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Cotransformation of Carbon Dots and Contaminant under Light in Aqueous Solutions: A Mechanistic Study

Chen, Xiru, Fang, Guodong, Liu, Cun, Dionysiou, Dionysios D., Wang, Xiaolei, Zhu, Changyin, Wang, Yujun, Gao, Juan, Zhou, Dongmei
Environmental science & technology 2019 v.53 no.11 pp. 6235-6244
anions, aquatic environment, aqueous solutions, carbon dioxide, carbon quantum dots, carbonates, chlorides, diethyl phthalate, electron paramagnetic resonance spectroscopy, free radicals, humic substances, hydroxyl radicals, mass spectrometry, mineralization, nitrates, photochemical reactions, pollutants, risk, singlet oxygen, solar radiation, superoxide anion, total organic carbon, ultraviolet radiation
In this study, the photochemistry of carbon dots (CDs) and their effects on pollutant transformation were systematically examined. Diethyl phthalate (DEP) degradation was strongly enhanced by CDs under UV light, with the observed reaction rate constant (kₒbₛ) increased by 2.4–15.1-fold by CDs at a concentration of 0.5–10 mg/L. Electron paramagnetic resonance (EPR) spectrometry combined with free radical quenching experiments with various chemical probes indicated the production of reactive oxygen species (ROS), including hydroxyl radicals (•OH), singlet oxygen (¹O₂), and superoxide radical anions (O₂•–), and these contributed to the enhanced DEP degradation. Meanwhile, CDs were also degraded to low-molecular-weight species and partially mineralized to CO₂ by ROS, as evidenced by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and total organic carbon (TOC) analysis, and transformation of CDs was accelerated by DEP. Furthermore, CDs were degraded rapidly under natural sunlight, accompanied by the formation of •OH and ¹O₂. Anions such as CO₃²–, NO₃–, and Cl– had limited effects on transformation of CDs, while humic substances greatly inhibited this process. Our results indicate that photoreactions of CDs play an important role in influencing the transformation of pollutants and CDs themselves in the natural aquatic environment. The findings provide invaluable information for evaluating risks associated with the release of CDs into the natural environment.