Removal of Phenols by Highly Active Periodate on Carbon Nanotubes: A Mechanistic Investigation

Carbon nanotubes (CNTs) and their derivatives have beenwidelyexploited to activate various oxidants for environmental remediation.However, the intrinsic mechanism of CNTs-driven periodate (PI) activationremains ambiguous, which significantly impedes their scientific progresstoward practical application. Here, we found that CNTs can stronglyboost PI activation for the oxidation of various phenols. Reactiveoxygen species analysis, in situ Raman characterization, galvanicoxidation process experiments, and electrochemical tests revealedthat CNTs could activate PI to form high-potential metastable intermediates(CNTs-PI*) rather than produce free radicals and O-1(2), thereby facilitating direct electron transfer fromthe pollutants to PI. Additionally, we analyzed quantitative structure-activityrelationships between rate constants of phenols oxidation and doubledescriptors (e.g., Hammett constants and logarithmof the octanol-water partition coefficient). The adsorptionof phenols on CNT surfaces and their electronic properties are criticalfactors affecting the oxidation process. Besides, in the CNTs/PI system,phenol adsorbed the CNT surfaces was oxidized by the CNTs-PI*complexes, and products were mainly generated via the coupling reactionof phenoxyl radical. Most of the products adsorbed and accumulatedon the CNT surfaces realized phenol removal from the bulk solution.Such a unique non-mineralization removal process achieved an extremelyhigh apparent electron utilization efficiency of 378%. The activityevaluation and theoretical calculations of CNT derivatives confirmedthat the carbonyl/ketonic functional groups and double-vacancy defectsof the CNTs were the primary active sites, where high-oxidation-potentialCNTs-PI* were formed. Further, the PI species could achievea stoichiometric decomposition into iodate, a safe sink of iodinespecies, without the generation of typical iodinated byproducts. Ourdiscovery provides new mechanistic insight into CNTs-driven PI activationfor the green future of environmental remediation. Knowledge of the CNTs/periodate systemcan help in understandingthe mechanism of activation and provide insight into pollutant removalin PI-based AOP systems.

Automated summary

The mechanism of carbon nanotubes (CNTs)-driven periodate (PI) activation for the oxidation of phenols was investigated. It was found that CNTs can strongly boost PI activation by forming high-potential metastable intermediates (CNTs-PI*). The adsorption of phenols on CNT surfaces and the electronic properties of CNTs play critical roles in the oxidation process.