Hydroxyl grafting on CN-coating surfaces and their contribution to O3 generation and oxygen transfer in plasma liquid film degradation

This study presents an innovative and eco-friendly method for wastewater treatment, emphasizing the degradation of tetracycline hydrochloride (TCH) through the use of monolithic materials in a trickle-bed dielectric-barrier-discharge (DBD)-based plasma reactor. Modifications to monolithic surface coatings can be utilized to investigate their impact their effects on plasma degradation and energy efficiency. Loading graphitic carbon nitride (g-C 3 N 4 , CN) onto the surface of a monolithic material allows for the modulation of functional groups on the material's surface and the homogeneity of liquid film flow, thereby influencing the generation of active substances and mass transfer at the gas-liquid interface. This study employed hydrothermal techniques to synthesize hydroxyl-grafted and fluorine-grafted g-C 3 N 4 coatings on the surface of open-cell ceramic foam (CN-OH@CF and CN-F@CF, respectively). The CN-OH-6@CF/plasma system exhibits the highest degradation efficiency at 91.1 % and an energy efficiency of 0.5 g/kWh. Active substances generated by plasma, such as ozone (O 3 ), hydroxyl radicals ( OH), and hydrogen peroxide (H 2 O 2 ), were quantified in a liquid falling film DBD-based plasma system. In the CN-OH-6@CF/plasma system, the concentration of OH was 4.55 mg/L at 60 min, the yield H 2 O 2 exceeded 4.04 mg/L and the concentration of O 3 reached 1.17 mmol/L at 60 min. These values were significantly higher than those observed in the CN@CF/plasma or CN-F@CF/plasma systems. The incorporation of – OH groups in CN coatings significantly enhances the production of the direct plasma product O 3 and optimizes O 3 mass transfer at the gas-liquid interface, achieving up to 85 % O 3 contribution in the CN-OH-6@CF/plasma system. The electron transfer number of 2.7 and the H 2 O 2 selectivity of 64.5 % for grafting − F on CN coatings support the selective generation of H 2 O 2 via a two-electron reaction process, as evidenced by rotating ring-disk electrode tests. The selective generation of H 2 O 2 is not conducive to energy efficiency and promotes accelerated degradation. O 3 , in conjunction with H 2 O 2 , promotes the acceleration of its decomposition and the production of OH.