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May 9, 2003 Zinc and Cadmium Associations with Reduced Forms of Sulfur in Organic Matter-Rich Aerobic SoilsC.E. Martínez1, M.B. McBride2, M.T. Kandianis2, J.M. Duxbury2, S. Yoon3, and W.F. Bleam3 Unusually high natural concentrations of zinc and cadmium have often been associated with soils high in organic matter and sulfur. For example, the soils from the Manning region of Western New York and from Guelph in Ontario concentrate zinc and cadmium by biogeochemical processes using sulfur. Scientists at The Pennsylvania State University in University Park, Cornell University in Ithaca, New York, and the University of Wisconsin in Madison have determined the oxidation states of sulfur in bulk soils, the distribution of these two metals in soil particles, and their solubility, toxicity and bioavailability. The researchers used spectroscopic, chemical, and bioassay approaches and illustrated how the macroscopic behavior of metals in soils can be explained by knowledge of their forms at the microscopic scale.
One possible concern with these metalliferous peats is the release of sulfur-bound cadmium and zinc during natural weathering conditions or when the bogs are drained for agricultural use. For example, in the Manning peatlands of western New York, this situation appears to have arisen as toxic metals were detected in vegetable crops in the early 1940s, not long after the bogs were drained for agriculture. Very high concentrations of zinc were measured in the plant shoots of some fields.
We recently collected peat soils from the Manning region and from Guelph in Ontario to investigate the processes controlling zinc and cadmium solubility, toxicity, and bioavailability in these soils.
Another technique called energy-dispersive x-ray absorption (EDX) also showed a close spatial distribution between zinc and sulfur in soil particles, thus suggesting their chemical association (Figure 3). But, despite this EDX evidence, conventional x-ray diffraction (XRD) analyses of the bulk soils did not detect a mineral phase of sphalerite (ZnS) in any of them. Chemical data showed that cadmium binds relatively more strongly in these sulfur-rich organic soils than in most mineral soils, suggesting that reduced sulfur could be limiting cadmium solubility more than zinc. These data also showed that most of the high-zinc soils were very toxic to plants, and that such toxicity has persisted for decades since the soils were drained.
We have also demonstrated high zinc solubility and plant availability but relatively low cadmium solubility and plant uptake, despite very high soil cadmium concentrations, as well as both low mobility and bioavailability of cadmium relative to zinc in these soils. We conclude that sulfur biogeochemical cycling may play an important role in zinc and cadmium retention in sulfur-rich organic soils. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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Peat deposits, such as those overlying the dolomite bedrock that
extends from eastern New York State into Ontario in Canada (Figure
1), contain unusually high concentrations of metals, such as zinc and
cadmium, leading to potentially hazardous concentrations of cadmium
in some crops. Scientists have determined that soluble zinc entered
these wetlands by drainage and groundwater emanating from weathering
dolomite [CaMg (CO3)2]. 
Reduced forms of sulfur (compared to their oxidized forms) are
potentially important to retain zinc and cadmium in organic soils.
By using x-ray absorption near-edge spectroscopy
(XANES) at NSLS beamline X19A, we have shown that 35 to 45 percent
of sulfur in these soils exists in the most reduced electronic
oxidation states 60 years after the soils were drained, while less
than five percent of sulfur exists in the most oxidized forms
(Figure 2). Although reduced forms of sulfur are not expected to
prevail in aerobic soils for long periods of time, their presence
may be the result of anaerobic microenvironments in otherwise
apparent oxygen-rich soils, or of slow oxygen diffusion into soil
aggregates. 