Insights into degradation pathways and toxicity changes during electro-catalytic degradation of tetracycline hydrochloride

The removal of antibiotics has attracted much attention due to their extremely high adverse impacts on the environment. However, the potential risks of degradation intermediates are seldom reported. In this work, the influence of different factors on the electro-catalytic degradation efficiency of tetracycline hydrochloride (TCH) by the prepared carbon nanotubes/agarose/indium tin oxide (CNTs/AG/ITO) electrode was investigated. Under optimal conditions (10 wt% CNTs dosage, pH = 7), the maximum degradation efficiency for TCH (10 mg L−1) reached up to 96% within 30 min treatment with 4 V potential. Superoxide anions (•O2−) played an important role in the electro-catalytic degradation. Totally 10 degradation intermediates were identified using HPLC-MS/MS, and the degradation pathway was proposed. Toxicities of the parent antibiotic and the identified intermediates were calculated using the ECOSAR (Ecological Structure Activity Relationship) program in EPISuite, and results showed that more toxic intermediates were generated. The maximal chronic toxicity for green algae of the intermediate increased 1439.92 times. Furthermore, antimicrobial activity was further verified by disk agar biocidal tests with Escherichia coli ATCC25922 and higher biotoxicity intermediates compared with parent compounds were confirmed to be formed. Therefore, more attention should be paid on the potential risk of degradation intermediates in the treatment of wastewater containing antibiotics.