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To determine further the enhancement and mitigation mechanisms of protein fouling, filtration experiments were carried out with polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes and bovine serum albumin (BSA) over a range of ionic strengths. The interaction forces, the adsorption behavior of BSA on the membrane surface, and the structure of the BSA adsorbed layers at corresponding ionic strengths were investigated. Results indicate that when the ionic strength increased from 0 to 1 mM, there was a decrease in the PVDF-BSA and BSA-BSA electrostatic repulsion forces, resulting in a higher deposition rate of BSA onto the membrane surface, and the formation of a denser BSA layer; consequently, membrane fouling was enhanced. However, at ionic strengths of 10 and 100 mM, membrane fouling and the BSA removal rate decreased significantly. This was mainly due to the increased hydration repulsion forces, which caused a decrease in the PVDF-BSA and BSA-BSA interaction forces accompanied by a decreased hydrodynamic radius and increased diffusion coefficient of BSA. Consequently, BSA passed more easily through the membrane and into permeate. There was less accumulation of BSA on the membrane surface. A more nonrigid and open structure BSA layer was formed on the membrane surface.

To further unravel the organic fouling behavior of polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes, the adhesion forces of membrane-foulant and foulant-foulant were investigated by atomic force microscopy (AFM) in conjunction with self-made PVDF colloidal probe and foulant-coated colloidal probe, respectively. Fouling experiments with bovine serum albumin, sodium alginate, humic acid, and secondary wastewater effluent organic matter (EfOM) were carried out with PVDF UF membrane. Results showed a positive correlation between the membrane-foulant adhesion force and the flux decline rate and extent in the initial filtration stage, whereas the foulant-foulant interaction force was closely related to the pseudostable flux and the cake layer structure in the later filtration stage. For each type of foulant used, the membrane-foulant adhesion force was much stronger than the foulant-foulant interaction force, and membrane flux decline mainly occurred in the earlier filtration stage indicating that elimination of the membrane-foulant interaction force is important for the control of membrane fouling. Upon considering the foulant-foulant interaction force and the membrane flux recovery rate of fouled membranes, it was evident that the main contributor to physically irreversible fouling is the foulant-foulant interaction force.