Modification of Ultrafiltration Membranes Based on Polyacrylonitrile

Three methods for modification of polyacrylonitrile (PAN) ultrafiltration membranes with polyelectrolytes are considered: (1) bulk modification by introducing polyacrylic acid (PAA) into the casting solution, (2) surface modification by using aqueous solutions of polyethyleneimine (PEI) as a coagulation bath, and (3) a combination of methods 1 and 2. In all three cases, modification of membranes with polyelectrolytes leads to effective hydrophilization of the surface of ultrafiltration membranes (the contact angle decreases from 41° to 15°–25°). It has been found that the bulk modification of PAN membranes by introducing 0.05–0.2 wt % PAA into the casting solution leads to a decrease in the pure water flux from 110 to 96 L/m² h. The maximum polyvinylpyrrolidone K30 rejection coefficient of 96% was observed at a PAA concentration of 0.05 wt %; with a subsequent increase in the PAA content, the rejection coefficient decreases to 70–73%. Surface modification of PAN membranes with polyethyleneimine leads to a more than twofold increase in their water flux (up to 233–294 L/m² h), while the rejection coefficient for polyvinylpyrrolidone K30 was 82–96% depending on the PEI concentration in the coagulation bath. It is shown that the combined modification method reduces the water flux to 44 L/m² h, which is associated with the formation of a polyelectrolyte complex and compaction of the membrane structure. It has been found that the combined modification method allows obtaining ultrafiltration PAN membranes with a high flux recovery ratio after filtration of model solutions of polyvinylpyrrolidone (73–100% compared to 65% for the unmodified membrane) and humic acids (80% compared to 73% for the unmodified membrane).