Evolution of Two Polymeric Nanofiltration Membranes Following Ion Beam Irradiation

For membranes to be competitive with conventional water treatment technologies, membrane processes need to operate at high flux rates, have high degrees of selectivity and have high fouling resistance. To accomplish these, modification of the membrane surface and/or microstructure may be necessary. One modification technique is ion beam irradiation. Ion beam irradiation is the bombardment of ions into the surface of the membrane. As the ions penetrate the membrane, they lose energy to the polymer. This energy transfer results in bond breaking, bond formation and microstructure alterations, which may result in changes in flux and selectivity as well as improve fouling resistance.

Ion beam irradiation was used to modify the surface of two nanofiltration membranes, one with a sulfonated polysulfone selective layer and the other with an aromatic polyamide selective layer. The effects of ion beam irradiation on surface morphology, microstructure, chemical structure, and performance were studied. A beam of 25 keV H⁺ ions with four irradiation fluences (1×10¹³ ions/cm², 5×10¹³ ions/cm², 1×10¹⁴ ions/cm² and 5×10¹⁴ ions/cm²) was used for membrane irradiation. ATR-FTIR and XPS analyses were performed to determine structural changes incurred by ion beam irradiation, and increased crosslinking of the surfaces was observed. AFM analysis showed that membrane roughness decreased after irradiation. Roughness changes were proportional to changes in ion fluence; that is, as the ion fluence increased, the magnitude of the effect of ion beam irradiation on the decrease in roughness increased. The hydrophobicity and the pore size distribution of the membranes were not affected by ion beam irradiation. Changes incurred by the membranes after ion beam irradiation resulted in decreases in fouling, as observed by lower flux declines during operation. The separation capabilities of the membranes were not affected by ion beam irradiation. There was a slight increase in the cleaning efficiency of the membrane after ion beam irradiation. Thus, ion beam irradiation was concluded to be an effective membrane modification technique.