Comparative sorption, desorption and leaching potential of aminocyclopyrachlor and picloram

Abstract

Sorption and desorption of aminocyclopyrachlor (6-amino-5-chloro-2-cyclopropylpyrimidine-4-carboxylic acid) were compared to that of the structurally similar herbicide picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) in three soils of differing origin and composition to determine if picloram data is representative of aminocyclopyrachlor behavior in soil. Aminocyclopyrachlor and picloram batch sorption data fit the Freundlich equation and was independent of concentration for aminocyclopyrachlor (1/n = 1), but not for picloram (1/n = 0.80–0.90). Freundlich sorption coefficients (K _f) for aminocyclopyrachlor were lowest in the eroded and depositional Minnesota soils (0.04 and 0.12 μmol ^(1–1/n) L^1/n kg⁻¹) and the highest in Molokai soil (0.31 μmol ^(1–1/n) L^1/n kg⁻¹). For picloram, K _f was lower in the eroded (0.28 μmol ^(1–1/n) L^1/n kg⁻¹) as compared to the depositional Minnesota soil (0.75 μmol ^(1–1/n) L^1/n kg⁻¹). Comparing soil to soil, K _f for picloram was consistently higher than those found for aminocyclopyrachlor. Desorption of aminocyclopyrachlor and picloram was hysteretic on all three soils. With regard to the theoretical leaching potential based on groundwater ubiquity score (GUS), leaching potential of both herbicides was considered to be similar. Aminocyclopyrachlor would be ranked as leacher in all three soils if t_1/2 was > 12.7 days. To be ranked as non-leacher in all three soils, aminocyclopyrachlor t_1/2 would have to be <3.3 days. Calculated half-life that would rank picloram as leacher was calculated to be ∼15.6 d. Using the current information for aminocycloprachlor, or using picloram data as representative of aminocycloprachlor behavior, scientists can now more accurately predict the potential for offsite transport of aminocycloprachlor.

Keywords:

• Adsorption
• hysteresis
• Pyrimidine carboxylic acids
• mobility
• soil properties.

Acknowledgment

We thank the CNPq (Brazil) for financial support for this research and DuPont (Wilmington, DE) for supplying ¹⁴C-labeled aminocyclopyrachlor.