Environ. Sci. Technol. 1997, 31, 2125-2129
Occurrence of Volatile Transition Metal Compounds in Landfill Gas: Synthesis of Molybdenum and Tungsten Carbonyls in the Environment JO ¨ RG FELDMANN AND WILLIAM R. CULLEN* Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada
Evidence for the occurrence of volatile molybdenum and tungsten compounds in the environment is presented for the first time. The gases from three different municipal waste deposits were sampled and analyzed for volatile metal and metalloid compounds by using gas chromatography coupled with inductively-coupled plasma mass spectrometry (GC-ICP-MS). In addition to the known hydrides and methylated compounds of As, Se, Sn, Sb, Te, Hg, Pb and Bi, volatile Mo and W compounds were found in concentrations of about 0.2-0.3 µg of Mo/m3 and 0.005-0.01 µg of W/m3. The isotopic fingerprint of the detected Mo and W from the samples matched perfectly with Mo and W standards. The correspondence of the samples’ retention times (GCICP-MS) with those of standards provides convincing evidence that Mo(CO)6 and W(CO)6 are present in landfill gas. The toxicity and origin of these compounds are discussed.
Introduction The volatilization pathway of metal and metalloids in the environment is reasonably well known for elements such as mercury (1), selenium (2), and arsenic (3, 4). These elements can form stable dimethylselenide, dimethylmercury, trimethylarsine, or other volatile inorganic species such as elemental mercury. Reports are also available about volatile methylated compounds of other elements such tin (5) and cadmium (6). In previous studies (7, 8), we found volatile compounds of the elements antimony (trimethylstibine), bismuth (trimethylbismuthine), and tellurium (dimethyltellurium). In addition to the methylated compounds, metal(loid) hydrides have been found in the environment, arsine is formed by rumen bacteria (4), and both arsine and stibine are produced by the process of charging batteries, (9, 10). Stannane (11) and phosphine (12) can also be formed under anaerobic conditions. In addition to the naturally formed volatile metal compounds, industrial products such as the gasoline antiknock additive tetraalkyllead are spread into the environment (13). However, there are no reports of the occurrence of volatile transition metal compounds in the environment other than the possibility of volatile nickel compounds being present in cigarette smoke (14), in carbon monoxide containing industrial hydrogen or nitrogen gas (15), in automobile exhaust (16), and in urban air (17). The industrial production of tetracarbonyl nickel (Mond process) is well documented (18). The objective of this study * Corresponding author fax: +604-822-2847; e-mail: wrc@chem. ubc.ca.
S0013-936X(96)00952-2 CCC: $14.00
1997 American Chemical Society
was to seek and identify any volatile transition metal compounds present in landfill gases by using gas chromatography coupled with inductively-coupled plasma mass spectrometry (GC-ICP-MS).
Experimental Section Sampling Sites. Gas samples were collected from three different municipal waste deposits that are located in British Columbia (Canada). Municipal garbage from the Greater Vancouver Regional District (GVRD) was dumped on all sites, but at different times. One site is located in Delta (Burns Bog, no. 1), where most of the municipal and industrial waste from the Vancouver area is currently dumped. The waste dumped in the second landfill (no. 2) represents the waste from the same area at least 30 years ago. The third sampling site (no. 3) is a landfill that was closed 8 years ago. Sampling sites 2 and 3 are located in North Vancouver (Premier’s Street). The gases were analyzed for the major components methane and carbon dioxide by using an infrared gas analyzer that was coupled with a galvanic cell for oxygen (Landtec Gem 500). Sampling Procedure. The gases from all three municipal waste deposits are collected into gas wells and pumped in pipelines either to a furnace or to a power station. The gases in the gas wells were sampled directly into Tedlar bags by using a membrane pump (AirPro 6000D, Bios Intrument Corp, NJ). Even when the gas is pumped out of the landfill, some gas can easily reach the surface of the landfill after migration through the waste and soil. The gas that migrated through the landfill was visibly bubbling through a rainwater puddle on the surface of the landfill. This stream was sampled by using a collecting device in a form of a cylinder (Plexiglas). A Teflon tube (o.d. 6 mm) was attached to the valve on the very top of this cylinder (i.d. 30 cm, height 10 cm). A styrofoam ring on the outside of the cylinder was used to float this device, and ropes held it in place on the water surface to collect the gas, which bubbled through the water. Basically the gas was collected under this device and pumped directly into a Tedlar bag. The flow rate of the membrane pump was adjusted to the same flow rate of the gases being generated under the sampling device by observing a constant water level in the Plexiglas cylinder during the sampling time. Ambient air above the landfill surface (0.5 m) was also sampled with the use of a Teflon pipe (o.d. 6 mm), the membrane pump, and a Tedlar bag (80 L). The flow rate was adjusted to 1 L/min. The volume of the samples varied from 4 L for the landfill gas to 80 L for the ambient air samples. The sampling bags were covered in black plastic bags to avoid the influence of UV light on the samples. The analysis of the samples was completed within 48 h of collection. Preconcentration. The gas samples were cryogenically preconcentrated by trapping the gases on Chromosorb (10% SP-2100 60/80 mesh, Supelco) at -78 °C (dry ice/acetone slush). This relatively high temperature was chosen to avoid condensation of carbon dioxide and methane, the major components of landfill gas (19). In a cryofocusing step, the volatile species were volatilized by increasing the temperature of the trap from -78 to 150 °C, and the released gas was frozen (liquid nitrogen) onto a second U-shaped trap (6 mm o.d., 31 cm length), which was packed with Chromosorb (10% SP-2100 45/60 mesh Supelco). The schematic diagram for the analytical setup is shown in Figure 1. Analytical Procedure. No cleanup procedure or derivatization was performed on the gas samples in order to avoid change in the molecular structure of volatile metal com-
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FIGURE 1. Analytical setup for GC-ICP-MS, which include a hydride generation system for the derivatization of liquid samples and the sampling and preconcentration apparatus for gaseous samples. F1 and F2 are gas flow meters. COL1 and COL2 are the columns. V1, V2, and V3 are three- and six-way valves. pounds. The analytical procedure applied was a combination of thermodesorption of the cryotrapped sample and separation by using a nonpolar chromatographic column. The column was heated from -196 to 150 °C within 3 min, and the gases were separated by using a He flow of 133 mL/min. The separated sample was transported through a heated Teflon transfer line (i.d. 0.3 mm) to the torch of the ICP-MS. In addition, an aqueous solution was introduced as a wet aerosol into the plasma by using a nebulizer. Both gas flows were mixed together in a tee-piece (o.d. 6 mm) inserted between the spray chamber and the torch, replacing the quartz elbow usually in this position. The operating parameters are shown in Table 1. In a screening analysis, the mass range between m/z 48 and m/z 209 was scanned by using 320 µs as the dwell time per channel. The following isotopes were detected in the peak hopping mode (20 ms dwell time per channel) for the determination of the isotopic fingerprint of the gas samples: 50Cr, 52Cr, 54Cr/Fe, 55Mn, 56Fe, 57Fe, 58Ni, 60Ni, 62Ni, 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, 100Mo, 103Rh, 120Sn, 121Sb, 180W, 182W, 183W, 184W, and 186W. However, for quantification only 52Cr, 53Cr, 57Fe, 58Ni, 59Co, 60Ni, 98Mo, 103Rh, and 184W were measured in order to enhance sensitivity.
Results and Discussion The gases from sites 1 and 2 were anaerobic (
