Design of ultrathin nanostructured polyelectrolyte-based membranes with high perchlorate rejection and high permeability

• Perchlorate rejection enhanced by surface modification via LbL assembly technique.
• Key parameters varied to optimize the permeability and rejection.
• Optimized membrane had much higher permeability and equivalent rejection than RO.
• Deposited nanothin film was characterized in terms of thickness and surface charge.

The presence of perchlorate in drinking water sources is an issue of overwhelming concern in United States. Commercial reverse osmosis (RO) membranes show high rejection of perchlorate ions but with very low water permeability. We propose the modification of commercial nanofiltration (NF) membranes by layer-by-layer (LbL) assembly in order to enhance the ion rejection of these membranes to the same level as commercial RO membranes, but with much higher permeability. Poly allylamine hydrochloride (PAH) and poly acrylic acid (PAA) were the two polyelectrolytes used for surface modification. We found that, when both these polyelectrolytes were deposited on a NF 90 membrane at a pH of 6.5 and crosslinked with glutaraldehyde, we were able to achieve around 93% perchlorate rejection at a pressure of 5 bar with 10 ppm feed concentration. This was almost equal to the rejection offered by SW 30 membrane and higher than a BW 30 membrane under the same conditions of pressure and feed concentration. Most importantly, the modified membranes had 1.5 times the permeability of BW 30 membrane and 6 times that of SW 30. These membranes, therefore, had much superior permselectivity than the commercial membranes. In fact, this was one of the highest values of permselectivity reported so far for a PEM-based RO membrane targeting monovalent ion removal. Only 3 bilayers, with an overall thickness of just 20 Å, were sufficient to achieve such a high rejection. The mechanism of ion rejection by these modified membranes was based on size-based exclusion rather than charge-based separation.

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