Surface modification of UF membranes with functionalized MWCNTs to control membrane fouling by NOM fractions

Abstract

To achieve greater hydrophilicity and improved antifouling ability, an ultrafiltration (UF) membrane was modified using a surface coating of raw and functionalized multi-walled carbon nanotubes (MWCNTs). Initially, raw MWCNTs were chemically treated to prepare MWCNTs with carboxylic groups (MWCNTs-COOH) and MWCNTs with polyethylene glycol groups (MWCNTs-PEG), and their physicochemical properties were systematically characterized. The results indicated that the MWCNTs-COOH exhibited the strongest negative charge among the three investigated MWCNTs, whereas the MWCNTs-PEG exhibited the best dispersion ability. Subsequently, the UF membranes were coated with the three MWCNTs, and the surface morphologies and permeability of the modified membranes under different transmembrane pressure (TMP) levels were compared. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images demonstrated that the modified membranes had a rougher surface than the virgin membrane because of the modification layer of MWCNTs. The pure water flux values of all the modified membranes were slightly reduced in comparison with that of the virgin membrane, but the flux sacrifice was very minor. Three natural organic matter (NOM) models, humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA), were employed to investigate the antifouling potentials of the membranes. The modified membranes exhibited strikingly improved antifouling properties compared with those of virgin membrane for all of the tested NOM foulants, which was probably a consequence of the decreased direct membrane–foulant contact due to the MWCNT coating layer. In addition, decreased surface negative charge and increased surface roughness were two important factors contributing to the enhanced antifouling properties. The r-membranes and the COOH-membrane exhibited satisfactory stabilities during pure-water backwashing at TMP of 60kPa, with dislodged ratios of 1.9% and 4.54%.