Mobilität von CeO2-Nanomaterialien in Böden

Der zunehmende Einsatz von Cerdioxid-Nanomaterialien (CeO2 NM) führt zu ihrer Freisetzung in die aquatische und terrestrische Umwelt. Aufgrund der ungeklärten human- und umwelttoxischen Wirkung von CeO2 NM muss deren Verbleib in der Umwelt untersucht werden. Es wurden zwei verschiedene CeO2 NM (OECD-Standard CeO2 NM-212, Europium-gedopte CeO2 NM (5 % Eu)) zu elf luftgetrockneten Ackerböden pulverförmig zugegeben und mit künstlichem Regenwasser dispergiert (Batchversuche), um Nachweismethoden für Bodenfestphase und Bodenlösung zu entwickeln. Darüber hinaus sollen diese Batchversuche erste Informationen über die Mobilität der NM in Böden liefern. Zusätzlich wurden Säulenversuche mit einer schluffigen Parabraunerde durchgeführt, um die Wiederfindung und mögliche Remobilisierung des Eu-gedopten CeO2 NM in Böden bzw. Bodenlösungen mit hohen Ce-Hintergrundgehalten zu untersuchen. Nach der pulverförmigen Applikation des gedopten NM wurde die Parabraunerde jeweils für 3 und 92 Tage inkubiert, der Wassergehalt wurde wöchentlich auf 55 % der maximalen Wasserhaltekapazität eingestellt. Anschließend erfolgte die Elution unter gesättigten Bedingungen mit künstlichem Regenwasser unter Austausch von maximal zwölf Porenvolumina. Die Bodenlösungen der Batchversuche und die Säuleneluate wurden nach der Fraktionierung (Filtration, Zentrifugation und Ultrazentrifugation) mit HNO3 und H2O2 aufgeschlossen. Die Messung der Ce- und Eu-Gehalte erfolgte mit ICP-OES und ICP-MS. Durch das Eu-Doping wird weder das Kristallgitter noch die Größe der Primärpartikel verändert. Die Bestimmungsgrenzen der applizierten NM in Böden und Bodenlösungen werden im Vergleich zum ungedopten Material, insbesondere bei hohen Ce-Hintergrundgehalten, deutlich erniedrigt. Dadurch können umweltrelevante Konzentrationen des NM getestet werden. Batchversuche sind für die Untersuchung der Mobilität von NM stark limitiert, dennoch deutet die variierende Freisetzung der NM aus den verschiedenen Bodenproben darauf hin, dass die unterschiedlichen Bodencharakteristika die NM Mobilität steuern. Die CeO2 NM Freisetzung aus den Säulenversuchen lag auf einem sehr niedrigen Niveau, war nach der Langzeitinkubation jedoch minimal erhöht. Aus den Ergebnissen lässt sich folgern, dass Böden wahrscheinlich eine deutliche Senke für die getesteten NM darstellen

Bioverfügbare Fraktionen von Arsenat, Kupfer und Nickel: Ergebnisse chemischer Extraktionsmethoden

In der Bundes-Bodenschutzverordnung sind für ausgewählte Metalle Vorsorgewerte festgeschrieben, bei deren Überschreiten „die Besorgnis einer schädlichen Bodenveränderung besteht“. Für deren Ableitung wurden die jeweiligen Gesamtgehalte im Boden (Königswasser-Aufschluss) verwendet. Allerdings ist für den Wirkungspfad Boden – Organismen der Zusammenhang zwischen Gesamtgehalten und einem potenziellen Effekt auf Mikroorganismen, Pflanzen und Invertebraten gering. Eine realistische Bewertung der Metallgehalte sollte deshalb die Bioverfügbarkeit von Metallen im Boden berücksichtigen. Um die geignetste Metallfraktion zur Bewertung der Bioverfügbarkeit von Metallen zu identifizieren, werden wir in unserem Projekt Ergebnisse verschiedener chemischer Extraktionsverfahren mit den Daten von mehreren ökotoxikologischen Tests auf den chronischen Effekt auf verschiedene Organismen miteinander korrelieren. In diesem Beitrag werden die Ergebnisse der chemischen Extraktionen präsentiert. Für die Untersuchungen wurden elf Böden ausgesucht, die einen großen Bereich der Eigenschaften von mitteleuropäischen Böden abdecken (z. B. 0.9‑23.3 % organischer Kohlenstoff; 5-36 % Ton). Teilproben wurden mit acht Konzentration der kationischen Metalle Kupfer und Nickel sowie dem Oxyanion Arsenat versetzt. Nach einer Woche Inkubationszeit wurden die Extraktionen durchgeführt. Diese umfassen neben dem Königswasser-Aufschluss, Extraktionen mit unterschiedlichen Salzlösungen (Ca(NO3)2 angepasst an die Ionenstärke des Bodens, 0.01 M CaCl2, and 1 M NH4NO3), mit Komplexbildner (DTPA/CaCl2) und mit schwach konzentrierter Säure (043 M HNO3). Die Extraktionsstärke der verschiedenen Methoden folgt bei den meisten Böden und Metallen der Reihenfolge HNO3 > DTPA > NH4NO3 > CaCl2. Die extrahierte Menge an Metall variert zwischen den Böden und hängt von deren Eigenschaften ab. Diese Einflussfaktoren werden weiter untersucht. Die Daten sollen dann zusammen mit den ökotoxikologischen Untersuchungen genutzt werden, um die Bioverfügbarkeit der einzelnen Metallfraktionen zu bewerten und zukünftig bei der Ableitung von Vorsorgewerten berücksichtigen zu können

Cluster Dynamical Mean-field calculations for TiOCl

Based on a combination of cluster dynamical mean field theory (DMFT) and density functional calculations, we calculated the angle-integrated spectral density in the layered $s=1/2$ quantum magnet TiOCl. The agreement with recent photoemission and oxygen K-edge X-ray absorption spectroscopy experiments is found to be good. Th e improvement achieved with this calculation with respect to previous single-site DMFT calculations is an indication of the correlated nature and low-dimensionality of TiOCl.Comment: 9 pages, 3 figures, improved version as publishe

Collective orbital excitations in orbitally ordered YVO3 and HoVO3

We study orbital excitations in the optical absorption spectra of YVO3 and HoVO3. We focus on an orbital absorption band observed at 0.4 eV for polarization E parallel c. This feature is only observed in the intermediate, monoclinic phase. By comparison with the local crystal-field excitations in VOCl and with recent theoretical predictions for the crystal-field levels we show that this absorption band cannot be interpreted in terms of a local crystal-field excitation. We discuss a microscopic model which attributes this absorption band to the exchange of two orbitals on adjacent sites, i.e., to the direct excitation of two orbitons. This model is strongly supported by the observed dependence on polarization and temperature. Moreover, the calculated spectral weight is in good agreement with the experimental result.Comment: 12 pages, 9 figure

How chemistry controls electron localization in 3d1 perovskites: A Wannier-function study

In the series of 3d1 t2g perovskites, SrVO3--CaVO3--LaTiO3--YTiO3 the transition-metal d electron becomes increasingly localized and undergoes a Mott transition between CaVO3 and LaTiO3. By defining a low-energy Hubbard Hamiltonian in the basis of Wannier functions for the t2g LDA band and solving it in the single-site DMFT approximation, it was recently shown[1] that simultaneously with the Mott transition there occurs a strong suppression of orbital fluctuations due to splitting of the t2g levels. The present paper reviews and expands this work, in particular in the direction of exposing the underlying chemical mechanisms by means of ab initio LDA Wannier functions generated with the NMTO method. The Wannier functions for the t2g band exhibit covalency between the transition-metal t2g, the large cation-d, and the oxygen-p states; this covalency, which increases along the series, turns out to be responsible not only for the splittings of the t2g levels, but also for non-cubic perturbations of the hopping integrals, both of which are decisive for the Mott transition. We find good agreement with the optical and photoemission spectra, with the crystal-field splittings and orbital polarizations recently measured for the titanates, and with the metallization volume for LaTiO3. The metallization volume for YTiO3 is predicted. Using super-exchange theory, we reproduce the observed magnetic orders in LaTiO3 and YTiO3, but the results are sensitive to detail, in particular for YTiO3 which, without the Jahn-Teller distortion, would be AFM C- or A-type, rather than FM. Finally, we show that it possible to unfold the orthorhombic t2g LDA bandstructure to a pseudocubic zone. In this zone, the lowest band is separated from the two others by a direct gap and has a width, W_I, which is significantly smaller than that, W, of the entire t2g band. The progressive GdFeO3-type distortion favours electron localization by decreasing W, by increasing the splitting of the t2g levels and by decreasing W_I. Our conclusions concerning the roles of GdFeO3-type and JT distortions agree with those of Mochizuki and Imada [2].Comment: Published version, final. For high resolution figures see http://www.fkf.mpg.de/andersen/docs/pub/abstract2004+/pavarini_02.pd

Study of the pressure effects in TiOCl by ab initio calculations

Electronic structure calculations on the low dimensional spin-1/2 compound TiOCl were performed at several pressures in the orthorhombic phase, finding that the structure is quasi-one-dimensional. The Ti3+ (d1) ions have one t2g orbital occupied (dyz) with a large hopping integral along the b direction of the crystal. The most important magnetic coupling is Ti-Ti along the b axis. The transition temperature (Tc) has a linear evolution with pressure, and at about 10 GPa this Tc is close to room temperature, leading to a room temperature spin-Peierls insulator-insulator transition, with an important reduction of the charge gap in agreement with the experiment. On the high-pressure monoclinic phase, TiOCl presents two possible dimerized structures, with a long or short dimerization. Long dimerized state occurs above 15 GPa, and below this pressure the short dimerized structure is the more stable phase.Comment: 3 pages, 3 embedded figures, 1 table. A. Pi\~neiro, et al.,J. Magn. Magn. Mater. (2009) (accepted

Optical study of orbital excitations in transition-metal oxides

The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yield good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g., the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular in case of the orbital excitations at about 0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.Comment: published version, discussion of TiOCl extended to low T, improved calculation of orbital excitation energies in TiOCl, figure 16 improved, references updated, 33 pages, 20 figure

Future Sea Level Change Under Coupled Model Intercomparison Project Phase 5 and Phase 6 Scenarios From the Greenland and Antarctic Ice Sheets

Projections of the sea level contribution from the Greenland and Antarctic ice sheets (GrIS and AIS) rely on atmospheric and oceanic drivers obtained from climate models. The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared with the previous Coupled Model Intercomparison Project phase 5 (CMIP5) effort. Here we use four CMIP6 models and a selection of CMIP5 models to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the ice sheet model ensemble under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for Greenland. Warmer atmosphere in CMIP6 models results in higher Greenland mass loss due to surface melt. For Antarctica, CMIP6 forcing is similar to CMIP5 and mass gain from increased snowfall counteracts increased loss due to ocean warming

Benchmark experiments for higher-order and full Stokes ice sheet models (ISMIP-HOM)

International audienceWe present the results of the first ice sheet model intercomparison project for higher-order and full Stokes ice sheet models. These models are validated in a series of six benchmark experiments of which one has an analytical solution under simplifying assumptions. Five of the tests are diagnostic and one experiment is prognostic or time dependent, for both 2-D and 3-D geometries. The results show a good convergence of the different models even for high aspect ratios. A clear distinction can be made between higher-order models and those that solve the full system of equations. The latter show a significantly better agreement with each other as well as with analytical solutions, which demonstrates that they are hardly influenced by the used numerics

Carbon balance under a changing light environment

The natural environment of Antarctic seaweeds is characterized by changing seasonal light conditions. The ability to adapt to this light regime is one of the most important prerequisites for their ecological success. Thus, the persistence of seaweeds depends on their capacity to maintain a positive carbon balance (CB)for buildup of biomass over the course of the year. A positive CB in Antarctica occurs only during the ice-free period in spring and summer, when photosynthetically active radiation (PAR, 400–700 nm) penetrates deeply into the water column. The accumulated carbon compounds during this period are stored and remobilized to support metabolism for the rest of the year. Over the last decades climate warming has induced a severe glacial retreat in Antarctica and has opened newly ice-free areas. Increased sediment runoff, and reduced light penetration due to melting during the warmer months, may lead to a negative CB with changes in the vertical distribution of seaweeds. Furthermore, warmer winters and springs result in earlier sea-ice melt, causing an abrupt increase in light, compensating the reduction in PAR in summer or increasing the annual light budget. Studies performed in Potter Cove, Isla 25 de Mayo/King George Island, reveal that algae growing in newly ice-free areas did not acclimate to the changing light conditions. Lower or even negative CB values in areas close to the glacier runoff seem to be primarily dependent on the incoming PAR that finally determines the lower distribution limit of seaweeds. The present chapter discusses how carbon balance respond to the changing Antarctic light environment and its potential implications for the fate of benthic algal communities