Environ. Sci. Technol. 2011, 45, 255–261
Time-Dependent Changes of Zinc Speciation in Four Soils Contaminated with Zincite or Sphalerite A N D R E A S V O E G E L I N , * ,† O L I V I E R J A C Q U A T , ⊥,‡ S A B I N A P F I S T E R , ‡ KURT BARMETTLER,‡ A N D R E A S C . S C H E I N O S T , §,| A N D RUBEN KRETZSCHMAR‡ Eawag, Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, CH-8600 Duebendorf, Switzerland, Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN, CH-8092 Zurich, Switzerland, Institute of Radiochemistry, FZD, Dresden, Germany, and Rossendorf Beamline, European Synchrotron Radiation Facility, Grenoble, France
Received April 14, 2010. Revised manuscript received November 12, 2010. Accepted November 22, 2010.
The long-term speciation of Zn in contaminated soils is strongly influenced by soil pH, clay, and organic matter content as well as Zn loading. In addition, the type of Zn-bearing contaminant entering the soil may influence the subsequent formation of pedogenic Zn species, but systematic studies on such effects are currently lacking. We therefore conducted a soil incubation study in which four soils, ranging from strongly acidic to calcareous, were spiked with 2000 mg/kg Zn using either ZnO (zincite) or ZnS (sphalerite) as the contamination source. The soils were incubated under aerated conditions in moist state for up to four years. The extractability and speciation of Zn were assessed after one, two, and four years using extractions with 0.01 M CaCl2 and Zn K-edge X-ray absorption fine structure (XAFS) spectroscopy, respectively. After four years, more than 90% of the added ZnO were dissolved in all soils, with the fastest dissolution occurring in the acidic soils. Contamination with ZnO favored the formation of Znbearing layered double hydroxides (LDH), even in acidic soils, and to a lesser degree Zn-phyllosilicates and adsorbed Zn species. This was explained by locally elevated pH and high Zn concentrations around dissolving ZnO particles. Except for the calcareous soil, ZnS dissolved more slowly than ZnO, reaching only 26 to 75% of the added ZnS after four years. ZnS dissolved more slowly in the two acidic soils than in the nearneutral and the calcareous soil. Also, the resulting Zn speciation was markedly different between these two pairs of soils: Whereas Zn bound to hydroxy-interlayered clay minerals (HIM) and octahedrally coordinated Zn sorption complexes * Corresponding author phone: ++41 44 823 54 70; fax: + +41 44 823 52 10; e-mail:
[email protected]. † Eawag, Swiss Federal Institute of Aquatic Science and Technology. ⊥ Present address: Swiss Federal Office for the Environment, CH3003 Bern, Switzerland. ‡ Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich. § Institute of Radiochemistry, FZD. | European Synchrotron Radiation Facility. 10.1021/es101189d
2011 American Chemical Society
Published on Web 12/09/2010
prevailed in the two acidic soils, Zn speciation in the neutral and the calcareous soil was dominated by Zn-LDH and tetrahedrally coordinated inner-sphere Zn complexes. Our results show that the type of Zn-bearing contaminant phase can have a significant influence on the formation of pedogenic Zn species in soils. Important factors include the rate of Zn release from the contaminant phases and effects of the contaminant phase on bulk soil properties and on local chemical conditions around weathering contaminant particles.
Introduction The bioavailability and mobility of zinc (Zn) in contaminated soils are strongly affected by its speciation, i.e., by the chemical forms in which it is present (1). Therefore, knowledge on Zn speciation in contaminated soils is essential in order to assess environmental impacts such as decreased soil microbial activity or reduced soil fertility. Using extended X-ray absorption fine structure (EXAFS) spectroscopy, element specific information on the speciation of Zn in complex soil materials can be obtained (2). Numerous EXAFS-based studies investigated the speciation of Zn in soils that had been contaminated by a variety of sources, ranging from mining, smelter, and foundry emissions to sewage sludge application (2-14). These studies identified a series of pedogenic Zn species: adsorbed Zn on a variety of inorganic and organic sorbents in tetrahedral and octahedral coordination, Zn-rich trioctahedral phyllosilicates (2), Zn-rich layered double hydroxides (LDH) (12), and Zn sorbed in the interlayers of hydroxy interlayered minerals (HIM) (10). Recent studies on an extensive set of soil materials contaminated by aqueous Zn from the runoff of galvanized powerline towers indicated a systematic dependence of pedogenic Zn speciation on soil pH as well as the level of Zn loading relative to the availability of sorption sites (15-17). At low Zn loadings in (typically acidic) HIM-containing soils, Zn-HIM was found to be an important Zn species with a high sorption affinity for Zn but a relatively limited sorption capacity (16, 17). Low levels of Zn contamination were also retained by adsorption or complexation on soil constituents. Over a wide range of soil pH, formation of minor fractions of Zn-rich trioctahedral phyllosilicates were observed. In soils with higher pH and especially high Zn loading relative to the available sorption sites, mainly Zn-LDH and adsorbed/ complexed Zn in tetrahedral coordination allowed for the accumulation of massive amounts of Zn (15, 17). Via its impact on soil chemical conditions, the type of Zn-bearing contaminant introduced into soil during contamination may also have an effect on pedogenic Zn species. Examples for such effects include changes in soil pH, e.g., acidification by the dissolution of Fe-bearing sulfides (10, 14), or increasing availability of sorption sites, e.g., on inorganic and organic sorbents introduced with sewage sludge (1). In addition to bulk chemical parameters, local chemical conditions around weathering or decomposing Zn-bearing contaminants may affect the types of pedogenic Zn species forming, depending on contaminant type. Zn-bearing contaminants may also vary in the rate at which they weather or decompose and thereby exhibit different rates of Zn release into soil. Zincite (ZnO) from foundry emissions or applied as fertilizer for instance represents a relatively soluble source of Zn (11, 18), whereas sphalerite (ZnS) introduced into soil with dredged sediments (7), animal wastes (19) or mine wastes (14) or spills (20, 21) may release Zn at lower rates. These differences in Zn release may affect the temporal evolution on Zn availability (22) as well as the formation of pedogenic VOL. 45, NO. 1, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
255
Zn species and may be further modulated by variations in the extent of aqueous solute transport among different soils. The aim of the present study was to assess the effect of two representative Zn sources on the types of pedogenic Zn species forming over time in different soils. For this purpose, we artificially contaminated four soils with different soil chemical and physical properties with either zincite (ZnO) or sphalerite (ZnS) and monitored the evolution of Zn speciation in the soils over four years of incubation in moist state. Changes in the speciation of Zn derived from Zn K-edge EXAFS spectroscopy were interpreted in relation to concomitant changes in the pH and Zn concentrations of CaCl2extracts.
Materials and Methods Soil Materials. The four soil materials used in this study were selected to span a wide range in chemical and physical soil properties (Table 1). The soil material Rie had the lowest pH and lowest organic C content among the studied soil materials. It was collected from the eluvial horizon of a Luvisol in northern Switzerland and exhibited a mixed clay mineralogy including HIM. This soil had previously been used in studies on cation transport and formation of Zn-bearing precipitates (23-26). The topsoil material U1 was collected from an acidic soil formed in saprolite residuum weathered from biotite-mica schist in the Piedmont region of North Carolina (A horizon of profile 2 in ref 27). The soil had a clay loam texture, and its clay mineralogy was dominated by kaolinite and hydroxy-interlayered vermiculite (HIV) (27). The topsoil material ZB has previously been used in a lysimeter study on the effect of metal contamination on ecosystem functioning and temporal changes in Zn speciation as well as for pot experiments on Zn uptake by plants (11, 22, 28). The noncalcareous soil had developed from a fluvioglacial deposit in northern Switzerland and was characterized by a circumneutral pH and a loamy texture. The topsoil material Hu was the only calcareous soil included in this study (containing ∼5% (w/w) CaCO3) and also had the highest soil pH and organic C content (likely also explaining the high field capacity, Table 1). It was collected from an arable soil developed on limestone in northern Switzerland. After air-drying, the soil was sieved to
