FEDERICO PERSICO^1,2, MAURIZIO SANSOTERA^1,2*, NICOLE SCHEPIS¹, LUCA MAGAGNIN¹, WALTER NAVARRINI^1,2*
* Corresponding authors
1. Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Via Mancinelli 7, 20131, Milano, Italy
2. Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, Via G. Giusti 9, 50121 Firenze, Italy

Abstract

The crystal violet (CV) dye has been chosen to study the photooxidation activity of titanium dioxide immobilized into a transparent fluoropolymeric matrix toward organic pollutants. The photoactive matrix was directly coated on the UV source by applying a TiO₂– containing fluorinated ionomeric dispersion and a perfluorinated transparent amorphous polymer in an appropriate sequence. The photocatalytic activity of the multilayered coating towards the hydrosoluble organic CV dye was evaluated in transparent solution and in highly turbid suspension. The turbidity was obtained by dispersing insoluble microparticles of calcium sulfate in the polluted aqueous solution. The photoabatement rates obtained in transparent and in turbid conditions were 0.0918 min^-1 and 0.0300 min^-1, respectively. The TiO₂ nanoparticle dispersed in the fluorinated matrix revealed a higher photocatalytic activity than simple dispersed TiO₂ in both transparent as well as turbid conditions. The immobilization of the catalyst in a stable fluorinated matrix avoided the TiO₂ separation and prevented catalyst losses. The synergism in the activity between the TiO₂-based photocatalyst and the fluorinated matrix was particularly evident at low pollutant concentrations.

INTRODUCTION
Advanced oxidation processes (AOPs) have been widely recognized as performing alternatives to conventional methods for removal of organic pollutants from water and wastewater (1-3). In particular, semiconducting transition metal oxides revealed remarkable catalytic properties in the presence of UV light, suggesting new promising applications in the field of photocatalysis (4-6). Anatase titanium dioxide, TiO₂, has a band gap of 3.2 eV and pairs of holes (h+) and electrons (e-) are efficiently formed when its surface is exposed to radiations with wavelengths below 385-410 nm. Electron-hole pairs can either recombine or react with adsorbed electron donor or acceptor molecules (7). In the presence of water and oxygen, hydroxyl radicals OH· and superoxide ions O₂-· can be generated (7). These radicalic intermediates are strong oxidizing species able to oxidize organic compounds (8, 9). Direct oxidation at the hole site is usually favored in case of highly adsorbed substrates, while the degradation of weakly adsorbed pollutants takes place mainly through hydroxyl radical mediated oxidation (10,11) ...