Abstract
Heterogeneous photocatalysis with titanium dioxide (TiO2) is considered one of the most promising Advanced Oxidation Processes (AOPs). In order to solve issues related to catalyst recovery and possible agglomeration, which are typical of catalysts in nanoparticle form, self-organized nanotubular TiO2 films directly immobilized on a metal substrate can be produced through anodization. In this study, a nanotubular anodic oxide was tested in the degradation of three organic dyes, namely Direct Red 80, Methylene Blue, and Rhodamine B, in single, binary, and ternary mixtures, to simulate industrial effluents with the co-presence of multiple dyes. To better understand the dyes’ behavior and possible interaction effects, spectrophotometry was used to analyze the degradation of each dye in the mixture. The zero-crossing first-order derivative approach and double divisor ratio spectra derivative method were used for the analysis of binary and ternary mixtures, respectively, to overcome quantification problems due to spectra overlapping. The photocatalytic system demonstrated good efficiency, supporting the use of nanotubular TiO2 as a photocatalyst for dye mixtures. Moreover, the interaction among dyes can actually affect, both positively and negatively, photodegradation kinetics, posing an issue in understanding the actual efficiency of the purification process as a function of the effluent composition.
Highlights
On a general basis,(Figure in all evaluated cases, high photodegradation efficiencies were obtained, salts addition and acidic condition; the only solution that behaves to the neutral condition supporting the use of nanotubular TiO2 as a photocatalyst for dye degradation, in complex mixed is the basicAll pHthe solution, since neitherDR80 single-dye binary mixtures to be affected by basicity
We presented the application of nanotubular TiO2 films for the photocatalytic degradation of multi-dye aqueous solutions
The results show that most dyes and dye mixtures degrade by more than 80%, confirming that titanium dioxide nanotubes are an efficient photocatalyst against simple dyes, and in the treatment of complex effluents
Summary
The necessity of improving photocatalytic efficiency has driven scientific research in the generation of nanostructured oxides for increasing the surface-to-volume ratio and so promoting interactions between photocatalyst and organic pollutants In this sense, anodic oxidation allows forming. Coatings 2020, 10, 919 self-organized nanotubular TiO2 films strongly immobilized on the substrate, facilitating the adsorption of organic pollutants on the photocatalyst surface, while enhancing light adsorption and avoiding recovery issues typical of nanoparticles These films are achieved by imposing a cell voltage between the titanium sample (anode) and a counter electrode (cathode) in a fluoride containing organic electrolyte; this causes the growth of an amorphous oxide layer on the Ti specimen, while fluorides dissolves locally the growing oxide, generating a nanotubular morphology. Calcination treatments promote oxide crystallization, improving the mobility of the photogenerated charge carriers [8,9,10,11,12,13]
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