Precise tuning of salt retention of backwashable polyelectrolyte multilayer hollow fiber nanofiltration membranes

•Constant flux deposition of layer-by-layer polyelectrolytes turn UF hollow fiber membranes into backwashable NF membranes.•Permeated volume and rejected polyelectrolyte mass precisely determine selectivity and flux.•Method can be applied on already existing hollow fiber membrane modules as a post-treatment.

We report a new dynamic Layer-by-Layer (LbL) fabrication method to molecularly engineer the layer build-up within the lumen of hollow fiber membrane involving constant flux permeation rather than the conventional static adsorption method. During the constant flux coating, the polyelectrolyte concentration at the membrane surface rises continuously due to its rejection causing a convective adsorption process of the polyelectrolyte onto the membrane. The retained polyelectrolyte amount correlates with the flux and retention properties of the resulting polyelectrolyte multilayer membranes irrespective of time span or flux level used for the convection step. This fabrication method enables the synthesis of new desalination membranes with unprecedented precision on ion transport properties. Furthermore the resulting hollow fiber nanofiltration membrane is stable towards backwashing. The method can be applied to already existing membrane modules as a finishing step or as an in-operando inline regeneration or repair step.

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