| Phenolic compounds are widely used in coking,pesticide,printing and dyeing industries,etc.,so phenolic wastewater is abundant.With the discharge of wastewater,phenolic substances enter the environment,causing them to be detected in rivers worldwide.Most phenolic compounds are highly toxic,and phenol,nitrophenols and chlorophenols are typical of China’s "priority control pollutants".If the wastewater enters the environment without control,it will cause great harm to the aquatic environment,crops and even human health.The phenolic compound has an inhibitory effect on microorganisms,so that its presence in the wastewater has a large impact on the biochemical treatment unit,so it is indispensable for the pretreatment process of the phenolic wastewater.Recently,electrochemical oxidation technology has drawn much attention due to its high efficiency,no secondary pollution,easy control,etc.At the same time,advances in the power industry will promote the development of its application.However,the catalytic activity of the electrode,the core of electrocatalytic oxidation technology,has also affected the development of this technology.In this paper,a new Ti3+self-doped TiO2 nanotube arrays(DNTA)anode with good electrooxidation performance was developed for the refractory phenols.The electrocatalytic oxidation performance of the anodes under different preparation parameters was systematically studied.Secondly,In the electrooxidation experiments of phenol and para-substituted phenol,the activity of DNTA and commercial anodes was compared,and the electrooxidation mechanism was further explored.Finally,combined with the electrochemical reduction process,the treatment effect of DNTA on the actual phenolic wastewater was investigated.The main findings are as follows:(1)Thoroughly polished and cleaned Titanium sheet was provided with anodization process to prepare TiO2 nanotube arrays,and then was calcined to turn amorphous TiO2 to anatase one.After nanoparticles on the electrode surface were removed by secondary anodization,cathodization was provided to obtain the Ti3+ selfdoped TiO2 nanotube arrays anode.SEM showed that uniform nanotubes was on the electrode surface,which was beneficial to the mass transfer during electrooxidation.It was confirmed by XRD and HRTEM that TiO2 was anatase and exposed(101)crystal plane.The Mott-Schottky curves showed that cathodization process greatly increased the carrier density of DNTA,which greatly improved the its conductivity(EIS).According to Tauc method,the band gap of TiO2 was not narrowed,which proved that the enhanced DNTA conductivity was attributed to the doping of Ti3+.Finally,in LSV,a high oxygen-evolution potential was observed on DNTA,indicating that DNTA had great potential as an anode for the electrochemical oxidation of organics.(2)The electrochemical oxidation performance of anodes under different preparation parameters was studied.The secondary anodization could clear surface nanoparticles,which was beneficial to the cathodization reaction of nanotubes,and finally improved the stability of the electrodes.Improper preparation parameters harmed the electrooxidation activity.Short anodization time leads to short nanotubes,and Ti3+ was susceptible to oxidation in short nanotubes,which resulted in poor electrochemical performance.During calcination,The temperature above 450℃could convert the surface TiO2 nanotubes to massive nanowires,so that Ti4+in the nanowires was reduced to Ti3+ instead of nanotubes,while Ti3+was more unstable in the TiO2 nanowires.For the cathodization,the electrolyte had little effect on the performance of the electrodes except for strong acid and weak electrolytes,and high cathodization current density could result in collapse of TiO2 skeleton.(3)In the experiments of degradation of phenol,removal efficiency of phenol on DNTA was 63.2%after 2 h,which was much higher than Ti/Ru-Ir,Ti./Sn-Sb,Ti/Ir-Ta and Ti/Pt,but lower than BDD.In terms of mineralization,the performance of DNTA was better than all electrodes.According to the mechanism study,the high electrocatalytic oxidation activity of DNTA was mainly attributed to the surface-bound·OH,while BDD mainly produced free ·OH.In the electrooxidation experiments of p-chlorophenol(PCP),p-nitrophenol(PNP),p-methoxyphenol(PMP)and phenol,the removal efficiency on DNTA followed the order PNP(70.5%)>PMP(63.4%)≈phenol(63.2%)≈PCP(61.0%).The comparison of DNTA,BDD and Ti/Pt showed that these phenols were mineralized on DNTA more easily because the produced intermediates were oxidized by surface-bound ·OH immediatedly when phenols were oxidized.Because free ·OH predominates in the BDD system and it can diffuse into the solution and react with the substrates,phenol in the solution is oxidized preferentially.However,it was difficult for Ti/Pt to degrade phenols.(4)In the experiments of actual phenolic wastewater treatment,taking the preferred material,graphite felt as cathode and DNTA as anode,the treatment effects of electrocatalytic oxidation,electrocatalytic reduction-oxidation and electrocatalytic oxidation-reduction were compared,and electrocatalytic reduction-oxidation performed best in terms of contaminants mineralization and toxicity reduction.Compared with commercial anodes,the performance of DNTA was similar to that of expensive BDD,and far superior to Ti/Pt,Ti/Ru-Ir and Ti/Sn-Sb.Finally,the initial pH of wastewater could influence the treatment effect,and the highest efficiency of UV254 removal and mineralization was observed when pH=2. |
