Single‐step polyelectrolyte complex coating on hollow fibers yields nanofiltration or biocatalytic properties

The modification of membranes with polyelectrolytes via the Layer-by-Layer (LBL) method has become state of the art in recent years. It is used to fabricate nanofiltration hollow fiber membranes or to immobilize biomolecules on a membrane surface. However, it still remains a time consuming process. In contrast, this work explores a single-step membrane modification with coating solutions containing both polyanions and polycations. High salt concentration in the coating solution suppresses the complexation of the polyelectrolytes prior to the coating. Then, the controlled reduction of the salt concentration during the coating triggers the formation of a polyelectrolyte complex layer on the membrane. Three coating methods are proposed: (1) In interfacial complexation (IC), the polyelectrolyte solution coats the membrane and is subsequently precipitated by flushing with water. (2) Diffusive desalination (DDS) uses the concentration difference between the coating solution in the lumen and a water stream in the shell side to remove salt ions continuously. (3) In polyelectrolyte concentration (PC), the polyelectrolyte solution is coated at a constant flux. Here, the membrane retains the polyelectrolyte while ions permeate through. First, we evaluate the coating methods regarding their ability to produce nanofiltration membranes, which varies depending on the coating method used. With PC, membranes with up to 79% MgCl₂ rejection and a permeability of 30 LMH/bar are obtained. Moreover, in-situ functionalization of the membranes is investigated by the addition of enzymes. Here, with DDS enzymes are immobilized, mostly achieved through adsorption via electrostatic interactions.