Removal of AOX from pulp mill industry effluents by heterogeneous photocatalysis

• Autor(a) principal: Leal, Cátia Daniela Jordão
• Data de Publicação: 2018
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/24480
• topic: Photocatalysis; Adsorbable organic halides; Bleaching effluent; Titanium dioxide; Magnetite

In recent years, pulp and paper mills have faced challenges on wastewater treatment management, due to the increase of effluents production and the environmental concerns, related with the need to meet progressively rigorous discharge limits imposed by new legislation. The effluents generated in pulp bleaching contain, among other organic pollutants, adsorbable organic halides (AOX) that can be photo-oxidized using ultraviolet radiation. There are strong evidences that confirm the efficiency of nanoparticles of titanium dioxide (TiO2) as photocatalyst in the photodegradation of AOX compounds. However, there is a need to develop supports for nano-TiO2, in order to allow its recovery and reutilization in consecutive photocatalytic tests, and this is the main aim of this thesis. Thus, this work investigated two independent strategies: (1) the preparation of nano-TiO2 with magnetic properties aiming easy and fast recovery by magnetic separation and (2) the use of geopolymer spheres containing TiO2, that were produced with a percentage of ash from residues of pulp and paper mills. All the materials were characterized using X-ray powder diffraction, spectroscopy (FTIR and UV-vis) and electron microscopy (SEM/TEM). The TiO2 nanoparticles with magnetic properties were prepared by in situ co-precipitation of magnetite (Fe3O4) in the presence of TiO2. The Fe3O4/TiO2 materials were prepared with variable TiO2 content (between 14.0 and 43.5wt%) and were composed of nanoparticles of magnetite and TiO2, in an irregular and non-homogeneous configuration. The highest TiO2 content was obtained for the sample Fe3O4/TiO2 (1:3) after centrifugation (43.5%). The geopolymer spheres containing TiO2 were around 3 mm average diameter, with porous surface. In terms of the chemical composition, the major crystalline compounds were TiO2 (24%), quartz (22%) and muscovite (20%). In a later stage, all materials were tested in the photodegradation of AOX present in real bleaching stream samples. The experiments were carried out for 1 hour in a laboratory photoreactor, with addition of H2O2 and using UV-light as radiation source. The percentage of AOX removal in all the supported photocatalysts was similar. The best results were attained with Fe3O4/TiO2 (1:3) centrifuged (73.3% AOX removal) and with the spheres (68.9% AOX removal). Comparing with TiO2 non-supported (76.6%), the removal efficiency on supported photocatalysts was slightly lower. In addition, it was also seen that the use of UV-light increases the performance of the photocatalysts, while the type of stirring (magnetic vs injection of compressed air) did not influence significantly the results. The percentage of catalyst magnetically recovered decreased with the increase of TiO2 content and with UV-light exposure, thus suggesting that these materials are sensitive to UV radiation and that they lose some of their magnetic force during the photocatalysis trials. Reusing the recovered photocatalysts in a second run, the AOX removal was relatively similar to the previous ones, with the best result obtained with Fe3O4/TiO2 (1:1), 75.8%, which proved their ability to be efficiently reused to remove AOX. The geopolymer spheres presented their surface and pores degraded after photocatalysis, but their chemical composition did not change significantly. These photocatalysts were also subjected to a second series of tests, which allowed to remove about 65.5% of AOX