Article pubs.acs.org/est
Characterizing Gas-Particle Interactions of Phthalate Plasticizer Emitted from Vinyl Flooring Jennifer L. Benning,*,† Zhe Liu,‡ Andrea Tiwari,‡ John C. Little,‡ and Linsey C. Marr‡ †
Department of Civil and Environmental Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, United States ‡ Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, Virginia 24061, United States S Supporting Information *
ABSTRACT: Phthalates are widely used as plasticizers, and improved ability to predict emissions of phthalates is of interest because of concern about their health effects. An experimental chamber was used to measure emissions of di-2ethylhexyl-phthalate (DEHP) from vinyl flooring, with ammonium sulfate particles introduced to examine their influence on the emission rate and to measure the partitioning of DEHP onto airborne particles. When particles were introduced to the chamber at concentrations of 100 to 245 μg/m3, the total (gas + particle) DEHP concentrations increased by a factor of 3 to 8; under these conditions, emissions were significantly enhanced compared to the condition without particles. The measured DEHP partition coefficient to ammonium sulfate particles with a median diameter of 45 ± 5 nm was 0.032 ± 0.003 m3/μg (95% confidence interval). The DEHP-particle sorption equilibration time was demonstrated to be less than 1 min. Both the partition coefficient and equilibration time agree well with predictions from the literature. This study represents the first known measurements of the particle-gas partition coefficient for DEHP. Furthermore, the results demonstrate that the emission rate of DEHP is substantially enhanced in the presence of particles. The particles rapidly sorb DEHP from the gas phase, allowing more to be emitted from the source, and also appear to enhance the convective mass-transfer coefficient itself. Airborne particles can influence SVOC fate and transport in the indoor environment, and these mechanisms must be considered in evaluating exposure and human health.
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INTRODUCTION Semivolatile organic compounds (SVOCs), such as plasticizers, flame retardants, and biocides, are present in many building materials and household products. Some of these SVOCs have been linked to endocrine disrupting behaviors, and due to their ubiquitous presence within the indoor environment and humans,1,2 there are serious concerns about their health implications. The plasticizer di-2-ethylhexyl phthalate (DEHP) is frequently used as a softener in polyvinyl chloride (PVC) products and building materials, such as vinyl flooring, and is also found in a variety of food packaging materials and personal care products.3,4 In vinyl flooring, DEHP may comprise 10−60% by weight of the material, and because it is not chemically bound within the product, it is emitted into indoor air, albeit at a slow rate.2,5,6 Exposure to DEHP has been associated with asthma and allergies,7−11 and studies have indicated that exposure may also affect reproductive development in humans12−17 and neurological development.18,19 When SVOCs are emitted into indoor air, they partition strongly to all interior surfaces, including airborne particles. Thus, understanding sorption phenomena is an important key to understanding transport in the indoor environment and consequential human exposures.20−22 Several © 2013 American Chemical Society
studies have investigated particle-gas partitioning of DEHP in the atmosphere,23−26 and several studies have developed theory on the partitioning of SVOCs in the gas and particle phases.27−29 It is well-recognized that particles may enhance SVOC emission rates from building materials,20,30 play an important role in the transport of SVOCs in the indoor environment,1,31−33 and contribute to inhalation exposure to SVOCs.21,34,35 Furthermore, Weschler and Nazaroff1 suggest that particles with sorbed SVOCs may deposit on skin and increase the rate of mass transfer of SVOCs from air to skin through rapid desorption from particles. This route may play an important, yet not well understood, role in dermal exposure. A subsequent study indicated that for some SVOCs, the uptake from air through the skin may be a greater contributor to human exposure than uptake through inhalation.36 Liu et al.30 developed a model that examined the potential impacts of the enhanced mass transfer of SVOCs between air and surfaces in Received: Revised: Accepted: Published: 2696
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Figure 1. Schematic and photograph of the emissions chamber.
Figure 2. Experimental sampling setup (left) for the measurement of gas-phase and gas- plus particle-phase DEHP. Particle size distributions (right) for the particles generated and for those remaining after passing through the sorbent tubes at 300 mL/min air flow rate.
source materials into settled dust that is in direct contact with the material. Xu and Little20 developed a mechanistic model to predict the emissions of SVOCs from polymeric materials and applied the model to the results of laboratory chamber studies to examine the emissions of DEHP from vinyl flooring. The experimentally validated model showed that the emissions of SVOCs, unlike more volatile compounds, are governed by external factors, such as partitioning into the gaseous phase, convective mass transfer, and sorption onto interior surfaces. When extended to include particles, the model predicted that partitioning onto airborne particulate matter would increase the rate of emissions, particularly for high particle-gas partition coefficients; however, this particle-induced enhancement of emissions had not been experimentally validated. The objective of this follow-up study is to quantify experimentally the effect of particles on DEHP emissions from vinyl flooring and to measure the DEHP particle-gas partition coefficient to ammonium sulfate particles; this paper expands on work originally described in Benning et al.42 The results of this study will provide an improved understanding of the effect of airborne particles on emissions and transport of SVOCs in the indoor environment, an essential step if mitigation strategies for improving indoor air quality and reducing human exposure are to be successful.
the presence of particles and showed that exposures may be four to ten times greater than predictions based on models that did not account for this effect. Despite the potential importance of particle-mediated transport to human exposure, few studies have quantified the partitioning of SVOCs between the gas- and particle-phases in indoor air, due to the inherent challenges associated with working with extremely low vapor pressure compounds.37 For example, Batterman et al.38 separately measured gas- and particle-phase concentrations of polybrominated diphenyl ethers (PBDEs) in the indoor air of homes and garages, though Weschler and Nazaroff39 suggested that the method used for separating gas- and particle-phases may have been subject to artifacts. Several studies have measured various SVOCs in settled dust and indoor air, both in buildings and in chambers;39,40 and Weschler et al.41 demonstrated that the concentrations in the gas-phase and particle-phase are correlated with concentrations in household dust. Applying the correlation, however, requires an estimate of the particle-gas partition coefficient (Kpart), which has not specifically been measured for any low-vapor pressure SVOCs to date, but has been estimated either from the SVOC’s saturation vapor pressure (ps) or its octanol-air partition coefficient (KOA). The correlations were successfully applied to measured data, although the authors acknowledged that universal correlations between airborne concentrations and dust may be uncertain because dust particles tend to vary in both size and chemical composition. Similarly, it is anticipated that dust particles that have settled to the floor will have different physical and chemical characteristics than airborne particles. Schripp et al.40 suggested that these correlations may be further complicated by the transfer of SVOCs from solid
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MATERIALS AND METHODS
Emissions Chamber. A 2-L chamber (Figure 1) with 0.25 m2 of DEHP-containing vinyl flooring and 0.02 m2 of stainless steel interior surface area was designed and tested by Xu et al.43 This chamber was designed to maximize the emissions surface 2697
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investigations of DEHP emissions and sorption. The retention efficiency of the sorbent tubes was >99% at a flow rate of 20 mL/min and >90% at a rate of 300 mL/min. In general, the separation of gas- and particle-phase SVOCs via filters is subject to two main artifacts. A negative artifact with respect to the filter, known as “blow-off,” can be caused by volatilization of the compound from particles or the resuspension and loss of particles with sorbed compound during the sampling period, while a positive artifact can be caused by the sorption of gas-phase SVOC to the filter and/or particles retained on it.46−48 Following the verification of particle retention in the sorbent tubes, the same tubes were experimentally tested for particle resuspension at a flow rate of 300 mL/min. Clean air was passed through the tubes, and particle concentrations were measured downstream of the tubes. For all sample tubes, no particles were detected downstream. Laboratory tests were not conducted to verify the potential influence of volatilization of the sorbed compound. Because experiments were conducted when the chamber was at steady-state with a constant gas-phase concentration, this phenomenon is not expected to contribute to biases in these investigations. Use of a Teflon membrane filter rather than other types of filters that may have greater total sorption surface areas should minimize the influence of the positive artifact (sorption of gas-phase DEHP to the filter).47 Prior to the experiments, ample time was allowed for the chamber and sampling train to reach steady-state at a flow rate of 850 mL/min, so that sorption onto the chamber walls and the interior of the sample tubing, filter holders, and the Teflon membrane filter was complete, and the measured gas-phase DEHP concentrations were stable. Particles were introduced, a total of 18 times, over time periods of 12 h or less, not inclusive of the time (2 h) allowed to achieve steady particle generation and steady-state particle loss within the chamber. The flow rate through the chamber, and thus the particle residence time, was varied in order to investigate the gas-particle DEHP sorption kinetics. The minimum flow rate was 110 mL/min, corresponding to a residence time of 18.2 min in the chamber, while the maximum flow rate was 4200 mL/min, corresponding to a residence time of 0.48 min. During experiments, the chamber flow rate and sample flow rates were controlled with mass flow controllers downstream of the sorbent tubes, allowing excess particle-laden air from the atomizer to be vented prior to entering the chamber. When the experiments with particles were completed, the chamber air flow rate was restored to the initial steady-state condition, and particle-free air was reintroduced to the chamber. Samples collected during and after the experiments with particles were used to verify that gas-phase DEHP concentrations in the chamber remained at their steady-state values. DEHP Analysis. The Tenax sorbent tubes were thermally desorbed in an automatic thermal desorber (ATD, PerkinElmer ATD 400), and DEHP was analyzed using a gas chromatograph with a flame ionization detector (GC-FID). The operation of the ATD-GC-FID system for analyses, calibration, and verification is described by Xu et al.43 Partition Coefficient. The DEHP particle-gas partition coefficient, Kpart, was calculated as
area, while minimizing the internal surface area available for sorption. The chamber geometry, test conditions, and operating protocol followed in these investigations are detailed in Xu et al.43 The vinyl flooring sample used in these studies came from the same roll of flooring as that used in previous studies43−45 and contains approximately 15% DEHP by mass. The laboratory temperature was 22 ± 0.3 °C and was monitored and logged throughout the duration of the chamber tests. Because similar testing with the same vinyl flooring material showed that emissions are not influenced by humidity, the humidity in the chamber was not monitored during the experiments.45 Clean, dry, compressed air was used in the experiments. Particle Generation and Measurement. Ammonium sulfate ((NH4)2SO4) particles were generated from an 11 mg/L ammonium sulfate solution in ultrapure water using a constant output atomizer (TSI 3076) with a pressure of 35 psi. The output was combined with clean dilution air at a flow rate of 1.2 L/min. The particle-laden airstream flowed through a diffusion dryer and a Krypton-85 neutralizer, and their size distributions and concentrations were measured with a Scanning Mobility Particle Sizer system (TSI 3936NL). Background particles derived from impurities in the water were verified to contribute less than 5% of the particles. Upon introduction into the chamber, the particles had a median diameter of 45 ± 5 nm. The total suspended particulate (TSP) mass concentration varied between 100 and 245 μg/m3. DEHP Sampling. For the measurement of gaseous and total DEHP, samples were collected from the chamber outlet using two different sampling trains (Figure 2). Both consisted of two sorbent tubes packed with Tenax in series, downstream of a 13mm stainless steel filter holder, but only one holder was actually loaded with a polytetrafluoroethylene (PTFE) membrane filter (Pall Life Sciences, 0.2 μm pore size) intended to capture particles. Potential variability in measurements associated with the sampling lines prior to the addition of the filter holders and filter was assessed by Xu et al.;43 there were no detectable differences between sampling lines. Therefore, the samples collected on the sorbent tubes downstream of the filter represent only gas-phase DEHP (potential artifacts are discussed later), while the samples collected from the second branch, without a filter, represent both gas- and particle-phase DEHP because the sorbent tubes effectively capture particles and the DEHP sorbed to them in addition to gas-phase DEHP. The particle-phase DEHP concentration can be calculated as the difference between the amounts captured on the two sets of tubes. Prior to introduction of particles into the chamber, air samples downstream of the atomizer, dryer, neutralizer, and all associated tubing were collected on sorbent tubes and analyzed for DEHP. These sample analyses ensured that there was negligible DEHP associated with the air and particles entering the chamber. The retention of particles in the sorbent tubes was verified through laboratory tests, and results are illustrated in Figure 2. Ammonium sulfate ((NH4)2SO4) particles were generated, and their size distributions and concentrations were measured, as described previously, at the start and end of each trial to ensure stability in concentrations. The particle size distributions and concentrations were measured downstream of each of six different sorbent tubes, and the difference in concentrations was used to calculate particle retention efficiency. The particle retention tests were conducted at various air flow rates that were representative of those used for sampling in the
K part = 2698
qpart y·TSP
(1)
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flowing continuously through the chamber over the entire year. The gas- plus particle-phase concentration data represent different experimental conditions of varying TSP (100−245 μg/m3) and varying chamber flow rates (110−4500 mL/min). As shown in Figure 3, when particles were introduced into the chamber, the system maintained the steady-state gas-phase DEHP concentration. A simple mass-balance indicates that the mass of DEHP in the particle-phase, which was 3 to 8 times the mass of DEHP in the gas-phase, cannot be accounted for solely by the rapid partitioning of DEHP from the gas-phase to the particle-phase in the chamber. In other words, the DEHP in the gas-phase would be rapidly depleted once the particles were introduced. To maintain the concentration at the former steady-state level, the gas-phase DEHP must have been rapidly resupplied to the air in the chamber. As will be discussed later, the gas-phase DEHP was most likely resupplied either by rapid desorption from the relatively large mass sorbed on the interior chamber walls, or by enhanced emissions from the vinyl flooring, or by a combination of these two processes. The effect of average residence time in the chamber, calculated as the chamber volume divided by the flow rate, on the measured Kpart is illustrated in Figure 4. Grubbs’ test for
where qpart is the particle-phase DEHP concentration, y is the gas-phase DEHP concentration, and TSP is the total suspended (airborne) particulate mass concentration.28,49 Corrections were applied, based on experimental measurements, to account for particle losses due to deposition and coagulation within the chamber. Particles were introduced into a second identical chamber at varying air flow rates, and size distributions were measured upstream and downstream of the chamber. A linear correlation was developed from this data to estimate particle losses in terms of number and mass at any chamber flow rate (Figure S1, Table S1). The estimated TSP mass losses within the chamber varied from 6.5% to 0% for chamber flow rates between 110 and 4200 mL/min, respectively. Due to inherent sampling and analytical errors associated with measuring gas-phase DEHP concentrations and because changes in the gas-phase concentration fell within the bounds of experimental uncertainty when particles were introduced, the gas-phase concentration, y, used in the calculations was the long-term average steady-state concentration. The TSP values used for individual estimates of Kpart were based on average measured particle concentrations (atomizer output) collected both before and after introducing particles into the chamber, with corrections for losses applied. The particle-phase DEHP concentrations, qpart, were calculated as the difference between the particle- plus gas-phase concentrations measured in the unfiltered sample branch and the long-term average gas-phase concentration, y.
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RESULTS
The DEHP concentrations in the gas-phase only and in the gasplus particle-phase (filtered and unfiltered sorbent tube samples, respectively) measured in the chamber exhaust over nearly a full year are shown in Figure 3. For the sampling periods when particles were present in the chamber, the experimental conditions, including chamber air flow rate, TSP, particle-phase DEHP concentration, and gas-phase concentration, are summarized in Table S2. Spikes in the gas- plus particle-phase concentration coincide with periods when particles were introduced to the chamber; particles were not
Figure 4. Experimentally determined Kpart for various chamber residence times.
outliers with significance level α = 0.05 indicated that measurements at the two shortest residence times and at 6 min are outliers; the latter of these was suspect, as incomplete particle drying was observed in the diffusion dryer upstream of the chamber. Excluding these outliers, the particle-gas partition coefficient for DEHP onto ammonium sulfate particles was found to be 0.032 ± 0.003 m3/μg (95% confidence interval). The results in Figure 4 indicate that there was no change in the value of Kpart, within reasonable error, with decreasing chamber residence times until approximately 0.6 min.
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DISCUSSION It is valuable to compare the Kpart measured in this investigation, 0.032 m3/μg, to those estimated for DEHP by Weschler et al.41 Weschler et al. summarize investigations that relate the Kpart for SVOCs to either the vapor pressure28,29,33,49−51 or the octanol-air partition coefficient, KOA.52−54 The measured value is considerably less than that derived from the saturation vapor pressure, 0.25 m3/μg, while it is within a factor of only 2 of that derived from the octanol-air partition coefficient, 0.064 m3/μg. Both of these estimates were derived from empirical relationships based on measurements of polycyclic aromatic hydrocarbons (PAHs), as a proxy to
Figure 3. Measured DEHP concentrations in the chamber exhaust in the gas-phase only and in the gas- plus particle-phases combined. Spikes in the latter coincide with periods of up to 12 h when particles were introduced to the chamber. 2699
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Figure 5. The DEHP loss rate in the chamber for experimental conditions and the predicted chamber DEHP resupply rates.
particle surface area. It is therefore estimated that the DEHP had formed as monolayer. The kinetics of sorption of SVOCs onto particles is generally assumed to be relatively fast; the results experimentally validate this assumption through close agreement with predictions in the literature. The application of eq 4.3 from Weschler and Nazaroff1 with the KOA from Weschler et al.41 and the mean particle diameter yields an estimated equilibration time for gasparticle reactions of 0.11 min. If the measured Kpart in these investigations is converted to a dimensionless coefficient and applied in place of the KOA, the estimated equilibration time is 0.14 min. A further comparison is made to the model predictions for equilibrium time shown in Figure 5 of Liu et al.,55 based on the measured Kpart and the mean particle diameter of 45 nm, the time scale required to reach gas/particle equilibrium is on the order of roughly 10−1 min. Figure 4 shows that the sorption of DEHP onto particles reaches equilibrium in less than 0.6 min. This is in reasonable agreement with the estimates from the literature, since it is possible that the results were subject to increased experimental error at the shorter residence times. The phenomenon of enhanced emissions of DEHP from source material in the presence of particles, as was confirmed in these experiments, was predicted by Xu and Little.20 However, the model developed by Liu et al.30 indicated that the earlier model may have underestimated the increase in emission rates in the presence of particles because it did not take into account the idea that particles can penetrate into the concentration boundary layers at surfaces and effectively increase the concentration gradient in the layer, thus leading to enhanced mass transfer of the SVOC to or from surfaces. A similar effect of particle-enhancement of convective heat transfer has been described by Hu and Zhang.56 From the results of this study, it is not possible to distinguish or measure the potential influence of enhanced mass transfer from surfaces in the presence of
phthalates, on ambient aerosol particles, and so some differences are expected due to differences in both the SVOC itself and the characteristics of the particles investigated. Liang et al.50 demonstrated that particle-gas partition coefficients may be different on organic versus inorganic particles, and so it is probable that the Kpart may be different either for particles that are comprised of organic matter or are covered with a thin organic film such as would be likely encountered in the atmosphere and unlike the inorganic particles used in this study. Furthermore, Weschler et al.41 also noted that the Kpart estimated from the KOA was derived for TSP, which is comparable to the particle measurements conducted in these investigations, while the Kpart derived from the vapor pressure was derived from PM2.5 measurements. The particles generated in these investigations are sufficiently small to qualify as PM2.5; however, it is expected that specific differences in particle sizes studied will influence the measured Kpart, as predicted by Liu et al.55 Another possible discrepancy in the estimated Kpart could result from uncertainties in the estimated saturation vapor pressures found in the literature, which span 2 orders of magnitude, as shown in Table 2 of Weschler et al.41 It is expected that the partitioning of DEHP onto particles could be affected by the particle surface area and furthermore could depend on the degree of coverage of the particles by sorbed DEHP (i.e., whether it formed a monolayer or more). These influences were not specifically investigated in this study. However, SMPS measurements indicated that the particle surface area concentration was roughly 12,000 mm2/m3 air. Applying the average measured qpart of 3.1 μg/m3, there were approximately 4.7 × 1015 DEHP molecules per m3 air. A spherical DEHP molecule with a diameter of approximately 1.08 nm was assumed, and thus, the cross sectional surface area was roughly 9.2 × 10−13 mm2. The DEHP molecules were estimated to cover an area of approximately 4300 mm2/m3, which is significantly less than the estimated 12,000 mm2/m3 of 2700
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changing TSP concentrations, although the general trend is an increase in the loss rate with increasing chamber air flow rate. The loss rate was compared to estimated DEHP resupply rates in the chamber or the sum of the DEHP emissions rate from the flooring material and DEHP desorption rate from the chamber stainless steel walls (eqs 2 and 3) in Figure 5. Four cases of DEHP resupply are illustrated in Figure 5. The “maximum resupply” case assumes that the gas-phase DEHP in the chamber is essentially instantly depleted (y = 0 μg/m3) due to sorption onto particles upon their introduction into the chamber. In this case, the DEHP emission rate from the flooring and desorption rate from the chamber walls is at a maximum. The estimated maximum release rate ranges from 8.7 × 10−5 to 5.4 × 10−4 μg/s or an emission rate per surface area of flooring, E/A, of 1.6 × 10−4 to 5.4 × 10−4 μg/m2·s. The “steady-state resupply” case (Figure 5) assumes that the steadystate gas-phase DEHP concentration in the chamber is quickly re-established after the introduction of particles (y = 0.65 μg/ m3), as is likely the case considering the consistency of the measured gas-phase outflow concentration during particle introduction into the chamber (see “gas-phase only” in Figure 3). In this case, the driving force for desorption from the stainless steel walls is essentially zero, following a period of rapid desorption, which will become minimal once the steadystate gas-phase DEHP concentration is re-established. Then, the DEHP release rate in the chamber, estimated to be 1.6 × 10−5 to 1.0 × 10−4 μg/s (emissions rate, E/A, of 6.5 × 10−5 to 4.0 × 10−4 μg/m2·s), occurs primarily through emissions from the vinyl flooring. In the “resupply under enhanced mass transfer (3x)” and the “resupply under enhanced mass transfer (8x)” cases, it is assumed that the release rate from the flooring is controlled by the measured chamber conditions (y = 0.65 μg/m3) (desorption from the chamber walls will become negligible) but that there is an enhancement of the mass transfer coefficient by three and eight times, respectively, for the transfer from the flooring to the air, as described by Liu et al.30 The range of increase of the mass transfer coefficient reported by Liu et al. was between three and eight. In these cases, the DEHP release rate ranges are 4.9 × 10−5 to 3.0 × 10−4 μg/s and 1.3 × 10−4 to 8.1 × 10−4 μg/s, respectively (E/A of 1.9 × 10−4 to 1.2 × 10−3 μg/m2·s and 5.2 × 10−4 to 3.2 × 10−3 μg/m2·s, respectively). In comparing the DEHP loss rate from the chamber to the four release rate scenarios, if the loss rate for all data points is less than the estimated resupply rate, then the assumptions can offer a plausible explanation for the system’s chamber-particle DEHP dynamics. It can be seen that in the case of the maximum emission and desorption rate (red line, Figure 5), where it is assumed that the gas-phase DEHP in the chamber instantaneously drops to zero as any available DEHP is taken up by particles, the resupply rate is greater than the loss rate for all experimental conditions. In examining the case of the rapid re-establishment of steadystate conditions where the DEHP resupply rate in the chamber is controlled by emissions from the flooring (green line, Figure 5), the DEHP loss rate is greater than the estimated resupply rate for all experimental conditions other than at the three lowest flow rates tested. This indicates that the predicted emissions are insufficient to account for the re-establishment of the steady-state DEHP gas-phase concentration in the chamber. Examination of the cases of the enhancement of the mass transfer coefficient by three and eight times (purple and black lines, Figure 5), as suggested by Liu et al.,30 shows that the
particles. However, it is reasonable to expect that the DEHP is readily available on the surface of the vinyl flooring as other studies have demonstrated that in a PVC material, the DEHP acts like a pure liquid.57,58 To further examine the results of this study with respect to the phenomenon of enhanced DEHP emissions in the presence of particles, the estimated rate of DEHP loss (i.e., removal via outflow) from the chamber was compared to the estimated rate of DEHP release from the chamber interior, applying the system parameters as determined by Xu et al.43 In short, this analysis is performed to determine whether the measured mass of DEHP sorbed to particles could have been provided by the gas-phase DEHP in the chamber or if it must have originated from the vinyl flooring itself or from the release of DEHP sorbed on the chamber walls. The DEHP resupply rate was calculated as the sum of the emission rate from the flooring and the desorption rate from the stainless steel interior walls of the chamber. The emission rate (E, μg/s) from the flooring was estimated as E = hm ·A ·(y0 − y)
(2)
where hm is the convective mass-transfer coefficient (4.0 × 10−4 m/s at 850 mL/min flow rate), A is the emissions surface area (0.25 m2), y0 is the concentration in the air immediately adjacent to the material surface (1.1 μg/m3),43 and y is the concentration of DEHP in the bulk air in the chamber. The emission rate (Es, μg/s) due to desorption from the stainless steel walls was estimated as Es = hs ·A s ·(y0s − y)
(3)
where hs is the convective mass-transfer coefficient near the sorption surface (0.01 m/s at 850 mL/min flow rate), As is the internal stainless steel surface area (0.02 m2), and y0s is the concentration of DEHP in the air immediately adjacent to the steel surface (0.65 μg/m3). The convective mass transfer rates, hm and hs, are influenced by the air velocity across these flat surfaces. When particles were introduced into the chamber, the air flow rate, or air exchange rate, through the chamber was simultaneously changed from the steady-state air flow rate of 850 mL/min, thus effectively changing the mass transfer coefficients. The mass transfer coefficients were determined for the chamber at the steady-state flow rate of 850 mL/min, as described in Xu et al.43 The mass transfer coefficients at other flow rates, Q, were approximated using the correlation developed by Axley,59 using the convective mass transfer coefficients, hm0 and hs0, as determined for the chamber at the “initial” condition with an air flow rate of 850 mL/min, Q0: ⎛ Q ⎞0.5 hs hm ⎟⎟ = = ⎜⎜ hm0 hs0 ⎝Q0 ⎠
(4)
Using eq 4, it was estimated that the mass transfer coefficient for the vinyl flooring varied from 1.4 × 10−4 to 8.9 × 10−4 m/s, and the mass transfer coefficient for the sorption surface varied from 3.6 × 10−3 to 2.2 × 10−2 m/s for the range of experimental chamber flow rates. The loss rate of DEHP from the chamber was calculated for each experimental flow rate and TSP concentration. DEHP was lost from the chamber through outflow in both the gas- and particle-phases; the experimentally determined Kpart was applied for chamber DEHP loss rate estimates. The loss rates, shown in Figure 5, range from 6.9 × 10−6 to 2.6 × 10−4 μg/s and reflect 2701
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DEHP loss rate in the chamber is always less than the predicted chamber DEHP resupply rate. Because the chamber gas-phase DEHP concentration must always lie between y = 0 and 0.65 μg/m3, the analysis suggests that the rate of mass transfer was being enhanced by the presence of particles during these experiments. This investigation has measured the particle-gas partition coefficient for DEHP onto laboratory-generated ammonium sulfate particles and has shown that mass transfer is enhanced in the presence of particles. However, there is much evidence in the literature that suggests that this partition coefficient will depend on additional factors that were not included in this study, such as the chemical composition, size, and surface area of the airborne particles. Further research to examine these influences and to improve understanding of the fate and transport of SVOCs in the indoor environment is needed. In addition, further research to experimentally verify and quantify the influence of enhanced mass transfer would provide valuable insight into the mechanisms controlling human exposure to SVOCs.
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1, which illustrates the experimentally determined particle losses in the chamber at various flow rates; Table S1, which summarizes the particle size distributions at the chamber inlet and outlet; and Table S2, which summarizes the chamber conditions when particles were present in the system. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Special thanks are due to Charles Weschler, Yinping Zhang, Cong Liu, and Glenn Morrison for valuable advice on these investigations and to Amara Holder, Marina Eller Quadros, Steven Cox, and Elizabeth Smiley for laboratory assistance. Financial support was provided by the National Science Foundation (CBET-0504167 and CBET-1066802).
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