Antimony- and Bismuth-Based Ionic Liquids as Efficient Adsorbents for the Removal of Dyes

A series of ionic liquids consisting of anilinium cations with varying alkyl chains and metallic (Sb and Bi) halides as anions have been synthesized and thoroughly characterized by using multinuclear (1H and 13C) NMR, FT-IR, Raman and XPS techniques. They have been exploited as adsorbents for the dye’s removal, such as malachite green, rhodamine B and Sudan II, from the aqueous solution. Various parameters like the effect of stirring rate, pH, reaction time, adsorbent amount and initial dye concentration have been optimized. Both antimony- and bismuth-based ionic liquids exhibit high adsorption efficiencies and have comparable performance for each dye. Kinetic data have been analyzed by applying kinetic models, and the best-fitted model was found to be pseudo-second order with an R2 value greater than 0.98. Adsorption capacity has been determined by analyzing the sorption data using the Langmuir and Freundlich equations, and the Langmuir isotherm model has been found to be the best fitting. The maximum adsorption capacities (qmax) derived from the Langmuir isotherm for malachite green, Sudan II and rhodamine B by M-Sb ILs were 217.36, 162.10 and 62.94 mg·g−1, whereas by M-Bi ILs, the adsorption capacities were slightly higher, at 230.18, 170.00 and 64.21 mg·g−1, respectively. Kinetic studies indicated pseudo-second-order behavior (R2 > 0.98), while thermodynamic analysis demonstrated an endothermic adsorption, and a spontaneous reaction was carried out by a physisorption process. These findings accentuate the potential of Sb- and Bi-based ionic liquids as efficient and reusable adsorbents for removing dyes from wastewater.