Rock−Water Interactions Controlling Zinc, Cadmium, and Lead Concentrations in Surface Waters and Sediments, U.S. Tri-State Mining District. 1. Molecular Identification Using X-ray Absorption Spectroscopy

Abstract

X-ray absorption spectroscopy (XAS) and X-ray absorption fine structure (XAFS) spectra of zinc-, cadmium-, and lead-bearing sediments from a mining-impacted site in the U.S. Tri-State Mining District (Kansas−Missouri−Oklahoma) are used to identify the local molecular coordination of metals in contaminated, untreated stream sediments. Quantitative analysis of the XAFS spectra, supplemented by elemental distributions on particles provided by electron microprobe and secondary ion mass spectrometry (SIMS), shows that zinc and cadmium occur in small (<1 μm), residual particles of the host ore, sphalerite (ZnS) in which cadmium substitutes for zinc in the mineral structure. In half of the samples studied, analyses indicate that zinc, as it weathers from sphalerite, is scavenged primarily by zinc hydroxide and/or zinc−iron oxyhydroxide phases, depending on the total amount of iron in the system. These phases probably form as amorphous or poorly crystal line coatings on mineral surfaces. There is no evidence that...