Zinc-sulfur and cadmium-sulfur association in metalliferous peats: evidence from spectroscopy, distribution coefficients, and phytoavailability.

Abstract

Peat deposits can concentrate chalcophilic metals such as Zn and Cd by biogeochemical processes; as a result, there is a possibility that the solubility, mobility, and bioavailability of these metals could increase when such deposits are drained and cropped, initiating oxidation of organic matter and sulfides under aerobic conditions. We use spectroscopic, chemical, and bioassay approaches to characterize high Zn (88-15,800 mg kg(-1)), Cd (0.55-83.0 mg kg(-1)), and S (3.52-9.54 g kg(-1)) peat soils collected from locations in New York State and Ontario that overlie Silurian-age metal-enriched dolomite bedrock. Total and KNO3-extractable trace metals were determined by ICP emission spectrometry, and labile Cd and Zn were measured in the KNO3 extracts by anodic stripping voltammetry. A greenhouse bioassay with maize and canola was conducted to determine the bioavailability and toxicity of the soil Zn and Cd. The electronic oxidation states of sulfur in the peat soils were determined by X-ray absorption near edge spectroscopy (XANES) and Zn and S distribution in soil particles by energy-dispersive X-ray absorption (EDX) spectroscopy. Sulfur-XANES analyses show that a high percentage (35-45%) of the total soil S exists in the most reduced electronic oxidation states (such as sulfides and thiols), while <5% exists in the most oxidized forms (such as sulfate). EDX analyses indicate a microscopic elemental association between Zn and S in these soils. Despite the EDX evidence of close association between Zn and S in soil particles, conventional X-ray diffraction analyses of the bulk soils did not detect a mineral phase of sphalerite (ZnS) in any of the soils. The distribution coefficients (Kd) for Zn and Cd increased with soil pH and indicated stronger Cd retention than Zn in the peats. The results of the bioassaytests showed that most of the high-Zn soils were very phytotoxic, with plant shoot Zn levels exceeding 400 mg kg(-1). Conversely, Cd concentrations in the plant shoots were generally below 2 mg kg(-1), showing a tendency toward low Cd phytoavailability relative to Zn. The information gained from the spectroscopic analyses (S-XANES and EDX) was used to explain the macroscopic observations (Cd and Zn Kd values and phytoavailability data) in these peat soils; we conclude that sulfur biogeochemical cycling may play an important role in Zn and Cd retention in these organic soils.