Zinc Speciation in Contaminated Sediments: Quantitative Determination of Zinc Coordination by X-ray Absorption Spectroscopy

X-ray absorption spectroscopy (XAS) was used to quantify the various proportions of the different ligation spheres of zinc in the sediments of Lake DePue (Illinois, USA), an environment contaminated by smelting operations. Samples were collected with a hand-held piston coring device, sub-cored, sectioned in thin, ~1-mm-thick sections, and kept under cryogenic conditions (liquid N₂) to prevent any chemical change. X-ray absorption spectra were recorded in the continuous XAS scanning mode, and the experimental errors of the data collection were directly measured. The extended X-ray absorption fine structure (EXAFS) was decomposed on a basis set of references’ EXAFS spectra. These reference standards were selected relying on geochemical analyses and analytical electron microscopy data. The propagation of experimental errors through the entire process allowed the assessment of the quality of the resulting Zn speciation. The results show that Zn is coordinated with a wide variety of ligands: carbonate, phosphate, sulfide, and water. The proportions of these various coordinative environments vary with both location and depth in the core. Close to the source of contamination, a significant fraction of Zn is present as an aqua-complex, suggesting that Zn is under a rather labile chemical form. Away from the source of contamination, Zn is predominantly bound to sulfide, which is rather inert under the reducing conditions prevailing in these sediments.