Can ionic liquids exist in the soil environment? Effect of quaternary ammonium cations on glyphosate sorption, mobility and toxicity in the selected herbicidal ionic liquids

Ionic liquids (ILs) are described as low-impact green chemicals in which the cation can influence the properties of the anion through ion pair formation. We decided to see how stable ILs are in the soil environment. Herbicidal ILs consisting of hydrophilic glyphosate [Glyph] paired with hydrophilic choline [Chol], hydrophobic C-12 alkyl modified choline [C(12)Chol] and hydrophobic quaternary ammonium cation [C(16)TMA] were synthesized. The calculated interaction energies between ions (75.9-88.9 kcal/mol) were typical for ILs. The results showed independent sorption of cations and anions and no statistical effect of cations on anion sorption. Hydrophobic [C(16)TMA], [C(12)Chol] cations followed 99 % adsorption, and hydrophilic [Chol] revealed 42-45 % adsorption. The Freundlich constants were [C(16)TMA] K-f approximate to 4000 > [C(12)Chol] K-f approximate to 3000 > [Chol] K-f approximate to 3 (mg(1-1/n) L1/n g-1). The preferential sorption of hydrophobic cations was also visible during leaching experiments and resembles the behavior of cationic surfactants. [Glyph] sorption was higher in organic-rich agricultural soil (approx. 45 %) compared to reference OECD soil (30 %), and the K-f values were constant, regardless of the presence of cations. The lack of ionic pair integrity was also visualized by the toxicity and phytotoxicity results. In the soil environment, ILs are a mixture of independent cations and anions rather than a novel emerging contaminant. (c) 2022 Elsevier B.V. All rights reserved.