Characterizing the transformation of aquatic humic substances exposed to ultraviolet radiation using excitation–emission matrix fluorescence spectroscopy and PARAFAC

Abstract

ABSTRACTIt is important to understand the change in aquatic humic substances (AHS) induced by light due to the upward trend in ultraviolet (UV) radiation reaching the surface of the Earth. Changes in the quantity and quality of AHS in AHS-rich wetland water exposed to UV-A or UV-B light were determined using 3-dimensional excitation–emission matrix (EEM) fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) and a resin isolation method. The dissolved organic carbon and AHS-carbon concentrations decreased via photodegradation with UV-A or UV-B exposure. The decreases in both carbon concentrations were greater when exposed to UV-B than when exposed to UV-A. Three AHS-like components were detected by EEM-PARAFAC: AHS-1, AHS-2, and AHS-3. AHS-1 and AHS-3 were degraded more by UV-A and UV-B exposure, respectively. AHS-2 was degraded slightly by UV-A exposure, whereas exposure to UV-B generated AHS-2 in the presence of low-molecular-weight compounds, and then underwent photodegradation. AHS-1 exposed to UV-A and AHS-3 exposed to UV-A or UV-B photoflocculated at day 14. The humification index indicated that UV-A exposure led to the preferential photodegradation of AHS, whereas UV-B exposure not only caused AHS degradation but also the degradation or photoflocculation of other compounds with simple structures. AHS changed little in the dark, demonstrating that bacterial effects on AHS were relatively unimportant. The processes of humification and flocculation are important in removing and sequestering carbon from its active cycle. Therefore, the degradation and transformation of AHS exposed to UV can have a major impact on aquatic ecosystem processes.