Abstract: Ionic liquids (ILs) have been reported to have an enhanced absorption capacity for SO₂ compared to that of traditional molecular solvents. This work evaluated the effect of water content in a 1-butyl-3-methyl imidazolium chloride [bmim][Cl] solution on the absorption of a diluted gas stream under dynamic conditions in a packed-bed column and analyzing chemical changes by FT-IR spectroscopy. For aqueous IL solutions, the absorption behavior shown a first fast-chemical reaction related to the SO_2–sulfite–bisulfite water equilibrium followed by physical absorption. The whole process is not limited by mass transfer. Conversely, in low water-content IL, the SO₂ interacts directly with the cation and absorption capacity is highly improved. In this case, mass transport limitations were detected as a consequence of high viscosity. For diluted IL solutions, only partial isothermal desorption could be reached by changing the solution pH. No desorption is obtained for highly concentrated solutions. The water content has a positive effect in reducing mass transfer limitation; however, changes the mechanism of absorption and reduce the capture capacity. The chemistry involved in aqueous IL capture of SO₂ seems to be analogous to amine-based technology; however, the non-volatility and thermal/chemical stability of the [bmim][Cl] allow minimize the generation of atmospheric and liquid phase pollutants during the absorption operation.

Keywords: 1-Butyl-3-methyl imidazolium chloride, bisulfite, desorption, gas absorption, ionic liquids, sulfur dioxide