Ageing of polyethersulfone ultrafiltration membranes under long-term exposures to alkaline and acidic cleaning solutions

Abstract

Six commercial ultrafiltration membranes made of polyethersulfone from Microdyn Nadir, Koch Membrane Systems and GE Osmonics were exposed to 0.01, 0.1 and 1.0M nitric acid (HNO3) and sodium hydroxide (NaOH) solutions at 50°C for 150 days. Water permeability, molecular weight cut-off curves, ATR-FTIR, SEM-EDS, AFM and contact angle measurements were employed to evaluate membrane ageing.All membranes showed a high resistance to acidic solutions keeping their chemical and morphological properties almost invariant. The alkaline ageing did not induce any substantial change in the Nadir membranes but, the stability of Koch and GE Osmonics support layers was compromised at high NaOH concentrations, leading to even the complete degradation of these supports in some cases. This was confirmed by ATR-FTIR, SEM and GC–MS analysis. The Nadir membrane resistance to alkaline conditions was found to be in the support layer material composition (mixture of polyethylene and polypropylene). Koch and GE Osmonics membranes support material (polyethylene terephthalate) broke down into its monomers under strong alkaline conditions via hydrolysis reaction.Almost all membranes showed changes in membrane performance which were attributed to their conditioning or their degradation. Regardless of the cause, those changes took place mainly in the first 10 days of exposure.The influence of temperature on the ageing was also studied under the strongest alkaline condition. Its effect on chemically stable membranes were negligible whereas for the others, higher temperatures led to faster support layer degradation.