Interaction of catalyst nanoparticles and pollutant molecules in photocatalytic wastewater treatment: Novel characterization via dynamic surface properties

Abstract

In photocatalytic processes for wastewater treatment, the degradation reactions mainly occurs at the surface of the catalyst and nearby regions. Therefore, understanding of the interactions between the catalyst and pollutant in a photocatalytic degradation system is of utmost importance, determining the affinity of the pollutant molecules to be attracted or repulsed from the photocatalyst particles. The aim of this experimental study is to get a better insight about the interactions between the catalyst nanoparticles and pollutant molecules via a novel characterization method based on dynamic surface phenomena analysis. Drop profile analysis tensiometry (PAT) is used for this purpose for dynamic surface tension and elasticity measurements of dye solutions (as pollutants), and in mixed solutions with catalyst particles at different pH conditions. The results demonstrate that the cationic dye Malachit Green (MG) has a low surface activity at pH 6.5, with an equilibrium surface tension about 69 mN/m. While it becomes much more surface active at pH 9 and surface tension is decreased to 59 mN/m. Adding TiO2 nanoparticles to MG solution at pH 9 causes a significant increase in surface tension, with much slower dynamics of adsorption at air/water interface. These results indicate a significant attraction and attachment of MG molecules to the surface of TiO2 nanoparticles, creating MG-TiO2 complexes, which have mobility much slower than the free MG molecules. This observation is in a good correlation with observed higher efficiency of MG degradation around pH 9. The capability of this novel experimental protocol is also examined for the anionic dye Eriochrom Black-T (EBT). According to the results, a significant interaction between TiO2 nanoparticles and EBT exist at pH 3, which is also well in accordance with reported higher efficiency of EBT degradation around pH 3. The elasticity measurements performed in this work, also support these results as an important characterization parameter for estimation of the molecular interactions in mixed adsorbed layers.