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A new optimized photocatalyst for wastewater remediation at neutral pH has been developed and fully characterized. The catalyst features microparticles of SiO2 as a supporting substrate and a uniform shell of WO3 nanoparticles externally decorated with Fe3O4 nanocrystals. The photocatalytic activity of SiO2@WO3@Fe3O4 was evaluated and compared to its SiO2@WO3 counterpart on the photodegradation of methylene blue (MB) under visible light, with and without H2O2. Best results were obtained for SiO2@WO3@Fe3O4 in the presence of H2O2, whose potential for wastewater remediation was further evaluated on the photodegradation and mineralization of a contaminant of emerging concern such as diclofenac (DCF). Furthermore, a photocatalytic mechanism was proposed based on the performed electron paramagnetic resonance (EPR) experiments, which provided evidence for the intermediates generated in all the involved photocatalytic processes. Hence it was proven that .OH is the species responsible for the photocatalyzed oxidation of MB and DCF. The generation of .OH is boosted by a synergistic effect between Fe3O4 and WO3 in the presence of light and H2O2.

Heterogeneous photocatalysts have attracted significant interest over the last few years. In this sense, improved photocatalytic properties for SiO2@TiO2 materials have been correlated with higher semiconductor's specific surface area and adsorption capability. However, the reported TiO2 shell optimizations did not consider the percentage of the photo-excited electron-hole (e¿-h+) pairs occurring in the inaccessible surface of the TiO2 shell. The present study aimed to find the best TiO2 shell thickness for the optimal harvest of light. For this purpose, the sol¿gel method was modified by applying a precise humid airflow to get a uniform deposition of titania on the surface of the SiO2 particles. As a result, photocatalysts with a robust and homogeneous TiO2 shell of different thicknesses (from 14 to 42 nm) with well-defined particle size for TiO2 crystals (ca. 12 nm) were obtained. The SiO2@TiO2 materials were extensively characterized. Finally, their photocatalytic activity was evaluated against methylene blue under irradiation by UVA light. Analysis of the results revealed that the most efficient photocatalyst has a TiO2 shell thickness of ca. 30 nm. Emission and photocurrent studies also agreed with the photocatalytic results suggesting a reduced e¿-h+ recombination rate. Thereby, for the first time, the relevance of shell thickness in supported photocatalysts has been demonstrated.

With the main objective of overcoming the main drawbacks in the actual application of semiconductors in slurry in solar photocatalysis, new immobilized photocatalysts based on TiO supported on glass wool and silica material have been evaluated for tertiary/quaternary treatment of urban wastewater treatment plant effluents. Preliminary tests were carried out under different operating conditions in a solar simulator SUNTEST XLS+ with 30 W/m irradiation average. Demineralized water was spiked with sulfamethoxazole (SMX) (at 1 mg/L initial concentration) as water model pollutant. The mere effect of only irradiance as well as the photocatalytic performance of TiO-P25 in slurry over SMX abatement were also investigated as references. The highest SMX removal (> 98% in 25 min) was obtained with SiO_TiO photocatalyst (0.5 g/L). The efficacy of SiO_TiO was demonstrated to be stable for 5 cycles of reusing and 66% of mineralization was also obtained through the degradation of 25 mg/L of SMX after 360 min of treatment. SiO_TiO photocatalyst was also assessed for the elimination of SMX present in a more complex water matrix, in simulated urban wastewater, obtaining 50% of SMX removal in 120 min. In addition, 7 transformation products (TPs) were identified by UPHLC-QTOF-MS/MS and a tentative degradation pathway was elucidated. Assessment on actual implementation of supported TiO photocatalysts in solar photoreactors must be tackled in the future.

Although TiO materials have been extensively studied as photocatalysts, there is not any commercial TiO-supported photocatalyst used for wastewater remediation. This fact set the goal to synthesize a new supported titanium dioxide photocatalyst which, besides being robust, also has a high reaction surface and a very efficient TiO shell thickness. Hence, we present a novel TiO-supported photocatalyst composed of titania-covered glass wool (GW) fibers decorated with SiO@TiO core-shell spheres. For optimizing the photocatalytic activity of this material, the surface of SiO microspheres, as well as, the highly mechanically resistant GW fibers, were covered by a robust titania layer of ca. 20–30 nm of thickness. This layer contains nanometric crystals, ca. 12 nm in size, linked to each other and to the surfaces of both SiO microspheres and GW. An exhaustive characterization of the physicochemical properties of this new SiO-TiO composite was performed to confirm the Si[sbnd]O[sbnd]Ti linkage and the anatase crystal phase. The photocatalytic activity was initially evaluated in batch through the photodegradation of Methylene Blue as a standard dye. Afterward, the addition of the new photocatalyst in a solid phase stationary (SPS) photoreactor coupled to a TOC detector allowed studying in situ the mineralization of the recalcitrant pollutant phenol under a continuous flow regime. Results revealed that the optimized TiO surface of the SiO-TiO composite produced the complete mineralization of phenol in less than three minutes. Even more, it was demonstrated that this photocatalyst is suitable for the industrial scale-up of wastewater treatment because it does not leach titania, its reuse does not require filtration procedures, and it can be easily implemented in SPS photoreactors at plant scale. In this context, the new material could be a good starting point to prepare other supported photocatalysts for wastewater remediation.

Organic photocatalysts could employ visible light to produce photodegradation of pollutants; however, their low photostability prevents their reuse even when they are supported on solid materials. Herein, to obtain a Riboflavin (RF)-based photocatalyst robust and recyclable, RF has been covalently anchored to silica particles, converting the known homogeneous photocatalyst into a new heterogeneous one (SiO-RF). To enhance the photostability of SiO-RF, generation of the RF triplet excited state and subsequent singlet oxygen were prevented by ensuring a complete shell of RF moieties on the silica spheres. Moreover, adsorption of organic pollutants such as phenol, ortho-phenylphenol, 2,4,6-trichlorophenol and pentachlorophenol on the surface of SiO-RF is favored in aqueous media, facilitating electron transfer from the pollutants to the short-lived first singlet excited state of RF. Photophysical measurements provided evidence on the photocatalytic mechanism for SiO-RF.