Study on the mechanism of tetracycline removal in electrocoagulation coupled with electro-fenton reaction system with Fe anode and carbon nanotube cathode

In this work, an efficient electrochemical system (Fe-CNT system) for the treatment of tetracycline (TC) was constructed using Fe plate anode and carbon nanotube (CNT) roller cathode. Compared with the traditional Fe-Fe electrocoagulation (EC) system, this Fe-CNT system exhibited higher TC removal rate and shorter reaction time. The best removal rate of 97.21% was achieved at pH of 7, the conductivity of 500 μS cm−1, initial TC concentration of 50 mg L-1, the voltage of 5 V, and reaction time of 25 min. The results demonstrated that the removal rate of total organic carbon was 52.79% at 25 min and 82.84% at 60 min, respectively. The excellent performance of the Fe-CNT system was mainly due to the use of CNT cathode slowed down the floatation in the system, which was conducive to the aggregation and growth of flocs. As a result, the flocs with larger size and more loose structure showed stronger adsorption and aggregation to organic molecules. At the same time, the concentration of hydrogen peroxide in-situ produced was in the range of 7.76 mg L-1 and 30.17 mg L-1. Furthermore, hydroxyl radical as the primary active specie was also produced on the cathode surface by electro-Fenton reaction, which could degrade TC rapidly. In addition, the intermediates involved in the degradation process of TC were identified, and the possible degradation paths were proposed. This work provides insights for improving the cathode utilization rate and removal ability of organic matter and adds a feasible option for the practical application of EC technology.