Collision theory and membrane photocatalysis: Steroid hormones meet singlet oxygen in palladium-porphyrin-coated polytetrafluoroethylene membranes

Photocatalytic membrane (PCM) reactors are an emerging technology for the continuous elimination of micropollutants from water. PCM material and process properties define performance limitations. A collision theory framework was established to elucidate the limiting factors of steroid hormone micropollutant photodegradation inside the pores (200 nm) of a palladium-porphyrin-coated polytetrafluoroethylene (PTFE) PCM under simulated sunlight. The collision theory can describe the degree of photodegradation of 17β-estradiol (E2). The production of singlet oxygen reactive species was limited by light intensity (up to 14 mW cm−2), porphyrin loading (up to 50 μmol g−1), and membrane layer thickness (46 μm, achieved via stacking thin membranes). Further increases in these parameters did not significantly enhance the removal of E2, because the quantity of singlet oxygen generated, and consequently the collision frequency, levelled off. Not all collisions result in photodegradation reaction. By reducing the reaction time via increasing the E2 molar flux, the rate of disappearance reached a threshold of 7 ± 2 nmol L−1 s−1. This is identified as the maximum effective collision frequency, implying that 11 % of the total collisions (64 ± 5 nmol L−1 s−1) resulted in successful reaction. ... mehrThe study presents a novel framework based on collision theory to predict the fundamental mechanisms of and limitations to photodegradation in porphyrin-PTFE membranes, providing unprecedented insight into the performance constraints of diverse membrane reactors for advances in materials and process engineering.