Removal of hydrophilic ionic liquids from aqueous solutions by adsorption onto high surface area oxygenated carbonaceous material

A carbonaceous material having a large surface area and high number of oxygenated groups was prepared by hydrothermal carbonization of cellulose and chemical activation with KOH with the goal to develop an adsorbent for hydrophilic ionic liquids. The as-prepared carbonaceous material (a-CM) was used to study the adsorption of representative imidazolium-based hydrophilic ionic liquids and exhibited adsorption capacity that was almost three times higher than that of commercial activated carbon (CAC) adsorbent. The large adsorption capacity of a-CM adsorbent for hydrophilic ionic liquids is attributed to its polar oxygenated surface groups (C-OH, CO and -COOH). The effect of ionic liquid chemical structure on a-CM adsorption was investigated in which it was found that adsorption capacity increased with increasing hydrophobicity of the anions and increasing alkyl chain length of the imidazolium cation. Hydrophilic ionic liquids were favorably adsorbed onto the highly-oxygenated a-CM adsorbent whereas hydrophobic ionic liquids were favorably adsorbed onto CAC adsorbent. The as-prepared adsorbent could be characterized by the point of zero charge (pH_PZC) derived from zeta potential. The results of this work demonstrate that ionic liquids can be efficiently removed from aqueous wastewater by tailoring adsorbent surface chemistry to the chemical characteristics of the ionic liquid.