Development and evaluation of a composite photocatalyst for water treatmment processes

Titanium dioxide (Ti0₂) has been extensively studied for photocatalytic oxidation of organic pollutants, in water and gas effluents, into smaller and harmless substances such as C0₂ and H₂0. However, there are still a number of drawbacks that hinder its large scale applications, for example expensive downstream filtration of the photocatalyst powder, mass transfer limitations, loss of activity, attrition, among others. Fluidized bed photoreactor (FBPR) with its advantages for potential commercial applications, suffers from the problem of attrition and elutriation of particles. This research focused on addressing this challenge and worked on the development of a template-free composite photocatalyst to be used in a FBPR. It involved the production of Ti0₂ nanoparticles, using sol-gel process, and mixing them with Ti0₂ pre-calcined powder (Degussa P-25), used as filler material bonded together with the gel-derived Ti0₂. This solution was then mixed with a polymeric structure to produce the spheres. A complete characterization of the Ti0₂ spheres was performed to determine their morphology (XRD - X Ray Diffraction, SEM - scanning electron microscopy), physicochemical properties (BET surface area, pore volume and pore diameter) and thermal behaviour (TGA - Thermo gravimetric analysis). The major achievement of this work was to modify the formulation and synthesis procédure of the composite Ti0₂ spheres, producing a photocatalyst with good attrition resistance and high photoactivity. The improvements were made by working on the preparation conditions (modifying the hydrolysis and condensation rates during sol-gel preparation, aging time, spheres formation, drying process, and calcination conditions) and evaluating their impacts on the attrition resistance (amount of Ti0₂ particles released to the solution during the normal operation of the FBPR). Photocatalyst activity was measured based on its efficacy at decomposing organic compounds such as formic acid (FA) and 2,4-Dichlorophenoacetic acid (2,4-D), as well as natural organic matter (NOM). The attrition resistance of the TiO₂ spheres was improved by 70%, corresponding to a reduction in the amount of TiO₂ particles released from the catalyst from 22 mg L-¹ to 7 mg L-¹. This was achieved via the following modifications in the formulation and synthesis procedure: • Accelerating the rate of hydrolysis and condensation reactions that occur during the formation of the sol-gel matrix, by increasing the amount of water from 0.040 g H₂O per mL-¹ of Ti precursor to 0.053 g H₂O per mL-¹ Ti precursor and reducing the amount of catalyst (HCI) from 0.20 mL HCI per mL-¹ Ti precursor to 0.13 mL HCI per mL-¹ Ti precursor. . Increasing pH of the solution, where the spheres are formed, from 11.75 to 12. • Accelerating the drying process from 15 days at 23°C to 20 h at 80°C. • Increasing the calcination time from 1 h to 3 h. In terms of the photocatalytic activity, the composite TiO₂ spheres demonstrated high activity to degrade either FA or 2,4-D. The degradation of both model pollutants followed first order kinetics with rates constants of 0.317 min-¹ (with fluence rate of 5.32 mW cm-²) and 0.736 min"1 (with a fluence rate of 4.16 mW cm-²) for FA and 2,4-D, respectively. A comparison with Degussa P-25 was made showing that the activity of the Ti0₂ sphères is higher than that of the commercial Ti0₂ powder for both model organic compounds. For NOM dégradation, water from Trepanier Creek in Central British Columbia (initial TOC of approximately 5 mg L-¹) was treated. There was about 50% reduction of NOM after 1 h, but this was primarily due to the adsorption of NOM on the spheres.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat