Ionic-liquid-crafted zeolite for the removal of anionic dye methyl orange

Abstract

In this study a clinoptilolite zeolite was modified by an ionic liquid 1-hexadecyl-3-methylimidazolium chloride monohydrate to 1.0 and 2.0 times its external cation exchange capacity (ECEC). The ionic-liquid-crafted zeolite (ILCZ) was tested for its removal of anionic dyes from solution using methyl orange (MO) as an example. The uptake of MO was fast and followed a pseudo-second-order kinetics with MO adsorption capacities of 45 and 116 mmol/kg on ILCZ modified to 1.0 and 2.0 ECEC, respectively. Stoichiometric desorption of counterion chloride accompanying MO uptake suggested an anion exchange mechanism of MO uptake on ILCZ modified to 2.0 ECEC. Solution pH in the range of 2–11 and ionic strength in the range of 0.001–0.01 M of background salt concentrations of varying charges had minimal effects on MO uptake, suggesting higher affinity of MO for ILCZ in comparison to other inorganic anions. As the MO adsorption capacity on ILCZ modified to 2.0 ECEC was about the same as the ECEC of the zeolite, the mechanism of removal of anionic dyes was attributed to surface anion exchange resulted from the positively charged ILCZ surfaces due to a bilayer IL formation on zeolite after being modified to 2.0 ECEC. In contrast, patchy admicelles of IL would form on zeolite after being modified to 1.0 ECEC, resulting in reduced MO uptake. The contrasting behaviors of IL modification on zeolite and subsequent MO adsorption on ILCZ suggested that modification of ILCZ to 2.0 ECEC was necessary for ILCZ to serve as a sorbent material for the removal of anionic dyes from solution.