Bicyclopyrone is a herbicide that is targeted for the control of herbicide-resistant weeds. However, there is a lack of extensive data on its sorption and factors that control its sorption in the soil system. In this study, we evaluated a series of 25 different soils, with a variety of soil properties to assess if an empirical relationship could be developed to predict the sorption coefficient for bicyclopyrone. Overall, there were no statistically significant relationships observed with organic carbon, cation exchange capacity, or clay content. There solely was a moderate negative correlation with soil pH (R=-0.65).  Additionally, Freundlich isotherm analysis suggests that the K_D could be adequate to characterize the sorption behavior for the range of soils evaluated here.

Bicyclopyrone sorption was determined on each soil in triplicate using the batch equilibration method with radiolabeled compounds. The ¹⁴C spiking solution for BCP was prepared by diluting ¹⁴C-bicyclopyrone [pyridinyl-3-¹⁴C; specific activity = 95.3 µCi mg^-1; 8,880 Bq mmol^-1; 99.5% purity] with unlabeled BCP (Syngenta Crop Protection, LLC) to result in a stock solution of 10 mg L⁻¹ with an overall activity of 1,090 Bq mL^-1 (65,450 DPM mL^-1). A 0.5 mL aliquot of this spiking solution was added to 9.5 mL of 0.01 M CaCl₂ to achieve a 10 mL solution volume. This solution (0.5 mg L⁻¹ BCP) was added to 1 g of soil in a 20 mL glass scintillation vial for a series of 25 different USDA soil series. Samples were then placed horizontally on a reciprocal shaker and allowed to equilibrate for 24 h (180 rev min^-1) in the dark. Following centrifugation (20 min, 1500 × g), 2 mL of supernatant was removed and filtered (0.45 μm). Then a 1 mL aliquot of the filtered solution was mixed with 5 mL of scintillation cocktail [EcoLite(+)™; MP Biomedicals, LLC, Solon, OH] and analyzed for ¹⁴C by liquid scintillation counting (LSC) (HITACHI AccuFLEX LSC-8000, GMI Ramsey, MN, 10-minute counting window).  No statistically significant sorption of the BCP to scintillation vials, syringes or syringe filters was observed (98-100% recovery, data not shown). The sorbed concentration of BCP on the soil was estimated by the following:

C_s=C_i-C_emV

,

where C_i is the initial concentration (mg L^-1), C_e is the equilibrium concentration (mg L^-1), V is the total volume of the liquid phase (L), and m is the mass of the soil (g). The equilibrium concentration was determined by the LSC of the liquid phase using the following equation:

C_e=LSC_e-Blank * C_iLSC_i ,

where LSC_e is the disintegrations per minute of the sample following equilibration with the soil, Blank is the disintegrations per minute of the scintillation cocktail and vial alone, C_i is the liquid phase concentration of the initial standard (mg L^-1), and LSC_i are the disintegration of the corresponding BCP standards (without soil added). The partitioning coefficient (K_D) is estimated by the following:

K_D= C_SC_e .

Sorption Isotherms – Multiple Concentrations

Sorption isotherms were generated for 7 selected soils which were selected based on the initial screening above for a range of observed K_D values. The isotherm was determined utilizing initial BCP solution concentrations of 0.1, 0.5, 1, 2.5, 5, 10, and 20 mg L⁻¹. Individual samples were run in duplicate using methods analogous to the single concentration batch K_D determination described earlier.  Equilibrium liquid (C_e) and solid (C_s) concentrations were then analyzed by fitting to different linear forms of four sorption models: Langmuir, Freundlich, Temkin, and the Dubinin-Radushkevich models (Horsfall, Spiff, & Abia, 2004; Hunt & He, 2015). 

Excel file with separate worksheets for the batch-isotherm (K_D) and the multi-point isotherm datasets.