Time-dependent changes of Zinc speciation in four soils contaminated with zincite or sphalerite


Time-dependent changes of Zinc speciation in four soils contaminated with zincite or sphalerite

Voegelin, A.; Jacquat, O.; Pfister, S.; Barmettler, K.; Scheinost, A. C.; Kretzschmar, R.

The speciation of Zn in contaminated soils depends on soil pH, clay content, and other soil properties. However, it is currently unclear how the type of Zn-bearing contaminant itself influences the Zn species newly formed in soils upon contamination. Therefore, we conducted a soil incubation study in which four soils (pH 4.2 to 7.7) were spiked either with ZnO (zincite) or ZnS (sphalerite) to a total Zn concentration of 2000 mg/kg and then incubated under aerated conditions near field capacity. The extractability and speciation of Zn were assessed after 1, 2, and 4 years of incubation using extractions with 0.01 M CaCl2 and Zn K-edge X-ray absorption fine structure (XAFS) spectroscopy, respectively. ZnO was dissolved most rapidly in the acidic soils, and in all soils more than 90% of the added ZnO were dissolved after four years. Fast dissolution of ZnO presumably results in elevated pH and high local Zn concentrations around dissolving ZnO particles, favoring the formation of Zn-LDH, and to a lesser degree Zn-phyllosilicates besides adsorbed Zn species. In contrast, ZnS dissolved much more slowly, reaching only 25 to 97% of the added ZnS after four years, with lower dissolution rates in the acidic soils. The resulting Zn speciation after 4 years was markedly different, with Zn-LDH only occurring in the neutral and alkaline soils. In both cases, adsorbed Zn was mainly octahedrally coordinated Zn bound as outer-sphere complex or Zn bound to HIM in the acidic soils, whereas tetrahedrally coordinated Zn sorbed as inner-sphere complex was found to be more important at higher soil pH. These results show that the type of Zn-bearing contaminant and its influence on local chemical conditions in addition to bulk soil properties control the types of Zn species forming in contaminated soil.

Keywords: XAS; zinc; soil; sphalerite; zincite

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Publ.-Id: 14051