Effect of Cation Sorption on 2,4-D Mobility of Herbicidal Ionic Liquids in Agricultural Soil Combined with Diversity of the Bacterial Community

Abstract

Ionic liquids (ILs) are thought to have unlimited configurability of the properties through selection of ions, e.g., thermal stability, nonflammability, and lower toxicity. For the example of a herbicidal ionic liquid, we decided to test if it is possible to increase the hydrophobicity of a soluble, easily leachable anionic herbicide, 2,4-dichlorophenoxyacetic acid [2,4-D], by pairing it with betaine [Bet] and modified betaine zwitterions characterized by increasing hydrophobicity. This would be important in terms of the environmental fate of herbicides. The resulting adsorption (Freundlich) in agricultural soil was equal to 87–95% for hydrophobic [C12Bet] (Kf = 80.2 mg1–1/n L1/n/g) and [CAPBet] (Kf = 31.4) and 3% for hydrophilic [Bet] (Kf < 1). The adsorption of [2,4-D] anions was minimal, indicating no statistical effect of the hydrophobicity of cation on the anion sorption. This was also confirmed by 1H–1H NOESY spectra and calculated interaction energies between ions. Moreover, toxicity, rhizosphere metabiome, and phytotoxicity studies revealed the dominant role of hydrophobic surface-active derivatives of betaines. We suggest the need for a better definition of ILs, as it is possible that many of the previously described ILs were no longer an IL after their introduction into the environment and instead became an independent mixture of ions.