Regeneration of carbon nanotube saturated with tetracycline by microwave-ultraviolet system: Performance and degradation pathway

Abstract

Regeneration of spent CNT is one of the most significant steps but usually neglected in adsorption application. In this study, microwave ultraviolet (MW-UV) system was applied to regenerate spent CNT and further degrade the desorbed tetracycline simultaneously. Both regeneration and mineralization ratios were detected to evaluate the regeneration performance. And they ascended with the increment of microwave power as well as regeneration time. Adsorption capacity of the regenerated CNT recovered to 100%, and 37% of tetracycline was mineralized to carbon dioxide at 500 W for 2.5 min. High total gas flow rate demonstrated negligible impact on mineralization ratio. After 5 adsorption-regeneration cycles, regeneration and mineralization ratios kept at approximately 80% and 27%, respectively, while BET surface area went up. Three kinds of reactive oxygen species (ROS) including ozone, hydroxyl (·OH) radicals and singlet oxygen (1O2) were detected during the regeneration. Ozone concentration declined with the enhancing CNT mass, total gas flow rate as well as relative humidity. A possible tetracycline degradation pathway under MW-UV irradiation was proposed according to the tetracycline structure, ROS chemistry and 11 by-products identified by Ultra High Performance Liquid Chromatography-Mass Spectrometry (UPLC-MS). Conclusively, our research provides new perspective into recycling and reusing spent CNT.