Metal-loaded And Heteroatom-doped Carbon Nanotubes For Catalytic Ozonation Of Organic Pollutants

Refractory organic pollutants in water are usually highly toxic and easy to be enriched in organisms,which endanger ecological safety and human health.Heterogeneous catalytic ozonation is an efficient technology for the treatment of wastewaters containing refractory organic pollutants.The catalysts play a key role in heterogeneous catalytic ozonation,which determine the feasibility and efficiency of this technology.However,the heterogeneous catalysts generally show low activity and poor stability.To address these issues,cerium or/and manganese-loaded carbon nanotubes and heteroatom-doped carbon nanotubes were designed and prepared for catalytic ozonation of organic pollutants.The relationship between the structure of catalysts and their catalytic performance was studied.And the catalytic mechanism was also explored.The main research contents and results are shown as follows:(1)Cerium dioxide loaded carbon nanotubes(CeO2-CNT)was prepared by a hydrothermal method.The catalytic performance of CeO2-CNT towards catalytic ozonation was evaluated.The results showed that with the increase of CeO2 loading content,the catalytic performance of CeO2-CNT illustrated a volcano-shaped variation.The CeO2-CNT exhibited best performance when the loading content of CeO2 was 9.0 wt%.After 60 min of reaction,the TOC removal efficiency of phenol on CeO2-OCNT was 96%,which was 2.9 and 2.0 times as much as that of pure CNT(33%)and CeO2(47%)respectively.The CeO2-CNT showed good stability and reusability.And the CeO2-CNT illustrated efficient mineralization of phenol under a wide range of pH 4.1-9.3.The CeO2-CNT/O3 system was highly effective for the treatment of a secondary effluent from coking wastewater.After 60 min of treatment,the CODcr value was decreased from 104.0 mg L-1 to 40.1 mg L-1,which was below the emission limit of Chinese National Standard for the Wastewater Discharge(GB 18918-2002,50 mg L-1).The results of EPR experiments revealed that hydroxyl radicals(·OH)was the reactive oxygen species(ROS)for organic pollutants degradation during CeO2-CNT/O3 system.During the CeO2-CNT/O3 system,the Ce4+ could be reduced to Ce3+ by electrons.The Ce3+catalyzed ozone decomposition to generate· OH and Ce4+.The results of XPS analysis showed that the Ce3+/Ce4+ratio for used CeO2-CNT was 1.2 times as much as that for CeO2.This indicated that CNT could favor the redox cycling of Ce4+/Ce3+,which benefited the conversion of ozone into ·OH and thereafter enhanced the removal of organic pollutants.(2)The cerium-manganese oxides complex loaded CNT@Fe3C(Ce-Mn-O/CNT@Fe3C)was synthesized with magnetic CNT@Fe3C as a supporter and manganese-cerium oxides complex as an active component.The Ce-Mn-O/CNT@Fe3C was employed as a catalyst for catalytic ozonation.After 45 min of reaction,the TOC removal efficiency of phenol on Ce-Mn-O/CNTs@Fe3C was 98%,which was 1.5 and 1.8 times as much as that of CeO2/CNT@Fe3C(65%)and MnxOy/CNT@Fe3C(54%)respectively,and was 1.3 times as much as that of CeO2-CNT.The Ce-Mn-O/CNT@Fe3C displayed good stability and reusability.And the Ce-Mn-O/CNT@Fe3C illustrated effcient mineralization of phenol under a wide range of pH 4.2-8.3.The results of EPR experiments revealed that ·OH was the ROS for organic pollutants degradation during Ce-Mn-O/CNT@Fe3C/O3 system.Additionally,the magnetic Ce-Mn-O/CNT@Fe3C is easy to be recovered and reused.(3)Heterogeneous Mn2+-bonded oxidized carbon nanotubes(Mn2+-OCNT)was synthesized by an ion-exchange method.Homogeneous catalytic ozonation has been achieved over heterogeneous Mn2+-OCNT through the reversible de-/ad-sorption of Mn2+on OCNT surfaces.The de-/ad-sorption reversibility of Mn2+was induced by the spontaneous variations of solution pH that caused by the formation and degradation of organic acid intermediates.The results showed that the Mn2+-OCNT displayed remarkably enhanced catalytic performance for phenol mineralization compared with that of pure OCNT and commercial Mn02,and was even comparable to that of hoUogeneous Mn2+/O3 system.Benefiting from the dropped solution pH(from 6.2 to 3.5)during phenol degradation,around 92%of Mn2+was rapidly desorbed from OCNT surfaces and accumulated in solution.The desorbed Mn2+in the solution catalyzed ozone decomposition to generate·OH for organic pollutants degradation.The solution pH was then increased from 3.5 to 4.5,which caused the re-sorption of 88%Mn2+.By increasing the solution pH from 4.5 to 5.0 with the treatment of NaOH(0.86 g m-3),100%of Mn2+recovery was achieved and the catalytic performance could be recoverd to its original level.(4)Fluorine-doped carbon nanotubes(F-CNT)were prepared and its catalytic performance towards catalytic ozonation was evaluated.The results showed that the catalytic performance of F-CNT illustrated a volcano-shaped variation with the increase of fluorine doping content.The F-CNT exhibited best performance when the doping content of fluorine was 2.17 at%.After 90 min of reaction,the TOC removal efficiency of oxalic acid on F-CNT was 99%,which was 2.2 times as much as that of pure CNT(45%),and was even superior to several typical metal oxides for catalytic ozonation(such as ZnO,Al2O3,Fe2O3 and MnO2).The prepared F-CNT showed good stability and reusability.The results of EPR experiments revealed that F-CNT could catalyze ozone decomposition to generate superoxide radicals(O2·-)and singlet oxygen(1O2)for organic pollutants degradation.Although the oxidation ability of O2·-and 1O2 is lower than that of ·OH,the metal-free F-CNT is extremely desirable in heterogeneous catalytic ozonation process because it can completely avoid metal leaching.