Understanding Selectivity in Solute-Solute Separation: Definitions, Measurements, and Comparability.

Area of Science:

• Membrane science and technology
• Separation processes
• Chemical engineering

Background:

• Precise solute-solute separation using membranes is crucial but lacks standardized evaluation methods.
• Existing membrane selectivity assessments are often incomparable across different techniques.

Purpose of the Study:

• To investigate and compare solute-solute selectivity of a nanofiltration membrane using diverse driving forces.
• To identify reasons for the incomparability of selectivity measurements obtained through different methods.

Main Methods:

• Utilized three membrane processes: pressure-driven filtration, concentration difference-driven diffusion, and electric field-driven ion migration.
• Characterized selectivity using both apparent and intrinsic definitions under varying conditions.
• Compared selectivity measurements from single-salt and mixed-salt feed solutions.

Main Results:

• Selectivity values were generally not comparable across different methods and even different conditions within the same method.
• Incomparability was observed for both apparent and intrinsic selectivity metrics.
• Differences in selectivity were attributed to variations in driving forces, water transport effects, and ion interactions.

Conclusions:

• Current methods for evaluating membrane selectivity are not standardized, leading to incomparable results.
• A need exists for a consistent protocol with standardized testing conditions for fair performance comparison.
• Standardized protocols are essential for advancing membrane technology and reliable performance assessment.