Effect of cationic alkyl chain lengths on the adsorption performance of supported imidazole-based ionic liquids for vanadium(V) recovery

Imidazole-based ionic liquids (ILs) with various cationic alkyl chain lengths were immobilized on polystyrene resins (PS[Cnmim][Cl], n = 2, 4, 6, 8, 12) and applied to separate vanadium(V) from vanadium-bearing solutions. The effects of alkyl chain lengths on the adsorption capacity, selectivity and stability of the supported ILs were thoroughly investigated and compared. As the alkyl chain length increases, the adsorption rate of the supported ILs decreases, leading to a longer adsorption equilibrium time. The alkyl chain length hardly affects the adsorption selectivity of vanadium over main impurities. However, the supported ILs with longer alkyl chain length exhibit greater cyclic stability. Among the synthesized five kinds supported ILs, PS[C6mim] [Cl] presents excellent adsorption performance toward vanadium(V). The adsorption capacity reaches 248.25 mg/g, which keeps 94.21 % after 15 adsorption-desorption cycles. Moreover, the separation coefficient of PS[C6mim][Cl] for V/Fe and V/Al reaches 24.22 and 7.7, severally. Vanadium(V) is mainly adsorbed onto the supported ILs by chemisorption as monolayer, the adsorption mechanism of which is barely affected by the alkyl chain lengths. The adsorption of vanadium is an endothermic process with increasing entropy, and elevated temperature is conducive to adsorption process. In addition, microscopic characterizations show that the structure of the supported ILs is stable, and the ILs are firmly loaded during multiple cycles. This study provides a reference with the design of ILs cationic alkyl chain for supported imidazolebased ILs in the field of vanadium separation and enrichment.