Sorption of fipronil and its metabolites on soils from South Australia

This paper reports on the sorption of fipronil [(+/-)-5-amino-1-(2,6-dichlo ro-alpha,alpha,alpha -trifluoro-p-tolyl)-4-trifluoromethyl-sulfinylpyrazole -3-carbonitrile] and its two main metabolites, desulfynil and sulfide deriv atives on a range of soils from South Australia. The Freundlich sorption co efficient (K-f) values for fipronil on the soils ranged from 1.94 to 4.84 u sing a 5% acetonitrile/water mixture as the soil solution. Its two metaboli tes had a higher sorption affinity for soils, with K-f values ranging from 11.09 to 23.49 for the sulfide derivative and from 4.70 to 11.77 for the de sulfynil derivative. Their sorption coefficients were found to be better re lated to the soil organic carbon than clay content. The presence of cosolvents in soil solutions had a significant influence on the sorption of fipronil. The Freundlich sorption coefficients showed a lo g linear relationship with the fractions of both acetonitrile and methanol in solutions. The sorption coefficient of fipronil on Turretfield soil in t he aqueous solution was estimated to be from 13.80 to 19.19. Methanol had l ess effect on the sorption of fipronil than acetonitrile. The K-d values fo r fipronil on the eight soils using a 5% methanol/water mixture were from 5 .34 to 13.85, which reflect more closely the sorption in the aqueous soluti on. The average K-oc, value for fipronil on the eight South Australian soil s was calculated to be 825 +/- 214.