Fate of Chemicals in the Environment - American Chemical Society

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12 A Comparative Study of the Relationships Between the Mobility of Alachlor, Butylate, and Metolachlor in Soil and Their Physicochemical Properties C. J. SPILLNER, V. M. THOMAS, and D. G. TAKAHASHI Stauffer Chemical Company, Mountain View Research Center, Pesticide Metabolism Section, Mountain View, CA 94042 H. B. SCHER Stauffer Chemical Company, DeGuigne Technical Center, Richmond, CA 94804

The order of the m o b i l i t i e s of a l a c h l o r , b u t y l a t e , and metolachlor i n columns of v a r i o u s s o i l s was metolachlor > a l a c h l o r > b u t y l a t e . This c o r r e l a t e s d i r e c t l y with the water solubilities and i n v e r s e l y to the adsorpt i o n c o e f f i c i e n t s and octanol/water p a r t i t i o n c o e f f i c i e n t s of these compounds. D i f f u s i o n of these compounds i n soil t h i n - l a y e r s was as f o l l o w s : b u t y l a t e > a l a c h l o r > metol a c h l o r , which c o r r e l a t e s d i r e c t l y with the vapor pressures of these compounds. S i g n i f i c a n t s o i l p r o p e r t i e s a f f e c t i n g d i f f u s i o n appeared to be bulk d e n s i t y and temperature. S o i l moisture i s a l s o probably important, but i t s e f f e c t on the d i f f u s i o n of these compounds was not determined. The physicochemical p r o p e r t i e s of a p e s t i c i d e and i t s i n t e r a c t i o n with s o i l g r e a t l y i n f l u e n c e s both i t s m o b i l i t y and b i o l o g i c a l av a i l a b i l i t y i n a s o i l environment (1_). Reviews on t h i s subject have been published by Goring and Hamaker (_2) and Greenland and Hayes ( 3 ) . The o b j e c t i v e s of t h i s study were to (a) determine the mobili t i e s of the h e r b i c i d e s , a l a c h l o r (2-chloro-2 ,6 -diethyl-N-(methoxymethyDacetanilide), butylate (S-ethyl diisobutylthiocarbamate), and metolachlor (2-chloro-N-(2-ethyl-6-methyl phenyl)-N(2-methoxy-l-methyl e t h y l ) acetamide i n the laboratory u s i n g s o i l leaching columns and s o i l t h i n - l a y e r vapor d i f f u s i o n techniques, (b) determine t h e i r s o i l adsorption c o e f f i c i e n t s and other physicochemical p r o p e r t i e s such as octanol/water p a r t i t i o n c o e f f i c i e n t s , water s o l u b i l i t i e s , vapor pressures, heats of adsorption and heats 1

1

0097-6156/83/0225-023l$06.00/0 © 1983 American Chemical Society

Swann and Eschenroeder; Fate of Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

232

FATE OF CHEMICALS

IN THE ENVIRONMENT

of s o l u t i o n and (c) c o r r e l a t e the m o b i l i t i e s and the physicochemi c a l p r o p e r t i e s of these compounds. M a t e r i a l s and Methods


1

Chemicals. P u r i f i e d , [ **C]-labelled a l a c h l o r ( s p e c i f i c a c t i v i t y = 17 mCi/mM), b u t y l a t e ( s p e c i f i c a c t i v i t y = 2.54 mCi/mM) and metolachlor ( s p e c i f i c a c t i v i t y = 4.5 mCi/mM) were used i n the leaching, adsorption, and d i f f u s i o n s t u d i e s . The r a d i o p u r i t y of these compounds was greater than 95% as determined by t h i n - l a y e r chromatography. A l l other studies were conducted using a n a l y t i c a l grade, non-radioactive m a t e r i a l ( p u r i t y > 95%). P h y s i c a l P r o p e r t i e s . Octanol/water p a r t i t i o n c o e f f i c i e n t s were determined f o l l o w i n g the method described by A. Leo, e_t al_. , (13). Samples were analyzed by gas chromatography (GC). Water s o l u b i l i t y was determined by e q u i l i b r a t i o n o f a n a l y t i c a l grade m a t e r i a l with water at constant temperature. E q u i l i b r i u m was approached from both under and super s a t u r a t i o n c o n d i t i o n s and samples were analyzed by GC. Vapor pressures were determined by the Knudsen e f f u s i o n method. Soils. The p h y s i c a l and chemical p r o p e r t i e s of the s o i l s used i n these studies are presented i n TABLE I . S o i l s were screened (500y) p r i o r to use. A n a l y t i c a l Methods. L i q u i d s c i n t i l l a t i o n counting (LSC) was done using Packard Models 3375 and 3380 L i q u i d S c i n t i l l a t i o n Spectrometers equipped with automatic e x t e r n a l standards. S o l i d samples were combusted i n a Packard model 306 Sample O x i d i z e r p r i o r to LSC a n a l y s i s . S o i l Thin-Layer Vapor D i f f u s i o n . Glass p l a t e s (20 x 20 cm) were covered with s o i l s l u r r i e s o f Keeton sandy loam and P r a i r i e s i l t y c l a y loam to a thickness o f 0.75 mm. The p l a t e s were allowed to dry overnight and then 10 mL a l i q u o t s o f acetone s o l u t i o n s o f l '•Clalachlor, [ ^ C ] b u t y l a t e , and [i^C]metolachlor (corresponding to 0.18 ymole and 1.0 x 10 dpm) were a p p l i e d to the s o i l . The s p e c i f i c a c t i v i t i e s of these compounds were a l l adjusted to 2.54 mCi/mM p r i o r to running these experiments. The treated p l a t e s were evenly sprayed to s a t u r a t i o n with d i s t i l l e d water, wrapped i n a p l a s t i c f i l m to reduce atmospheric v o l a t i l i t y , and then held i n a dark growth chamber maintained at 24°C. At 0-, 12-, and 24-hour i n t e r v a l s , the p l a t e s were removed, and placed under x-ray f i l m . They were stored i n a f r e e z e r compartment (-4°C) f o r three days before developing the f i l m . l

1

6

S o i l Column Leaching. Glass tubing (diameter = 1 cm) was cut i n to 50 cm lengths, and one end was plugged with glass wool and Miracloth®. A i r dry s o i l (percent moisture = 2%, 1%, 4% ,for F e l ton, Keeton, P r a i r i e , r e s p e c t i v e l y ) was packed i n t o the tubes to a depth of 30 cm. A small layer o f white b u i l d e r s sand was then


Swann and Eschenroeder; Fate of Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 1.2

1.4

g/cc

g/cc

(approximately twice f i e l d

S a t u r a t i o n percentage-amount

d -

of water to s a t u r a t e 100 g of s o i l

Organic matter determined by combusting a 2.0 g sample at 550-600°C.

S o i l c o l l e c t e d from R. P o r t e r Farm, Thurman, Iowa.

58

38

capacity).

and S o i l and P l a n t Labs, Santa C l a r a , CA.

6.1

-

8.2%

6.7

Analyses performed by Perry Laboratory, Los Gatos, CA.,

29%

4.2%

-

loam-

59%

8%

g/cc

BP

to

2 ^ 21

^*

3.

£

12%

1.7

Prairie silty

SP28

| S §

26%

6.1

pH

66%

3.8%

P.M.-

Keeton sandy loam

4%

Clay

o

6%

Silt

H >

w

90%

Sand

Mechanical and Chemical P r o p e r t i e s of S o i l s . -

r

Felton sand

Name

TABLE I.


234

FATE OF CHEMICALS IN THE ENVIRONMENT

added to mark the treatment zone. A weighed 5 cm equivalent of s o i l was then t r e a t e d with 0.5 mL of acetone c o n t a i n i n g enough of the C h e r b i c i d e s to provide the appropriate treatment r a t e . The treated s o i l was added to the column, and a 15 cm equivalent of water (12 mL) was added so that the s o i l was j u s t saturated (1 mL leachate was c o l l e c t e d from the F e l t o n s o i l columns). After a 3-hour e q u i l i b r a t i o n p e r i o d , the columns were broken i n t o 2.5 or 5 cm s e c t i o n s . Each s e c t i o n was extracted with 10 mL of acetone by shaking f o r 3 hours, the suspension was allowed to s e t t l e , and a l i q u o t s of the e x t r a c t s were analyzed by LSC. A l l leaching was done i n an environmental chamber held at 26°C. The average percent recovery of r a d i o a c t i v i t y was 93.4%.


11+

S o i l Adsorption. S o i l (2.5 g) and 10 mL of aqueous p e s t i c i d e sol u t i o n were combined i n 30 mL screw cap ( t e f l o n - l i n e d ) c e n t r i f u g e tubes which were then a g i t a t e d f o r 3 hours, i n darkness, i n a growth chamber set at 10 ± 1°C, 19 ± 1°C, or 30 ± 2°C. The tubes were c e n t r i f u g e d and the supernatants were analyzed by LSC. Cont r o l experiments included untreated s o l u t i o n / s o i l mixtures used for LSC background determnations and t r e a t e d s o l u t i o n s without s o i l used to determine the extent of p e s t i c i d e adsorption by the glass tubes. The adsorption s o l u t i o n s from the highest concentrations runs were e x t r a c t e d with ether. The ether e x t r a c t s were concent r a t e d and analyzed by TLC. S i m i l a r l y , the corresponding s o i l s were extracted with ether and the ether e x t r a c t s were analyzed by TLC. Other s o i l samples were analyzed by combustion i n order to determine d i r e c t l y the amount of adsorbed h e r b i c i d e . The adsorption c o e f f i c i e n t s (K) were determined using the equation for the F r e u n d l i c h adsorption isotherm: C where C

s w

and C

= KC

1 / n

(1) w = e q u i l i b r i u m s o l u t i o n concentration M

= weight absorbed

solute/weight s o l i d (at e q u i l i b r i u m ) .

Least-squares l i n e a r r e g r e s s i o n a n a l y s i s was performed on the data. The thermodynamic heats of adsorption (AH) were c a l c u l a t e d using equation 3, which i s derived as follows from the r e l a t i o n ship between free energy and the e q u i l i b r i u m constant: AG

= -RTln(Kd) = AH-TAS

t h e r e f o r e , ln(Kd) = -

AH RT

(2) AS R

(3)

Kd = C /C (4) s w Equation 3 i s analogous to the Clausius-Clapeyron equation f o r e q u i l i b r i u m of a substance i n the vapor and condensed phases ( 4 ) .


12.

SPILLNER E T A L .

235

Mobilities and Physicochemical Properties

Where AG i s f r e e energy, R i s gas constant (1.987 cal/deg K m o l e " ) , T i s degrees K e l v i n , and AS i s entropy. Kd i s the d i s t r i b u t i o n constant o f the h e r b i c i d e between the s o l u t i o n phase and the adsorbed phase (equation 4 ) . Thus, l e a s t squares l i n e a r r e g r e s s i o n a n a l y s i s o f ln(Kd) vs. 1/T y i e l d e d values f o r heats of a d s o r p t i o n (AH) f o r the h e r b i c i d e s i n Keeton s o i l . 1


Results Physical Properties. F i g u r e 1.

Results o f these measurements are given i n

S o i l Thin-Layer Vapor D i f f u s i o n . An example o f an autoradiogram obtained from a d i f f u s i o n experiment i s shown i n F i g u r e 2. The extent of d i f f u s i o n of metolachlor, a l a c h l o r , and b u t y l a t e i s given i n TABLE I I . B u t y l a t e d i f f u s i o n increased during the 24

TABLE I I .

D i f f u s i o n of A l a c h l o r , B u t y l a t e , and M e t o l a c h l o r i n S o i l Thin-Layers 2

Area of D i f f u s i o n (cm ) Compound Butylate Alachlor Metolachlor

Soil Keeton

O-Dry 1.1 1.1 1.0

0-Moist 2.0 2.0 1.1

6 Hr 8.0 2.0 1.3

12 Hr 5.3 2.3 1.3

24 Hr 19 2.8 3.3

Butylate Alachlor Metolachlor

Prairie

1.3 1.3 1.5

1.5 1.3 1.5

11 2.3 2.0

13 2.6 1.5

25 5.7 5.2

hour t e s t period while the d i f f u s i o n o f a l a c h l o r and metolachlor was r a t h e r l i m i t e d i n s o i l p l a t e s which were saturated with water. No d i f f u s i o n was detected i n dry s o i l . Under a l l o f the c o n d i t i o n s considered, the r e l a t i v e degrees of d i f f u s i o n through a moist t h i n - l a y e r of s o i l , was b u t y l a t e > a l a c h l o r % m e t o l a c h l o r . In a second experiment, the extent of d i f f u s i o n i n each s o i l ( s a turated with water) was measured as a f u n c t i o n of temperature. These r e s u l t s are shown i n TABLE I I I . Temperature had the greatest a f f e c t on the d i f f u s i o n o f b u t y l a t e and l e s s i n f l u e n c e on the TABLE I I I .

D i f f u s i o n of A l a c h l o r , B u t y l a t e , and M e t o l a c h l o r i n S o i l Thin-Layers at Various Temperatures2

Area of D i f f u s i o n (cm )

Compound Butylate Alachlor Metolachlor -

Keeton S o i l 13 C 18 C 29 C 13 28 36 3.5 4.5 3.8 3.8 4.5 4.2

Prairie Soil 13 C 18 C 29 C 20 39 20 4.2 7.6 7.6 3.5 8.6 8.6

Extent of d i f f u s i o n i n 24 hours.


Swann and Eschenroeder; Fate of Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 3

o

C - CH-CI II '

1.1 X 10

Octanol/Water Partition Coefficient (P at 20°C)

* Increase in water solubility with increase in temperature.

1.

Chemical

Metolachlor.

Figure

3

3

2

H

C H

14.0 X 10

3

Negative**

46 ppm

1.3 X 1 0 m m H g

_2

2

3

CH.-C-CH. , 3

217.4

S

CH CH S — C — N

Butylate

p r o p e r t i e s of A l a c h l o r , B u t y l a t e , and

* * Decrease in water solubility with increase in temperature.

3

Positive*

Positive*

Heat of Solution

0.8 X 10

200 ppm

5

2

3.1 X 10" mmHg

269.8

3

550 ppm

3

2

^CH C,

Solubility in Water (20°C)

5

2

/

3 CH OCH

CH CH

C H

3.2 X 1 1 i n d i c a t e s that as the s o l u t i o n concentration increases the s o r p t i o n s i t e s become saturated, r e s u l t i n g i n a d i s p r o p o r t i o n a t e amount of chemical being d i s s o l v e d . Since n i s n e a r l y equal to 1 i n these s t u d i e s , the adsorption isotherms are n e a r l y l i n e a r and the values f o r Kd (shown i n TABLE IV) correspond c l o s e l y to K. These Kd values were used to c a l c u l a t e heats of adsorption (AH). TABLE IV.

-

Adsorption C o e f f i c i e n t s f o r B u t y l a t e , A l a c h l o r , and Metolachlor i n Keeton S o i l at Various Temperatures Obtained Using the F r e u n d l i c h Equation. Compound

T(°C)

K

n

r

Kd-

Butylate

10 20 30

2. 79 3. 18 3. 28

1 .02 1 .01 1 .01

0 .99 0 .99 0 .99

2 .86 3 .14 3 .26

Alachlor

10 20 30

1. 65 1.62 1.45

1 .13 1 .08 1 .09

0 .99 0 .99 0 .99

2 .00 1 .80 1 .65

Metolachlor

10 20 30

1. 98 1.87 1.54

1 .03 1 .05 1 .06

0 .99 0 .99 0 .99

2 .10 1 .98 1 .64

The average d i s t r i b u t i o n constant, c a l c u l a t e d from the r a t i o of s o i l concentration (moles/kg):solution concent r a t i o n (moles/L).



12.

SPILLNER E T AL.


239

Heats o f Adsorption. Temperature e f f e c t s were determined by meas u r i n g adsorption at three temperatures. As seen from TABLE IV, the K values vary with temperature such that f o r b u t y l a t e , K i n creases with temperature, while f o r a l a c h l o r and metolachlor, K decreases with temperature. These r e s u l t s i n d i c a t e that b u t y l a t e becomes more adsorbed to Keeton s o i l as the temperature increases while a l a c h l o r and metolachlor become l e s s adsorbed as temperature increases. In order to obtain a q u a n t i t a t i v e measure o f these e f f e c t s , heats of adsorption (AH) were c a l c u l a t e d as desc r i b e d p r e v i o u s l y i n the M a t e r i a l s and Methods s e c t i o n (equation 3). TABLE IV contains values f o r the average molar d i s t r i b u t i o n constants (Kd) f o r b u t y l a t e , a l a c h l o r , and metolachlor which were p l o t t e d vs the inverse temperatures (1/°K) to obtain the AH s shown i n Figure 3. f

S o i l Column Leaching. The d i s t r i b u t i o n o f r a d i o a c t i v i t y from [ C ] b u t y l a t e a p p l i e d at 4.5 KG/HA and [ C ] a l a c h l o r and [ C ] metolachlor a p p l i e d at 2.25 KG/HA and leached with 15 cm o f water i n F e l t o n sand, i s shown i n Figure 4. Although a l l three h e r b i cides are mobile i n t h i s s o i l type, b u t y l a t e showed l e s s mobili t y , with 59.6% of the a p p l i e d r a i d o a c t i v i t y found i n the upper 10 cm of the column, while 28.4% and 24.3% of the a p p l i e d C was found i n the upper 10 cm of the a l a c h l o r and metolachlor columns, r e s p e c t i v e l y . In Keeton sandy loam, m o b i l i t y was reduced (Figure 4 ) , but 54.6% of the b u t y l a t e remained i n the 5 cm treated area, while 36.3% and 28.2% of the a p p l i e d a l a c h l o r and metolachlor remained in this section. In P r a i r i e s i l t y c l a y loam (Figure 4 ) , the m o b i l i t y o f a l l three h e r b i c i d e s was g r e a t l y reduced due to the s o i l s ' high o r ganic matter content (8.2%). Most o f the a p p l i e d r a d i o a c t i v i t y was found i n the upper 10 cm of the column f o r each compound. Rf values, c a l c u l a t e d by d i v i d i n g the distance moved by the water front by the distance moved by the compounds are given i n TABLE V. These values can be used to v e r i f y various models. For ltf

1 4

1 4

lk

TABLE V.

Rf o f B u t y l a t e , A l a c h l o r , and Metolachlor i n Various S o i l Columns S o i l Columns Felton Keeton Prairie Calc. Rf? Butylate 0.6 0.33 0.17 0.26 Alachlor 0.9 0.5 0.25 0.41 Metolachlor 0.9 0.8 0.33 0.37 -

Rf i n Keeton s o i l were c a l c u l a t e d as follows (see r e f . 17); Rf = ( l + ( K ) ( a S ) (

1

1_ ) ) " where P2/3 ~ Rf of the p e s t i c i d e i n the s o i l column. F r e u n d l i c h adsorption c o e f f i c i e n t from TABLE IV. bulk density of s o i l from TABLE I . s o i l pore f r a c t i o n (0.476). L

Rf K ds p

= = = -




240

AH (cal/mole) Butylate

3.0

284

Alachlor

-538

Metolachlor

-415

3.5 (1/T) X 1 0

3

F i g u r e 3. H e a t s o f a d s o r p t i o n f o r B u t y l a t e , A l a c h l o r , and M e t o l a c h l o r i n sandy loam s o i l .


Swann and Eschenroeder; Fate of Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1983. Percent of Applied Radioactivity

Sandy Loam

Percent of Applied Radioactivity

Silty Clay Loam

Figure 4. Leaching of Butylate, A l a c h l o r , and Metolachlor i n sand, sandy loam, and s i l t y c l a y loam s o i l columns.

Percent of Applied Radioactivity

Sand


242


example, Hamaker (6) gives an equation f o r converting from ads o r p t i o n c o e f f i c i e n t s (K) to Rf values. In view of the r e l a t i v e s i m p l i c i t y of t h i s model, the c a l c u l a t e d Rf values presented i n TABLE V, (determined from the K values i n TABLE V at 20°C) are good approximations of the a c t u a l Rf determined i n these leaching s t u d i e s .


Discussion S o i l D i f f u s i o n . The r e s u l t s from the s o i l t h i n - l a y e r d i f f u s i o n study of b u t y l a t e , a l a c h l o r , and metolachlor appear to be c o r r e l a t e d d i r e c t l y to the vapor pressues and i n v e r s e l y r e l a t e d to water s o l u b i l i t i e s (TABLE I I ) i n accord with Henry's law of s o l u t e / solvent i n t e r a c t i o n s (.5). Thus, d i f f u s i o n i s the r e s u l t of pesti c i d e vapor movement i n e q u i l i b r i u m with the l i q u i d phase of the s o i l environment, r a t h e r than d i f f u s i o n i n the l i q u i d phase or movement with the l i q u i d phase. These conclusions are supported by the f o l l o w i n g observations: 1) movement with the l i q u i d phase can be r u l e d out i n these d i f f u s i o n s t u d i e s since i n the s o i l c o l umn leaching s t u d i e s a l a c h l o r and metolachlor leached more than b u t y l a t e , but d i f f u s e d l e s s ; 2) d i f f u s i o n i n the l i q u i d phase i s not s i g n i f i c a n t since adsorption to s o i l organic matter would be expected to play a predominant r o l e , and the r e s u l t s i n d i c a t e that s o i l organic matter had no a f f e c t on the d i f f u s i o n of these compounds ( i . e . , greater d i f f u s i o n occurred i n P r a i r i e s o i l which had the greatest percent organic matter). The p o s s i b i l i t y of d i f f u s i o n o c c u r r i n g i n the space between the s o i l l a y e r and the p l a s t i c wrap covering the s o i l , was r u l e d out by adding an a d d i t i o n a l l a y e r of moist s o i l over the a p p l i e d h e r b i c i d e s and by observing that the area of d i f f u s i o n d i d not change. The various s o i l prop e r t i e s which appear to be important i n the d i f f u s i o n of these h e r b i c i d e s are s o i l moisture, s o i l temperature, and s o i l bulk d e n s i t y . Although not enough d i f f e r e n t s o i l s were tested to est a b l i s h these c o r r e l a t i o n s . The absence of d i f f u s i o n i n a i r - d r y s o i l s was determined i n a p r e l i m i n a r y experiment and the d i r e c t c o r r e l a t i o n with temperature i s c l e a r i n TABLE I I I . There a l s o appears to be an inverse c o r r e l a t i o n between d i f f u s i o n and s o i l bulk density since greater d i f f u s i o n was observed i n the P r a i r i e s o i l compared to the Keeton s o i l . The e f f e c t of temperature i s not s u r p r i s i n g i n view of the r e l a t i o n s h i p between vapor pressure (p) and temperature (T) shown i n equation 5: log

p = A-B/T

(5)

where A and B are constants and T i s absolute temperature (17). These f i n d i n g s are c o n s i s t e n t with the work of Farmer, et a l . , (JJS, lj)) and Igue, et a l . , (20) who have reported on the importance of these f a c t o r s (vapor pressure, temperature, s o i l moisture content, and s o i l bulk d e n s i t y ) and t h e i r e f f e c t on the d i f f u s i o n of p e s t i c i d e s i n s o i l . In a d d i t i o n to water s o l u b i l i t y ,


12.

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these are some of the important f a c t o r s which must be considered when developing a comprehensive environmental model which i n cludes p e s t i c i d e d i f f u s i o n i n the s o i l . Soil Mobility. The m o b i l i t y of these compounds i n s o i l l e a c h i n g columns can be d i r e c t l y c o r r e l a t e d to t h e i r r e s p e c t i v e water s o l u b i l i t i e s (TABLE I I ) . In a l l cases, i n c r e a s i n g l e a c h i n g was observed as f o l l o w s : metolachlor > a l a c h l o r > b u t y l a t e . F u r t h e r more, s o i l organic matter appears to be the s i n g l e most important s o i l f a c t o r a f f e c t i n g the v e r t i c a l m o b i l i t y of these compounds. This i s demonstrated by the s l i g h t l e a c h i n g o f a l l three compounds observed i n P r a i r i e s o i l (OM - 8.2%) i n which the d i f f e r ences i n m o b i l i t y are minimal. Adsorption o f these compounds i n the s o i l i s a predominant f a c t o r i n t h e i r m o b i l i t i e s . A thorough understanding of these processes r e s u l t s i n a b e t t e r understanding of the m o b i l i t y o f these compounds i n s o i l . The K values recorded i n TABLE IV are r e l a t e d to a d s o r p t i o n such that i n c r e a s i n g values i n d i c a t e greater adsorption. In the present study, b u t y l a t e e x h i b i t e d the l a r g e s t K values i n Keeton s o i l and i s t h e r e f o r e the most s t r o n g l y adsorbed o f the three compounds s t u d i e d . These r e s u l t s i n d i c a t e that b u t y l a t e would be the l e a s t mobile of these three compounds i n that s o i l type. This i s c o n s i s t e n t with the r e s u l t s from the comparative l e a c h ing of b u t y l a t e , a l a c h l o r , and metolachlor i n three s o i l types ( i n c l u d i n g Keeton s o i l ) . In a l l s o i l s , b u t y l a t e e x h i b i t e d the l e a s t m o b i l i t y . Adsorption p r o p e r t i e s o f p e s t i c i d e s have been shown to most uniformly c o r r e l a t e with the organic matter content of the s o i l (6). Obrigawitch et a l . , concluded that s o i l organic matter was the s i n g l e most important s o i l property a f f e c t i n g met o l a c h l o r adsorption and m o b i l i t y ( 7 ) . In a s i n g l e t e s t at one c o n c e n t r a t i o n , the adsorption o f b u t y l a t e and a l a c h l o r was greater i n P r a i r i e loam s o i l (OM = 8.2%) compared to Keeton s o i l (OM = 4.2%). The r e s u l t s from the present study i n d i c a t e that i n creased adsorption (corresponding to the increase i n s o i l OM) i s r e s p o n s i b l e f o r the decreased l e a c h i n g observed i n the s o i l s with the greatest OM. Other p h y s i c a l p r o p e r t i e s of these compounds which are a l s o c o r r e l a t e d with t h e i r adsorption p r o p e r t i e s (K) are water s o l u b i l i t y ( S H 0 ) and octanol/water p a r t i t i o n c o e f f i c i e n t (Pow). Evidence f o r these types of c o r r e l a t i o n abound i n the l i t e r a t u r e ( 8 ) . I t i s not s u r p r i s i n g that these p h y s i c a l p r o p e r t i e s are c o r r e l a t e d since they a l l r e f l e c t the s o l u t i o n p r o p e r t i e s o f the compound. In systems where s o i l organic matter i s the p r i n c i p l e s o i l c o n s t i t u e n t r e s p o n s i b l e f o r a d s o r p t i o n , the c o r r e l a t i o n between adsorption and octanol/water p a r t i t i o n coeff i c i e n t i s reasonable, since octanol as a sorbant simulates the s o i l organic matter (9). Recently, a method has been proposed whereby a l l of these p h y s i c a l p r o p e r t i e s (K, Pow, SH2O) he estimated from the reverse phase-HPLC r e t e n t i o n time o f a compound (10). T h i s i s i n d i c a t i v e of the s i m i l a r i t y i n the p h y s i c a l 2

c

a


n


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processes involved i n the p a r t i t i o n i n g of a compound i n an RPHPLC column (aqueous/organic phase), p a r t i t i o n i n g i n w a t e r / s o i l and water/octanol systems, and movement i n the s o i l . Heats of a d s o r p t i o n are shown i n F i g u r e 3 f o r these compounds. These modest AH's probably i n d i c a t e that hydrophobic bonding i s r e s p o n s i b l e f o r adsorption (12). This i s c o n s i s t e n t with the non-polar nature of these compounds and the important r o l e of s o i l organic matter i n a d s o r p t i o n . S o i l organic matter i s considered to be hydrophobic (JL1_). One u s u a l l y obtains negat i v e heats of a d s o r p t i o n f o r p e s t i c i d e s (L2) i n d i c a t i n g that heat i s evolved during the process (exothermic). However, with b u t y l ate, a p o s i t i v e AH was observed, which i n d i c a t e s an endothermic process; thus, heat was absorbed from the system when b u t y l a t e was adsorbed by the Keeton s o i l . The AH observed i s a l s o i n correspondence with the heats of s o l u t i o n measured f o r these compounds. A l a c h l o r and metolachlor e x h i b i t .positive heats of s o l u t i o n (greater s o l u b i l i t y at higher temperatures) while b u t y l a t e e x h i b i t s a negative heat of s o l u t i o n , so that i t s s o l u b i l i t y decreases at e l e v a t e d temperatures. The e f f e c t of temperature on adsorption i s d i r e c t l y l i n k e d to the s o l u t i o n p r o p e r t i e s of these compounds at v a r i o u s temperatures. Butylate probably e x h i b i t s a negative heat of s o l u t i o n (and hence a p o s t i v e heat of adsorption) due to the hydroponic e f f e c t described by Tanford (14). This e f f e c t i s caused by the d i s r u p t ed water molecules r e a r r a n g i n g themselves i n t o a lower energy s t a t e at the hydrophobic surface of the b u t y l a t e molecule. In a d d i t i o n , there i s probably a negative entropy of s o l u t i o n as the water molecules f i n d themselves i n a more ordered state at the hydrophobic surface of the b u t y l a t e molecule (JL5). The b u t y l a t e molecule presents a hydrophobic surface from a l l d i r e c t i o n s but metolachlor and a l a c h l o r do not (Figure 5 ) . Conclusions The good c o r r e l a t i o n of the r e s u l t s of vapor d i f f u s i o n and leaching experiments f o r b u t y l a t e , a l a c h l o r , and metolachlor with t h e i r p h y s i c a l p r o p e r t i e s has given support to the value of phys i c a l property measurements to p r e d i c t p e s t i c i d e movement i n the soil. Transport of the h e r b i c i d e s by vapor d i f f u s i o n on moist s o i l was shown to be d i r e c t l y r e l a t e d to vapor pressure and i n v e r s e l y r e l a t e d to water s o l u b i l i t y . Transport of the h e r b i c i d e s by l e a c h i n g was shown to be i n v e r s e l y r e l a t e d to the F r e u n d l i c h ads o r p t i o n c o e f f i c i e n t which i n turn was d i r e c t l y r e l a t e d to the octanol/water p a r t i t i o n c o e f f i c i e n t and i n v e r s e l y r e l a t e d to water s o l u b i l i t y (16). Another i n t e r e s t i n g r e s u l t was the observed p o s i t i v e heat of adsorption f o r b u t y l a t e (negative heat of s o l u t i o n ) and negative heat of a d s o r p t i o n f o r a l a c h l o r and metolachlor ( p o s i t i v e heat of s o l u t i o n ) . T h i s r e s u l t i n d i c a t e s that at low temperatures (near


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12.

Figure 5. Computer generated minimum energy c o n f i g u r a t i o n s of Metolachlor, A l a c h l o r , and Butylate.


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0°C) t h e i r r e l a t i v e a d s o r p t i v i t i e s w i l l converge and that as the temperature increases, t h e i r r e l a t i v e a d s o r p t i v i t i e s w i l l diverge as b u t y l a t e becomes more strongly adsorbed and a l a c h l o r and metol a c h l o r become l e s s s t r o n g l y adsorbed. This r e s u l t should transl a t e i n t o a r e d u c t i o n o f leaching of b u t y l a t e (compared to a l a c h l o r and metolachlor) as the temperature of the s o i l system i s r a i s e d . Thus, the e f f e c t of temperature can be handled by an environmental model f o r s o i l m o b i l i t y by i n c l u d i n g the heat o f adsorption o f the p e s t i c i d e . Acknowledgments The authors would l i k e to acknowledge the c o n t r i b u t i o n s o f the f o l l o w i n g r e s e a r c h e r s : J.R. DeBaun and L.S. Mullen-Rokita, f o r h e l p f u l d i s c u s s i o n s ; E.B. Cramer, f o r a s s i s t i n g with the adsorpt i o n measurements; L.-S. Yu-Farina, f o r the water s o l u b i l i t y and p a r t i t i o n c o e f f i c i e n t measurements; H. Myers, f o r the vapor pressure measurements; and R.R. Winter, f o r running the MACCS molecul a r s t r u c t u r e analyses. Literature Cited 1. 2.

3. 4. 5. 6.

7. 8. 9. 10.

11.

Haque, R. and V.H.E. Freed, "Environmental Dynamics o f P e s t i c i d e s " , Plenum Press, New York (1975). Goring, C.A.I, and J.W. Hamaker. "Organic Chemicals i n the S o i l Environment". Volume 1. Marcel Dekker, Inc., New York (1972). Greenland, D.J. and M.H.B. Hayes. "The Chemistry o f S o i l Processes". John Wiley and Sons, New York (1981). K l u t z , I . , " I n t r o d u c t i o n to Chemical Thermodynamics", W.A. Benjamin, Inc., New York (1964). Williamson, .G., "An I n t r o d u c t i o n to N o n - E l e c t r o l y t e Solut i o n s " , John Wiley and Sons, New York (1967). Hamaker, J.W., i n "Environmental Dynamics of P e s t i c i d e s " , edited by R.Haque and V.H. Freed, Plenum Press, New York (1975) page 115. Obrigawitch, T., F.M. Hons, J.R. Abernathy, and J.R. Gispon, Weed Science, 29, 32 (1981). H a s s e t t , J . J . , J.C. means, W.L. Banwart, and S.G. Wood, EPA/390-041 (1980). Briggs, G.G., Proceedings 7th British I n s e c t i c i d e and Fungicide Conference, pages 83-86 (1973). Swann, R.L., D.A. L a s k o s k i , P.J. McCall,K. Vander Kuy, H.J. Dishburger, PEST No. 34, N a t i o n a l American Chemical Society Meeting, New York, August 23-28 (1981). A h l r i c h s , J.L., Chapter 1 i n "Organic Chemicals i n the S o i l Environment", e d i t e d by Hamaker, J.W. and Goring, C.A.I., Marcel Dekker, Inc., New York (1972).


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15. 16. 17.

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Hamaker J.W. and J.M. Thompson, Chapter 2 in "Organic Chemic a l s i n the S o i l Environment", e d i t e d by Hamaker, J.W. and Goring, C.A.I., Marcel Dekker, Inc., New York (1972). Leo, A., C. Hausch, and D. E l k i n s , Chemical Reviews, 71, 525 (1971). Tanford, C., "The Hydrophobic E f f e c t " , John Wiley and Sons, New York (1973), pages 20-21. Freed, V.H., R. Hague, J . V e r n e t t i , J . A g r i c . Food Chem., 15, 1121 (1967). Khan, S.V, " P e s t i c i d e s i n the S o i l Environment", E1 Sevior S c i e n t i f i c P u b l i s h i n g Co., New York (1980), page 41. Spencer, W.F. and M.M. C l i a t h , "Environmental Dynamics o f P e s t i c i d e s " , edited by R. Haque and V.H. Freed, Plenum Press (1975) p. 61. Farmer, W.J., K. Igue, and W.F. Spencer, J . Environ. Qual., 2, 107 (1973). Farmer, W.J., K. Igue, W.F. Spencer and J.P. M a r t i n , S o i l S c i . Soc. Amer. Proc, 36, 443 (1972). K. Igue, W.J. Farmer, W.F. Spencer and J.P. M a r t i n , I b i d . , 36, 447 (1972).

R E C E I V E D April 29,

1983.

American Chemical Society Library 1155

16th St. N. W.

Washington, 0 . C. 20038 Swann and Eschenroeder; Fate of Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Fate of Chemicals in the Environment - American Chemical Society

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