Efficient Depolymerization and Low-Toxicity Leaching of Polyester Microplastics through Alkali-Hydrothermal Treatment of Sewage Sludge

Most microplastics (MPs) in wastewater are retained within the sewage sludge. These MPs enter the soil environment through land application, posing a threat to ecosystems. This study proposes an effective control strategy using alkali pretreatment (pH 10, 5 days) followed by hydrothermal treatment at 180 °C (AHT), achieving a degradation rate of 81.83% for polyethylene terephthalate MPs (PET-MPs) in sludge. AHT promotes the formation and solubilization of key active components in sludge, such as alkalinity, which drives nucleophilic attack, metal ions, which catalyze reactions, and organic matter, which acts as radical donors. These components synergistically disintegrate PET-MPs through hydrolysis and radical oxidation pathways during hydrothermal treatment. Meanwhile, hydrothermal treatment induces polymer chain motion and physical structural disruption, accelerating the penetration and reaction of active components, thereby achieving efficient degradation of PET-MPs. Spectral and high-resolution mass spectrometry analyses reveal that sludge AHT facilitates the transformation of MP-derived dissolved organic matter (MP-DOM) into molecules characterized by low-aromaticity, low-molecular-weight, high-saturation, and high-bioavailability. Concurrently, MP-DOM exhibits low acute toxicity toward aquatic organisms and the immortalized human liver cell line (THLE-2 cells). Therefore, sludge AHT effectively degrades and converts polyester MPs into MP-DOM with low-toxicity, thereby mitigating the risks of sludge-based MPs to ecosystems.