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Chemical and Microbial Degradation of Malaoxon in an Illinois Soil¹

ABSTRACT

Chemical and microbial degradation of malaoxon was studied in an Illinois silty loam. This soil sample is typical of the glacial till soil used for agriculture in the Midwest. This sample had no history of pesticide application and demonstrates high microbial activity as evidenced by rapid degradation of malathion. To differentiate between microbial degradation and chemical decomposition, both heat-sterilized and nonsterile soil samples were prepared. Because pH has been shown to affect the rate of chemical hydrolysis, samples were adjusted to three different values of pH: 6.2, 7.2, and 8.3. Malaoxon (10 ppm) was added to both sterile and nonsterile samples, and chemical and microbial analyses were performed over a 5-day period. Chemical analyses were performed as hexane extractions followed by electron capture gas chromatography. Microbial numbers were estimated by conventional dilution and spread-plate techniques.

Half-lives for malaoxon were determined for the samples. A pronounced effect was noted with pH; malaoxon has a half-live of approximately 3 days in the basic samples and a half-life of approximately 7 days in acidic samples. Only a minor effect on half-lives was noticed from microbial activity. This may be due in part to a demonstrated biocidal effect of malaoxon on soil microorganisms. The disappearance of malaoxon in soil was interpreted to be chemical hydrolysis. The persistence of malaoxon in acidic soils may be of significance in agricultural usage where acidic soils is desirable.

Key Words: malathion • organophosphorous insecticides • oxidative desulfuration • biotransformation

¹ This research was supported in part by Illinois State Univ. Grant No. ISU-75-32.

² Assistant Professor of Chemistry, Dep. of Chem., and Assistant Professor of Microbiology, Dep. of Biol. Sci., respectively, Illinois State Univ., Normal, IL 61761.

Received for publication October 28, 1975.