Improving the hydrophilicity and fouling resistance of RO membranes by surface immobilization of PVP based on a metal-polyphenol precursor layer

• PVP was immobilized on RO membrane surface based on TA–Fe-PEI precursor layer.
• The flux was slightly decreased and the rejection was almost unchanged.
• PVP immobilization significantly improved the surface hydrophilicity.
• The modified membrane exhibited excellent fouling resistance and stability.

A commercial polyamide reverse osmosis (RO) membrane was surface modified in a sequential two-step dip coating process for improvement of the hydrophilicity and fouling resistance. In the first step, an ultrathin metal-polyphenol precursor layer was deposited onto the membrane surface via the self-assembly of tannic acid (TA) and Fe(III) ions, and polyethylenimine (PEI) was added as a covalent cross-linker. In the second step, poly(N-vinylpyrrolidone) (PVP), a common hydrophilic polymer, was immobilized onto the metal-polyphenol precursor layer through the strong hydrogen bonding interactions between the lactam groups in PVP and the phenolic hydroxyl groups in the precursor layer. X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscopy (TEM) and zeta potential measurements confirmed the successful immobilization of the precursor layer and PVP. Atomic force microscopy (AFM) test indicated a smoother surface of the modified membrane. Furthermore, the water flux of the modified membrane was only slightly decreased and the salt rejection was almost unchanged. More importantly, static contact angle measurement showed that the membrane surface hydrophilicity was significantly improved after the PVP immobilization (contact angle from 59.0±1.2° to 14.5±2.4°). Membrane fouling experiments were performed over three “filtration–rinsing” cycles using two proteins (lysozyme and bovine serum albumin) and one polysaccharide (sodium alginate) as model foulants. The results of the fouling experiments revealed that the fouling resistance of the reverse osmosis membrane was obviously improved by the surface immobilization of PVP. Besides, cross-flow filtration test under high hydrodynamic shear condition for 15 days indicated the excellent stability of the immobilized PVP. This work provided a new potential strategy for improving the fouling resistance of RO membranes.