Biocatalytic and salt selective multilayer polyelectrolyte nanofiltration membrane

We used layer-by-layer (LbL) self-assembly to fabricate a polyelectrolyte (PE) nanofiltration membrane for salt rejection and to immobilize trypsin on the membrane outer layer for biocatalytic activity. 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. 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) was systematically investigated. A maximum rejection of 12.7%, 45.2%, 85.5%, 94.0%, and 100% of MgCl2, NaCl, MgSO4, Na2SO4, and BSA, respectively, was obtained by the PDADMAC-PSS membrane with four bilayers. Trypsin (TRY) was immobilized on the membrane surface by electrostatic attraction or covalent bonding to produce a biocatalytic membrane and to alleviate protein fouling. 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 enzyme activity in the presence of salt and BSA. BSA was used as a model protein for organic fouling experiments, and flux decline rate of the membranes was determined. Our results show that LbL-modified membranes with covalent enzyme immobilization had the lowest protein fouling rate, which we attribute to the biocatalytic activity of the immobilized trypsin.