Environ. Sci. Technol. 2008, 42, 5706–5711
Development of a Silicone Membrane Tube Equilibrator for Measuring Partial Pressures of Volatile Organic Compounds in Natural Water ATSUSHI OOKI* AND YOKO YOKOUCHI National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki, 305-0053 Japan
Received April 1, 2008. Revised manuscript received May 12, 2008. Accepted May 14, 2008.
Methods for determining volatile organic compounds (VOCs) in water and air are required so that the VOCs’ fluxes in water environments can be estimated. We developed a silicone membrane tube equilibrator for collecting gas-phase samples containing VOCs at equilibrium with natural water. The equilibrator consists of six silicone tubes housed in a polyvinyl chloride pipe. Equilibrated air samples collected from the equilibrator were analyzed with an automated preconcentration gas chromatography-mass spectrometry system for hourly measurements of VOC partial pressures. The partial pressures of all the target VOCs reached equilibrium within 1 h in the equilibrator. The system was used to determine VOC partial pressures in Lake Kasumigaura, a shallow eutrophic lake with a high concentration of suspended particulate matter (SPM). Compressed air was used daily to remove SPM deposited on the inner wall of the equilibrator and to maintain the equilibrium conditions for more than a week without the need to shut the system down. CH2Br2, CHCl3, CHBrCl2, CH2BrCl, C2H5I, C2Cl4, CH3I, and CH3Br in the lake were supersaturated with respect to the air, whereas CH3Cl was undersaturated. CHCl3 had the highest flux (6.2 nmol m-2 hr-1) during the observation period.
Introduction Various volatile organic compounds (VOCs) found in water are emitted from the ocean surface into the atmosphere. These VOCs include halocarbons (e.g., bromoform, methyl iodide, methyl bromide) (1, 2), sulfur-containing compounds (e.g., dimethyl sulfide, carbonyl sulfide) (3), and hydrocarbons (e.g., isoprene) (4). These compounds are believed to have substantial effects on the atmosphere; for example, they contribute to ozone depletion in the troposphere and the stratosphere by emitting halogens (1), and aerosols derived from these VOCs influence the solar radiation budget (5). Estimates of the air-water flux of VOCs are usually derived from differences between the partial pressures of the VOCs in water and air. Many studies have employed a purge-andtrap method to measure the concentrations of VOCs in water samples (6). With this method, discrete water samples are drawn into a bottle or syringe, hence, applying the purgeand-trap method to an automated online measurement system is difficult. In some studies, liquid-gas equilibrators * Corresponding author tel: +81-29-850-2549; fax: +81-29-8502549; e-mail: Ooki
[email protected]. 5706
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have been used to collect gas-phase sample of VOCs equilibrated with the water sample. The equilibrators are linked to an online measurement system by means of a flowthrough design, which permits automatic measurement of the partial pressures of the soluble gases in the water (pGaswater) (7). There are two types of equilibrators: showerhead equilibrators and membrane tube equilibrators. The former is represented by the Weiss-type equilibrator. In this type of equilibrator (2, 7, 8), a seawater sample is continuously showered through the headspace of a tank, and the gasphase air equilibrates with the seawater. In a membrane tube equilibrator, air is passed through a membrane tube immersed in a liquid sample stream, which allows the exchange of low-molecular-weight compounds across the membrane wall. Groszko and Moore (9) pointed out several benefits of the membrane tube equilibrator: the simple draining and pressure-stabilization system, a relatively compact body, and a reduced potential for contamination in the equilibrator because the air passes through the tube only once. For example, a porous Gore-Tex tube has been used to measure pCO2water (10) and pCH3Brwater (11). Groszko and Moore built a silicone membrane tube equilibrator to determine VOCs (CH3Cl, CH3Br, CH3I, and CFCl3) (9). Considering that VOCs in the liquid phase permeate the silicone membrane by dissolving into the silicone and then evaporate into the gas phase from the surface of the membrane (pervaporation mechanism) (12), the use of a silicone membrane can be expected to eliminate the risk of adsorptive loss of VOCs to the surface of the membrane. In this study, we developed a silicone membrane tube equilibrator for automatic measurement of the partial pressures of various VOCs in natural water, and we used the equilibrator to study the lake-air flux of VOCs in a shallow eutrophic lake.
Experimental Procedures Silicone Membrane Tube Equilibrator. Figure 1shows a schematic of our silicone membrane tube equilibrator. To establish efficient contact between the sample water and the membrane surface, we employed multiple silicone membrane tubes (Fuji System Co., material Q7-4780, 80SH) having an outer diameter of 2.0 mm, a tube wall thickness of 250 µm, and a length of 1, 4, or 10 m. The outlet and inlet of each silicone tube were connected to 1/16 in. stainless steel tubes, which were joined into one pipe. The silicone tubes were housed in a polyvinyl chloride (PVC) pipe with an inner diameter of 16 mm. Pure air was continuously supplied from an air cylinder to the inlet of the equilibrator at a flow rate of 25, 50, or 75 mL min-1. The air pressure was regulated at 0.14 MPa (absolute pressure). Sample water was passed through the PVC pipe at 10 L min-1, which corresponds to a linear speed of about 1 m/s in the pipe. The air collected from the outlet of the equilibrator was drawn into an analytical system. To remove any suspended particulate matter (SPM) deposited on the inner wall of the equilibrator, compressed air was occasionally supplied to the water stream in the equilibrator at a flow rate of about 40 L min-1 for 1 min. Analytical Procedure. An air sample that had passed through the equilibrator was dehumidified by means of a Nafion dryer or with magnesium perchlorate (Mg(ClO4)2) and was transferred to a preconcentration/capillary gas chromatography-mass spectrometry (GC-MS) system developed for automated measurement of atmospheric halocarbons (13). The air sample (750 mL) was collected in a trap containing Carboxene 1000 and Carbopak B cooled to -50 10.1021/es800912j CCC: $40.75
2008 American Chemical Society
Published on Web 06/18/2008
FIGURE 1. Structure of the silicone membrane equilibrator.
FIGURE 2. System for alternating measurement of pVOCair and pVOCwater. °C in a small freezer. Concentrated VOCs in this first trap were thermally (200 °C) desorbed and transferred to a second trap, which contained Tenax TA and Carboxene 1000 cooled to -50 °C. Then the second trap was heated to 200 °C, and the desorbed components were transferred to a capillary column (Porabond Q, 0.32 mm × 50 m) for GC-MS (selectedion monitoring) analysis. Target compounds included CH3Cl, CHClF2 (HCFC-22), CCl3F (CFC-11), CH2Br2, CHCl3, C2Cl4, CH2Cl2, CHBrCl2, CH2BrCl, C2H5I, CH3I, CHBr3, (CH3)2S (DMS), and CH3Br. A gravimetrically prepared standard gas containing CH3Cl, HCFC-22, CFC-11, CH2Br2, CHCl3, C2Cl4, CH2Cl2, CH3I, CHBr3, DMS, and CH3Br at 100-500 ppt (Taiyo Nissan, Inc.) was analyzed for quantification according to the same procedure. To calibrate the concentrations of CHBrCl2, CH2BrCl, and C2H5I, which were not contained in the standard gas, we prepared a liquid standard containing these compounds, along with C2Cl4. We employed the relative ratios of their responses to that of C2Cl4. The detection limits (S/N ) 3) were 0.1-1 ppt for all species. Test for Equilibrium in the Silicone Membrane Tube Equilibrator. In the silicone membrane tube equilibrator, equilibration of the VOCs between the gas phase and the liquid phase occurred in two stages: between the gas phase and the silicone membrane, and between the silicone membrane and the liquid phase. Equilibration of the VOCs between the gas phase and the silicone membrane depended on the residence time of the air flowing through the silicone tube. Therefore, we examined the residence time required to establish equilibrium between the gas phase and the
silicone membrane. We adjusted the residence time by changing the tube length (L ) 1, 4, or 10 m) and the flow rate of the air (R ) 25-75 mL min-1 or 50-75 mL min-1). The combinations of tube lengths and flow rates result in residence times (T) between 8.5 and 25 s (L ) 1 m, R ) 2575 mL min-1), between 34 and 102 s (L ) 4 m, R ) 25-75 mL min-1), and between 127 and 254 s (L ) 10 m, R ) 25-50 mL min-1). The partial pressures of VOCs in the air that passed through the equilibrator having a tube length of 1, 4, or 10 m were measured for 2-4 days while the T was switched between the above time sets every 3 measurements. This measurement was done three times using three different tube lengths of the equilibrator. We measured the partial pressures of HCFC-22, CH2Br2, CHCl3, C2Cl4, CH2Cl2, CHBrCl2, CH2BrCl, C2H5I, CH3I, and CH3Br in these experiments, which were done using water collected from Lake Kasumigaura, as described below. Lake Water Sampling. Lake Kasumigaura, which has a surface area of 220 km2, used to be brackish; however, after the construction of a water gate in 1963, the lake water became fresh. It is a shallow eutrophic lake with a mean depth of 4 m, and it has high biological activity, especially in the summer. Water at a depth of 2 m was collected from an intake tower which is located 150 m offshore, and was intermittently pumped into a tank with a volume of 15 m3 at approximately 2-h intervals keeping the amount of water in the tank more than 10 m3. The untreated water in the tank was supplied to the equilibrator at a flow rate of 10 L min-1 and to other research equipment at a flow rate of approximately 40-70 L min-1. The residence time of water in the tank was approximately 3-5 h. Because we expected the VOC concentrations in the lake water to vary somewhat over time, we measured the partial pressures of VOCs in the lake water for several days to catch the natural variations so that we could distinguish them from variations due to conditioning of the equilibrator. Equilibrium of VOCs between the Liquid Phase and the Gas Phase through the Membrane. We used the following procedure to confirm that the VOCs had equilibrated between the liquid phase and the gas phase through the membrane in the equilibrator. A water sample that had been equilibrated with ambient air was introduced to the equilibrator, and then the partial pressures of VOCs in the gas phase sample collected from the equilibrator (pVOCwater) were compared with those in the ambient air sample (pVOCair). If the gasphase sample from the equilibrator had reached equilibrium with the liquid phase, pVOCwater should have been the same as pVOCair. This experiment was done using the system shown in Figure 2. The water equilibrated with the ambient air was continuously prepared by means of a silicone hollow fiber VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Partial pressures of CH3Cl and HCFC-22 in the gas-phase air collected from the equilibrator: (a) L ) 1 m, T ) 25 s (white bars) and T ) 8.5 s (gray bars); and (b) L ) 4 m, T ) 102 s (white bars) and T ) 34 s (gray bars). membrane module (NAGASEP, Nagayanagi-Kogyo Ltd.) containing 6000 silicone 20-cm tubes (o.d. 0.25 mm; i.d. 0.17 mm). Filtered ambient air was supplied to the hollow fiber module at a flow rate of 10 L min-1, and the water was supplied to the housing of the module at a flow rate of 8 L min-1. The ambient air that had passed through the hollow fiber module was bubbled through water in a bucket. VOCs in the ambient air permeated the silicone of the hollow fiber module and equilibrated between the water and the ambient air. We expected that the silicone hollow fiber module would allow the system to equilibrate efficiently, but measuring the partial pressure of VOCs in natural water is not always appropriate, because the numerous hollow fibers can catch SPM contained in natural water. The water was circulated through the hollow fiber module, the equilibrator, and the bucket for 24 h before the experiment was started. pVOCwater at T ) 254 s and pVOCair were alternately measured by the automated analytical system at 1-h intervals. We quantified the partial pressures of DMS, CFC-11, and CHBr3 in addition to the VOCs measured during the residence time test described above. Since the total pressure of the gas phase in the equilibrator was maintained at 0.14 MPa, the partial pressures of VOCs in the gas phase equilibrated with the water at 0.1 MPa were converted from the partial pressures at 0.14 MPa.
Results and Discussion Residence Time of the Air Flowing in the Silicone Tube. The partial pressures of CH3Cl and HCFC-22 in the gas-phase samples collected from equilibrators with tube lengths of 1 m (T ) 8.5 and 25 s), 4 m (T ) 34 and 102 s), and 10 m (T ) 127 and 254 s) are shown in Figures 3a, 3b, and 4, respectively. It can be seen that those partial pressures discontinuously decreased (or increased) with the decrease (or increase) of residence time from T ) 25 to 8.5 s (or T ) 8.5 to 25 s) (Figure 3a). These results indicate that neither of these VOCs reached equilibrium within 25 s. In contrast, no substantial difference was observed between the partial pressures of CH3Cl at T ) 34 and 102 s (Figure 3b), and there were no differences between the partial pressures of HCFC22 at T ) 127 and 254 s (Figure 4). These results indicate that CH3Cl and HCFC-22 had equilibrated between the silicone and the gas phase by T ) 34 s and T ) 127 s, respectively; at these T values, the partial pressures showed smooth 5708
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variations (which correspond to natural variations). CH2Br2, CHCl3, CHBrCl2, CH2BrCl, C2H5I, C2Cl4, CH2Cl2, CH3I, and CH3Br reached equilibrium within 8.5 s. The difference between the partial pressures of the VOCs at T ) 127 s and T ) 254 s were within approximately 1% of each other which includes natural differences. Therefore, the rates of equilibration were more than 99%. For this equilibrator, a residence time of 254 s was considered to be sufficient for the target VOCs to equilibrate between the gas phase and the silicone. Equilibrium of VOCs between the Liquid Phase and the Gas Phase through the Membrane. The values for pVOCair in ambient air and pVOCwater (T ) 254 s) in water assumed to be equilibrated with the ambient air are shown in Figure 5. The variations of pVOCwater followed those of pVOCair in less than an hour. The high values for pCHBr3water were probably caused by contamination from the stainless steel tubes; this possibility was confirmed by the fact that replacement of the tubes with pipes coated with fused silica (Silicosteel) on December 15 resulted in some improvement. These results indicate that the hollow fiber module could continuously supply water equilibrated with ambient air, and that the equilibrium of VOCs between the liquid phase and the gas phase in the equilibrator was reached within an hour. The Efficiency of Cleaning. We observed high concentrations of SPM (approximately 15 mg L-1) in Lake Kasumigaura in September 2007 during the residence time test. To remove SPM deposited on the inner wall of the equilibrator, compressed air was occasionally supplied to the water stream in the equilibrator at a flow rate of about 40 L min-1 every day for 1 min without interrupting the measurement cycle. This means that SPM was deposited on the inner surface of the equilibrator for 24 h between cleanings. To determine the efficiency of the cleaning procedure, we collected the SPM in the clean-out water on September 14 and 21. In addition, the equilibrator was taken apart and wiped out with a wet sponge after the sequential measurements on both days; we then squeezed the water out of the sponge. It is probable that the large fraction of SPM wiped with the sponge was collected in the squeezed water. The dry weights of SPM in the clean-out water (SPMclean-out) and the wiping water (SPMwipe) were 7800 mg and 90 mg, respectively; therefore, the clean-out efficiency (SPMclean out/[SPMclean-out + SPMwipe])) was 98.9%. Although the cleaning procedure was
FIGURE 4. Partial pressures of CH3Cl, HCFC-22, CH2Br2, CHCl3, CHBrCl2, CH2BrCl, C2H5I, C2Cl4, CH2Cl2, CH3I, and CH3Br in the gas-phase air collected from the equilibrator: L ) 10 m; T ) 254 s (white bars) and T ) 127 s (gray bars). The equilibrator used for 5 days was switched to a new one at 15:00 on September 14. The last two samples on 14 September were obtained with the new equilibrator. highly efficient, small amounts of SPM accumulated in the equilibrator during the sequential measurements. In addition, the surface of the silicone membrane would be coated with a biofilm, and the bacteria in the biofilm may consume or produce halocarbons. In order to study the influence of the accumulated SPM and the biofilm on the pVOCwater measurement, we changed the equilibrator that had been used for the preceding 5 days (with cleaning as usual) to a new one at 15:00 on September 14. There were no discontinuous changes in pVOCwater when the equilibrator was switched (Figure 4), indicating that the bacterial production or consumption of halocabons in the equilibrator would not have serious effects on the measurements. VOC Flux between the Lake and the Air. We used the pVOCwater results for Lake Kasumigaura obtained on September 10-21, 2007 during the residence time test (Figure
4) to calculate the VOC flux between the lake and the air. Lake water that had been stored in the water tank (15 m3) was supplied to the equilibrator. Production and consumption of VOCs by organisms in the tank probably occurred to some extent, but we assumed that the residence time of water in the tank (3-5 h) was short relative to the production and consumption rates of VOCs in the tank. To calculate anomalies between the pVOC values for the lake and the air, we measured partial pressures of VOC in the air (pVOCair) at the shore of Lake Kasumigaura on September 11, 12, 19, and 20, 2007 (n ) 4). Saturation anomalies for the VOCs (SVOC, n ) 4) were calculated with the following equation and are summarized in Table 1: SVOC ) (pVOCwater- pVOCair) ⁄ pVOCair VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
(1)
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TABLE 1. Saturation Anomalies for VOCs between the Lake and the Air
CH3Cl CH2Cl2 HCFC-22 CH2Br2 C2Cl4 CH2BrCl CHCl3 CH3Br CHBrCl2 CH3I C2H5I
mean
range
-0.45 +0.01 +0.02 +1.8 +3.2 +6.0 +6.7 +7.5 +11 +29 +a
-0.42 - -0.51 -0.67 - +0.45 -0.09 - +0.10 +1.3 - +2.7 +0.4 - +6.1 +3.7 - +8.1 +4.1 - +7.9 +6.2 - +11 +6.2 - +22 +19 - +47
a The partial pressure of C2H5I in the air was below the detection limit.
FIGURE 6. VOC species as percentages of total halogens (I, Br, and Cl) exchanged between the lake and the air.
FIGURE 5. Alternating measurements of partial pressures in ambient air, pVOCair (O), and partial pressures in water assumed to have equilibrated with the ambient air, pVOCwater (2). HCFC-22 and CH2Cl2, which are mainly derived from anthropogenic emissions, were nearly equilibrated between the air and the lake on average (SVOC ) +0.02 and +0.01, respectively). In contrast, C2Cl4, the dominant source of which is also anthropogenic, was supersaturated in the lake (SC2Cl4 ) +3.2). The high level of C2Cl4 in the lake was likely to have been caused by an input of C2Cl4 from some additional source other than the air, perhaps a river or from the groundwater. The undersaturation of CH3Cl in the lake water (SCH3Cl ) -0.45) can be explained by chemical or biological decomposition that occurred in the lake. The hydrolysis rate of 5710
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CH3Cl (3 × 10-8 s-1) in natural water (14) is much slower than the biological decomposition rate of 9 × 10-7 s-1 (15). Several studies have indicated that uptake of CH3Cl by bacteria in seawater (15, 16) and lake water (17) decreases pCH3Clwater sufficiently to result in undersaturation. CH3Br is also consumed by bacteria in seawater (16); however, it was supersaturated in Lake Kasumigaura (SCH3Br ) +7.5), which indicates that biological production of CH3Br must have exceeded biological consumption. Biological production of other VOCs (CH2Br2, CHCl3, CHBrCl2, CH2BrCl, C2H5I, and CH3I), of which global sources are known to be mainly biogenic (1), would have raised the partial pressures to supersaturation levels, and made the lake a local source of VOCs. Using pVOCwater and pVOCair, we calculated the air-lake fluxes of VOCs based on the theory of air-sea gas exchange (18). Henry’s law constants of target VOCs (except for CH2BrCl) were obtained from Sander (19), and calculated by means of a linear algebraic equation (20) for CH2BrCl. The wind speed was continuously measured at the roof of the intake tower over the lake approximately 3 m height. Total fluxes of halogens (I, Br, and Cl) derived from VOCs exchanged between the lake and the air are +2.0, +4.0, and +19 nmol m-2 hr-1, respectively. We found that Cl as chloroform was the most abundant halogen emitted from the lake. Percentages of VOC species relative to the total flux of each halogen are shown in Figure 6. CHCl3 accounted for 77% of the total flux of Cl. The flux of CHCl3 per unit area from Lake Kasumigaura, 6.2 nmol m-2 hr-1, was several times the value estimated for the global ocean average (21). The flux of CH3I, 1.9 nmol m-2 hr-1, which accounted for 97% of the total flux of I, was comparable to the flux from the open ocean, 0-2 nmol m-2 hr-1 (2, 6). The percentage of the total flux of Br was shared among several brominated VOCs (CH3Br, CH2BrCl, CHBrCl2, and CH2Br2).
Acknowledgments We thank Dr. K. Matsushige (National Institute for Environmental Studies) for enabling us to conduct the observation at the water research station of Lake Kasumigaura. This work was supported by KAKENHI (18067012) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.
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