Immobilized nano-TiO2 photocatalysts for the degradation of three organic dyes in single, binary and ternary mixtures

U. Bellè, F. Pelizzari, M.V. Diamanti, M. Ormellese, M. Pedeferri
Politecnico Di Milano,
Italy

Keywords: titanium dioxide, nanotubes, photocatalysis, dyes, textile wastewaters

Summary:

Heterogeneous titanium dioxide (TiO2) photocatalysis is considered as one of the most promising advanced oxidation processes (AOPs) due to its capability to produce charge carriers, i.e. electrons and holes, when irradiated by UV light, leading to the generation of radical species able to degrade organic pollutants. In order to increase the photodegradation process efficiency, nanoscale morphologies characterized by a high surface area are exploited. Particularly, anodization in a fluoride-containing electrolyte leads to the generation of self-organized nanotubular TiO2 films directly immobilized on a substrate, avoiding the drawbacks of TiO2 suspensions, such as agglomeration and issues in catalyst recovery [1-3]. Photocatalytic properties of these films are widely exploited for the degradation of aqueous solutions containing organic dyes; these pollutants are abundantly present in textile wastewaters, resulting hazardous for living organisms due to their carcinogenicity and harmfulness [4]. In the current study, anodization of titanium was achieved in ethylene glycol-based electrolyte containing NH4F (0.2 M) in order to get a titanium dioxide film with a nanotubular morphology. Annealing treatment was carried out at 500 °C to in order to obtain the photoactive anatase crystalline structure. The photoactivity of the TiO2 nanotubular arrays were then tested in the degradation of aqueous solutions containing three different organic dyes, namely Direct Red 80 (DR), Methylene Blue (MB) and Rhodamine B (RhB), by monitoring the dye absorbance variation in the visible range with spectrophotometric analyses. A UV LED lamp with emission peak at 365 nm was positioned 3 cm away from the photoactive nanotubular film to induce photoactivation. Three single-dye aqueous solution, having an initial concentration of 10-5 M, were firstly tested; then three binary mixtures, obtained by mixing all combinations of single-dye solutions (50/50 vol%), and one ternary mixture, containing all single-dye solutions in the same volumetric quantity, were prepared in order to analyse possible interactions among different dyes during the photodegradation process. First order derivative approach and ratio spectra derivative method were used, respectively, for the analysis of binary and ternary mixtures in order to solve any possible spectra overlapping and so to get a simultaneous quantification of two or more compounds in the same solution, whereas Beer Lambert law was exploited for the analyses of single-dye solutions [5-7]. In general, after 3 h irradiation most dyes are degraded by almost 80%, supporting the use of nanotubular TiO2 as a photocatalyst for dye mixtures. Moreover, looking at Figure 1 which shows the degradation kinetics of each dye either in single solution or in mixtures, it is possible to notice that the interaction among dyes can affect the photodegradation kinetics; particularly, DR80 negatively affects the photocatalytic degradation of other dyes co-present in the mixture, while it is not affected itself by the presence of other contaminants; on the other hand, RhB degradation is positively influenced by the addition of MB both in binary and in ternary mixtures. Spectroscopic analyses are underway in order to better understand the chemical interactions among dyes.