Assessing the efficiency of photocatalytic degradation via TiO2 nanoparticles and nanotubes for complete mineralization of nanoplastics in drinking water

Abstract

Natural and engineered nanoplastics are persistent organic pollutants that form a risk to human health and the ecosystem due to their bioaccumulation and recalcitrant nature. Although conventional water and wastewater treatment methods remove nanoplastics from water, these methods cause secondary pollution by transferring the pollutants in concentrated form to other environments. Advanced oxidation processes, on the other hand, are methods that result in high treatment efficiencies for the degradation and mineralization of organic pollutants in an aqueous medium by the formation of free radicals in the presence of a photocatalyst. However, in the literature, it has been observed that only up to 50% mineralization is achieved when this method is used to degrade microplastics in water. Therefore, in this thesis, instead of assessing the treatment efficiency of the better-known microplastics, it is aimed to investigate the treatment of not well-known nanoplastics in water using multiple characterization techniques, especially HRTEM, GC-MS, Zetasizer, and FTIR. In the study, the effects of operating parameters on the mineralization were examined. For the sustainability of the process, the reusability of the photocatalyst with and without recovery was investigated. In this context, a comparison was made between the nanoparticle (anatase and a mixture of anatase/rutile) that stands out as the best photocatalyst and nanotube form of TiO2. The anatase (10-20 nm, 80 m2/g) and the mixture of 80/20% anatase/rutile (18 nm, 60-90%) form of TiO2 nanoparticles, and TiO2 nanotubes (15-20 µm length, 50-100 nm diameter, and 15 nm wall thickness) were used for degradation of 22 nm engineered polystyrene nanoplastics as a model for treating nanoplastic pollution in the aqueous environment. Langmuir-Hinshelwood kinetic model was found suitable for photocatalytic degradation, and reaction rate constants were calculated. The results showed that the mixture of anatase/rutile form of TiO2 had a mineralization efficiency of 90% for polystyrene nanoplastics under optimum operating parameters (96 mg/L TiO2 dose, pH 6.35, 6 hours irradiation) under UV irradiation from 4.5 cm distance and 300 rpm magnetic stirring. Reusability tests showed the potential of TiO2 nanoparticles with high treatment efficiency after 5 reuse cycles. Under optimum operating parameters, it was determined that the method without recovery performed better than the method with recovery.