[Show abstract][Hide abstract] ABSTRACT: Lead (Pb) and other heavy metals represent a great source of concern in agriculture, because
they may disperse from polluted sources and accumulate in crop organs. This research study
was performed with three edible crops and one pasture species (lettuce: Lactuca sativa L. cv.
Romana; radish: Raphanus sativus L. var. radicicola; tomato: Lycopersicon lycopersicum L.
Karst.; Italian ryegrass: Lolium multiflorum Lam). It was aimed at (1) assessing how species affect
Pb distribution among plant organs, (2) determining the extent to which Pb is localized in edible
organs, and (3) ascertaining whether it could be possible to distinguish which compounds
are responsible for the transport of Pb from one plant organ to another and which compounds
are responsible for the accumulation of this metal inside each plant organ. The experiment was
conducted in the greenhouse. Plants were grown in plastic pots using a Pb-spiked sandy soil as
substrate. Total Pb concentrations in different plant organs and in soil were determined. Within
plants, the maximum accumulation of Pb was found in roots while the remaining part of Pb was
mainly located in leaves. Pb LIII edge XANES (X-ray Absorption Near Edge Spectroscopy) was
applied to identify the principal Pb carrier molecules in the different plant organs. The data suggest
that in roots Pb immobilization is mainly due to the complexing ability of histidine, which
binds the metal and, to a lesser extent, to precipitation of Pb as carbonate. The transport to the
upper plant organs is mainly attributed to Pb complexes with organic acids. In stems and leaves,
Pb bonding is mainly carboxylic and amino acid-like, thus confirming the role of these substances
in promoting Pb mobility. Thio amino acidic (glutathione and cysteine-like) Pb complexes,
which in this study were only found in stems, can also be held responsible for Pb long-distance
transport from roots to shoots.
Full-text · Article · Oct 2014 · Journal of Plant Nutrition and Soil Science
[Show abstract][Hide abstract] ABSTRACT: The Pb absorption processes on a heavy textured calcareous soil, typical of central Italy, were studied using synchrotron X-ray absorption spectroscopy (XAS) in order to probe, at molecular scale, the structure and chemical nature of Pb in contaminated soils and achieve precise description of Pb ions localization into these contaminated soils.
In order to distinguish the role of the different components of soils in Pb retention, samples were prepared from the original soils removing the carbonate fractions, the organic matter, the metal oxides, or selecting the clay fractions. Then these samples were fortified with Pb simulating the natural interactions processes of heavy metal solutions with soils. The quantitative analysis of near edge (XANES) as well extended (EXAFS) regions of Pb L(III) edge absorption spectra, in comparison with Pb XAS data of selected reference compounds, allowed the precise determination of local structure and chemical environment of Pb ions in these soil samples.
Four components were individuated as the major responsible of Pb retention in calcareous soils: the carbonates, the metal oxide surfaces, the organic matter, and the colloidal inorganic surfaces containing clay components. The structural analysis suggests that, within these experimental conditions, the Pb adsorbed on the soil is generally present as Pb hydroxide with poor crystallization degree. However, the presence of carbonates (CaCO3) induces the co-precipitation of PbCO3-like phases with some degree of crystallinity.
Full-text · Article · Nov 2011 · Environmental Science and Pollution Research
[Show abstract][Hide abstract] ABSTRACT: We present a computational approach for the simulation of extended x-ray absorption fine structure (EXAFS) spectra of nanoparticles directly from molecular dynamics simulations without fitting any of the structural parameters of the nanoparticle to experimental data. The calculation consists of two stages. First, a molecular dynamics simulation of the nanoparticle is performed and then the EXAFS spectrum is computed from ``snapshots'' of structures extracted from the simulation. A probability distribution function approach calculated directly from the molecular dynamics simulations is used to ensure a balanced sampling of photoabsorbing atoms and their surrounding scattering atoms while keeping the number of EXAFS calculations that need to be performed to a manageable level. The average spectrum from all configurations and photoabsorbing atoms is computed as an Au L3-edge EXAFS spectrum with the FEFF 8.4 package, which includes the self-consistent calculation of atomic potentials. We validate and apply this approach in simulations of EXAFS spectra of gold nanoparticles with sizes between 20 and 60 Å. We investigate the effect of size, structural anisotropy, and thermal motion on the gold nanoparticle EXAFS spectra and we find that our simulations closely reproduce the experimentally determined spectra.
[Show abstract][Hide abstract] ABSTRACT: Materials showing negative thermal expansion (NTE) coefficient over large temperature ranges are nowadays of great interest for their possible applications. Small nanoparticles show changes in their properties with respect to the corresponding bulk, mainly due to the high surface to volume ratio and to the confinement of electrons in a small volume. In the present paper we report a x-ray absorption fine structure (XAFS) study on the thermal expansion coefficient of an Au foil and of Au nanoparticles of very small dimensions ranging from 2.4 nm and 5.0 nm. Their L3 edge has been investigated in the temperature range 20K – 300K and a very accurate data analysis has been performed taking into account the presence of asymmetry effects. All clusters showed a thermal trend of the first shell distance significantly different from that of the bulk. The larger clusters were characterized by a reduction of the thermal expansion coefficient with respect to bulk; in the smallest samples the crossover from a thermal expansion to a NTE effect was observed. A simple model, based on the contribution of localized states induced by the finite size of the clusters, qualitatively accounts for the observed behaviour.
No preview · Article · Nov 2009 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: The structural parameters of the first five coordination shells of an Au bulk obtained from high accuracy
L3-edge extended x-ray absorption fine structure (EXAFS) spectra in the temperature range
20–300 K are reported. Good agreement with previously reported studies is found. The
effective second and third order force constants evaluated using EXAFS data
are compatible with those calculated from phonon dispersion curves. A careful
comparison of the variations of the EXAFS first shell distance with x-ray diffraction
data provided the mean squared relative displacement of the atomic vibrations
perpendicular to the first interatomic bond. An alternative new approach that is useful in
achieving this parameter when x-ray diffraction data are not available is proposed.
No preview · Article · Aug 2009 · Journal of Physics Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: The L3 edge of Au nanoparticles, having sizes ranging from 2.4 to 5.0 nm, have been investigated by x-ray absorption fine structure spectroscopy in the temperature range of 20–300 K. Data were recorded at the European Synchrotron Radiation Facility with a very good signal to noise ratio. To achieve a very high accuracy in the determination of the first shell distance, a very careful data analysis was performed also taking into account the presence of asymmetry effects. In all samples, the temperature dependence of the first neighbor distance results is different from that of the macrocrystalline counterpart. In the largest size samples, a reduction of the thermal expansion was found, whereas in the smallest ones, the presence of a crossover from an initial thermal expansion to a thermal contraction was observed. Calculations based on a simple model show that localization effects that increase as the nanoparticle size decreases can explain the reported thermal effects.
No preview · Article · Feb 2008 · Physical review. B, Condensed matter