Phototransformation of benzodiazepines in water mediated by dissolved black carbon: Mechanisms and the structure-reactivity relationship

Abstract

Dissolved black carbon (DBC) within aquatic dissolved organic matters demonstrates potent photochemical activity, yet its effects on emerging contaminant transformation and structure-reactivity relationships remain inadequately characterized. In this study, benzodiazepines were employed as model emerging contaminants to investigate the photoactivity, mediation effect, and structural-activity relationships of five DBC samples and four well-studied dissolved humic substances (DHSs) under simulated sunlight irradiation. DBC efficiently generated superoxide anions (O₂^·-), singlet oxygen (¹O₂), and triplet excited states, thereby facilitating the phototransformation of benzodiazepines. Under DBC mediation, midazolam and flurazepam achieved transformation rates of 61.1-99.5% and 19.0-84.6%, respectively, within 8 h, exceeding direct phototransformation efficiencies (1.3-13.3%). Triplet excited states were identified as the dominant reactive intermediates, contributing more than 46.7% to the photoreactions. At equivalent total organic carbon levels, DBCs averagely exhibited 1.3-, 2.7-, and 3.3-fold higher photoactivity (O₂^·-, ¹O₂, and triplet excited states) alongside 2.5- and 4.7-fold enhanced benzodiazepine (midazolam and flurazepam) transformation compared to DHSs based on quantum yield measurements. Phototransformation pathways of benzodiazepines via DBC-mediation included ¹O₂ oxidation and charge transfer with triplet excited states, generating charge-separated intermediates that subsequently induce ring cleavage and coupling reactions. The molecular structures of DBC and DHS were characterized using UV-visible spectroscopy and Fourier transform-ion cyclotron resonance mass spectrometry. To elucidate the structure-reactivity relationship, Spearman rank correlation analysis, structural equation modeling, and orthogonal partial least squares regression were employed. The results identify molecular weight, aromaticity, and oxidation degree as key structural determinants of photoactivity. Low-molecular-weight compounds containing condensed aromatic structures and lignins demonstrate superior photoactivity in DBC/DHS structures, driving triplet excited-state and ¹O₂ formation, while O₂^·- might be generated from different structures. This study elucidated DBC mediation mechanisms and structure-reactivity relationships in benzodiazepine phototransformation, identifying low-molecular-weight fractions containing condensed aromatic and lignin moieties as dominant photoactive drivers in aquatic environment.