Catalyzed Photodegradation of the Herbicides Molinate and

Chapter 18

Catalyzed Photodegradation of the Herbicides Molinate and Thiobencarb 1

2

R. Barton Draper and Donald G. Crosby

Downloaded by PENNSYLVANIA STATE UNIV on March 8, 2013 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch018

Department of Environmental Toxicology, University of California, Davis, CA 95616

A survey of oxidizing agents capable of supplementing the natural oxidants of field water showed that the rice herbicides molinate (I) and thiobencarb (VIII) were degraded rapidly in sunlight-irradiated suspensions of TiO and ZnO. ZnO served both as a semiconductor photooxidant and as the Zn(II) fertilizer normally applied for plant nutrition. In a flooded rice field, isolated basins were treated with Ordram 10G (molinate); after 3 days, an aqueous ZnO suspension, stable in field water at pH 8 to 9, was applied. The resulting immediate decrease in molinate half-life from 60 h to 1.5 h indicates that applying ZnO before releasing agricultural wastewater may provide an economical means of intentionally degrading persistent chemical residues. 2

Herbicides are applied extensively to protect C a l i f o r n i a ' s valuable r i c e crop. During 1983, for example, flooded r i c e f i e l d s i n the Sacramento Valley received a e r i a l applications of 422 metric tons of molinate (_S_-ethyl hexahydro-1 j£-azepine-1 -carbothioate) ( I ) and 100 metric tons of thiobencarb [^-(4-chlorophenyl)methyl-N,N^ diethylcarbamothioate](VIII) as the 10% (a.i.) granular formulations Ordram 10G and Bolero 10G, respectively. After holding i r r i g a t i o n flood water for the four to eight days required for residue d i s s i p a t i o n and e f f e c t i v e h e r b i c i d a l a c t i v i t y , farmers release i t into a g r i c u l t u r a l drains which empty into the Sacramento River. Seasonal f i s h k i l l s i n these a g r i c u l t u r a l drains are attributed to the r i c e herbicides. The molinate concentration i n the Colusa Basin Drain has approached 350 Pg/L during early June when herbicide use i s heaviest. Farther downstream i n the Sacramento River, near Sacramento's drinking water treatment f a c i l i t y , concentrations of molinate and thiobencarb have exceeded 20 ug/L 'Current address: Department of Chemistry, University of Texas, Austin, TX 78712 Correspondence should be addressed to this author. 0097-6156/87/0327-0240$06.00/0 © 1987 American Chemical Society

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

18.

DRAPER AND CROSBY

Catalyzed Photodegradation of Herbicides

241

and 6 ug/L, respectively. Removal of these herbicides from i r r i g a t i o n waste-water by natural v o l a t i l i z a t i o n , hydrolysis, photolysis, and microbial breakdown proceeds so slowly that adequate d i s s i p a t i o n does not occur before w i l d l i f e and human drinking water supplies are threatened. Our investigation evaluated the f e a s i b i l i t y of enhancing the natural oxidizing a c t i v i t y of r i c e f i e l d water i n order to i n t e n t i o n a l l y degrade r i c e herbicides.


Expérimenta Herbicide solutions were prepared by d i l u t i o n of a concentrated aqueous stock solution with d i s t i l l e d deionized water or r i c e f i e l d water. Solutions or suspensions i n gas-tight Pyrex vessels were s t i r r e d and irradiated i n a photoreactor (_2), a 150 χ 36 cm chromelined cylinder f i t t e d with s i x F40BL fluorescent lamps. A 4nitroanisole/pyridine actinometer (_3) had a h a l f - l i f e of 50 min i n the reactor, equivalent to spring sunlight i n the Sacramento Valley. Rate Measurements. Suspensions of titanium dioxide ( T i 0 ) or zinc oxide (ZnO) i n d i s t i l l e d water or r i c e f i e l d water containing the herbicide were subjected to i r r a d i a t i o n for 2 h i n the photoreactor either (1) wrapped i n f o i l to exclude l i g h t , (2) exposed to l i g h t with no semiconductor, or (3) exposed to l i g h t with semiconductor added. Samples were p e r i o d i c a l l y withdrawn, extracted onto bondedphase extraction cartridges, and analyzed by gas chromatography (glc). 2

Ti02"Induced Photoproducts of Molinate. A 100 mg/L suspension of T i 0 i n 6.4 χ 10~^ j4 aqueous molinate, irradiated 40 min i n the photoreactor, provided a mixture of photoproducts. Molinate sulfoxide and hexamethyleneimine were extracted, derivatized, and detected according to a procedure described by Soderquist, et a l . (4_) · A second irradiated suspension, extracted onto a C-8 bondedphase cartridge and eluted with methanol, provided a concentrated solution of photoproducts suitable for analysis by high-performance l i q u i d chromatography (HPLC), g l c , and glc with a mass spectrometer detector (GC-MS). 2

ZnO-Induced Photoproducts of Thiobencarb. A 100 mg/L suspension of ZnO i n 4.7 χ 10" JM aqueous thiobencarb, irradiated 15 min i n the photoreactor, provided a mixture of photoproducts. After a c i d i f i c a t i o n , extraction with a bonded-phase cartridge, and e l u t i o n , eluates were dried over anhyd sodium sulfate and concentrated under Ν . The C-8 cartridge eluate (methanol) was treated with diazomethane and the cyclohexyl cartridge eluate (ethyl acetate) was treated with ^-methyl-N-(Jt-butyldimethylsilyl) trifluoroacetamide (MTBSTFA) for glc (FID detector) and GC-MS analysis. F i e l d Application of ZnO. F i e l d plots consisted of four 2.4 m diameter aluminum rings placed i n a flooded r i c e f i e l d at the UCD Rice Research F a c i l i t y . In August, 1984, the r i c e plants were removed from these isolated basins, and granular molinate (Ordram


PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS


242

10G) was applied at an a g r i c u l t u r a l rate equivalent to 6.3 kg(a.i.)/ha. Water samples were collected from each basin at six to nine h intervals and stored i n glass at -4°C u n t i l analyzed. pH, dissolved oxygen (DO), a i r temperature, and water temperature were intensively monitored. After 3 d, two basins were treated with an aqueous suspension of ZnO (1 g/L) at an application rate equivalent to 14 kg(Zn)/ha, giving a 8.3 mg(ZnO)/L suspension i n the basins. Water samples from both treated and control basins were taken at 0, 5, 15, 45, 120, 240, 411, and 1300 min after the ZnO application and analyzed for molinate and 4-ketomolinate. In a similar experiment, performed several days l a t e r , an application of 6.7 kg(Zn)/ha provided a 4 mg(ZnO)/L suspension i n the basins. Results and Discussion, In our investigation of enhanced photodegradation, molinate and thiobencarb proved to be p a r t i c u l a r l y valuable probes. Neither herbicide was degraded appreciably i n sunlight by primary photolysis, and both broke down only slowly when exposed to natural photosensitizers (£, 5_, 6_, T_, ) · In addition to their satisfactory physical-chemical properties, molinate and thiobencarb are key pesticides for protection of a r i c e crop with a potential annual market value over 500 m i l l i o n d o l l a r s . The herbicides, used on 500,000 acres of r i c e i n C a l i f o r n i a , control Echinochloa c r u s - g a l l i (barnyard grass) and Leptochloa f a s c i c u l a r i s (sprangletop), contributing to a r i c e y i e l d that i s three times the world average {S)· However, molinate and thiobencarb also contribute to serious l o c a l water p o l l u t i o n problems. For the intentional destruction of these herbicide residues, the i d e a l agent would be inexpensive, rapid acting, d i s s i p a t i v e or controllable, non-polluting, and added d i r e c t l y to the rice f i e l d or the adjacent drains. From a survey of a variety of reagents, the semiconductor photocatalysts titanium dioxide and zinc oxide emerged as a t t r a c t i v e candidates. Photooxidation by Titanium Dioxide and Zinc Oxide. Titanium dioxide and zinc oxide have the a b i l i t y to photo-induce the oxidation of organic compounds (9_, 10, 11 , ) · The redox reaction involves the absorption of l i g h t by the semiconductor s o l i d which results i n promotion of an electron from the valence band to the conduction band, leaving an electron-deficiency or "hole". In such n-type semiconductors (those made electron-rich by doping with an electron donor), the photogenerated hole migrates towards the solution interface where oxidations occur. The electron moves away from the interface into the bulk of the semiconductor p a r t i c l e . Ultimately, a negatively charged aggregate i s formed which can act as a reducing center and e f f e c t solution-phase reductions, such as that of oxygen to superoxide. Surface-sensitized oxidations should occur with adsorbed organic molecules having oxidation potentials less positive than that of the semiconductor valence band. The valence band positions of T i 0 *d ZnO, measured i n water at pH 1, are reported to be +3.1 and +3.0 V ar

2


18.

DRAPER AND CROSBY


243

vs. SCE respectively ( 11 ), making the photoexcited catalysts potential oxidants of a wide variety of organic compounds (Table I ) . In water, reactions may proceed by surface-sensitized oxidation, i n i t i a l oxidation of water to hydroxyl r a d i c a l (12), or possibly by reduction of oxygen with subsequent disproportionation to hydrogen peroxide and subsequent photolysis to hydroxyl r a d i c a l s .


f

Table I . Anodic oxidation potentials of some environmental pollutants (19). E

p nV

Compound

2-Chlorophenol 2,4-Dichlorophenol 3-Chlorophenol 4,4*-Dimethoxybiphenyl 4-Hydroxybiphenyl 2-Hydroxybiphenyl 3-Chloro-4-hydroxybiphenyl 3-Hydroxybiphenyl Biphenyl 4,4'-Dichlorobiphenyl 2,5,2 ,5'-Tetrachlorobiphenyl Decachlorobiphenyl Benzene Arochlor 1232 Arochlor 1242 Arochlor 1254 Arochlor 1260 1

+ + + + + + + + + + + + + + + + +

a/2

.V vs. SCE

0.63 0.65 0.73 1.18 1 .24 1.27 1 .30 1.47 1 .82 1 .88 2.38 2.63 2.32 2.23 2.39 2.42 2.43

In our experiments, T i 0 and ZnO did not perform equivalently; a 100 mg/L suspension of TiO^ provided the same molinate degradation rate as d i d a 5 mg/L suspension of ZnO. A further advantage of ZnO as an intentional additive was i t s i n s t a b i l i t y i n l i g h t and water (13, 14); as i t slowly dissolves, Zn(II) would become available f o r r i c e plant n u t r i t i o n . The pH dependence of ZnO s t a b i l i t y could, however, l i m i t the u t i l i t y of this catalyst; a t pH

NCSC H 2

5

/==\ ? I

N(C S C H 2

^ > Γ

245

5

IV

ν p0 NCSC H 2

5

Γ

NH

Molinate I


VI Figure 1.

F i g u r e 2.

Major products of the TiC^-catalyzed photooxidation of molinate.

Major products of the ZnO-catalyzed photooxidation of thiobencarb.

1

«

Γ

χ

TIME (days) F i g u r e 3.

Diurnal variation of pH i n r i c e f i e l d basins.


VII

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS


246

Figure 5 ·

E f f e c t of ZnO (14 kg/ha Zn) on molinate concentration i n f i e l d water (•) when applied on Day 3 (j) compared to an untreated control (#) ·


18.

DRAPER A N D CROSBY


247

Literature Cited 1. 2. 3. 4. 5.


6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19.

Draper, W.M.; Crosby, D.G. Arch, Environ, Contarn, T o x i c o l , 1983, 12, 121-126. Crosby, D . G . ; Wong, A . S . J. Agr. Food Chem. 1973, 21, 10521054. Dulin, D . ; Mill, T. Environ. S c i . Technol. 1982, 16, 815-820. Soderquist, C.J.; Bowers, J.B.; Crosby, D . G . J. A g r i c . Food Chem. 1977, 25, 940-945. Draper, W.M.; Crosby, D.G. J. Agr. Food Chem. 1981, 29, 699702. Draper, W.M.; Crosby, D.G. J. A g r i c . Food Chem. 1984, 32, 231237. Ross, R . D . ; Crosby, D.G. Environ. T o x i c o l . Chem. 1985, 4, 773778. Rutger, J.N.; Brandon, D.M. S c i . Am. 1981, 244, 43-51. Carey, J.H.; Lawrence, J.; Tosine, H.M. B u l l . Environ. Contarn. Toxicol. 1976, 16, 697-701. Fox, M.A. Acc. Chem. Res. 1983, 16, 314-321. Fox, M.A.; Chen, C.-C; Younathan, J.N.N. J. Org. Chem. 1984, 49, 1969-1974. Jaeger, C.D.; Bard, A.J. J. Phys. Chem. 1979, 83, 3146-3152. Blok, L.; de Bruyn, P . L . J. C o l l o i d Interface S c i . 1970, 32, 518. Gerischer, H. J. E l e c t r o a n a l . Chem. 1977, 82, 133-143. DeBaun, J.R.; Bova, D . L . ; Tseng, C . K . ; Menn, J.J. J. Agr. Food Chem 1978, 26, 1098-1104. Lay, M.-M.; Niland, A . M . ; DeBaun, J.R.; Menn, J.J. In "Pesticide and Xenobiotic Metabolism i n Aquatic Organisms"; Khan, M.A.Q.; Lech, J.J.; Menn, J.J., eds; ACS SYMPOSIUM SERIES No. 99, American Chemical Society: Washington, D.C.,1979; pp. 95-119. Crosby, D . G . ; Bowers, J.B. J. Agr. Food Chem. 1968, 16, 839843. Mikkelson, D . S . ; De Datta, S . K . ; Obsemea, W.N. S o i l S c i . Soc. Am. J. 1978, 42, 775-730. Fenn, R.J.; Krantz, K.W.; Stuart, J.D. J. Electrochem. Soc. 1976, 123, 1643-1647.

R E C E I V E D June 2 7 , 1 9 8 6


Catalyzed Photodegradation of the Herbicides Molinate and

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