Photochemistry of plateau lake-derived dissolved organic matter: Reactive species generation and effects on 17ß-estradiol photodegradation.

Naturally strong ultraviolet irradiation at high altitudes causes photobleaching of plateau lake DOM (P-DOM) and affects its photochemical activity. However, the photoreactivity of P-DOM has remained unclear under natural photobleaching condition. Here, six P-DOM samples isolated from plateau lakes in Yunnan Province, China as well as two reference DOM as comparisons were used to explore the photogeneration of reactive species (RS) and their effects on 17ß-estradiol photodegradation. Compared with SRHA/SRFA, P-DOM has lower aromaticity, average molecular weight, and electron-donating capacity. The quantum yields of triplet state P-DOM (3P-DOM*), 1O2, and ∙OH produced in P-DOM solutions were greatly higher than those of reference DOM. The RS quantum yields had positive linear correlations with E2/E3 and SR, whereas were negatively linear correlated with SUVA25. Radical quenching experiments showed that 3P-DOM* was the prominent RS for 17ß-estradiol photodegradation, and its contribution exceeded 70% for each of P-DOM. 3P-DOM*-mediated photodegradation was mainly attributed to the electron-transfer reactions with an average second-order rate constant of 4.62 × 109 M-1s-1, indicating the strong photoreactivity towards 17ß-estradiol. These findings demonstrate that P-DOM is an efficient photosensitizer for RS production, among which 3P-DOM* may play an important role in enhanced photodegradation for organic micropollutants in plateau lake enriched with DOM.

Inhibiting mechanisms of metal ion complexation on photogenerated reactive intermediates derived from dissolved black carbon.

Dissolved black carbon (DBC), an important photosensitizer in surface waters, can influence the photodegradation of various organic micropollutants. In natural water systems, DBC often co-occurs with metal ions as DBC-metal ion complexes; however, the influence of metal ion complexation on the photochemical activity of DBC is still unclear. Herein, the effects of metal ion complexation were investigated using common metal ions (Mn2+, Cr3+, Cu2+, Fe3+, Zn2+, Al3+, Ca2+, and Mg2+). Complexation constants (logKM) derived from three-dimensional fluorescence spectra revealed that Mn2+, Cr3+, Cu2+, Fe3+, Zn2+, and Al3+ quenched the fluorescence components of DBC via static quenching. The steady-state radical experiment suggested that in the complex systems of DBC with various metal ions, Mn2+, Cr3+, Cu2+, Fe3+, Zn2+ and Al3+ inhibited the photogeneration of 3DBC* via dynamic quenching, which reduced the yields of 3DBC*-derived 1O2 and O2·-. Moreover, 3DBC* quenching by metal ions was associated with the complexation constant. A strong positive linear relationship existed between logKM and the dynamic quenching rate constant of metal ions. These results indicate that the strong complexation ability of metal ions enabled 3DBC quenching, which highlights the photochemical activity of DBC in natural aquatic environments enriched with metal ions.

Dual roles of Cu2+ complexation with dissolved organic matter on the photodegradation of trace organic pollutants: Triplet- and OH-induced reactions.

The triplet excited state of dissolved organic matter (3DOM⁎) is highly effective in the photodegradation of a broad spectrum of trace organic pollutants (TOPs), and its photoactivity is affected by concomitant metal ions in surface waters. However, the impact of environmental metal ions on the 3DOM⁎-induced photodegradation of TOPs has not been systemically explored. Herein, we investigated the effect of environmental Cu2+ on the 3DOM⁎-induced photodegradation kinetics of 16 TOPs. A fluorescence quenching experiment showed that a Cu(II)-DOM complex was formed. For the TOPs with stronger electron-donating groups (triplet-labile moieties, e.g., phenols and anilines), Cu2+ complexation notably inhibited 3DOM⁎-induced photodegradation. This may be ascribed to the decrease of 3DOM⁎ steady-state concentration because Cu2+ complexation reduces its formation rates and enhances scavenging rates tested by sorbic acid isomerization experiment. Meanwhile, it was found that Cu2+ complexation facilitated the photolysis of refractory TOPs (lower triplet reactivity) because of enhanced electron transfer between DOM and Cu(II), causing photoinduced OH formation. These findings implied that 3DOM⁎ reactivity differences in TOPs could affect the photodegradation rates in the complex system, which was confirmed via a linear correlation of photodegradation rate ratios for 16 TOPs induced by 3DOM⁎ in the presence/absence of Cu2+ with their 3DOM⁎ reactivity. These findings helped to improve our understanding of the photochemical reactivity of 3DOM⁎ in natural waters, especially the effects of environmentally concomitant metal ions.

Enhancive and inhibitory effects of copper complexation on triplet dissolved black carbon-sensitized photodegradation of organic micropollutants.

Excited-triplet dissolved black carbon (DBC) was deemed as a significant reactive intermediate in the phototransformation of environmental micropollutants, but the impacts of concomitant metal ions on photochemical behavior of excited-triplet DBC (3DBC*) are poorly understood. Here, the photolytic kinetics of sulfadiazine and carbamazepine induced by 3DBC* involving Cu2+ was explored. The presence of Cu2+ reduced the 3DBC*-induced photodegradation rate of sulfadiazine; whereas for carbamazepine, Cu2+ enhanced 3DBC*-induced photodegradation. Cu(II)-DBC complex was formed due to the decreasing fluorescence intensities of DBC in the presence of Cu2+. Cu2+ complexation caused the decrease of 3DBC* steady-state concentrations, which markedly reduced 3DBC*-induced photodegradation rate of sulfadiazine due to its high triplet reactivity. Kinetic model showed that 3DBC* quenching rate by Cu2+ was 7.98 × 109 M-1 s-1. Cu2+ complexation can also enhance the electron transfer ability, thereby producing more ∙OH in Cu(II)-DBC complex, which explains the promoting effect of Cu2+ complexation on carbamazepine photodegradation in view of its low triplet reaction rate. These indicate that 3DBC* reactivity differences of organic micropollutants may explain their photodegradation kinetics differences in DBC system with/without Cu2+, which was supported by the linearized relationship between the photodegradation rate ratios of ten micropollutants with/without Cu2+ and their triplet reaction activity.