Toxicity change patterns and its mechanism during the degradation of nitrogen-heterocyclic compounds by O3/UV

Many nitrogen-heterocyclic compounds (NHCs) are toxic and recalcitrant contaminants that need to be degraded by advanced oxidation processes. In this study, quinoline, isoquinoline and indole were selected to investigate their toxicity patterns during the degradation by O3/UV. It was found that for all the three NHCs there was some H2O2 formed in the degradation process, which caused the sharp increase of toxicity to Photobacterium phosphoreum. The toxicity decreasing patterns after H2O2 elimination were different for quinoline (or isoquinoline) and indole. After H2O2 elimination, for quinoline or isoquinoline the toxicity decreased concurrently with the decrease of its concentration, while for indole the toxicity lagged behind its removal rate. The rate constant of the NHC with O3 (kD) was the decisive parameter of its toxicity pattern because of its critical role in determining the degradation rate of the NHC. Two quantitative structure–activity relationship equations for the kD values of simple NHCs and homocyclic aromatics were successfully established, which would be useful to predict their toxicity patterns.