Saturday, 15 July 2006

Fate of Fipronil in Soils under Sugar Cane Cultivation from the Northeast of Brazil: Adsorption and Degradation.

Carmem S. M. Masutti, Fundação Universidade Federal do Vale do São Francisco, Av. Tancredo Neves, 100 - Centro, Petrolina, PE, 56.306-410, Brazil and Ahmet Mermut, Dept of Soil Science, Univ of Saskatchewan, College Drive, Saskatoon, SK S7N5A8, Canada.

FipronilTM, (±) -5- amino -1- (2,6-dichloro-a,a,a-trifluoro-p-tolyl) -4- [(trifluoro-methyl) sulfinyl] pyrazole-3-carbonitrile, has been applied to sugar cane plantations at stand establishment in Brazil for recent years. However, there is lack of information on fipronil dynamics in humic tropical soils, particularly its behavior at the soil-solution interface and degradation. The objectives of this research were to: study the adsorption-desorption behavior of fipronil and its sulfide derivative, and evaluate fipronil biodegradation in soils from the sugar cane industry in the Northeast of Brazil. Surface soils from summit (Ustox) and footslope (Aquept) landscapes were collected and characterized. The adsorption-desorption study was conducted on two selected tropical soils (Ustox and Aquept) and goethite to establish isotherms of fipronil and the sulfide derivative using the batch equilibrium technique. Fipronil biodegradation experiments were carried out under laboratory conditions (controlled temperature and in the dark), where sterile and non-sterile soils (Ustox) were incubated (under moisture content of 55% of the water holding capacity) and analyzed for fipronil disappearance and metabolite formation. Footslope soils had the highest clay, OC, CEC, and oxalate-Fe values; however, it had a lower dithionite-citrate extractable Fe content, compared to summit soils. Following the Giles classification, the fipronil isotherms were type C and L for the soils and goethite, whereas sulfide derivative adsorption isothems for soils were type L. L type isotherm indicates higher affinity of the pesticide molecules for soil surfaces. Freundlich adsorption-desorption isotherms fitted the fipronil and sulfide derivative data (R2 > 0.94). Freundlich adsorption coefficients (Kf) for fipronil ranged from 14.68 to 30.63 while for the sulfide derivative they ranged from 7.13 to 10.34 for the Ustox and Aquept, respectively. The Aquept sample had a higher amount of poorly crystalline Fe-oxyhydroxides. Adsorption was higher on goethite (Kf = 38.70) than on soils due to its high adsorptive surface. Fipronil Kfd for both soils and goethite was higher than its Kf, indicating that its adsorption was hysteretic despite the reversibility. The sulfide derivative Kfd values (12.24 – 17.65) were also higher than its Kf. Desorption of the sulfide derivative was slightly lower than fipronil desorption, especially for the Aquept sample, despite the presence of methanol, confirming the sulfide derivative has higher affinity for soil surfaces, relative to the parent compound. Soil microbial communities degraded fipronil, with levels decreasing from 0.689 to 0.399 μg g-1 after 120 days of incubation in non-sterile conditions. However, biodegradation seems to be dependent on the bioavailability of the fipronil. The half-life of fipronil ranged from 83 days (initial concentration = 978 μg g-1; short-term experiment) to 200 days (initial concentration = 689 μg g-1; long-term experiment), according to the zero-order model. Fipronil degradation rate appeared to be biphasic with an initial slower rate followed by a faster rate after 90 days of incubation, which may lead to shorter half-life than that calculated with the zero-order model. The sulfone derivative (an oxidation product) was the predominant metabolite, but the sulfide (a reduction product) and amide (a hydrolysis product) derivatives also were formed under non-sterile conditions after 120 days of incubation. Despite the fine texture of Ustoxs, more aerobic sites were present due to its microaggregation, thus favouring the formation of the sulfone metabolite over that of the sulfide metabolite. Following formation, the metabolites underwent further biodegradation, particularly the sulfone derivative. Bioavailability appears to affect fipronil degradation in soils with an effective capacity to adsorb fipronil (such as Ustoxs), while redox potential was important when determining the metabolites formed. Fipronil displayed affinity and effective binding to soil particles in the humid tropics, and underwent biodegradation, which reduced the residence time of the insecticide in the soil. Special thanks to CAPES/Brazil, Bayer CropScience, and NSERC for financial support.

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