Elimination of nanoplastics by ozonation for water purification

Nanoplastics—plastic particles smaller than 100 nanometers—have emerged as a significant environmental contaminant, especially in aquatic systems. These tiny fragments originate from the degradation of larger plastics or are manufactured intentionally for industrial use. Their minute size allows them to penetrate biological membranes, potentially causing cellular damage in aquatic organisms and posing risks to human health through drinking water and the food chain. Addressing the issue of nanoplastics requires innovative and effective removal techniques. Among these, ozonation has gained attention as a promising advanced oxidation process (AOP) capable of degrading and eliminating nanoplastics from water systems.

Understanding ozonation

Ozonation involves the use of ozone (O₃), a powerful oxidizing agent, to break down contaminants in water. Ozone can be generated on-site using ozone generators, typically via corona discharge or ultraviolet radiation, by splitting oxygen molecules (O₂) into atomic oxygen (O) which then recombines with O₂ to form O₃. When dissolved in water, ozone reacts with organic pollutants through direct oxidation or indirectly via the generation of hydroxyl radicals (•OH), which are even more reactive.

The oxidation potential of ozone (2.07 V) and hydroxyl radicals (2.8 V) enables them to cleave carbon-carbon bonds, modify polymer structures, and convert harmful compounds into carbon dioxide, water, and other benign substances.

Mechanism of nanoplastic degradation by ozonation

The degradation of nanoplastics through ozonation primarily occurs via:

1. Direct Reaction with Ozone: Ozone can attack electron-rich sites in plastic polymers, such as double bonds or aromatic rings. For example, in polystyrene nanoplastics, ozone reacts with the phenyl rings, leading to ring opening and fragmentation.
2. Indirect Reaction via Hydroxyl Radicals: In aqueous solutions, ozone decomposes to form hydroxyl radicals, especially under alkaline conditions or in the presence of catalysts like hydrogen peroxide (O₃/H₂O₂) or UV light (UV/O₃). These radicals nonspecifically attack polymer chains, leading to chain scission and the formation of smaller, more biodegradable molecules.
3. Surface Functionalization and Fragmentation: As ozonation progresses, the surface of nanoplastics becomes functionalized with oxygen-containing groups like carbonyls, carboxyls, and hydroxyls. This functionalization increases hydrophilicity, facilitates aggregation, and enhances susceptibility to further degradation or removal via filtration.

Experimental evidence

A study conducted by Kalčíková et al. (2021) demonstrated the effectiveness of ozonation in degrading polyethylene and polystyrene nanoplastics in synthetic wastewater. Ozone treatment led to a significant reduction in nanoplastic particle size and number within 30 to 60 minutes of exposure. Fourier Transform Infrared Spectroscopy (FTIR) confirmed the formation of oxidized functional groups, while Scanning Electron Microscopy (SEM) revealed morphological damage and surface erosion of plastic particles.

Another study explored the combination of ozonation with membrane filtration and found that the pre-treatment of water with ozone enhanced the retention of nanoplastics by ultrafiltration membranes due to increased particle aggregation.

Advantages of ozonation

• High Oxidation Potential: Ozone and its reactive species can degrade a wide range of polymers, including polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS).
• No Residual Chemicals: Unlike chlorine or other disinfectants, ozone decomposes into oxygen, leaving no harmful residues.
• Synergy with Other AOPs: Ozonation can be combined with UV radiation, hydrogen peroxide, or photocatalysts (like TiO₂) for enhanced degradation.
• Enhanced Biodegradability: Oxidation can transform persistent plastics into smaller, biodegradable molecules, aiding subsequent biological treatment processes.

Future directions

To enhance the practical application of ozonation for nanoplastic removal, researchers are focusing on:

• Catalytic Ozonation: Using catalysts like manganese oxides or carbon-based materials to improve ozone decomposition and increase radical yield.
• Hybrid Systems: Integrating ozonation with biological reactors, membranes, or adsorption systems to achieve complete purification.
• Automation and Sensing: Developing real-time sensors for nanoplastics and ozone levels to optimize treatment processes.

Ozonation presents a powerful and environmentally friendly approach for mitigating the growing problem of nanoplastics in water systems. By leveraging the high oxidative potential of ozone and its derivatives, it is possible to break down these persistent pollutants and reduce their ecological and health impacts. Continued research, process optimization, and integration with other treatment technologies will be key to scaling up ozonation for widespread environmental application.