Environ. Sci. Technol. 1997, 31, 71-74
Modeling Volatilization of PCDD/F from Soil and Uptake into Vegetation STEFAN TRAPP* AND MICHAEL MATTHIES Institute of Environmental Systems Research, University of Osnabru ¨ ck, D-49069 Osnabru ¨ ck, Germany
Soil contamination is one of the long-term environmental problems generated by anthropogenic activities of the last decades. Standards for some organic compounds are being discussed. We investigated the question of whether volatilization of organic chemicalsshere PCDD/Fsfrom soil and subsequent sorption to leaves may be a significant pathway for contamination. This transfer pathway is investigated with a mathematical model. The model is based on an analytical solution of the diffusion/dispersion equation for two media and equilibrium assumptions. A calculation is made for 2,3,7,8-TCDD uptake into herbage. It can be shown that this transfer pathway may only be important for highly contaminated soils. The aim of reducing the daily intake of PCDD/F by contaminated food is best reached by an effective emissions control. Volatilization of persistent lipophilic chemicals from the soil is a long-term process and can lead to a secondary pollution of the atmosphere.
Introduction Soil contamination is one of the long-term environmental problems generated by anthropogenic activities of the last decades. Standards for some organic compounds are being discussed in the European Union (1) and elsewhere but have not yet been agreed upon. For a long time, soils have been considered as a filter or a sink for pollutants. However, pollutants may desorb from soil and pollute the air or vegetation. Compartment models for uptake from soil into plants have been developed (2), but they do not include the pathway of volatilization from soil and subsequent uptake into plants. The competitive uptake of 2,3,7,8-TCDD from soil and air into leaves has been calculated with a generic one-compartment model. It was found that uptake from air is the faster process (3). The question arose whether volatilization from soil and subsequent sorption to leaves may be a significant pathway for contamination.
Analytical Solution of Diffusion-Advection Equation for Two Neighboring Media Bulk Soil and Vegetation Soils are usually not well mixed. Volatilization from soils, therefore, cannot be calculated with Ficks first law but requires Ficks second law. The problem of volatilization from soil has been addressed by Jury et al. (4, 5). In their solution, the air was considered as a perfect sink (i.e., the air concentration is zero). Here, we try to find a solution for the calculation of concentrations in the vegetation covering the soil. A ‘standard solution’ for the diffusion between two neighboring but not well-mixed phases is used originating from Jost (1937), cited in refs 6 and 7. * Corresponding author e-mail:
[email protected]; telephone: +49/(0)541 969 2574; fax: +49/(0)541 969 2599.
S0013-936X(96)00133-2 CCC: $14.00
1996 American Chemical Society
Composed Media. Originally, the solution is for two pure phases. Bulk soil and vegetation are composed of more than one phase. This needs to be considered. The medium bulk soil, index B, is composed of the phases soil matrix, soil solution (which is neglected here), and gas phase. CB is the mass of chemical per volume of bulk soil (kg/m3 of soil). The boundary between the two media is the soil surface at z ) 0. The second medium, the so-called vegetation layer (index V), is composed of plants and air. We use a help variable, CV, which is the mass of chemical per volume of vegetation. This is the sum of the masses in air and sorbed to plant material divided by the considered volume. We assume immediate and completely reversible local equilibrium between the several phases. The equilibrium condition is described by the partition coefficient:
KIJ ) CI/CJ
(kg/m3 of phase I:kg/m3 of phase J)
(1)
where CI and CJ are the concentrations in phase I and J, respectively (kg/m3 phase). Only the gaseous fraction of the chemical in each medium is considered to be mobile. Principially, the methodology is identical to that used by Jury et al. (4, 5). It is applied now to vegetation, too. Analytical Solution. The analytical solution of the diffusion equation requires the following conditions. Initial Conditions. Two semi-infinite vertical layers. Medium 1 (z < 0, bulk soil B) is burdened with CB (z < 0, t ) 0) ) CB(0). Medium 2 (z > 0, vegetation layer V) is clean, CV (z > 0, t ) 0) ) 0. Boundary Conditions. At z ) 0, the concentration is assumed to be in local equilibrium for all t > 0, described by the partition coefficient KVB (kg/m3 of vegetation layer:kg/m3 of bulk soil):
KVB ) CV/CB
z ) 0, t > 0
The diffusive substance flux out of soil is the substance flux into the vegetation layer (and vice versa):
DB ∂CB/∂z ) DV ∂CV/∂z
z ) 0, t > 0
where DB is the effective bulk soil gas diffusion coefficient (m2/d) and DV is the effective gas diffusion/dispersion coefficient in the vegetation (m2/d), see below. Both are constant in space and time. The analytical solution of this problem is
for z < 0, bulk soil CB(z,t) ) CB(0) CB(0)
KVBxDV KVB xDV + xDB
[ (x )] z
1 + erf
(2a)
(4DBt)
and for z > 0, vegetation layer
[ (x )]
KVBxDB CV(z,t) ) CB(0) 1 - erf KVBxDV + xDB
z
(2b)
(4DVt)
where erf is the error function that is available from tables or as an approximation (8).
Parametrization for Volatilization into Vegetation It has been shown that very lipophilic chemicals are usually not well translocated within plants (9). However, they may
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strongly accumulate from air into leaves (10), and this may also occur after volatilization from soil. To simplify the problem, we restrict ourselves to chemicals that are not well translocated within plants (log Kow is large), do not move with soil water (log Kow is large), have a tendency to volatilize (Kaw is not too small, i.e., > 10-4), and sorb to plant leaves (log Kow is large, vapor pressure is small). We neglect ploughing, kryoturbation, and bioturbation. If we have these preconditions, we can restrict ourselves to transport in the gas phase of the soil. An example of this type of chemical is 2,3,7,8-TCDD. Basic physicochemical properties needed for the following calculations are (11) as follows: partition coefficient octanol to water log Kow ) 6.76; dimensionless Henry’s law constant Kaw ) 0.0015. For illustration, values for the chemical 2,3,7,8-TCDD are calculated. Diffusion in Soil. An estimation for the gaseous diffusion coefficient from molar mass M is
DG,TCDD ) DG,H2O x(18/322)
(m2/d)
(3)
Water vapor is used as the reference substance, with DG,H2O ) 2.2 m2/d. It follows that DG of 2,3,7,8-TCDD is 0.5 m2/d. To yield the ‘effective’ diffusion coefficient, a correction with the labyrinth or tortuosity factor is used (4):
T ) ( - θ)10/3/2
(4)
where is the total porosity in soil (m3 of pores/m3 of soil) and θ is the fraction of water-filled pores (m3/m3). For sandy soil where is 0.50 m3/m3 and θ is 0.15 m3/m3, T is 0.12 and
DG,eff ) TDG ) 0.06 m2/d
(2,3,7,8-TCDD) (5)
Diffusion in the gas phase of the soil only occurs for the gaseous fraction fg of the chemical (mass in gas phase divided by total mass in soil). An estimate of fg for strongly sorbing chemicals (soil solution neglected) is
fg ≈ ( - θ)/KBA
(6)
where KBA is the partition coefficient bulk soil/air ≈ FKd/Kaw, F is the soil density (here, 1.3 kg/L), Kd is the linear ad/desorption coefficient (L of water/kg of matrix) ≈ OC Koc (with Koc ) 0.411 Kow ) 2 365 058 L/kg), and OC is the organic carbon content (here, 0.01 kg/kg). For 2,3,7,8-TCDD, it follows that Kd is approximately 23 650 L/kg, the partition coefficient between bulk soil and air KBA is 2.05 × 107, fg is 1.7 × 10-8, and the effective bulk soil gas diffusion coefficient of the chemical is
DB ) DG,eff fg ) 10-9 m2/d
(7)
Transport in Vegetation Layer. For the calculation of the effective gas diffusion/dispersion coefficient in the vegetation layer DV (m2/d), we use the same approach. Gaseous and Sorbed Fraction. The space above the soil surface, the vegetation layer, is considered to be composed of gas phase and the sorbing plant tissue. Since plants are immobile (apart from growth, harvest, and wind-induced movement), only molecules in the gas phase are transported. The help variable CV, which is the mass of chemical per volume of vegetation layer, is found from
CV ) (CPVP + CAVA)/VV
(8)
where VV ) VP + VA is the total volume of the vegetation layer, VP is the volume of the aerial plant tissue, and VA is the volume of air (m3); CA and CP are the concentrations in air and plant tissue, respectively (kg/m3 phase). Using the equilibrium condition, we can replace CP by CP ) CAKPA, where KPA is the partition coefficient between plants and air; when we restrict
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ourselves to leaves, a recently calculated value of approximately 2.1 × 107 can be used (3). The growth density on a meadow is approximately 1 kg of herbage/m2. The height of vegetation is taken to be 0.3 m, and the density of plant leaves is 500 kg/m3. Therefore, VP/VV in the segment 0 < z < 0.3 m is 6.67 × 10-3 m3/m3, and VA/VV is 1 - VP/VV and is approximately 1. Now we can simplifiy eq 8 to
CV ) KPACAVP/VV + CA
(9)
The mobile gaseous fraction of chemical fa (mass in gas phase divided by total mass in vegetation layer) is approximately
fa ) CA/CV ) 1/(KPAVP/VV + 1)
(10)
It follows for 2,3,7,8-TCDD that fa is 7 × 10-6. Partition Coefficient between Vegetation Layer and Soil. The partition coefficient between vegetation layer and bulk soil is
KVB ) CV/CB ) (1 - VP/VV)KAB + VP/VVKPB ≈ KAB + VP/VV KPB (11) where KAB ) 1/KBA ) 4.9 × 10-8; KPB ) KPA/KBA ) 1.0 f KVB ≈ VP/VV ) 0.0067. The concentration in plant material (kg of substance/m3 of plant) is found from
CP ) CV (1 - fa)VV/VP
(12)
Diffusion and Dispersion in Vegetation Layer. In a thin layer above the leaf surface, no dispersive mixing occurs [maximally a few cm (12)], and diffusion is responsible for transport. The effective molecular diffusion coefficient in the vegetation layer is
DV ) DG fa ) 3.5 × 10-6
m2/d
(13)
where DG is the molecular diffusion coefficent in the gas phase (0.5 m2 /d). Above the roughness height (a few cm), turbulence is the dominant transport mechanism. Instead of the diffusion coefficient DG, a dispersion coefficient DDisp is applied. An estimation method of DDisp is (13)
DDisp )
ku* (z - d) Φ (z - d/L)
(here, m2/s)
(14)
where k is the Karman constant (0.41), u* is the shear velocity of the wind [m/s; approximation, u* ) 0.0359u100.93 (14)], z is the height above surface (m), d is the ‘displacement height’ (roughness height, m), and Φ (z - d/L) is a term accounting for the stability conditions of the atmosphere. For neutral stabilities, it is approximately 1. If for a meadow we take the displacement height d as 0.01 m, the height z as 0.1 m, the wind velocity in 10 m height u10 as 4 m/s, and neutral stability conditions, we get a value of DDisp ) 415 m2/d. This is several orders of magnitude above the molecular diffusion. By the assumption that only the gaseous fraction of 2,3,7,8-TCDD moves, the value of DV is found to be
DV ) DDisp fa + DG fa ) 2.9 × 10-3
m2/d
(15)
Since the analytical solution allows only onesand constantsdiffusion/dispersion coefficient, we will investigate the sensitivity of results against the variation of this parameter. Background Air Concentration. In the analytical solution, CV (z > 0, t ) 0) is zero. A background air concentration
FIGURE 1. Volatilization of 2,3,7,8-TCDD from soil and uptake into vegetation; diffusive DV ) 3.5 × 10-6 m2/d, dispersive DV ) 2.9 × 10-3 m2/d. C 0A can be introduced by the recalculation of CB and CP:
new CB(0) ) old CB (z < 0, t ) 0) - C 0AKBA
(16a)
new CB (z < 0,t > 0) ) old CB (z < 0,t > 0) + C 0AKBA (16b) new CP (z > 0, t > 0) ) old CV (z > 0, t > 0)(1 - fa)VV/VP + C 0AKPA (16c) The solution is only valid for new CB (0) > 0, otherwise there is a net flux from air to soil.
Simulation of the Volatilization of 2,3,7,8-TCDD As a scenario, we use a soil that has an initial concentration CB (z < 0, t ) 0) of 700 pg of 2,3,7,8-TCDD per kg of soil or 910 ng/m3. The background concentration in air C 0A is 3 fg/m3 (the ‘new’ CB(0) is 848.5 ng/m3, eq 16a). The initial concentration ratio between soil and air is 15-fold higher than the thermodynamic equilibrium. Results: Concentrations. At the boundary (z ) 0, t > 0), the concentration is constant over time (erf(0) ) 0) and is
CB(0, t > 0) )
CB(0) 1 + KVB xDV/DB
+ C 0AKBA
CP (0, t > 0) ) KPB CB(0, t > 0)
(17a) (17b)
It follows for the concentrations at the boundary (z ) 0):
CB(0, t > 0) ≈ CP(0, t > 0) ) 675 ng/m3 (diffusive DV ) 3.5 × 10-6 m2/d) CB(0, t > 0) ≈ CP(0, t > 0) ) 134 ng/m3 (dispersive DV ) 2.9 × 10-3 m2/d) These are the maximum concentrations of CP. Values of CB(z,t) and CP(z,t) for -0.05 m < z < 0.30 m and t ) 60 days, and for both values of DV, are plotted in Figure 1. Case 1, Diffusion in the Vegetation Layer. Within 60 days, the concentration of 2,3,7,8-TCDD decreases only in the upper millimeters of the soil layer. This is enough to contaminate the lower centimeters of the plants. Above a height of approximately 5 cm, the concentration of 2,3,7,8-TCDD is only influenced by background air concentration. In the layer
close to the soil surface, concentrations in plants are up to 6715 ng/m3. Usually, herbage is cut some cm above the surface. The average concentration from z ) 0 to z ) 30 cm is 104 ng/m3. Case 2, Dispersion Added: Adding dispersion changes the pattern: flux out of the soil is somewhat higher. Maximum concentrations in plants are much lower (one-fifth), but concentration decreases only slowly with height. The average concentration from z ) 0 to z ) 30 cm is 120 ng/m3. Variation of DV gives the average concentration of 2,3,7,8TCDD in plant tissue is maximal (153 ng/m3) for dispersion coefficients that are around 100 times above the molecular diffusion coefficient. The average CP is not very sensitive to the (critical to estimate) value of DV. It should be noted that dispersion induced by wind and advection will take away the contaminant, leading to a long-range transport but also to a significant decrease of concentrations in the vegetation. A numerical simulation with diffusive DV close to the soil surface (z < 1 cm) and increasing with height (according to eq 14) gave a result similar to the simulation with constant diffusive DV, but with slightly slower average concentrations. Results: Transfer Factors. It is convenient to use a ‘transfer factor’ to calculate concentrations in plant material in relation to those in soil and air (15). For the path soil f air f plant, the use of a transfer factor is not really justified because the chemical is not transferred directly from soil to plant. The transfer factor can be found by looking at the net contribution of the soil contamination to plant contamination. When grass is cut at the soil surface, the average concentration of 2,3,7,8-TCDD between 0 and 30 cm increases by volatilization from soil (contaminated 15-fold above equilibrium) from 62 to maximally 153 ng/m3 (DV optimal). The net contribution of soil contaminated with 910 ng/m3 2,3,7,8-TCDD on concentration in plant is then maximally 91 ng/m3. The transfer factor is maximally 0.1 (based on mass per volume and fresh plant). Grass is usually cut at several centimeters above the soil surface. For DV similar to molecular diffusion, there is no influence of the contaminated soil on plant material above 5 cm height. For DV calculated from turbulent dispersion, the transfer factor is 0.06. Extrapolation to Other PCDD/F Congeners. The same calculations can be carried out for other PCDD/F congeners (or other persistent organic chemicals) when physicochemical properties are available. For our purposes, it is sufficient to assume that higher chlorinated 2,3,7,8-substituted congeners will sorb stronger and volatilize more slowly than 2,3,7,8TCDD. Therefore, our calculation is an upper limit for the transfer of toxic equivalents (TE) of PCDD/F.
Weakness of Method and Plausibility Control The above calculation is very approximate and includes only diffusive and dispersive vapor-phase transport between soil and vegetation and immediate equilibrium sorption processes. It does not include several processes known to be relevant, among others degradation, e.g., by photolysis; dilution by growth; advective transport, e.g., by wind; boundary resistance against uptake into leaves; and advection by barometric pressure changes (which increases flux out of the soil). In a recent publication (3), we were able to show that, due to growth dilution, transfer resistances, and possibly photodegradation, concentrations in leaves are a factor 10 or more below equilibrium to air. This means that the solution most likely overestimates concentrations in leaves. To control the plausibility of the calculation, the results can be compared with experimental studies. Several authors assume a volatilization of PCDD/F from soil (16). Freeman and Schroy (17) showed that 2,3,7,8-TCDD diffuses very slowly in soil air (a maximum of 0.1 m in sandy soil in 12 years). Krause et al. (18) made several glasshouse experiments with filtered air. Ray grass (Lolium perenne) was grown in pots
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with soils contaminated with PCDD/F between 12 and 600 ng of TE/kg dry wt. The contents of PCDD/F in washed grass were measured after 48 days exposure. With plants grown in the soil with 600 ng of TE/kg dw, only 1.8 ng of TE/kg dw were analyzed. From this and other findings, the authors concluded that the transfer factor for the pathway soil to plants (all processes) is at maximum 0.01 (ng of TE/kg of dry plant material:ng TE/kg of dry soil). This is in the same order of magnitude as calculated here.
Conclusions for Real Systems The competitive uptake of 2,3,7,8-TCDD from soil and air was calculated recently (3). It was found that (a) PCDD/F are (at least for herbage) not transported in significant amounts within the transpiration stream of the plant. (b) The uptake of PCDD/F from air into leaves is mainly responsible for a contamination of aerial plant parts. Now we can conclude that (a) For background conditions, air and soil are close to equilibrium. The main entrance of PCDD/F into the agricultural food chain is therefore directly from air into leaves. (b) Desorption from soil plays a minor role, even when soil concentrations are above chemical equilibrium to air. But when highly polluted soils are covered with vegetation, in particular, the lower plant parts may undergo significant contamination. (c) Volatilization from highly polluted soils is a secondary source for atmospheric pollution. A depletion of soils via volatilization into air takes a long time. In equilibrium, 1 mm of soil contains about the same amount of 2,3,7,8-TCDD as 20 km of atmosphere (KBA ) 20 × 106). (d) The aim of reducing the daily intake of PCDD/F by contaminated food below 1 pg of I-TEq per kg of body weight and day is best reached by an effective emissions control.
(2) Trapp, S.; Mc Farlane, J. C. Plant Contamination: Modeling and Simulation of Organic Chemicals Processes; Lewis Publishers: Boca Raton, 1995. (3) Trapp, S.; Matthies, M. Environ. Sci. Technol. 1995, 29, 23332338; Erratum Environ. Sci. Technol. 1996, 30, 360. (4) Jury, W. A.; Spencer, W. F.; Farmer, W. J. J. Environ. Qual. 1983, 12, 558-564; Erratum J. Environ. Qual. 1987, 16, 448. (5) Jury, W. A.; Russo, D.; Streile, G.; Hesham, el Abd Water Resour. Res. 1990, 26, 13-20. (6) Jost, W. Diffusion in Solids, Liquids and Gases, 3rd ed.; Academic Press: New York, 1960; p 69. (7) Crank, J. The Mathematics of Diffusion, 2nd ed.; Oxford University Press: London, 1979, p 39. (8) Abramowitz, M., Stegun, I., Eds. Handbook of Mathematical Functions with Formulas, Graphs and Mathematical Tables. 10th ed.; John Wiley and Sons: New York, 1972. (9) Briggs, G. G.; Bromilow, R. H.; Evans, A. A. Pestic. Sci. 1982, 13, 495-504. (10) Simonich, S. L.; Hites, R. A. Environ. Sci. Technol. 1995, 29, 29052914. (11) Rippen, G. Handbuch Umweltchemikalien; ecomed: Landsberg, Lech, FRG, 1994 (permanently updated). (12) Gates, D. M. Biophysical Ecology; New York: Springer, 1980. (13) Joss, U. Mikrometereologie, Profile und Flu ¨ sse von CO2, H2O, NO2, O3 in zwei mitteleuropa¨ischen Nadelwa¨ldern. Thesis, University of Basel, Switzerland, 1995. (14) Monim and Yaglom, cited in Rippen, G.; Flothmann, D.; Witt, W. UBA-Report 106 02024/06, 1984. (15) Matthies, M.; Trapp, S. UWSF-Z. Umweltchem. O ¨ kotox. 1994, 6, 297-303. (16) Fiedler, H.; Hutzinger, O.; Kaune, A. Organohalogen Compd. 1991, 5, 297-303.
Acknowledgments
(17) Freeman, R. A.; Schroy, J. M. Environ. Prog. 1986, 5 (1), 28-33.
Thanks to Andreas Kaune and Hans-Ulrich Wolter for their support. Thanks to Teresa Gehrs and Horst Malchow for pre-reviewing the manuscript. Thanks to R. Eiden, University of Bayreuth, for useful information about micrometeorology. Thanks to the Deutsche Forschungsgemeinschaft for sponsoring this work.
(18) Krause, G. H. M.; Prinz, B.; Rademacher, L. Kriterien zur Beurteilung organischer Bodenkontaminationen: Dioxine, PCDD/F und Phthalate; Dechema eV: Frankfurt, 1995; ISBN 3-92695951-7, pp 287-299.
Literature Cited (1) Visser, W. J. F. Contaminated land policies in some industrialized countries; Technical Soil Protection Committee: The Netherlands, 1993.
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Received for review February 12, 1996. Revised manuscript received September 9, 1996. Accepted September 13, 1996.X ES960133D X
Abstract published in Advance ACS Abstracts, December 1, 1996.
