Removal of 2,4-dichlorophenoxyacetic acid by the boron-nitrogen co-doped carbon nanotubes: Insights into peroxymonosulfate adsorption and activation

B, N co-doped CNTs were synthesized by a simple one-step thermal process and the advanced oxidation processes (AOPs) of 2,4-dichlorophenoxyacetic acid (2,4-D) over the catalyst involved peroxymonosulfate (PMS) activation was investigated. The co-doped catalysts showed higher isoelectric points (IEPs) than those of CNTs and single doped CNTs, facilitating the adsorption of PMS anion (HSO5-) on the catalyst surface, and the AOPs of 2,4-D depended on PMS concentration could be illustrated by the Langmuir-Hinshelwood model. The removal efficiency of 2,4-D over CNTs, CB450, CN450 and CBN450-2 was 20%, 23%, 34% and 68%, respectively. The enhancement of catalytic activity was due to the synergistic effect between B and N dopants and both the pyrrolic N and B-N complex played roles in the catalytic reaction. The quenching test and electron paramagnetic resonance (EPR) results showed that O-1(2) contributed most to the oxidation of 2,4-D and the reaction experienced both the free radical and non-radical pathways. Additionally, the DFT calculation results showed that the adsorption energy of PMS on the surface N dopants decreased by B co-doping, implying that a moderate adsorption energy may facilitate the release of active species generated by PMS activation.