Sabina Pfister | PubMed Health

PubMed2005

Changes in zinc speciation in field soil after contamination with zinc oxide.

Recent studies on the speciation of Zn in contaminated soils confirmed the formation of Zn-layered double hydroxide (LDH) and Zn-phyllosilicate phases. However, no information on the kinetics of the formation of those phases under field conditions is currently available. In the present study, the transformation of Zn in a field soil artificially contaminated with ZnO containing filter dust from a brass foundry was monitored during 4 years using extended X-ray absorption fine structure (EXAFS) spectroscopy. Soil sections were studied by micro-X-ray fluorescence (micro-XRF) and micro-EXAFS spectroscopy. EXAFS spectra were analyzed by principal component analysis (PCA) and linear combination fitting (LCF). The results show that ZnO dissolved within 9 months and that half of the total Zn reprecipitated. The precipitate was mainly of the Zn-LDH type (>75%). Only a minor fraction (<25%) may be of Zn-phyllosilicate type. The remaining Zn was adsorbed to soil organic and inorganic particles. No significant changes in Zn speciation occurred from 9 to 47 months after the contamination. Thermodynamic calculations show that both Zn-LDH and Zn-phyllosilicate may form in the presence of ZnO but that the formation of Zn-phyllosilicate would be thermodynamically favored. Thus, the dominance of Zn-LDH found by spectroscopy suggests that the formation of the Zn precipitates was not solely controlled bythermodynamics but also contained a kinetic component. The rate-limiting step could be the supply of Al and Si from soil minerals to the Zn-rich solutions around dissolving ZnO grains.

Absorption↗

PubMed2002

Combining selective sequential extractions, X-ray absorption spectroscopy, and principal component analysis for quantitative zinc speciation in soil.

Selective sequential extractions (SSE) and, more recently, X-ray absorption fine-structure IXAFS) 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.

Adsorption↗