Photochemical transformation of pyridinium ionic liquids in aqueous phase: Kinetics, products and mechanism

Abstract

Pyridinium ionic liquids (Pyr+ ILs) with promising applications are likely to appear in large quantity in water environment and become emergent pollutants due to their high hydrophilicity and low biodegradability. Hence, we studied the indirect photochemical degradation of Pyr+ ILs, including their kinetics of reactions with transient oxidants (•OH, CO3•–, triplet stated sodium anthraquinone-2-sulfonate (3AQ2S*) and 1O2), modeling studies on contribution of the transient oxidants to their degradation and their half-lives, as well as mechanistic study and toxicity estimation of the ILs and their products. Half-lives of these Pyr+ ILs were estimated to be 5–600 days under different scenarios, indicating persistence of the Pyr+ ILs in natural waters. Moreover, major contributors for degradation of the ILs are triplet stated dissolved organic matter (3DOM*) and CO3•―. For reaction with •OH, products identified by secondary mass spectrometry (MS2) and hydrogen nuclear magnetic resonance (1H NMR) indicate that the reactions proceed via hydrogen-atom transfer at the initial stage, and further transformation of the alkyl radicals lead to products by dehydrogenation, hydroxylation, and further oxidation. Additionally, model study shows that 1-butyl-Pyr+ and 1-butyl-3-methyl-Pyr+ IL are toxic to the aquatic organisms, and their products possess even higher toxicity, indicating that the Pyr+ ILs in natural waters may pose threat to aquatic organisms. Potential eco-risk of Pyr+ ILs should be fully considered before large-scale industrial application.