Overlooked fluorescence spectra at short excitation wavelengths: Unraveled by uranium-mediated fluorescence quenching...

Uranium release from uranium resource extraction into aquatic environments has markedly elevated its ecological risks in natural waters. Chromophoric dissolved organic matter (CDOM), a major optically active fraction of dissolved organic matter (DOM), plays a key role in the carbon cycle of natural aquatic environments and can complex with specific heavy metals via complexation. This study investigated the binding mechanisms between uranium and CDOM in natural waters using the porewater collected from the Yellow River Estuarine wetland. It highlights the crucial role of the fluorescence spectra at shorter excitation wavelengths (<240 nm). Four components of CDOM were identified: microbial humic-like (C1), terrestrial humic-like (C2), tryptophan-like (C3), and an unknown humic-like (C4, potentially a photoproduct of terrestrial DOM). Among them, only the fluorescence intensity of the C4 component decreases with increasing dissolved uranium concentration. The positive curvature of the Stern-Volmer plot indicates that the quenching of the C4 by uranium does not follow the typical features of dynamic quenching. Uranium quenches the C4 component in <1 kDa fraction more efficiently compared with in >1 kDa fractions (1-3 kDa, 3-10 kDa and >10 kDa). Furthermore, this study found that using excitation-emission matrices (EEMs) with different starting excitation wavelengths in PARAFAC analysis may yield different component results. Consequently, the fluorescence quenching results derived from different PARAFAC models also showed marked differences. In particular, excluding fluorescence data below 240 nm excitation wavelength in PARAFAC analysis may cause loss or misestimate of information on metal-CDOM binding capacity and reactivity in natural waters, as a result of fluorescence peak reorganization across components. This study provides a unique perspective for understanding the biogeochemical behavior of CDOM and uranium. It highlights the necessity of paying greater attention to fluorescence spectra at shorter excitation wavelengths (<240 nm) in CDOM analysis, which can yield new insights into CDOM reactivity and the environmental fate of metals.