Layer-By-Layer Assembled Biocatalytic and Salt Selective Nanofiltration Membrane

We used layer-by-layer (LbL) assembly to produce a polyelectrolytic (PE) nanofiltration membrane with salt rejection and antifouling capabilities. Poly(ethylene imine) (PEI) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) were used as cationic PE while poly(acrylic acid) (PAA) and poly(styrene sulfonate) (PSS) were used as anionic PE. We systematically investigated the impact of PE type, number of PE bilayers, and PE concentration on the rejection of inorganic salts (NaCl, MgCl₂, Na₂SO₄, and MgSO₄) and protein (bovine serum albumin, BSA). The PDADMAC-PSS membrane with four bilayers demonstrated the maximum rejections of MgCl₂, NaCl, MgSO₄, Na₂SO₄, and BSA, at 12.7%, 45.2%, 85.5%, 94.0%, and 100%, respectively. For reduction of organic fouling through enzymatic activity, we immobilized trypsin on the membrane outer layer by both electrostatic attraction and covalent bonding. Important parameters for enzymatic activity, such as immobilization time, pH, temperature, salt concentration and type, as well as the reuse number and storage time were investigated to expound the mechanism of enzymatic activity in the presence of salt and BSA. To investigate flux decline rates due to organic fouling, BSA was used as a model protein. Our results show that LbL-modified membranes with covalently bound trypsin had the lowest protein fouling rate.