Enhancing nanoplastics removal by metal ion-catalyzed ozonation

Nanoplastics (NPs), characterized by sizes < 1 µm, are not completely removed in conventional drinking water treatment plants affecting human health. Catalytic ozonation appears as a promising alternative due to its ability to oxidize emerging pollutants by generating hydroxyl radicals with higher removal rate and mineralization than single ozonation. In this work, catalytic ozonation was investigated for the removal of polystyrene nanoplastics (PSNPs) by employing transition metal ion catalysts, Fe³⁺, Co²⁺, Ni²⁺, and Zn²⁺. The single ozonation of PSNPs, in the absence of catalyst, led to low turbidity reduction (33 %) and low mineralization rate (16 %) even after 2 h reaction time. Nevertheless, the rapid size reduction of PSNPs by >99 % in less than 5 min of ozonation (TOD: 30 mg L⁻¹) was confirmed. This fact could imply a new issue under the ozone disinfection conditions by the formation of smaller-size particles. However, in the presence of Co²⁺ (1 mM), the highest PSNPs ozonation performance was achieved, decreasing the turbidity up to 65 % and achieving 70 % of mineralization in the same ozonation time. The scavenger experiments with methanol confirmed that direct ozonation and catalyst role were responsible for PSNPs degradation, with the generation of •OH being the most dominant catalytic mechanism. Therefore, although single ozonation at disinfection doses reduced the particle size of PSNPs, the catalytic process demonstrated greater efficacy in the total removal of PSNPs. These results highlight the need to further investigate ozonation of NPs intermediates, and the synergic improvements of catalytic ozonation for their mineralization.