Advanced oxidation processes for the removal of [bmim][Sal] third generation ionic liquids: effect of water matrices and intermediates identification

Abstract

Unique properties of ionic liquids make them green alternatives for conventional volatile organic compounds. Due to increased production and the high stability of these substances, they could be classified as persistent pollutants and could break through classical treatment systems into natural waters. A preliminary ionic liquid hydrolysis study demonstrated a pH dependent degradation profile with a significant decrease in hydrolysis efficiency as pH lowered from 10.0 to 2.8. In order to examine future prospects for ionic liquid removal, different advanced oxidation processes (TiO₂ Degussa P25/H₂O₂, TiO₂ Degussa P25, 7.2Fe/TiO₂/H₂O₂, and H₂O₂) were studied for their applicability in the degradation of imidazolium-based ionic liquids in aqueous solution. These processes were conducted in the dark as well as in the presence of UVA and simulated sunlight (SS) radiation. Among the investigated dark processes, the 7.2Fe/TiO₂/H₂O₂ system showed the highest efficiency, which can be attributed to a dark heterogeneous Fenton process. Otherwise, the most efficient among all the studied degradation processes was the UVA/TiO₂ Degussa P25/H₂O₂ process. In order to make degradation processes more similar to that of the practical process SS radiation was used. Among studied processes, the 7.2Fe/TiO₂/H₂O₂ system showed the greatest potential for the removal of ionic liquids. Also, it was observed that the impact of anions on the cation degradation efficiency was much more pronounced. Due to the possible fate of ionic liquids in the environment, for five different waters (pond, rain, tap, river, and condensate) degradations in the dark and under simulated sunlight were studied. For all processes, and all water types in the presence of SS radiation a remarkable positive effect of naturally dissolved organic matter on the degradation efficiency was observed. Also, in all experiments, the anion was less stable than the cation. The major photodegradation products identified using liquid chromatography-mass spectrometry (HPLC-MS/MS) techniques were hydroxylated compounds.