Assessment of physicochemical interactions in hollow fibre ultrafiltration membrane by contact angle analysis

One critical issue faced during membrane operation is the occurrence of fouling, which partially results from the foulant-membrane affinity, often broadly attributed to “hydrophobicity”. Not only the simple concept of hydrophobicity fails to accurately assess membrane performance, but it also leads to over-simplified assumptions on phenomena occurring at the membrane surface during filtration. Foulant deposition on membrane surface is generally controlled by short-range non-electrostatic interactions. The energy related to the interactions between foulant and membrane surface in water can easily be described by the various surface tension components of the different phases. However, the determination of these surface tension components has been, so far, difficult to assess. In this study, specific experimental conditions were used to better characterise the surface tensions of membrane and foulants through contact angle analysis. Polyvinylidene fluoride (PVDF) hollow fibre ultrafiltration (UF) membrane was used with sodium alginate (SA), bovine serum albumin (BSA) and humic acid (HA) as foulants, representing polysaccharides, proteins and humic substances respectively. The novel approach proposed in this paper was to characterise simultaneously the initial and the subsequent membrane and foulant interactions during constant flux filtration with contact angle analysis and calculation of surface free energy of adhesion and cohesion for various foulants. This was possible by considering the free energy of interactions of extended Derjaguin–Landau–Verwey–Overbeek (xDLVO) theory to predict and evaluate fouling mechanisms. Real wastewater samples were also used for fouling experiments, to assess the applicability of xDLVO theory for complex mixture of organics. This study demonstrated that the surface free energy components calculated for the range of wastewaters tested successfully predicted the organic deposition on membrane surface, as higher organic deposition was observed for high surface free energy surfaces.