Thermal Hydrolysis-Induced Molecular Transformations in Sludge: Implications for Photochemical Reactivity and Dissolved Antibiotics Photodissipation

Abstract

Thermal hydrolysis is an effective technique for enhancing the solubilization of sewage sludge and improving the safety of biosolids for disposal or reuse. However, the release of various dissolved organic matter (DOM) at different TH temperatures, along with their properties that may influence intrinsic photochemical characteristics, remains poorly understood. This study investigates the temperature-dependent molecular evolution of sludge DOM (90–220°C) and its impact on antibiotic photodegradation. FT-ICR MS and ETC analysis were employed to explore the structural evolution, redox properties, and reactive oxygen species generation of DOM. The results reveal that 150°C represents a critical threshold for optimal photochemical activity. At this temperature, proteinaceous substances undergo decarboxylation and denitration, reducing polar functional groups and enhancing electron donor capacity (30.424 μmol e⁻ (mg C)⁻¹). Simultaneously, this molecular transformation facilitates the generation of excited triplet states (³DOM⁎) and significantly enhances the production efficiency of key reactive oxygen species (ROS), such as ¹O₂ and ·O₂^-. These properties significantly improved sulfamethoxazole photodegradation (kobs=0.2587 h^-1). Below 150°C, limited DOM release and reduced ROS production hinder photochemical activity, whereas above 180°C, the increased aromaticity and molecular stability of humic-like substances inhibited photochemical reactivity due to light-shielding effects. This study offers a theoretical basis for optimizing sludge thermal hydrolysis conditions and links DOM molecular structures to the fate of dissolved antibiotics during photodegradation.