| Chlorophenol pollutants are highly toxic and difficult to be biodegraded,posing a serious threat to environment and human health.Therefore,the development of efficient removal technologies is extremely important.In recent years,metal-free doped carbon nanotubes have demonstrated good performance in persulfate-based advanced oxidation process for degrading chlorophenol pollutants.However,current modification technologies still exist problems such as complex preparation process and harsh modification conditions.In addition,the influence of non-metal doping on persulfate activation mechanism still not clear and degradation rule of pollutants needs to be further studied.This paper used convenient and efficient plasma etching method to prepare oxygen-doped(E-CNTs)and oxygen-nitrogen co-doped carbon nanotubes(E-N-CNTs).2,4-dichlorophenol(2,4-DCP)was selected as a typical chlorophenol pollutant to explore the modified carbon nanotubes performance on activating persulfate.The main active species were studied and the influence of non-metal doping on the degradation mechanism was revealed.The degradation rule of phenolic pollutants in the system was discussed,and the toxicity of intermediate products were further evaluated.The main results are as follows:Characterization results demonstrated that oxygen atoms were successfully doped into carbon nanotubes by oxygen plasma etching.E-CNTs exhibited excellent activating persulfate performance.EC30 was obtained by oxygen etching CNTs for 3000 s，which can remove 97.0%of 2,4-DCP in 20 min.Its reaction kinetic constant was 4.1 times of initial carbon nanotubes(CNTs).Moreover,E-CNTs exhibited good application in a range of pH 3-9 and actual water.Quenching experiments,electron paramagnetic resonance(EPR)and electrochemical tests indicated that the main active species in system were singlet oxygen(1O2).Through steady-state concentration calculations,electron transfer oxidation contributed most in CNTs/persulfate system(73.49%),while 1O2 oxidation contributed most in EC30/persulfate system(54.93%).Oxygen atoms doping could improve catalytic performance of carbon nanotubes and activation mechanism was changed.To further investigate effect of non-metal atoms co-doping on the efficiency and mechanism of persulfate activation,E-N-CNTs were prepared by nitrogen plasma etching and the content of nitrogen increaseed from 0.41%to 5.90%.ENC24 exhibited better catalytic activity.The kinetic constant of ENC24 was 2.1 times of EC30.The main degradation mechanism of ENC24/persulfate system was 1O2 oxidation with 69.93%contribution rate.ENC24/persulfate system showed excellent anti-interference ability due to high substrate selectivity of 1O2.Density functional theory calculations(DFT)showed that the doping of heteroatoms could significantly change the charge distribution of carbon nanotubes and improved adsorption capacity for persulfate.4-chlorophenol,4-bromophenol,2,4-dichlorophenol and acetaminophen were selected according to substituents.The quantum chemical properties such as highest occupied molecular orbital(HOMO),lowest unoccupied molecular orbital(LUMO),natural bond orbital(NBO)charge distribution,electrostatic potential and Fukui function were calculated.The results revealed that the main reactive sites on these pollutants located on benzene ring and its connection with heteroatoms.The oxidation reaction is mainly hydroxylation,heteroatom removal and coupling reaction in 1O2 dominated system.After degradation,most of the intermediates were less toxic than parent pollutantsThis study provided new idea for the modification of non-metal doped carbon nanotubes in persulfate activation,and offered theoretical support for deeper understanding in the degradation rules of pollutants and the assessment of environmental risks in carbon nanotubes activating persulfate system. |
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