Toxicity change patterns and its mechanism during the degradation of nitrogen-heterocyclic compounds by O(3)/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 O(3)/UV. It was found that for all the three NHCs there was some H(2)O(2) formed in the degradation process, which caused the sharp increase of toxicity to Photobacterium phosphoreum. The toxicity decreasing patterns after H(2)O(2) elimination were different for quinoline (or isoquinoline) and indole. After H(2)O(2) 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 O(3) (k(D)) 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 k(D) values of simple NHCs and homocyclic aromatics were successfully established, which would be useful to predict their toxicity patterns.