Soil properties controlling Zn speciation and fractionation in contaminated soils
Introduction
Soil contamination with zinc (Zn) is widespread and in many regions still increasing due to emissions from mining, smelting, chemical and processing industry, agriculture, and sewage sludge application. Elevated Zn concentrations in soils can pose long-term risks for soil fertility and ecosystem health, e.g., by affecting soil microbial activity or causing phytotoxicity. The speciation of Zn exerts a strong influence on the mobility, bioavailability and toxicity of Zn in contaminated soils. Knowledge on the types of Zn species and their chemical transformation is therefore essential for the reliable assessment of Zn dynamics and impact in terrestrial systems.
Single and sequential extractions are widely used for the assessment of metal availability in soils (Young et al., 2005). Extractions are also commonly used as a cost-effective method for the determination of metal fractionation, often serving as a proxy to metal speciation (Tessier et al., 1979, Shuman, 1985, Zeien and Brümmer, 1989, Young et al., 2005). Due to the limited selectivity of extraction procedures (Gleyzes et al., 2002, Young et al., 2005), however, the types and concentrations of different molecular-level Zn species cannot be unequivocally inferred from extraction data (Scheinost et al., 2002, Kirpichtchikova et al., 2006, Voegelin et al., 2008). Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy on the other hand allows for the in situ identification and quantification of the dominant metal species (Manceau et al., 2000), though its sensitivity may be limited with respect to the most labile species controlling metal availability (Scheinost et al., 2002). Over the past two decades, EXAFS spectroscopy greatly contributed to advances in elucidating the fate of heavy metals in contaminated soils and sediments (Brown et al., 1988, Fendorf et al., 1994, Schulze and Bertsch, 1995, O’Day, 1999, Manceau et al., 2002b).
To date, about a dozen studies (summarized in Table 1) have addressed the speciation of Zn in contaminated upland soils using EXAFS spectroscopy. Based on the interpretation of bulk and micro-focused soil EXAFS spectra by linear combination fits (LCF) of reference spectra, Manceau and coworkers (Manceau et al., 2000) first identified and quantified different Zn species present in the clay and silt fractions of contaminated soils. Besides Zn-bearing minerals from smelter emissions, several pedogenic (soil-formed) Zn species were identified, including Zn-bearing phyllosilicate-type precipitates, Zn in the structure of goethite, and Zn sorbed to Fe and Mn oxides (Manceau et al., 2000). Further studies on the forms of Zn in soils contaminated by mining, smelting, or foundry emissions, deposition of dredged sediments or sewage irrigation confirmed the formation and quantitative relevance of Zn-bearing precipitates and sorbed/complexed Zn species upon weathering of the Zn-bearing phases emitted by the respective contamination source (Isaure et al., 2002, Roberts et al., 2002, Juillot et al., 2003, Sarret et al., 2004, Isaure et al., 2005, Nachtegaal et al., 2005, Panfili et al., 2005, Voegelin et al., 2005, Kirpichtchikova et al., 2006, Schuwirth et al., 2007) (Table 1). Pedogenic Zn precipitates identified in these slightly acidic to neutral soils included Zn-layered double hydroxides (Zn-LDH) and Zn-phyllosilicates. In an acidic contaminated soil, Zn was shown to be strongly retained by incorporation into the Al-hydroxy-interlayer of hydroxy-interlayered minerals (HIM) (Scheinost et al., 2002). The formation of Zn-HIM, as well as the uptake of Zn into the gibbsitic sheets of the phyllomanganate lithiophorite, have also been reported for pristine soils (Manceau et al., 2003, Manceau et al., 2004, Manceau et al., 2005).
Most spectroscopic studies on Zn speciation in soils so far considered soils contaminated by emissions from mining and smelting. In such soils, Zn-bearing phases stemming from the emission source may still dominate the Zn speciation, thereby, lowering the sensitivity and accuracy of bulk EXAFS spectroscopy with respect to the identification and quantification of pedogenic Zn species (Manceau et al., 2000). In addition, even though the results obtained to date provide detailed insight into the diversity of pedogenic Zn species, comprehensive data on their occurrence and abundance as a function of soil chemical parameters is still lacking. With respect to the wide use of single and sequential extractions for the assessment of Zn availability in contaminated soils and related research, it is also essential to know how differences in molecular-level Zn speciation affect Zn fractionation in such extraction schemes.
The main objective of the current study therefore was to establish how pedogenic Zn speciation depends on soil chemical parameters and how differences in Zn speciation affected Zn fractionation by investigating the speciation of Zn in 49 contaminated soils spanning a wide range in composition and Zn contamination level. The soils had all been contaminated by inputs of aqueous Zn from the corrosion of galvanized power line towers. Pedogenic Zn species could thus be studied without interference by Zn-bearing phases from the contamination source. In a first step, Zn K-edge EXAFS spectra from all soils were analyzed simultaneously using principle component analysis (PCA), followed by target transform testing (TT), and linear combination fitting (LCF) based on a large set of Zn reference spectra. We previously investigated two subsets of these soils with respect to Zn speciation in five calcareous (Jacquat et al., 2008) and eight (mostly acidic) HIM-containing soils (Jacquat et al., 2009). In a second step, we investigated how Zn speciation in this wide range of soils related to soil properties and Zn content. Using the same set of soils, we have previously studied how soil properties and Zn contamination level affected Zn fractionation in single and sequential extractions (Voegelin et al., 2008). Combined with the EXAFS results from the current work, this enabled us to determine how differences in molecular-level Zn speciation as a function of soil properties were reflected in a typical sequential Zn fractionation scheme used for the assessment of Zn availability in soil.
Section snippets
Soil sampling and soil properties
Forty-nine topsoils (0–5 cm) were collected underneath 17–74 years-old galvanized power line towers across different geologic and climatic regions of Switzerland. The samples were air-dried, sieved to <2 mm size, and homogenized. Powdered subsamples <50 μm were prepared from the <2 mm fraction using an agate disk swing mill. Soil pH was determined after equilibration of 1 g soil with 10 mL 0.01 M CaCl2 for 30 min. Powdered soil material was pressed into pellets for the analysis of total element
Soil properties
Selected physical and chemical properties and the Zn contents of the 49 soils are summarized in Table 2. The data for all individual soils are provided in the electronic annex information (Tables EA1–EA4). The soils differed strongly in pH, ranging from acidic to weakly alkaline (pH 4.1–7.7), and covered a wide range in TOC (9–102 g/kg), TIC (0–89 g/kg), texture (clay: 38–451 g/kg, silt: 111–700 g/kg, sand: 184–781 g/kg), and ECEC (8–464 mmolc/kg). The total Zn contents ranged from 251 to 30090 mg/kg,
Differences in soil properties between LCF-derived soil groups
The physical and chemical properties of the four soil groups derived from LCF results were compared to examine if differences in Zn speciation were related to differences in soil properties. Group-wise average, minimum and maximum values for soil parameters and LCF results are provided in Table 2. Corresponding box-plots indicating the variation in selected soil physical and chemical parameters between soil groups are provided in the electronic annex (Fig. EA2). Zn-HIM was identified as a major
Conclusions
In contaminated soils, soil pH and Zn content relative to the amount of sorption sites are the most important parameters influencing pedogenic Zn speciation. Formation of Zn-HIM is restricted to moderately contaminated HIM-containing soils. Zn-phyllosilicates form over a wide range of soil pH, but are quantitatively less relevant than Zn-LDH, which is the dominant Zn-precipitate at higher soil pH and/or Zn loading. The fractionation of Zn as observed by sequential extraction depends on Zn
Acknowledgments
Evert Elzinga (ETH Zurich) kindly provided the EXAFS spectra of Zn sorbed calcite. Kurt Barmettler is acknowledged for technical support in the laboratory. We thank Stefan Mangold from the XAS beamline at ANKA (Germany), the staff from the SNBL beamline at the ESRF (France) and Kumi Pandya from the beamline X11A at NSLS (USA) for their help with EXAFS data acquisition. The ANKA (Germany) and SNBL at the ESRF (France) are acknowledged for the provision of beamtime. This project was financially
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