Preparation and characterization of a neutrally charged antifouling nanofiltration membrane by coating a layer of sulfonated poly(ether ether ketone) on a positively charged nanofiltration membrane

In this study, a nearly neutrally charged NF membrane was developed by adsorption of a layer of negatively charged sulfonated poly(ether ether ketone) (SPEEK) onto the surface of a positively charged NF membrane. It was found that surface charge had a significant influence on multivalent ion rejection and fouling resistance. The neutrally charged membrane could remove multivalent ions (both cations and anions) more efficiently (>95%) than monovalent ions (80%) due to the size effect; a rejection sequence of CaCl₂ ≈ MgSO₄ ≈ Na₂SO₄ > NaCl was observed. When using bovine serum albumin (BSA), humic acid and sodium alginate as the model foulants, the neutrally charged membrane exhibited much better fouling resistance than both the positively and the negatively charged membranes. This result suggests that the foulants would less likely deposit onto a neutral membrane due to the elimination of the charge interaction between the membrane and the foulants. In addition, the neutrally charged NF membrane showed better resistance to activated sludge than the commercial NTR-7450 membrane, which suggests this membrane may be applied in an MBR process.