Degradation of glucocorticoids in aqueous solution by dielectric barrier discharge: Kinetics, mechanisms, and degradation pathways

Performance and mechanism of non-thermal plasma (NTP) technology in removing glucocorticoids (GCs) was investigated using a dielectric barrier discharge (DBD) reactor with fluocinolone acetonide (FA), triamcinolone acetonide (TA) and clobetasol propionate (CP) as representative compounds. Effects of discharge power, plasma-working gases, initial pH, coexistence of ions, and various water matrices (ultrapure water, lake water, drinking water, wastewater effluent) on GC removal and energy yield were evaluated. The results confirm that DBD treatment could efficiently remove FA, TA, and CP, achieving efficiency of 72% (k = 0.0126 min(-1)), 71% (k = 0.0096 min(-1)), and 74% (k = 0.0116 min(-1)), respectively in air-DBD system at 45.2 W, with the process following the first order kinetics and energy yield of 6 mg kW(-1) h(-1). The removal efficiency decreased when adding radical scavengers, indicating that hydroxyl radicals played an important role in GC degradation, while other active species (such as solvated electrons (e(aq)(-)), ozone (O-3), hydrogen peroxide (H2O2) and ultraviolet photolysis (UV)) also contribute to GC degradation. The intermediates generated during the process were analyzed using quadrupole time-of-flight mass spectrometry (QTOF-MS). A total of 23 transformation products of FA, TA and CP were identified, and it was noted that substitution of halogen atoms with center dot OH, oxidation of hydroxyl group to keto acid, decarboxylation of the keto acid, addition of center dot OH, intramolecular cyclization, and hydrolysis of esters occurred during GC degradation by DBD treatment.