Nitrogen doped biochar derived from algae as persulfate activator for the degradation of tetracycline: Role of exogenous N doping and electron transfer pathway

The recycling of biochar for wastewater treatment has attracted much attention due to the persistent increase of organic waste. Recently, it has been demonstrated that nitrogen (N)-doping biochar has high catalytic activity, however, the role of exogenous N and the reaction mechanism are not yet clear. In this study, biochar was manufactured from cyanobacteria from Lake Dianchi via thermal pyrolysis and decorated with N atoms to enhance its catalytic performance in peroxydisulfate (PDS) activation. The optimal synthesis of C/N = 1:2 (CN12) with high N loading, large specific area, and excellent electron transfer rate was achieved. A comprehensive study of the mechanism was conducted by integrating selective ESR, radical scavenging experiment, and electrochemical measurements, which demonstrated that the degradation of TC includes free radical and nonradical pathways. The formation of 1O2 and surficial reactive complexes derived from PDS performs a crucial role by attacking the adsorbed TC molecules. It is proved that N-doping accelerates the electron transfer between TC and PDS by increasing electrochemical specific area and mediation of a metastable CN12/PDS complex. Moreover, N-containing functional groups such as graphitic N, pyridinic-N and CO/CC/CN functional groups strengthened the nonradical pathway mediated by sp2 defects. The influence of an alkaline environment indicates that electrostatic interaction between TC and CN12 determines the adsorption capacity and formation of surface reactive complexes. This study helps to understand the mechanism of an algal biochar catalyst for PDS activation and degradation of organic pollutants, and proposes a new approach for solid waste recycling.