Changes in Zinc Speciation in Field Soil after Contamination with Zinc Oxide

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Abstract
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 μ-X-ray fluorescence (μ-XRF) and μ-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 by thermodynamics 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.