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.