Insight into the role of ion-pairing in the adsorption of imidazolium derivative-based ionic liquids by activated carbon

Mengping Liu; Ling Zhu; Xiaoxian Zhang; Wenhui Han; Yuping Qiu

doi:10.1016/j.scitotenv.2020.140644

Insight into the role of ion-pairing in the adsorption of imidazolium derivative-based ionic liquids by activated carbon

Sci Total Environ. 2020 Nov 15:743:140644. doi: 10.1016/j.scitotenv.2020.140644. Epub 2020 Jul 4.

Authors

Mengping Liu¹, Ling Zhu¹, Xiaoxian Zhang¹, Wenhui Han¹, Yuping Qiu²

Affiliations

¹ College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China; Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China.
² College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China; Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China. Electronic address: ypqiu@tongji.edu.cn.

PMID: 32653710
DOI: 10.1016/j.scitotenv.2020.140644

Abstract

The association of the cation and anion of ionic liquids (ILs) dominates the absorbability of ILs by activated carbon (AC). Nevertheless, the mechanism behind the role of ion-pairs is largely unknown. In this study, the adsorption of a series of imidazolium derivative-based ILs by AC was involved in response to the octanol-water partition coefficient (K_OW), hydrogen bonding acidity (α), ion-pair binding constants (K_IP), binding energy of ion-pairs (E_binding) and density functional theory (DFT) calculation of ILs. A significant positive correlation between lg K_OW and K_d and between K_IP and lg K_OW was observed (p < 0.05). However, both E_binding and α was inversely proportional to K_IP. Hence, the substitution of oxygen-containing functional groups, such as carboxyethyl, 1-methoxyethyl, and 1-(ethoxycarbonyl)methyl, on imidazolium ring enhanced the hydrogen bond interaction with water molecules and then weakened the binding of imidazolium cation and [NTf₂]^-, thereby reducing the adsorption of ILs to AC. DFT calculation further revealed that the polar substitution improved the electron density and electronegativity of imidazolium skeleton. By contrast, the ILs functionalized with non-polar groups (e.g., butyl, allyl, and benzyl) generally displayed high K_IP values and low α values. Consequently, the formation of hydrogen bond between the oxygen-containing functional groups of IL cation and water would facilitate the dissociation of IL ion-pairs and then decrease the adsorption of ILs on AC.

Keywords: Activated carbon; Adsorption; Hydrogen bonding; Ion-pairs; Ionic liquids.