Harnessing the potential of thin film composite membranes for efficient treatment of aqueous streams containing polar aprotic organic solvents

Designing high-permeance nanofiltration (NF) and reverse osmosis (RO) membranes without compromising their selectivity toward ions and small organic molecules is needed for the efficient separation of ions and molecules. Thin-film composite membranes derived from polyamide and used for water desalination have their limitation for organic solvent filtration because of the instability of the support membranes which serve as a foundation for the polyamide separation layer. Although pure organic solvent filtration demands designing new membranes, the aqueous mixture of organic solvents can be treated with these membranes, where the support membrane remains stable. This could open a new application area of the existing NF and RO membranes and expand their applications in the separation industry. Herein, we report the effect of solvent treatment on the desalination and molecular separation performance of a few commercial polyamide NF and RO membranes and their applications in aqueous mixtures of organic solvents containing up to 70% (v/v) of polar aprotic solvents. Upon solvent treatment, no significant reduction in the mean pore radius (0.72-0.69 nm) and the molecular weight cutoff was observed, while up to similar to 100% enhancement in water permeance was achieved without loss in the rejection of Na2SO4 (similar to 99.3%) for NF and NaCl (similar to 98%) for BWRO membranes. Molecular separation in aqueous mixtures of polar aprotic organic solvents signifies the utility of these membranes for the separation of small organics with a molecular weight of < 300 Da. The findings from an X-ray photoelectron spectroscopic study indicate that upon solvent treatment the chemical modification of the polyamide layer in the membranes is highly unlikely. Instead, the observed performance enhancement was coined to the physical restructuring of the polyamide layer and modifications at the interlayer. We expect such findings would find several new applications of the NF and RO membranes including desalination, wastewater treatment, and solvent recycling.

Automated summary

Designing high-permeance nanofiltration (NF) and reverse osmosis (RO) membranes without compromising selectivity is important for efficient separation. Existing membranes have limitations for organic solvent filtration, but solvent treatment can enhance desalination and molecular separation without compromising rejection. This expands the applications of NF and RO membranes in aqueous mixtures of organic solvents.