Environ. Sci. Technol. 1990, 2 4 , 1246-1252
as the kinetics of sorption are not rate-limiting. Conclusions
This study has focused on the kinetics of biodegradation (mineralization to COz) of 10 14C-ring-labeled LAS homologues and phenyl isomers in river water and river water/sediment systems at concentrations approximating realistic environmental levels. The homologues and isomers tested bracket the range of materials present in commercial LAS formulations. Based on the results of this study, the following conclusions can be drawn: (1) Mineralization of LAS homologues and phenyl isomers was extensive (-80%) and followed apparent firstorder kinetics in river water and river water/sediment systems. Half-lives for mineralization of the benzene ring ranged from 15 to 33 h. (2) The rate and extent of mineralization were not significantly affected by the length of the alkyl chain or the point of attachment of the benzene ring to the alkyl chain (phenyl position). (3) Mineralization rates for individual homologues were not adversely affected by the presence of high concentrations of suspended sediments or competing homologues and biodegradation appeared to occur independently for different homologues. Acknowledgments
My appreciation is expressed to Daniel H. Davidson for completing the technical aspects of this work. Registry No. Clo LAS, 2627-06-7; Cll LAS, 20466-34-6;Clz LAS, 2211-98-5; Cis LAS, 14356-40-2;C14 LAS, 1797-33-7;Cia2-phenyl LAS, 73602-65-0;Clo-5-phenylLAS, 73602-67-2;C12-2phenyl LAS, 2211-99-6;CIz-6-phenylLAS, 2212-52-4;CI4-2-phenyl LAS, 13419-31-3.
Literature Cited (1) Painter, H. A.; Zabel, T. F. Reuiew of the Enuironmental Safety of LAS; Co 1659-m/l/EV 8658; Water Research Center: Medmenham, UK, 1988; p 1. (2) Boatman, R. J.; Cunningham, S. C.; Ziegler, D. A. Environ, Toxicol. Chem. 1986,5, 233-243. (3) Gerike, P.; Fischer, W. K. Ecotoxicol. Enuiron. Saf. 1979, 3, 159-173. (4) Gerike, P.; Fischer, W. K. Ecotoxicol. Enuiron. Saf. 1981,
5 , 45-55. ( 5 ) Larson, R. J. Appl. Enuiron. Microbiol. 1979,38,1153-1161. ( 6 ) Larson, R. J. Residue Reu. 1983,85, 159-171. (7) Swisher, R. D. Surfactant Biodegradation; Marcel Dekker, Inc.: New York, 1987. (8) Hand, V. C.; Williams, G. K. Environ. Sci. Technol. 1987, 21, 370-373.
(9) Games, L. M. In Aquatic Toxicology and Hazard Assess-
ment: Sixth Symposium;ASTM STP 802; Bishop, W. E., Cardwell, R. D., Heidolph, B. B., Eds.; American Society for Testing and Materials: Philadelphia, PA, 1983; pp
282-299. (10) Larson,R. J.; Payne, A. G. Appl. Enuiron. Microbiol. 1981, 41,621-627. (11) De Henau, H.; Matthijs, E.; Hopping, W. D. Znt. J. Enuiron.
Anal. Chem. 1986,26, 279-293. (12) Larson, R. J. In Current Perspectives in Microbial Ecology; Klug, M. J., Reddy, C. A., Eds.; American Society for Microbiology: Washington, DC, 1984; pp 677-688. (13) Ventullo, R. M.; Larson, R. J.; Beba, A. D., in press. (14) Ward, T. E.; Larson, R. J. Ecotoxicol. Enuiron. Saf. 1989, 17, 119-130.
Waters, J.; Holt, M. S.; Matthijs. Tenside, Surfactants, Deterg. 1989, 26, 129-135. (16) Abe, S . Nippon Kagaku Kaishi 1984, 9, 1465-1470. (15)
Received for review August 28,1989. Revised manuscript received February 22, 1990. Accepted March 26, 1990.
Modeling the Bioconcentration of Organic Chemicals in Plants Stefan Trapp, * # +Mlchael Matthies,t Irene Scheunert,$and Eva M. Topp5 Projektgruppe Umweltgefahrdungspotentiale von Chemikalien, Institut fur Bodenokologie, and Institut fur Okologische Chemie, Gesellschaft fur Strahlen- und Umweltforschung, Ingolstadter Landstrasse 1, D-8042 Neuherberg, West Germany
Uptake of several organic chemicals by growing barley was investigated in a closed laboratory model ecosystem (volume 9.4 L). To discriminate between root and foliar uptake, the soil was divided into test soil contaminated with 2 mg/kg 14C-labeledchemicals and unpolluted control soil. Air was exchanged at a rate of 10 mL/min. Concentrations of 14Cchemicals in air, soil, roots, and shoots and bioconcentration factors were simulated with a mathematical model based on fugacity. Bioconcentration in the plant is dependent on transfer rates and the ratio of KO,and K , values. Uptake into foliage is mainly from air for chemicals with a high KO,and a Henry’s law constant high enough for volatilization. Chemicals with medium KO,are translocated with the transpiration stream. Metabolites are also taken up. Chemicals in the roots of the barley plants reach equilibrium with the soil, whereas those in foliage are in equilibrium with the air in most cases. Introduction
The uptake of chemicals into plants is the first step for accumulation in the terrestrial food chain. Chemicals can Projektgruppe Umweltgefahrdungspotentiale von Chemikalien.
* Institut fur Bodenokologie.
5Institut fur Okologische Chemie. 1246
Environ. Sci. Technol., Vol. 24, No. 8, 1990
enter plants via roots or via foliage. Various processes of partitioning, transfer, and transformation that govern uptake have been investigated (1-11). Recently a theoretical model was published but not yet validated (12).In this paper a model based on fugacity is presented for the calculation of transfer and distribution of chemicals between soil, air, water, and plants. Laboratory experiments with radiolabeled chemicals (1)support the results of the model calculation. Experimental Section
The aim of the laboratory experiments was to yield data on the material balance of chemicals in a soil-waterplant-air model ecosystem including uptake by plants, volatilization, transformation in plants and soil, and the formation of unextractable residues. Model Ecosystem. The composition of the soil was as follows: particle size distribution, clay (2 mm) 6.6%;organic carbon content, 2.06%;soil water content, 30 vol %; pH 6.4 (soil properties measured with methods described in ref 13). The plant was barley (Hordeum uulgare). Uptake of chemicals by plants was determined by short-term studies in a closed aerated laboratory apparatus as described elsewhere (10). To
0013-936X/90/0924-1246$02.50/0
0 1990 American Chemical Society
inftow/outflow A 10 ml/minute
P2
ontroi shoots
/
air
Table 11. Physicochemical Properties of the Investigated Chemicals"
- - - _l--
,/'
V=9 liter
I
/test shoots lg 87.5% water
test soil 600 g \ \ 20% water \ \
test roots 2 9 87.5% water I
control soil - -
/
400 g
20% water
contlol roots Flgure 1. Properties of the model ecosystem.
Table I. Radiopurities of the Test Compounds" chemical
position of the 14Clabel
radiopurity, %
atrazine triCB tetraCB dieldrin PCB HCB DDT
ring at C-2 benzene ring benzene ring chlorinated ring dichlorinated ring benzene ring benzene rings
99.8 98 >99 >99 >99 >99 >99
" Abbreviations: triCB, 1,2,4-trichlorobenzene; tetraCB, 1,2,3,5tetrachlorobenzene; PCB, 2,2',4,4',6-pentachlorobiphenyl;HCB, hexachlorobenzene. discriminate between root and foliar uptake, the soil was divided into contaminated test soil and control soil. A 400-g sample of the control soil was filled into a Petri dish (-11 cm in diameter and 8 cm high) and placed in a desiccator (diameter -30 cm, volume 9.4 L). Around the dish was placed 600 g of the test soil containing 2 mg/kg (dry weight) 14C-labeledchemicals. Ten barley seeds were plated in the treated test soil to determine the total uptake via both roots and leaves. The untreated control soil in the dish was covered with a plate containing 10 holes where 10 barley grains were seeded. Thus, transfer of chemicals to control soil from the air and hence uptake by control plants from the soil was reduced. Air was pumped through the apparatus at a low rate (10 mL/min). All experiments were carried out in replications (Table 111). After 1week, the desiccator was opened and samples were taken for analysis. The properties of the model ecosystem are shown in Figure 1. Chemicals. 14C-labeledcompounds were either commercially purchased or newly synthesized (10, 11) and purified to radiopurities between 98 and 99.8% (Table I). The 14C-labeledchemicals were mixed with commercially available nonradioactive compounds and solved in acetone by using ultrasound. Before treatment of the soil, the test system was cooled to minimize losses by volatilization. A 500-pL aliquot of the radioactive solution was applied with a Hamilton syringe and mixed into the test soil with a spatula. The application rate was -1 pCi per test. Altogether, 16 substances were tested in the system (I). For the simulations, chemicals with slow conversion rates were selected. The chemicals studied and their physicochemical properties are listed in Table 11. Substances were excluded when accurate physicochemical data were not available. Analytical Methods. Concentrations of 14C-labeled organic compounds in the air of the apparatus were determined by counting 14C trapped in tubes containing
chemical
log KO,
log K,
KAW
MW
atrazine triCB tetraCB dieldrin PCB HCB DDT
2.71 (14) 3.98 (16) 4.65 (16) 5.48 (18) 5.92c 5.47 (16) 6.19 (22)
2.33 (14) 3.35b 3.&lb 4.44* 4.75b 4.43b 5.14 (22)
8.05 X lo4 0.17 (17) 0.1 (17) 4.4 X lo4 (17) 0.0076a 0.054 (21) 2.14 X (17)
215.7 (1) 181.5 (1) 215.8 (1) 380.9 (1) 326.4 (1) 284.8 ( 1 ) 354.5 (1)
"log KO, is the logarithm of the partition constant between noctanol and water, log K , is the logarithm of the partition constant between soil organic carbon and water, KAW = H / ( R T ) and is the dimensionless partition constant between air and water, and MW is the molecular weight. bEstimated; ref 22: log K , = 0.72 log KO, + 0.49. CValuefor the isomer 2,2',4,5,5'-PCB (19). dValue for Aroclor 1254 (20).
ethylene glycol monomethyl ether. The glass walls of the apparatus were rinsed with methanol and acetone. The radioactivity of these rinsing solutions was also determined and added to that of the ethylene glycol monomethyl ether. 14C02,which had been formed in small amounts by biomineralization of the chemicals, was trapped in separate tubes containing Carbosorb/Permafluor V 812 (Packard). Radioactivity was determined in a liquid scintillation counter (Berthold/Frieseke Betaszint BF 8000) by use of a dioxane-based scintillation liquid. The sum of volatile organic 14C was related to the volume of air passing through the apparatus during the experiment. Total 14C residues in the soil were determined after Soxhlet extraction of samples with methanol for 48 h. Plants were homogenized with an Ultra Turrax prior to Soxhlet extraction. Radioactivity in extracts was determined in a liquid scintillation counter by use of a dioxane-based scintillation liquid. Unextractable 14C in soil and plants was determined in an automatic sample oxidizer Tri-Carb B 306 (Packard). Thin-layer chromatography was used for the separation and quantification of parent compounds and of conversion products, except for 1,2,4trichlorobenzene. For this compound, the portion of unchanged parent compound was measured by gas chromatography after cleanup by various partition steps. Conversion ratios were expressed as percent conversion products of total 14C in each sample.
Theoretical Section The Fugacity Concept. The model for the simulation of the test ecosystem is based on the fugacity concept (23, 24), in which
c =fZ (1) where C is the concentration (mol/m3), f is the fugacity of the chemical. (Pa), and 2 is the capacity (m~l-m-~.Pa-') Capacities of the Compartments. The model system is divided into the compartments air, test soil, test roots, and test shoots, and control soil, control roots, and control shoots (Figure 1). Every compartment has its own capacity Z for a chemical (control soil and control plants have the same properties as test soil and test plants). The Z values are computed as follows. Air. The capacity of air is the same for all chemicals: ZA = 1/(RT) (2) in which R is the universal gas constant (J-mol-'-K-l) and T is the temperature (K). ZA is 4.1 X low4for 20 OC. Soil. Chemicals may sorb to the organic carbon in the soil, dissolve in the soil solution, or partition into the soil air: Environ. Sci. Technol., Vol. 24, No. 8, 1990 1247
ZS= pKoc[CorgIZw + OwZw + OAZA
(3)
where p is the bulk density of the soil (1.5 g/cm3),KO,is the partition constant between soil organic carbon and water (cm3/g), [Cor,] is the organic carbon content of the soil (0.0206 g/g of dry soil), Ow is the volumetric water content of the soil (30 vol TO),1 9 is ~ the air-filled pore fraction (20 vol W).2, is the capacity of water and is 1/H, where H is Henry’s law constant (m3.Pa/mol). Roots. The distribution of chemicals between barley roots and water was measured by Briggs et al. (3). It follows ZR = (lO(0.77 log K m - 1.52) + WR)z, (4) where WR is the volumetric water content of the barley roots (87.5%). The first term represents the capacity of the root matrix, the second term ( WR) that for root water. The density of the plant is equal to that of water and does not appear in the equations. Shoots. The distribution was also measured ( 4 ) : Sh = (lO(0.95log Kow - 2.05) + WSh)Zw (5)
z
with Wsh is the volumetric water content of the barley shoots (87.5%). This distribution equation can also be derived from thermodynamic considerations. Assuming that the partitioning of a chemical between lipid and water is similar to that between n-octanol and water we obtain ZSh
= [(lipid content)Ko,
+ WS&Z,
(6)
With a lipid content of 1% in barley shoots we can write ZSh = (0.01Kow + Wsh)zw (7) Equation 7 is nearly identical with eq 5 and can be used for all plants with known lipid and water content. An analogous equation for the distribution into roots is derived in ref 25. Transfer between Compartments. Other important elements of the model are the transfer values between the compartments. They are also calculated by following the fugacity concept (15): (8) Nij = Dij(fi - f j ) in which N is the flux of chemical between two compartments (mol/s) and Dij is the “conductivity” between compartments i and j (mo1.s-’.Pa-’). Soil-Air. Diffusion takes place in the water or air-filled pores (15): DSA = ASA(DGeffZA + DWeffZw) /
(9)
fR is fs). This is taken NSR between soil and
into account in form of a mass flux growing roots:
NSR= (dVR/dt)Zds
(10)
in which dVR/dt is the change of the root volume with time (3.3 X m3/s). This pathway is important in particular for substances that are immobile in soil. Chemicals dissolved in the soil water can enter the roots with the water taken up to form the transpiration stream. The accompanying conductivity DTsT can be described with DTsT = TSTZ,
(11)
where TST is the transpiration stream in m3/s [was not measured, so an average value of 650 L/kg of dry mass of barley was used (28)]. Through uptake, a chemical gradient develops in the vicinity of the roots (“deficiency zone“). For phosphorus, zones with a radius of 1.5 mm were found (29). Since phosphorus is a fertilizer and is preferentially taken up, this can be regarded as a maximum value for organic chemicals. Chemicals diffuse from outside the deficiency zone to the root surface. Diffusion in soil water can be neglected in this case because the movement of water taken up by roots is much faster. A solution of Fick‘s law for gaseous diffusion to a root or cylindrical surface is given by (30) DSR= DGeffZ~2Lr/ln(r2/?1)
(12)
in which DsR is the conductivity soil-roots, L is the length of the roots (0.1 m), r1 is the radius of the roots (0.0003 m), and r2 is the radius of the deficiency zone added to rl (0.0018 m). Water-soluble substances are preferably transported with the water stream into the roots (DTST is larger), whereas chemicals with a high tendency to partition into the vapor phase rapidly diffuse into the roots (DSRis larger). Transport with the transpiration stream occurs in one direction only (into the roots), whereas diffusion can occur in both directions. Roots-Shoots. The chemical is transported with the transpiration stream. The ratio of the concentration in the transpiration stream CTsT to the concentration of the chemical in the external soil solution Cs was expressed by Briggs (3) as TSCF (transpiration stream concentration factor): TSCF = CTST/CS TSCF = 0.784 exp[-(log KO,- 1.78)2/2.44] (13)
Equation 15 gives a Gaussian curve with a maximum at in which ASA is the area between soil and air (test soil-air log KO, of 1.78. 2.21 X m2); DGeffis the effective diffusion constant in air pores of the soil (m2/s),calculated as DG,R = D G ~ A * ~ ~ ; With the capacity of the soil solution and of the transpiration stream being that of water Z,, the flux into shoots DWeffis the effective diffusion constant in water pores of NRsh can be expressed as the soil (m2/s), DW8w1,5;DG and DW are the binary diffusion coefficients of the chemical in air and water, NRsh = TST TSCFZ,(f, - fSh) (14) respectively, and are estimated with the program DTEST (26);Y is the diffusion length (half soil depth, for the test Air-Foliage. The transfer depends on the resistances soil 0.009 m). This simplification is justified only for small of the air layer, the stomata, and the cuticula. An estisoil depths. For deeper soils, the transport in the soil must mation of the maximum transfer velocity of 0.005 m/s is be considered (27). The resistance of the air layer is negiven in ref 5. This value is for pesticides with a molecular glegible here. weight of 300 and can be adapted to other molecular Soil-Roots. The chemical can enter the roots through weights: root growth, with the water taken up by the roots (tranDAF = A,O.OO54-Z, (15) spiration stream) and via diffusion. When roots grow in polluted soil, they take up a part where AF is the area of the foliage (70.3 X lo4 m2). of the soil-borne chemical. It is assumed that the root tips For simplicity it is assumed that the plant’s growth is achieve equilibrium with the soil during growth (which linear, i.e., volumes and transfer rates change in a linear means that they have the same fugacity as the soil, Le., 1248
Environ. Sci. Technol., Vol. 24, No. 8 , 1990
Table 111. Measured Concentrations in Plants and Soil chemical
N"
mean
DDT
2 2 2 4 2 3 3
0.875 1.24 2.77
dieldrin PCB HCB atrazine triCB tetraCB ON,
Cpht, ppm fresh max 0.89 1.37 2.99 2.43 2.60 1.80 3.79
2.21
2.53 1.33 3.54
min
mean
P P dry ~ max
min
recovery, %
0.86
2.16 2.075 2.16 1.88 0.98 0.745 1.18
2.20 2.10 2.20 1.93 0.99 0.78 1.22
2.12 2.05 2.12 1.85 0.97b 0.70 1.15
106.6 106.95 105.1 96.4 97.9 69.9 96.1
Cwil,
1.11
2.54 1.99 2.46 0.91 3.37
number of experiments. *Starting concentration in soil 1 ppm. All others 2 ppm.
Table IV. Percent Metabolism and Type of Products Formed % metabolized
% insoluble
% polar
%
coz
name
soil
plant
soil
plant
soil
plant
air
atrazine triCB tetraCB dieldrin PCB HCB
14.7 12.7 4.5 0.4 0.5 0.8 2.9
59.5 47.2 45.6 8.3 8.3 0.8 20.7
10.5 0.95 0.2 0.2 0.3 naa 2.5
48.9 36.6 40.9 8.1
4.2 11.75 4.3 0.2 0.2 0.8 0.4
10.6 10.6 4.7 0.2 0.2 0.8 0.3
0.48 0.46 0.01 0.01 0.03
