14 Abiotic Hydrolysis of Sorbed Pesticides D. L. MACALADY Department of Chemistry and Geochemistry, Colorado School of Mines, Golden,CO80401 N. L. WOLFE Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, GA 30613
The hydrolysis of pesticides which are sorbed to sterilized natural sediments has been investigated in aqueous systems at acid, neutral and alkaline pH's. The results show that the rate constants of pH independent ("neutral") hydrolyses are the same within experimental uncertainties as the corresponding rate constants for dissolved aqueous phase pesticides. Base-catalyzed rates, on the other hand, are substantially retarded by sorption and acid-catalyzed rates are substantially enhanced. A large body of evidence w i l l be presented which substantiates these conclusions for a variety of pesticide types sorbed to several well-characterized sediments. The significance of our results for the evaluation of the effects of sorption on the degradation of pesticides in waste treatment systems and natural water bodies w i l l also be discussed.
Whether such d i s p o s a l i s i n t e n t i o n a l or i n c i d e n t a l , s i g n i f i c a n t q u a n t i t i e s of p e s t i c i d e s and p e s t i c i d e wastes end up i n n a t u r a l and a r t i f i c i a l aquatic systems. Thus, any c o n s i d e r a t i o n of the d i s p o s a l of t h i s broad category of anthropogenic chemicals must include an understanding of the r e a c t i o n mechanisms and p r i n c i p a l pathways f o r degradation of p e s t i c i d e s i n aquatic systems. Of the degradative pathways relevant to such systems, h y d r o l y s i s reactions are perhaps the most important type o f chemical decomposition process (Jj-7). Since many p e s t i c i d e s are compounds of low water s o l u b i l i t y , t h e i r form i n aquatic systems i s often dominated not by m a t e r i a l i n aqueous s o l u t i o n , but rather by m a t e r i a l sorbed to suspended or bottom sediments (8_-9). Thus, an understanding of the h y d r o l y t i c reactions of p e s t i c i d e s which are sorbed to 0097-6156/84/0259-0221$07.00/0 © 1984 American Chemical Society
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T R E A T M E N T A N D DISPOSAL O F PESTICIDE WASTES
sediments i s c r u c i a l for an adequate representation of the dominant chemical dégradâtive pathways for these compounds i n aquatic systems. It i s the purpose of t h i s a r t i c l e to summarize the present status of our understanding of the f a c t o r s governing the rates of h y d r o l y s i s of p e s t i c i d e s which are sorbed to sediments. The work reported herein deals s p e c i f i c a l l y with a b i o t i c h y d r o l y s i s r e a c t i o n s , which for some p e s t i c i d e s , may be as important or more important than b i o l o g i c a l l y mediated h y d r o l y s i s r e a c t i o n s (7_, 10-13). U n t i l r e c e n t l y , no e f f o r t s to measure the rates of h y d r o l y t i c degradation of sorbed p e s t i c i d e s have been reported. Indeed, i t has been widely assumed that h y d r o l y t i c reactions are important only i n the aqueous phase and that h y d r o l y s i s of sorbed p e s t i c i d e s proceeds at an i n s i g n i f i c a n t rate (13). The only a v a i l a b l e evidence i n the l i t e r a t u r e which r e l a t e s , though i n d i r e c t l y , to h y d r o l y s i s of sorbed p e s t i c i d e s concerns p e s t i c i d e s i n s o i l systems (see for example 14, 15). Though the r e s u l t s of such studies are not d i r e c t l y a p p l i c a b l e to aquatic systems, they do, i n general, show that c e r t a i n p e s t i c i d e s undergo a b i o t i c r e a c t i o n s i n s o i l - s o r b e d s t a t e s . This review, then, reports r e s u l t s of experiments which provide information that can be used to test the hypothesis that h y d r o l y s i s r e a c t i o n s proceed at s u b s t a n t i a l l y reduced rates when the molecules undergoing h y d r o l y s i s are sorbed to sediments. Results are reported for a v a r i e t y of p e s t i c i d e s and for model compounds that are s i m i l a r i n s t r u c t u r a l features to pesticides. Included are n e u t r a l , base-catalyzed and, to a l i m i t e d extent, a c i d - c a t a l y z e d h y d r o l y s i s r e a c t i o n s . Preliminary
Considerations
Three general c l a s s e s of h y d r o l y t i c r e a c t i o n s in aqueous s o l u t i o n s have been c h a r a c t e r i z e d . In n e u t r a l , or pH independent h y d r o l y s i s , the rate of disappearance of a p e s t i c i d e , P, i s given by
where kj i s the f i r s t - o r d e r disappearance rate constant. For base-mediated h y d r o l y s i s , the corresponding expression i s 4?*-
= -k_[B][P]
=
-k
,
[P]
(2)
dt Β obs where Β represents a generalized base. For n a t u r a l waters and the experimental systems relevant to t h i s report, the only base of s i g n i f i c a n c e for such r e a c t i o n s i s the hydroxide ion, OH (16). In equation 2, k ^ represents a pseudo f i r s t - o r d e r rate constant, v a l i d at f i x e d pH (or [B]). Q
s
14.
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Abiotic Hydrolysis of Sorbed Pesticides
223
Recently reported r e s u l t s for the h y d r o l y s i s k i n e t i c s of c h l o r p y r i f o s (7) suggest that equation 2 may not be a v a l i d r e p r e s e n t a t i o n of a l k a l i n e h y d r o l y s i s k i n e t i c s for at l e a s t one c l a s s of p e s t i c i d e s (organophosphorothioates). In short, kg may be pH dependent. However, disappearance k i n e t i c s for such molecules are s t i l l adequately described at f i x e d pH by pseudo first-order kinetics. A c i d - c a t a l y z e d h y d r o l y s i s k i n e t i c s are described by the expression -
dt
"k
+
a
lH ][P]
- -k
. [P] obs
(3)
+
where [H ] represents the hydrogen ion a c t i v i t y , and k ^ the pseudo f i r s t - o r d p r disappearance rate constant at f i x e d pH. For a given p e s t i c i d e which undergoes h y d r o l y s i s , any or a l l of these h y d r o l y t i c pathways may be relevant at various pH's. Organophosphorothioates, for example, have measurable n e u t r a l and a l k a l i n e h y d r o l y s i s rate constants (7)· Esters of 2,4-dichlorophenoxyacetic a c i d (2,4-D), on the other hand, hydrolyze by acid and a l k a l i n e catalyzed r e a c t i o n s , but have extremely small n e u t r a l h y d r o l y s i s rate constants (17). Thus, any study of the h y d r o l y s i s of sorbed p e s t i c i d e s must be prefaced by an understanding of the h y d r o l y t i c behavior of i n d i v i d u a l p e s t i c i d e s i n aqueous s o l u t i o n . Another important c o n s i d e r a t i o n i n i n v e s t i g a t i o n of the r e a c t i o n of sorbed p e s t i c i d e s i s the nature of the s o r p t i o n process i t s e l f . Sorption/desorption k i n e t i c s and the physicochemical c h a r a c t e r i s t i c s of the p e s t i c i d e molecules i n the sediment-sorbed state can be expected to i n f l u e n c e the k i n e t i c observations made i n experimental systems. Sorption has been commonly described as an e q u i l i b r i u m process, in which the p e s t i c i d e molecules are r a p i d l y and r e a d i l y exchanged between the sediment and aqueous phases. In t h i s approach (8), the e q u i l i b r i u m water phase concentration, C (expressed r e l a t i v e to suspension volume) i s r e l a t e d to the sediment phase concentration, C (expressed r e l a t i v e to dry weight sediment), through 0
g
w
g
K
p
= C /C s
(4)
w
where Κ i s the e q u i l i b r i u m p a r t i t i o n c o e f f i c i e n t (L-g~*) f o r the p e s t i c i d e . C i s then r e l a t e d to the t o t a l concentration of p e s t i c i d e , C , by w
T
C
w
T 1+pK
Ρ where ρ i s the sediment-to-water r a t i o . K has been shown to be d i r e c t l y p r o p o r t i o n a l to the weight f r a c t i o n of organic carbon i n the sediments (O.C.) p
TREATMENT AND DISPOSAL OF PESTICIDE WASTES
224
the p e s t i c i d e (18). Several i n v e s t i g a t i o n s have, however, v e r i f i e d the inadequacy of t h i s r e p r e s e n t a t i o n of the s o r p t i o n process. V a r i a t i o n s of "K " with sediment concentration Cl_9,_20) have been reported. More importantly, the rate of the sorption process has been shown to be more complex than a simple rapid e q u i l i b r i u m between sediment and aqueous phases (9_ lOj 21). The fact that s o r p t i v e e q u i l i b r i u m can be approached quite slowly i s i l l u s t r a t e d d r a m a t i c a l l y by data for the system i n which c h l o r p y r i f o s i s sorbed to EPA-14, one of a group of sediments c o l l e c t e d and c h a r a c t e r i z e d for the U. S. Environmental P r o t e c t i o n Agency (22). Figure 1 i s a plot of the sediment/aqueous concentration r a t i o versus time for t h i s system. It i s c h a r a c t e r i z e d by a r a p i d s o r p t i o n process and a much slower s o r p t i o n process which does not reach e q u i l i b r i u m u n t i l about 10 days a f t e r i n i t i a l mixing of the sediment and chlorpyrifos solution. Though t h i s system i s perhaps an extreme example of slow s o r p t i o n k i n e t i c s , i t i l l u s t r a t e s that the assumption of r a p i d e q u i l i b r i u m between the sediment and aqueous phases i s questionable. The importance of such an observation to the i n v e s t i g a t i o n of h y d r o l y s i s k i n e t i c s i n sediment/water systems must be emphasized. C e r t a i n l y , any model of h y d r o l y s i s k i n e t i c s i n sediment/water systems must include e x p l i c i t expressions for the k i n e t i c s of the s o r p t i o n / d e s o r p t i o n process. Unfortunately, our present understanding of sorption k i n e t i c s i s inadequate to allow unambiguous representation of the s o r p t i o n - d e s o r p t i o n process. C l e a r l y the states of sorbed p e s t i c i d e s include f r a c t i o n s which vary i n t h e i r l a b i l i t y with respect to desorption (9, 10, 21). The f r a c t i o n of the sorbed molecules i n r e l a t i v e l y l a b i l e and n o n - l a b i l e states i s a f u n c t i o n of the nature of the p e s t i c i d e and sediment and the time of contact between the sediment and p e s t i c i d e s o l u t i o n . With these l i m i t a t i o n s i n mind, however, we have used the f o l l o w i n g model to represent the k i n e t i c s of the h y d r o l y s i s of p e s t i c i d e s i n sediment/water suspensions (10) 9
C, w
C
(7)
s k, s
*w Products
Here, k j represents a pseudo f i r s t - o r d e r rate constant ( l i n e a r in sediment concentration) for the s o r p t i o n process, k the Q
14.
MACALADY AND WOLFE
Abiotic Hydrolysis of Sorbed Pesticides
1 5
1
1
1
1
1
10
15
20
25
30
1 35
Time (K min) Figure 1. K i n e t i c s o f the s o r p t i o n of c h l o r p y r i f o s sediment, P = 0 . 2 0 , t = 25 ° C .
to EPA-14
225
TREATMENT A N D DISPOSAL OF PESTICIDE WASTES
226
f i r s t order rate constant f o r d e s o r p t i o n , k the f i r s t or pseudo f i r s t - o r d e r rate constant f o r h y d r o l y s i s of the aqueous ( d i s s o l v e d ) p e s t i c i d e and k the corresponding rate constant for h y d r o l y s i s of the sorbed p e s t i c i d e . T h i s model, i n l i g h t of the d i s c u s s i o n above, i s c l e a r l y not r e p r e s e n t a t i v e of a l l of the k i n e t i c processes which are o c c u r r i n g i n sediment/water systems c o n t a i n i n g hydrophobic pesticides. However, i t does include at l e a s t the more l a b i l e f r a c t i o n of the sorbed p e s t i c i d e i n the o v e r a l l k i n e t i c model. Complications due to the inadequacy of t h i s r e p r e s e n t a t i o n w i l l be i l l u s t r a t e d and discussed below. Based on t h i s model, the f o l l o w i n g rate equations r e l a t i n g the h y d r o l y t i c degradation of p e s t i c i d e s from sediment water suspensions can be w r i t t e n : dC ~Τ7Γ~ = ~(k + k ) C + k C dt w I w os w
g
dC -Jdt 1
= -(k +k )C s o s
+ k C I w T
(8)
14,000 minutes) the disappearance rate i s f i r s t order f o r both the water and sediment phases. A l s o , the aqueous disappearance rate constant c a l c u l a t e d from the slope of the l i n e a r p o r t i o n of the n a t u r a l log aqueous c o n c e n t r a t i o n versus time plot i s 0.5±0.2 χ 10" min" , which i s s i m i l a r to the values measured i n sediment-free EPA-14 supernatant (Table II). A plot summarizing two experiments using EPA-23 sediment i s shown i n Figure 4. The value of 1^ c a l c u l a t e d from the n a t u r a l log water c o n c e n t r a t i o n vs. time plot i n t h i s f i g u r e i s (1.9*0.2) χ 10" m i n " . Data from these studies were analyzed by a computer using equations 8 based on our simple k i n e t i c model for the sediment/water systems (eqn. 7). The computer program (23) uses concentrations of c h l o r p y r i f o s i n the water and sediment phases and product concentrations (obtained by d i f f e r e n c e ) as a 5
1
14.
Abiotic Hydrolysis of Sorbed Pesticides
MACALADY A N D WOLFE
231
8
11
•
Sediment Phase (moles/Kg
•
Water Phase (moles/l x
x
10 ) 6
10 ) 7
Time (K minutes)
Figure 3. C h l o r p y r i f o s disappearance from an EPA-14 sediment/ water system, P= 0 . 2 0 , t = 25 ° C .
io.h
8-
ο
76-
#
Log sediment phase concentration (moles/kg χ 1 0 ) 5
4H 3H
•
Log water phase concentration 7
(moles/1 χ 10 )
I
10
20
30
40
50
60
Time (K min) Figure 4. C h l o r p y r i f o s disappearance from EPA-23 sediment/ water systems, P= 0.016, t = 25 °C.
232
T R E A T M E N T A N D DISPOSAL O F PESTICIDE WASTES
f u n c t i o n of time to c a l c u l a t e values f o r any three of the rate constants k j , k , k and k . For the purposes of our c a l c u l a t i o n s , a "known" value of 1.0x10 min" f o r 1^ was used to enable c a l c u l a t i o n of k j , k and k . The r e s u l t s of these c a l c u l a t i o n s are shown i n Table I I I . A l s o shown i n Table I I I are values f o r k , the o v e r a l l c h l o r p y r i f o s disappearance r a t e constant, and a value c a l c u l a t e d f o r k using equation 10, which i s based on k^O, i . e . no h y d r o l y s i s of sorbed chlorpyrifos. Q
w
Q
g
o b
o b s
Table I I I .
Observed and C a l c u l a t e d Values of Rate Constants (min ) f o r C h l o r p y r i f o s i n Sediment/Water Systems at Non-adjusted p H s . f
sediment/ water (p) fraction sorbed sterile? k o b g
k (calc.) k k k (fixed) k PH (H 0 phase)
a
EPA 14
EPA 23
EPA 23
0.20
0.016
0.016
0.94 yes l.OxlO"
0.87 no 1.7xl0~
0.87 yes 1.6xl0~"
5
5
5
Q b
b
x
Q
c
w
s
2
7
6.0xl0~ 6 ± 2)xl0~ (6 ± 2 ) x l 0 " l.OxlO' (6.9 ± 0.9)xl0""
6
3
6
- 3
4
4
5
6
7.2 ± 0.2
1.3xl0" (4 ± D x l O " (4 ± 2 ) x l 0 " l.OxlO" (1.2 ± O . D x l O " 3
4
5
4.1 ± 0.4
1.3xl0" (4 ± D x l O (9 ± 3 ) x l 0 " l.OxlO"" (1.8 ± O.DxlO
5
5
5
7.4 ± 0.4
j*(See text f o r Symbol D e f i n i t i o n s ) . Assuming sediment/water e q u i l i b r i u m , no h y d r o l y s i s i n the sorbed s t a t e , and k - 1.0x10 , k , = k /(1+pK ). C„ , , η , . ODS W Ρ For computer model c a l c u l a t i o n s . W
Several features of these c a l c u l a t i o n s are important. F i r s t , the computer c a l c u l a t e d u n c e r t a i n t i e s shown f o r the c a l c u l a t e d values of k , k and k are an i n d i c a t i o n that the 1 0 s . . model has considerable v a l i d i t y f o r d e s c r i b i n g the k i n e t i c s of the system, at l e a s t over one h a l f - l i f e i n the disappearance of chlorpyrifos. Second, the values of k and k^ are a l l s i m i l a r and t h e i r magnitude i n d i c a t e s that i n t h i s case the assumption of r a p i d s o r p t i o n / d e s o r p t i o n k i n e t i c s compared to h y d r o l y s i s i s valid. More importantly, the c a l c u l a t e d values of k are a l l s i m i l a r i n magnitude to 1^. Coupled with the fact that the T
Q
g
14.
Abiotic Hydrolysis of Sorbed Pesticides
MACALADY AND WOLFE
233
values c a l c u l a t e d for k ^ assuming no sediment phase h y d r o l y s i s are a l l c o n s i d e r a b l y lower than the actual values for k ^ > these k values i n d i c a t e that n e u t r a l h y d r o l y s i s of c h l o r p y r i f o s i n the sorbed state proceeds at a rate that i s the same as the disappearance rate i n the aqueous phase. 2. Diazinon and Ronnel. The c o n c l u s i o n that n e u t r a l h y d r o l y s i s of sorbed c h l o r p y r i f o s i s c h a r a c t e r i z e d by a f i r s t order rate constant s i m i l a r to the aqueous phase value i s strengthened and made more general by the r e s u l t s f o r d i a z i n o n , 0,0-diethyl 0-(2-iso-propyl-4-methyl-6-pyrimidyl) phosphorothioate, and Ronnel, 0,0-dimethyl 0-(2,4,5t r i c h l o r o p h e n y l ) phosphorothioate (10). The r e s u l t s f o r the pH independent h y d r o l y s i s at 35°C f o r these compounds i n an EPA-26 sediment/water system (p=0.040) are summarized i n Table IV. Because the aqueous ( d i s t i l l e d ) values of k f o r d i a z i n o n and Ronnel are s i m i l a r i n magnitude to the value for c h l o r p y r i f o s , and because these values were shown by the c h l o r p y r i f o s study to be slow compared to s o r p t i o n / d e s o r p t i o n k i n e t i c s , computer c a l c u l a t i o n s of k were not deemed necessary and were not made for these data. Q
s
Q
s
g
w
g
Table IV.
Experimental Values for Neutral H y d r o l y s i s Disappearance Rate Constants (min ) i n an EPA 26 Sediment/Water System for Diazinon and Ronnel at 35°C a
Diazinon 0.040 0.64 (3 ± D x l O " 1.3xl0~ (1.2 ± 0.2)xl0~^ (3.8 ± 0.6)xl0 (2.9 ± 0 . 5 ) x l 0 "
sediment/waterip) f r a c t i o n sorbed
5
k
obs , obs ( l l d) k^ U i s t i l l e d ) k^ (observed)
k
c a
c u
5
a t e
5
5
k
s
Ronnel 0.040 0.96 (2.7 ± 0 . 4 ) x l 0 " 2.3xl0"
5
6
(2.0 ± 0.2)xl0J (3.8 ± 1.8)xl0 I (2.6 ± 0 . 3 ) x l 0 "
5
Symbols Defined i n t e x t . ^Assuming sediment/water e q u i l i b r i u m , no h y d r o l y s i s i n the sorbed state and k = 3 χ k ( d i s t i l l e d ) - s e e d i s c u s s i o n . k b w
Q
s
~
Again the values of k , c a l c u l a t e d from the log C v s . time p l o t s , were s i m i l a r i n magnitude to the value of k calculated from the log C vs. time p l o t s . A l s o the values or k ^ c a l c u l a t e d from the k =0 assumption i m p l i c i t i n equation 10 were lower than the experimental k ^ v a l u e s . This l a t t e r e f f e c t i s less dramatic for d i a z i n o n since i t s lower Κ value r e s u l t s i n ρ an e q u i l i b r i u m f r a c t i o n sorbed of only 0.64. Note also that the g
w
Q
g
Q
s
g
234
T R E A T M E N T A N D DISPOSAL O F PESTICIDE WASTES
values f o r k are 2-3 times the d i s t i l l e d water values. This v observation i s also c o n s i s t e n t with the rate enhancements observed f o r c h l o r p y r i f o s i n n a t u r a l ( c f . d i s t i l l e d ) waters. Thus, for c h l o r p y r i f o s , d i a z i n o n , Ronnel (and by extension, other organophosphorothioate p e s t i c i d e s ) , n e u t r a l h y d r o l y s i s proceeds at s i m i l a r rates i n both the aqueous and sediment phases of sediment/water systems. 3. Experiments on the h y d r o l y s i s of 4-(p-chlorophenoxy) b u t y l bromide, (PCBB) which proceeds v i a an S 2 s u b s t i t u t i o n mechanism (11) were s i m i l a r i n design and data a n a l y s i s procedures to the c h l o r p y r i f o s experiments d e t a i l e d above. Results from a study at 35°C using EPA-12 sediment with 80% of the compound i n the sorbed state are i l l u s t r a t e d i n Figure 5. C a l c u l a t e d and observed values from t h i s study, using the d i s t i l l e d water value for 1^ of (7.9±0.5xl0~ ) min" as a "known value for computer c a l c u l a t i o n s are: N
5
1
11
5
k j » 8.1 χ 10" min"J 5
1
k = 1.6 χ 10" min k = 5.1 χ 10 min" Again, the value of k i s s i m i l a r i n magnitude to the value of 1^. Other studies using EPA-12 (80-95% sorbed) at 25°C and EPA-10 (90% sorbed) at 35° also i n d i c a t e s i m i l a r values for k and k . Several features of the PCBB experiments are d i f f e r e n t than those f o r c h l o r p y r i f o s . The h y d r o l y s i s r e a c t i o n proceeds v i a a d i f f e r e n t mechanism. The rate enhancements observed for c h l o r p y r i f o s i n n a t u r a l waters and the aqueous phases of the sediment/water systems (as compared to s t e r i l e d i s t i l l e d water) are not observed f o r PCBB. The values of kj. and k c a l c u l a t e d for PCBB are slower than those for c h l o r p y r i f o s anS s i m i l a r i n magnitude to the h y d r o l y s i s r a t e s . In s p i t e of these d i f f e r e n c e s , n e u t r a l h y d r o l y s i s i s s t i l l c h a r a c t e r i z e d by s i m i l a r rate constants f o r both sediment-sorbed and aqueous PCBB. 4. Benzyl c h l o r i d e h y d r o l y s i s proceeds v i a a t h i r d mechanism (S*,l). Results of studies of benzyl c h l o r i d e h y d r o l y s i s ( Π ) i n d i s t i l l e d water and EPA-13 and EPA-2 sediment/water systems are summarized i n Table V. Results f o r t h i s compound include only o v e r a l l f i r s t - o r d e r disappearance rate constants, but the data c l e a r l y show that the h y d r o l y s i s rate i s independent of the f r a c t i o n sorbed to sediment. Thus, the c o n c l u s i o n i s again made that n e u t r a l h y d r o l y s i s proceeds v i a s i m i l a r rate constants i n both the aqueous and sedimentsorbed phases. 0
5
1
s
g
g
w
14.
MACALADY A N D WOLFE
Table V.
k
Abiotic Hydrolysis of Sorbed Pesticides
H y d r o l y s i s of Benzyl Chloride Systems at 25°C
obs, m i n ^ x l O
3
i n Sediment/Water
Fraction Sorbed
Sediment to Water R a t i o , p(Sed. *)
1.18 ± 0.05 1.33 ± 0.03 1.1 ± 0.1 1.4 ± 0.1 1.15 ± .05 1.10 ± .08
235
0 0 0.15 0.25 0.87 0.87
0 0 0.025 (EPA 13) 0.05 (EPA 13) 1.0 (EPA 2) 1.0 (EPA 2)
5. The h y d r o l y s i s of hexachlorocyclopentadiene (HEX) represents a fourth h y d r o l y s i s mechanism (S^2*). Studies of the o v e r a l l disappearance k i n e t i c s of HEX from s t e r i l e d i s t i l l e d water and EPA-13 sediment/water systems (12) are summarized i n Table VI. Again, the rate constants are e s s e n t i a l l y independent of the sediment concentration and therefore independent of the f r a c t i o n of the HEX which i s sorbed to the sediment. This i n d i c a t e s that n e u t r a l h y d r o l y s i s of HEX i s also c h a r a c t e r i z e d by s i m i l a r rate constants for both the sediment-sorbed and aqueous phases.
Table VI.
H y d r o l y s i s of Hexachlorocyctopentadiene i n EPA-13 Sediment/Water Systems at 30°C
Sediment to Water Ratio, ρ
0 0.05 0.10 0.15 0.20 0.40 1.0 2.0
k
obs
m
n
i
9 21 26 27 22 16 20 13
± ± ± ± ± ± ± ±
x
^
3 2 2 4 2 1 2 2
In summary, n e u t r a l h y d r o l y s i s rate constants for s i x d i f f e r e n t compounds which hydrolyze by four d i f f e r e n t h y d r o l y s i s mechanisms were determined i n sediment/water systems using seven
236
TREATMENT AND
DISPOSAL OF PESTICIDE WASTES
d i f f e r e n t sediments. Sediment to water r a t i o s varying from 5x10 to 2.0 were used. Yet, i n each case, n e u t r a l h y d r o l y s i s of the sorbed compounds was shown to be c h a r a c t e r i z e d by rate constants which were very n e a r l y equal to the rate constants f o r h y d r o l y s i s i n the aqueous phase of these systems. The conclusion i s that n e u t r a l h y d r o l y s i s reactions are not quenched when the molecules undergoing h y d r o l y s i s are sorbed to sediments. In fact the rate of n e u t r a l h y d r o l y t i c r e a c t i o n s appears to be unaffected by s o r p t i o n .
A l k a l i n e H y d r o l y s i s Studies. A l k a l i n e catalyzed h y d r o l y s i s k i n e t i c s i n sediment/water systems have been i n v e s t i g a t e d for c h l o r p y r i f o s and the methyl and n - o c t y l esters of 2,4dichlorophenoxyacetic acid (2,4-D). 1. C h l o r p y r i f o s . As was the case for the n e u t r a l h y d r o l y s i s s t u d i e s , the most d e t a i l e d k i n e t i c i n v e s t i g a t i o n s of a l k a l i n e h y d r o l y s i s k i n e t i c s i n sediment/water systems have been conducted using c h l o r p y r i f o s (10). As can be seen from Figure 2, a l k a l i n e h y d r o l y s i s of c h l o r p y r i f o s i s not second-order, so the value s e l e c t e d for 1^ cannot be c a l c u l a t e d from the pH and a second-order rate constant. Nevertheless, since aqueous k i n e t i c s at a l k a l i n e pH's for c h l o r p y r i f o s was always pseudof i r s t order, c a r e f u l pH measurements and Figure 2 can be used to s e l e c t accurate values for 1^ at any pH. A p r e l i m i n a r y c o n s i d e r a t i o n for studies at a l k a l i n e pH's i s the e f f e c t of pH on Kp values and on sorption/desorption rate constants. Studies using c h l o r p y r i f o s and EPA-26 (p=0.0150) i n d i c a t e no measurable e f f e c t s on Κ over the pH range 5.5-10.8 (K -250±37 for f i v e determinations). K i n e t i c e f f e c t s are a l s o minor, as i l l u s t r a t e d by the s i m i l a r i t y in the c a l c u l a t e d values of k j and k for EPA-23 at pH's of 7.2 and 7.4 (Table III) and 10.67 (Table V I I ) . Two types of i n v e s t i g a t i o n s of the a l k a l i n e h y d r o l y s i s of c h l o r p y r i f o s i n sediment/water systems were made, a l l at pH's between 10.6 and 10.8. F i r s t , studies were conducted i n which the pH was adjusted (using a carbonate b u f f e r ) immediately upon mixing the sediments (EPA-23 and EPA-26) with the c h l o r p y r i f o s solution. Second, a study using EPA-26 was made i n which the a l k a l i n e b u f f e r was not added u n t i l three days a f t e r mixing the sediment with the c h l o r p y r i f o s s o l u t i o n . Three days represents a time which i s long with respect to the achievement of sediment-water e q u i l i b r i u m for t h i s system, yet short compared to the n e u t r a l h y d r o l y s i s h a l f l i f e (~50 days).
14.
MACALADY AND WOLFE
Table VII.
Abiotic Hydrolysis of Sorbed Pesticides
237
Experimental and C a l c u l a t e d Values of the Rate Constants f o r the A l k a l i n e H y d r o l y s i s of C h l o r p y r i f o s i n Sediment/Water Systems 3
EPA pH Ρ K
P
ave f r . sorbed
23
EPA
10.67 ± 0 04 0.019 ± 0 001 453 ± 59 0.90 (1.05 ± 0 l l ) x l 0 ~ 5.0xl0" 3 (1.3 ± 0 8)xl0 (1.8 ± 1 D x l O " 4.8xl0" (7.1 ± 0 8)xl0 -5 5
k^g(calculated)
b
4
k (fixed)
0
4
w
k
s
4
26
10.60 ± 0 ,04 0.031 ± 0 ,001 191 ± 4 0.85 (1.10 ± 0 04)xl0"" 6.2x10 (2.5 ± 0.9)xl0 -3 (4.4 ± 1.7)xl0 4.3xl0" (4.1 ± 0.3)xl0"
4
4
^Symbols defined i n the t e x t . No p r e - e q u i l i b r i u m between gediment and c h l o r p y r i f o s p r i o r to pH adjustment. Assuming sediment/water e q u i l i b r i u m , no h y d r o l y s i s i n the sorbed s t a t e , k ( c a l e . ) = k /(1+pK ). Q ops w ρ For computer c a l c u l a t i o n s . The value of i s the expected d i s t i l l e d water h y d r o l y s i s rate constant at t h i s pH. K
In the f i r s t type of study, pseudo f i r s t - o r d e r k i n e t i c s were observed i n both the sediment and aqueous phases from t=0 through two h a l f - l i v e s i n o v e r a l l c h l o r p y r i f o s disappearance ( t o t a l time -8 days). For these s t u d i e s , computer c a l c u l a t i o n s using the model i l l u s t r a t e d i n equations 7 were again used to c a l c u l a t e values for k j , k and k , assuming a value of k equal to the pseudo f i r s t - o r d e r rate constant i n d i s t i l l e d water buffered to the same pH. Values were also c a l c u l a t e d f o r k ^ assuming kg^O (equation 10) f o r comparison to the experimental k v a l u e s . The r e s u l t s of these c a l c u l a t i o n s are shown i n Table VII. The contrast between these a l k a l i n e h y d r o l y s i s r e s u l t s and the n e u t r a l h y d r o l y s i s r e s u l t s i s s t r i k i n g . The c a l c u l a t e d values of k are lower by f a c t o r s of 7-10 than the corresponding k v a l u e s . The values c a l c u l a t e d f o r k ^ g assuming k =0 are only 1.8-2.1 times smaller than the experimental k ^ values ( c f . c a l c u l a t e d values 12-17 times lower than observed f o r n e u t r a l h y d r o l y s i s at s i m i l a r f r a c t i o n s sorbed). These r e s u l t s , t h e r e f o r e , show that a l k a l i n e h y d r o l y s i s i s c o n s i d e r a b l y slowed when the c h l o r p y r i f o s i s sorbed to sediments. The r e s u l t s from the study f e a t u r i n g s e d i m e n t - c h l o r p y r i f o s e q u i l i b r a t i o n p r i o r to pH adjustment (Figure 6) are Q
g
w
s
g
w
g
Q
s
238
T R E A T M E N T A N D DISPOSAL O F PESTICIDE WASTES -4.8-1
-5.0-Τ
-5.2Η c
Φ
-5.4Η
υ c ο ο
σ> ο
Jj
-5.6Η
-5.8Η
•
Sediment layer (moles/Kg)
^
Water layer (moles/1) "Γ
-6.0-
ιο
12
Time (K minutes) Figure 5. PCBB disappearance from 3 s t e r i l i z e d EPA-12 sediment/ water system, P= 0.050, t = 35 ° C .
c ο k.
c φ υ C
• Total Concentration -7.6-
A Sediment Concentration
Ο
σ> ο
-
1
• Water Concentration -7.8Η
-8.0Η -8.210
20
30
Time (K minutes)
40
"I 50
Figure 6. C h l o r p y r i f o s disappearance from an EPA-26 sediment/ water system e q u i l i b r a t e d f o r three days p r i o r to pH adjustment to 10.6; P= 0.031, t = 25 o c .
14.
MACALADY AND WOLFE
Abiotic Hydrolysis of Sorbed Pesticides
239
q u a l i t a t i v e l y d i f f e r e n t than those from the studies without preequilibration. Attempts to f i t these data to our simple k i n e t i c model gave a very poor f i t . The model i s c l e a r l y inadequate i n t h i s case. Though t h i s i s a very l i m i t e d data set, the k i n e t i c s appear to f o l l o w an i n i t i a l disappearance of c h l o r p y r i f o s dominated by a l k a l i n e h y d r o l y s i s of the aqueous phase m a t e r i a l and d e s o r p t i o n of a f r a c t i o n of the sorbed m a t e r i a l . Through approximately one h a l f - l i f e , k ^ i s 2xlO"~ min" , a value s i m i l a r to the k ^ measured f o r the EPA-26 study without pree q u i l i b r a t i o n (1x10 min ). Subsequently, however, k falls to 5x10 min , a number quite s i m i l a r to the k value c a l c u l a t e d f o r the p a r a l l e l study without p r e - e q u i l i b r a t i o n (4xl0~ min" ). These o b s e r v a t i o n s , though t e n t a t i v e , suggest the existence of a s u b s t a n t i a l f r a c t i o n of the sorbed m a t e r i a l which i s c o n s i d e r a b l y less l a b i l e with respect to desorption than the m a t e r i a l i n i t i a l l y sorbed to the sediments. Further study of these e f f e c t s i s c l e a r l y needed. 2. E s t e r s of 2,4-D. Studies of the a l k a l i n e h y d r o l y s i s of the methyl and n - o c t y l e s t e r s of 2,4-D i n sediment/water systems (24), though less d e t a i l e d than the c h l o r p y r i f o s s t u d i e s , show similar effects. Results from i n v e s t i g a t i o n s using EPA-13 at pH's near 10 f o r the methyl and o c t y l e s t e r s of 2,4-D are summarized i n Figure 7. Under the conditons i n these experiments, the f r a c t i o n s of the methyl and o c t y l e s t e r s which are sorbed to the sediment are 0.10 and 0.87, r e s p e c t i v e l y . The aqueous h y d r o l y s i s h a l f - l i v e s of the methyl and o c t y l esters at pH=10 are 3.6 and 27 minutes, r e s p e c t i v e l y . In the sediment/water system, the methyl e s t e r , which i s mainly i n the d i s s o l v e d phase, hydrolyzes at a rate s i m i l a r to that expected for the sediment-free system at the same pH. The o c t y l e s t e r , on the other hand, hydrolyses at a rate which i s c o n s i d e r a b l y retarded (and n o n - f i r s t - o r d e r ) when compared to the expected aqueous phase r a t e . Though the data are l e s s d e t a i l e d and do not permit c a l c u l a t i o n s s i m i l a r to those conducted f o r c h l o r p y r i f o s , i t i s c l e a r that the e f f e c t of s o r p t i o n i s to c o n s i d e r a b l y slow the a l k a l i n e h y d r o l y s i s r a t e . Studies of the disappearance of the o c t y l ester at pH 9.8 i n sediment/water systems aged 3 days p r i o r to pH adjustment are summarized i n Figure 8. For the systems with p=0.013 and 0.005 ( f r a c t i o n s sorbed = .978 and .945) the rate i s pseudo f i r s t order, but the rate constant i s 1 0 times smaller than the aqueous value (1.6x10 min ) at t h i s pH. As was suggested f o r chlorpyrifos, this k | value may be c h a r a c t e r i s t i c of the a c t u a l value of k . At p=0.001, ( f r a c t i o n sorbed = 0.78), the disappearance k i n e t i c s i s not f i r s t order, but shows r a p i d disappearance of the aqueous e s t e r , followed by disappearance of the sorbed e s t e r at a rate s i m i l a r to the studies with higher sediment to water r a t i o s . 4
Q
1
Q
Q
Q b s
g
5
1
4
Q
g
)s
240
TREATMENT AND DISPOSAL OF PESTICIDE WASTES
Figure 7. A l k a l i n e h y d r o l y s i s o f the 2,4-D methyl e s t e r ÇpH 10.01) and 2,4-D n-octyl e s t e r (pH 10.12) from h e a t - s t e r i l ized EPA-13 sediment/water systems.
14.
M A CA L A D Y A N D W O L F E
Ο * •
Abiotic Hydrolysis of Sorbed Pesticides
2,4-DOE 2,4-DOE 2,4-DOE
ρ = 0.013 ρ = 0.005 ρ = 0.001
k
p
241
= 3500
Time, min Figure 8. A l k a l i n e h y d r o l y s i s o f the 2,4-D n-octyl e s t e r (pH 9.8) in EPA-13 sediment/water systems e q u i l i b r a t e d 3 days p r i o r to pH adjustment.
242
TREATMENT A N D DISPOSAL OF PESTICIDE WASTES
Based on experiments f o r these three compounds, i t i s c l e a r that the a l k a l i n e h y d r o l y s i s of sorbed molecules i s s u b s t a n t i a l l y retarded with respect to the rate for d i s s o l v e d molecules. The extent of t h i s r e t a r d a t i o n cannot be q u a n t i t a t i v e l y discussed at t h i s time, however, due to lack of a s u f f i c i e n t l y broad set of d e t a i l e d experimental data. 3. Acid H y d r o l y s i s . Considering the observed r e s u l t s f o r n e u t r a l and a l k a l i n e h y d r o l y s i s , i t i s i n t e r e s t i n g to speculate as to the expected e f f e c t s of s o r p t i o n on a c i d - c a t a l y z e d hydrolysis. Since most sediments are of predominantly c l a y mineralogy, sediment grains e x h i b i t g e n e r a l l y negative surface changes at pH s common i n n a t u r a l waters (25). When t h i s i s the case, one would expect the concentration of negative ions such as OH to be lower near the sediment p a r t i c l e surface. In a d d i t i o n , the ( a c i d i c ) f u n c t i o n a l groups of the organic matter associated with the sediments would be expected to be n e g a t i v e l y charged at a l k a l i n e pH's. These e f f e c t s alone might be expected to reduce the rate of a l k a l i n e h y d r o l y t i c processes occuring at these s u r f a c e s . A l s o , r e c a l l that a l k a l i n e h y d r o l y s i s of organophosphorothioates has been shown to involve a n e g a t i v e l y charged intermediate (v.s. and 7_). Such an intermediate would be expected to be less stable i n the n e g a t i v e l y charged environment of the sediment p a r t i c l e surface. On the other hand, the concentration of p o s i t i v e i o n s , i n c l u d i n g H^O , near sediment p a r t i c l e surfaces would be expected to be enhanced r e l a t i v e to the bulk s o l u t i o n concentrations. From t h i s c o n s i d e r a t i o n , we would p r e d i c t that a c i d - c a t a l y z e d h y d r o l y s i s r e a c t i o n s should occur at enhanced rates f o r sorbed molecules. Unfortunately, there i s p r e s e n t l y only a very t e n t a t i v e b i t of evidence a v a i l a b l e to substantiate t h i s p r e d i c t i o n . In one experiment at an a c i d pH, the o v e r a l l rate of h y d r o l y s i s of the n - o c t y l ester of 2,4-D was measured i n a sediment/water s l u r r y i n which a s u b s t a n t i a l f r a c t i o n of the ester was sorbed. The rate was observed to be s u b s t a n t i a l l y f a s t e r than the p r e d i c t e d aqueous phase r a t e . Though t h i s i s an i n d i c a t i o n that the above p r e d i c t i o n i s c o r r e c t , much more experimental work i s needed to s u b s t a n t i a t e and q u a n t i f y t h i s predicted rate enhancement. f
Summary and Conclusions The hypothesis that h y d r o l y s i s of sorbed molecules occurs at rates i n s i g n i f i c a n t with respect to aqueous phase h y d r o l y s i s has been demonstrated to be i n c o r r e c t for n e u t r a l (pH-independent) h y d r o l y s i s r e a c t i o n s . The rate-constants for sorbed s t a t e n e u t r a l h y d r o l y s i s are, on the contrary, s i m i l a r i n magnitude to those for h y d r o l y s i s i n the aqueous phase. The hypothesis has been shown to be more nearly correct f o r a l k a l i n e h y d r o l y s i s r e a c t i o n s , since s i g n i f i c a n t rate
14.
MACALADY AND WOLFE
Abiotic Hydrolysis of Sorbed Pesticides
243
r e t a r d a t i o n s , corresponding to g r e a t l y reduced rate constants for sorbed molecules, occur when s u b s t a n t i a l f r a c t i o n s of the h y d r o l y z i n g molecules are sorbed to sediments. Inadequate understanding of the k i n e t i c s of the s o r p t i o n / d e s o r p t i o n process detracts from our a b i l i t y to completely understand the e f f e c t s of s o r p t i o n on h y d r o l y t i c r a t e s , and more research i s needed i n t h i s regard. Limited experimental evidence and t h e o r e t i c a l c o n s i d e r a t i o n s suggest that a c i d - c a t a l y z e d h y d r o l y s i s rates are s u b s a n t i a l l y enhanced f o r sorbed molecules. Much more experimental evidence i s necessary, however, to v e r i f y t h i s effect. These conclusions have several i m p l i c a t i o n s f o r p e s t i c i d e waste d i s p o s a l c o n s i d e r a t i o n s . For i n c i d e n t a l or a c c i d e n t a l d i s p o s a l of p e s t i c i d e s i n n a t u r a l aquatic systems, the r e s u l t s suggest that model c a l c u l a t i o n s using aqueous s o l u t i o n values f o r a b i o t i c n e u t r a l h y d r o l y s i s rate constants can be used without regard to s o r p t i o n to sediments. For a l k a l i n e h y d r o l y s i s , on the other hand, models must e x p l i c i t l y include s o r p t i o n phenomena and the corresponding rate reductions i n order to a c c u r a t e l y p r e d i c t h y d r o l y t i c degadation. The l i m i t e d a c i d - h y d r o l y s i s r e s u l t s , i f substantiated, have broader i m p l i c a t i o n s . They suggest that rapid h y d r o l y s i s of p e s t i c i d e wastes i n a c i d i f i e d a r t i f i c i a l sediment/water s l u r r i e s may be an a t t r a c t i v e method f o r the i n t e n t i o n a l d i s p o s a l and degradation of p e s t i c i d e wastes. Acknowledgments Most of the work reviewed i n t h i s a r t i c l e was performed at the U. S. Environmental Research Laboratory i n Athens, GA. Support f o r Donald Macalady's work at t h i s laboratory was provided by a Senior A s s o c i a t e s h i p award from the National Rsearch C o u n c i l . NOTE : Mention of trade names or commercial products does not c o n s t i t u t e endorsement or recommendation f o r use by the U. S. Environmental P r o t e c t i o n Agency.
Literature Cited 1. 2. 3. 4. 5. 6.
Wolfe, N.L.; Zepp, R.G.; Paris, D.F. Water Research, 1978, 12, 561. Wolfe, N.L.; Burns, L.A.; Steen, W.C. Chemosphere, 1980, 9, 393. Wolfe, N.L.; Zepp, R.G.; Doster, J.C.; H o l l i s , R.C. J. Agric. Food Chem., 1976, 24, 1041. Wolfe, N.L.; Zepp, R.G.; Paris, D.F. Water Reserch, 1978, 12, 565. Maquire, R.J.; Hale, E.J. J. Agric. Food Chem., 1980, 28, 372. Ou, L.T.; Gancarz, D.H.; Wheeler, W.B.; Rao, P.S.C.; Davidson, J.M. J. Environ. Quality, 1982, 11, 293.
TREATMENT AND DISPOSAL OF PESTICIDE WASTES
244
7. 8. 9.
10.
11.
12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
22.
23. 24.
25.
Macalady, D.L.; Wolfe, N.L., J. Agric. Food Chem., 1983, 31, 1139. Karickhoff, S.W.; Brown, D.S.; Scott, T.A. Water Research, 1979, 13, 241. Karickhoff, S.W. In Contaminants and Sediments, Baker, R.A., Ed. Ann Arbor Science, Ann Arbor, MI, 1980; Vol. 2, Chapter II. Macalady, D.L.; Wolfe, N.L. "Effects of Sediment Sorption on Abiotic Hydrolyses: I. Organophosphorothioate Esters", submitted to Environ. S c i . Technol., 1983. Pierce, J.H.; Wolfe, N.L. "Effects of Sediment Sorption on Abiotic Hydrolyses: II. 4-(p-chlorophenoxy) butyl bromide and Benzyl Chloride", paper in preparation, 1984. Wolfe, N.L.; Zepp, R.G.; Schlotzhauer, P.; Sink, M. Chemosphere 1982, 11, 91. Wolfe, N.L.; Zepp, R.G.; Paris, D.F.; Baughman, G.L.; H o l l i s , R.C. Environ. S c i . Technol. 1977, 11, 1077. Sethunan, N.; MacRae, I.C., J. Agric. Food Chem. 1969, 17, 221. Mingelgrin, U.; Saltzman, S.; Yaron, B. Soil Sci. Soc. Am. J. 1977, 41, 519. Perdue, E . M . ; Wolfe, N.L. Environ. Sci. Technol. 1983, 11, 635. Wolfe, N.L., U.S. Environmental Protection Agency, Athens, GA. Unpublished results, 1982. Karickhoff, S.W. Chemosphere, 1981, 10, 833. O'Connor, D.J.; Connolly, J.P. Water Research, 1980, 14, 1517. Voice, T.C.; Rice, C . P . ; Weber, W . J . , J r . Environ. Sci. Technol., 1983, 17, 513. M i l l e r , C.T.; Weber, W . J . , J r . 186th National Meeting of the American Chemical Society, Washington, D.C., August, 1983; Abstr. PEST 73. Hassett, J.J.; Means, J.C.; Banwort, W.L.; Wood, S.G. "Sorption Properties of Sediments and Energy Related Pollutants"; U.S. Environmental Protection Agency, Athens, Georgia. EPA-600/3-80-041, 1980. Knott, G.; Reece, D. "MLAB", Division of Computer Research and Technology, National Institute of Health, Bethesda, MD 20014, 7th ed., 1977. Wolfe, N.L. "Effects of Sediment Sorption on Abiotic Hydrolyses: III. Carboxylic Acid Esters", paper in preparation, 1984. Stumm, W.; Morgan, J.J. "Aquatic Chemistry", 2nd E d . , John Wiley & Sons, New York, 1981; Ch. 10.
RECEIVED April 16, 1984
