Annals of Occupational Hygiene Advance Access originally published online on October 10, 2006
Annals of Occupational Hygiene 2007 51(1):81-89; doi:10.1093/annhyg/mel066
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Size-Resolved Sulfuric Acid Mist Concentrations At Phosphate Fertilizer Manufacturing Facilities In Florida
1 University of Florida, Environmental Engineering Sciences Gainesville, FL, USA
2 Florida Institute of Phosphate Research, Bartow FL, USA
*Author to whom correspondence should be addressed. E-mail: cywu{at}ufl.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTS AND DISCUSSIONSCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Strong inorganic acid mists containing sulfuric acid were identified as a ‘known human carcinogen’ in a National Toxicology Program (NTP) report where phosphate fertilizer manufacture was listed as one of many occupational exposures to strong acids. To properly assess the occupational exposure to sulfuric acid mists in modern facilities, approved National Institute for Occupational Safety and Health (NIOSH) Method 7903 and a cascade impactor were used for measuring the total sulfuric acid mist concentration and size-resolved sulfuric acid mist concentration, respectively. Sampling was conducted at eight phosphate fertilizer plants and two background sites in Florida and there were 24 sampling sites in these plants. Samples were analyzed by ion chromatography (IC) to quantify the water-soluble ion species. The highest sulfuric acid concentrations by the cascade impactor were obtained at the sulfuric acid pump tank area. When high aerosol mass concentrations (100 µg m–3) were observed at this area, the sulfuric acid mists were in the coarse mode. The geometric mean sulfuric acid concentrations (±geometric standard deviation) of PM23 (aerodynamic cut size smaller than 23 µm), PM10 and PM2.5 from the cascade impactor were 41.7 (±5.5), 37.9 (±5.8) and 22.1 (±4.5) µg m–3, respectively. The geometric mean (±geometric standard deviation) for total sulfuric acid concentration from the NIOSH method samples was 143 (±5.08) µg m–3. Sulfuric acid mist concentrations varied significantly among the plants and even at the same location. The measurements by the NIOSH method were 1.5–229 times higher than those by the cascade impactor. Moreover, using the NIOSH method, the sulfuric acid concentrations measured at the lower flow rate (0.30 Lpm) were higher than those at the higher flow rate (0.45 Lpm). One possible reason for the significant differences between the results from the cascade impactor and the NIOSH method is the potential artifact resulting from the interaction of SO2 with silica gel and glass fiber used in the NIOSH method.
Keywords: sulfuric acid mist • phosphate fertilizer facilities • NIOSH method 7903 • cascade impactor
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Strong inorganic acid mists containing sulfuric acid (H2SO4) have been reported to correlate with lung and laryngeal cancer in humans (Blair and Kazerouni, 1997; Sathiakumar et al., 1997; Steenland, 1997) and are identified as a human carcinogen as reported by the National Toxicology Program (NTP) (USDHHS, 2005). Sulfuric acid is typically present in the air in mist form. Its chemical characteristics include low volatility, high acidity, high reactivity, high corrosivity and high affinity for water.
Phosphate fertilizer manufacture is listed in the NTP report as one of the industries that has sulfuric acid mist exposure potential. The Occupational Safety & Health Administration (OSHA) has established an 8 h Time-Weighted Average (TWA) of Permissible Exposure Limit (PEL) of sulfuric acid mist at 1 mg m–3. It is well known that the deposition of an aerosol in the respiratory system depends on its aerodynamic behavior. Considering the effects of aerosol size, the American Conference of Governmental Industrial Hygienists (ACGIH) has adopted a Threshold Limit Value-Time Weighted Average (TLV-TWA) of 0.2 mg m–3 for the thoracic particulate fraction of sulfuric acid mist (ACGIH, 2004).
National Institute for Occupational Safety and Health (NIOSH) Method 7903 is an OSHA-approved method that is commonly used by the health and safety staff in industries to measure the total sulfuric acid mist concentration. The sampler of NIOSH Method 7903 is a silica gel tube consisting of one section of glass fiber filter plug followed by two sections of silica gel (commercially available: ORBO-53 tube, Supelco, and SKC silica gel tube, SKC). The glass fiber filter plug is designed to filter out the majority of acid aerosols while the silica gel sections are used mainly to adsorb acid gases. The collected samples are desorbed in eluent and the aliquots are analyzed by ion chromatography (IC). NIOSH researchers who developed the method reported 
90% collection efficiency for acidic aerosols with 0.4 µm volume median diameter (94.8 ± 4.8% for H3PO4 and 86 ± 4.6% for H2SO4) when the samples collected on the glass fiber section and the front silica gel section (400 mg) were combined (Cassinelli and Taylor, 1981; Cassinelli, 1986).
Cascade impactors are commonly used for characterizing aerosol size distribution (Swietlicki et al., 1997; Dibb et al., 2002). Large particles are collected on a substrate by inertial impaction, while small particles can better follow the changes in the flow direction of a curved air stream. By adopting a series of impactor stages with increasing flow velocities, the aerosol size distribution can be classified.
The approved NIOSH method only provides total sulfuric acid mist concentration, but not size-dependent information. The comparison of the total mist concentration with the size-fractionated measurement by the cascade impactor may provide a convenient tool for correlating exposure levels based on the simpler NIOSH method. This information can be applied to develop informed policies with respect to respiratory protection in the workplace.
To properly assess the occupational exposure of workers to sulfuric acid mist in phosphate fertilizer manufacturing facilities, the objectives of this study were to determine the total sulfuric acid mist concentrations using the approved NIOSH method and to characterize the size distributions of sulfuric acid mist by cascade impactor sampling. Furthermore, the feasibility of using a correlation factor between these two measurements was examined.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Sampling sites
The final products of phosphate fertilizer facilities are mono-ammonium phosphate (MAP), di-ammonium phosphate (DAP), phosphoric acid and sulfuric acid. The manufacturing processes have been described in Hsu et al. (2006). Five types of locations with the potential of H2SO4 exposure corresponding to the manufacturing process were selected for sampling. These locations (the number of the sampling sites) include the sulfuric acid pump tank area (9), rotating table/belt filter floor (9), attack tank area (2), truck station for loading/unloading sulfuric acid (3), and the granulator on a "scrub" day (1). Sampling was carried out at eight plants. Seven of them are located in central Florida, and one of them is located in north Florida. In addition, Gainesville, FL, and Winter Haven, FL, were chosen as the background sites.
Sampling and analysis
A University of Washington Source Test cascade impactor (Mark III) was used as an area sampler to sample mist aerosols for the size distribution. The inlet of the cascade impactor was set at 1.5 m from the floor. The impactor was operated at 25 Lpm with aerodynamic cut sizes (d50) of 0.20, 0.48, 0.98, 1.8, 3.8, 10 and 23 µm for the seven stages. The impactor with glass fiber filter can provide high collection efficiency for aerosols; however, glass fiber filter is well known to adsorb acidic gas, such as sulfur dioxide (Chow, 1995; Lee and Mukund, 2001). Therefore, Teflon membrane filters (ZeflourTM, 8'' x10'', pore size: 2 µm, Pall Corp., Ann Arbor, MI)that provide high collection efficiency and low reactivity with acidic gases (Chow, 1995; Lee and Mukund, 2001) were applied for the collection substrate. Those filters were cut to fit onto the impactor stages. The collection efficiencies of the cascade impactor for liquid (substrate: a glass fiber filter) and solid aerosols (substrate: an aluminum foil with silicone coating) were 97.2 ± 11.9 and 94.1 ± 17.3%, respectively (Pauluhn, 2005). Droplet break-up in this instrument is negligible for large droplets (10 and 25 µm) even when a high sampling flow rate (28.3 Lpm) is applied (Horton and Mitchell, 1989). A final Teflon filter (ZefluorTM, 47 mm, pore size: 2 µm, Pall Corp., Ann Arbor, MI) was placed after the impactor to collect penetrating aerosols. Filters were placed in a desiccator at room temperature for pre- and post-conditioning for 24 h before weighing to reduce the effect of water collected by the filter. The vapor pressure of sulfuric acid mist is low and it remains in the mist form under these conditions. The lower limit of particle size collected on the final filter was assigned to be 0.03 µm, a typical value of those employed in other research studies (Divita et al., 1996; Howell et al., 1998; Wagner and Leith, 2001). The aerodynamic diameters of collected particles were from 0.03 to 23 µm. PM23 is the aerosol mass concentration with an aerodynamic diameter smaller than 23 µm, which was the largest aerosol size collected using this methodology. PM23 sulfuric acid mist concentration was used to compare the total sulfuric acid mist concentration provided by NIOSH Method 7903.
NIOSH Method 7903 was applied for the sampling of total sulfuric acid mist concentration using a commercially available silica gel tube (ORBO 53 tube, Supelco). The sampling flow rate was 0.45 Lpm for 72 sets of samples. Six sets of samples were also collected at 0.3 Lpm in order to verify whether the results were the same at different flow rates in the recommended range (0.2–0.5 Lpm) (Cassinelli and Eller, 1979; Cassinelli and Taylor, 1980; Cassinelli, 1986; NIOSH, 1994).
Gravimetric measurement for sample mass was carried out using a micro-balance (Model MC 210 S, Sartorius Corp., Edgewood, NY; readability— 10 µg), and the analysis of sulfate concentration was accomplished by using an IC system (Dionex ICS 1500, Dionex Corp., Atlanta, GA). The analytical columns of anion species [nitrate (
), sulfate (
), fluoride (F–), phosphate (
), and chloride (Cl–)] and cation species [potassium (K+), calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and ammonium (
)] are IonPac AS9-HC (Dionex Corp., Atlanta, GA) and IonPac CS12A (Dionex Corp., Atlanta, GA), respectively. The detection limit for sulfate was determined to be 0.12 ppm for this system.
The sampling time was 24 h and three successive samples were obtained for each sampler at each site. Totally, there were 72 sets of impactor samples and 78 silica gel tube samples in those plants. In background locations, there were six sets of impactor samples and six silica gel tube samples.
Calculation of PM2.5
Because 2.5 µm was not one of the cascade impactor cut-sizes, PM2.5 was determined by interpolating the size bin that covers 2.5 µm (i.e. 1.81 and 3.76 µm). Assuming a uniform distribution in this size range in log-scale, PM2.5 can be obtained according to the following relationship:
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Rearranging the formula, PM2.5 can be derived as:
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Calculation of sulfuric acid mist concentration
According to NIOSH Method 7903, sulfuric acid mist concentration is converted from the sulfate concentration determined by IC. Although the sulfate may not necessarily come from sulfuric acid (i.e. it can be ammonium sulfate or calcium sulfate), any sulfate determined by this method is conservatively assumed to be sulfuric acid. In this study, the same protocol was followed for all samples from the fertilizer plants.
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RESULTS AND DISCUSSIONS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Background site
In general, the mass concentrations and sulfate concentrations were low at both background locations. The PM23, PM10 and PM2.5 were 20.6–53.2, 18.7–36.9 and 15.1–26.4 µg m–3 in Gainesville, and 19.1–27.0, 16.3–27.0 and 11.5–22.8 µg m–3 in Winter Haven. The corresponding sulfate concentrations were 5.4–8.6 (PM23), 5.2–7.1 (PM10) and 5.0–6.3 (PM2.5) µg m–3 in Gainesville, and they were lower in Winter Haven: 3.0–3.2 (PM23), 2.7–2.8 (PM10) and 2.4–2.5 (PM2.5) µg m–3. The ratios of sulfate concentration to the sum of all ionic species concentrations (
, , F–, , Cl–, K+, Ca2+, Mg2+, Na+, and ) were 0.44–0.46 in Gainesville and 0.16–0.39 in Winter Haven. For NIOSH method samples, the total sulfate concentrations ranged from 6.81 to 10.5 µg m–3 in Gainesville and much higher in Winter Haven, 31.2–46.0 µg m–3. Compared to the cascade impactor results, the measurements were closer in Gainesville than those in Winter Haven. The ratio of sulfate from the impactor to sulfate from the NIOSH method sampler ranged from 0.67 to 0.82 in Gainesville and from 0.069 to 0.096 in Winter Haven. It should also be noted that while the impactor results showed higher sulfate concentrations in Gainesville than in Winter Haven, the NIOSH method measurements showed the opposite. It is suspected that the NIOSH Method 7903 might have interferences from SO2 gas. This will be further discussed in later sections.
Plants: cascade impactor samples
The sampling results of the cascade impactor at all locations are shown in Fig. 1a. The highest median sulfuric acid mist concentration was observed at the sulfuric acid pump tank areas where two sulfuric acid mist concentrations from the cascade impactor were higher than the OSHA standard, 1 mg m–3. The size information at each type of location will be discussed as follows.
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(i) Attack tank area
Sampling for the attack tank area was carried out at two plants. The aerosol and sulfuric acid PM23, PM10 and PM2.5 mass concentrations are listed in Table 1. Aerosols at the attack tank areas had high mass loadings but low sulfuric acid concentrations. The violent reaction in the attack tank releases heat and causes a significant amount of volatile species to evaporate. These species, such as fluoride gases, condense on existing aerosols when they encounter cooler ambient air, thus resulting in high aerosol mass concentrations. However, the temperature in the process was not high enough for the evaporation of sulfuric acid or sulfate salt that has lower vapor pressure. Hence, sulfuric acid mist concentrations were low at this location.
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(ii) Sulfuric acid pump tank area
Sampling at the sulfuric acid pump tank area was carried out at all eight plants. The PM23, PM10 and PM2.5 mass concentrations are listed in Table 2. The geometric mean PM23, PM10 and PM2.5 sulfuric acid concentrations (±geometric standard deviation) were 41.7 (±5.5), 37.9 (±5.8), 22.1 (±4.5) µg m–3. The highest geometric mean sulfuric acid concentration from cascade impactor measurement among all types of locations was indeed observed at the pump tank area. The large geometric standard deviation implies that the sulfuric acid concentrations at these nine sulfuric acid pump tank areas differed greatly. The geometric mean ratios of sulfate concentration to all ionic species were greater than 0.50, which indicated that sulfate was the predominant ion accounting for the aerosol mass at this type of location.
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The aerosol mass size distributions and sulfuric acid size distributions are shown in Fig. 2. They are plotted in two ranges: larger than and smaller than 100 µg m–3. The size distribution maintained the same pattern at a given site, but not from site to site. It should also be noted that most of the sulfuric acid size distributions resemble the aerosol mass size distributions at the same site. At plants B1, D, H and B2, Fig. 2a, both the aerosol mass concentrations and sulfuric acid mass concentrations were higher than 100 µg m–3, and the aerosols were predominantly supermicron particles. The sulfate/aerosol mass ratios were greater than 0.5 (Table 3). The high ratios indicate that sulfuric acid was the major species and that these locations might have sulfuric acid emission sources. At the pump tank area, the possible sulfuric acid emission source is the leakage of SO3 that will quickly combine with water molecules to form H2SO4.
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At other plants, Fig. 2b, the sulfuric acid concentrations were lower than 100 µg m–3 and their sulfuric acid aerosols were mainly in the submicron range or presented no specific pattern. In the case of a very low aerosol mass loading, the sulfuric acid aerosols could very likely be affected by the ambient aerosols at this outdoor location. The geometric mean sulfuric acid concentration at Plant F was 6.8 µg m–3 (Table 3), which is as low as that at the background sites.
(iii) Belt or rotating table filter floor
Sampling at the belt or rotating table filter floor area was carried out at all eight plants. The aerosol mass concentrations were high: the PM23, PM10 and PM2.5 mass concentrations ranged from 57.4 to 2535, 54.0–1857 and 28.3–358 µg m–3, respectively; their geometric mean concentrations (±geometric standard deviation) were 225.3 (±2.3), 187.0 (±2.2) and 94.7 (±1.8) µg m–3, respectively. PM23, PM10 and PM2.5 sulfuric acid concentrations were 7.1–575, 4.9–419 and 2.4–60.0 µg m–3; the geometric mean sulfuric acid concentrations (±geometric standard deviation) of PM23, PM10 and PM2.5 were 17.9 (±2.7), 14.4 (±2.7) and 6.6 (±2.1) µg m–3.
The ratios of sulfate to all ions ranged from 0.07 to 0.32 (geometric mean: 0.16). The low ratios indicate that sulfuric acid was not a major compound at the filter floor area. The fractional size distributions of aerosol mass and sulfuric acid at nine belt/rotating table filter floors are shown in Fig. 2c. Sulfuric acid was dominantly present in the coarse mode. During this process, gypsum is filtered out by belt or rotating table filter, and the aerosols are formed by mechanical agitation. The similarity in sulfuric acid and aerosol mass size distributions indicates that the chemical might be from the residual content in the product.
(iv) Sulfuric acid truck loading/unloading station
Sampling at the truck loading/unloading station was conducted at three plants. The aerosol mass concentrations were low and their PM23, PM10 and PM2.5 ranged from 19.9 to 174, 15.8–131 and 10.0–69.7 µg m–3. Sulfuric acid was the major species (the ratios of sulfate to total ion mass: 0.28–0.42, median: 0.36); the concentrations ranged from 3.9 to 24.5 (PM23), 3.5–23.9 (PM10) and 3.1–20.6 (PM2.5) µg m–3, which were close to the measurements at the background locations. All size distributions of sulfuric acid were similar: the mode size was 0.48–0.98 µm. During loading and unloading, sulfuric acid is transferred from the storage tank in liquid form to the truck. The only possible pathway that workers can be exposed to sulfuric acid is the spray of sulfuric acid from the truck loading nozzle. Normally, the connection is well sealed and the workers are well protected to avoid contact with liquid sulfuric acid. The measurements verified that the sulfuric acid concentrations were very low.
(v) Granulator on a scrub day
The mass concentrations ranged from 126–835 (PM23), 90.2–578 (PM10) and 59.7–303 (PM2.5) µg m–3. The sulfuric acid concentration ranges were 7.63–87.9 (PM23), 5.73–59.7 (PM10) and 3.85–51.6 (PM2.5) µg m–3. The source of sulfuric acid mists is the spray from the scrubbing process (a weak acid solution) which is not a steady operation. Hence, the concentrations varied significantly, but they were all below the standards.
Plants: NIOSH method samples
Sampling results of the NIOSH samples at five types of locations are shown in Fig. 1b. Generally, the sulfuric acid pump tank area had the higher concentrations. However, the maximum sulfuric acid concentration (11225 µg m–3) measured by the NIOSH method for all locations was obtained at the filter floor area. The largest geometric mean sulfuric acid concentration was obtained at the pump tank area (143.2 µg m–3), followed by the granulator on a scrub day (122.4 µg m–3) and then at the filter floor (108.7 µg m–3). The geometric mean concentrations at the granulator on a scrub day and sulfuric acid truck loading/unloading stations were at lower levels.
Comparisons of the results from the cascade impactor and the NIOSH method
The paired measurements of PM23 sulfuric acid concentrations from the cascade impactor and total sulfuric acid concentrations from the NIOSH method at all sampling locations are shown in Fig. 3. As shown, 71 out of 72 impactor samples had a lower concentration than the NIOSH method measurement. The ratios of sulfuric acid mist concentrations from the NIOSH to the cascade impactor were 1.5–229.0 for 71 impactor samples. The largest difference was over two orders of magnitude, and many of the NIOSH measurements were more than 10 times larger the impactor results. The substantial difference was quite unexpected. To quantitatively compare these two measurements and evaluate their correlation, three ratios were used which are defined as:
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Table 4 displays the R23, R10 and R2.5 at five types of sampling locations. A large variation at each type of location is observed, e.g. from 0.00 to 0.67 at the filter floor areas for R23. A strong relationship between methods would be indicated by a constant correlation factor; the wide variation of the ratios does not allow any meaningful correlation of these two types of measurements. The much higher values by the NIOSH method also prompted further investigation of the data.
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In examining the data, it was found that in many cases the silica gel sections collected more sulfuric acid than the glass fiber section (see examples in Table 5). The results of the NIOSH method imply that sulfuric acid as well as sulfate is partially measured as a gas. In the ambient condition, there is no known sulfate species that exists in the gas phase. Even sulfuric acid exists in the condensed phase because it has a high boiling point of 327°C and a very low vapor pressure at room temperature (<0.001 mmHg) (Weast, 1988). Furthermore, the hygroscopic sulfuric acid will rapidly pick up moisture in the air and remain in the aerosol phase. It should be noted again that according to Ortiz and Fairchild (1976) the majority of the aerosol mass is collected on the glass fiber filter plug. Chen et al. (2002) reported that aerosol penetration across the filter section of the silica gel tube (SKC 226-10-03 tube) at the most penetrating size was lower than 5%. The collection efficiency of large particles (>3 µm) is higher than 99%. Sulfuric acid mists mainly exist as coarse aerosols at the pump tank area (Fig. 2a); hence, aerosol penetration cannot explain the high sulfuric acid concentration sampled by silica gel. Thus, the situation that more sulfuric acid mist is collected in the silica gel section than the glass fiber section is highly unlikely. The adsorption of a significant amount of interfering gases by the silica gel is therefore suspected to be the reason for the observed phenomenon.
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Another unexpected phenomenon was observed when comparing the results obtained from the NIOSH method at different sampling flow rates. In the recommended range of sampling flow rate (0.2–0.5 Lpm), concentrations should be similar when different flow rates are used. Two different flow rates, 0.3 and 0.45 Lpm, were employed for six sets of the NIOSH method sampling, and the results are shown in Table 5. These paired samples were taken concurrently (e.g. #1-low and #1-high were taken at the same time), and three consecutive samples were carried out (i.e. #1 followed by #2 and then by #3). As shown, sulfuric acid concentrations at the lower flow rate (0.3 Lpm) were higher than those at the higher flow rate (0.45 Lpm). Most of the ratios (C@ 0.3 Lpm/C@ 0.45 Lpm) were larger than 1, and they were much higher in the silica gel section than those in the glass tube section. The concentrations at two different flow rates do not exhibit any direct proportion between the gas phase and the particulate phase. Hence, systemic errors can be neglected. The collection mechanism of acid gases on the silica gel is diffusion, and its efficiency decreases as the flow rate increases (due to shorter residence time). The observations support the hypothesis that the higher measurement in the silica gel section is probably from the collection of gas.
Silica gel, a high surface area material, can adsorb SO2 (Stratmann and Buck, 1965; Kopac and Kocabas, 2002). The hydrophilic property of silica gel can effectively attract moisture which can enhance the absorption of soluble species, such as SO2 (Tsai et al., 2001). The adsorbed or absorbed SO2 can be further oxidized to form sulfate (Lunsford, 1979) that causes overestimation.
Annual SO2 concentrations (2003) were monitored by the state of Florida and the results indicated that central FL had higher SO2 concentrations (2–6 ppb) than north FL (2–3 ppb) (FDEP, 2003). If the above hypothesis is true, this might explain why sulfate concentrations in Winter Haven measured by the NIOSH method were much higher than those in Gainesville but the impactor results did not exhibit such a pattern.
Limited SO2 concentrations in fertilizer facilities are available in the literature. The SO2 concentration in a fertilizer factory in India was 41.7 mg m–3 (Yadav and Kaushik, 1996) while those in China and Sweden were 0.34–11.97 and 3.6 mg m–3, respectively (Englander et al., 1988; Meng and Zhang, 1990). Atmospheric dispersion can quickly reduce the concentration to a much lower level as reported in a study near a fertilizer factory in Zimbabwe (Jonnalagadda et al., 1991). If the hypothesis holds true, the sulfuric acid concentration in the fertilizer facilities can be expected to be overestimated when SO2 is present in the studied facilities. Further investigation of this subject is warranted.
Comparisons of sulfuric acid mist concentrations with the OSHA and the ACGIH regulations
The number of samples with concentrations higher than the OSHA regulation (total sulfuric acid mist concentration <1 mg m–3) was 7 of the 78 samples collected. According to the cascade impactor sampling, 90% of the samples were lower than the ACGIH standard and 97% of the samples were lower than the OSHA regulation. The results of the NIOSH method samples obtained concurrently with the impactor samples had a smaller percentage of samples with concentrations lower than the OSHA regulation. The only location where the sulfuric acid mist concentrations from the cascade impactor exceeded both the OSHA and ACGIH standards was the sulfuric acid pump tank area. For the NIOSH method samples, the locations included sulfuric acid pump tank areas, belt/rotating table filter floors and the granulator on a scrub day.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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In this study, the total sulfuric acid mist concentration and sulfuric acid mist size distributions at modern phosphate fertilizer manufacturing facilities in Florida were characterized by using NIOSH Method 7903 and a cascade impactor, respectively. Sampling was carried out at five types of locations in eight facilities and two background sites.
Based on cascade impactor sampling, sulfuric acid pump tank areas had higher sulfuric acid mist concentrations than other types of locations and sulfuric acid was the dominant chemical species. When high sulfuric acid concentrations were identified, the aerosols were dominantly in the coarse mode. The most likely cause of high sulfuric acid concentrations at this location is the leakage of SO3. Constant inspection of the tubing around this location and immediate repair may provide an effective measure to further lower the concentrations. According to the impactor sampling results, seven samples (total: 72) exceeded the ACGIH standard (0.2 mg m–3, thoracic fraction), and two samples (total: 72) were above the OSHA regulation (1 mg m–3, total concentration). Meanwhile, there were seven samples (total: 78) by the NIOSH method that exceeded the OSHA regulation. It should be noted at these locations, workers typically stay for about 1–2 h per day and respirators for sulfuric acid mist are required in this area. Consequently, the real time-weighted exposure to sulfuric acid mist is likely to be lower than the concentrations from the stationary sampling conducted in this study.
The results from the cascade impactor and the NIOSH method were compared to determine if a correlation factor could be established. The sulfuric acid mist concentrations from the NIOSH method were higher than those from the cascade impactor for the dominating majority of samples. No specific trend of systemic error was observed between these two methods. In many cases, the silica gel section collected more ‘‘sulfuric acid’’ than the glass fiber filter plug. This is highly unlikely because sulfuric acid or sulfates are not known to exist in the gas form in the ambient condition, and should not be collected in the silica gel section. Moreover, the sulfuric acid concentrations at 0.30 Lpm were higher than the concentrations at 0.45 Lpm in the NIOSH method sampling, indicating the influence of diffusing gases. The possible reason for the variation is the interaction between SO2 and silica gel/glass fiber. Further investigation is recommended to verify the causes.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The authors greatly appreciate the efforts of Cheng-Chuan Wang and the environmental/safety staff at fertilizer plants who helped carry out the sampling. The authors are also grateful to the insightful knowledge provided by Mr Alan Pratt at CF Industries, Inc. This project is funded by the Florida Institute of Phosphate Research under FIPR# 04-05-066.
Received March 14, 2006; in final form July 31, 2006
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