The influence of ion identity and ionic strength on membrane biofouling of a binary protein solution

Abstract Studying biofouling is important for increased implementation of membrane-based water purification technologies. A binary protein solution (bovine serum albumin (BSA) and hemoglobin (Hb)) was used to study biofouling of hydrophilic and hydrophobic PVDF membranes. By fitting normalized plots of flux vs time to an exponential decay, the rate of fouling and long-term steady-state flux were compared in the presence of different ions and at different ionic strengths. The presence of ions causes a lower steady-state flux but rate effects are ion identity dependent. Equally important is membrane type. For the hydrophobic membrane, the charge of the cation dictates different mechanisms affecting the rate or steady-state flux. With the hydrophilic membrane, the presence of cations results in a direct correlation between the rate of fouling and steady-state flux while the presence of different anions leads to no apparent trends. Thus, the Hofmeister series does not explain the influence of ions on the fouling of the binary protein solution. Finally, studies at higher ionic strengths (150–600 mM) show that fouling rates decrease likely due to charge shielding. The ionic strength where this effect is apparent depends on ion identity. The limited flux model supports these conclusions, though membrane type is important. Therefore, while protein-protein interactions are crucial, ion-membrane interactions cannot be ignored either. Conclusions about the role of ions and ionic strength on fouling must be made with care as multiple factors contribute to the rate and extent of fouling of a binary protein solution.