Photocatalytic decolorization and mineralization of malachite green in an aqueous suspension of titanium(IV) oxide nano-particles under aerated conditions: correlation between some physical properties and their photocatalytic activity

Titanium(IV) oxide (TiO2) nano-particles with various physical properties, which had been prepared by hydrothermal crystallization in organic media (HyCOM) and post-calcination, were used for photocatalytic decomposition of malachite green (MG) in an aqueous suspension under aerated conditions. The amount of MG adsorbed on TiO2 ([MG]ad) increased as the surface area of HyCOM TiO2 increased. The apparent initial rate constant of pseudo zero-order kinetics, k0, for decolorization of MG in an early stage (∼15 min) coincided well with [MG]ad, indicating that adsorptivity is a decisive factor for the initial bleaching of MG. However, in a 1 h irradiation experiment, a HyCOM sample with a smaller [MG]ad but improved crystallinity exhibited higher decolorization activity than did HyCOM TiO2 with the largest [MG]ad, suggesting that this reaction process includes deactivation of the photocatalyst due to deposition of intermediate(s) that had been formed by degradation of MG. Longer irradiation was required for mineralization of MG. Total organic carbon (TOC) in the suspension of HyCOM TiO2 with improved crystallinity continuously decreased whereas TOC in Degussa P-25 TiO2 did not decrease any more after 5 h of irradiation, although one-third of the initial TOC still remained in the suspension.