Reaction rates and product formation during advanced oxidation of ionic liquid cations by UV/peroxide, UV/persulfate, and UV/chlorine

Abstract

Ionic liquids (ILs) are expected to be used increasingly in the coming years for industrial chemical applications as replacements for volatile organic solvents. The organic cations used in ILs typically contain quaternary ammonium groups and may reach aquatic environments due to their high water solubility and limited biodegradability. Given the persistence of IL cations in aquatic environments and the potential of quaternary ammonium compounds to form nitrosamines, the potential of advanced oxidation processes with UV irradiation (UV/AOP) to remove IL cations from drinking water sources was assessed. We found that IL cations react readily with hydroxyl and sulfate radicals with bimolecular reactions rate constants ranging from (1.2 ± 0.6) × 10⁹ to (8.5 ± 1.0) × 10⁹ M⁻¹ s⁻¹ and from (0.08 ± 0.06) × 10⁹ to (1.7 ± 0.2) × 10⁹ M⁻¹ s⁻¹, respectively. Consequently, half-lives in the order of minutes are expected for all IL cations in UV/AOP applications with hydrogen peroxide, persulfate, or free chlorine. In addition to efficient removal of the parent compounds, most transformation products of IL cations are formed through sequential hydroxylation reactions, which could ultimately lead to small, benign end-products. While we did not find evidence supporting the direct reaction of IL cations with reactive halogen species, a series of chlorinated transformation products were identified in UV/chlorine experiments. We hypothesize that these compounds are formed through the reactions of radical intermediates with free chlorine.