4 Quantifying Pesticide Adsorption and Degradation during Transport through Soil to Ground Water 1
1
2
W. Z. Zhong , A . T. Lemley , and R. J. Wagenet 1
Downloaded by SUFFOLK UNIV on January 19, 2018 | http://pubs.acs.org Publication Date: July 17, 1986 | doi: 10.1021/bk-1986-0315.ch004
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College of Human Ecology, Cornell University, Ithaca, NY 14853 Department of Agronomy, Cornell University, Ithaca, NY 14853
We describe a rapid experimental method and a basis for analyzing the results that will allow quantitative determination of pesticide adsorption and degradation during displacement through soil. A soil column methodology employing commercially available equipment developed for high-pressure liquid chromatography studies was used in the experiments. The movement and transformation of aldicarb, aldicarb sulfoxide and aldicarb sulfone were studied with this approach. Analytical solutions to the convection-dispersion equation that included description of linear adsorption and firstorder transformation were used to interpret the data. Experiments were conducted in two soils, at two flow velocities, in sterile and non-sterile conditions, and at two initial influent concentrations to demonstrate the usefulness of the method, and to better define the adsorption and microbial and chemical conversion of aldicarb. Data from these experiments are presented and discussed in the context of this method and other studies on aldicarb. The method appears useful as a generalizeable protocol for the study of any pesticide in soil. P e s t i c i d e l e a c h i n g from t h e r o o t zone and t h r o u g h t h e s o i l p r o f i l e i n t o groundwater i s a s e r i o u s c o n c e r n . C o n t a m i n a t i o n o f groundwater by such c h e m i c a l s may r e n d e r t h a t s u p p l y h a z a r d o u s f o r human consumption. A d d i t i o n a l l y , passage o f t h e c h e m i c a l beyond t h e r o o t zone p r e v e n t s f u r t h e r impact upon t h e t a r g e t p e s t . The r e s i d e n c e time o f a p p l i e d p e s t i c i d e s w i t h i n the r o o t zone s h o u l d t h e r e f o r e be maximized so t h a t t h e d u a l o b j e c t i v e s o f h i g h a g r i c u l t u r a l p r o d u c t i v i t y and low e n v i r o n m e n t a l h a z a r d from l e a c h i n g can be met. P e s t i c i d e and s o i l p r o p e r t i e s determine the m o b i l i t y and d e g r a dation o f applied chemicals. The i n t e r a c t i o n o f the o r g a n i c molec u l e s with s o i l s o l i d s v a r i e s according t o chemical s t r u c t u r e , organi c matter and c l a y c o n t e n t , s o i l pH, and i n some c a s e s c o n c e n t r a t i o n . D e g r a d a t i o n r a t e s a r e i n f l u e n c e d by pH, s u b s t r a t e c o n c e n t r a t i o n , 0097-6156/ 86/ 0315-0061 $06.00/0 © 1986 A m e r i c a n C h e m i c a l Society
Garner et al.; Evaluation of Pesticides in Ground Water ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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E V A L U A T I O N O F PESTICIDES IN G R O U N D WATER
t e m p e r a t u r e , s o i l m i c r o b i o l o g i c a l p o p u l a t i o n s , and s o i l water content. A l t h o u g h i t i s p o s s i b l e to measure the r e l a t i v e e f f e c t o f t h e s e v a r i a b l e s i n i n c u b a t i o n - t y p e l a b o r a t o r y s t u d i e s , i t i s not obv i o u s how to e x t r a p o l a t e the r e s u l t s o f such e x p e r i m e n t s to f i e l d conditions. A r e l a t i v e l y r a p i d method i s needed f o r s t u d y i n g both m o b i l i t y and d e g r a d a t i o n under c o n d i t i o n s t h a t more a c c u r a t e l y repres e n t the f i e l d regime. S o i l column t e c h n i q u e s p r o v i d e the experiment a l framework f o r such a method. A l t h o u g h s o i l columns do not comp l e t e l y r e p r e s e n t the f i e l d c o n d i t i o n s , they do approximate t r a n s i e n t f l o w i n g c o n d i t i o n s b e t t e r than i n c u b a t i o n - t y p e e x p e r i m e n t s . They may t h e r e f o r e be more u s e f u l i n i n f e r r i n g the f i e l d c o n d i t i o n . Analytic s o l u t i o n s o f t r a n s p o r t e q u a t i o n s t h a t c o n s i d e r a d s o r p t i o n and d e g r a d a t i o n p r o v i d e t h e o r e t i c a l models t h a t can be used to i n t e r p r e t the results quantitatively. T h i s paper p r e s e n t s an e x p e r i m e n t a l and t h e o r e t i c a l methodology t h a t a l l o w s r e l a t i v e l y r a p i d assessment o f pest i c i d e f a t e i n s o i l , and i l l u s t r a t e s the a p p l i c a t i o n o f the method t o a l d i c a r b , a p e s t i c i d e o f both e n v i r o n m e n t a l and a g r i c u l t u r a l i n t e r est. Theoretical
Methods
A l d i c a r b (A) i s both c h e m i c a l l y and b i o l o g i c a l l y o x i d i z e d t o a l d i c a r b s u l f o x i d e (A-SO), which i s then f u r t h e r o x i d i z e d by s i m i l a r p r o c e s s e s to a l d i c a r b sulfone ( A - S 0 ) . These compounds are s i m u l t a n e o u s l y subj e c t to o t h e r dégradâtive c h e m i c a l p r o c e s s e s dominated by h y d r o l y sis. These r e a c t i o n s have been s u c c e s s f u l l y d e s c r i b e d by f i r s t - o r d e r k i n e t i c s (J[), and can be g e n e r a l l y summarized as: 2
k*
k *
x
2
A
A-SO
+
k
Ψ
l
A-oxime
A-SO
A-S0 k
Ψ
2
oxime
k
2
(1)
3
A-S0
oxime
2
where k k and k r e p r e s e n t h y d r o l y s i s , and k * and k * i n d i c a t e oxidation. E x p r e s s e d i n f i r s t - o r d e r k i n e t i c terms, assuming t h a t o x i d a t i o n and h y d r o l y s i s o c c u r i n s o l u t i o n , and w i t h r e f e r e n c e to a s o i l system i n which o n l y p a r t o f the t o t a l volume i s o c c u p i e d by water, the f o l l o w i n g can be f o r m u l a t e d : l f
2
3
x
θ
= -Θ
Θ
d
θ
d (
(
^
*~
S
Q
)
(k
+ k^)(A)
x
= 6k * x
S O
a>
2
(A)
- 9(k
(2)
+ k *)(A-SO)
2
2
= 9k *(A-S0) - 9 k ( A - S 0 ) 2
3
2
Garner et al.; Evaluation of Pesticides in Ground Water ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
(3)
(4)
4.
63
Quantifying Pesticide Adsorption and Degradation
Z H O N G ET A L .
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3
Downloaded by SUFFOLK UNIV on January 19, 2018 | http://pubs.acs.org Publication Date: July 17, 1986 | doi: 10.1021/bk-1986-0315.ch004
where θ = v o l u m e t r i c water c o n t e n t ( c m c m " ) , t h e p a r e n t h e s e s i n d i c a t e c o n c e n t r a t i o n , and t h e t r a n s f o r m a t i o n o f c h e m i c a l i s d e f i n e d w i t h r e s p e c t t o t h e u n i t volume o f t h e system. The assumption t h a t d e g r a d a t i o n o c c u r s i n s o l u t i o n i s a good one i n t h e case o f a l d i c a r b and i t s m e t a b o l i t e s s i n c e t h e r e i s l i t t l e a d s o r p t i o n . These e q u a t i o n s p r e d i c t t h a t t h e c o n c e n t r a t i o n o f a l d i c a r b i s m o n o t o n i c a l l y d e c r e a s i n g , w h i l e t h e s u l f o n e and s u l f o x i d e forms change i n c o n c e n t r a t i o n depending on t h e v a l u e o f t h e r a t e coefficients. The d e s c r i p t i o n o f t h e s e s e q u e n t i a l and s i m u l t a n e o u s r e a c t i o n s d u r i n g t r a n s p o r t through a s o i l system i s a c c o m p l i s h e d u s i n g m i s c i b l e d i s p l a c e m e n t t h e o r y , which can be f o r m u l a t e d f o r an i n t e r a c t i n g , d e g r a d i n g s o l u t e ( e . g . an o r g a n i c m o l e c u l e ) a s ( 2 ) :
Ψ ' W- • i 1
M
