Detection, occurrence and fate of indirubin in municipal sewage treatment plants

Indirubin which has been isolated from human urine is an extremely potent AhR agonist. This paper first established an analytical method based on liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) for indirubin in complex environmental waters, and then applied this method to investigate its occurrence and fate in sewage treatment plants (STPs). For the various types of aqueous matrices considered, the absolute recoveries were from 64 to 81%, and the limits of quantification were below 0.05 ng/L. Among the seven STPs studied, the average concentrations of indirubin in influents ranged from 8.3 to 29.7 ng/L, and their aqueous-phase removal rates were 72-91 %. In the receiving waters, the Tonghui and Qinghe Rivers, the concentrations of indirubin (0.65-3.7 ng/L) in some samples were much higher than those in their corresponding STP effluents, suggesting that there is random discharging of untreated sewage. The fate of indirubin was investigated in mechanical and biological sewage treatment as well as in sewage-sludge treatment at a STP consisting of anoxic, anaerobic, and aerobic tanks. The indirubin was largely removed in the anoxic tank and the secondary clarifier mainly due to the biodegradation and sorption on sludge, respectively. An increase of indirubin was observed in the aerobic tank, which was due to the cleavage of indirubin conjugates.