Degradation of the Insecticide Pirimicarb in Soil-Characterization of

Jul 23, 2009 - Each soil type was treated with 1 Kg ai/ha of the insecticide radiolabeled at the pyrimidine ... Degradation was most rapid in alkaline...
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25 Degradation of the Insecticide Pirimicarb in SoilCharacterization of "Bound" Residues I. R. H I L L

Downloaded by FUDAN UNIV on February 6, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0029.ch025

ICI Plant Protection Division, Jealott's Hill Research Station, Bracknell, Berkshire, England

The degradation of pirimicarb (2-dimethylamino-5,6dimethylpyrimidin-4-yl dimethylcarbamate), a fast acting selec­ tive aphicide, has been studied in several s t e r i l e and non­ -sterile soils maintained in controlled laboratory conditions. Each soil type was treated with 1 Kg ai/ha of the insecticide radiolabeled at the pyrimidine ring-2-carbon at the carbamate­ carbonyl-carbon, and incubated 'aerobically' (at 40% of moisture holding capacity) and after flooding the soil with water to a depth of 2.5 cm. Pirimicarb and any degradation products were extracted directly from the moist or wet soils (air drying can result in 'loss' of radioactivity) by refluxing in acetone:water (5:1) (selected after studies of the efficiency of a range of solvents). Chromatographic analyses of the soil extracts i n d i ­ cated that pirimicarb degradation proceeded via cleavage of the carbamate moiety from the ring, N-dealkylation of the 2-dimethyl= amino group and pyrimidine ring cleavage. Degradation was most rapid in alkaline s o i l s . After extraction the soils were freeze-dried, ground to a powder in a hammer m i l l and thoroughly mixed by rotation. As this procedure 'homogenized' the soil sample, making subsequent analyses more accurate, and did not result in any loss of radio­ activity, all measurements of unextractable radioactivity were made by dry combustion. Radioactivity remaining after exhaustive extraction was considered to be 'bound'. In 'aerobically' incubated s o i l s considerable amounts of the ring-2- C, but relatively little (less than 10% of that applied) of the carbamate- C, radiolabel were present as 'bound' residues. Within an incubation period of 1 year, approximately 20-60% of the applied ring- C became 'bound' in a range of 9 soil types of pH 5.5-8.1 and organic matter content 1.7-10.4%. In one alkaline soil (Gore, pH 7.8), in which the insecticide was very rapidly degraded (50% in 1-2 weeks, 90% in 3-5 weeks), over half of the applied ring- C was 'bound' within 5 weeks incu­ bation. In s t e r i l e soils (autoclaved and γ-irradiated) both the rate of pirimicarb degradation and the accumulation of 'bound' residues were considerably less than in their non-sterile 14

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equivalents. An a c i d soil (Broadricks, pH 5.5) and an a l k a l i n e soil (Gore) were a l s o incubated under flooded c o n d i t i o n s . In B r o a d r i c k s , the amount o f ' b i n d i n g was very s i m i l a r t o t h a t observed under aerobic c o n d i t i o n s . However, in Gore 20-30%of both the r i n g - and carbamate- radiocarbons became 'bound during an i n c u b a t i o n p e r i o d of 20 weeks. In most s o i l s the amount o f 'bound r a d i o a c t i v i t y increased p r o g r e s s i v e l y throughout the i n c u b a t i o n p e r i o d . I n Broadricks t r e a t e d w i t h r i n g - 2 - l ^ C p i r i m i c a r b and incubated a e r o b i c a l l y f o r 2 years, 70% o f the a p p l i e d r a d i o a c t i v i t y was 'bound . In Gore, whether incubated ' a e r o b i c a l l y ' o r in flooded soil, the amount o f 'bound' r a d i o a c t i v i t y reached a maximum w i t h i n 5-10 weeks, but t h e r e a f t e r d e c l i n e d s t e a d i l y f o r the remainder o f the i n c u b a t i o n p e r i o d . There was no simple r e l a t i o n s h i p between t h e amount o r r a t e o f formation o f 'bound' residues and e i t h e r the r a t e o f p i r i m i c a r b degradation o r the soil pH and organic matter content. Attempts were made t o e x t r a c t and c h a r a c t e r i z e t h e 'bound' p e s t i c i d e residues in the f o l l o w i n g e x t r a c t e d , f r e e z e - d r i e d s o i l s (composite samples from s o i l s incubated f o r a minimum o f 5 weeks): a. Gore ' a e r o b i c a l l y ' incubated w i t h r i n g - 2 - l ^ C p i r i m i c a r b (GAR) ; b. Broadricks ' a e r o b i c a l l y ' incubated w i t h r i n g - 2 - C p i r i m i c a r b (BAR); c. Gore incubated in flooded s t a t e w i t h r i n g - 2- C p i r i m i c a r b (GFR); and d. Gore incubated in flooded s t a t e w i t h carbamate-^C p i r i m i c a r b (GFC). E x t r a c t i o n s were c a r r i e d out w i t h acetone:water (5:1), a c i d , a l k a l i and s a l t s o l u t i o n s by shaking, r e f l u x i n g and u l t r a s o n i c a t i o n . Forallbut the u l t r a s o n i c a t e d treatments e x t r a c t i o n s were c a r r i e d out in 'enclosed' v e s s e l s through which a stream o f a i r was passed. The e f f l u e n t a i r was bubbled through tubes cont a i n i n g , in s u c c e s s i o n , methoxyethanol and ethanolamine o r O.5 M sodium hydroxide, t o t r a p products evolved i n t o the a i r stream during e x t r a c t i o n . S o i l s GAR and BAR contained approximately 40% and 35%, r e s p e c t i v e l y , o f the r a d i o l a b e l e d carbon o r i g i n a l l y a p p l i e d t o the soil as p i r i m i c a r b . V i r t u a l l y all o f t h i s 'bound' r a d i o a c t i v i t y was e x t r a c t e d w i t h O.1 M NaOH and w i t h O.1 M N a P 0 , 60-80% w i t h O.5 M H S 0 and o n l y 10-33% w i t h 1.0 M HC1. No more than 1% o f the r a d i o a c t i v i t y was evolved during a c i d e x t r a c t i o n and no more than 3% during a l k a l i n e o r s a l t e x t r a c t i o n . A f t e r a c i d i f i c a t i o n (to pH 1.0) o f the NaOH e x t r a c t s o f GAR and BAR, almost all o f the r a d i o a c t i v i t y was present in the a c i d a l k a l i s o l u b l e f u l v i c a c i d component. Any 'bound' p y r i m i d i n - 4 - y l carbamate may w e l l be hydrolyzed t o a hydroxypyrimidine during NaOH e x t r a c t i o n which, together w i t h any 'bound' hydroxyp y r i m i d i n e s , would f r a c t i o n a t e w i t h the f u l v i c a c i d s . Refluxing of GAR and BAR with acetone:water e x t r a c t e d 63% and 78%, 1

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r e s p e c t i v e l y , o f the bound r a d i o a c t i v i t y . C l e a r l y the freeze d r y i n g and/or g r i n d i n g had a l t e r e d the s t a t e o f these r e s i d u e s , f o r p r e v i o u s l y these s o i l s had been e x h a u s t i v e l y e x t r a c t e d in the same s o l v e n t . From chromatographic analyses o f these acetone:water e x t r a c t s , 8% and 18% o f the 'bound r a d i o a c t i v i t y in GAR and BAR r e s p e c t i v e l y was i d e n t i f i e d as p i r i m i c a r b , 5% and l e s s than 1% as i t s 2-methylformamido and 2-methylamino d e r i v a t i v e s , and 8% and 38% as hydroxypyrimidines formed by decarbamoylation o f the parent and i t s 2-methylamino degradation product. Thus in both GAR and BAR approximately 5% o f the o r i g i n a l l y a p p l i e d r a d i o a c t i v i t y was present in the 'bound' residues as i d e n t i f i e d p y r i m i d i n - 4 - y l carbamates. This compares approximately w i t h the t o t a l 'bound' r a d i o a c t i v i t y in s o i l s incubated 'aerobically' with carbamate-^^C-pirimicarb. Subsequently, Gore soil t r e a t e d w i t h r i n g - C - p i r i m i c a r b has been used t o i n v e s t i g a t e the e f f e c t o f s e q u e n t i a l acetone :water e x t r a c t i o n s , w i t h combinations o f a i r - d r y i n g , f r e e z e - d r y i n g and g r i n d i n g the soil, on the e x t r a c t a b i l i t y o f p i r i m i c a r b and i t s degradation products. Both methods o f d r y i n g the soil enabled f u r t h e r amounts o f r a d i o a c t i v i t y t o be e x t r a c t e d by repeated acetonerwater r e f l u x . Grinding the soil caused a f u r t h e r but small increase in e x t r a c t a b i l i t y . A f t e r 15 weeks i n c u b a t i o n , 54% of the r a d i o a c t i v i t y present was e x t r a c t e d from the moist soil. Subsequent s e q u e n t i a l d r y i n g and r e f l u x i n g (3 times) e x t r a c t e d an a d d i t i o n a l 15% o f the r a d i o a c t i v i t y . Although d r y i n g may have r e l e a s e d C - m a t e r i a l bound t o soil s u r f a c e s , it is a l s o a p o s s i b i l i t y t h a t the p e s t i c i d e and/or i t s degradation products can accumulate i n s i d e m i c r o b i a l c e l l s . These c e l l s might not all be ruptured without an e x t r a c t i o n from the d r i e d s t a t e . Studies are in progress t o examine t h i s p o s s i b i l i t y . S o i l s GFR and GFC contained approximately 14% and 21%, r e s p e c t i v e l y , o f r a d i o a c t i v i t y derived from the r i n g - ^ c and carbamate-^C r a d i o l a b e l e d p i r i m i c a r b o r i g i n a l l y a p p l i e d . E x t r a c t i o n o f GFC w i t h 1.0 M HC1 (shaken a t room temperature) r e s u l t e d in a l o s s o f 75% o f the bound r a d i o a c t i v i t y from the e x t r a c t i o n v e s s e l , and i t s recovery in the ethanolamine 'trapping' s o l u t i o n . I t was, however, observed t h a t 98% o f the r a d i o a c t i v i t y was a l s o evolved from soil t r e a t e d w i t h carbamate-**C-pirimicarb and ext r a c t e d w i t h 1.0 M HC1 a f t e r o n l y 4 hours i n c u b a t i o n . As 1.0 M CH COONa (adjusted t o pH 5 w i t h g l a c i a l CH3COOH) d i d n o t cause any such ' l o s s ' o f r a d i o a c t i v i t y , soil GFC was a l s o shaken w i t h the acetate b u f f e r . During t h i s e x t r a c t i o n , 55% o f the 'bound' r a d i o a c t i v i t y was evolved and recovered in e i t h e r ethanolamine o r NaOH s o l u t i o n s . Almost all o f the r a d i o a c t i v i t y in the NaOH 'trap' was i d e n t i f i e d as Na2^C03. I t was thus concluded t h a t approximately h a l f o f the 'bound' r a d i o a c t i v i t y in GFC was probably present as -carbonate, d e r i v e d from f i x a t i o n o f ^ C C ^ evolved a f t e r h y d r o l y t i c cleavage o f the l^C-carbamate e s t e r under flooded c o n d i t i o n s . As expected, a s i m i l a r amount o f r a d i o a c t i v i t y (approx 50%) remained in the soil residue a f t e r

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NaOH e x t r a c t i o n . The remaining r a d i o l a b e l e d residues were a l k a l i e x t r a c t e d , and a f t e r a c i d i f i c a t i o n were recovered from the f u l v i c a c i d f r a c t i o n . Acetone:water r e f l u x i n g e x t r a c t e d only 20% o f the GFC 'bound r e s i d u e s , o f which three quarters was i d e n t i f i e d as pirimicarb. During a c i d e x t r a c t i o n o f GFR, l e s s than 2% o f the r a d i o a c t i v i t y was evolved i n t o the a i r stream. Instead, almost all o f the 'bound' C c o n s t i t u e n t s behaved as those in BAR and GAR, being e x t r a c t e d w i t h a l k a l i and s o l u b l e in a c i d s . From acetone: water e x t r a c t i o n , 10% o f the 'bound' r a d i o a c t i v i t y in GFR was i d e n t i f i e d as p i r i m i c a r b and 25% as 5,6-dimethyl-2-dimethylamino4-hydroxypyrimidine. A f u r t h e r 20% was e x t r a c t e d but not characterized. It has thus been p o s s i b l e t o i d e n t i f y between 20% and 65% o f what was o r i g i n a l l y b e l i e v e d t o be the 'bound' r a d i o a c t i v e residues o f p i r i m i c a r b r e s u l t i n g from i n c u b a t i o n o f soil under 'aerobic' and flooded c o n d i t i o n s . The 'bound' -residues o f p i r i m i c a r b remaining unidentf i e d are a t present under i n v e s t i g a t i o n using chromatographic and isotope d i l u t i o n techniques. 1

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Downloaded by FUDAN UNIV on February 6, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0029.ch025

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Kaufman et al.; Bound and Conjugated Pesticide Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1976.