Absorption mechanism and thermodynamics of SO2 using pH-buffered ionic liquid aqueous solution

The emission of sulfur dioxide (SO2), a major atmospheric pollutant, has caused wide concern in recent years. Wet flue gas desulfurization is used to remove SO2 from combustion exhaust, but this process produces numerous secondary pollutants. Ionic liquids (ILs) are green organic solvents that can selectively absorb acidic gases. Triethanolamine citrate ([TEOA][CA]) is an IL that contains O and N electron-rich sites with a high binding to SO2; however, the high viscosity of ILs limits their industrial applications. An aqueous solution of [TEOA][CA] exhibits a low viscosity at 25 °C and good pH-buffering properties, which favors SO2 absorption. This solution reached SO2 absorption equilibrium in 50 min, with molar absorption values of 7.64 and 1.75 mol SO2/mol ILs in 10 wt% and 50 wt% [TEOA][CA] aqueous solutions at 25℃ and 2 kPa SO2 partial pressure. High molar absorption attributed to the direct chemical absorption of [TEOA][CA] with SO2 in water. This chemical reaction formed multiple bonds, involving –O–(S=O)–OH, O–S(SO2), H–O(SO2), and N–O(SO2). The thermodynamic parameters were calculated and indicated that SO2 absorption was an exothermic, orderly, and spontaneous process with an absorption enthalpy of − 44.20 kJ mol−1 in 50 wt% [TEOA][CA] aqueous solution at 25℃. [TEOA][CA] aqueous solution accelerated the mass transfer processing and exhibited high absorption capacity of SO2.