Self-assembled thin films of PAA/PAH/TiO2 for the photooxidation of ibuprofen. Part I: Optimization of photoactivity using design of experiments and surface response methodology

Abstract

The degradation of ibuprofen by photocatalysis using solar energy and TiO2 nanoparticles is a promising energy-efficient alternative to conventional water treatments. However, nano-TiO2 is toxic and can disturb the redox cycles of micro-ecosystems, affecting the local microbiota. By considering these aspects, the present study introduces the preparation and optimization of novel nanostructured self-assembled thin films (SATFs) for the immobilization of TiO2, aiming to prevent the leaching of this photocatalyst in the environment. In the Part I of this study, we focused more specifically on the deposition and application of SATFs obtained by a combination of poly(acrylic acid), poly(allylamine hydrochloride) and TiO2 through the layer-by-layer technique, using response surface methodology (RSM) and design of experiments (DOE) to evaluate the deposition parameters that lead to the best properties for photocatalytic application. These tools were employed to optimize the properties of this kind of photoactive material with high statistical confidence. The pH of the cationic solution and the TiO2 concentration were the only statistically significant factors affecting the photoactivity of the SATFs. The optimized SATF presented degradation efficiencies similar to those of colloidal systems, being able to reduce the concentration of ibuprofen by 95% and degrade 50% of its aromatic centres, after 150 min and at a neutral pH of approximately 6.5. The high efficiency of the SATFs is attributed to its nanostructured topography, which provides a large active surface for the photocatalytic degradation of contaminants. Moreover, for the first time, the photoactivity and thickness of the SATF were mathematically modelled through DOE and RSM and can be predicted as a function of pH and TiO2 concentration.