Combining Selective Sequential Extractions, X-ray Absorption Spectroscopy, and Principal Component Analysis for Quantitative Zinc Speciation in Soil

• Published in: Environmental science & technology Vol. 36; no. 23; pp. 5021 - 5028
• Main Authors: Scheinost, Andreas C, Kretzschmar, Ruben, Pfister, Sabina, Roberts, Darryl R
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.12.2002
• Subjects: Adsorption; Analysis; Applied sciences; Biological and physicochemical properties of pollutants. Interaction in the soil; Chemistry Techniques, Analytical; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental Monitoring - methods; Exact sciences and technology; Geologic Sediments - chemistry; Industry; Pollution; Pollution, environment geology; Principal components analysis; Reproducibility of Results; Scientific imaging; Sediments; Sensitivity and Specificity; Soil and sediments pollution; Soil Pollutants - analysis; Soils; Spectrum Analysis; X-rays; Zinc; Zinc - analysis; Zinc - chemistry; Selective extraction; Fractionation; XANES spectrometry; Pollutant behavior; Geochemistry; Sequential extraction; Soil pollution; Sediments; EXAFS spectrometry; Zinc; Heavy metal; Statistical method; X ray absorption spectrometry; Investigation method; Speciation; Principal component analysis
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es025669f

Selective sequential extractions (SSE) and, more recently, X-ray absorption fine-structure (XAFS) spectroscopy have been used to characterize the speciation of metal contaminants in soils and sediments. However, both methods have specific limitations when multiple metal species coexist in soils and sediments. In this study, we tested a combined approach, in which XAFS spectra were collected after each of 6 SSE steps, and then analyzed by multishell fitting, principal component analysis (PCA) and linear combination fits (LCF), to determine the Zn speciation in a smelter-contaminated, strongly acidic soil. In the topsoil, Zn was predominately found in the smelter-emitted minerals franklinite (60%) and sphalerite (30%) and as aqueous or outer-sphere Zn2+ (10%). In the subsoil, aqueous or outer-sphere Zn2+ prevailed (55%), but 45% of Zn was incorporated by hydroxy-Al interlayers of phyllosilicates. Formation of such Zn-bearing hydroxy-interlayers, which has been observed here for the first time, may be an important mechanism to reduce the solubility of Zn in those soils, which are too acidic to retain Zn by formation of inner-sphere sorption complexes, layered double hydroxides or phyllosilicates. The stepwise removal of Zn fractions by SSE significantly improved the identification of species by XAFS and PCA and their subsequent quantification by LCF. While SSE alone provided excellent estimates of the amount of mobile Zn species, it failed to identify and quantify Zn associated with mineral phases because of nonspecific dissolution and the precipitation of Zn oxalate. The systematic combination of chemical extraction, spectroscopy, and advanced statistical analysis allowed us to identify and quantify both mobile and recalcitrant species with high reliability and precision.