Photochemical Transformation of Four Ionic Liquid Cation Structures in Aqueous Solution

Area of Science:

• Environmental Chemistry
• Photochemistry
• Green Chemistry

Background:

• Ionic liquids (ILs) are emerging solvents with increasing industrial use.
• IL cations may enter aquatic environments due to slow biodegradation and limited sorption.
• Understanding ILs' environmental fate is crucial for assessing their ecological impact.

Purpose of the Study:

• To determine the photochemical transformation rates of common IL cations in sunlit surface water.
• To elucidate the role of direct and indirect photodegradation pathways.
• To assess the potential persistence of IL cations in aquatic ecosystems.

Main Methods:

• Investigated photodegradation of imidazolium, pyridinium, pyrrolidinium, and piperidinium cations.
• Compared degradation rates in ultrapure water versus solutions with dissolved organic matter (DOM).
• Identified dominant transformation mechanisms using model sensitizers and DOM isolates.

Main Results:

• Photodegradation was significantly faster in the presence of DOM, indicating the importance of indirect processes.
• Hydroxyl radical reactions were identified as the primary transformation pathway for IL cations.
• Bimolecular rate constants with hydroxyl radicals increased with IL cation carbon side-chain length.
• Estimated half-lives in sunlit surface water ranged from 32 to 135 days.

Conclusions:

• IL cations undergo significant photochemical transformation in surface waters, primarily through reactions with hydroxyl radicals.
• Dissolved organic matter enhances IL cation photodegradation.
• The estimated half-lives suggest IL cations have the potential to persist in aquatic environments.