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 (Mn, Cr, Cu, Fe, Zn, Al, Ca, and Mg). Complexation constants (logK) derived from three-dimensional fluorescence spectra revealed that Mn, Cr, Cu, Fe, Zn, and Al 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, Mn, Cr, Cu, Fe, Zn and Al inhibited the photogeneration of DBC* via dynamic quenching, which reduced the yields of DBC*-derived O and O·. Moreover, DBC* quenching by metal ions was associated with the complexation constant. A strong positive linear relationship existed between logK and the dynamic quenching rate constant of metal ions. These results indicate that the strong complexation ability of metal ions enabled DBC quenching, which highlights the photochemical activity of DBC in natural aquatic environments enriched with metal ions.