Decolorization of aqueous solution containing Organic Synthetic-Dye via Dark-Catalysis process using hydrothermally synthesized semiconductor-oxides nanotubes

Abstract

The removal of an organic synthetic-dye from an aqueous solution via the adsorption and decolorization processes, typically occurring in the dark-condition on the surface of semiconductor-oxides nanotubes-based adsorbents, has been demonstrated for the first time without the use of any external power-source such as the radiation, potential-difference, microwave-generator, and ultrasonicator. “Two-step” and “one-step” dyeremoval methods have been developed by utilizing the hydrothermally processed hydrogen titanate (H2Ti3O7)and anatase-titania (TiO2) nanotubes in combination with the strong oxidizer such as the hydrogen peroxide (H2O2). In the “two-step” dye-removal method, conducted in the dark-condition, the organic synthetic-dye is adsorbed from an aqueous solution on the surface of nanotubes and decolorized in another aqueous solution containing H2O2. In contrast to this, in the “one-step” dye-removal method, the simultaneous dye-adsorption and dye-decomposition (decolorization) take place in the dark-condition on the surface of nanotubes only in one aqueous solution containing H2O2. The comparison shows that the dye-removal rate-capacity in the “onestep”dye-removal method is ∼2–3 times higher relative to that in the “two-step” dye-removal method. In both the methods, the nanotubes recovered after the end of the first dye-removal cycle can be reused as catalyst for the multiple dye-removal cycles operating under the dark-condition. The underlying dye-removal mechanism is termed as “dark-catalysis” in which the presence of both the nanotubes and H2O2 has been shown to be an essential condition for the complete decolorization in the dark-condition. Due to its several benefits, the dark-catalysis mechanism appears to be a commercially viable process compared with the conventional photocatalysis mechanism.