Activation Of Persulfate On Carbon Nanotubes For Degradation Of Typical Organic Pollutants

With the development of urbanization and industrialization,the total emissions of domestic sewage and industrial wastewater increases continuously.Meanwhile,the water pollution with complex composition,high concentration as well as difficult biodegradability,is also quite different from before,resulting in the inability of traditional wastewater treatment technologies to meet the increasingly strict discharge standards.Phenolic compounds are a kind of typical refractory organic pollutants with high biotoxicity and wide pollution range,which need to be removed by some novel water treatment technologies.Due to elemental abundance,high efficiency and environmental friendliness,metal-free persulfate-based Advanced Oxidation Processes(PS-AOPs)is a promising green wastewater purification,which have attracted considerable attentions.However,the activated mechanisms of persulfate by metal-free catalysts are still debatable.(i)Due to the lack of reliable electrochemical characterization techniques,it is difficult to lucubrate the electron transfer process in carbon/persulfate system;(ii)The active center on carbon surface is not clear,especially for the surface oxygen-and nitrogen-containing functional groups;(iii)Carbon/persulfate system can remove organic pollutants effectively and quickly,but the reuses of most carbon materials are not ideal;(iv)Due to the limited studies on boron/PMS system,its reaction mechanism and practical water applications are still unclear.Based on the above problems,specific researches are as follows:(1)The mechanism study on selective oxidation of phenolic compounds by activation of persulfate on carbon materials.The second chapter proposed an electrochemical technique for investigating the mechanism of nonradical oxidation of organics with peroxydisulfate(PDS)activated by carbon nanotubes(CNT).The electrochemical property of twelve phenolic compounds(PCs)were evaluated by their half-wave potentials,which were then correlated to their kinetic rate constants in the PDS/CNT system.Integrated with quantitative structure-activity relationships(QSARs),electron paramagnetic resonance(EPR)and radical scavenging tests,the nature of nonradical pathways of phenolic compound oxidation was unveiled to be an electron-transfer regime other than a singlet oxygenation process.A facile electrochemical analysis method(open circuit potential combined with chronoamperometry)was also employed to probe the mechanism,suggesting that PDS was catalyzed initially by CNT to form a CNT surface-confined and activated PDS(CNT-PDS*)complex with a high redox potential.Then,the CNT-PDS* complex selectively abstracted electrons from the co-adsorbed PCs to initiate the oxidation.Finally,a comparison of PDS/CNT and graphite anodic oxidation under constant potentials was comprehensively analyzed to unveil the relative activity of the nonradical CNT-PDS* complex toward the oxidation of different PCs,which was found to be dependent on the oxidative potentials of the CNT-PDS* complex and the adsorbed organics.(2)Insights into the electron-transfer regime of peroxydisulfate activation on carbon nanotubes: The role of oxygen functional groups.The third chapter 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.Eighteen analogical CNT 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 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 CNT-activated PDS complexes(CNT-PDS*).Firstly,a decline in the concentration of oxygen groups in CNT will make the Zeta potential of the CNT become less negative in neutral solutions,which facilitated the adsorption of PDS due to the weaker electrostatic repulsion.Then,the metastable CNT-PDS* 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 complexes,suggesting that the nonradical oxidation by the CNT-PDS* undergoes a mechanism analogous to anodic oxidation.(3)The intrinsic nature of persulfate activation and N-doping in carbocatalysis.The fourth chapter was dedicated to elucidating the roles of nitrogen species in CNT-based persulfate systems are intrinsically different.In peroxymonosulfate(PMS)activation mediated by nitrogen-doped CNT(N-CNT),surface chemical modification(N-doping)can profoundly promote the adsorption quantity of PMS,consequently elevate potential of derived nonradical N-CNT-PMS* complexes and boosted organic oxidation efficiency via an electron-transfer regime.In contrast,PDS adsorption was not enhanced upon incorporating N into CNT due to the limited equilibrium adsorption quantity of PDS,leading to a relatively lower oxidative potential of PDS/N-CNT system and a mediocre degradation rate.However,with equivalent persulfate adsorption on N-CNT at a low quantity,PDS/NCNT exhibited a stronger oxidizing capacity than PMS/N-CNT because of the intrinsic higher redox potential of PDS than PMS.The oxidation rates of the two systems were in great linearity with the potentials of carbon-persulfate* complexes,suggesting N-CNT activation of PMS and PDS shared the similar electron-transfer oxidation mechanism.(4)Degradation of chlorinated aromatic compounds via activation of peroxydisulfate on carbon nanotubes under electric field.In The fifth chapter,a novel wastewater purification for the degradation of chloroaromatic compounds by peroxydisulfate activation on carbon nanotubes under electric field(E/PDS/CNT).Under electric field,the CNT could be polarized into CNT particle electrode,which could remarkably improve its catalytic activity.Integrated with EPR,chemical quenching and divided electrochemical chambers,the E/PDS/CNT involved radical(adsorbed hydroxyl and sulfate radicals on anode)and nonradical(electron-transfer process of PDS/CNT system and anode oxidation)pathways.Moreover,2.4-dichlorophen could be removed in water not only through coupling reaction on CNT surface by electron-transfer process,but also through selective dechlorination by anode oxidation and adsorbed radicals.Most importantly,the system exhibited excellent performances for the degradation of other chlorinated aromatic compounds(clofibric acid and 4-chlorophenol)both in oxidation rate and dechlorination efficiency.The applicability and feasibility of metal-free PS-AOPs in wastewater purification were evaluated by the selection of metal-free catalysts,the dosage and consumption of persulfate,the kinds of ROS,the oxidation pathways of organics,as well as the operational costs.