2.4. Photocatalytic Activity Evaluation

Tetracycline (TC) was degraded by CeO₂ QDs, Ag₂Se, and CeO₂ QDs/Ag₂Se with an 8 W halogen lamp (KLD-08L/P/N, Gyeonggi-do, Korea) as illuminant. A total of 50 mg of the photocatalyst were scattered in 50 mL of a solution with a TC concentration of 0.02 g/L. The beaker was placed in the box in which the lamp was placed 100 mm above the solution. The luminescent efficiency of the lamp was 80 lm/W and the wavelength range was between 400 and 790 nm. Prior to the photocatalytic experiment, the solution was magnetically stirred in the dark for 60 minutes to establish an adsorption-desorption equilibrium. Additionally, 10 ml of the sample were extracted from the solution and spun at 10,000 rpm in the rotator to eliminate the solid materials. Afterward, the lamp was turned on and photocatalytic degradation proceeded under magnetic stirring. The specimens were taken from the beaker at regular intervals, centrifuged for 10 min to remove the powder, and carried out on UV-vis spectrophotometry at 357 nm.

The intermediate products during the degradation of TC were identified using LC-MS (LC-Agilent Technologies 1290 Infinity, Palo Alto, CA, USA; MS-AB SCIEX QTR AP4500, Waltham, MA, USA), ZORBAX Eclipse Plus C18 Column (150 mm × 2.1 mm, 3.5 μm) (Agilent, Santa Clara, CA, USA). During the process, formic acid water: acetonitrile: methanol (45:35:20) was used as a mobile phase. The flow rate was 0.2 mL/min. The mass spectra with a scan range m/z 50–500 were recorded.