Accelerated ageing method for the determination of photostability of polymer-based photocatalytic membranes

Abstract

Photocatalytic membranes (PM) are promising for the removal of micropollutants from water. Limited photostability of polymeric PM systems is an obstacle to widespread application where a lifetime of years is required, maintaining good filtration and photodegradation efficiencies during exposure to both ultraviolet light and reactive oxygen species. The monitoring of physicochemical properties with commonly available tools, combined with a proposed method to evaluate PM stability using accelerated ageing is reported. Three PMs based on combinations of support membranes (polyethersulphone – PES, polyvinylidene fluoride – PVDF) and photocatalysts (TiO2, palladium porphyrin). Accelerated ageing was implemented through intense UV-violet illumination (2223 W m−2 of combined 365 nm and 405 nm) for 250 h. The PM photostability was evaluated by i) morphology and surface characteristics, ii) chemical composition, iii) optical properties, and iv) photodegradation of methylene blue (MB) dye. The three PMs exhibited distinct differences in photostability. The PVDF support membrane was stable, however, the degradation of hydrophilic additives led to TiO2 leaching and porphyrin exhibited limited stability due to photobleaching. TiO2-PVDF retained its initial MB removal performance after ageing, while TiO2-PES and porphyrin-PVDF had lower or completely depleted photocatalytic activity. The proposed experimental method realises photostability screening of PMs in a laboratory environment within a short period of time, allowing for early-stage weaknesses to be identified.