Integrating Mechanochemistry with Advanced Oxidation for Mild-Condition Degradation of Inert Polyolefins

Plastic waste is one of the most persistent pollutants in the environment. Despite global efforts to enhance recycling practices, the recycling rate remains low. Over time, plastic waste fragments into microplastics that are difficult to remove from ecosystems. According to a 2021 study, microplastic concentrations in sludge have reached 4.40 × 10³–2.40 × 10⁵ particles/kg. These particles can adsorb contaminants such as heavy metals and toxic hydrocarbons, posing significant threats to environmental and human health by entering the food chain. Recent studies have explored microplastic removal through physical, mechanical, and chemical methods. Physical methods include flocculation, membrane filtration, and physisorption, while chemical methods involve advanced oxidation processes, pyrolysis, hydrogenolysis, cracking, and biodegradation. However, most chemical degradation methods require high temperatures (up to 300°C), making them energy-intensive and costly. This study aims to develop a microplastic degradation process through advanced oxidation processes (AOPs) via two different approaches: (1) AOPs under hydrothermal condition and (2) mechanochemical-assisted AOP degradation via ball milling. Previously, we have attempted the degradation of PS microplastic in light under ambient temperature. Gel permeation chromatography (GPC) analysis of the molecular weight suggests that SR-AOP using SO4 radicals is not effective in degrading polystyrene microplastic at room temperature in water. Further increasing the reaction temperature to 80°C and maintaining for 48 hours showed no significant changes in the molecular weight of the microplastic. Our result is consistent with literatures, demonstrating the inherent chemical inertness of polystyrene, coupled with its limited swellability in water, restricts the penetration of reactive oxygen species (ROS) into the bulk of the microplastic. To overcome this challenge, we implemented an AOP under hydrothermal conditions at temperatures exceeding the polymer's glass transition temperature (Tg). This approach aims to induce microplastic swelling, thereby increasing the exposure of reactive surfaces and enhancing degradation. In our experiment, 1 g/L of commercial CoO was used as a catalyst, along with 200 mM of potassium peroxymonosulfate (PMS), in 150 mL of DI water containing 1 g/L of polystyrene. After 2 days of reaction at 140°C, a 14% weight loss was observed (Figure 1a). Additionally, changes in molecular weight distribution observed in GPC analysis suggest that the polymer chain underwent degradation, breaking down into smaller molecular fragments (Figure 1b). Higher dosage of CoO and PMS led to further increase in weight loss and a more pronounced shift in molecular weight distribution (Figure 2, Figure 3) In our second approach, we integrated AOP with ball milling to enhance the degradation of inert microplastic more effectively. Ball milling generates high shear forces that mechanically cleave polymer chains by breaking carbon-carbon bonds. In our preliminary study, 150 mg of CoO and 18.45 g of PMS were added to a milling jar containing 150 mg of polystyrene. To prevent powder adhesion to the jar walls, 10 mL of deionized water was introduced. The mixture was subjected to 2D ball milling at 133 rpm using 20 stainless steel balls (10 mm diameter) for 48 hours. GPC analysis revealed no significant change in molecular weight. However, a substantial increase in pressure developed in the reaction jar suggested that a reaction had occurred. Gas chromatography analysis of a 50 µL gas sample extracted from the milling jar confirmed the generation of CO₂ (Figure 4a) , a phenomenon observed only when both the catalyst and oxidant were present. To enhance reaction efficiency, we are currently investigating the use of a planetary ball mill and vibrational ball mill, which operates at higher energy levels. The success of this research strategy will be presented, highlighting its potential as a viable approach for plastic degradation through the integration of AOP and mechanochemistry. To enhance the mechanochemical assisted AOP degradation of microplastic, we are developing a BiFeO3 catalysts as a multifunctional piezo-photocatalytic PMS activator to exert synergistic effect in enhancing the degradation efficiency under mechanochemical-assisted AOPs. When BiFeO₃ is subjected to strain, its piezoelectric properties generate internal electric field that facilitates charge separation and transfer to the surface. The resulting e⁻/h⁺ pairs enhance redox reactions at Bi/Fe sites, thereby improving PMS activation efficiency. Additionally, ball milling provides several advantages, including catalyst particle refinement to increase active site availability, enhanced interfacial interactions, improved surface adsorption, and high energy efficiency. Collectively, these advantages could contribute to the enhancement in the degradation efficiency of plastic. Key words: microplastic degradation, advanced oxidation processes, PMS activation