A comparative study of ionic pesticide sorption and degradation in contrasting Brazilian soils and the development of a novel 3-Phase Assay to assess sorption reversibility.

Brazilian soils have distinctive characteristics to European and North American soils which are typically used to investigate pesticide fate. This study aimed to compare soil-water partition coefficient (Kd), reversibility of adsorption and degradation half-life (DT50) of 5 pesticides covering a wide range of physico-chemical properties in contrasting Brazilian soils (Argissolo, Gleissolo, Latossolo and Neossolo) and a temperate (UK) alfisol soil, and to study their relationship with soil OM, clay and expandable clay content, CEC and pH. In addition, we used a novel laboratory test to evaluate sorption reversibility, the 3-Phase Assay (3PA). This stationary extraction system uses an organic solvent that sequesters compounds present in the soil aqueous solution and forces the diffusion of reversibly sorbed compounds. Kd (mL/g) values ranged from 4.85 to 79.26 for ametryn, 0.80 to 37.58 for clodinafop, 1.49 to 6.57 for fomesafen, 9.93 to 488.90 for thiabendazole and 2.52 to 6.77 for trifloxysulfuron. Desorption of the test compound (% applied radioactivity) ranged from 37.29 to 101.91 for ametryn, 32.05 to 100.67 for clodinafop, 34.73 to 73.33 for fomesafen and 4.15 to 66.87 for thiabendazole. Sorption reversibility was not assessed for trifloxysulfuron due to hydrolytic instability. DT50 (days) ranged from 29 to 90 for ametryn, 1 to 466 for clodinafop, 49 to 601 for fomesafen, 4 to 342 for thiabendazole and 22 to 38 for trifloxysulfuron. The data generated gives an overview of pesticide fate in Brazilian soils used for regulatory testing and is helpful for exposure risk assessment. The results showed that pesticide behaviour in Brazilian soils was not systematically different from those in European and North American soils. The 3PA was shown to be a reliable and simple method for assessing pesticide desorption in soil and could be adapted to assess pesticide bioavailability. The use of the 3PA allowed a more thorough explanation of the observed differences in degradation behaviour between the compounds.