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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 due to the metal-free feature of the carbocatalysts. However, the regime of the emerging nonradical pathway is ambiguous due to the complex carbon structure. To this end, this study was dedicated to unveiling the intrinsic structure-performance relationship of peroxydisulfate (PDS) activation by carbon nanotubes (CNT) toward nonradical oxidation of organics such as phenol via electron transfer. Fifteen analogical CNT were synthesized and functionalized with different categories and contents of oxygen species. The quenching tests and chronopotentiometry suggest the improved reactivity of surface regulated CNT was attributed to the reinforced electron-transfer regime without generation of free radicals and singlet oxygen. The quantitative structure-activity relationships (QSARs) were established and correlated to the Tafel equation, which unveils the nature of the nonradical oxidation by a CNT-PDS* complex. Firstly, the decline in oxygen groups in CNT will make the Zeta potential of CNT become less negative in neutral solutions, which facilitated the adsorption of PDS due to the weaker electrostatic repulsion. Then, the metastable CNT-PDS* complex was formed, which elevated the oxidation capacity of CNT. Finally, phenol would be oxidized over the 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 complex, suggesting that the nonradical oxidation by the CNT-PDS* undergoes a mechanism analogous to anodic oxidation.