Fouling of reverse osmosis membrane by protein (BSA): Effects of pH, calcium, magnesium, ionic strength and temperature

Protein fouling using bovine serum albumin (BSA) in reverse osmosis (RO) under various chemical (pH, cation species and ionic strength) and physical (temperature) conditions was investigated using crossflow RO filtration setup. The results showed that BSA was readily adsorbed onto the RO membrane over the range of pH tested. Interactions between BSA molecules became the dominant mechanism for subsequent BSA fouling. It was observed that the most severe BSA fouling occurred at pH near to the iso-electric point (IEP) of BSA, where electrostatic repulsion between BSA molecules was the weakest. Increase in the ionic strength enhanced the double layer compression and charge-shielding effect that led to weaker electrostatic repulsion, and hence more rapid fouling. Fouling was observed to be more severe in the presence of calcium ions compared to monovalent ions due to the extra positive charge and its ability to form ionic bridges that resulted in denser fouling layer. In the presence of magnesium, fouling of RO membrane by BSA was not as severe as the ones observed with calcium. In addition, fouling by BSA did not seem to be affected by the change in pH in the presence of magnesium. Experiments also demonstrated that higher temperature could increase the rate of BSA fouling at tested pHs above the IEP of BSA by reducing the zeta potential on the membrane surface. This was attributed to the unfolding of the compact BSA molecule that exposed the charged functional groups to the aqueous solution for charge shielding.