Fouling mechanism of low-pressure hollow fiber membranes used in separating nanosized photocatalysts

Low-pressure hollow fiber membranes were applied to separate and recover the nanosized TiO2 photocatalysts in a photocatalysis and membrane separation coupling process, and the fouling mechanism of membranes with different material and pore structure was investigated in detail. The cross-flow filtration experiments were performed on polyacrylonitrile (PAN) ultrafiltration membrane with MWCO 60,000 Da, polyether sulfone (PES) ultrafiltration membrane with MWCO 65,000 Da, and other two kinds of polyether sulfone (PES) microfiltration membrane with pore size 0.2 μm and 0.4 μm. Rapid flux decline happened after TiO2 particles entered into the membrane, blocked internal pores and formed cake layer on the top of membrane. The fouling behaviors of membranes were explored and associated with membrane pore size as well as the interaction between TiO2 particles and membrane. The fundamental relationship was set up between various operating conditions and the characteristics of cake layer formed on membrane by analyzing the forces exerted on the TiO2 particles. It was found that TiO2 aggregates presented a stable diameter larger than 0.2 μm in nature aqueous media, and the porosity of TiO2 cake increased along the height of the cake in the presence of flux decline. Hydrodynamic backflush was unable to completely recover TiO2 catalysts and eliminate the filtration resistance due to the adhesion of nanosized TiO2 catalysts in/on the membranes. The progresses in this research are of potential benefit to solve the membrane fouling problems caused by nanosized TiO2 photocatalysts in the coming application of photocatalytic membrane reactor.