1198 J. Agric. Food Chem., Vol. 46, No. 3, 1998
Hong and Pehkonen
(
Gosselin and Gleason, 1976). Considering that the
Beynon, K. I.; Hutson, D. H.; Wright, A. N. The Metabolism
and Degradation of Vinyl Phosphate Insecticides. Residue
Rev. 1973, 47, 55-142.
Brindley, G. W., Beward, A. S., Berry, L. G., Post, B.,
Weissmann, S., Cohen, G. E., Eds. Index to the X-ray Powder
Data File; American Society for Testing Materials; Phila-
delphia, PA, 1958.
thiol group is a common molecular segment of many
organophosphorus pesticides, it seems reasonable to say
that the potential of seemingly environmentally friendly
organophosphorus pesticides to be the precursors to
toxic compounds has been underestimated.
Budavari, S., O’Neil, M. J ., Smith, A., Heckelman, P. E., Eds.
The Merck Index, 11th ed.; Merck: Rahway, NJ , 1989.
Calleja, A.; Baldasano, J . M.; Mulet, A. Toxicity Analysis of
Leachates from Hazardous Wastes via Microtox and Daph-
nia magna. Toxic Assess. 1986, 1, 73-83.
Capozzi, G.; Modena, G. Oxidation of thiols. In The Chemistry
of the Thiol Group; Patai, S., Ed.; Wiley, New York, 1974.
Chen, K. Y.; Morris J . C. Kinetics of Oxidation of Aqueous
CONCLUSIONS
At room temperature, the hydrolysis rates of phorate
at pH 5.7 and at pH 8.5 were found to be comparable.
The basic and neutral hydrolysis rate constants of
phorate are similar in magnitude to those of other
phosphoric and thiophosphoric triesters (Faust and
Gomaa, 1972; Mabey and Mill, 1978; Wanner et al.,
Sulfide by O . Environ. Sci. Technol. 1972, 6, 529-537.
2
Cowart, R. P.; Bonner, F. L.; Epp, E. A. Rate of Hydrolysis of
Seven Organophosphate Pesticides. Bull. Environ. Contam.
Toxicol. 1971, 6, 231-234.
1
989). While most of the organophosphorus triesters
are relatively insensitive to acidic hydrolysis, the acidic
hydrolysis rate constant of phorate was found to be quite
significant. Interestingly the acidic hydrolysis of disul-
foton, which is only one -CH2- different from phorate,
is negligible (Faust and Gomaa, 1972). Some of the
major metal (hydr)oxides found in the natural environ-
ment do not seem to have significant influence on the
half-life of phorate under the experimental conditions
employed here.
Dannenberg, A.; Pehkonen, S. O. Investigation of the Hetero-
geneously Catalyzed Hydrolysis of Organophosphorus Pes-
ticides. J . Agric. Food Chem. 1998, 46, 325-334.
Davis, J . A.; Hayes, K. F. Geochemical Processes at Mineral
Surfaces; ACS Symposium Series 323; American Chemical
Society: Washington, D.C., 1986.
Faust, S. D.; Gomaa, H. M. Chemical Hydrolysis of Some
Organic Phosphorus and Carbamate Pesticides in Aquatic
Environments. Environ. Lett. 1972, 3, 171-201.
Fernandez-Casalderrey, A.; Ferrando, M. D.; Andreu-Moliner,
E. Endosulfan and Diazinon Toxicity to the Freshwater
Rotifier Brachionus calyciflorus. J . Environ. Sci. Health:
Pestic. Food Contam. Agric. Wastes 1992, B27, 155-164.
Fernandez-Casalderrey, A.; Ferrando, M. D.; Andreu-Moliner,
E. Effect of the Insecticide Methylparathion on Filtration
and Ingestion Rates of Brachionus calyciflorus and Daphnia
magna. In Science of Total Environment, 2nd European
Conference on Ecotoxicology; Society for Ecotoxicology &
Environment Safety et al.: Amsterdam, Netherlands, 1993;
p 867.
Gomaa, H. M.; Faust, S. D. Chemical Hydrolysis and Oxidation
of Parathion and Paraoxon in Aquatic Environment. In Fate
of Organic Pesticides in the Aquatic Environment; Gould,
R. F., Ed.; American Chemical Society: Washington, D.C.
1972.
Gosselin, R. E., Gleason, M. N., Eds. Clinical Toxicology of
Commercial Products; 4th ed.; Williams & Wilkins: Balti-
more, 1976.
Hay, R. W. Lewis Acid Catalysis and the Reactions of
Coordinated Ligands. In Comprehensive Coordination Chem-
istry; Wilkinson, G., Gillard, R. D., McCleverty, J . A., Eds.;
Pergamon: Oxford, U.K. 1987.
Hine, J ., Ed. Physical Organic Chemistry McGraw-Hill: New
York, 1962.
Abiotic hydrolysis of phorate under alkaline condi-
tions produces diethyl disulfide, hydrogen sulfide, and
formaldehyde, all of which are toxic compounds. What
is also noteworthy is the uniqueness of degradation
products identified here. Neither diethyl disulfide nor
formaldehyde has ever been reported as the degradation
product of organophosphorus pesticides. Even H2S is
rarely mentioned in the literature as a product of
organophosphorus pesticide hydrolysis. While the struc-
ture of the phosphorus-containing bulky product is
important, it is also more predictable on the basis of
previous work. The more variable side chain gives each
organophosphorus pesticide its distinct physical, chemi-
cal, and pesticidal activity, and the hydrolysis products
derived from the side chain deserve more attention. For
example, compared to phorate, both malathion and
disulfoton are structurally similar dithiophosphates, yet
both produce very different hydrolysis products (Wolfe
et al, 1977; Dannenberg and Pehkonen, 1998). The
current study underscores the importance of research
on the hydrolysis pathways, product identification, and
product toxicity analysis prior to the introduction of new
organophosphorus pesticides into the environment.
Indorato, A. M.; Snyder, K. B.; Usinowics, P. J . Toxicity
Screening Using Microtox Analyzer. In Toxicity Screening
Procedures Using Bacterial Systems; Liu, D., Dutka, P. J .,
Eds.; Dekker: New York, 1984.
J uarez, S. L. M.; Sanchez, J . Toxicity of the Organophosphorus
Insecticide Metamidophos (O,S-Dimethyl Phosphoramido-
thioate) to Larvae of the Freshwater Prawn Macrobrachium
rosenbergii (De Man) and the Blue Shrimp Penaeus styl-
irostris. Bull Environ. Contam. Toxicol. 1989, 43, 302-308.
Khan, S. U., Ed. Pesticides in the Soil Environment; Elsevier:
Amsterdam, Netherlands, 1980.
ACKNOWLEDGMENT
We thank Prof. Allan Pinhas for many helpful discus-
sion sections. We also thank Mr. Michael Menard for
technical assistance in XRD analysis and Ms. Vicki
Steed for assistance in sulfide analysis.
LITERATURE CITED
Almgren, T.; Hagstr o¨ m, I. The Oxidation Rate of Sulfide in
Seawater. Water Res. 1974, 8, 395-400.
Backhus, D. A.; Ryan, J . N.; Groher, D. M.; Macfarlane, J . K.;
Gschwend, P. M. Sampling Colloids and Colloids-Associated
Contaminants in Groundwater. Ground Water 1993, 31,
Kramer, K. J . M.; Botterweg, J . Aquatic Biological Early
Warning System: An Overview. In Bioindicators and
Environmental Management; J effrey, D. W., Madden, B.,
Eds.; Academic Press: New York, 1991.
Kummert, R.; Stumm, W. The Surface Complexation of
Organic Acids on Hydrous γ-Al O . J . Colloid Interface Sci.
4
66-479.
2
3
Barnard, P. W. C.; Burton, C. A.; L’lewellyn, D. R.; Vernon,
C. A.; Welch, V. A. The Reactions of Organic Phosphates.
Part V. The Hydrolysis of Triphenyl and Trimethyl Phos-
phate. J . Chem. Soc. 1961, 2070-2076.
1980, 75, 373-385.
Mabey, W.; Mill, T. Critical Review of Hydrolysis of Organic
Compounds in Water under Environmental Conditions. J .
Phys. Ref. Data 1978, 7, 383-415.