A comparative study on the removal of cylindrospermopsin and microcystins from water with NF-TiO2-P25 composite films with visible and UV–vis light photocatalytic activity

Abstract

In this investigation, in order to develop photocatalyst materials with improved photo-efficiency and visible light response compared to the state of the art materials, the role of Evonik Aeroxide® P25-TiO2 (P25) nanoparticles incorporated in a modified sol–gel process to yield composite nitrogen and fluorine doped TiO2-P25 (NF-TiO2-P25) films was investigated. The addition of P25 nanoparticles in the sol leads to higher BET surface area, pore volume, porosity and total TiO2 mass, as well as larger thickness and roughness of the films after heat treatment. Microscopy techniques confirmed partial sintering of NF-TiO2 sol–gel formed and P25 nanoparticles having different average size. The existence of well defined regions of only anatase from NF-TiO2 and anatase–rutile mix from P25 was verified in the micro-Raman spectra. The photocatalytic degradation of four microcystins (microcystin-LR, -RR, -LA and -YR) and cylindrospermopsin was evaluated with NF-TiO2 and NF-TiO2-P25 films under visible and UV–vis light. The general reactivity for the microcystins under acidic conditions (pH 3.0) was: MC-LA>MC-LR≥MC-YR>MC-RR where the highest initial degradation rate was achieved with the NF-TiO2-P25 films (5gL−1 of P25 in sol when irradiated with visible light and 15gL−1 of P25 in sol when irradiated with UV–vis light). Cylindrospermopsin showed negligible adsorption at pH 3.0 for all films. Nevertheless, significant photocatalytic removal was found under UV–vis light with maximum P25 loaded films indicating that the degradation was mediated by the involvement of photogenerated reactive oxygen species and not by the trapping reaction of the positive hole.