Agricultural dissolved black carbon modulates photodegradation of antiviral drug: Insights from optical and molecular signatures

Dissolved black carbon (DBC) plays a critical role in the photochemical transformation of emerging contaminants in aquatic systems. However, the source-dependent molecular composition of DBC and its mechanistic influence on contaminant photodegradation remain poorly understood. Here, we characterize the optical properties and molecular structure of DBC derived from five crop straws and evaluate their effects on the photodegradation of antiviral drug abacavir. Results demonstrate that light absorption capacity of five DBC follows the order: DBC rice >DBC peanut >DBC sorghum >DBC wheat >DBC bamboo (DBC X , where X = crop straw). DBC rice is characterized by low molecular weight, high nitrogen content, and lignin-like components, with unsaturation provided by the carbon backbone. In contrast, DBC bamboo exhibits strong reducibility, high sulfur content, and protein-like constituents, where unsaturation is attributed to oxygen-containing functional groups. The distinct molecular composition and structure of DBC rice lead to a higher steady-state concentration of reactive intermediates than DBC bamboo , which accounts for the enhanced photodegradation rates of abacavir. The abacavir is easily transformed via photolysis in waters due to the presence of cyclopropyl and amino functional groups, and reaction pathways mainly involves with hydroxylation, oxidation, hydrogen abstraction, and C-N bond cleavage. This study highlights the critical role of source-specific molecular composition in dictating the photochemical activity of DBC, which is essential for accurately predicting the environmental fate of antiviral drugs in sunlit waters.