Synthesis of anionic ionic liquids@TpBd-(SO3)2 for the selective adsorption of cationic dyes with superior capacity

Abstract

The discharge of industrial printing and dyeing wastewater is one of the main reasons for the increasing water shortage and deterioration. The treatment of dyestuff wastewater is an issue and needs to be urgently solved. In this work, anionic ionic liquid functional covalent organic materials (COMs) were firstly synthesized and used for the selective adsorption of cationic dyes. First, a series of sulfonic acid group (SO₃H)-functionalized anionic TpPa-SO₃, TpBd-(SO₃)₂, and TpCR-(SO₃)₂ were prepared, respectively, and then imidazole was grafted onto TpBd-(SO₃)₂ to obtain ImI@TpBd-(SO₃)₂. The full characterization using X-ray diffraction, FT-IR spectroscopy, ¹³C cross-polarization magic-angle spinning NMR spectroscopy, zeta-potentials, BET surface and pore analysis indicated that these COMs and ImI@TpBd-(SO₃)₂ exhibited different morphologies, porosities, and potentials. The effects of the type of dye, adsorption time, initial dye concentration, and pH on the adsorption of dyes on ImI@TpBd-(SO₃)₂ were systematically investigated, respectively. The results revealed that ImI@TpBd-(SO₃)₂ possessed good adsorption performance for nine different cationic dyes with adsorption capacities in the range from 2865.3 mg g⁻¹ for methylene blue (MB) to 597.9 mg g⁻¹ for basic orange 2 (BO), but little adsorption for anionic and neutral dyes, revealing charge selectivity. The adsorption ratio of ImI@TpBd-(SO₃)₂ for MB was as high as 74.0% at 10 min by using 1.0 mg material, owing to the post modification of TpBd-(SO₃)₂ with imidazole. The adsorption of MB on ImI@TpBd-(SO₃)₂ was pH dependent. The adsorption isotherm and kinetics fitted well with the Freundlich and pseudo second-order kinetic model, respectively. Finally, the very outstanding advantages of superior selective adsorption, desorption, convenient preparation, and low density of ImI@TpBd-(SO₃)₂ predicted its research and application potential in dye wastewater recovery.