Environ. Sci. Technol. 2011, 45, 943–950
Volatilization of Organotin Species from Municipal Waste Deposits: Novel Species Identification and Modeling of Atmospheric Stability E V A M . K R U P P , * ,†,‡ J O H N K . M E R L E , § KARSTEN HAAS,† GARY FOOTE,† NICOLAS MAUBEC,† AND ¨ R G F E L D M A N N * ,† JO TESLA (Trace Element Speciation Laboratory), Chemistry, College of Physical Science, and ACES (Aberdeen Centre of Environmental Sustainability), University of Aberdeen, Aberdeen AB24 3UE, Scotland, United Kingdom, and Winston-Salem State University, 601 Martin Luther King Jr. Drive, Winston-Salem, North Carolina 27110, United States
Received July 23, 2010. Revised manuscript received November 21, 2010. Accepted November 30, 2010.
Organotin compounds are used as pesticides and fungicides as well as additives in plastics. This study identifies the de novo generation of novel volatile organotins in municipal waste deposits and their release via landfill gas. Besides tetramethyltin (Me4Sn), a strong neurotoxin, and 5 previously reported organotins, 13 novel ethylated, propylated, and butylated tetraalkyltin compounds were identified. A concentration of 2-4 µg of Sn m-3 landfill gas was estimated for two landfill sites in Scotland. The atmospheric stability of Me4Sn and methylated tin hydrides was determined empirically in a static atmosphere in the dark and under UV light to simulate night- and daytime conditions. Theoretical calculations were carried out to help predict the experimentally obtained stabilities and to estimate the relative stabilities of other alkylated species. Assuming firstorder kinetics, the atmospheric half-life for Me3SnH was found to be 33 ( 16 and 1311 ( 111 h during day- and nighttime conditions, respectively. Polyalkylation and larger alkyl substitutes tend to reducetheatmosphericstability.Theseresultsshowthatsubstantial concentrations of neurotoxic organotin compounds can be released from landfill sites and are sufficiently stable in the atmosphere to travel over large distances in night- and daytime conditions to populated areas.
Introduction The environmental chemistry of tin has been focused on organotin compounds which have been deliberately introduced as fungicides into the environment, mainly in the form of tributyltin or phenyltin species (1, 2). Emphasis here has been on the comparison of the lipophilicity, water solubility, and biodegradability of those compounds to those properties of the benign inorganic Sn (3). Limited information is
available on the formation of tetraalkylated Sn compounds in sediments and algae (4, 5). However, organotins are widely used as stabilizers in plastics (e.g., PVC) and fungicides in a variety of materials, including diapers and clothing, which usually end up on landfill sites (6). Besides leaching of various organotin compounds from municipal landfills into water bodies (7-9), volatile organotin compounds partitioning into the landfill gas have been evidenced (7, 8, 10, 11). The primary tin species in landfill gas has previously been identified as tetramethyltin (Me4Sn), a neurotoxic compound. However, many more volatile and unidentified tin compounds have been shown to exist in landfill gas, making up more than 50% of all volatile Sn compounds. It was speculated that some of the compounds are stannanes (tin hydrides) (4, 10), while other studies imply that the Sn compounds are mixed methylated/alkylated Sn compounds (5, 11, 12). Although it is assumed that most of these compounds are fully alkylated, it is of interest to identify the molecular form of the individual Sn species since the toxicity and stability of known organotin compounds are strongly dependent on the number and type of the alkyl groups (13). The in situ generation of tetraalkyltin or tin hydride species reveals a new pathway for introducing tin compounds into the atmosphere under ambient atmospheric conditions and possibility for the occurrence of organotin compounds in remote areas (14, 15). In this paper, we report the identification and quantification of novel volatile organotin compounds generated in waste deposits and estimate their atmospheric stability to assess their impact on the biogeochemical cycle of tin and the risk associated with their mobility. The first objective of this study was to identify and quantify the plethora of volatile tin species in landfill gas. The gas emitted from two Scottish landfill sites was sampled in Tedlar bags and subsequently analyzed by cryotrapping-capillary gas chromatography (GC) coupled to inductively coupled plasma mass spectrometry (ICPMS), enabling tin-specific detection. This technique has been proven to allow quantitative recoveries for volatile tin species (16). The gas samples were spiked with known concentrations of selected tetraalkyltin species to confirm their identity via retention time and also serve as standards for quantification. The second objective was to determine the atmospheric half-life of some volatile tin compounds. Here, the kinetic stability of five different methylated tin hydrides (MexSnH4-x) and nBuSnH3 was determined under simulated atmospheric conditions in a microcosm experiment, described in detail elsewhere for volatile arsines (17). The reason for preferred use of tin hydrides over tetraalkyltin species is detailed in the Supporting Information, S1. Briefly, the latter are more prone to surface adsorption in the relatively small gas volumes used and thus would compromise half-life determinations. The third objective was to calculate bond dissociation energies and model possible atmospheric reaction pathways for tin compounds using quantum mechanical electronic structure methods to understand possible degradation processes and to estimate the stability of novel identified tin compounds, which were not determined empirically.
Experimental Section * Address correpondence to either author. E-mail: e.krupp@ abdn.ac.uk (E.M.K.);
[email protected] (J.F.). Phone: +44-1224272901 (E.M.K.); +44-1224-272911 (J.F.). Fax for both authors: +441224-272921. † TESLA, Chemistry, College of Physical Science, University of Aberdeen. ‡ ACES, University of Aberdeen. § Winston-Salem State University. 10.1021/es102512u
2011 American Chemical Society
Published on Web 12/21/2010
Landfill Gas and Gas Condensate. Landfill gas from two municipal and industrial waste receiving landfill sites located north of Aberdeen, Scotland, was sampled. One is a mature site (referred to as TH), where dumping stopped approximately five years prior to sampling in spring 2006. The other landfill (WH) had been opened in 2002 and was still in operation during the gas sampling period. Both sites are VOL. 45, NO. 3, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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equipped with gas wells, and the landfill gas is actively pumped out and transported to a furnace. Landfill gas and gas condensate were sampled directly from the gas wells near the furnace, which allowed a mixed pooled gas sample representing the overall landfill site. The concentration of methane was generally around 55%. Gas was sampled at the head of the gas wells directly into 10 L Teflon-coated gas bags (SKC, Dorset, U.K.). Gas condensate was sampled from TH only at a condensate collection point. The condensate contains mainly water and small amounts (∼5%) of organic phase, which may contain less volatile lipophilic organic compounds. Condensate was sampled into glass vials and stored at 4 °C. Experimental Setup To Measure the Atmospheric Stability of Volatile Sn Species. The experiments were conducted in pillow-shaped 10 L gas bags suitable for the simulation of atmospheric reactions (18). The bags were transparent for UV and visible light down to 290 nm, and the experiments were carried out indoors at 20 and 50 °C with the absence of artificial light. Exposure of the gas bags to UV light was realized using a 300 W halogen lamp at a fixed distance of 45 cm, giving a light intensity of around 5000 lx. Gas bags were filled with air containing stannanes (tin hydrides) in relevant atmospheric concentrations. For the generation of the methylated Sn compounds, a static hydride generation method was used as described by Haas and Feldmann (16) and given in the Supporting Information. Speciation Analysis of Volatile Sn Compounds. Gas samples were taken from the gas bags with a 100 mL gastight syringe and injected into a U-shaped cryotrap, which was subsequently heated to submit the gas to GC separation. Two different gas chromatographic conditions were used which are described in detail in the Supporting Information. Briefly, method A (16, 19) was used for the volatile tin hydrides with a slow heating rate, while in method B used for the semivolatile tetraalkyltins a fast heating rate was used and a higher final temperature employed (20). Less volatile Sn compounds condensed with the water in the gas wells were extracted with pentane prior to direct injection into the GC-ICPMS system. Briefly, 10 mL of condensate was extracted into 2 mL of pentane. The organic layer of the gas condensate was directly injected into the GC-ICPMS system. Chemicals and Tin Standards. All chemicals used were of analytical grade and, if not mentioned otherwise, were obtained from Sigma-Aldrich (Gillingham, U.K.). Three different strategies were used for the synthesis of Sn standards. The starting compounds and the experimental conditions are given in Table S1 and explained in detail in the Supporting Information. Briefly, hydride generation with sodium borohydride (NaBH4) was used to produce volatile tin hydrides. Sodium tetraethylborate (NaBEt4) and sodium tetra(n-propyl)borate (NaB(nPr)4) were used for aequeous alkylation of tin compounds. Grignard reagents (e.g., isopropylmagnesium chloride (iPrMgCl)) were used to synthesize other alkylated Sn compounds. Details can be found in the Supporting Information. In all, 28 different volatile tin compounds were synthesized. Theoretical Methods. All geometries and energies for tin compounds and products resulting from homolytic bond scission were optimized via the B3LYP (21, 22) hybrid density functional theory (DFT) (23) method as implemented in the PC GAMESS (24) software program. Density functional theory is a method which determines a molecule’s electron density to predict molecular properties (23). The electron wave functions were described using a 6-311G(d,p) (25) basis set for all other atoms except Sn, which used an aug-cc-pVTZPP (26) basis set that describes the first 28 core electrons with a relativistic pseudopotential. All homolytic bond dissociation energies (BDEs) are calculated by taking the 944
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difference between the sum of energies for each radical species formed by bond scission and the energy of the parent molecule. For the reaction R-X f R• + •X BDE(R-X) ) ER• + EX• - ERX Throughout the results and discussion BDE refers to ∆H298 for the above reaction. For the reactions of tin compounds with hydroxyl radical (•OH), the geometries and energies for H-atom abstraction pathways (R-H + •OH f R• + H2O) were calculated using the Moller-Plesset second-order perturbation theory (MP2) (27) method with excitations from core electrons fixed and the 6-311G (d,p) (25) basis set for all other atoms except Sn, which used an aug-cc-pVTZ-PP (26) basis set. The MP2 methodsand not the B3LYP methodswas chosen to obtain clear reaction barrier trends because the B3LYP method tends to provide low reaction barrier heights for H-atom transfer reactions and sometimes predicts no reaction barrier (28). The Gaussian 03 (29) software program was used for the MP2 calculations. Vibrational frequency calculations were performed on all geometries to confirm the nature of the geometry. Minima have all real vibrational frequencies and transition states to have one imaginary vibrational frequency corresponding to a maximum on the reaction coordinate. The thermal contributions were calculated using unscaled harmonic vibrational frequencies and rotational constants assuming an ideal gas at 1 atm (30). Zero-point vibrational energies (ZPEs) were not scaled.
Results and Discussion Speciation Analysis of Landfill Gas. Using the low-temperature method (A), tin hydride standards including Me4Sn, MexSnH4-x, and nBuSnH3 were separated by GC-ICPMS (Figure S1, Supporting Information) as shown in previous studies (10, 16, 31). However, the separation power of this method is limited, and higher alkylated species with less volatility cannot be detected; therefore, some tin species proposed in previous work were not unambiguously identified (12). This was the incentive to develop a method for the separation of less volatile tin compounds. Tin-specific detection with ICPMS is a powerful tool to detect and identify volatile Sn species in the complex matrix represented by landfill gas. The multitude of volatile organic compounds present in landfill gas severely hampers compound identification with molecular mass spectrometry (GC-MS), as this technique is not sensitive and selective enough for the identification of the trace amounts of Sn in complex matrixes. However, species identification with GC-ICPMS requires high chromatographic resolution since it is purely based on retention times. Here, this was achieved by using cryofocusing prior to capillary GC coupled to ICPMS via a heated transfer line (method B), which dramatically enhanced the chromatographic resolution and delivered peaks of primary > secondary > tertiary carbons. To explain the lower stability at daytime in the presence of UV irradiation and the scarcity of hydrides in landfill samples, reactions of the Sn species with the •OH radical were modeled for the methylated tin hydrides. The reaction barriers and energies for the Sn-H H-atom abstraction reactions with hydroxyl radical (SnMexH4-x + •OH) were calculated with the MP2 method (Table 3). It is this H-atom that should be most reactive with •OH since the Sn-H bond is much weaker (by ∼110 kJ mol-1) than the C-H bonds. As the degree of methylation is increased, the reaction barrier becomes smaller (from 20 to 11 kJ mol-1). These calculations agree with the experimental data from the atmospheric
stability tests for hydrides (Table 2) and provide a likely reaction mechanism of how UV light exposure decreases the atmospheric stability of higher alkylated tin hydrides. Due to the greater bond strength of C-H bonds (relative to Sn-H bonds), hydrogen abstraction reactions by •OH will not be responsible for the low half-life of (Me)4Sn (Table 2); rather some other mechanism (such as thermal decomposition or photodecomposition) is responsible for nonhydride Sn compounds. While the primary pathway for atmospheric decomposition is via reaction with •OH, the short half-life for tetramethyltin, relative to the hydrides, hints at an active decomposition mechanism other than abstraction reaction via •OH. It has been shown that Me4Sn and ethylated, propylated, and butylated Sn species could be identified in gas generated from two municipal landfills, which make up between 35% and 53% of all volatile Sn compounds in the biogas emitted. The atmospheric half-lives of alkylated Sn compounds under atmospheric conditions in the dark are comparable to those of benzene and toluene. Thus, if landfill gas escapes from the dump, nighttime distribution can cover a vast area and possibly reach populated areas near landfill sites. Although most modern landfills are equipped with gas collection systems, these are never quantitative and usually 20-40% of the landfill gas escapes uncontrolled. How widespread the generation of alkylated Sn species is and whether their generation is restricted to landfill sites needs further investigation.
Acknowledgments We thank the waste management companies for providing us the permission to sample the municipal waste deposits. For financial support we acknowledge the TESLA research funding; N.M. was supported by the EU Erasmus Programme.
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Supporting Information Available Description of the synthesis of the Sn standards and their retention times, details about the modeling, and calculated DBEs and time-dependent recoveries of volatile Sn compounds in moisturized air. This material is available free of charge via the Internet at http://pubs.acs.org.
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ES102512U
