Natural sunlight-driven photocatalytic degradation of polypropylene microplastics over ZnO nanorods

Microplastic pollution has emerged as a major global issue, with over 5 trillion plastic particles contaminating oceans and seas, posing significant risks to both ecosystems and human health. Recent studies have highlighted advanced oxidation processes as a promising technology to tackle this issue. Among these, heterogeneous photocatalysis has been identified as a particularly effective process. In this study, we fabricated ZnO-based photocatalysts, hydrothermally coated onto a glass fiber meshes with a thickness of approximately 16 microm. The ZnO nanorods exhibited an average length (l) of ∼3.2 microm and an average diameter (d) of ∼94 nm, resulting in a high length-to-diameter (l/d) ratio of 34 to enhance photocatalytic efficiency. The photocatalytic degradation of low-density polypropylene MPs (300 microm, 905 kg/m³) was tested using a 100 mL batch photoreactor in which 27 particles (∼26 mg) were suspended under natural solar irradiation for a total of 196 h. The oxidation of microplastics was quantified by monitoring the carbonyl index (CI) over a period of 16 days. The results showed a remarkable increase in the CI reaching a value of 57, approximately five times higher than that observed in the natural degradation process without the presence of a photocatalyst, and higher than in almost all recent studies.