Enhanced activation of persulfate by nitric acid/annealing modified multi-walled carbon nanotubes via non-radical...

This study aim to explore an effective approach of persulfate (PS) activation based on multi-walled carbon nanotubes (CNTs) for the phenol degradation. The nitric acid/annealing modified CNTs were obtained by oxidation with concentrated nitric acid followed by annealing at various high temperatures (400-1000 °C). The modified CNTs catalysts were characterized and their catalytic degradation performances of phenol with PS were investigated. The results reveal that the modified CNTs can obviously enhance the phenol removal. The catalytic activity is improved by nitric acid modification and obviously influenced by the annealing temperature. PS concentration and solution pH could also affect the catalytic degradation of phenol. The mechanism of enhanced PS activation by the nitric acid/annealing modified CNTs was proposed that the nitric acid modification resulted in more defective edges and oxygen-containing groups, and subsequent annealing at high temperate facilitated the conversion of sp³ to sp² carbon, then the catalytic activity for PS activation was enhanced by the active sites of CO group and sp²-hybridized carbon at the defective edges. In addition, a non-radical process of phenol degradation was demonstrated in terms of the radical quenching tests and electron paramagnetic resonance (EPR) spectra analyses. It is suggested that ¹O₂ is likely the main reactive specie generated in PS activation by the modified CNTs for the phenol removal.