Changes in Zn speciation during soil formation from Zn-rich limestones

In order to better understand the long-term speciation and fractionation of Zn in soils, we investigated three soils naturally enriched in Zn (237-864 mg/kg Zn) from the weathering of Zn-rich limestones (40-207 mg/kg Zn) using extended X-ray absorption. ne structure (EXAFS) spectroscopy and sequential extractions. The analysis of bulk EXAFS spectra by linear combination fitting (LCF) indicated that Zn in the oolitic limestones was mainly present as Zn-containing calcite (at site Dornach), Zn-containing goethite (Gurnigel) and Zn-containing goethite and sphalerite (Liestal). Correspondingly, extraction of the powdered rocks with 1 M NH4-acetate at pH 6.0 mobilized only minor fractions of Zn from the Gurnigel and Liestal limestones (<30%), but most Zn from the Dornach rock (81%). In the Dornach soil, part of the Zn released from the dissolving limestone was subsequently incorporated into pedogenic hydroxy-interlayered vermiculite (Zn-HIV, similar to 30%) and Zn-containing kaolinite (similar to 30%) and adsorbed or complexed by soil organic and inorganic components (similar to 40%). The Gurnigel and Liestal soils contained substantial amounts of Zn-containing goethite (similar to 50%) stemming from the parent rock, smaller amounts (similar to 20%) of Zn-containing kaolinite (and possibly Zn-HIV), as well as adsorbed or complexed Zn-species (similar to 30%). In the soil from Liestal, sphalerite was only found in trace amounts, indicating its dissolution during soil formation. In sequential extractions, large percentages of Zn (similar to 55-85%) were extracted in recalcitrant extraction steps, confirming that Zn-HIV, Zn-containing kaolinite and Zn-containing goethite are highly resistant to weathering. These Zn-bearing phases thus represent long-term hosts for Zn in soils over thousands of years. The capability of these phases to immobilize Zn in heavily contaminated soils may however be limited by their uptake capacity (especially HIV and kaolinite) or their abundance in soil. (C) 2009 Elsevier Ltd. All rights reserved.