Distribution and Speciation of Metals in Annual Rings of Black Willow
Tracy Punshon,* Antonio Lanzirotti, Steve Harper, Paul M. Bertsch, and
Joanna Burger
ABSTRACT
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.
SREL Reprint #2866
Punshon, T., A. Lanzirotti, S. Harper, P. M. Bertsch and J. Burger. 2005. Distribution and speciation of metals in annual rings of black willow. Journal of Environmental Quality 34:1165-1173.
