Nitrogen-Doped Graphdiyne Nanotubes for Metal-Free Activation of Peroxymonosulfate and Enhanced Degradation of Recalcitrant Heterocyclic Contaminants

Pollution removal via peroxymonosulfate (PMS)-mediated advanced oxidation processes provides an effective approach for water remediation. Although numerous carbon allotropes have been applied in PMS activation, the application of graphdiyne-based catalysts has been practically unexplored in this process despite their excellent electronic properties. In this study, the first instance of exploiting N-doped graphdiyne nanotubes (NGNTs) for metal-free PMS activation is presented to remove recalcitrant heterocyclic contaminants in aqueous environments. This mechanistic study demonstrates that the NGNT–PMS system decomposes heterocyclic contaminants via electron-transfer-mediated nonradical oxidation, where metastable NGNT–PMS* complexes were the primary oxidants. The apparent reaction rate constant for the NGNT–PMS system is 2.4 and 3.1 times higher than those for the counterpart systems by N-doped carbon nanotubes (NCNTs) and graphene oxide (NGO) systems, respectively. The NGNT exhibits excellent N-doping potential, demonstrated by a 7-fold increase in atomic nitrogen compared to NCNT in an identical N-doping procedure. The possible N-doping sites in NGNT were determined as mainly sp-N-1, sp-N-2, pyridinic-N, amino-N, and graphitic-N atoms by the N K-edge X-ray absorption near-edge structure spectra and X-ray photoelectron spectroscopy. Theoretical calculations demonstrate the relatively obvious electron transfer between PMS and different N-sites in NGNT, indicating the possible formation of NGNT–PMS* complexes. Additionally, the NGNT–PMS system plays an important role in the cleavage of pyridine heterocycles and C–Cl bonds as demonstrated by the destruction of nitenpyram. This study provides mechanistic insights into the roles of NGNT in the metal-free activation of PMS for the remediation of recalcitrant heterocyclic contaminants.