Effect of charge-transfer complex on ultraviolet-visible (UV-Vis) absorption property of chromophoric dissolved organic matter (CDOM) in waters of typical water-level fluctuation zones of the Three Gorges Reservoir areas

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© 2016, Science Press. All right reserved. As an important fraction of dissolved organic matter (DOM), chromophoric dissolved organic matter (CDOM) plays a key role in decision of the optical properties and photogeochemistry of DOM, and further affects pollutant fate and global carbon cycle. These optical properties are ascribed to two chromophoric systems including superposition of individual chromophores and charge-transfer (CT) complexation between electron donor (e. g., phenols and indoles) and acceptor (e. g., quinones and other oxidized aromatics) in DOM structures. Thus in this study, based on the “double-chromophoric system” model, DOM samples from four typical water-level fluctuation zones of Three Gorges Reservoir (TGR) areas were selected, to investigate the effect and contribution of charge-transfer complex to ultraviolet-visible (UV-Vis) absorption property of CDOM. Using NaBH4 reduction method, original featureless absorption curve was classified into two independent curves caused by individual chromophoric group, which were derived from a simple superposition of independent chromophore and charge-transfer complex, respectively. Also, the changes in curve properties and specific parameters before and after NaBH4 reduction were compared. The results showed that in all DOM samples from the four sites of TGR, more than 35% of absorption was attributed from CT complex. Shibaozhai of Zhongxian and Zhenxi of Fuling showed the highest proportion (> 50%). It suggested that the role of CT complex in CDOM property could not be neglected. After removal of CT complex, absorption curve showed blue-shift and CDOM concentration [a(355)] decreased significantly. Meanwhile, because of deforming of bonds by reduction, DOM structures became more dispersive and the molecular size was decreased, resulting in the lower spectral slope (S) observed, which evidentially supported that the supermolecular association structure of DOM was self-assembled through CT complex. Meanwhile, deceasing hydrophobic components led to decreased apparent aromaticity (lower SUVA values), whereas specific parameters including SUVA, CDOM and SR still were applicable for comparison among different DOM samples instead of the same sample without consideration of “double-chromophoric system” model involving the role of CT complex. Comparatively, S275-295 was dynamic due to the impact of CT effect. Furthermore, establishing DOC estimation model by short-wavelength range of CDOM was recommended because of its stability despite of CT complex.

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Huanjing Kexue/Environmental Science