Studies On Residual Dynamics Of Fipronil In Rice And Screening For Degrading Bacteria

In this study, a method was developed to analysis residue of fipronil in cereal and soil.The degradation and final residues of fipronil in rice and soil were studied through field experiments in Anhui and Hunan with different dosages and spraying methods.Μsing enrichment medium and inorganic salt medium, a fipronil degrading strain was isolated from sludge and identified as actinomyces. The main results of this paper were shown as follows:1. A method was developed to analysis residue of fipronil in cereal and soil. After acetonitrile was added, the samples were sonicated in anμltrasonic water bath and then centrifuged, filtered, and cleaned up by a column containing neutral alumina and active carbon, successive eluted by ethyl acetate /n-hexane (3:5,V/V). The pesticide residues were determined by gas chromatography with electron-capture detection. Recoveries at spiked concentrations between 0.01 to 0.5mg/kg were 80.60%~99.66% with RSDs less than 9%. The limit of detection was 0.01mg/kg. The sensitivity and accuracy of the proposed method were fit for analysis of pesticide residue in cereal and soil.2. The degradation of fipronil in plant was fit for first-order kinetic model, that in Hefei was slower (the half-lives were 10 days) than in Hunan (the half-lives were 4.6 days).After spraying fipronil 80% WG at recommended double dosage (equivalent to 72 g a.i/hm2), the residue in soil was increasing until 7th day, and then decreased. The final residue in soil was about 0.2mg/kg after 45 days.3. Fipronil 80% WG was sprayed into the rice field once and twice at 36 g a.i/hm2 and 72 g a.i/hm2. The maximum residue levels of fipronil in soil, straw, rice and bran were 1.14~6.85mg/kg, 0.03~0.32mg/kg, 0.01~0.11mg/kg and 0.07~1.42mg/kg twenty-one days later.4. A strain which can degrade fipronil and grow rapidly was isolated and selected from the soil of pesticide factory. The degradation rate for 50mg/L fipronil can reach 81% in 5 days. Compared to the unspiked one, the rate enhanced 1.9 times.5. The strain lived at the condition of 30℃,PH 7.0 and NaCl of 1%(W/V).It is straight rod-shaped which is round at both sides.The colony is opaque with the color of pink.It showed negative by gram stain. It was preliminarily identified as S.Marcescens.