Photo-catalytic degradation of microplastics in wastewater using Metal Modified Titanium (IV) oxide catalysts

Abstract

Microplastics (MPs) are increasingly recognized as emerging environmental pollutants due to their ubiquitous distribution, persistence, and potential impacts on ecosystems and human health. These particles, resulting from the breakdown of larger plastic materials, are small enough to be ingested by organisms, often being mistaken for food. In addition to their physical presence, MPs act as vectors for other pollutants such as heavy metals and persistent organic pollutants, which can bioaccumulate and transfer through the food chain. Although wastewater treatment plants (WWTPs) serve as barriers to MPs entering aquatic ecosystems, they are not specifically designed for MP removal and can therefore serve as point sources of MP pollution in receiving water bodies. This study assessed a novel approach to MP remediation by employing metal-modified Titanium (IV) oxide (TiO₂) nanophotocatalysts activated under visible light. High-density polyethylene (HDPE) and polyethylene terephthalate (PET) microplastics were used as model polymers to optimize degradation conditions, which were subsequently applied to environmental MP samples. The photocatalysts were synthesized using the sol-gel method with vanadium (V), cobalt (Co), and molybdenum (Mo) as modifying metal ions, a combination not extensively studied in MP degradation research. Characterization of the materials was performed using X-ray diffraction (XRD) for crystal structure, scanning electron microscopy (SEM) for morphology, and UV- Vis spectrophotometry for optical properties. Metal modification with V, Co, and Mo resulted in complete transformation of TiO₂ to the anatase phase and narrowing of the band gap from 3.2 eV to 2.7 eV, enhancing its activity under visible light. Degradation was found to be pH-dependent: acidic conditions favored HDPE degradation, while neutral to alkaline conditions enhanced PET breakdown. Fourier-transform infrared spectroscopy (FTIR) revealed formation of carbonyl (C=O) and hydroxyl (OH) groups, indicating oxidative degradation. Visual observations under an optical microscope showed yellowing, cracking, and surface deformation of the MPs. An increase in chemical oxygen demand (COD) to 230.0 mg/L in treated samples confirmed the disintegration of MPs into smaller organic compounds. Significant mass reductions were observed after 200 hours of exposure, with environmental MP samples showing 83.0 to 91.7% degradation depending on the photocatalyst used. These results demonstrate the novel potential of visible-light-driven, metal-modified TiO₂ photocatalysis as an effective method for MP degradation in wastewater. The findings suggest that this method could be incorporated into existing WWTPs as an additional treatment step to enhance the removal of MPs from wastewater.