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TECHNICAL REPORTS

Heavy Metals in the Environment

Distribution and Speciation of Metals in Annual Rings of Black Willow

^a Consortium for Risk Evaluation with Stake Holder Participation, Environmental and Occupational Health Science Institute, Division of Life Sciences, Rutgers University, 604 Allison Road, Piscataway, NJ 08854
^b Consortium for Advanced Radiation Sources, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637
^c Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, SC 29802

^* Corresponding author (punshon{at}srel.edu)

Received for publication December 2, 2004. Information on the spatial distribution and speciation of metals in nonhyperaccumulator plants is lacking. This study used synchrotron X-ray fluorescence (SXRF) compositional imaging to investigate the spatial distribution of Ni, Mn, Cu, Zn, and Fe in annual rings of black willow (Salix nigra L.) collected from a metal-contaminated area, and used X-ray absorption spectroscopy (XAS) to investigate Ni and Mn speciation in regions of the annual rings with elevated Ni concentrations. Annual rings were recollected in early 2003 from an individual known to be enriched with Ni from previous studies. Compositional imaging showed Ni and associated co-contaminants conservatively located in an annual ring. When compared with a corresponding photomicrograph, SXRF compositional images showed that metals were sharply constrained by the boundaries of the annual ring, indicating a sudden onset and cessation of uptake, and a lack of post-growth mobility of the metals. There was a particularly strong correlation between Ni and Mn in the metal-enriched annual ring (r = 0.8822), which suggested similar transport and binding behavior of these elements. X-ray absorption spectroscopy showed Ni and Mn to be present in the 2+ oxidation state. X-ray absorption near edge structure spectroscopy (XANES) fingerprinting of localized, highly Ni-enriched regions within the lumen of willow xylem vessels found similarities with Ni–pectic acid complexes, Ni–histidine, and NiSO₄.

Abbreviations: SXRF, synchrotron X-ray fluorescence • XANES, X-ray absorption near edge structure spectroscopy • XAS, X-ray absorption spectroscopy

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