Effect of cation hydrophobicity in dicamba-based ionic liquids on herbicide accumulation and bioavailability in soil

Abstract

One of the main environmental problems due to using herbicides in agriculture is their mobility and leachability from crop fields. Using an ‘ionic liquid’ strategy, we set out to see if it was possible to increase the hydrophobicity of the soluble, easily leached anionic herbicide, dicamba, by combining it with a cation exhibiting increased hydrophobicity, through the incorporation of a C8, C10, C12 and C14 alkyl chain into the structure of naturally derived choline. Hydrophobic modifications of the choline cation were quantitatively sorbed in agricultural soil, posing a long-term danger of accumulation. In addition, the possibility of negative effects on microorganisms, as well as plant germination and development were noted for all tested ionic liquids. Nevertheless, the parameters Kd (0.7–0.8 [L/kg]), Koc (54–58 [L/kg]) and GUS (4.5–3.5) and LIX (0.34) determined for dicamba clearly indicated that this herbicide functions in the soil independently of the cation with which it was originally paired in the ionic liquid. These observations are consistent with the absence of intermolecular interactions between ionic protons, confirmed by the 1H–1H NOESY spectra. The multi-level analysis of ionic liquids, containing herbicidal anion, and the subsequent interactions showed that they are not a new and distinct group of emerging contaminants, but a mixture of known cations and anions that function independently of each other in the environment.