Poly(ether sulfone) (PES)/polyvinylpyrrolidone (PVP) membranes are widely used in various industrial fields such as drinking water production and in the dairy industry. However, the use of oxidants to sanitize the processing equipment is known to impair the integrity and lifespan of polymer membranes. In this work we showed how thorough electrokinetic measurements can provide essential information regarding the mechanism of degradation of PES/PVP membranes by sodium hypochlorite. Tangential streaming current measurements were performed with ultrafiltration and nanofiltration PES/PVP membranes for various aging times. The electrokinetic characterization of membranes was complemented by FTIR-ATR spectroscopy. Results confirmed that sodium hypochlorite induces the degradation of both PES and PVP. This latter is easily oxidized by sodium hypochlorite, which leads to an increase in the negative charge density of the membrane due to the formation of carboxylic acid groups. The PVP was also found to be partly released from the membrane with aging time. Thanks to the advanced electrokinetic characterization implemented in this work it was possible for the first time to demonstrate that two different mechanisms are involved in the degradation of PES. Phenol groups were first formed as a result of the oxidation of PES aromatic rings by substitution of hydrogen by hydroxyl radicals. For more severe aging conditions, this membrane degradation mechanism was followed by the formation of sulfonic acid functions, thus indicating a second degradation process through scission of PES chains.

International audienceThis study focused on the impact of ageing solution pH (200 ppm TFC sodium hypochlorite) on the electrokinetic properties of a commercial PES/PVP UF membrane. PVP oxidation, leading to an increase in the negative charge density of aged membranes, was pointed out whatever the ageing solution pH although different mechanisms might be involved depending on the ageing pH. PES degradation was also demonstrated. Electrokinetic measurements highlighted the formation of functional groups with very weak acid properties on the surface of membranes aged in sodium hypochlorite at pH 8.0 and to a lesser extent at pH 6.0 and 11.5. These results were found to be consistent with the formation of phenol groups due to the radical hydroxylation of PES aromatic rings. Moreover, the disappearance of the isoelectric point of membranes aged in sodium hypochlorite at pH 6.0 and 8.0 gave evidence for the formation of strong acid groups such as sulfonic acids. These results suggested some PES-chain scissions, which was confirmed by XPS measurements. The disappearance of the isoelectric point was not observed for membranes aged in sodium hypochlorite at pH 11.5, thus indicating that ClO− was not involved in PES-chain scissions for the ageing conditions considered in this work. Finally, electrokinetic measurements performed with the addition of tertiobutanol (free radical scavenger) and thermo-oxidation experiments revealed for the first time that, although both HClO and free radicals species contributed to PES-chain scissions, HClO had the greater impact on PES degradation