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The fate and chemical speciation of arsenic (As) during uptake, translocation, and storage by the As hyperaccumulating fern Pityrogramma calomelanos var. austroamericana (Pteridaceae) were examined using inductively coupled plasma-atomic emission spectrometry (ICP-AES) and synchrotron-based micro-X-ray absorption near edge structure (micro-XANES) and micro-X-ray fluorescence (micro-XRF) spectroscopies. Chemical analysis revealed total As concentration was ca. 6.5 times greater in young fronds (5845 mg kg(-1) dry weight (DW)) than in old fronds (903 mg kg(-1) DW). In pinnae, As concentration decreased from the base (6822 mg kg(-1) DW) to the apex (4301 mg kg(-1) DW) of the fronds. The results from micro-XANES and micro-XRF of living tissues suggested that more than 60% of arsenate (As(V)) absorbed was reduced to arsenite (As(III)) in roots, prior to transport through vascular tissues as As(V) and As(III). In pinnules, As(III) was the predominant redox species (72-90%), presumably as solvated, oxygen coordinated compounds. The presence of putative As(III)-sulphide (S(2-)) coordination throughout the fern tissues (4-25%) suggests that S(2-) functional groups may contribute in the biochemical reduction of As(V) to As(III) during uptake and transport at a whole-plant level. Organic arsenicals and thiol-rich compounds were not detected in the species and are unlikely to play a role in As hyperaccumulation in this fern. The study provides important insights into homeostatic regulation of As following As uptake in P. calomelanos var. austroamericana.

The consumption of arsenic (As) contaminated rice is an important exposure route for humans in countries where rice cultivation employs As contaminated irrigation water. Arsenic toxicity and mobility are a function of its chemical-speciation. The distribution and identification of As in the rice plant are hence necessary to determine the uptake, transformation and potential risk posed by As contaminated rice. In this study we report on the distribution and chemical-speciation of As in rice (Oryza sativa Quest) by X-ray fluorescence (XRF) and X-ray absorption near edge structure (XANES) measurements of rice plants grown in As contaminated paddy water. Investigations of muXRF images from rice tissues found that As was present in all rice tissues, and its presence correlated with the presence of iron at the root surface and copper in the rice leaf. X-ray absorption near edge structure analysis of rice tissues identified that inorganic As was the predominant form of As in all rice tissues studied, and that arsenite became increasingly dominant in the aerial portion of the rice plant.

The speciation of arsenic (As) in a copper-chromated-arsenate (CCA) contaminated soil was investigated using micro-focused X-ray fluorescence (microXRF) and micro-focused X-ray absorption fine structure (microXAFS) spectroscopies to determine if and how the co-contaminating metal cations (Cu, Zn, Cr) influenced the speciation of As. 15 microXRF images were collected on 30-mum polished thin sections and powder-on-tape samples from which Pearson correlation coefficients (rho) between As and various metal species were determined based on the fluorescence intensity of each element in each image pixel. 29 microXAFS and two bulk-XAFS spectra were collected from depths of 0-20 cm (LM-A) and 20-40 cm (LM-B) to determine the chemical speciation of As in the soil by target analyses of principal components with circa 52 reference spectra and linear least-square combination fitting of individual experimental spectra with a refined reference phase list (32) of likely As species. Arsenic and metal cations (Cr, Mn, Fe, Cu, Zn) accumulated in distinct, isolated areas often not larger than 50 x 50 microm in which the Pearson correlation between the elements was strongly positive (rho>0.75). The correlation of As to Zn and Cr decreased from >0.9 to <0.8 and increased to Cu from approximately 0.6 to >0.8 with depth. Arsenic occurred predominantly in the +5 oxidation state. Abstract factor analysis and linear least square combination fit analysis suggested that As occurred as a continuum of fully and poorly-ordered copper-arsenate precipitates with additional components being characterized by surface adsorption complexes on goethite and gibbsite in the presence and absence of Zn. Precipitates other than copper-based ones, e.g., scorodite, adamite and ojuelaite were also identified. The significant co-localization and chemical speciation of As with Cu suggest that the speciation of As in a contaminated soils is not solely controlled by surface adsorption reactions, but significantly influenced by the co-contaminating metal cation fraction. Future studies into As contaminated soil therefore need to focus on identifying the speciation of As and the co-localizing metal cations.

Little information is available concerning cosorbing oxyanion and metal contaminants in the environment, yet in most metal-contaminated areas, cocontamination by arsenate [AsO4, As(V)] is common. This study investigated the cosorption of As(V) and Zn on goethite at pH 4 and 7 as a function of final solution concentration. Complimentary extended X-ray absorption fine structure (EXAFS) spectroscopic data were collected at the As and Zn K-edges in order to glean information about the coordination environment of As and Zn at the goethite-water interface. Macroscopic sorption studies revealed that As(V) and Zn sorption on goethite increased in cosorption experiments beyond that suggested by single sorption isotherms. At pH 4 and 7, As(V) surface saturation was 3.2 and 2.2 micromol m(-2), respectively, and Zn surface saturation was absent at pH 4 and approximately 1.0 micromol m(-2) at pH 7. Arsenate sorption on goethite increased in the presence of Zn by 29% and by more than 500% at pH 4 and 7, respectively. In the presence of As(V), Zn sorption on goethite increased by 800 and 1300% at pH 4 and 7, respectively. More As(V) than Zn sorbed on goethite below surface saturation at pH 7. Above surface saturation, the Zn:As surface density ratio (SDR) remained constant at 0.91 +/- 0.03. At pH 4, the Zn:As SDR was less than 1 throughout the concentration range. Below As(V) surface saturation on goethite, As(V) formed bidentate binuclear bridging complexes on Fe and/or Zn octahedra, while Zn mainly formed edge-sharing complexes with Fe at the goethite surface. Above surface saturation, Zn was increasingly complexed by AsO4, gradually forming an adamite-like [Zn2(AsO4)OH] surface precipitate on goethite. Precipitated contaminants are more stable due to the limited dissolution kinetics of their solid phase. This study may therefore prove useful in remediation strategies of sites knowingly contaminated with oxyanions and metals.