Insights into the Electron-Transfer Regime of Peroxydisulfate Activation on Carbon Nanotubes: The Role of Oxygen Functional Groups

Carbon-driven advanced oxidation processes are appealing in wastewater purification because of the metal-free feature of the carbocatalysts. However, the regime of the emerging nonradical pathway is ambiguous because of the intricate carbon structure. To this end, this study was dedicated to unveil the intrinsic structure-performance relationship of peroxydisulfate (PDS) activation by carbon nanotubes (CNTs) toward nonradical oxidation of organics such as phenol (PE) via electron transfer. Eighteen analogical CNTs were synthesized and functionalized with different categories and contents of oxygen species. The quenching tests and chronopotentiometry suggest that an improved reactivity of surface-regulated CNTs was attributed to the reinforced electron-transfer regime without generation of free radicals and singlet oxygen. The quantitative structure–activity relationships were established and correlated to the Tafel equation, which unveils the nature of the nonradical oxidation by CNT-activated PDS complexes (CNT-PDS*). First, a decline in the concentration of oxygen groups in CNTs will make the zeta potential of the CNT become less negative in neutral solutions, which facilitated the adsorption of PDS because of weaker electrostatic repulsion. Then, the metastable CNT-PDS* was formed, which elevated the oxidation capacity of the CNT. Finally, PE would be oxidized over CNT-PDS* via electron transfer to fulfill the redox cycle. Moreover, the nonradical oxidation rate was uncovered to be exponentially related with the potential of the complexes, suggesting that the nonradical oxidation by the CNT-PDS* undergoes a mechanism analogous to anodic oxidation.