(108d) Photocatalytic Degradation of Organic Contaminants In Wastewater Using Visible Light Irradiation

Much attention has been given in recent years to improve the treatment of industrial wastewaters. Although biological treatment is commonly employed in most waste water plants to remove organic compounds tested by BOD and COD, most organic contaminants cannot be fully degraded by such methods alone. In order to meet increasingly more stringent environmental regulations and laws, more efficient processes for the removal of organic pollutants are needed. Among the most promising and advanced methods being investigated, photocatalysis at semiconductors and at TiO₂, in particular, receives a considerable attention as reflected by the recent large body of research published in this field. Titanium dioxide, however, can be easily degraded when subjected to the relatively harsh UV irradiation.

In this work TiO₂ nanoparticles anchored into 3-D mesoporous material (TUD-1) using a sol-gel preparation strategy were prepared and tested as a photocatalyst using only visible light (λ = 447 nm) for the treatment of wastewaters containing different organic contaminants, such as phenol and azodyes. Samples with different loadings of titania, 10, 25, 50 and 75 w./w.%, were prepared and characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet spectroscopy (UV), and nitrogen adsorption at 77K.  Only samples of high loadings of titania were found to have anatase in very small amount, while low loading samples didn't show separate phases of TiO_2.

The newly developed Ti-TUD-1 photocatalysts showed excellent activity towards the removal of the tested organic compounds. Up to 97% of phenol and 94% of azodyes were successfully removed from the contaminated wastewater. Moreover, the catalysts showed remarkable stability under the relatively mild irradiation conditions used. Repeated photodegradation tests demonstrated remarkable reproducibility and showed no decrease in photocatalytic activity after 5 cycles.

TiO₂ is known to be photocatalytically active only in the UV region. The resulting photoactivity of Ti-TUD-1 in the visible light region can be attributed to a reduction in the band gap of TiO₂ upon tetrahedral attachment to TUD-1 framework and the formation of tiny small nanoparticles of titania in the pores of TUD-1.