Environ. Sci. Technol. 1998, 32, 640-646
Kinetics and Potential Significance of Polycyclic Aromatic Hydrocarbon Desorption from Creosote-Treated Wood BONDI GEVAO AND KEVIN C. JONES* Environmental Science Department, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, United Kingdom
PAHs are major constituents of creosote, and it has been suggested that their volatilization from treated wood could make a major contribution to the U.K. atmospheric emission inventories. This paper reports a study to elucidate the volatilization characteristics of selected PAHs from creosotetreated wood and to estimate the potential significance of this source to the U.K. atmospheric source inventory. Desorption kinetics from untreated and treated wood were measured at 4 and 30 °C for five PAHs (acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene) using chambers in the laboratory. Rates of desorption followed first-order kinetics for all compounds and were higher at 30 than 4 °C. Mean ∑PAH fluxes varied from 2.57 ( 1.52 to 29.5 ( 6.1 mg/m2 treated wood/day at 4 and 30 °C, respectively. A prolonged study of the kinetics at 4 °C showed that the volatilization rate remained constant over about 7 weeks, after which >85% of the compounds still remained sorbed on the wood. Initial desorption rates were governed by partitioning between the substrate (wood) and air and subsequently by the rates of compound diffusion from the interstices of the wood. A strong negative correlation was observed when the percent increment in the desorption rate from 4 to 30 °C was plotted as a function of the octanol-air partition coefficient (KOA) (r 2 ) -0.93); vapor pressure was strongly positively correlated (r 2 ) 0.92). Desorption half-lives ranged from 0.7 to 31 yr at 4 °C and from 0.3 to 1 yr at 30 °C for fluoranthene and acenaphthene, respectively. Estimates of annual U.K. emissions for the compounds specified from freshly treated wood (i.e., ignoring long-term releases from previously treated wood) were subject to a number of important assumptions, but were in the range of ∼100 t ∑PAH. This is less than the annual U.K. emissions of ∑PAHs estimated for domestic heating (∼600 t) and believed to be similar to that from vehicle emissions (∼80 t) at the present time.
Introduction Creosote is a widely used wood-preserving product that is derived from the distillation of coal tar, a byproduct of the coking process. It is a brownish-black, oily liquid with a density slightly greater than that of water. It is a complex mixture of many different organic chemicals, including polycyclic aromatic hydrocarbons (PAHs) (80-85%), mono* Corresponding author e-mail:
[email protected]; tel: +44 1524 593972; fax: +44 1524 593985. 640
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cyclic aromatic hydrocarbons (5-15%), phenolic compounds (5-12%), and 5% N-, S-, and O-heterocyclics (1-4). Certain components in creosote, e.g., high molecular weight PAHs, persist in contaminated environments and are toxic, carcinogenic, or teratogenic (4-7). Since the beginning of the construction of railways, wooden sleepers (railway cross ties) have been impregnated with creosote before being placed under the rails. Depending on the type of wood, it is estimated that between 20 and 25 L/m3 creosote is pressed into the sleepers by the vacuum high-pressure method (8). Other preservative application modes include dipping, deluging, and spraying. This prevents them from rotting for decades, especially in regions of moderate climate (8, 9). Creosote is also used to treat utility poles, bridge timbers, and garden and household timber. Following treatment, the timber is dried before use. This is achieved by allowing the solvent to evaporate from the treated timber in the open atmosphere (8), which potentially leads to the volatilization of constituents into the atmosphere. Volatilization of PAHs and other constituents from creosotetreated wood has a potential influence on indoor air quality (7). Much research effort has been focused on remediation of creosote-contaminated land, the dissolution behavior of creosote, and its potential to contaminate groundwater at these sites (4, 10-12). However, very little is known about the volatilization of PAHs from creosote-treated wood. This is somewhat surprising given the substantial environmental ‘reservoir’ of PAHs that treated wood (and soils) represent. The U.K. Department of the Environment (DoE) (13), for example, has estimated that approximately 40 000 t of creosote are manufactured each year in the U.K., 25% of which is exported, 25% is used by industry, and 50% is used for immersion treatment and retail domestic use. Wild and Jones (14) estimated the input of PAHs associated with creosote into the U.K. environment to be 25 000 t of ∑PAHs [sum of 12 individual parental PAHssnaphthalene (Naph), acenaphthene (Ace), fluorene (Fluo), phenanthrene (Phen), anthracene (Anth), fluoranthene (Fla), pyrene (Py), benz[a]anthracene (B[a]A), chrysene (Chry), benzo[b]fluoranthene (B[b]F), benzo[a]pyrene (B[a]P), benzo[ghi]perylene (B[ghi]P)] annually and noted that the volatilization of PAHs from treated wood was a potentially major but very uncertain source to the U.K. atmospheric emissions inventory. This study therefore presents the first results of laboratory measurements for the non-steady-state desorption/volatilization of PAHs from the surface of freshly creosote-treated wood and assesses their potential importance to the atmospheric emissions of PAHs. It is to be noted, however, that the rate of release of PAHs from painted wood (as in this study) may not accurately model release rates from the vacuum high-pressure system industrially in wood treatment, since impregnation is designed to reduce releases.
Experimental Section Experimental Setup. Glass chambers (h ) 24.5 cm, l ) 27.5 cm, w ) 7 cm) were constructed with two orifices to serve as the inlet and outlet for air. Each chamber was made airtight with a metallic casing, and a peristaltic pump was used to induce air flow through the chambers. The inlet air was cleaned prior to entering the chamber by passing it through a pre-extracted polyurethane foam (PUF) trap, and the target compounds volatilizing from the wood were trapped from the outlet air on a second PUF trap placed after the chamber. Flow meters were introduced at the entrance of the chamber and after the second PUF trap. Steady-state airflow conditions of 10-3 m3/min were maintained throughout the S0013-936X(97)00641-X CCC: $15.00
1998 American Chemical Society Published on Web 01/22/1998
FIGURE 1. PAH flux from untreated wood (mg m-2 h-1). experiment, which ensured that there was no buildup of the volatilizing compounds. All the tubing was made of Teflon, and connectors were stainless steel to minimize compound sorption. The total exposed surface area of wood was 0.118 m2 painted with approximately 120 mL of creosote of density 0.924 g/mL. Planks of Yellow Pine (Pinus strobus) with moisture content of 9.6% were used in this study. Sampling. Experiments were carried out in duplicate in two different rooms maintained constantly at 4 and 30 °C to approximately simulate U.K. average winter and maximum summer temperatures. Prior to use of treated wood in the chambers, blank levels were established. Blanks consisted of untreated wood of the same dimensions as those of the treated wood. Wood was placed in the chambers, and air was circulated for 90 h. PAH data from the first PUF was used to determine the concentrations of the target compounds in the different rooms prior to the start of the experiments, while the second trap was used to assess the flux of compounds from the untreated wood. Following this, the treated wood that had been painted with 110 g of creosote was placed in the chambers within 5 min following treatment, and PUF samples were collected by changing the traps after certain predetermined time intervals. The samples were stored for between 2 and 4 weeks in clean glass jars with hexane-rinsed aluminum foil at -17 °C prior to extraction. Extraction and Analysis. The samples were extracted in a Soxhlet apparatus for 18 h with dichloromethane (DCM), rotary evaporated, and transferred to 15-mL vials. The samples were then blown down under a gentle stream of nitrogen to about 1 mL and chromatographed on a silica gel column. The 35 cm long column was slurry packed (to the height of 30 cm from the sinter) with silica gel that had been activated at 120 °C for 17 h using hexane. The silica was capped with 2 mm of anhydrous sodium sulfate to prevent it from going dry and to prevent the disturbance of the column (by resuspension) when the sample was added. The samples were charged on top of the column and allowed to drain through the bed of the Na2SO4. This was followed by the addition of 30 mL of hexane (F1) and 15 mL of 60% hexane/
40% DCM (F2) in sequence. Both fractions (F1 and F2) were discarded as they contained mainly the aliphatics and other nontarget compounds. The PAHs were then eluted with 25 mL of 60:40 hexane:DCM and collected in a volumetric flask. The solvent was reduced under a stream of nitrogen and solvent exchanged to hexane. The samples were diluted accordingly and analyzed on a Hewlett-Packard HP5890A gas chromatograph equipped with a flame ionization detector using splitless injection on a cross-linked 5% phenyl methyl silicone chromatographic column for separation (30 m × 0.25 mm i.d. and 0.25 mm film thickness). The temperature program was as follows: 50 °C for 2 min, then 20 °C/min to 130 °C, then 6 °C/min to 310 °C, and held at this temperature for 10 min. The injector temperature was kept at 290 °C, and the FID was at 300 °C. The column flow was maintained at 1 mL/min with a head pressure of 20 psi, and the N2 makeup gas was maintained at 36 mL/min. Air and H2 pressures were 320 and 185 kpa with flow rates of 400 and 48 mL/min, respectively. The septum was purged at a rate of 1.8 mL/ min, and samples were purged on at 2 min and off at 45 min. The total run time was 46 min. Identification of PAHs was carried out by overlaying the chromatogram of a standard mix of 11 compounds onto the sample chromatogram by matching the peaks by their retention times. The PAHs routinely present in the samples were the low molecular weight compounds Naph, Ace, Fluo, Phen, Anth, Fla, and Py. Naphthalene was omitted from the analysis because its recoveries were not consistent due to its high volatility. Quantification was by external calibration against a set of five standards prepared from the standard mix. The analytical procedure was assessed by spiking five PUF plugs with the standard mix, which was then extracted and analyzed as described above. Mean and standard deviations of recoveries for five replicate analyses ranged from 84 ( 5% for the low molecular weight, tricyclic PAHs to 104 ( 0.9% for the higher molecular weight (g 4 rings) compounds. The accuracy and precision of the method employed in the analysis were further assessed by extracting two marine sediment reference materials (HS6 and HS4) that have been VOL. 32, NO. 5, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. (A-F) First-order plots for desorption of PAHs from creosote-treated wood at 4 °C. certified for certain PAHs. The results obtained were within 10-20% of the quoted value for >80% of the certified PAHs in the reference materials.
area of the wood (m2), and Cout and Cin are the spatial average or representative concentrations of the target compounds (ng/m3) measured in a time interval (t2 - t1).
Results and Discussion
Compound-Specific Desorption Rates. It was assumed that volatilization of compounds from the treated wood could be approximated to a first-order process. The data obtained were therefore tested against the first-order model:
Background and Blank Concentrations. Background PAH fluxes from the untreated wood (blanks) over a 90-h period show that lower molecular weight PAHs (notably phenanthrene, fluorene, and pyrene) dominated the volatilized fraction and were enhanced at 30 °C (Figure 1). As mentioned earlier, the chambers were at steady state with respect to the rate of air flow through them throughout the experiment. The fluxes were thus calculated from
J ) Q(Cout - Cin)/A
(1)
where J is the mass flux (ng m-2 h-1), Q is the constant flow rate of air through the chamber (m3 h-1), A is the total surface 642
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dx/dt ) k[a]
(2)
where dx/dt is the rate of desorption, x is the amount desorbed at time (t), a is the initial amount present, and k is the first-order rate constant. Thus, plotting the integral of eq 2 gives a straight line whose slope is the desorption rate constant
ln a/(a - x) ) kt
(3)
FIGURE 3. (A-F) First-order plots for desorption of PAHs from creosote-treated wood at 30 °C. To test the data against the first-order model, it was assumed that the individual PAH compounds behave independently in the creosote mixture, degradation of the compounds is negligible, and that local equilibrium occurs with respect to partitioning between the air and wood/ creosote phases. Thus the release rates were obtained individually by plotting ln a/(a - x) vs t as shown in Figures
2(a-f) and 3(a-f). The desorption of PAHs from creosotetreated wood was first order over the study period of 50 h at 30 °C (Figure 3). At 4 °C, the desorption rate was initially first order for 1000 h for Ace and Fluo but became rate limited after this time (Figure 2). The rate of the other compounds at this temperature were still first order even after 2000 h (see Figure 2). The experiment at 30 °C was terminated after VOL. 32, NO. 5, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. (a) Graph of percent increase in rate from 4 to 30 °C vs vapor pressure. (b) Graph of incremental factor in rate from 4 to 30 °C vs log KOA of the PAHs monitored in the study. 5 days and therefore presumably did not have long enough to become rate limited. Presumably volatilization of the PAHs from the treated wood may be controlled by a number of complex processes occurring at the creosote-wood/air interface including transport to the surface of the wood, desorption from the surface, diffusion through the stagnant boundary layer, and convection transport by turbulent air (15). We hypothesize that the rate of desorption is initially controlled by partitioning of the PAHs between the wood surface and the air in contact with it, but that the kinetics is thereafter controlled by slow diffusion of the compounds from the interstices of the wood to the wood/air interface. It is worth noting that at the time the rates attain a plateau (e.g., see Figure 2) >85% of the compounds originally present remained sorbed onto the wood. Presumably treated wood may then still continue to release PAHs to the atmosphere, but at a slower rate. 644
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Temperature and Vapor Pressure Dependence of Volatilization. The rate constants for the PAH volatilization fluxes monitored together with their half-lives and selected physicochemical properties are presented in Table 2. Clearly initial volatilization rates are higher at 30 °C than at 4 °C, and the half-lives are higher at 4 °C than at 30 °C. The percentage increase in the rate of desorption between 30 and 4 °C is strongly positively correlated (r 2 ) 0.92) with vapor pressure (Figure 4a). The deviation from a straight line relationship in the plot may be due to the reduction in the effective vapor pressure of the PAHs as compared with those of their pure compounds, due to adsorptive effects of the wood (16). Thus the type of wood and its adsorptive capacity is expected to have a marked influence on volatilization. The strong negative correlation (r 2 ) 0.93) between the increase in desorption rate from 4 to 30 °C and log KOA (Figure 4b) indicates that creosote has properties that approximate that
TABLE 1. PAH Compound-Specific Chemical Composition of Commercial Creosote Used in the Study and an Estimate of the Amount of PAHs Entering the U.K. Environment from Creosote Use (t) compound
concn (µg/g)
wt %
ton of PAHs entering U.K. environment from current creosote usea
acenaphthene fluorene phenanthrene anthracene fluoranthene pyrene benz[a]anthracene chrysene benzo[a]pyrene benzo[ghi]perylene
1080 1650 1210 959 69 81 18 11 5.4 3.8
21 32 23 18 1.3 1.6 0.34 0.22 0.1 0.07
27 41 30 24 1.7 2 0.5 0.3 0.1 0.1
a Derived from the product of the concentration and the DoE (13) estimate of current creosote use in the U.K.
TABLE 2. Experimental and Calculated Volatilization Rate Constants, Estimated Half-Lives, and Some Physicochemical Properties of the PAHs Studied rate constant (h-1)
compound
4
°Ca
10
°Cb
18
°Cb
30
°Ca
acenaphthene 2.55E-06 2.67E-05 5.37E-05 7.54E-05 fluorene 2.65E-05 2.27E-04 4.31E-04 5.87E-04 phenanthrene 4.87E-05 2.16E-04 3.36E-04 4.17E-04 anthracene 1.45E-05 3.14E-05 3.95E-05 4.42E-05 fluoranthene 1.14E-04 2.06E-04 2.46E-04 2.68E-04 a
Experimental.
b
half-life in the log wood (yr) KOA 35.4 3.5 2.2 10.0 1.4
6.23 6.68 7.45 7.34 8.6
Calculated.
FIGURE 5. Percent relative contribution from different sources to the U.K. PAH atmospheric burden (adapted from ref 14).
of octanol, and the KOA or Henry’s law constant of the compounds influences desorption. An approach To Estimate U.K. Emissions of PAHs from Creosote-Treated Wood. The contribution of any PAH source to the atmosphere will depend on a number of factors including the emission rate of the source, its geographical location, and the local climatic conditions. As stated earlier, the rate of volatilization of PAHs from treated wood approximates to a first-order process. The first-order rate equation has a solution of the form:
The estimates were carried out on a ‘seasonal’ basis because the rate of release was shown to be temperature dependent. The average temperature in the U.K. for November, December, January, and February was assumed to be 4 °C; in September, October, May and April to be 10 °C; and in June, July, and August to be 18 °C. Using the experimental rates obtained at 4 and 30 °C, the compoundspecific rates at 10 and 18 °C were calculated using the integral form of the Arrhenius equation:
C ) Coe-kt
ln k ) Ea/RT + ln A
(4)
where C is the concentration volatilized after time t, Co is the initial concentration of the compound, and k is the compound-specific rate constant (h-1). The following assumptions were made in carrying out the emission estimates: (a) that all the creosote marketed in the U.K. is used for either ‘brush’ (e.g., the painting of fences) or industrial applications; (b) that degradation of the PAHs were negligible while sorbed onto the surface of application; (c) that the individual compounds behave independent of each other in the creosote mixture; and (d) that no creosote is imported into the U.K. The contribution of old treated wood to the emission estimates was omitted, because we were uncertain about the long-term kinetic behavior of PAHs from treated wood and the historical use pattern of creosote in the U.K. Nonetheless, it should be noted that this omission will underestimate the annual emission estimates, possibly substantially. Rotard and Mailahn (9), for example, found significant emission of PAHs from treated timber 30 yr after treatment, and our estimates above showed that treated wood remained a substantial reservoir over many months. The estimates below are therefore for current annual releases from creosote use to the U.K. atmosphere.
(5)
where k is the rate constant (h-1), A is the preexponential factor, Ea is the activation energy, and R is the molar gas constant (J mol-1 K-1). At two different temperatures T1 and T2:
ln(k1/k2) ) Ea/R(T2 - T1/T2T1)
(6)
where T2 > T1. Using the rates at 4 and 30 °C, the compound-specific activation energies of desorption were calculated from eq 6 and subsequently used to estimate the desorption rates at 10 and 18 °C from the same equation. Using the DoE (13) data mentioned in the Introduction, we estimated that ∼30 000 t of creosote is used in the U.K. each year. From the PAH compound-specific chemical analysis of creosote from Table 1, the annual burden (ton) of individual PAHs entering the U.K. environment each year in treated wood was calculated and given in Table 1. From this and the desorption rates at 4, 10, and 18 °C, the compound-specific emissions were calculated for the different defined seasons and combined to give the annual emissions (Figure 5). It is estimated that PAHs entering the U.K. environment from this source is ∼100 t (Ace, 5.6; Fluo, VOL. 32, NO. 5, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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51; Phen, 36; Anth, 5.5; Fla, 2 t). The estimates suggest that, depending on the volatility of the compound, the amounts desorbed from creosoted wood in warmer periods can be a factor of 2-14 that in the colder periods. It has been demonstrated that the use of creosote has the potential to release considerable quantities of PAHs to the U.K. environment. Although our estimates probably underestimate the importance of PAH emission from treated wood (because of uncertainties of the contribution from older treated wood), they still show volatilization from creosote to be of the same order as vehicle emissions (see Figure 5). Clearly these estimates need to be substantiated and updated, but they serve to indicate the potential contribution of this diffuse source and widespread activity to the U.K. ∑PAH emission inventory. It should also be noted that the release to air is dominated by the lighter compounds that are generally believed to be of less toxicological concern than the higher molecular weight PAHs.
Acknowledgments We wish to thank Paula Woolgar for undertaking the analysis of the commercial creosote product and Ruth Alcock for helping with information on current creosote use in the U.K.
Literature Cited (1) Davis, M. W.; Glaser, J. A.; Evans, J. W.; Lamar, R. T. Environ. Sci. Technol. 1993, 27, 2572-2576. (2) Deschenes, L.; Lafrance, P.; Villeneuve, J. P.; Samson, R. Hydrol. Sci. J. 1995, 40, 471-484. (3) Kawahara, F. K.; Davila, B.; Alabed, S. R.; Vesper, S. J.; Ireland, J. C.; Rock, S. Chemosphere 1995, 31, 4131-4142. (4) Mueller, J. G.; Chapman, P. J.; Pritchard, P. H. Environ. Sci. Technol. 1989, 23, 1197-1201.
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(5) Mueller, J. G.; Middaugh, D. P.; Lantz, S. E.; Chapman, P. J. Appl. Environ. Microbiol. 1991, 57, 1277-1285. (6) Mueller, J. G.; Lantz, S. E.; Ross, D.; Colvin, R. J.; Middaugh, D. P.; Pritchard, P. H. Environ. Sci. Technol. 1993, 27, 691-698. (7) Jones, P. W.; Freundenthal, R. I. Carcinogens: A comprehensive survey, Vol. 3, Polynuclear aromatic hydrocarbons; Raven Press: New York, 1978. (8) Her Majesty’s Inspectorate of Pollution. Timber preservation processes; EPA 1990; Process Guidance Note IPR 6/3; London, U.K. (9) Rotard, W.; Mailahn, W. Anal. Chem. 1987, 59, 65-69. (10) Priddle, M. W.; Macquarrie, K. T. B. J. Contam. Hydrol. 1994, 15, 27-56 (11) Fowler, M. G.; Brooks, P. W.; Northcott, M.; King, M. W. G.; Barker, J. F.; Snowdon, L. R. Org. Geochem. 1994, 22, 641649. (12) Kiilerich, O.; Arvin, E. Ground Water Monit. Rem. 1996, 16, 112117. (13) Department of Environment (DoE) Report, 1988. (14) Wild, S. R.; Jones, K. C. Environ. Pollut. 1995, 88, 91-108. (15) Stork, A.; Witte, R.; Fuhr, F. Environ. Sci. Pollut. Res. 1994, 1, 234-245. (16) Spenser, W. F.; Farmer, W. J.; Cliath, M. M. Residue Rev. 1973, 49, 1-8. (17) Mackay, D.; Shiu, W. Y.; Ma, K. C. Illustrated handbook of physical-chemical properties and environmental fate for organic chemicals, Vol. II, polynuclear aromatic hydrocarbons, polychlorinated dioxins and dibenzofurans; Lewis Publishers: Chelsea, MI, 1992.
Received for review July 22, 1997. Revised manuscript received November 19, 1997. Accepted November 20, 1997. ES9706413
