Development of a novel electrophoresis-UV grafting technique to modify PES UF membranes used for NOM removal

Abstract

Membrane surface modification is an attractive approach to reduce membrane fouling by foulants such as natural organic matters (NOM). A novel technique which combines controlled deposition by electrophoresis of charged moieties, with UV grafting on the membrane surface has been applied to modify poly(ether sulfone) membranes with several different monomers. The monomers included three strong polyelectrolytes [methacrylic acid (MA), acrylic acid (AA) and 2-acryamido glycolic acid (AAG)] and two weak polyelectrolytes [(2-hydroxyethyl)-methacrylate (HEMA) and N-vinylformamide (NVF)]. The molecular weight cut-off, pure water flux, wettability, surface charge and surface morphology were measured for modified and unmodified membranes. The filtration and fouling performance was tested and compared using surface water from a Singapore reservoir. It was found that the modified membrane surfaces exhibited more hydrophilic and negative charged features after the electrophoresis-UV grafting treatment. The MA and AAG grafting reduced the MWCO of the PES membranes to improve NOM retention, and decreased the permeability to less than half of the original. The reason for this was the formation of a thin layer on the membrane surface which reduced the porosity and pore size of the treated membranes. In contrast, the HEMA grafted membrane showed increased MWCO and permeability as a result of the monomer dissolution which enlarged the membrane pore sizes. Membrane characterization by FE-SEM observation confirmed these two mechanisms. With electrophoresis assistance, the modified membrane exhibited more homogeneous surface properties than that without electrophoresis. The modified PES membranes also exhibited lower fouling tendency than the unmodified membranes when filtering NOM due to their hydrophilic and negative charge character after modification. Although the MA modified commercial PW membrane had the same MWCO and pure water flux as the unmodified commercial PT membrane, it had a much lower NOM fouling tendency.