Vacancies-rich MOFs-derived magnetic CoFe encapsulated in N-doped carbon nanotubes as peroxymonosulfate activator for p-arsanilic acid removal

A rational design of high-performance catalysts with peroxymonosulfate (PMS) activation and arsenic adsorption capacities for the removal of p-arsanilic acid (p-ASA) is in high demand but remains a significant challenge. Herein, vacancies-rich metal–organic frameworks (MOFs) were constructed by room-temperature co-precipitation procedure, and the magnetic CoFe encapsulated in N-doped carbon nanotubes (CoFe-N-CNTs) were prepared using MOFs template by an annealing process under a nitrogen atmosphere. The synthesis conditions of CoFe-N-CNTs were optimized by tuning Fe/Co ratio and carbonization temperature. The CoFe-N-CNTs/PMS system showed favorable removal performance, achieving complete degradation of 46 μM p-ASA within 3 min and a total arsenic removal efficiency of 98% within 30 min at 0.1 g/L catalyst and 400 μM PMS. The degradation rate (kobs) and total arsenic removal efficiency towards p-ASA by CoFe-N-CNTs/PMS system were 6.7 and 2.7 times of those in the MOFs without vacancies-derived catalyst/PMS system, respectively. CoFe-N-CNTs/PMS system exhibited robust and efficient performance over a broad range of pH (4.5–9.0) and in different water matrices. Scavenging experiments and electron paramagnetic resonance (EPR) results demonstrated that non-radical singlet oxygen (1O2), sulfate radicals (SO4−), hydroxyl radicals (OH), and superoxide (O2−) were involved in p-ASA decomposition. Electron transfer and direct oxidative transfer process (DOTP) also contributed to p-ASA degradation. High-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) analysis indicated that the arsenic moiety was cleaved from p-ASA in the form of As(III) and then rapidly oxidized to As(V). X-ray photoelectron spectroscopy (XPS) analysis indicated the removal of inorganic arsenic followed the mechanism of the inner-sphere complex. This study highlights the in-situ synthesis of CoFe-N-CNTs with outstanding activities for catalytic degradation and arsenic removal of organoarsenicals.