New Indoor Carpet as an Adsorptive Reservoir for Volatile Organic

Carpet is generally recognized as a source of volatile organic compounds (VOCs) in indoor air. However, carpet systems can also serve as adsorptive si...
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Environ. Sci. Technol. 2000, 34, 4193-4198

New Indoor Carpet as an Adsorptive Reservoir for Volatile Organic Compounds DOYUN WON, RICHARD L. CORSI,* AND MIKE RYNES Texas Institute for the Indoor Environment, Department of Civil Engineering, The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78758

Carpet is generally recognized as a source of volatile organic compounds (VOCs) in indoor air. However, carpet systems can also serve as adsorptive sinks with the potential for reductions in peak VOC concentrations and subsequent re-emission of VOCs over prolonged periods of time. A series of experiments involving eight VOCs, several carpet systems, and environmental conditions were completed using a set of parallel chambers to characterize the sorptive interaction between VOCs and carpet. A linear adsorption/desorption model was observed to be appropriate for short-term sorption events. New carpet fibers treated with stain protection generally accounted for only a small fraction of mass sorbed to carpet. Most of the sorbed mass was accounted for by either the underlying pad (cushion) or a combination of the pad and structural backing. Equilibrium partition coefficients were observed to be correlated to chemical vapor pressure and octanolair partition coefficient. Variations in relative humidity (RH) had a significant effect on the degree of sorption for a highly soluble VOC (2-propanol). However, RH had little apparent effect on other VOCs. Inlet concentrations generally had little effect on sorption.

dCg ) QgCg,in - QgCg - kaCgA + kdMnA dt

V

(1)

dM ) kaCg - kdMn dt

(2)

where Cg ) VOC concentration in chamber air (mg/m3), Cg,in ) VOC concentration in inlet air (mg/m3), V ) chamber volume (m3), Qg ) air flow rate through the chamber (m3/h), A ) sorptive sink area (m2), ka ) adsorption rate coefficient (m/h), kd ) desorption rate coefficient (1/h if n ) 1), M ) mass collected on the sink per unit area (mg/m2), and n ) a constant that accounts for nonlinearities in the desorption process (dimensionless). The area (A) is typically assumed to be the horizontally projected area of a material surface, i.e., as opposed to the true area that accounts for interior pores and irregularities (roughness) of exposed surfaces. For all intents and purposes, increased surface areas due to surface irregularities and internal pores, as well as bulk mass transport and diffusive transport processes, are implicitly “lumped” into the adsorption and desorption coefficients. Furthermore, “n” is typically assumed to equal unity (6), allowing for the material/ air equilibrium partition coefficient (Keq) for a specific VOC to be defined by the ratio ka/kd. The results of a study intended to improve existing knowledge of the sorptive behavior of carpet systems are presented in this paper. Preliminary experiments indicated that multicomponent interactions did not have a significant effect on sorption parameters. Linear adsorption and desorption coefficients were thus calculated for a large number of chemical/carpet combinations and were used to determine equilibrium partition coefficients. Sorption parameters and the relative contributions of three carpet components to overall sorbed mass are described herein. Attempts to relate chemical properties to equilibrium partition coefficients between VOCs and carpet are also discussed. The influences of relative humidity and VOC concentrations are also presented.

Methodology Introduction Several researchers have studied carpet as a potential source of volatile organic compounds (VOCs) in indoor air (1-3). Carpet has also been reported to exhibit the greatest sorption capacity among various indoor surface materials (4-6). However, little research has been completed on the contributions of individual carpet components to the overall sorptive capacity of carpet. Sorptive interactions may lead to reductions in peak concentrations of VOCs and hence reductions in occupant inhalation exposures. However, sorption may also lead to significant increases in chemical retention time within a building and subsequent re-emission over prolonged time periods. A practical example of such effects is the prolonged occurrence of airborne chemicals, often above odor thresholds, associated with cigarette smoke or moth cakes long after source termination (5, 7). The most widely used model for predicting sorptive interactions is given by eqs 1 and 2, which define mass balances on the headspace of an inert and well-mixed chamber and a sorptive material placed within that chamber, respectively * Corresponding author phone: (512)475-8617; fax: (512)471-1720; e-mail: [email protected]. 10.1021/es9910412 CCC: $19.00 Published on Web 09/06/2000

 2000 American Chemical Society

The experimental system consisted of four 50-L electropolished stainless steel chambers arranged in parallel (Figure 1). During each experiment, one of the four chambers was used as a blank (no carpet added), and the other three contained carpet or carpet component specimens. Carpet components were heated in a drying oven for 7 days at 35 °C prior to each experiment. The air flow rate through the oven was approximately 0.6 m3/h, with a corresponding air exchange rate of between 4 and 5 air changes per hour. A syringe pump was used to deliver a mixture of target VOCs into a stainless steel manifold, where the chemicals were volatilized into an air stream that was split evenly for delivery to each chamber. The air stream was provided by pressurized air cylinders (Air Liquide Zero Air). Exhaust from each chamber was directed to individual Atkomatic electrically actuated three-way valves where it was split to a waste line and a position valve (VICI SC P type multiposition valve with an electronic actuator) to prevent pressure build-up. One exhaust stream was programmed to be selected for analysis, allowing for sequential sampling from each chamber. The selected exhaust stream was conveyed to a sampling loop (1.0 mL) inside an online gas chromatography system equipped with a flame ionization detector (GC/FID) (SRI 8610C). One side of the sampling loop was connected to a VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Flow diagram of experimental system. GC capillary column (Restek 30 m length, 0.53 mm ID, 1 µm film of MXT-5) for several seconds when the GC oven was activated or directed to room air otherwise. The GC temperature program included an initial oven temperature of 35 °C that was held constant for 1.3 min. To expedite sample analysis, the oven temperature was then ramped at 50 °C/ min to a final temperature of 110 °C. This final temperature was held constant for 2.4 min. The GC oven temperature was then reduced at a rate of 15-16 °C/min until it reached 35 °C, where it was held for 4.8 min prior to initiation of the next analysis. Each experiment lasted 18 h and included an adsorption stage (with chemical injection) and a desorption stage (no chemical injection). Equations 1 and 2 and gas concentration (Cg) profiles for the adsorption stage were used along with a SAS software routine to determine ka and kd based on a best-fit, nonlinear regression analysis using Marquardt’s method (8). The resulting sorption parameters were used to compare predicted and measured desorption stage concentration profiles. The inlet gas concentration (Cg,in) was determined as the average of two data points associated with the steady-state exhaust of the reference chamber (no material), i.e., when the inlet and exhaust concentrations were theoretically equivalent. Three carpets and two carpet pads (cushions) were tested as sorptive materials. Two of the carpet specimens (carpet 1 and carpet 3) consisted of nylon cut-pile fibers, while the third (carpet 2) was comprised of 90% olefin/10% nylon looppile fibers. Carpet fibers were treated with either Stainmaster (carpet 1) or Scotchgard (carpets 2 and 3) at the manufacturer. Two carpet cushions were used, each comprised of bonded polyurethane with densities of 96 and 112 mg/cm3. The three carpet specimens were all tufted with polypropylene backing. In addition to carpet and carpet with an underlying cushion, fibers were removed from carpet 1 to allow for separate testing of fibers, structural backing, and cushions. The cut edges and reverse side of each specimen were sealed with sodium silicate and allowed to dry prior to each experiment. Target VOCs were selected to allow for a broad range of chemical properties. These chemicals included methyl tertbutyl ether (MTBE), cyclohexane (CH), 2-propanol (IP), 4194

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toluene (TOL), tetrachloroethene (PCE), ethylbenzene (EB), o-dichlorobenzene (DCB), and 1,2,4-trichlorobenzene (TCB). Physicochemical properties of each chemical are provided in Table 1. Over all experiments, standard conditions were defined as an air temperature of 25 °C, relative humidity of 0%, and air exchange rate of 0.5/h. Variations to these conditions are described below. Preliminary experiments indicated that multicomponent effects of chemicals were insignificant. One experiment was first completed with carpet 1 exposed to a gaseous mixture containing toluene, tetrachloroethene, and ethylbenzene under standard conditions. The results of this single experiment were then compared with three replicate experiments involving a mixture of eight VOCs under otherwise identical conditions. A statistical comparison of the differences in sorption parameters for three and eight chemical mixtures was not possible given that only a single experiment was completed for the three chemical mixture. However, with the exception of ka and kd for toluene, the individual sorption parameters (ka, kd, Keq) for the three chemical experiment were all within the range of values observed for the eight chemical mixture, i.e., for the same three chemicals. The average ratio (over all three chemicals) of equilibrium partition coefficients for the three chemical mixture relative to the eight chemical mixture was 0.97 m, with a standard deviation (3 values) of 0.37 m. A second set of experiments was completed under standard conditions using carpet 1. For this set, three experiments were completed using DCB as the only chemical and five additional experiments were completed using an eight chemical gas mixture. A test of the null hypothesis, i.e., no difference in the mean of measured sorption parameters (ka, kd, Keq), was completed using a two-sided t-test with a 95% confidence level (R ) 0.05 level of significance). The null hypothesis could not be rejected for any of the three sorption parameters, indicating that there was not strong evidence of multicomponent effects on the sorption process. This finding is consistent with observations made by van der Wal et al. (4) for woolen carpet exposed to single and multicomponent gas streams containing toluene, m- and

TABLE 1. Physicochemical Properties of Experimental Chemicals characteristics

MTBEk

CHl

IPm

TOLl

PCEl

EBl

DCBl

TCBl

formula MWa Po b Kowc Koad Hce Tbf Cwg Dah Dwi

C5H12O 88.2 3.2E-1 15.8 9.7E+1 0.02 55.0 4.9E-1 7.9E-2 9.4E-5

C6H12 84.2 1.3E-1 2754.2 3.8E+2 7.2 80.7 7.08E-4 8.0E-2 8.5E-6

C3H8O 60.1 4.2E-2 0.692 2.0E+3 0.0055 82.4 1.27E+1j

C7H8 92.1 3.8E-2 489.8 1.8E+3 0.3 110.6 5.62E-3 8.7E-2 8.6E-6

C2Cl4 165.8 2.5E-2 758.6 6.7E+2 1.1 121.0 9.12E-4 7.2E-2 8.2E-6

C8H10 106.2 1.3E-2 1412.5 4.3E+3 0.3 136.2 1.58E-3 7.5E-2 7.8E-6

C6H4Cl2 147.0 2.0E-3 2398.8 1.9E+4 0.1 180.0 6.31E-4 6.9E-2 7.9E-6

C6H3Cl3 181.5 4.5E-4 4466.8 1.9E+4 0.2 213.0 4.68E-4 3.0E-2 8.2E-6

a MW: molecular weight. b Po: vapor pressure at 25 oC (atm). c K : octanol-water partition coefficient at 25 °C (L d K : octanol-air ow water/Loctanol). oa partition coefficient at 25 °C, estimated by Kow/Hc (Lair/Loctanol). e Hc: Henry’s law constant at 25 °C (Lwater/Lair). f Tb: boiling point (°C). g Cw: solubility h 2 i 2 j k (mol/L). Da: diffusivity in air (cm /s). Dw: diffusivity in water (cm /s). Estimated from 1/Cw ) 1.113 log Kow - 0.926 in ref 9. Reference 10 except boiling point, Hc, Da, and Dw. l Reference 11. m Reference 12.

FIGURE 2. Example concentration profiles (ethylbenzene, carpet 1/pad 1). p-xylenes, mesitylene, n-decane, n-undecane, and 2-ethylhexanol. While the effects of multicomponent interactions between VOCs and indoor materials is an area that has not been extensively reported in the published literature and is worthy of additional study, based on our preliminary experiments and recent studies by van der Wal et al. (4), it was decided that multicomponent gas streams would be used for this study. The obvious benefit of such an approach is reflected in the large number of chemical/carpet combinations and corresponding sorption parameters presented herein.

Results and Discussion Normalized Concentration Profiles. Normalized concentration profiles (gas concentration in chamber divided by inlet gas concentration) for ethylbenzene are presented in Figure 2. The upper adsorption curve corresponds to the reference (empty) chamber and includes measured data and a dashed line that corresponds to a theoretical well-mixed and inert reactor at the experimental air exchange rate of 0.5 ACH. The lower curve corresponds to a chamber containing carpet 1 with an underlying cushion. Open circles depict measured data. The solid line corresponds to model predictions based on values of ka and kd that were determined during the adsorption stage (first 10 hours). Similar plots were developed for every combination of VOC and carpet system. Data presented for the reference chamber in Figure 2 compare favorably with an assumed well-mixed and inert chamber. This was true for all VOCs other than 1,2,4trichlorobenzene, which generally exhibited a sigmoidal adsorption curve indicative of some level of sorption to the experimental and/or analytical systems. This problem made it difficult to accurately determine sorption parameters for 1,2,4-trichlorobenzene during some experiments, and the reader is therefore cautioned that results for this one

experimental chemical are subject to greater uncertainty than results for the other seven VOCs. The extent of ethylbenzene sorption to carpet 1 with pad 1 is clear in Figure 2, as over 50% of the inlet mass sorbed to the material during the adsorption stage. Furthermore, values of ka and kd determined from the adsorption stage led to a good level of agreement between predicted and measured desorption profiles. Desorption stage data also illustrate that sorptive sinks can increase chemical retention times in an indoor environment. This is reflected by higher gas concentrations in the material chamber than in the reference chamber after hour 14. Sorption Parameters. Average sorption parameters and corresponding coefficients of variation are summarized in Table 2. The two carpets with treated nylon-based fibers (carpets 1 and 3) exhibited very similar sorption capacities as defined by Keq. The carpet with primarily olefin-based fibers (carpet 2) exhibited a greater sorption capacity than the other two carpets for all VOCs. However, there was no significant difference in sorption capacity when padding was added to each of the three carpets. This result is attributed to the fact that the underlying cushion tends to be the dominant sorptive component for a carpet system. Contribution of Carpet Components. Concentration profiles for ethylbenzene and several combinations of carpet components are presented in Figure 3. All chemicals except MTBE and 2-propanol exhibited a similar trend to that for ethylbenzene. Despite the large surface area of fibers, their contribution to sorptive interaction was almost negligible for all chemicals except 1,2,4-trichlorobenzene. The structural backing exhibited little sorptive interaction with cyclohexane but led to significantly greater interaction, relative to fibers, for toluene, tetrachloroethene, and ethylbenzene. However, a significant amount of sorption to the structural backing was observed for both o-dichlorobenzene and 1,2,4-trichlorobenzene. As expected, greater sorption was always observed for fibers + backing, than for either fibers or backing. However, the sum of the individual contributions from fibers and backing was consistently less than that of the composite fiber and backing system. Although the difference between the summed and composite systems was small, the discrepancy may have been due to differences in the geometric configurations of the fibers. For individual fiber experiments, the fibers were randomly scattered, generally lying horizontal, within the chamber. This may have affected the rate of mass transfer to, or total available surface area of, the fibers relative to the composite system in which fibers were held upright, i.e., vertical, by the structural backing. Padding appears to be responsible for a great deal of the adsorptive interaction between VOCs and carpet systems. This is clearly illustrated by examination of the concentration profile (Figure 3) for systems including fiber, structural VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Summary of Sorption Parameters for Carpets with and without Paddinga chemical material carpet 1 (Cp1)

AVG CV

carpet 2 (Cp2)

n AVG CV

parameter

MTBE

CH

IP

TOL

PCE

EB

DCB

TCB

ka (m/h) kd (1/h) Keq (m) ka kd Keq

ns ns ns ns ns ns 5 nd nd nd b b b 2 nd nd nd 1 0.76 4.9 0.15 0.45 0.53 0.21 6 0.10 0.45 0.22 0.71 19 18 2 0.10 0.69 0.14 3.7 5.2 8.8 2

ns ns ns ns ns ns 6 nd nd nd b b b 2 ns ns ns 1 0.39 1.7 0.24 0.64 0.61 0.20 9 0.10 0.67 0.15 0.0 15 15 2 0.10 0.60 0.16 7.4 0.0 7.4 2

b b b b b b b b b b b b b b b b b b 0.75 1.1 0.78 0.15 0.12 0.30 5 0.36 1.2 0.29 51 43 10 2 0.27 0.47 0.57 42 32 11 2

0.11 0.56 0.22 46 64 30 7 0.26 0.44 0.60 12 15 2.5 2 0.18 0.65 0.28 1 0.49 0.29 1.7 14 17 11 10 0.42 0.23 1.9 3.4 9.4 6.1 2 0.23 0.16 1.5 16 23 7.2 2

0.17 0.47 0.36 31 57 25 9 0.31 0.32 0.97 7.0 6.7 0.2 2 0.16 0.42 0.38 1 0.44 0.25 1.8 12 15 10 10 0.45 0.22 2.1 3.1 10 6.7 2 0.23 0.17 1.4 16 21 5.8 2

0.30 0.62 0.46 57 63 28 9 0.41 0.34 1.2 12 17 4.5 2 0.17 0.37 0.46 1 0.48 0.15 3.3 11 14 13 10 0.49 0.15 3.4 4.4 15 10 2 0.29 0.14 2.1 12 16 3.3 2

0.52 0.25 2.1 41 71 73 8 0.80 0.17 4.7 5.3 0.0 5.3 2 0.43 0.21 2.0 1 0.60 0.083 8.0 10 32 35 10 0.96 0.15 6.6 6.7 15 8.0 2 0.62 0.13 4.9 18 34 16 2

0.58 0.10 5.9 b b b 1 0.91 0.13 7.8 31 63 35 2 0.49 0.16 3.1 1 1.5 0.46 4.9 80 120 68 10 2.1 0.36 6.1 43 54 13 2 1.4 0.42 3.5 66 77 15 2

ka (m/h) kd (1/h) Keq (m) ka kd Keq

n

ka (m/h) kd (1/h) Keq (m)

carpet 3 (Cp3)

carpet 1 w/pad (Cp1.p)

n AVG CV

carpet 2 w/pad (Cp2.p)

n AVG CV

carpet 3 w/pad (Cp3.p)

n AVG CV n

ka (m/h) kd (1/h) Keq (m) ka kd Keq ka (m/h) kd (1/h) Keq (m) ka kd Keq ka (m/h) kd (1/h) Keq (m) ka kd Keq

AVG: average, CV: coefficient of variation (as %) ) (standard deviation/average)*100, n: number of data points available to determine average, nd: not determined (no convergence), ns: no sorption. b Failure of quality assurance protocols. a

FIGURE 3. Concentration profiles for ethylbenzene for components of carpet system. backing, and either pad 1 or pad 2. Small differences in cushion densities had no effect on levels of sorption. For all chemicals other than MTBE, for which the extent of sorption was generally low for all components, and 1,2,4-trichlorobenzene, the presence of a urethane cushion led to a significant increase in the sorbed mass. The use of two layers of pad 1 as part of a fiber, backing, and cushion system led to only a slight increase in the degree of sorption, i.e., relative to the use of only one layer. This suggests that only a very small fraction of the VOCs was able to diffuse through the 4196

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first layer of padding during the 10 h adsorption stage. The degree of sorption to padding (alone) was generally greater than to fiber/backing/padding systems. This is likely due to the processes of VOC transport into carpet systems, i.e., the chemicals must first negotiate their way through a layer of fibers and the corresponding structural backing before reaching the underlying cushion. In contrast, the rate of mass transfer to the surface of the padding was faster in the absence of overlying carpet. In effect, the carpet fibers and backing act as resistances to mass transfer to the underlying cushion. The extent of sorption is compared in Table 3. Total mass sorbed was normalized by total mass introduced during the same period. Total sorbed mass was determined as the difference in mass that did not sorb between the reference chamber and chamber containing material. For each chamber, the mass that did not sorb was determined as the total mass outflow during the adsorption stage plus the mass in the chamber at the end of the adsorption stage. Two values are listed for each component and correspond to replicate experiments. Padding was estimated to be responsible for 4 to 8% (MTBE) to 77% (o-dichlorobenzene) of total introduced mass, while structural backing sorbed from 0 to 6% (MTBE) to 42% (1,2,4-trichlorobenzene). Effects of Chemical Properties. Over all experiments, only a small amount of sorption occurred for MTBE and cyclohexane. The greatest amount of sorption was observed for

TABLE 3. Summary of the Ratio of Total Sorbed Mass and Total Introduced Mass (M/Min) for a Carpet System and Individual Components

TABLE 5. Correlation between Keq and Octanol-Air Partition Coefficienta

chemicals material

MTBE

fibers only

0.11 0.02 backing only 0.06 0.00 pad 1 only 0.08 0.04 carpet 1/pad 1 0.04 0.07

CH

IP

TOL

PCE

EB

DCB TCB

0.01 0.00 0.01 0.00 0.10 0.09 0.10 0.11

0.09 0.14 0.23 0.31

0.01 0.01 0.03 0.02 0.46 0.45 0.41 0.43

0.01 0.01 0.05 0.02 0.46 0.46 0.41 0.42

0.02 0.01 0.08 0.03 0.61 0.60 0.51 0.50

0.03 0.03 0.30 0.18 0.77 0.77 0.59 0.60

0.06 0.06 0.42 0.32 0.68 0.73 0.60 0.58

materials

parameter

intercept

slope

R2

carpet 1 (Cp1) carpet 1/padding (Cp1.p) carpet 2 (Cp2) carpet 2/padding (Cp2.p) carpet 3 (Cp3) carpet 3/padding (Cp3.p)

Keq Keq Keq Keq Keq Keq

0.12 0.67 0.44 0.81 0.16 0.59

1E-4 4E-4 2E-4 3E-4 1E-4 2E-4

0.98 0.94 0.98 0.84 0.98 0.90

a Relationships shown in this table are between the equilibrium partition coefficient and octanol-air partition coefficients (no logarithms used).

TABLE 4. Correlation between Sorption Parameters and Vapor Pressurea materials carpet 1 (Cp1) carpet 1/padding (Cp1.p) carpet 2 (Cp2) carpet 2/padding (Cp2.p) carpet 3 (Cp3) carpet 3/padding (Cp3.p)

parameters intercept

Keq Keq Keq Keq Keq Keq

-3.88 -2.76 -2.63 -2.67 -3.52 -2.74

slope

R2

-0.74 -0.82 -0.67 -0.78 -0.67 -0.75

0.99 0.94 0.99 0.76 0.98 0.89

a Relationships shown in this table are between the natural logarithms of equilibrium partition coefficient and vapor pressure.

FIGURE 4. Relationship between Keq and vapor pressure (atm) for carpet 1. o-dichlorobenzene and 1,2,4-trichlorobenzene. In general, sorption capacities, i.e., Keq, were inversely related to vapor pressure. These results are consistent with those reported by van der Wal et al. (4) who observed an increase in the sorption capacity of indoor materials with increasing adsorbate boiling point (decreasing vapor pressure). Empirical relationships between Keq and vapor pressure were determined for the specific carpets that were tested. As listed in Table 2, values of Keq could not be determined for MTBE, cyclohexane, and 2-propanol when a carpet cushion was not used. Furthermore, as described previously, analytical/experimental problems associated with 1,2,4-trichlorobenzene led to relatively high coefficients of variation between several replicate experiments for this compound. For this reason, 1,2,4-trichlorobenzene was omitted from the regression analysis. Thus, the number of data points used for each analysis ranged from 4 to 7. An example relationship between Keq and vapor pressure is illustrated in Figure 4 for carpet 1. Correlation results between ln(Keq) and ln(vapor pressure) are summarized in Table 4. The arithmetic mean R2 value for all Keq analyses was 0.93, with the highest R2 values being associated with carpet without an underlying cushion. This may be due to the additional phenomenon of molecular diffusion into

FIGURE 5. Effects of relative humidity on sorption (o-dichlorobenzene, carpet 1/pad 1). porous urethane cushions, i.e., reducing the correlation between a single chemical property (vapor pressure) and Keq. The slope of the logarithmic relationship between Keq and vapor pressure fell within a fairly narrow range (-0.67 to -0.82) for the six carpet systems. These values are slightly less than a slope of unity that was derived theoretically based on fundamental thermodynamic principles (for a single adsorbent) and n ) 1 by An et al. (13). Similar to the way that octanol-water partition coefficients are used to predict equilibrium partitioning between chemicals in the aqueous phase and adjacent organic phases, the octanol-air partition coefficient (Koa) may be a useful parameter for estimating the extent to which airborne gases will partition to an organic phase at equilibrium (14). Since many indoor materials have an organic phase, e.g., indoor plants, or may be purposely, e.g., wax on wood floors, or inadvertently, e.g., dirt or dust, coated with an organic phase, a relationship between Koa and Keq would not be surprising. For this study, linear relationships were also observed between Koa and Keq for the carpet systems that were tested. The results of a least-squares linear regression analysis are summarized in Table 5. While these results appear promising, there was a significant gap in Koa values between 5000 and 20,000 Lair/Loctanol for the chemicals that were studied. Additional experiments should be completed to improve the relationship for chemicals with Koa in this range and for Koa greater than 20,000 Lair/Loctanol. Effects of Relative Humidity. Carpet 1 with pad 1 was tested with three different relative humidity profiles, each beginning at 50-70% and varying to 0, 50, and 80% RH within hour six of the adsorption stage. Relative humidity had no observable effect on sorption parameters for all nonpolar VOCs. Figure 5 provides an example involving carpet 1/pad 1 and o-dichlorobenzene. The only VOC that was significantly influenced by RH was 2-propanol. Values of Keq actually increased with increasing RH (0.68 m at 0% RH; 1.1 m at 50% RH; 1.4 m at 80% RH) as shown in Figure 6. It is conceivable that this was due to the absorption of 2-propanol into water that condensed in the pores of the urethane cushion or to VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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centrations that were tested in this study appeared to be low enough to exhibit a linear relationship between concentration in air and mass on the material phase. van der Wal et al. (4) reported similar results for TVOC concentrations that ranged from 10.5 to 43.6 mg/m3.

Acknowledgments This study was funded by BP Oil and Exploration, Inc (BP). The authors thank Mr. Jim Rocco and Ms. Lesley Hay-Wilson of Sage Environmental (formerly of BP Oil and Exploration) for their insightful comments, enthusiasm, and patience. FIGURE 6. Effects of relative humidity on sorption of 2-propanol (carpet 1/pad 1).

FIGURE 7. Effects of inlet gas concentration on sorption (tetrachloroethene, carpet 1/pad 1). the carpet surface. The solubility of 2-propanol is 26 times greater than that of the second most soluble target VOC (MTBE), and it has a much lower Henry’s law constant than any of the other VOCs. Effects of Inlet Gas Concentration. Three inlet gas concentrations were tested with carpet 1/pad 1. Normalized gas concentration profiles for tetrachloroethene are plotted in Figure 7 for 2.5, 5, and 15 ppm. Inlet gas concentration did not have a significant effect on the degree of sorption. A similar trend was observed for other VOCs. Thus, con-

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Literature Cited (1) Dietert, R. R.; Hedge, A. Crit. Rev. Toxicol. 1996, 26(6), 633. (2) Hodgson, J. D.; Wooley, J. D.; Daisey, J. M. J. Air Waste Manage. Assoc. 1993, 43, 316. (3) Sollinger, S.; Levsen, K.; Wunsch, G. Atmos. Environ. 1994, 28, 2369. (4) van der Wal, J. F.; Hoogeveen, A. W.; van Leeuwen, L. Indoor Air 1998, 8, 103. (5) Sparks, L. E.; Tichenor, B. A.; White, J. B.; Jackson, M. D. Indoor Air 1991, 1, 577. (6) Tichenor, B. A.; Guo, Z.; Dunn, J. E. Indoor Air 1991, 1, 23. (7) Meyer, B. Indoor Air Quality; Addison-Wesley Publishing Co., Inc.: 1983; p 301. (8) SAS Institute. In SAS/STAT User’s Guide; 1990; pp 1154-1155. (9) Lyman, W. J.; Reehl, W. F.; Rosenblatt, D. H. In Handbook of Chemical Property Estimation Methods: Environmental Behavior of Organic Compounds; McGraw-Hill: 1982; pp 2.12.52. (10) Buckley, T. J.; Prah, J. D.; Ashley, D.; Zweidinger, R. A.; Wallace, L. A. J. Waste Manage. Assoc. 1997, 47, 739. (11) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. In Environmental Organic Chemistry; John Wiley & Sons: New York, 1993; pp 618-625. (12) Verschueren, K. In Handbook of environmental data on organic chemicals, 3rd ed.; Van Nostrand Reinhold: 1996. (13) An, Y.; Zhang, J. S.; Shaw, C. Y. HVAC&R Res. 1999, 5(4), 297. (14) Harner, T.; Mackay, D. Environ. Sci. Technol. 1995, 29(6), 1599.

Received for review September 9, 1999. Revised manuscript received July 12, 2000. Accepted July 24, 2000. ES9910412