Figure 4
(a) Normalized EXAFS and corresponding radial structure functions (uncorrected for phase shift of backscattering atoms) for cadmium in two NAS Alameda sediment samples from core SC4-12 (34.5 cm) and core C1-2 (5.5 cm) compared to reference precipitates, cadmium sulfide (CdS(ppc)) and a mixture of cadmium and iron sulfide (CdS-FeS) precipitated from solution immediately before data collection, and crystalline cadmium sulfate (CdSO4(s)). Dashed red line is the non-linear least-squares best fit. (b) Normalized EXAFS and corresponding radial structure functions (uncorrected for phase shift of backscattering atoms) for zinc in sediment samples from core SC4 (at 1.5 and 34.5 cm) and core C3-9 (55 cm) compared to a reference spectrum of crystalline sphalerite (ZnS(s)). Dashed red line is the non-linear least-squares best fit; green line is the fit component corresponding to sphalerite; blue line is the fit for a zinc-oxygen component that probably represents zinc substitution in detrital phyllosilicate or oxide minerals. (c) Normalized XANES spectra and corresponding first-derivative spectra for chromium in samples from cores SC4 (at 1.5 and 34.5 cm), SC9 (at 2.5 and 30 cm) and SC7 (at 18.5 cm) compared with reference spectra for crystalline chromium sulfide and potassium chromate compounds, (d) Normalized EXAFS and corresponding radial structure functions (uncorrected for phase shift of backscattering atoms) for chromium in sediment samples SC2-7, SC4-1, and SC4-12 compared to reference crystalline potassium chromate. Dashed red line is the non-linear least-squares best fit. Oxidation of Cr(III) to Cr(VI) results in a significant shift of the first-shell oxygen peak to shorter interatomic distance. (e) Normalized XANES spectra for lead in samples from core SC4 (at 1.5, 31.5, and 34.5 cm) compared with reference spectra for crystalline lead sulfide, oxide, and carbonate compounds.