No abstract

No abstract

No abstract

The mechanisms whereby As(III) and As(V) in aqueous solution (pH 5.5-6.5) interact with the surfaces of goethite, lepidocrocite, mackinawite, and pyrite have been investigated using As K-edge EXAFS and XANES spectroscopy. Arsenic species retain original oxidation states and occupy similar environments on the oxyhydroxide substrates, with first-shell coordination to four oxygens at 1.78 A for As(III) and 1.69 A for As(V). In agreement with other workers, we find that inner sphere complexes form, apparently involving bidentate (bridging) arsenate or arsenite. Interaction of As(III) and As(V) with the sulfide surfaces shows primary coordination to four oxygens (As-O: 1.69-1.76 A) with further sulfur (approximately 3.1 A) and iron (3.4-3.5 A) shells suggesting outer sphere complexation. Arsenic species were also coprecipitated with mackinawite (pH 4.0), and these samples were further studied following oxidation. At high As(III) or As(V) concentrations, arsenate or arsenite species form, probably as sorption complexes, along with poorly crystalline arsenic sulfide (the only product at low As(V) concentrations). All oxidized samples show primary coordination to four oxygens at 1.7 A, indicating As(V); these arsenates may show both outer sphere complexation with residual mackinawite and inner sphere complexation with new oxyhydroxides. These experiments help to clarify our understanding of As mobility in near-surface environments.

A compartmented soil-glass bead culture system was used to investigate characteristics of iron plaque and arsenic accumulation and speciation in mature rice plants with different capacities of forming iron plaque on their roots. X-ray absorption near-edge structure spectra and extended X-ray absorption fine structure were utilized to identify the mineralogical characteristics of iron plaque and arsenic sequestration in plaque on the rice roots. Iron plaque was dominated by (oxyhydr)oxides, which were composed of ferrihydrite (81-100%), with a minor amount of goethite (19%) fitted in one of the samples. Sequential extraction and XANES data showed that arsenic in iron plaque was sequestered mainly with amorphous and crystalline iron (oxyhydr)oxides, and that arsenate was the predominant species. There was significant variation in iron plaque formation between genotypes, and the distribution of arsenic in different components of mature rice plants followed the following order: iron plaque > root > straw > husk > grain for all genotypes. Arsenic accumulation in grain differed significantly among genotypes. Inorganic arsenic and dimethylarsinic acid (DMA) were the main arsenic species in rice grain for six genotypes, and there were large genotypic differences in levels of DMA and inorganic arsenic in grain.

Rice (Oryza sativa) is the staple food for over half the world's population yet may represent a significant dietary source of inorganic arsenic (As), a nonthreshold, class 1 human carcinogen. Rice grain As is dominated by the inorganic species, and the organic species dimethylarsinic acid (DMA). To investigate how As species are unloaded into grain rice, panicles were excised during grain filling and hydroponically pulsed with arsenite, arsenate, glutathione-complexed As, or DMA. Total As concentrations in flag leaf, grain, and husk, were quantified by inductively coupled plasma mass spectroscopy and As speciation in the fresh grain was determined by x-ray absorption near-edge spectroscopy. The roles of phloem and xylem transport were investigated by applying a 6 stem-girdling treatment to a second set of panicles, limiting phloem transport to the grain in panicles pulsed with arsenite or DMA. The results demonstrate that DMA is translocated to the rice grain with over an order magnitude greater efficiency than inorganic species and is more mobile than arsenite in both the phloem and the xylem. Phloem transport accounted for 90% of arsenite, and 55% of DMA, transport to the grain. Synchrotron x-ray fluorescence mapping and fluorescence microtomography revealed marked differences in the pattern of As unloading into the grain between DMA and arsenite-challenged grain. Arsenite was retained in the ovular vascular trace and DMA dispersed throughout the external grain parts and into the endosperm. This study also demonstrates that DMA speciation is altered in planta, potentially through complexation with thiols.

The release of uranium and other transuranics into the environment, and their subsequent mobility, are subjects of intense public concern. Uranium dominates the inventory of most medium- and low-level radioactive waste sites and under oxic conditions is highly mobile as U(VI), the soluble uranyl dioxocation (UO2)2+. Specialist anaerobic bacteria are, however, able to reduce U(VI)to insoluble U(IV), offering a strategy for the bioremediation of uranium-contaminated groundwater and a potential mechanism for the biodeposition of uranium ores. Despite the environmental importance of U(VI) bioreduction, there is little information on the mechanism of this transformation. In the course of this study we used X-ray absorption spectroscopy (XAS) to show that the subsurface metal-reducing bacterium Geobacter sulfurreducens reduces U(VI) by a one-electron reduction, forming an unstable (UO2)+ species. The final, insoluble U(IV) product could be formed either through further reduction of U(V) or through its disproportionation. When G. sulfurreducens was challenged with the chemically analogous (NpO2)+, which is stable with respect to disproportionation, it was not reduced, suggesting that it is disproportionation of U(V) which leads to the U(IV) product. This surprising discrimination between U and Np illustrates the need for mechanistic understanding and care in devising in situ bioremediation strategies for complex wastes containing other redox-active actinides, including plutonium.

The retention of radionuclides by interaction with mineral phases has significant consequences for the planning of their short-and long-term disposal to geological systems. An understanding of binding mechanisms is important in determining the ultimate fate of radionuclides following release into natural systems and will give increased confidence in predictive models. X-ray absorption spectroscopy (XAS) has been used to study the local environment of uranium taken up from aqueous solution by the surfaces of goethite, lepidocrocite, muscovite, and mackinawite. On both iron hydroxides uranium uptake occurs by surface complexation and ceases when the surface is saturated. The muscovite surface does not become saturated and uptake increases linearly suggesting formation of a uranium phase on the surface. Uranium uptake on mackinawite also suggests a replacement or precipitation process. XAS indicates that bidentate inner-sphere surface complexes are formed on the iron hydroxides by coordination of two surface oxygens from an iron octahedron in the equatorial plane of the complex. Uranium uptake on muscovite may occur through surface precipitation, the first layer of uranium atoms binding through equatorial coordination of two adjacent surface oxygens from a silicate tetrahedron, with the axial oxygens of the uranyl unit aligned across the hexagonal "cavities" created by the rings of tetrahedra. At low concentrations, uptake on mackinawite occurs at locally oxidized regions on the surface via a similar mechanism to that on iron hydroxides. At the highest concentrations, equatorial oxygen bond distances around 2.0-2.1 Å are observed, inconsistent with the presence of uranyl species. The average number of axial oxygens also decreases with increasing concentration, and these results suggest partial reduction of uranium. The nature of these different surface reactions plays an important role in assessing the geochemical behavior of uranium in natural systems, particularly under reducing conditions.