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Opis: Selenium (Se) supplied in inorganic form (as selenate or selenite) was shown to decrease mercury (Hg) toxicity by forming HgSe in soils as well as in animal and human tissues, while for plants there is no evidence of Hg-Se complexation. Although Se in not an essential element for plants it was shown to counteract various abiotic stresses when applied at trace amounts. The aim of this work was therefore to study physiological responses and Hg speciation in plant/ fungi-animal food chain. Lettuce (Lactuca sativa) and porcini mushrooms (Boletus edulis) were taken as model plant/ fungal species and Spanish slug (Arion vulgaris) as a model animal species. The plants, fed to the slugs, were grown in HgCl2 contaminated soil or soil from the vicinity of Hg mine in Idrija with traces of HgS and methyl Hg). Physiological parameters of plants and slugs were monitored during the experiment. At the end the biological material was frozen in LN2 and freeze dried. Hg L3-edge (12284 eV) XANES and EXAFS spectra of the biological samples and standards were measured at liquid helium temperature in fluorescence detection mode at the BM30B beamline of the ESRF synchrotron in Grenoble, using the 30-segment germanium solid state detector [1]. The results showed that addition of Se alleviated Hg toxic effects in the food chain started at HgCl2-contaminated soil, while for the soil from Idrija, containing low amounts of highly toxic methyl-Hg, the beneficial effect was less prominent [2]. No Hg-Se complexes were detected in plants, while in mushrooms and slugs the complexation was confirmed. Addition of Se to the plants, however, changed Hg ligand environment in plant tissues from sulphur to nitrogen ligands. Hg and Se both target the -SH functional groups in the plant tissues, so toxic effects of Hg are rather enhanced than alleviated by addition of Se. Nevertheless, the addition of Se to the plants is beneficial for higher trophic levels and lowers Hg toxicity for the primary consumers, the slugs.
Ključne besede: mercury, toxicity, ligand environment, XANES, EXAFS, food chain, plant, slug, fungi
Objavljeno v RUNG: 05.12.2022; Ogledov: 2256; Prenosov: 0
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Opis: An absorption cell is constructed for x-ray absorption spectroscopy of reactive, unstable or hazardous gases at room temperature. In conjunction with in-situ micro-synthesis technique relying on handling the gas in syringes it enabled a first measurement of x-ray absorption spectra in the region of K and L edges for the series of hydrides of 5p elements (SnH4, SbH3, TeH2, HI). The signal-to-noise ratio above 103 was achieved, whereby fine detail is discerned in the spectra, in particular the small sharp features above each absorption edge, testifying of coexcitations of outer electrons in the core photoeffect.
Ključne besede: X-ray absorption spectroscopy Micro-synthesis absorption cell Gaseous hydrides Multielectron photoexcitations
Objavljeno v RUNG: 10.02.2020; Ogledov: 4474; Prenosov: 0
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Opis: Photoabsorption spectra of gaseous hydrides of 3p (PH3, H2S, HCl) and 4p elements (GeH4, AsH3 , H2Se, HBr) are measured in the energy region within 50 eV above the K edge, to study coexcitations of valence electrons by photoeffect in the K shell. The analysis of the valence coexcitations is extended to Ar, Kr, and SiH4. Relative probabilities and energies of states in the individual coexcitation channels are recovered by modeling the spectral features with a minimal ansatz based on the features in the contiguous noble gas. The extracted parameters are compared to the results of theoretical calculations for molecules (ORCA code) and free atoms (Hartree-Fock code). The experimental results confirm that the valence coexcitations in the 3p and 4p hydride molecules can be satisfactorily described by a two-step process, with the shake of the outer electron following the excitation of the core electron. The total probability—relative to the K-edge jump—of the shake-up processes shows a steady decrease from 19% in Si to 14% in Cl, and from 15% in Ge to 12% in Br. The experimental values for Ar (12%) and Kr (10%) are in accord with the trend. The dominant contribution is the transition to quasiatomic orbitals, in contrast with the deeper coexcitation channels in hydride molecules where transition to molecular orbitals prevails.
Ključne besede: X-ray absorption spectra, gaseous hydrides, 3p K-edge spectra, DFT
Objavljeno v RUNG: 05.09.2019; Ogledov: 3702; Prenosov: 0
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Opis: The fine detail in the x-ray absorption spectra in the energy region of absorption edges provides the insight into the mechanism of inner-shell photoexcitation: in particular in spectra of free atoms or simple molecules, the simplest being gaseous hydrides [1-2]. Measured K edge absorption spectra of hydrides of 3p (PH3, H2S in HCl) and 4p (GeH4, AsH3, H2Se, HBr) elements, and published data of 2p hydrides (CH4, NH3, H2O, HF) as well as SiH4 [3-6] and the noble gases at the end of the isoelectronic series (Ne, Ar, Kr) are compared to the respective calculated spectra, obtained by atomic HF86, GRASP codes [7] and molecular DFT (Density functional theory) ORCA code [8]. For a clearer view of intraatomic processes, the weak and simple structural (XAFS) signal of the molecule is removed from the spectra. Among the spectral features below the continuum limit, those with the lowest energy belong to the transition of the core electron to the lowermost free orbitals with the molecular character. They are, as a rule, wider than the transitions to the higher orbitals with prevailing atomic character. The theoretical description with DFT code without specific adaptations is sufficient for a qualitative picture of the pre-edge structure. The fine structure immediately above the K edge stems from the coexcitation of valence electrons. We have proved that the coexcitations can be explained as a two-step process: the inner-shell photoeffect followed by the shake-up of a valence electron predominantly to a free atomic orbital. This process is markedly different from coexcitations of more tightly bound electrons [9]. In the collection of consecutive and homologous data, analyzed by a common procedure, the reaction channels can be identified with better precision and reliability than in analysis of individual spectra. Our analysis showed that the energies and probabilities of single-electron transitions into the molecular orbitals are strongly affected by the symmetry of the molecule, essentially in the same way in 3p and 4p homologues, but not in 2p homologues with a stronger influence of the core charge. In transitions to atomic orbitals the influence of the molecular field is negligible.
Ključne besede: hidridi, rentgenska spektroskopija, XAFS
Objavljeno v RUNG: 12.09.2018; Ogledov: 4791; Prenosov: 0
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Ključne besede: nickel, hyperaccumulator plants
Objavljeno v RUNG: 12.09.2018; Ogledov: 3875; Prenosov: 0
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