Environ. Sci. Technol. 2007, 41, 6205-6209
Positive Artifact Sulfate Formation from SO2 Adsorption in the Silica Gel Sampler Used in NIOSH Method 7903 YU-MEI HSU,† JOSHUA KOLLETT,‡ K A T H E R I N E W Y S O C K I , ‡ C H A N G - Y U W U , * ,† DALE A. LUNDGREN,† AND BRIAN K. BIRKY§ Department of Environmental Engineering Sciences, University of Florida, Gainesville, Florida 32611-6450, Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611-6005, and Florida Institute of Phosphate Research, Bartow, Florida 33830
NIOSH Method 7903, which uses one section of glass fiber filter and two sections of silica gel, has been developed to determine the total concentrations of acid mists in workplace air, although certain gases are suspected to cause interference. In this study, experiments were performed to investigate the roles of sulfur(IV) oxidation and sulfur dioxide (SO2) adsorption in causing artifacts in sulfuric acid measurement. First, sulfur(IV) oxidation, under four combinations of water bath temperature and Na2CO3 solution concentration, was examined to investigate the effect of the extraction process of NIOSH Method 7903. It was shown that sulfur(IV) oxidation to form sulfate could reach 100% within just 2-3 min, following the extraction process of NIOSH Method 7903. The results demonstrate that, using the procedure, SO2 adsorbed by the silica gel and the glass fiber filter easily yields artifact sulfate. Sulfur dioxide adsorption under various flow rates, SO2 concentrations, and sampling times was also investigated. The experimental data were fitted to a deactivation model to determine the adsorption rate constant and the deactivation rate constant. The model can serve as a tool for estimating the artifact sulfate if the SO2 concentration is available.
Introduction NIOSH (National Institute for Occupational Safety and Health) Method 7903 (1) is the approved method set by the Occupational Safety and Health Administration (OSHA) for measuring the total concentration of acidic aerosols and gases, including hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr), nitric acid (HNO3), sulfuric acid (H2SO4), and phosphoric acid (H3PO4). It is the method commonly used by the health and safety staff in the phosphate industry, as well as other occupational environments, such as semiconductor industry, lead battery factories, aluminum smelting, machining, electroplating processes, and * Corresponding author. Phone: (352)392-0845; fax: (352)3923076; e-mail address:
[email protected]. † Department of Environmental Engineering Sciences, University of Florida. ‡ Department of Chemical Engineering, University of Florida. § Florida Institute of Phosphate Research. 10.1021/es070265e CCC: $37.00 Published on Web 08/03/2007
2007 American Chemical Society
even disaster response (2-6). The sampler of NIOSH Method 7903 consists of one section of glass fiber filter plug, followed by two sections of silica gel. The glass fiber filter plug is designed to filter out the majority of aerosols, whereas the silica gel sections are used mainly to adsorb acidic gases. The NIOSH recommended sampling flow rate range is 0.20.5 Lpm (except a rate of 0.92.
TABLE 3. Rate Parameters Obtained Using eq 6 Glass Fiber Filter
Q (Lpm) ksSo (mlpm) kd (min-1) 0.2 0.3 0.4 0.5
0.0254 0.0226 0.0169 0.0160
0.0026 0.0025 0.0025 0.0026
mean stdeva
0.0202 0.0045
0.0026 5.8 × 10-5
a
Silica Gel
R2 0.95 0.93 0.94 0.92
ksSo (mlpm) kd (min-1) 0.0119 0.0114 0.0096 0.0089
0.0012 0.0013 0.0015 0.0014
0.0104 0.0014
0.0014 0.0001
R2 0.81 0.87 0.92 0.81
Standard deviation.
FIGURE 5. Relationship between the sulfate concentrations from the measurement and from the model. The relationship between the measured sulfate concentration from the silica gel tube and the concentration from the model prediction is displayed in Figure 5. Most predictions are in the range of (35% artifact sulfate concentration from the measurement. As demonstrated, the deactivation model gives reasonably good predictions of artifact sulfate concentrations obtained in the silica gel tube. Thus, it can be used for correcting the artifacts in sulfuric acid sampling if the SO2 concentration is available.
Acknowledgments This project is funded by the Florida Institute of Phosphate Research (FIPR), under FIPR No. 05-05-068. Authors are grateful to Dr. Eric Allen for the knowledgeable instruction. Authors J.K. and K.W. are grateful to the Particle Engineering Research Center at University of Florida for the Undergraduate Research Scholarship.
Literature Cited (1) NIOSH Method No. 7903. In NIOSH Manual of Analytical Methods, 4th Edition; National Institute for Occupational Safety and Health (NIOSH): Cincinnati, OH, 1994.
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Received for review February 2, 2007. Revised manuscript received June 6, 2007. Accepted July 2, 2007. ES070265E
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