Biochemical and spectroscopic studies of the response of Convolvulus arvensis L. to chromium(III) and chromium(VI) stress.

The objective of the present study was to determine the oxidative stress caused by hexavalent chromium (Cr[VI]), the chromium (Cr) uptake, and the Cr speciation in Convolvulus arvensis L. plants grown in hydroponics media containing either Cr(VI) or Cr(III). The results demonstrated that C. arvensis plants exposed to Cr(VI) concentrations ranging from 0 to 40 mg/L expressed higher ascorbate peroxidase specific activity in roots than in shoots. On the other hand, catalase activity monitored in plants exposed to 2 mg/L of Cr(VI) for 24 h increased in roots after a few hours of exposure. However, catalase activity in shoots revealed a decrement almost immediately after treatment was initiated. The results from x-ray absorption spectroscopic studies indicated that the oxidation state of the supplied Cr(III) remained the same in plant tissues. The supplied Cr(VI), however, was reduced to the trivalent form in plant tissues. The results of inductively coupled plasma/optical emission spectroscopy demonstrated that after 5 d, the roots of plants exposed to 40 mg/L of Cr(III) or Cr(VI) accumulated approximately 25,000 and 3,500 mg/kg dry weight of Cr, respectively. Nevertheless, shoots concentrated 1,500 and 2,000 mg/kg dry weight of Cr from Cr(III) and Cr(VI), respectively, which indicated that Cr moved faster into C. arvensis plants when supplied as Cr(VI).

Use of ICP and XAS to determine the enhancement of gold phytoextraction by Chilopsis linearis using thiocyanate as a complexing agent.

Under natural conditions gold has low solubility that reduces its bioavailability, a critical factor for phytoextraction. Researchers have found that phytoextraction can be improved by using synthetic chelating agents. Preliminary studies have shown that desert willow (Chilopsis linearis), a common inhabitant of the Chihuahuan Desert, is able to extract gold from a gold-enriched medium. The objective of the present study was to determine the ability of thiocyanate to enhance the gold-uptake capacity of C. linearis. Seedlings of this plant were exposed to the following hydroponics treatment: (1) 5 mg Au L(-1) (2.5x10(-5) mol L(-1)), (2) 5 mg Au L(-1) + 10(-5) mol L(-1) NH4SCN, (3) 5 mg Au L(-1) + 5x10(-5) mol L(-1) NH4SCN, and (4) 5 mg Au L(-1) + 10(-4) mol L(-1) NH4SCN. Each treatment had its respective control. After 2 weeks we determined the effect of the treatment on plant growth and gold content by inductively coupled plasma-optical emission spectroscopy (ICP-OES). No signs of shoot-growth inhibition were observed at any NH4SCN treatment level. The ICP-OES analysis showed that addition of 10(-4) mol L(-1) NH4SCN increased the concentration of gold by about 595, 396, and 467% in roots, stems, and leaves, respectively. X-ray absorption spectroscopy (XAS) studies showed that the oxidation state of gold was Au(0) and that gold nanoparticles were formed inside the plants.

Synchrotron and simulations techniques applied to problems in materials science: catalysts and Azul Maya pigments.

Development of synchrotron techniques for the determination of the structure of disordered, amorphous and surface materials has exploded over the past 20 years owing to the increasing availability of high-flux synchrotron radiation and the continuing development of increasingly powerful synchrotron techniques. These techniques are available to materials scientists who are not necessarily synchrotron scientists through interaction with effective user communities that exist at synchrotrons such as the Stanford Synchrotron Radiation Laboratory. In this article the application of multiple synchrotron characterization techniques to two classes of materials defined as 'surface compounds' is reviewed. One class of surface compounds are materials like MoS(2-x)C(x) that are widely used petroleum catalysts, used to improve the environmental properties of transportation fuels. These compounds may be viewed as 'sulfide-supported carbides' in their catalytically active states. The second class of 'surface compounds' are the 'Maya blue' pigments that are based on technology created by the ancient Maya. These compounds are organic/inorganic 'surface complexes' consisting of the dye indigo and palygorskite, common clay. The identification of both surface compounds relies on the application of synchrotron techniques as described here.

An X-ray absorption study of two VOCl3-modified silicas: evidence for chloride-silica interactions.

The structures of the sites formed in the gas-solid reactions of VOCl3 with the surfaces of a fumed silica (Aerosil) and a silica gel (Sylopol) were investigated by using X-ray absorption spectroscopy. XANES and EXAFS analysis at the vanadium K-edge reveal that the sites have a uniform first coordination sphere regardless of the origin or the extent of hydroxylation of the silica support (controlled by thermal treatment in vacuo at 100 and 500 degrees C). Analysis of the second coordination sphere was limited by the lack of structural uniformity. EXAFS curve-fitting confirmed that the sites are [triple bond]SiOVOCl2, but revealed an unexpected asymmetry in the V-Cl bond distances. The latter is suggested to be a manifestation of silicon-chloride interactions.