S. J. Naftel

M3,2‐edge x‐ray absorption near‐edge structure spectroscopy: An alternative probe to the L3,2‐edge near‐edge structure for the unoccupied densities of d states of 5d metals

We report M3,2‐edge x‐ray absorption near‐edge structure (XANES) measurements for a series of 5d metals, Ta, W, Pt, and Au using a total electron yield technique. It is found that the M3,2 XANES absorption features and their systematics are comparable to their L3,2 counterparts. These results clearly indicate that M3,2‐edge XANES can be used as an alternative probe for the unoccupied densities of d states of the 5d metals and their compounds. The implication of these results is discussed.

Mineral Weathering in the Rhizosphere of Forested Soils

ABSTRACT The rhizosphere is a microenvironment enriched in organic matter and generally more acidic than the bulk soil. In this chapter, we submit that mineral weathering and metal fractionation differ in the rhizosphere compared to the bulk soil, a change that could impact on plant nutrition and element toxicity. The objective of the study is to establish the nature of the effect of roots on mineral weathering in the rhizosphere of forested soils based on differences in (1) mineralogical composition and (2) the chemical forms of metals between the rhizosphere and the bulk soil. The study area was located in Rouyn–Noranda (Canada), where samples were collected under Populus tremuloides growing on Luvisolic soils. X-ray diffraction (XRD), time of flight secondary-ion mass spectroscopy (TOF-SIMS) and X-ray absorption near-edge structure (XANES) analyses were performed. The concentrations of Al, Ca, Cd, Co, Cr, Cu, Fe, Li, Mg, Mn, Ni, Pb, Si and Zn were obtained from an acid ammonium oxalate (AAO) extraction. The XRD results show differences in mineralogical abundance, particularly of chlorite and amphibole, which is interpreted as an increase in mineral weathering in the rhizosphere. It was suggested in the literature that the higher alteration in the rhizosphere could be related to K uptake by roots. However, our results show greater BaCl2-extractable K in the rhizosphere, an observation in opposition to this nutrient-depletion hypothesis. The AAO extraction reveals higher contentrations of Fe and Mn in the rhizosphere. These data support XRD results and suggest the formation of secondary oxides in the rhizosphere through weathering. In turn, the greater abundance of oxides creates absorption sites for trace elements such as Cu and Zn as supported by the AAO extractions. The TOF-SIMS mapping also shows an accumulation of total metals at the soil–root interface. The XANES analysis of Mn further indicates that metals tend to be oxidized in the rhizosphere, whereas they are found in organic forms in the root or as a mixture of both at the soil–root interface. The presence of oxidized forms of Mn in the rhizosphere is in agreement with the results of the AAO extraction. In summary, weathering is shown to be higher in the rhizosphere, favors the formation of oxides, notably Mn oxides and, hence, the retention of trace metals.

Semiconductor Growth and Junction Formation within Nano-Porous Oxides

We report a systematic study of the chemical properties of porous silicon as a moderate reducing agent and a nano-structure template for the reductive formation of nano-metal cluster aggregates. This study investigates the consequences of the interaction of silver ions in aqueous solution with porous silicon. The nature of the redox reaction, especially the role of surface hydrogen and silicon atoms, is established through the study of the morphology, structure, and electronic and optical properties of Ag on porous silicon. The specimens were characterized with SEM and X-ray absorption fine structure (XAFS) spectroscopy using synchrotron radiation

Structure and electronic properties of SiO2/Si multilayer superlattices: Si K edge and L3,2 edge x-ray absorption fine structure study

We report an x-ray absorption fine structure study at the Si K and L3,2 edges of a series of Si/SiO2 superlattices (SL). The SL system comprises four periods of elemental silicon with a spacing of 1, 1.4, 2.2, and 2.6 nm sandwiched by a 1.5 nm silicon oxide and capped by a 3 nm silicon oxide layer. These systems exhibit electroluminescence and photoluminescence. X-ray absorption near edge structure (XANES) at both the Si K and L3,2 edge confirms that the Si layers are amorphous. Polarization dependent measurement at the Si K edge reveals that a distinct Si/SiO2 interface exists with strong Si–O bonding oriented preferentially closer to the surface normal. High resolution XANES at the Si L3,2 edge shows a noticeable blueshift of the edge threshold as the lattice spacing decreases, in good accord with quantum confinement. The results and their implications for the origin (quantum confinement and interface/oxide defects) of luminescence in these superlattice systems are discussed.

X-ray excited optical luminescence (XEOL) studies of CaF2 at the Ca L3,2-edge

We report a study of the optical response of CaF2 specimens (crystal and powder) with excitation photon energy tuned across the Ca L3,2-edge. The luminescence was in turn used to monitor the absorption across the Ca L-edge (optical XAFS). It is found that the photoluminescence yield produces an inverted spectrum. This observation is interpreted in terms of total absorption (thickness effect), the change in decay dynamics below and above the edge and the atomic origin of the luminescence. By comparing crystal and powder results, we find that the surface component of CaF2 exhibits a positive shift (∼0.15 eV) relative to the bulk at the p–d resonance at the Ca L3,2-edge. A DFT calculation has been carried out and it is in good accord with the experiment. The implications of these results are discussed.

Micro-synchrotron x-ray fluorescence of cadmium-challenged corn roots

Abstract Micro-synchrotron X-ray fluorescence was carried out to examine the metal distribution in cadmium-challenged corn ( Zea mays ) roots. The results suggest that cadmium alters calcium distribution in the root tissue.

Comparison of the distributions of bromine, lead, and zinc in tooth and bone from an ancient Peruvian burial site by X-ray fluorescence

Synchrotron micro X-ray fluorescence was used to study the distribution of selected trace elements (Zn, Pb, and Br) in tooth and bone samples obtained from an individual from a pre-Columbian archaeological site (Cabur) located on the north coast of Peru. The results show that Zn, Pb, and Br are present in both the teeth and bone samples and that the Zn and Pb seem to be confined to similar regions (cementum and periostium), while Br shows a novel distribution with enrichment close to the Haversian canals and (or) in regions that appear to be Ca deficient.Key words: teeth, bone, metals, XRF, Br, Zn, and Pb.

Chapter 7 Frontiers in assessing the role of chemical speciation and natural attenuation on the Bioavailability of Contaminants in the Terrestrial Environment

Publisher Summary This chapter provides an overview of the current state-of-the-art for assessing speciation and attenuation of contaminants in the terrestrial environment. Given that speciation and attenuation influence chemical bioavailability, the chapter discusses application of isotopic dilution techniques to bioavailability assessment. A series of case studies is provided that exemplify cutting-edge techniques and methodologies for assessing the role of chemical speciation and natural attenuation on the bioavailability of contaminants in the terrestrial environment. A summary is presented of the findings of the techniques and case studies presented to reinforce the strength of the techniques described. The strength of synchrotron-based spectroscopic techniques is explained. Through the application of XANES, it is demonstrated that Cr is present in ambient particulate matter in a wide variety of forms that probably represent an equally broad set of sources. Basics of the isotopic dilution principle are elaborated. Also,details of microbeam synchrotron X-ray fluorescence of human teeth are provided. An overview of XANES and EXAFS analyses of arsenic in soils is also presented.