Fast selective homogeneous extraction of UO₂²⁺ with carboxyl-functionalised task-specific ionic liquids

Area of Science:

• Materials Science
• Nuclear Chemistry
• Separation Science

Background:

• Ionic liquids are emerging as versatile solvents for various chemical applications.
• Efficient extraction and separation of uranyl ions (UO₂²⁺) are crucial in nuclear fuel reprocessing and waste management.
• Task-specific ionic liquids offer tailored properties for targeted chemical processes.

Purpose of the Study:

• To investigate the efficacy of a carboxyl-functionalised ionic liquid, 1-carboxymethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide ([HOOCmim][NTf₂]), as a solvent and extractant for UO₂²⁺.
• To evaluate the extraction efficiency, selectivity, and stripping performance of the [HOOCmim][NTf₂]-H₂O system for UO₂²⁺.
• To explore the underlying mechanisms of selectivity using experimental and theoretical approaches.

Main Methods:

• Homogeneous phase formation of the [HOOCmim][NTf₂]-H₂O system at elevated temperatures.
• Rapid extraction of UO₂²⁺ via mechanical vibration.
• Stripping of UO₂²⁺ using nitric acid solutions.
• Investigation of interference effects from various metal ions (K⁺, Na⁺, Mg²⁺, Dy³⁺, La³⁺, Eu³⁺).
• Density functional theory (DFT) calculations to understand selectivity.

Main Results:

• The [HOOCmim][NTf₂]-H₂O system achieved a homogeneous phase at 75°C.
• Over 86% UO₂²⁺ extraction was accomplished in just 1 minute of vibration.
• Stripping efficiency of UO₂²⁺ from the ionic liquid phase reached nearly 97% using 1 M HNO₃.
• The extraction process showed high tolerance to common metal ions, with UO₂²⁺ extraction efficiency remaining around 80% in their presence.
• DFT calculations revealed the significant role of solvent effects in the system's selectivity.

Conclusions:

• The carboxyl-functionalised ionic liquid [HOOCmim][NTf₂] is a highly effective and rapid extractant for UO₂²⁺.
• The developed system demonstrates excellent selectivity and stripping capabilities, suitable for nuclear applications.
• The study provides fundamental insights into the solvent effects governing the selectivity of ionic liquid-based extraction systems.