Microspheroidization treatment of macroporous TiO2 to enhance its recycling and prevent membrane fouling of photocatalysis–membrane system

Patterned porous (PP)-TiO2/calcium–alginate (CA) microspheres with an average diameter of 3 mm were prepared by immobilizing PP-TiO2 into CA balls to improve their recycling and photoactivity. The microspheres demonstrate higher degradation activity on methylene blue (MB) than Degussa P-25 nanoparticle/CA (P25-TiO2/CA) spheres, freely suspended PP-TiO2, and P25-TiO2 fine particles. Furthermore, the microspheres can be easily recycled. Meanwhile, the energy cost in the recycling of PP-TiO2 is 1.23 × 108 J per gram PP-TiO2. The PP-TiO2/CA microsphere combined polyacrylonitrile (PAN) microfiltration membrane system was established to dispose of the MB wastewater. Compared with the PP-TiO2 catalyst–membrane system, the PP-TiO2/CA microsphere–membrane system can achieve almost 90% degradation ratio of MB. The permeation flux for the combined system is approximately the same as that of the PAN membrane and remains at 90 L/m2 h. However, the permeation flux decreases to 70 L/m2 h when freely suspended PP-TiO2 is used as photocatalyst. The reason is that with the operation going on, the PP-TiO2 cake layer is formed by a growing number of suspended PP-TiO2 particles depositing on the membrane surface. The deposition induces membrane fouling and inhibits the photocatalysis activity of PP-TiO2 in the solution, whereas PP-TiO2/CA microspheres remain suspended in the solution under gentle stirring. Moreover, the sum of drag force FD and buoyancy Fb (FD + Fb = 1.552 × 10−4 N) imposed on the microsphere was calculated to be comparable with the gravity G (1.566 × 10−4 N) of the PP-TiO2/CA microsphere. This result further reveals that the microspheres are suspended in the dye solution, thereby preventing membrane fouling. We believe that the microspheroidization of PP-TiO2 and the combined microsphere–membrane system provide a new idea for practical wastewater treatment.