Hydrothermal versus photo-aged polyethylene microplastics: Aging mode-dependent oxidative stress and enzyme...

The aging of microplastics (MPs) inevitably occurs during sludge treatment, leading to the leaching of dissolved organic matter (DOM) and associated chemical additives. However, the impact and mechanism of aged leachate on anaerobic fermentation of waste activated sludge (WAS), particularly its ecological risks and implications for sludge resource recovery, remain poorly understood. This study investigated the effects of leachates prepared from photochemically (PC) and hydrothermally (HT) aged polyethylene (PE) MPs at an initial PE-MPs concentration of 5 g/L on short-chain fatty acids (SCFAs) production during WAS anaerobic fermentation. The results showed that PC and HT leachates reduced SCFAs production by 21% and 30.6%, respectively, and shifted the product composition from acetate-dominant to a mixed-acid profile. Biochemical analyses revealed that the aged leachates induced oxidative stress and inhibited protease and α-glucosidase activities, resulting in only 2.3%-6.0% protein degradation and a 23.1%-23.8% decrease in carbohydrate hydrolysis efficiency. Microbial community analysis further demonstrated that both PC and HT leachates reduced microbial diversity and the abundance of key hydrolytic genera, thereby compromising the functions of acid-producing and nitrogen-cycling bacteria. Analysis of leachate composition indicated that HT leachates released more oxygen-containing depolymerization products and additives, leading to stronger inhibition of microbial activity and hydrolysis processes. ECOSAR predictions suggested that several phenolic antioxidants and phthalate plasticizers may pose relatively high aquatic hazards. Their presence may partly contribute to the stronger biological inhibition observed in the aged-leachate treatments. These findings provide critical insights into the toxicity mechanisms of aged MP leachates in sludge fermentation systems.