Degradation of Pharmaceutical Contaminant Tetracycline in Aqueous Solution by Coaxial-Type DBD Plasma Reactor

Tetracycline is one of the widely used antibiotic drugs, but it also causes severe water pollution. This research applied a pulsed dielectric barrier discharge (DBD) plasma reactor of coaxial configuration with a screw-type high-voltage electrode to treat tetracycline in an aqueous solution, which was recycled to flow over the surface of the inner electrode. The effects of discharge voltage, initial concentration, and pH value of tetracycline on the degradation efficiency, total organic carbon (TOC) removal rate, kinetic rate constant ( ${k}$ ), and energy efficiency were investigated. The results show that 92.3% degradation efficiency and 65.01% TOC removal rate of tetracycline were achieved at the initial concentration of 50 mg/L, corresponding to the energy efficiency of 20.24 g/(kW $\cdot \text{h}$ ). The tetracycline degradation efficiencies were dependent on the initial concentration from 5 to 75 mg/L, which led to the decrease of the degradation kinetic rate constant from 0.518 to 0.077 min $^{-1}$ . The TOC removal rate of tetracycline was strongly enhanced at higher pH values from 1.5 to 10.5, with the optimum value at 7.5. The intermediate products of tetracycline produced by the plasma treatment were analyzed by liquid chromatography mass spectrometry (LC-MS), which show that the aromatic ring, amino group, and the double bonds were mainly attacked by the hydroxyl radical and other reactive oxygen species. A possible degradation pathway has also been proposed.