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Organic matter fractions controlling the sorption of atrazine in sandy soils
Chemosphere,Vol.25,No.6, pp 887-898, 1992 Printedin GreatBritain
0045-6535/92$5.00+ 0.00 PergamonPressLtd.
ORGANIC MATTER FRACTIONS CONTROLLING THE SORPTION OF ATRAZINE IN SANDY SOILS
A.B. Payh-P~rez; A. Cortes; M.N. Sala and B. Larsen
Commission of the European Communities. Joint Research Center. Environment Institute. Ispra Establishment. 21020 Ispra (VA), ITALY (Received in Germany 20 July 1992)
ABSTRACT The soil-water distribution (Kd) of atrazine has been studied in 24 soil profiles, comprising 109 soil horizons derived from granodioritic materials; Kd ranged from 0.01 I./kg to 64 L/kg with a mean value of (2.4 5: 7.3) l_/kg. The concentrations of organic carbon, cation exchange capacity, clay, and oxides of iron, aluminum, and manganese were determined. In a multiple linear regression analysis Ka was strongly correlated with the organic carbon content (foe) and weakly correlated with aluminum oxides. No other soil components were correlated with Kd. The organic carbon soil sorption coefficient (Koc) was estimated to be 216 _+9 L/kg (95% confidence limits) from a linear plot of Ka versus the foc- In a logarithmic correlation between Kd and foc the non-linearity constant proved statistically significant from unity: logKd = (1.81 + 0.20) + (0.75 5: 0.07) × logfoc which indicates that not only the concentration, but also the composition of the organic matter play important roles in soil sorption of atrazine. In 48 of the 109 soil horizons which contained more than 0.1% of organic carbon, the organic matter was fractionated into humic acids, fulvic acids, humin and free organic matter. A multiple linear regression analysis between Kd and the various organic matter fractions revealed that humic acid (Ha) explained 71% of the variance compared to 26% for humin plus free organic matter (Hum + Fom). In addition, 3% of the variance was explained by manganese oxide: Kd= (167 + 11) x Ha - (0.74 5: 0.09) x (Hum + Fom) + (21 + 16) x MnO - (1.6 + 1.0) Previously published equations for predictions of Koc of a compound from its water solubility and its octanol-water partition coefficient (Kow) were evaluated. Kow proved to be the best predictor for atrazine. Key words: atrazine, soil sorption, organic matter fractions, correlations
INTRODUCTION Atrazine is a herbicide controlling broadleaf weeds and some grasses used particulary in corn and sorghum cultivation. Atrazine is also used at higher rates of application as a non selective herbicide, and may remain in the soil for several months after the application (Frank and Sirons, 1985
and
Neugebaur,
1990).
Groundwater
contamination
with
atrazine
[2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine],is frequently reported as a result of its 887
888
widespread agricultural use (Buser, 1990; and Bacci, 1989). The mobility of atrazine by surface runoff or leaching through the soil, is important for the assessment of its impact on the environment (Bowman, 1990; Alhajjar, 1990 and Starr, 1990). The parameter most often used as an indicator of the mobility of soil contaminants is the soil-water distribution coefficient (Kd) calculated from the Freundlich sorption isotherms (Karickhoff, 1981; Clay, 1988; Lagas, 1988; Calvet, 1989 and Oepen, 1991): K d = Xeq/Ceq ( 1) where X and C are the concentration of atrazine in soil and in solution, respectively. The soil-water distribution coefficient of a broad range of chemicals has been shown to be related to the organic carbon content of the soil (Briggs, 1981 and Singh, 1990): K d = Koc x fo~ (2) where foc is the organic carbon fraction on weight basis. Assuming that the carbon content of soil organic matter in "ordinary soils" is approximately constant, the organic carbon soil sorption coefficient (Koc) can be computed from plots of Kd versus foc when the obtained linear regression lines intercept the origin. A more general formulation of Eq. (2) has been proposed: Kd = Koc x foc + K° (3) where K° describes adsorption on components other than organic carbon e.g. active surfaces (Lee, 1979 and Pay~i-PErez et al. 1991). These other soil parameters are expected to contribute to the atrazine soil sorption, in soils with a low organic carbon content (Hasset & Bandward, 1989). The effect of organic matter composition on Kd has been illustrated in studies of organic solute (partition) interactions with humic and fulvic acids where Kd decreased with increasing polar group content of the humic and fulvic acids (Chiou et al. 1986; Garbarini and Lion, 1986; Chiou et al. 1987). Similar findings have recently been published for indigenous organic matter of soils (Rutherford et al. 1992). These results call into question the assumption in Eqs. 2 and 3 that Koc is a universal constant independent of the soil quality. The composition of organic matter in soil is changing continuously through chemical and physical processes (humification) which eventually lead to a decrease in the polar group content of the organic matter (Grathwohl, 1990). Thus, the solute partition efficiency of the organic matter increases with the age of a soil, and normally also with the depth of a soil horizon. Since the concentration of organic matter in soils is typically decreasing with the depth, a non-linear correlation between K d and foc is expected: K d = Koc x (foe)n (4) where "n" is less than unity. For the soil-water distribution of single families of compounds in a limited number of soils of the same type (e.g. horizons from the same profile) such non-linear correlations have recently been reported with values of "n" around 0.7 (Lara and Ernst, 1991; Larsen and Hansen, 1992). A re-elaboration of the data published by Larsen et al. (1990) and Kishi et al. (1990) also point to a value of "n" between 0.6 and 0.8.
In the present study we have determined K d for atrazine in 24 sandy soils, subdivided into 109 soil horizons and investigated the correlations between Kd, soil organic carbon, and mineral soil components (clay, iron oxides, aluminum oxides, manganese oxides and cation exchange capacity). For soil horizons containing more than 0.1% organic carbon the organic matter was fractionated into humic acids, fulvic acids, humin and free organic matter, and the role of each of these fractions in the soil-water distribution of atrazine was evaluated using multiple linear regression analyses. Several equations have been proposed in the literature for the estimation of the soil sorption coefficient (Koc) of organic compounds from their water solubility and from their octanol-water
889
partition coefficient. In order to test the validity of such equations, the water solubility and the octanol-water partition coefficient of atrazine were determined and the predicted Koc values were compared to our experimentally determined values.
MATERIALS AND METHODS
Soils: A total of 109 soil horizons from 24 soil profiles were collected in NE Spain from 1984 to 1987. These soils represent a broad range of soil-forming factors i.e. topography, orientation, vegetation and human activity. The soils are mostly classified as Xerorthents, Xeropsamments and Xerochrepts, with a few Haploxeralfs and Palexeralfs (Soil Taxonomy USDA, 1987) according to their morphological and physicochemical characteristics. The soils were formed on granodioritic materials rich in plagioclases. They are sandy soils with pH between 5 and 8. The most common clay minerals in these soils are mica-illite, vermiculite and kaolinite. The soil samples were air dried and sieved to 2 mm. Selected soil characteristics that may affect the soil-water distribution of atrazine were determined in triplicate according to standard methods, (Cort6s, 1989), and are given in Table 1. Table 1. Characteristics of the 109 soil horizons
MEAN
SD
RANGE
Organic carbon (%)
0.81
+ 2.9
0.01-23.4
Clay (%)
10.5
+ 5.1
1.8-24.1
CEC (meq/100g)
15.3
+ 5.8
5.3-42.7
CaCO3(%)
2.4
+ 5.8
0-19
Total A1203 (%)
14.9
+ 1.3
11.1-17.8
Fe203 (%)
4.1
5:0.8
1.6-5.7
MnO (%)
0.07
5:0.02
0.03-0,11
Organic carbon was determined by the Walkley-Black method (SSSA, 1986) using potassium dichromate as
oxidant. Samples with more than 12% organic carbon were reanalyzed by the
combustion method (SSSA, 1986). Four major fractions of soil organic matter have been defined in terms of their solubilities (Aiken et al., 1985); Humic Acid (Ha): The fraction of humic substances insoluble in water at a pH below 2 but soluble at a higher pH. Fulvic Acid (Fa): The fraction of humic substances soluble at all pH conditions. Humin (Hum): The fraction of humic substances insoluble in water at any pH but soluble in ethanol-bromoform 1:1. Free Organic matter (Fom): The fraction of humic substances insoluble in both water at any pH and ethanol-bromoform 1:1. These organic matter fractions (Ha, Fa, Hum and Fom) were determined in 48 of the 109 soil horizons which contained more than 0.1% of organic carbon. Humic and fulvic acids were extracted using 0.1N NaOH and IM Na4P207 in a ratio of 1:1. Humin and free organic matter were separated by densimetry using ethanol-bromoform 1:1 . The various fractions were quantified colorimetrically at 620 nm, (Con6s, 1989).
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Total iron, aluminum and manganese oxides were determined by X-Ray fluorescence. Tablets of soil compressed at 20 metric tons during one minute were used. In some cases it was necessary to add a binding material to facilitate the compression and preservation of the samples. Samples were analyzed using a Philips PW 400 spectrophotometer with a gold tube calibrated to standards obtained from the U.S. Geological Service. Partition experiments: 14C-Labeled atrazine [2-chloro-4-(ethyl-l-14C-amino)-6-(isopropylamino) -s-triazine] (925 MBq/mmol) of a 98% radiochemical purity was purchased from Amersham International, Ltd., The identity and purity were tested by gas chromatography-mass spectrometry (GC-MS). A mother solution of 14C-labelled atrazine in deionized and distilled water (0.56 ~mol /L) was prepared. For the determination of the soil-water distribution coefficient, a 10 mL aliquot of the mother solution was added to 1 g of soil in teflon-capped centrifuge tubes (in triplicate). The tubes were shaken for 24 hours at 22 + I°C to assure equilibrium (Singh, 1990), and centrifuged at 4300 rpm for 10 minutes at 22 + I°C. A 1 mL aliquot of the centrifugate was mixed with 10 mL of liquid scintillator and counted for 1 hour in a liquid scintillation analyzer (standard error less than 0.8 %). Data were corrected for quenching and background. The amount of atrazine adsorbed in the soil was calculated by difference and the soil-water distribution coefficient (Kd) computed from Eq.(1). For control of the mass balance the desorption was studied for soils rich in organic matter (Clay et al., 1988). A total of 81% + 7% could be recovered after 4 aqueous desorption steps and a further 8.0% + 0.3% after desorption with acetonitrile.
Kow and S: The octanol/water partition coefficient (Kow) was determined by a shake-flask method. Stock solutions of (19,2 ixmol/L) 14C-atrazine were prepared using buffer solutions from pH 5 to pH 9. 1 mL of n-octanol was added to 10 mL of stock solution in teflon-capped centrifuge tubes. The tubes were equilibrated for 19 hours in an end-over-end shaker at about 30 rpm at 22°C + I°C. The phases were separated by centrifugation at 3400 rpm for 1 hour. 14C-atrazine was measured in both phases, and the Kow was calculated after correcting for quenching and background (four replicates).The aqueous solubility of atrazine was determined (five replicates) at 22°C + I°C. Water was saturated with atrazine at an equilibrium period of 7 days, and aliquots (1 mL) were extracted with dichloromethane (9 mL), diluted 1:10 with iso-octane, and analyzed by GC-MS on a Carlo-Erba QMD 1000 instrument in the single ion recording mode at 200 m/z.
RESULTS AND DISCUSSION
Horizons: The soil-water distribution coefficients (Kd) for atrazine in the studied soil horizons ranged from 0.01 to 64.3 L/kg, with a mean value of 2.4 -+ 7.3 L/kg, and tended to decrease when moving from the surface to the parent material (Table 2). The organic carbon distribution coefficient (Koc) calculated from Eq.(2) for each individual soil ranged from 11 to 25800 L/kg, with a mean value of 566 + 278 L/kg (excluding data from parent material, type Diaclase). It must be noted that this method tends to overestimate Koc. A better way is to calculate Koc as the slope in linear plots of Kd versus foc as in Eq.(3). However, due to the low number of soils in some of the horizon types linear plots could not be constructed for all horizon types.
891
TABLE 2. Organic carbon (%), K d and Koc of the different soil horizons HORIZON
% OC
Kd
Koc
number of
TYPE a
M E A N SD
MEAN SD
MEAN b SD
soil horizons
O
16.5
+ 8.4
38
+ 16
274
+ 148
3
A
0.91
+ 0.94
2.0
+ 2.2
253
+ 161
38
AC
0.14
+ 0.09
0.78
+ 0.49
864
+ 702
11
BA
0.29
+ 0.13
0.76
+ 0.22
322
+ 212
3
B
0.12
+ 0.09
0.41
+ 0.37
403
+ 461
10
Bt
0.09
+ 0.05
0.53
+ 0.38
664
+ 514
7
Bw
0.17
+ 0.06
0.63
+ 0.09
404
+ 192
2
C
0.10
+ 0.09
0.30
+ 0.25
691
+ 693
11
0.07
+ 0.11
0.44
+ 0.37
1092
+ 1026
20
0.03
+ 0.01
0.22
+ 0.07
11900
+ 13600
4
Cr Diaclase aUSDA, 1987.
b calculated from Eq.(2")for each individual horizon.
Koc varied with a factor of four between the different horizons (Table 2), and tended to be higher for the lower horizons closer to the parent material (AC, Bt, C and Cr). This indicates that other factors than organic carbon are important in controlling K d.
The role of organic carbon and other soil components: A multiple linear correlation analysis for all 109 soil horizons between K d and all the considered soil factors showed that K d was strongly correlated with the soil organic carbon, and weakly correlated with aluminum oxides Eq.(5). No other soil components revealed significant correlations (estimates of coefficients + 95% confidence limits): Kd= (214 + 12) × foc + (0.10 + 0.07) × A1203 -(1.26 + 1.00)
(5)
with N=327, r2= 0.82, and a standard error of estimation of 1.29. More than 99% of the variance in Eq. (5) is explained by the organic carbon, thus the aluminum oxides may be eliminated from the equation without reducing the correlation (estimates of coefficients + 95% confidence limits): K d = (216 + 9)foe - (0.27 + 0.27) with N=327, r2= 0.88 and a standard error of estimation of 1.30. Eq.(6) is plotted in Figure la:
(6)
892
a) all soils
@@ !L I
b) soils with low organic carbon content
• ,
LI
I 6e
F
bL
o
/. "/ //z
L
! \ ..l v "0 xl
I
\
48
L
// /-
i-
~~Jr
h L
._1
/-
1:1 v
//
!
i
z
,-
=
u
o
~"
jf i
i
J @
@. 0 4
0.08
@.£2
0.16
0.2
l [
0.24
I
O
,
,
,
,
0.0~.
fO~
,
0.02
,
i
O.O3
fOC:
Figure la and lb. Atrazine soil-water distribution coefficient (Kd) versus the soil organic carbon fraction (fo~) of 109 granodioritic soil horizons.
Most of the data in Figs.la and lb are clustered around the origin. This brings about two implications for the linear regression analysis: (i) that the slope (Ko¢) will be determined mainly by the (few) data points for soils with high organic carbon content and (ii), that the regression coefficient is (artificially) high. In a sensitivity analysis where data points were eliminated randomly, the estimated Koc varied significantly (from 206 to 332 L/kg). Koc for atrazine has previously been determined to be 149 by Kenaga and Goring (1980), 216 by Brown and Flag (1981), from 23 to 128 by Singh et al. (1990), and from 57 to 636 by Muntau et al. (1992) in a number of different soil types. This variability in data on Koc, may be explained, partly by differences in the experimental techniques used (as reviewed by Singh et al., 1990) and partly by differences in the solute partition efficiency of the organic matter from the various soil types. The non-linearity parameter "n" in Eq. (4) was revealed to be significantly different from unity (99.9%) in a logarithmic regression analysis. For the 109 soil horizons the best estimates of the coefficients in Eq. (4) were (95% confidence intervals): Kd = 10O81+°2°) × (foe)(0"75-'0"03)
(7)
A non linearity factor less than unity supports the hypothesis of an enhanced efficiency in adsorption of atrazine in the lower profiles due to a higher degree of humification of the organic matter in these horizons and thus a higher number of adsorption sites per unit of mass of organic matter. The present value of 0.75 is in agreement with published data (Larsen and Hansen, 1992; Lara and Ernst, 1991). A plot on a logarithmic form of Eq. (7) is shown in Figure 2.
0.04
893
2
tI
/~/
t
i
z v
~
o-9/"
-
e~
0 -4
,
-i
4"
o
o
~./o [
°°
=
-2 -4.5
-3.G
-2.5
log
-1.5
-0.,~
(foc)
Figure 2. Atrazine soil-water distribution coefficient (log Kd) versus the soil organic carbon fraction (log foc) of 109 granodioritic soil horizons.
The role of the various organic matter fractions: For the soils rich in organic carbon (0.1% or higher), the concentration of the various fractions of humic substances was measured. The results are summarized in Table 3.
TABLE 3. Composition (%) of fractionated organic matter in 48 soil horizons MEAN
SD
RANGE
Humic acids
0.030
+ 0.040
0.002-0.18
Fulvic acids
0.010
+ 0.016
0.001-0.086
Humin
1.21
+ 1.66
0.001-7.9
Free organic matter
1.11
+ 3.07
0.001-17.1
Humin (Hum) and free organic matter (Fom) were present at concentrations of approximately two orders of magnitude higher than humic acids (Ha) and fulvic acids (Fa). Nevertheless, a multiple linear regression analysis between Kd and the various organic matter fractions indicated that humic acid explained 71% of the variance compared to 26% for humin plus free organic matter and 0% for fulvic acids (estimates of coefficients + 95% confidence limits): Ka= (167 + 11) x H a - (0.74 + 0.09) × (Hum + Fom) + (21 + 16) × MnO- (1.6 + 1.0)
(8)
with n= 96, r2= 0.92 and a standard error of estimation of 1.5. The amount of humic acids increases in soil organic matter by humification processes (Grathwohl, 1990). The identification of humic acids as the dominating factor for soil sorption of atrazine is therefore enterily consistent with the previously mentioned hypothesis, which leads to a non-linear relationship between Kd and foc. Manganese oxides (MnO) explained 3% of the total variance in Eq. (8). A similar correlation was
894
found with iron oxides. Since manganese and iron oxides are not independent variables (inter-correlation with r 2 = 0.71) they cannot be included simultaneously in the regression analysis, but in any case, they have little significance. Prediction o f soil sorption: The aqueous solubility of atrazine at 22°C was determined to be 34.2 _+
3.2 mg/L which is in agreement with the value of 33 mg/L as published by Kenaga and Goring (1980). Predicted Koc values for atrazine based on our experimental water solubility data and empirical models are shown in Table 4.
Table 4. Comparison of sorption coeffidents for atrazine found in this study (Koc= 216 + 9 L/kg) and predicted by empirical models based on water solubility (S = 34.2 + 3.2 mg/L). Predicted Koe- range 3100-3400 1500-1700 870-970 970-1100 580-630
Equation log Koc= -0.54 log S a + 4.70 n=10 r2=0.99 log Koc= -0.69 log Sc + 4.27 n=22 r2=0.93 log Ko¢= -0.58 log Sa + 4.24 n=15 r2=0.64 log Koc= -0.73 log S b + 0.24 n=12 r2=0.99 log Koc= -0.46 log Sa + 3.79 n=10 r2=0.80
Reference Karickhoff et aL(1979) Means et al.(1982) Mingelgrin et a/.(1983) Chiou et al. (1983) Kawamoto et a/.(1989)
Units for S are: a) urnol]L; b) mol/L ; c) mg]L
The predicted Koc-values for atrazine of 580 to 3400 L/kg are much higher than the experimental value from the present investigation of 216 + 9 L/kg and higher than the values of 23-636 ldkg reported in the literature (Kenaga and Goring, 1980; Rao, 1982, Brown and Flagg, 1981 and Muntau et al; 1992). The solubilities of s-triazines are strongly influenced by their crystal energy. The empirical equations in Table 4 have been derived mainly from data on hydrophobic compounds, for which, the crystal energy plays a lesser role, and this may explain the over-estimation of Ko¢ for atrazine from its solubility (Karickhoff, 1981). The octanol/water partition coefficient (Kow) of atrazine was determined at different pH values; log Kow gave a mean value of 2.35 + 0.05 for pH values ranging from 5 to 9. No significant differences in this pH range were observed. The log Kow value is in accordance with log Kow of 2.33 found by Rao (1982). Predicted Koc values for atrazine, based on our experimental water octanol-water partition coefficient and empirical models, are shown in Table 5.
895
TABLE 5. Comparison of sorption coefficients for atrazine found in this study (Ko~ = 216 + 9 L/kg) and predicted by empirical models based on the n-oetanol/water partition coefficient (log Kow = 2.35 + 0.05). Predicted Koc- range
Equation
120-150
log Ko¢= 1.00 log Kow-0.21
Reference n=10 r2=l.00
Karickhoff et aL(1979)
140-170
log Koc= 0.72 log Kow+0.49
n=13 r2=0.95
Schwarzenbach (1981)
12-14
log Koc= 0.90 logKow- 1.01
n=12 r2=0.99
Chiou
370-430 37-46
log Ko¢= 0.64 log Kow+ 1.1 log Koc= 0.87 logKow- 0.43
n=10 r2=0.87 n=17 r2=0.73
Kawamoto e t a/.(1989) Mingelgrin e t al.(1983)
120-130
log Koc= 0.52 logKow+ 0.88
n=105 r2=0.95
Briggs (1981)
140-180
log Koc= 0.94 logKow-0.006
n=9 rZ=0.95
Brown
et
et
al.(1983)
al.(1979)
The predicted Koc values for atrazine from empirical models based on our experimental Kow data are in a similar range as the experimental values found in the literature (23-636). The sorption of hydrophobic compounds in soil has been expressed as a partitioning of the solute between water and a hydrophobic phase found in the soil organic matter, akin to a non-miscible organic solvent (reviewed by Chiou, 1989). The good prediction power of Kow for Koc found in the present study supports the above explanation.
CONCLUSIONS The soil-water distribution (Kd) of atrazine in granodioritic soils is governed by the soil organic matter. Other soil components like the oxides of aluminum, manganese, or iron, play a minor role, and are only important for soils with extremely low organic matter contents. Not only the amount of organic matter in a soil, but also its composition determines K d. In particular, the humic acids are strongly correlated with K d. This underlines that caution should be taken when soil sorption of atrazine and possibly other related compounds are to be estimated from published organic carbon soil sorption coefficients (Koc). In particular, the assumption that soil organic matter in "ordinary soils" has a constant composition with an invariable efficiency of solute partition can result in overestimation of soil sorption. Consequently misleading results can be obtained for the assessment of groundwater contamination. An analysis of the concentration of various organic matter fractions will improve estimations of the soil sorption. In the absence of sorption data, predictions of the soil sorption for atrazine and possibly other s-triazines are more precise when based on the octanol/water partition coefficient of the compound, rather than on the aqueous solubility.
ACKNOWLEDGMENTS The authors wish to thank A. Nielsen and H. SkejO-Andresen for their skilful technical assistance.
896
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Aiken, G.R., McKnight, D.M., and Wershaw., 1985. An introduction to humic substances in soil, sediments, and water. A Wiley-lnterscience Publication .ISBN 0-471-88274-7. pp. 1-9. Alhajjar, B.J., Simsiman, G.V., and Chester, G., 1990. Fate and transport of alachlor, metolachlor and atrazine in large columns. War. Sci. Tech. 22, 87-94. Bacci, E., Renzoni, A., Gaggi, C., Calamari, D., Franchi, A., Vighi, M., and Severi, A., 1989. Models, field studies, laboratory experiments: an integrated approach to evaluate the environmental fate of atrazine (s-triazine herbicide). Agr. Ecosyst. Environ. 27,513-522. Bowman, B.T., 1990. Mobility and persistence of alachlor, atrazine and metolachlor in plainfield sand, and atrazine and isazofos in honeywood silt loam, using field lysimeters. Environ. Toxicol. Chem. 9, 453-461. Buser, H., 1990. Atrazine and other s-triazine herbicides in lakes and in rain in Switzerland. Environ. Sci. TecnoL 24, 1049-1058. Briggs, G.G., 1981. Theoretical and experimental relationships between soil adsorption, octanol-water partition coefficients, water solubilities, bioconcentration factors and the parachor. J. Environ. Agric. Food Chem. 29, 1050-1059. Calvet, R., 1989. Adsorption of organic chemicals in soils. Environ. Health Persp. 83, 145-177. Chiou, C.T., Porter, P.E., and Schmedding, D.W., 1983. Partition equilibria of non-ionic organic compounds between soil organic matter and water. Environ. Sci. Technol., 17:227-231. Chiou, C.T.; Malcom, R.L.; Brinton, T.I.; Kile, D.E., 1986. Water solubility enhancement of some organic pollutants and pesticides by dissolved humic and fulvic acids. Environ. Sci. TechnoL 20, 502-5O8. Chiou, C.T.; Kile, D.E.; Brinton, T.I.; Malcom, R.L.; Leenheer, J.A.; MacCarthy, P., 1987. A comparison of water solubility enhancements of organic solutes by aquatic humic materials and commercial humic acids. Environ. Sci. Technol., 21, 1231-1234. Chiou, C.T., 1989. Theoretical considerations of the partition uptake of nonionic organic compounds by soil organic matter. In Reactions and Movement of Organic Chemicals in Soils; Sawhney, B.L., Brown,K., Eds.; Special Pubblication 22; Soil Science Society of America: Madison, WI, 1989; Chapter 1, pp 1-29 Clay, S.A., Allmaras, R.R., Koskinen, W.C., and Wyse, D.L., 1988. Desorption of atrazine and cyanazine from soil. J. Environ. Qual. 17, 719-723. Cortts, A. GEnesis y evolucitn de suelos desarrollados a partir de granodioritas en el area de influencia mediterranea (NE. de Espafia). Doctoral thesis (Universitat de Barcelona), 1989., 482 PP. Frank, R. and Sirons, J.G., 1985. Dissipation of atrazine residues from soils. Bull. Environ. Contain. Toxicol. 34, 541-548. Garbarini, D.R.; Lion, L.W., 1986. Influence of the nature of soil organics on the sorption of toluene and trichloroethylene. Environ Sci. Technol. 20, 1263-1269. Grathwohl, P., 1990. Influence of organic matter from soils and sediments from various origins on the sorption of some chlorinated aliphatic hydrocarbons: implications on Koc correlations. Environ. Sci. Technol. 24, 1678-1693. Hassett, J.J. and Banwart, W.L., 1989. The sorption of nonpolar organics by soils and sediments. Reactions and movement of organic chemicals in soils. SSSA Publication 22, 31-44. Hodson, J. and Williams, N.A., 1988. The estimation of the adsorption coefficient (Koc) for soils by high performance liquid chromatography. Chemosphere. 17, 67-77.
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Karickhoff, S.W., Brown, D.S., and Scott, T.A., 1979. Sorption of hydrophobic pollutants on natural sediments. War. Res. 13, 241-248. Karickhoff, S.W., 1981. Semi-empirical estimation of sorption of hydrophobic pollutants on natural sediments and soils. Chemosphere. 10, 833-846. Kawamoto, K. and Urano, K., 1989. Parameters for predicting fate of organochlorine pesticides in the environment (II). Adsorption constant to soil. Chemosphere, 19, 1223-1231. Kenaga, E.E. and Goring, C.A.I., 1980. Relationship between water solubility, soil sorption, octanol-water partitioning and concentration of chemicals in biota. Aquatic Toxicol. ASTM STP 707, 78-115. Kishi, H.; Kogure, N., and Hashimoto, Y., 1990. Contribution of soil constituents in adsorption coefficient of aromatic compounds, halogenated alicyclic and aromatic compounds to soil. Chemosphere. 7, 867-876. Lagas, P., 1988. Sorption of chlorophenols in the soil. Chemosphere. 17, 205-216. Lara, R., and Ernst, W., 1990. Sorption of polychlorinated biphenyls on marine sediments. I. The r61e of the organic carbon content. Environ. Technol. 11, 83-92. Larsen, B.; Pay~i-P6rez, A.B.; Cort6s, A., and Riego, J., Correlation studies for soil sorption of PCB congeners. In Organohalogen Compounds, Vol. 4:Dioxin'90 - EPRI SEMINAR, Ed. by O. Hutzinger and H. Fiedler.- Bayreuth: Ecoinforma Press, 1990, pp 215-217. Larsen, B., and Hansen, B., 1992. Estimations of physico-chemical properties of 209 chorobiphenyls from Gas-Chromatographic retention times, total molecular surface area and molecular structure analyzed by the Bootstrap method. Chemosphere, (in preparation). Lee, M.C., Griffin, R.A., Miller, M.L. and Chian, E.S.K., 1979. Adsorption of water-soluble polychorinated biphenyl, Arochlor 1242 and used capacitor fluid by soil materials and coal chars. J. Environ. Sci. Health. 1,415-442. Means, J.C., Wood, S.G., Hasset, J.J. and Banwart, W.L., 1982. Sorption of amino- and carboxy-substiruted polynuclear aromatic hydrocarbons by sediments and soils. Environ. Sci. Technol. 16, 93-98. Mingelgrin, U. and Gerstl, Z., 1983. Reevaluation of partitioning as a mechanism of nonionic chemicals adsorption in soil. J. Environ. Qual.,12, 1-11. Muntau, H., Kuhnt, O.F., and Murphy, P., 1992. Eurosoils: identification, collection, treatment and characterization. EUR. Report (1992). Neugebaur, K., Zieris, F. and Huber, W., 1990. Ecological effects of atrazine on two outdoor artificial freshwater ecosystems. Z. Wasser-Abwasser-Forsch. 23, 11-17. Oepen, B. von, K6rdel, W., and Klein, W., 1991. Sorption of nonpolar and polar compounds to soils: Processes, measurements and experience with the applicability of the modified OECD-guideline 106. Chernosphere. 22, 285-304. Pay~-P6rez, A.B., Riaz, M., and Larsen, B.R., 1991. Soil sorption of 20 PCB congeners and six chlorobenzenes. Ecotox. Environ. Safe. 21, 1-17. Rao, P.S.C., Davidson, J.M., Berkheiser, V.E., Ou, L.T., Street, J.J., Wheeler, W.B., and Yuan, T.L., 1982. Retention and transformation of selected pesticides and phosphorous in soil-water systems: a critical review, "U.S. Environmental ProtectiOn Agency Report - 600/3-82-060. USEPA, Washington, D.C. Rutherford, D.W.; Chiou, C.T., and Kile, D.E., 1992. Influence of soil organic matter composition on the partition of organic compounds. Environ. Sci. Technol. 26, 336-340. Schwarzenbach, R.P. and Westall, J., 1981. Transport of non polar organic compounds from surface water to groundwater. Laboratory sorption studies. Environ. Sci. Technol. 15, 1360-1367.
898
Singh, G., Spencer, W.F., Cliath., and van Genuchten, M. Th., 1990. Sorption behaviour of s-triazine and thiocarbamate herbicides on soils. J. Environ. Qual. 19, 520-525. Soil Science Society of America (SSSA), 1986. Methods of soil analysis, Part. 2. Ed. A. Klute. 570-571. Starr, J.L. and Glotfelty, D.E., 1990. Atrazine and Bromide movement through a silt loam soil. J.
Environ. Qual. 19, 552-558. U.S. Department of Agriculture (USDA), 1987. Soil Survey Staff. Keys to soil taxonomy (third printing). SMSS technical monograph, n° 6. Ithaca, New York. 280 pp.
0045-6535/92$5.00+ 0.00 PergamonPressLtd.
ORGANIC MATTER FRACTIONS CONTROLLING THE SORPTION OF ATRAZINE IN SANDY SOILS
A.B. Payh-P~rez; A. Cortes; M.N. Sala and B. Larsen
Commission of the European Communities. Joint Research Center. Environment Institute. Ispra Establishment. 21020 Ispra (VA), ITALY (Received in Germany 20 July 1992)
ABSTRACT The soil-water distribution (Kd) of atrazine has been studied in 24 soil profiles, comprising 109 soil horizons derived from granodioritic materials; Kd ranged from 0.01 I./kg to 64 L/kg with a mean value of (2.4 5: 7.3) l_/kg. The concentrations of organic carbon, cation exchange capacity, clay, and oxides of iron, aluminum, and manganese were determined. In a multiple linear regression analysis Ka was strongly correlated with the organic carbon content (foe) and weakly correlated with aluminum oxides. No other soil components were correlated with Kd. The organic carbon soil sorption coefficient (Koc) was estimated to be 216 _+9 L/kg (95% confidence limits) from a linear plot of Ka versus the foc- In a logarithmic correlation between Kd and foc the non-linearity constant proved statistically significant from unity: logKd = (1.81 + 0.20) + (0.75 5: 0.07) × logfoc which indicates that not only the concentration, but also the composition of the organic matter play important roles in soil sorption of atrazine. In 48 of the 109 soil horizons which contained more than 0.1% of organic carbon, the organic matter was fractionated into humic acids, fulvic acids, humin and free organic matter. A multiple linear regression analysis between Kd and the various organic matter fractions revealed that humic acid (Ha) explained 71% of the variance compared to 26% for humin plus free organic matter (Hum + Fom). In addition, 3% of the variance was explained by manganese oxide: Kd= (167 + 11) x Ha - (0.74 5: 0.09) x (Hum + Fom) + (21 + 16) x MnO - (1.6 + 1.0) Previously published equations for predictions of Koc of a compound from its water solubility and its octanol-water partition coefficient (Kow) were evaluated. Kow proved to be the best predictor for atrazine. Key words: atrazine, soil sorption, organic matter fractions, correlations
INTRODUCTION Atrazine is a herbicide controlling broadleaf weeds and some grasses used particulary in corn and sorghum cultivation. Atrazine is also used at higher rates of application as a non selective herbicide, and may remain in the soil for several months after the application (Frank and Sirons, 1985
and
Neugebaur,
1990).
Groundwater
contamination
with
atrazine
[2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine],is frequently reported as a result of its 887
888
widespread agricultural use (Buser, 1990; and Bacci, 1989). The mobility of atrazine by surface runoff or leaching through the soil, is important for the assessment of its impact on the environment (Bowman, 1990; Alhajjar, 1990 and Starr, 1990). The parameter most often used as an indicator of the mobility of soil contaminants is the soil-water distribution coefficient (Kd) calculated from the Freundlich sorption isotherms (Karickhoff, 1981; Clay, 1988; Lagas, 1988; Calvet, 1989 and Oepen, 1991): K d = Xeq/Ceq ( 1) where X and C are the concentration of atrazine in soil and in solution, respectively. The soil-water distribution coefficient of a broad range of chemicals has been shown to be related to the organic carbon content of the soil (Briggs, 1981 and Singh, 1990): K d = Koc x fo~ (2) where foc is the organic carbon fraction on weight basis. Assuming that the carbon content of soil organic matter in "ordinary soils" is approximately constant, the organic carbon soil sorption coefficient (Koc) can be computed from plots of Kd versus foc when the obtained linear regression lines intercept the origin. A more general formulation of Eq. (2) has been proposed: Kd = Koc x foc + K° (3) where K° describes adsorption on components other than organic carbon e.g. active surfaces (Lee, 1979 and Pay~i-PErez et al. 1991). These other soil parameters are expected to contribute to the atrazine soil sorption, in soils with a low organic carbon content (Hasset & Bandward, 1989). The effect of organic matter composition on Kd has been illustrated in studies of organic solute (partition) interactions with humic and fulvic acids where Kd decreased with increasing polar group content of the humic and fulvic acids (Chiou et al. 1986; Garbarini and Lion, 1986; Chiou et al. 1987). Similar findings have recently been published for indigenous organic matter of soils (Rutherford et al. 1992). These results call into question the assumption in Eqs. 2 and 3 that Koc is a universal constant independent of the soil quality. The composition of organic matter in soil is changing continuously through chemical and physical processes (humification) which eventually lead to a decrease in the polar group content of the organic matter (Grathwohl, 1990). Thus, the solute partition efficiency of the organic matter increases with the age of a soil, and normally also with the depth of a soil horizon. Since the concentration of organic matter in soils is typically decreasing with the depth, a non-linear correlation between K d and foc is expected: K d = Koc x (foe)n (4) where "n" is less than unity. For the soil-water distribution of single families of compounds in a limited number of soils of the same type (e.g. horizons from the same profile) such non-linear correlations have recently been reported with values of "n" around 0.7 (Lara and Ernst, 1991; Larsen and Hansen, 1992). A re-elaboration of the data published by Larsen et al. (1990) and Kishi et al. (1990) also point to a value of "n" between 0.6 and 0.8.
In the present study we have determined K d for atrazine in 24 sandy soils, subdivided into 109 soil horizons and investigated the correlations between Kd, soil organic carbon, and mineral soil components (clay, iron oxides, aluminum oxides, manganese oxides and cation exchange capacity). For soil horizons containing more than 0.1% organic carbon the organic matter was fractionated into humic acids, fulvic acids, humin and free organic matter, and the role of each of these fractions in the soil-water distribution of atrazine was evaluated using multiple linear regression analyses. Several equations have been proposed in the literature for the estimation of the soil sorption coefficient (Koc) of organic compounds from their water solubility and from their octanol-water
889
partition coefficient. In order to test the validity of such equations, the water solubility and the octanol-water partition coefficient of atrazine were determined and the predicted Koc values were compared to our experimentally determined values.
MATERIALS AND METHODS
Soils: A total of 109 soil horizons from 24 soil profiles were collected in NE Spain from 1984 to 1987. These soils represent a broad range of soil-forming factors i.e. topography, orientation, vegetation and human activity. The soils are mostly classified as Xerorthents, Xeropsamments and Xerochrepts, with a few Haploxeralfs and Palexeralfs (Soil Taxonomy USDA, 1987) according to their morphological and physicochemical characteristics. The soils were formed on granodioritic materials rich in plagioclases. They are sandy soils with pH between 5 and 8. The most common clay minerals in these soils are mica-illite, vermiculite and kaolinite. The soil samples were air dried and sieved to 2 mm. Selected soil characteristics that may affect the soil-water distribution of atrazine were determined in triplicate according to standard methods, (Cort6s, 1989), and are given in Table 1. Table 1. Characteristics of the 109 soil horizons
MEAN
SD
RANGE
Organic carbon (%)
0.81
+ 2.9
0.01-23.4
Clay (%)
10.5
+ 5.1
1.8-24.1
CEC (meq/100g)
15.3
+ 5.8
5.3-42.7
CaCO3(%)
2.4
+ 5.8
0-19
Total A1203 (%)
14.9
+ 1.3
11.1-17.8
Fe203 (%)
4.1
5:0.8
1.6-5.7
MnO (%)
0.07
5:0.02
0.03-0,11
Organic carbon was determined by the Walkley-Black method (SSSA, 1986) using potassium dichromate as
oxidant. Samples with more than 12% organic carbon were reanalyzed by the
combustion method (SSSA, 1986). Four major fractions of soil organic matter have been defined in terms of their solubilities (Aiken et al., 1985); Humic Acid (Ha): The fraction of humic substances insoluble in water at a pH below 2 but soluble at a higher pH. Fulvic Acid (Fa): The fraction of humic substances soluble at all pH conditions. Humin (Hum): The fraction of humic substances insoluble in water at any pH but soluble in ethanol-bromoform 1:1. Free Organic matter (Fom): The fraction of humic substances insoluble in both water at any pH and ethanol-bromoform 1:1. These organic matter fractions (Ha, Fa, Hum and Fom) were determined in 48 of the 109 soil horizons which contained more than 0.1% of organic carbon. Humic and fulvic acids were extracted using 0.1N NaOH and IM Na4P207 in a ratio of 1:1. Humin and free organic matter were separated by densimetry using ethanol-bromoform 1:1 . The various fractions were quantified colorimetrically at 620 nm, (Con6s, 1989).
890
Total iron, aluminum and manganese oxides were determined by X-Ray fluorescence. Tablets of soil compressed at 20 metric tons during one minute were used. In some cases it was necessary to add a binding material to facilitate the compression and preservation of the samples. Samples were analyzed using a Philips PW 400 spectrophotometer with a gold tube calibrated to standards obtained from the U.S. Geological Service. Partition experiments: 14C-Labeled atrazine [2-chloro-4-(ethyl-l-14C-amino)-6-(isopropylamino) -s-triazine] (925 MBq/mmol) of a 98% radiochemical purity was purchased from Amersham International, Ltd., The identity and purity were tested by gas chromatography-mass spectrometry (GC-MS). A mother solution of 14C-labelled atrazine in deionized and distilled water (0.56 ~mol /L) was prepared. For the determination of the soil-water distribution coefficient, a 10 mL aliquot of the mother solution was added to 1 g of soil in teflon-capped centrifuge tubes (in triplicate). The tubes were shaken for 24 hours at 22 + I°C to assure equilibrium (Singh, 1990), and centrifuged at 4300 rpm for 10 minutes at 22 + I°C. A 1 mL aliquot of the centrifugate was mixed with 10 mL of liquid scintillator and counted for 1 hour in a liquid scintillation analyzer (standard error less than 0.8 %). Data were corrected for quenching and background. The amount of atrazine adsorbed in the soil was calculated by difference and the soil-water distribution coefficient (Kd) computed from Eq.(1). For control of the mass balance the desorption was studied for soils rich in organic matter (Clay et al., 1988). A total of 81% + 7% could be recovered after 4 aqueous desorption steps and a further 8.0% + 0.3% after desorption with acetonitrile.
Kow and S: The octanol/water partition coefficient (Kow) was determined by a shake-flask method. Stock solutions of (19,2 ixmol/L) 14C-atrazine were prepared using buffer solutions from pH 5 to pH 9. 1 mL of n-octanol was added to 10 mL of stock solution in teflon-capped centrifuge tubes. The tubes were equilibrated for 19 hours in an end-over-end shaker at about 30 rpm at 22°C + I°C. The phases were separated by centrifugation at 3400 rpm for 1 hour. 14C-atrazine was measured in both phases, and the Kow was calculated after correcting for quenching and background (four replicates).The aqueous solubility of atrazine was determined (five replicates) at 22°C + I°C. Water was saturated with atrazine at an equilibrium period of 7 days, and aliquots (1 mL) were extracted with dichloromethane (9 mL), diluted 1:10 with iso-octane, and analyzed by GC-MS on a Carlo-Erba QMD 1000 instrument in the single ion recording mode at 200 m/z.
RESULTS AND DISCUSSION
Horizons: The soil-water distribution coefficients (Kd) for atrazine in the studied soil horizons ranged from 0.01 to 64.3 L/kg, with a mean value of 2.4 -+ 7.3 L/kg, and tended to decrease when moving from the surface to the parent material (Table 2). The organic carbon distribution coefficient (Koc) calculated from Eq.(2) for each individual soil ranged from 11 to 25800 L/kg, with a mean value of 566 + 278 L/kg (excluding data from parent material, type Diaclase). It must be noted that this method tends to overestimate Koc. A better way is to calculate Koc as the slope in linear plots of Kd versus foc as in Eq.(3). However, due to the low number of soils in some of the horizon types linear plots could not be constructed for all horizon types.
891
TABLE 2. Organic carbon (%), K d and Koc of the different soil horizons HORIZON
% OC
Kd
Koc
number of
TYPE a
M E A N SD
MEAN SD
MEAN b SD
soil horizons
O
16.5
+ 8.4
38
+ 16
274
+ 148
3
A
0.91
+ 0.94
2.0
+ 2.2
253
+ 161
38
AC
0.14
+ 0.09
0.78
+ 0.49
864
+ 702
11
BA
0.29
+ 0.13
0.76
+ 0.22
322
+ 212
3
B
0.12
+ 0.09
0.41
+ 0.37
403
+ 461
10
Bt
0.09
+ 0.05
0.53
+ 0.38
664
+ 514
7
Bw
0.17
+ 0.06
0.63
+ 0.09
404
+ 192
2
C
0.10
+ 0.09
0.30
+ 0.25
691
+ 693
11
0.07
+ 0.11
0.44
+ 0.37
1092
+ 1026
20
0.03
+ 0.01
0.22
+ 0.07
11900
+ 13600
4
Cr Diaclase aUSDA, 1987.
b calculated from Eq.(2")for each individual horizon.
Koc varied with a factor of four between the different horizons (Table 2), and tended to be higher for the lower horizons closer to the parent material (AC, Bt, C and Cr). This indicates that other factors than organic carbon are important in controlling K d.
The role of organic carbon and other soil components: A multiple linear correlation analysis for all 109 soil horizons between K d and all the considered soil factors showed that K d was strongly correlated with the soil organic carbon, and weakly correlated with aluminum oxides Eq.(5). No other soil components revealed significant correlations (estimates of coefficients + 95% confidence limits): Kd= (214 + 12) × foc + (0.10 + 0.07) × A1203 -(1.26 + 1.00)
(5)
with N=327, r2= 0.82, and a standard error of estimation of 1.29. More than 99% of the variance in Eq. (5) is explained by the organic carbon, thus the aluminum oxides may be eliminated from the equation without reducing the correlation (estimates of coefficients + 95% confidence limits): K d = (216 + 9)foe - (0.27 + 0.27) with N=327, r2= 0.88 and a standard error of estimation of 1.30. Eq.(6) is plotted in Figure la:
(6)
892
a) all soils
@@ !L I
b) soils with low organic carbon content
• ,
LI
I 6e
F
bL
o
/. "/ //z
L
! \ ..l v "0 xl
I
\
48
L
// /-
i-
~~Jr
h L
._1
/-
1:1 v
//
!
i
z
,-
=
u
o
~"
jf i
i
J @
@. 0 4
0.08
@.£2
0.16
0.2
l [
0.24
I
O
,
,
,
,
0.0~.
fO~
,
0.02
,
i
O.O3
fOC:
Figure la and lb. Atrazine soil-water distribution coefficient (Kd) versus the soil organic carbon fraction (fo~) of 109 granodioritic soil horizons.
Most of the data in Figs.la and lb are clustered around the origin. This brings about two implications for the linear regression analysis: (i) that the slope (Ko¢) will be determined mainly by the (few) data points for soils with high organic carbon content and (ii), that the regression coefficient is (artificially) high. In a sensitivity analysis where data points were eliminated randomly, the estimated Koc varied significantly (from 206 to 332 L/kg). Koc for atrazine has previously been determined to be 149 by Kenaga and Goring (1980), 216 by Brown and Flag (1981), from 23 to 128 by Singh et al. (1990), and from 57 to 636 by Muntau et al. (1992) in a number of different soil types. This variability in data on Koc, may be explained, partly by differences in the experimental techniques used (as reviewed by Singh et al., 1990) and partly by differences in the solute partition efficiency of the organic matter from the various soil types. The non-linearity parameter "n" in Eq. (4) was revealed to be significantly different from unity (99.9%) in a logarithmic regression analysis. For the 109 soil horizons the best estimates of the coefficients in Eq. (4) were (95% confidence intervals): Kd = 10O81+°2°) × (foe)(0"75-'0"03)
(7)
A non linearity factor less than unity supports the hypothesis of an enhanced efficiency in adsorption of atrazine in the lower profiles due to a higher degree of humification of the organic matter in these horizons and thus a higher number of adsorption sites per unit of mass of organic matter. The present value of 0.75 is in agreement with published data (Larsen and Hansen, 1992; Lara and Ernst, 1991). A plot on a logarithmic form of Eq. (7) is shown in Figure 2.
0.04
893
2
tI
/~/
t
i
z v
~
o-9/"
-
e~
0 -4
,
-i
4"
o
o
~./o [
°°
=
-2 -4.5
-3.G
-2.5
log
-1.5
-0.,~
(foc)
Figure 2. Atrazine soil-water distribution coefficient (log Kd) versus the soil organic carbon fraction (log foc) of 109 granodioritic soil horizons.
The role of the various organic matter fractions: For the soils rich in organic carbon (0.1% or higher), the concentration of the various fractions of humic substances was measured. The results are summarized in Table 3.
TABLE 3. Composition (%) of fractionated organic matter in 48 soil horizons MEAN
SD
RANGE
Humic acids
0.030
+ 0.040
0.002-0.18
Fulvic acids
0.010
+ 0.016
0.001-0.086
Humin
1.21
+ 1.66
0.001-7.9
Free organic matter
1.11
+ 3.07
0.001-17.1
Humin (Hum) and free organic matter (Fom) were present at concentrations of approximately two orders of magnitude higher than humic acids (Ha) and fulvic acids (Fa). Nevertheless, a multiple linear regression analysis between Kd and the various organic matter fractions indicated that humic acid explained 71% of the variance compared to 26% for humin plus free organic matter and 0% for fulvic acids (estimates of coefficients + 95% confidence limits): Ka= (167 + 11) x H a - (0.74 + 0.09) × (Hum + Fom) + (21 + 16) × MnO- (1.6 + 1.0)
(8)
with n= 96, r2= 0.92 and a standard error of estimation of 1.5. The amount of humic acids increases in soil organic matter by humification processes (Grathwohl, 1990). The identification of humic acids as the dominating factor for soil sorption of atrazine is therefore enterily consistent with the previously mentioned hypothesis, which leads to a non-linear relationship between Kd and foc. Manganese oxides (MnO) explained 3% of the total variance in Eq. (8). A similar correlation was
894
found with iron oxides. Since manganese and iron oxides are not independent variables (inter-correlation with r 2 = 0.71) they cannot be included simultaneously in the regression analysis, but in any case, they have little significance. Prediction o f soil sorption: The aqueous solubility of atrazine at 22°C was determined to be 34.2 _+
3.2 mg/L which is in agreement with the value of 33 mg/L as published by Kenaga and Goring (1980). Predicted Koc values for atrazine based on our experimental water solubility data and empirical models are shown in Table 4.
Table 4. Comparison of sorption coeffidents for atrazine found in this study (Koc= 216 + 9 L/kg) and predicted by empirical models based on water solubility (S = 34.2 + 3.2 mg/L). Predicted Koe- range 3100-3400 1500-1700 870-970 970-1100 580-630
Equation log Koc= -0.54 log S a + 4.70 n=10 r2=0.99 log Koc= -0.69 log Sc + 4.27 n=22 r2=0.93 log Ko¢= -0.58 log Sa + 4.24 n=15 r2=0.64 log Koc= -0.73 log S b + 0.24 n=12 r2=0.99 log Koc= -0.46 log Sa + 3.79 n=10 r2=0.80
Reference Karickhoff et aL(1979) Means et al.(1982) Mingelgrin et a/.(1983) Chiou et al. (1983) Kawamoto et a/.(1989)
Units for S are: a) urnol]L; b) mol/L ; c) mg]L
The predicted Koc-values for atrazine of 580 to 3400 L/kg are much higher than the experimental value from the present investigation of 216 + 9 L/kg and higher than the values of 23-636 ldkg reported in the literature (Kenaga and Goring, 1980; Rao, 1982, Brown and Flagg, 1981 and Muntau et al; 1992). The solubilities of s-triazines are strongly influenced by their crystal energy. The empirical equations in Table 4 have been derived mainly from data on hydrophobic compounds, for which, the crystal energy plays a lesser role, and this may explain the over-estimation of Ko¢ for atrazine from its solubility (Karickhoff, 1981). The octanol/water partition coefficient (Kow) of atrazine was determined at different pH values; log Kow gave a mean value of 2.35 + 0.05 for pH values ranging from 5 to 9. No significant differences in this pH range were observed. The log Kow value is in accordance with log Kow of 2.33 found by Rao (1982). Predicted Koc values for atrazine, based on our experimental water octanol-water partition coefficient and empirical models, are shown in Table 5.
895
TABLE 5. Comparison of sorption coefficients for atrazine found in this study (Ko~ = 216 + 9 L/kg) and predicted by empirical models based on the n-oetanol/water partition coefficient (log Kow = 2.35 + 0.05). Predicted Koc- range
Equation
120-150
log Ko¢= 1.00 log Kow-0.21
Reference n=10 r2=l.00
Karickhoff et aL(1979)
140-170
log Koc= 0.72 log Kow+0.49
n=13 r2=0.95
Schwarzenbach (1981)
12-14
log Koc= 0.90 logKow- 1.01
n=12 r2=0.99
Chiou
370-430 37-46
log Ko¢= 0.64 log Kow+ 1.1 log Koc= 0.87 logKow- 0.43
n=10 r2=0.87 n=17 r2=0.73
Kawamoto e t a/.(1989) Mingelgrin e t al.(1983)
120-130
log Koc= 0.52 logKow+ 0.88
n=105 r2=0.95
Briggs (1981)
140-180
log Koc= 0.94 logKow-0.006
n=9 rZ=0.95
Brown
et
et
al.(1983)
al.(1979)
The predicted Koc values for atrazine from empirical models based on our experimental Kow data are in a similar range as the experimental values found in the literature (23-636). The sorption of hydrophobic compounds in soil has been expressed as a partitioning of the solute between water and a hydrophobic phase found in the soil organic matter, akin to a non-miscible organic solvent (reviewed by Chiou, 1989). The good prediction power of Kow for Koc found in the present study supports the above explanation.
CONCLUSIONS The soil-water distribution (Kd) of atrazine in granodioritic soils is governed by the soil organic matter. Other soil components like the oxides of aluminum, manganese, or iron, play a minor role, and are only important for soils with extremely low organic matter contents. Not only the amount of organic matter in a soil, but also its composition determines K d. In particular, the humic acids are strongly correlated with K d. This underlines that caution should be taken when soil sorption of atrazine and possibly other related compounds are to be estimated from published organic carbon soil sorption coefficients (Koc). In particular, the assumption that soil organic matter in "ordinary soils" has a constant composition with an invariable efficiency of solute partition can result in overestimation of soil sorption. Consequently misleading results can be obtained for the assessment of groundwater contamination. An analysis of the concentration of various organic matter fractions will improve estimations of the soil sorption. In the absence of sorption data, predictions of the soil sorption for atrazine and possibly other s-triazines are more precise when based on the octanol/water partition coefficient of the compound, rather than on the aqueous solubility.
ACKNOWLEDGMENTS The authors wish to thank A. Nielsen and H. SkejO-Andresen for their skilful technical assistance.
896
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