Degradation of Poly(Ether Sulfone)/Polyvinylpyrrolidone Membranes by Sodium Hypochlorite: Insight from Advanced Electrokinetic Characterizations

Abstract

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.

Influence of PVP content on degradation of PES/PVP membranes: Insights from characterization of membranes with controlled composition

“…In more drastic ageing conditions dislodgement of PVP from the membrane matrix has even been reported [3,10,13,15,20].…”

“…Sodium hypochlorite is one of the most widely-used disinfecting/cleaning agents owing to its high efficiency and low cost [8][9][10]. However, it is well known that sodium hypochlorite has a strong impact on membranes properties such as structure [2,11,12], surface charge [2,13,14] and chemical composition [3,10,13,15], which directly impacts their filtration performance and their mechanical strength as well [3,15,16].…”

“…Several studies pointed out the occurrence of a PES-chain scission mechanism [2,3,10,13,14,21,22]. For PES (MW = 30,000 g mol -1 ) dense films containing small amounts of polyetherethersulfone (PEES), PES-chain scissions were observed after long term ageing (8 months) in 28,500 ppm total free chlorine (TFC) NaOCl solutions at pH 9 and 12 [23,24].…”

Multi-stage Mass Spectrometry of Poly(vinyl pyrrolidone) and Its Vinyl Succinimide Copolymer Formed upon Exposure to Sodium Hypochlorite

“…4) Such chemical cleaning is not inconsequential and turns out to be deleterious for the membrane, ultimately leading to its degradation. Numerous studies have dealt with the ageing of PVP exposed to sodium hypochlorite [5][6][7][8] under various conditions (e.g. pH, concentration, exposure time…) and proposed several degradation routes and degradation products based on physico-chemical characterization such as infrared spectroscopy or electrochemistry.…”

“…pH, concentration, exposure time…) and proposed several degradation routes and degradation products based on physico-chemical characterization such as infrared spectroscopy or electrochemistry. 6) e two main degradation pathways reported in the literature concern 1) the vinyl pyrrolidone repeating unit which is modi ed in carboxylate/ carboxylic acid or succinimide 5) and 2) the polymeric backbone which undergoes chain scission. 4) In other words, the chemical ageing of PVP leads to a changing of the repeating unit or of the end-groups of the polymer, two main pieces of information readily collected using mass spectrometry (MS).…”