TiO2 photocatalytic membranes

Since typical organic and inorganic membranes have a physical separation function toward water contaminants (i.e., no chemical decomposition), the pollutants retained by the membrane are concentrated in the system while the treated water is clean enough for its specific usage. The retentate with high organic concentration should be treated appropriately before its discharge into the environment. As a result, special attention is given to the fabrication of TiO2 photocatalytic membranes, which interestingly combine the physical separation of organic contaminants and their simultaneous chemical decomposition [71-74]. As illustrated in Fig. 5, TiO2 is immobilized on a porous substrate, allowing water molecules to pass through the porous TiO2 active layer followed by the porous support when a pressure across the membrane is introduced. The photocatalytic membrane reactors may gain tremendous popularity because of their multiple functions: photocatalysis of organic compounds and physical separation of target contaminants and reaction intermediates.

Influent

Retentate

TiO2 Skin Layer Porous Support

Retentate

TiO2 Skin Layer Porous Support

Permeate (Effluent)

Figure 5 Concept of a TiO2 photocatalytic membrane, exhibiting multifunction of photocatalysis, separation, and antibiofouling. Photocatalytic decomposition of organic chemicals results in the formation of simpler intermediates. Note cascaded changes in the color of contaminants with different sizes represent their photocatalytic decomposition and increase in the number of contaminants expresses their retention and accumulation when passing the membrane in parallel.

Choi et al. demonstrated such a multifunction of TiO2 membranes in terms of the decomposition of methylene blue dye and creatinine, destruction of biological toxins (microcystin-LR), and inactivation of pathogenic microorganisms (E. coli) [75]. They also observed high and reliable organic rejection efficiency of the TiO2 membrane when it was used to treat dissolved organic carbon solution obtained from an activated sludge plant for wastewater treatment. Interestingly, the inherent antibiofouling properties of TiO2 membrane, which are highly important in membrane research and industry, were investigated [76]. TiO2 membranes irradiated by UV exhibited less flux decline over time. The antibiofouling property can be explained by the fact that the decomposition of the organic foulants accumulated on the TiO2 membrane surface simultaneously occurs due to hydroxyl radical attack during their filtration.

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