U. Bellè, L. De Flaviis, M.V. Diamanti, M. Ormellese, M. Pedeferri
Politecnico di Milano,
Italy
Keywords: titanium dioxide, photocatalysis, dyes, nanotubes
Summary:
Titanium dioxide is widely investigated for its photocatalytic properties, i.e. its capability to generate radical species able to degrade organic or inorganic pollutants by means of light-produced charge carriers. In recent years, titanium anodization has attracted a large interest due to the possibility of obtaining nanotubular oxide films, allowing to obtain photocatalyst nanostructures with high specific surface area directly immobilized on a substrate, hence avoiding the drawbacks of suspended titanium dioxide powders, such as possible agglomeration and the necessary step of water filtration after photocatalysis [1,2]. In the present work, nanotubular titanium dioxide was obtained through anodization of titanium in ethylene glycol-based electrolyte containing NH4F (0.2 M). Annealing was performed to induce the crystallization of the nanobubes array to the photoactive anatase structure. Subsequently, the photocatalytic properties of the so-obtained titanium dioxide nanotubular arrays were tested in the degradation of solutions containing different organic dyes. These ones are widely used by the scientific community for the evaluation of photocatalysts efficiency because they represent a class of pollutants extensively emitted into wastewaters from different industries. Moreover, their photodegradation process can be easily monitored through spectrophotometric analyses, allowing a simple control of the variation of dye concentration inside the solution, which is a valid estimation of the photocatalysis efficiency [3]. Photodegradation tests were performed on four different organic dyes, respectively Rhodamine B (RhB), Methylene Blue (MB), Direct Red 80 (DR) and Acid Orange 7 (AO7). Four single-dye solutions, characterized by an initial absorbance of 0.5 (in arbitrary units), were prepared and tested to verify the degradation behavior of the single molecules in presence of TiO2. Then six binary solutions were also tested, obtained by mixing of different single-dye solutions of the previously determined initial concentration with a 50/50 percentage in volume. A batch reactor built during the experimental stage, characterized by a 450 mW UV LED, with emission peak at 365 nm positioned at a distance of 3 cm from the sample, and by a stirred solution in laminar regime, was used for the whole set of experiments. As shown in Figure 1, which reports the degradation kinetics of two sample dyes either in single solution or in mixture, the degradation rate is similar in the mixed solutions, although some differences can be noticed. In general, the obtained results show that, after 3 h irradiation, most dyes are degraded by almost 90, confirming that nanotubular TiO2 can be well – exploited as a photocatalyst both for single dye solutions and for binary solutions (Figure 2). Furthermore, dyes can degrade with different kinetics depending on the type of binary solution in which they are mixed – for instance, MB/DR mixtures inhibit overall efficiency. This means that the interaction between different dyes inside a solution can affect the overall kinetics of the photodegradation process, which needs to be taken into account in order to transfer this technology from laboratory experiments to field applications. Further analyses are suggested in order to investigate the nature of these interactions.