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Degradation pathway of quinolines in a biofilm system under denitrifying conditions

This article reports for the first time the degradation pathways of quinoline, isoquinoline, and methylquinolines by a mixed culture in a biofilm under nitrate‐reducing conditions. A simple reverse‐phase high‐performance liquid chromatography method using ultraviolet detection at 223 nm for determination of seven quinoline analogues and 15 metabolites was developed, and gas chromatography–mass spectrometry and thin‐layer chromatography analyses were used for identification. The inhibition of nitrification by the parent compounds and their degradation products was assessed by a nitrification toxicity test called MINNTOX. Quinoline and 3‐, 4‐, 6‐, and 8‐methylquinoline were all transformed by hydroxylation into their 2‐hydroxyquinoline analogues (2‐quinolinones), and isoquinoline was transformed into 1‐hydroxyisoquinoline. 2‐Methylquinoline was not transformed by this microcosm, likely due to the blockage at position 2 by the methyl group. The hydroxylated metabolites of isoquinoline and quinolines methylated at the heterocyclic ring were not transformed further, whereas metabolites of quinoline and quinolines methylated at the homocyclic ring were hydrogenated at position 3 and 4, and the resulting 3,4‐dihydro‐2‐quinolinone analogues accumulated. Of these metabolites, only 3,4‐dihydro‐2‐quinolinone from the degradation of quinoline was further transformed into unidentified products. All quinolines and their metabolites had inhibiting effects on the nitrifying bacteria at the same level (ppm) in the applied bioassay, indicating that the inhibition of the compounds was not influenced by the initial transformation reactions.