Transport of imidazolium-based ionic liquids with different anion/cation species in sand/soil columns

Ionic liquids (ILs) have been widely used as environmentally friendly solvents to replace volatile organic solvents in the chemistry industries. They have a high water solubility and potential risk to organisms in the soil-water environment. At present, most studies focused on the batch sorption of ILs in soil and neglected the investigation of IL transports in soil, which results in a lack of understanding of the structure-dependent mobility of ILs in the environment. Laboratory-scale sand/soil column experiments were performed to study the transport of imidazolium-based ILs, such as [C₄mim][OTF], [C₄mim][TOS], [C₄mim][MeSO₃], [C₄mim][BF₄], [C₂mim][BF₄], and [C₆mim][BF₄] including different counteranions and alkyl chain lengths of IL cations. Batch experiments were also carried out to compare the difference of sorption distribution coefficient (K_d) between the batch and column experiments. A one-dimensional convective-dispersive model using CXTFIT code was created based on the measured breakthrough curves (BTCs) to estimate the column transport parameters. For the anion, [BF₄^-], the K_d of ILs in both batch and column experiments increased with increasing alkyl chain lengths of the IL cation. In batch tests, counteranions showed no influence on the K_d of [C₄mim][OTF], [C₄mim][TOS], [C₄mim][MeSO₃], [C₄mim][BF₄], [C₂mim][BF₄], and [C₆mim][BF₄]. However, in column tests, the BTCs of 1-butyl-3-methylimidazolium-based ILs were anion dependent as evidenced by the change of retardation factor (R) for different counteranions. Furthermore, the effects of transport distance (11cm, 15cm, 19cm, and 24cm) on the mobility of ILs were estimated. The longer distances signified an increase in the contact time and more binding sites for ILs and therefore, the smoother shapes of BTCs in column experiments.