Titanate nanotubes for removal of methylene blue dye by combined adsorption and photocatalysis

The removal of dyes from water effluents is an actual challenge in the wastewater treatment. The complete degradation of organic pollutants into gaseous CO2 and inorganic ions (mineralization) is desirable. Generally, photocatalysis and adsorption are used to achieve this aim. However, it is difficult to reach the complete mineralization of dyes. In the present work, the removal of the commercial methylene blue (MB) dye from aqueous solutions was studied upon UV irradiation in presence of trititanate nanotubes with different sodium contents. It was observed that both dye adsorption and photocatalytic degradation took place. At high sodium content in the titanate nanotubes, high dye adsorption capacity was observed, where the MB adsorption followed the cation exchange mechanism. However, a deep decolorization of the aqueous solution was not accompanied by the complete mineralization of the dye. This can be attributed to a screen effect that avoids the generation and diffusion of the oxidative species (OH radicals) on the catalyst surface. On the other hand, hydrogen titanate nanotubes with almost zero sodium content showed a low adsorption capacity, but a high degree of mineralization of the MB dye. Probably, the low band gap energy of low-Na titanate nanotubes promotes formation of a larger amount of OH radicals (non-selective and highly oxidative species). In the same reaction conditions, hydrogen titanate nanotubes allowed to mineralize 65% of the initial organic carbon, whereas the reference commercial TiO2 Degussa P-25 gave only 41% of mineralization. In the latter case, instead of the complete mineralization of the MB the formation of demethylated dyes was detected. Finally, it was shown that the balance between MB dye adsorption and photodegradation can be easily tuned by the modification of the Na content in trititanate nanotubes.