Electron beam induced degradation of indomethacin in aqueous solution: kinetics, degradation mechanism, and toxicity assessment

Pharmaceutical compounds were emerging contaminants, and the accumulation of pharmaceutical compounds in the environment increased the risk to humans and ecosystems. In this study, electron beam irradiation was applied to degrade indomethacin (IDM) in aqueous solution. IDM degradation followed pseudo-first-order kinetics and 300 mu M IDM could be completely degraded at only 2 kGy. According to the quenching experiment, the dose constant ratios of oxidative radicals (center dot OH) and reductive radicals (e(-)aq and center dot H) could be calculated as k(center dot OH): k(e)(aq and center dot H)=4.79:1. As the concentration of H2O2 increased from 0 to 10 mM, the dose constant increased from 1.883 to 2.582 kGy(-1). However, degradation effect would be restrained in the existence of NO3-, NO2-, CO32-, HCO3-, SO2-, and humic acid due to their competition for the active species. Theoretical calculation revealed the radical attacking sites of IDM molecule and the most probable pathways were proposed with identification of intermediates. The attack of center dot OH mainly resulted in the cleavage of amide bond, indole ring opening, demethoxylation, and center dot OH addition. Dechlorination and the reduction of the carbonyl group occurred on IDM molecular through the reduction of e(aq)(-) and center dot H. The intermediates could continue to be degraded to small molecule acid, such as formic acid, acetic acid, and oxalic acid. Furthermore, highly toxic IDM transformed into less toxic products during the irradiation process.