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 mu m), 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 crystalline coatings on mineral surfaces. There is no evidence that zinc sorption or substitution in other mineral phases is a significant mode of uptake. In contrast, there is no spectral evidence for the association of cadmium with secondary oxide or oxyhydroxide phases in both high- and low-iron samples. Cadmium bound in sphalerite is found in all samples; evidence for cadmium uptake into a carbonate phase (in addition to sphalerite) is found in only one sample. This result may suggest preferential partitioning of cadmium, relative to zinc, into the aqueous phase as sphalerite weathers. XAFS spectra of lead in sediments with low total iron concentrations indicate no evidence for Pb bonding in galena (PbS), the host ore, and suggest lead uptake in secondary carbonate and/or oxide phases. Uptake of metal ions from solution into secondary phases is apparently governed by competition between iron oxyhydroxide and carbonate phases that can be related to total iron in the sediments and to stream pH. This work highlights the differential chemical behavior of three divalent metal cations in a contaminated system and demonstrates the need for direct molecular identification of metal coordination in order to generate accurate geochemical predictions.
