A comparison study: The different impacts of sodium hypochlorite on PVDF and PSF ultrafiltration (UF) membranes

Chemical ageing of ultrafiltration membranes related to drinking water operation is not a new problem, but literature documenting sieving properties and fouling behaviors of aged membranes are scarce. This paper shows how the surface chemistry evolution of polysulfone (PSF) and polyvinylidene fluoride (PVDF) membranes can lead to the formation of distinct membrane properties when exposed to sodium hypochlorite solutions. The surface physicochemical evolutions of aged membranes are characterized based on chemical composition (ATR-FTIR and XPS), water contact angle analysis, and surface zeta potential test. The results indicate the ageing mechanism and evolutions between PSF and PVDF membranes are totally different. In addition, the retention ability and fouling propensity of the aged membrane are also investigated by three different kinds of foulant (Dextran/HA/BSA). Surface hydrophilization of aged PVDF membrane enhanced its retention toward neutral (Dextran) and hydrophilic charged foulant (BSA), but the fouling type of aged PVDF membranes remained the same, in spite of the initial flux increasing greatly. In terms of PSF membranes, pore enlargement could be verified by a seemingly inconsistent result of dextran sieving experiments and the decreased HA rejection rate. Moreover, the increased negative charge on aged PSF membrane surface have a great impact on its sieving property and fouling propensity, especially for the charged and hydrophilic BSA. With the regression data of Hermia's model, the main fouling type of aged PSF membranes was proven changed to pore blocking, and the changes can ascribe to its altered surface chemical characteristic and pore diameter.