Study On Photoelectrocatalysis Degradation Of Tetracycline By TiO₂ Nanotube Electrode

Pharmaceutical wastewater is a kind of difficultly degraded industrial wastewater,because it is intermittent discharged and has a bad biodegradability.It contains a large number of organic compounds,such as antibiotics.Pharmaceutical wastewater is complex,and has a strong biological toxicity.It is difficult to deal by single biochemical treatment,so effective physico-chemical pretreatment technologies must be adopted to destroy the molecular structure of antibiotic,to reduce the toxicity,and to improve the biodegradability.In this paper,theinfluence factors and the degradation mechanisms of refractory tetracycline pharmaceutical wastewater in the photoelectrochemical processes were studied in details.By means of anodic oxidation,highly ordered TiO₂ nanotube array electrodes were prepared in the electrolytes,respectively with HF acid solution and NH4 F glycerol solution.Scanning electron microscope（SEM）results show that TiO₂ nanotube arrays are of uniform morphology.The length of TiO₂ nanotube arrays prepared in organic electrolyte is 3 μ m,while the length prepared in aqueous solution is 300 nm.XRD patterns show obvious peaks of anatase after heat treating at 450 ℃.TiO₂ nanotube array electrodes also have good photoelectrocatalytic activities.At the bias potential of 4 V,the photocurrents are 1.37 m A ·cm-2 and 0.83 m A ·cm-2,respectively.By using the electrode of 3 μ m,the removal efficiency of 50 mg · L-1 tetracycline reaches about 93% within 180 min.The electrodes are highly stable and they can be used repeatedly.The TiO₂ nanotube electrodes were modified by Ag doping,and the samples were characterized by scanning electron microscope and X-ray diffraction.The reduction of Ag deposited in TiO₂ nanotubes,and the morphology of the growth was good.The results of TOC removal rate and three-dimensional fluorescence spectra showed that the tetracycline degradated gradually in thephotoelectrocatalysis process.Using the single-factor test and response surface methods based on central composite design,the influence of bias,initial p H,Clconcentration,reaction time and its interactive effects was detected.Furthermore,a second-order polynomial regression equation was developed to describe the TOC removal rate of photoelectrocatalysis process and validated by analysis of variance.The results showed that the significance of influence factor followed the order: the reaction time,the concentration of Cl-,initial p H value and bias potential,the interactive influence between the concentration of Cl-and the reaction time was the most significant.The optimal conditions with the predicted TOC removal of 86.72 % were determined as bias potential of 3.12 V,initial p H of 3.7,initial Cl-concentration of 62.5 mmol/L,and reaction time of 133.2 min.Under these conditions,the actual TOC removal rate was 83.56 %,which was highly consistent with the predicted result of the model equation.It also showed that the Ag-TiO₂ electrode has a good photoelectrocatalysis activity.The analysis results of the three-dimensional fluorescence spectroscopy and GC-MS of wastewater showed that photocatalytic reaction may be preferentially damaged the conjugated double bond structure of humic acids,eventually transformed fulvic acids and other small molecules.Finally most of these organics were mineralized.