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Downloaded by NATL UNIV OF SINGAPORE on November 10, 2017 | http://pubs.acs.org Publication Date: July 17, 1986 | doi: 10.1021/bk-1986-0315.ch015
Chemical and Microbial Degradation of 1,2-Dibromoethane (EDB) in Florida Ground Water, Soil, and Sludge R. A . Weintraub, G. W. Jex, and H . A . Moye Pesticide Research Laboratory, University of Florida, Gainesville, F L 32611
The chemical and microbial degradation of 1,2-dibromoethane (EDB) in the subsurface environment was studied in laboratory incubations of groundwater, soil suspensions and sludge suspensions. EDB has been determined to have a chemical half-life of 1.5 to 2 years in Florida groundwaters (22°C). Rate constants for degradation determined at elevated temperatures were used to obtain extrapolated values via Arrhenius kinetics. Hydrolysis is the major mode of degradation, giving ethylene glycol and bromide ion, and is not pH-dependent between pH 4 and 9. Formaldehyde, an oxidation product of ethylene glycol, was shown to be a degradation product during extended incubation at elevated temperatures. Concentrations of 1 to 2 ppm of EDB were anaerobically degraded to ethylene by either methanogenic or facultative sludge in about 60 days, whereas Florida soils examined were nearly incapable of degrading EDB under these conditions. 1,2-Dibromoethane ( e t h y l e n e d i b r o m i d e ; EDB) i s used as a l e a d scavenger i n g a s o l i n e , a s o i l fumigant n e m a t i c i d e , a fumigant f o r s t o r e d g r a i n s , as a treatment t o conform w i t h q u a r a n t i n e r e g u l a t i o n s f o r c e r t a i n f r u i t shipments and a s a means o f k e e p i n g m i l l i n g m a c h i n e r y f r e e from i n s e c t s . By l a t e 1983, EDB r e c e i v e d g r e a t n o t o r i e t y when i t was r e p o r t e d t h a t t r a c e amounts o f t h i s c h e m i c a l were d e t e c t e d i n g r a i n s and g r a i n p r o d u c t s i n t h e U.S. and i n groundwater i n F l o r i d a , G e o r g i a , C a l i f o r n i a , South C a r o l i n a , New York, and H a w a i i (I). By March of 1984, a l l r e g i s t e r e d a g r i c u l t u r a l u s e s o f EDB were phased o u t as a r e s u l t o f t h e U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency's (EPA) d e t e r m i n a t i o n t h a t E D B s a g r i c u l t u r a l u s e s p r e s e n t e d an "imminent h a z a r d " t o t h e h e a l t h o f humans. The c h e m i c a l had been shown t o have a h i g h a c u t e t o x i c i t y i n a l l animals t e s t e d , o r a l L D v a l u e s r a n g i n g from 50 mg/kg i n t h e r a b b i t t o 420 mg/kg i n female mice (2) · I t s mutagenicity toward b a c t e r i a (3) and c a r c i n o g e n i c i t y i n r a t s and mice (4) have 1
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0097-6156/86/0315-O294$06.00/0 © 1986 American Chemical Society
Garner et al.; Evaluation of Pesticides in Ground Water ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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been demonstrated and t h e mechanisms o f t o x i c i t y have been de scribed i n p r o g r e s s i v e d e t a i l since the I960 s (5,6). Consistent measurable levels o f EDB i n groundwater have sparked c o n c e r n o v e r t h e p e r s i s t e n c e o f t h a t p o r t i o n o f c h e m i c a l t h a t does n o t v o l a t i l i z e o r i s n o t r e a d i l y degraded. I t s chemical p r o p e r t i e s make i t f a i r l y m o b i l e i n t h e s u b s u r f a c e environment, h a v i n g a s o l u b i l i t y i n water o f 4300 ppm a t 30°C (7) and a v a p o r p r e s s u r e o f 11 mm Hg a t 25°C ( 8 ) , g i v i n g i t a H e n r y s c o n s t a n t o f a p p r o x i m a t e l y 6.3 χ I0~ atm-m /mol. A Κ v a l u e o f 58 i s o b t a i n e d u s i n g t h e a p p r o p r i a t e p r e d i c t i v e model (9?Y A Κ v a l u e o f 66 mL/g has been e x p e r i m e n t a l l y o b t a i n e d i n two v a r y i n g s o i l s ( 1 0 ) . Such c h e m i c a l and f i e l d characteristics p l a c e EDB on t h e l i s t o f pesticides with the potential t o be groundwater contamination h a z a r d s by t h e c r i t e r i a e s t a b l i s h e d by U.S. EPA O f f i c e o f P e s t i c i d e Programs (1) I n v e s t i g a t i o n s i n t h e p a s t have have f o c u s e d r e s i d u a l EDB on c r o p s and on p r o c e s s e d o r s t o r e d p r o d u c t s (11-13). Values o f the o r d e r o f 5-10 days (14) and 14 y e a r s (15) have been r e p o r t e d as t h e h a l f - l i f e due t o h y d r o l y s i s i n n e u t r a l aqueous s o l u t i o n s a t ambient temperature. The b i o l o g i c a l c o n v e r s i o n o f EDB t o e t h y l e n e and bromide i o n by an a n a e r o b i c s o i l - w a t e r c u l t u r e h a s been r e p o r t e d t o o c c u r i n two months ( 1 6 ) . In another study, b a c t e r i a l c u l t u r e s under d e n i t r i f y i n g c o n d i t i o n s f a i l e d t o show any p o t e n t i a l f o r d e g r a d i n g EDB ( 1 7 ) . F u r t h e r , i t was found t h a t under methanogenic i n c u b a t i o n c o n d i t i o n s , EDB was t r a n s f o r m e d to a water-insoluble non-halogenated g a s , which was thought t o be e t h y l e n e (18,19). Recent a c c o u n t s of photodegradation showed EDB was c o m p l e t e l y m i n e r a l i z e d t o HBr and C 0 by h e t e r o g e n e o u s p h o t o c a t a l y s i s (on T1O2) i n aqueous s o l u t i o n (20),while i n another study, the p h o t o r e a c t i o n was r e p o r t e d t o p r o c e e d v i a t h e c o n v e r s i o n o f EDB t o bromoethanol f o l l o w e d by t h e c y c l i z a t i o n t o e t h y l e n e o x i d e w h i c h was h y d r o l y z e d t o e t h y l e n e g l y c o l by a p r o c e s s n o t enhanced by l i g h t (21). Two y e a r s a f t e r t h e b a n on a g r i c u l t u r a l u s e o f EDB, c o n c e n t r a t i o n s r a n g i n g from 0.02 t o about 600 ppb have been d e t e c t e d i n well-water samples b e i n g t a k e n by t h e s t a t e o f F l o r i d a ' s EDB monitoring program. Figure 1 gives a representation of the l o c a t i o n s and f r e q u e n c y o f s t a t e - c o n t r o l l e d EDB a p p l i c a t i o n s and water w e l l s i t e s found t o be contaminated w i t h EDB as o f March 1985. Estimated amounts of applied EDB i n Florida total 600-700,000 L / y e a r . G r e a t e r than one-half o f the contaminated w e l l s i t e s have been below 0.20 ppb and most o f t h e r e m a i n i n g p o s i t i v e s have been between 0.20 and 10 ppb. Depths o f t h e w e l l s sampled v a r y w i d e l y from 3 t o 300 m ( 2 2 ) . F l o r i d a ' s c i t r u s , peanut, and soybean farming a r e a s , where most o f t h e EDB u s e h a s been c o n c e n t r a t e d f o r 40 t o 50 y e a r s , a r e l o c a t e d on p r e d o m i n a n t l y sandy s o i l s w i t h r e l a t i v e l y low o r g a n i c c o n t e n t ( i . e .
