An AE/Fe-MOF/PVDF heterojunction self-cleaning photocatalytic membrane for rapid electron transfer to enhance photo-Fenton degradation of tannic acid

Abstract

As the traditional Chinese medicine (TCM) industry develops at a fast pace, its wastewater discharge has become an increasingly severe environmental problem. Membrane technology has been applied in the treatment of TCM wastewater; however, the high concentration of macromolecular organics, such as tannic acid, pectin, and protein, readily triggers severe pore blockage and fouling of polyvinylidene fluoride (PVDF) separation membranes. To address these challenges, this study innovatively integrates the natural photosensitizer aloe-emodin (AE) with an Fe-MOF to construct an AE/Fe-MOF type-I heterojunction photocatalyst directly immobilized onto a PVDF membrane substrate. Under visible-light irradiation with H₂O₂, the fabricated AE/Fe-MOF/PVDF membrane achieves 87.29% degradation efficiency toward tannic acid (TA) within 90 minutes. It also exhibits excellent removal performance for simulated TCM wastewater derived from Rheum officinale Baill. and Polygonum cuspidatum Sieb. et Zucc., achieving chemical oxygen demand (COD) removal rates of 74.44% and 96.61%, respectively. Furthermore, the AE/Fe-MOF/PVDF membrane demonstrates remarkable self-cleaning capability, achieving a flux recovery ratio (FRR) of up to 96.5% following in situ photo-Fenton degradation, which is far superior to conventional hydraulic cleaning (65.7%). Stable filtration performance is maintained across multiple filtration–cleaning cycles. Mechanistic investigations reveal that the AE/Fe-MOF type-I heterojunction enhances the spatial separation of photoinduced carriers and accelerates the Fe³⁺/Fe²⁺ redox cycle, thereby synergistically driving a sustained and efficient photo-Fenton process that enables simultaneous pollutant degradation and in situ membrane fouling mitigation. This work not only provides a novel heterojunction-based self-cleaning photocatalytic membrane for the efficient treatment of TCM wastewater but also deepens the understanding of the synergistic mechanism between charge transfer and the iron cycle in natural photosensitizer and MOF hybrid systems.