Pyrolysis temperature dictates the molecular evolution of dissolved black carbon in UV/chlorine system using FT-ICR MS

Dissolved black carbon (DBC), particularly its optically active fraction, plays a crucial role in the photodegradation of organic pollutants. However, the transformation mechanisms of DBC's optical properties and molecular composition in UV/chlorine process remain poorly understood. In this study, we systematically investigated the photochemical evolution of rice straw-derived DBC prepared at two pyrolysis temperatures (300 °C and 500 °C) under UV/chlorine treatment, as well as their impacts on the photodegradation of acetamiprid (ACT). The results demonstrate that DBC 300 exhibited strong light absorption capacity, high aromaticity, large molecular weight, and abundant lignin-like components, whereas DBC 500 contained higher heteroatom (N, S) content, greater protein-like constituents, and enhanced reducing capacity and electron transfer capability. Under UV irradiation alone, the content of condensed aromatics, lipid-like compounds, and molecular diversity in DBC were enhanced. In contrast, chlorination reduced the light absorption capacity, humification degree, and molecular diversity of DBC. Photodegradation of ACT was primarily promoted by the generation of excited triplet DBC ( 3 DBC ⁎ ). With increasing chlorine concentration, the electron transfer capability of 3 DBC 300 ⁎ decreased, whereas that of 3 DBC 500 ⁎ increased, leading to weakened and enhanced promotion effects on ACT photodegradation, respectively. The photodegradation pathways of ACT mainly involved demethylation, hydroxylation, oxidation and C N bond cleavage. Furthermore, the photodegradation products of ACT exhibited increased toxicity toward rat and Daphnia magna , while demonstrating reduced mutagenic potential. This study provides critical insights into the photochemical transformation processes and underlying molecular mechanisms of DBC during UV/chlorine treatment.