| The existence of toxic and refractory organic pollutants increases the difficulty of wastewater treatment and generates potentially severe risks to both human body and the whole ecosystem.Electrochemical advanced oxidation pocesses(EAOPs)have been regarded as an effective technology in the degradation of a wide variety of organic pollutants,which involves in situ generation of strong active oxidants and direct or indirect reaction with organic pollutants.Among the electrode materials,boron-doped diamond(BDD)electrode is considered as an optimal electrode material for electrochemical oxidation organic contaminants in the aquatic environment due to its well physical and chemical properties,and the fundamental research and application of BDD electrode into the mineralization of organic pollutants have been well developed.However,the specific electrochemical oxidation process,mechanism and enhancement approaches are always neglected by the researchers,which could also give the enlightenment to the development of electrode materials and oxidation procecess.Aiming to treating organic wastewater with high efficiency,the research are focused on the fundamental research including the construction of high active BDD electrodes,the relatonship between electrochemical activity and titanium substrate and the following application in electrochemical oxidation.The enhancement approaches are further developed for the electrochemical oxidation mechanism and kinetics on electrode/solution interface based on the experimental results.The main research achievements are show as the following:(1)BDD film was deposited on porous Ti substrate via hot filament chemical vapor deposition(HFCVD)method.The morphology,phase composition and quality were studied via SEM,XRD and Raman spectroscopy technology,respectively.The structure of the porous Ti substrate endowed the BDD electrode 3D porous performance,which enhanced the surface area compared with flat Ti substrate.The effective electrochemical area of porous BDD electrode was 8.37 cm2 cm-2,3 times as much as that of conventional flat BDD electrode while smaller electron transfer resistance(31.3 ? cm2)for Fe(CN)63-/Fe(CN)64-redox couple was obtained compared to 128.3 ? cm2,exhibiting outstanding electrochemical activity.The enhancement of electrocatalytic activity was correlated to the porous structure of titanium substrate,which improved the electrochemical performance of BDD electrodes due to providing more active sites for electrochemical reactions.Meanwhile,the deposition parameters,including the reactor pressure,carbon and boron source concentration,aslo play an important role in the surface morphology and quality of the film via affecting the deposition and growth rate of diamond grains.The micro-and nano-diamond film could be controllable realized by the adjusting the deposition parameters.(2)The electrochemical oxidation of paracetamol,a kind potentially dangerous pharmaceutical compound,is conducted on flat and porous Ti/BDD electrodes to investigate the effect of electrode structure on the electrochemical oxidation behavior.Oxidative peak corresponding to the electron transfer of paracetamol could be observed around 0.90 V on BDD electrodes.The current responses from chronoamperometric curves are proportionally linear in the concentration range from 0 to 3 mmol L-1 and higher response slope could be obtained on porous Ti/BDD electrode.Complete mineralization could be obtained on Ti/BDD electrodes while porous Ti/BDD presents higher COD removal rate and current efficiency compared with flat BDD electrode.The decay kinetics of acetaminophen on BDD electrodes both follow a pseudo-first-order behavior,and the corresponding apparent rate constants are 0.208 and 0.344 h-1 for flat and porous Ti/BDD electrodes at 30 mA cm-2.The current density is found to have a positive effect on the COD removal and a negative effect on the current efficiency due to the high opportunity of oxygen evolution under high current density.From the detection of hydroxyl radicals,higher generation rate and amount can be achieved on porous Ti/BDD electrode.The enhancement for electrochemical oxidation is related to the porous structure of 3D-Ti/BDD electrode,which provides more active sites for hydroxyl radicals(·OH)generation and degradation rate in the following.Finally,the degradation mechanism and pathway are put forward based on the intermediates during oxidation process.(3)There is a crucial important relationship between electrochemical activity and oxygen evolution potential or generation ability of hydroxyl radicals based on the electrochemical mineralization of hydroquinone on different electrode materials.Specific relationship between the substituent group and the electrochemical oxidation activity are also systematically studied,where the mineralization of substituted phenols is indirect conducted by hydroxyl radicals and the divorce of substituent group becomes the determining step for the mineralization.Meanwhile,electrochemical activity towards substituted phenol contaminants is limited by the electronic effect of substituent and there is liner relationship between the reaction kinetic constant and Hammett constant during degradation of substituted phenols.The generation ability and rate of ·OH on porous BDD electrode were enhanced due to the porous structure of BDD film.The step currents on porous Ti/BDD were about 2 times of those on flat BDD electrode from chronoamperometry curves,which implied porous Ti/BDD exhibited superior indirect electrochemical oxidation ability.The results of electrodegradation of several typical organic pollutants revealed that current efficiencies on porous Ti/BDD electrode were higher compared with flat BDD electrode,corroborating the enhanced of electrochemical oxidation as the introduction of porous Ti substrate.Moreover,the discrepancy between promotion of COD removal rate and enhancement of ·OH was discussed,which could be explained in terms of in situ generation and utilization rate of hydroxyl radicals on the surface of electrode materials and limited mass transfer process in the inner pores of porous electrode.(4)To further improve the mass transfer process on electrode/solution interface during electrochemical oxidation,BDD electrode with three-dimensionally network was fabricated on mesh titanium substrate.These fascinating properties inherent to the nature of the original format properties endowed the prepared mesh BDD electrode excellent electrochemical performance and oxidation ability towards various contaminants during electrochmemical tests and actual degradation experiments.The mechanisms for enhancement of electrochemical oxidation contaminants are discussed and proved,which could be speculated as the follow:(i)Microstructure of mesh electrode providi higher surface area and active sites for electrochemical reaction.(ii)High utilization of ·OH due to the change of wettability property and active ·OH could divorce from hydrophobic surface and then react with contaminants in aquatic environment.(iii)The enhancement of mass transfer of the solution originated from network structure.Meanwhile,the mesh Ti substrate above could also be expanded to other active coatings and mesh Ti subtstrate with different sizes.Especially,mesh Ti/PbO2 electrode show efficient electrochemical oxidation ability towards aspirin and the performance could compare with BDD electrode to some extent,demonstrating the effectiveness and suitability of mesh electrodes. |
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