Polyphenol engineered membranes with dually charged sandwich structure for low-pressure molecular separation

A dual-charged composite membrane was engineered for efficient separation of both cationic and anionic molecules. Alternately charged functional layer with sandwich structure was assembled onto porous membrane via chemical-mediated polyphenol surface engineering. The surface morphology, element variation and surface charge properties of the functional layer were respectively analysed using SEM, FTIR and streaming zeta potential measurements. The as-prepared “sandwich”-type composite membrane showed preferable rejection to multiple charged dyes (99.6% Alcian blue, 99.3% Brilliant blue, 92.21% Methyl blue, 89.91% Rhodanile blue). Both Donnan repulsion by dual-charged functional layer and entrapment effect by micro electric fields contributed to the outstanding separation performance. Compared with generally needed high pressure of 4–8 bar for the current nanofiltration separation, the developed composite membrane achieved desirable rejection under a low operating pressure of 1.5 bar, as well as favorable recycling performance for long-term use. This study provides an energy-efficient membrane candidate for comprehensive separation of charged molecules in practical applications such as dye wastewater treatment.