pure ferrihydrite. We then combine our end-member models for Cu adsorption on B. subtilis (Moon and Peacock, 2011) and ferrihydrite to model the observed Cu adsorption on the composites, adopting a component linear additivity approach. By comparing observed Cu adsorption to that predicted by our composite model, constrained to the exact best fitting end-member stability constants, we find that Cu adsorption behaviour on ferrihydrite–B. subtilis composites deviates from additivity. Specifically, Cu adsorption on composites composed mainly of ferrihydrite is enhanced across the adsorption pH edge (pH ?3–6), while on our composite composed mainly of bacteria adsorption is enhanced at mid-high pH (pH ?5–6) but diminished at mid-low pH (pH ?5–3), compared to additivity. In current surface complexation modelling constructs, Cu adsorption on composites composed mainly of ferrihydrite can be modelled in a component additivity approach, by optimising the stability constants for Cu adsorption on the ferrihydrite and bacteria fractions to values that are within the uncertainty on the end-member stability constant values. The deviation from additivity of these composites, apparent when using the exact best fitting end-member stability constants, is therefore either a modelling artefact due to uncertainties in the surface adsorption properties of the end-member phases, or is relatively minor and cannot be separated from these uncertainties that are inherently present in the parameterisation of the surface adsorption properties in the modelling. In contrast, composites composed mainly of bacteria express significant deviation from Cu adsorption additivity that cannot be modelled in a component additivity approach. We propose that the deviations in Cu adsorption from additivity are the result of physiochemical interactions between the composite fractions that change the surface charge of the ferrihydrite and B. subtilis fractions compared to the isolated end-member phases. The magnitude and direction of the additivity deviations due to these electrostatic effects are then a function of the affinity of Cu for the bacterial fraction and the mass fraction of bacteria in the composite
Adsorption of Cu(II) to ferrihydrite and ferrihydrite–bacteria composites: Importance of the carboxyl group for Cu mobility in natural environments
No full textBacterially associated iron (hydr)oxide composites are widespread in natural environments, and by analogy with isolated iron (hydr)oxides and bacteria, are important scavengers of dissolved trace-metals. We precipitated ferrihydrite via rapid Fe(III) hydrolysis in the absence and presence of the non-Fe metabolising, Gram-positive bacterium Bacillus subtilis, commonly found in natural waters, soils and sediments. We combined XRD, SEM, BET and Fe K-edge EXAFS to examine the mineralogy, morphology and crystallinity of the ferrihydrite composites. We find that the mineral fraction of the composites is unaltered in primary mineralogy, morphology and crystallinity compared to pure ferrihydrite. We then measured the adsorption of Cu to ferrihydrite and the ferrihydrite–B. subtilis composites as a function of pH and the ferrihydrite:bacteria mass ratio of the composites, and used EXAFS to determine the molecular mechanisms of Cu adsorption. We determine directly for the first time that Cu uptake by ferrihydrite–B. subtilis composites is the result of adsorption to both the ferrihydrite and B. subtilis fractions. Adsorption of Cu by the B. subtilis fraction results in significant Cu uptake in the low pH regime (pH ?4, ?20% of [Cu]total) and significantly enhanced Cu uptake in the mid pH regime. This composite sorption behaviour is in stark contrast to pure ferrihydrite, where Cu adsorption is negligible at low pH. Overall, for composites dominated by either ferrihydrite or B. subtilis, the bacterial fraction is exclusively responsible for Cu adsorption at low pH while the ferrihydrite fraction is predominantly responsible for adsorption at high pH. Furthermore, with an increased mass ratio of bacteria, the dominance of Cu adsorption to the bacterial fraction persists into the mid pH regime and extends significantly into the upper pH region. As such, the distribution of the total adsorbed Cu between the composite fractions is a function of both pH and the ferrihydrite:bacteria mass ratio of the composite. EXAFS shows that Cu adsorbs to ferrihydrite as an inner-sphere, (CuO4Hn)n ? 6 bidentate edge-sharing complex; and to ferrihydrite composites as an inner-sphere, (CuO5Hn)n ? 8 monodentate complex with carboxyl surface functional groups present on the bacterial fraction plus the bidentate edge-sharing complex on the ferrihydrite fraction. Our new results combined with previous work on Cu sorption to bacteria, humic substances and iron (hydr)oxides coated with humics, demonstrate the universal importance of the carboxyl moiety for Cu sorption and mobility in natural environments. Taken together these results show that Cu-carboxyl binding is the predominant mechanism by which Cu interacts with abiotic and biotic organic matter, and provides a ubiquitous control on Cu fate and mobility in natural waters, soils and sediments. Our results indicate that in environments where a significant proportion of iron (hydr)oxides are intimately intermixed with an organic fraction, we must consider Cu sequestration by these composites in addition to pure mineral phases
Oxidative scavenging of thallium by birnessite: Explanation for thallium enrichment and stable isotope fractionation in marine ferromanganese precipitates
No full textTl stable isotopes recorded in marine ferromanganese crusts show great promise as a tracer of past marine and climatic conditions. Key to interpreting recent Tl stable isotope time-series data is a detailed, molecular-level understanding of Tl scavenging by ferromanganese crust minerals and Tl stable isotope fractionation occurring during uptake. To this end, we determine the mechanism of Tl sorption to the primary ferromanganese minerals in crusts, namely hexagonal birnessite, todorokite and ferrihydrite, using XAS. We compliment our data with micro-focus XAS of a Tl-enriched hydrogenetic ferromanganese crust. We show that Tl(I) is oxidised to Tl(III) during sorption to hexagonal birnessite, but not during sorption to todorokite, triclinic birnessite and ferrihydrite. Tl(III) forms an inner-sphere complex at the hexagonal birnessite surface, located at vacant octahedral sites in the phyllomanganate sheets. We show that oxidation of Tl(I) to Tl(III) during reductive dissolution of birnessite is thermodynamically unfavourable; and propose that oxidation of Tl(I) is driven by the formation of the Tl(III) surface complex. Recent theoretical calculations predict a large equilibrium stable isotope fractionation between Tl(I) and Tl(III), leading to Tl(III) species that are enriched in the heavy 205Tl isotope. In light of this work, we propose a molecular sorption–oxidation–fractionation mechanism that provides a unifying explanation for the recently observed geochemical behaviour of Tl in marine ferromanganese-rich sediments. In this mechanism, the proportion of hexagonal birnessite dictates the extent of Tl oxidation, which controls the extent of Tl enrichment and isotope fractionation. This work is among the first to provide a molecular explanation for reported trends in trace element enrichments and stable isotope compositions in geologic deposits. Our molecular sorption–oxidation–fractionation mechanism will ultimately help interpret Tl signals in marine sedimentary archives to provide new constraints on past oceanic and climatic change. In addition, our mechanism should also help explain compositional relationships of other redox-sensitive elements in ferromanganese-rich marine sediments that might also be used as paleoceanographic and paleoclimate proxies
Good governance and e-government
No full text노트 : Please send an email to the following address to contact the author M. Jae Moon : [email protected]
The on-nara system for task and document management
No full text노트 : Please send an email to the following address to contact the author M. Jae Moon : [email protected]
Towards an understanding of thallium isotope fractionation during adsorption to manganese oxides
No full textWe have conducted the first study of Tl isotope fractionation during sorption of aqueous Tl(I) onto the manganese oxide hexagonal birnessite. The experiments had different initial Tl concentrations, amounts of birnessite, experimental durations, and temperatures, but all of them exhibit heavy Tl isotope compositions for the sorbed Tl compared with the solution, which is consistent with the direction of isotope fractionation observed between seawater and natural ferromanganese sediments. However, the magnitude of fractionation in all experiments (? ? 1.0002–1.0015, where ?=205Tl/203Tlsolid/205Tl/203Tlliq is smaller than observed between seawater and natural sediments (? ? 1.0019–1.0021; Rehkämper et al., 2002, Earth. Planet. Sci. Lett. 197, 65–81). The experimental results display a strong correlation between the concentration of Tl in the resulting Tl-sorbed birnessite and the magnitude of fractionation. This correlation is best explained by sorption of Tl to two sites on birnessite, one with large isotope fractionation and one with little or no isotope fractionation. Previous work (Peacock and Moon, 2012, Geochim. Cosmochim. Acta 84, 297–313) indicates that Tl in natural ferromanganese sediments is oxidized to Tl(III) and adsorbed over Mn vacancy sites in the phyllomanganate sheets of birnessite, and we hypothesize that this site is strongly fractionated from Tl in solution due to the change in oxidation state from aqueous Tl(I). In most experiments, which have orders of magnitude more Tl associated with the solid than in nature, these vacancy sites are probably fully saturated, so various amounts of additional Tl are likely sorbed to either edge sites on the birnessite or triclinic birnessite formed through oxidative ripening of the hexagonal starting material, with unknown oxidation state and little or no isotopic fractionation. Thus each experiment displays isotopic fractionation governed by the proportions of Tl in the fractionated and slightly fractionated sites, and those proportions are controlled by how much total Tl is sorbed per unit of birnessite. In the experiments with the lowest initial Tl concentrations in solution (?0.15–0.4 ?g/g) and the lowest concentrations of Tl in the resulting Tl-sorbed birnessite (?17 ?g Tl/mg birnessite), we observed the largest isotopic fractionations, and fractionation is inversely proportional to the initial aqueous Tl concentration. Again, this correlation can be explained by the simultaneous occupation of two different sorption sites; vacancy sites that carry isotopically fractionated Tl and a second site carrying slightly fractionated Tl. The fractionation factors observed in nature exceed those in the experiments likely because the Tl concentrations in seawater and in ferromanganese sediments are three to four orders of magnitude lower than in our experiments, and therefore the second slightly fractionated sorption site is not significantly utilized. Temperature (6–40 °C) and experimental duration (3 min–72 h) appear to have little or no effects on isotope behaviour in this system
Evolution of Korea's E-government
No full text노트 : Please send an email to the following address to contact the author M. Jae Moon : [email protected]
Governance in the information age
No full text노트 : Please send an email to the following address to contact the author M. Jae Moon : [email protected]
Impact evaluation and new approaches to governance
No full text노트 : Please send an email to the following address to contact the author M. Jae Moon : [email protected]
Government reforms and national competitiveness
No full text노트 : Please send an email to the following address to contact the author M. Jae Moon : [email protected]
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