Activation of persulfates on carbon nanotubes for water decontamination: Is the non-radical process consistently considered across different pH levels?

Carbon nanotubes-driven persulfates oxidation processes (CNTs/PS) have been extensively studied for environmental remediation. Solution pH is one of the main factors affecting wastewater treatment, but it is often overlooked. Herein, we report the effect laws of pH on the mechanism of peroxymonosulfate (PMS) or peroxydisulfate (PDS) activation by CNTs. The oxidation of organics (e.g., phenol) in the CNTs/PDS system involves an electron transfer process mediated by metastable intermediates (CNTs-PDS*), whose potential is influenced by pH, reaching the highest oxidation potential under neutral condition. In the CNTs/PMS system, the active species (i.e., SO₄^•-, CNTs-PMS*) shifted with pH variations. In acidic environment, phenol oxidation is governed by CNTs-PMS* . As pH increases, PMS undergoes accelerated decomposition, generating SO₄^•-, which plays a crucial role in pollutant oxidation. Moreover, in the CNTs/PMS system, the oxidation products of phenol were not easily accumulated on CNT surfaces, contributing to a lower total organic carbon removal in solution. Additionally, the oxidation rates of phenolic compounds in the CNTs/PMS system, which involve more complex mechanisms, exhibited a weaker correlation with their descriptors (e.g., the octanol-water partition coefficient) compared to CNTs/PDS system. This comprehensive investigation deepens our understanding of CNTs/PS systems and provides guidance for selecting superior oxidants for wastewater treatment.