Preparation Of Carbon Nanotube Modified PbO₂Electrode And Its Application For The Degradation Of Phenolic Pollutants In Aqueous Solutions

The electro-catalytic oxidation technology attracts extensive attention becauseof its effectiveness in the degradation of bio-refractory organic pollutants,environmental compatibility and easy implementation. The material of electrode, asthe key of electro-catalytic oxidation, is closely related to the degradation efficiencyof pollutants and the current efficiency of electro-catalytic oxidation process.Therefore, in present work, the lead dioxide electrode was modified, theelectrodeposition conditions of lead dioxide electrode were optimized, and theelectrochemical degradation of phenolic pollutants was studied with the lead dioxideelectrodes. The main research contents and results were summarized as follows:The SnO₂-Sb₂O₃interlayer of PbO₂electrode was prepared by thermaldecomposition. The α-PbO₂intermediate layer and β-PbO₂top layer of PbO₂electrode were prepared by electrodeposition. Scanning electronic microscopy（SEM）, X-ray diffraction （XRD）, open-circuit potential and electrochemicaloxidation of phenol were used to investigate the effect of electrodepositionconditions on the morphology, crystal structure and properties of α-PbO₂intermediate layer and β-PbO₂top layer. The experimental results showed that theα-PbO₂interlayer was prepared under optimum conditions: current density is3mA/cm², temperature is40℃and time is1h; β-PbO₂interlayer was prepared underoptimum conditions: current density is15mA/cm², temperature is65℃and time is1h.The electrochemical oxidation of phenol in aqueous solution was studied usingβ-PbO₂electrode as anode. The removal, mineralization and degradation kinetics ofphenol were studied and the current efficiency and energy consumption werecalculated during the course of electrolysis. In addition, the intermediates generatedin the degradation of phenol were indentified using high-performance liquidchromatography （HPLC） and a general pathway for the electrochemical degradationof phenol on β-PbO₂anode was proposed. The cyclic voltammetric curves indicatedthat phenol could be directly oxidized on β-PbO₂electrode, and this oxidationprocess was controlled by adsorption process. The phenol degradation on β-PbO₂electrode always followed a pseudo-first-order kinetics.Carbon nanotube （CNT） modified PbO₂electrodes （CNT-PbO₂） were fabricatedby adding CNT into the electrodeposition solution. However, CNT couldn’t bedoped into β-PbO₂films when the CNT was added alone. Thus, the surfactants werealso added into electrodeposition solution, and then CNT could be successfully doped into β-PbO₂films under the synergistic effect of surfactants. The influence ofcationic surfactant etyltrimethylammonium bromide （CTAB） and anionic surfactantlauryl benzene sulfonic acid sodium （LAS） on the doping of CNT was compared.The results of SEM and Energy Dispersive X-ray Spectrometer （EDS） showed thatmore CNT could be doped into β-PbO₂film under the synergistic effect of LAS.Polarization curves and cyclic voltammetry were adopted to measure theelectrochemical properties of these PbO₂electrodes. The CNT-PbO₂electrodeprepared with LAS （LAS-CNT-PbO₂） had higher activity, higher current efficiencyand longer lifetime. The degradation rate of4-CP and lifetime of LAS-CNT-PbO₂electrode was2.35and1.87times higher than those of β-PbO₂electrode,respectively.The oxidation process of4-chlorophenol （4-CP） was studied onLAS-CNT-PbO₂electrode in detail. First, the cyclic voltammetry technology wasexclusively used to study the effect of pH, temperature and4-CP concentration onthe oxidation of4-CP. The results indicated that4-CP was oxidized more easily inalkaline medium than acidic and neutral mediums, and the oxidation peaks of4-CPshifted toward lower potential values with increasing temperature and4-CPconcentration. Secondly, the effect of operating conditions of initial4-CPconcentration, current density, supporting electrolyte concentration, and temperatureon the removal and mineralization of4-CP, current efficiency, and kinetics were alsoinvestigated during electro-catalytic oxidation of4-CP on LAS-CNT-PbO₂electrode.The experimental results showed that higher mineralization of organic compoundsand lower current efficiency was obtained by higher current density and lower initial4-CP concentration; the higher the temperature, the higher the4-CP and TOCremoval ratios; the concentration of supporting electrolyte is not the significantparameter in this process. The4-CP degradation always followed apseudo-first-order kinetics. HPLC was employed to identify the products resultingfrom the electrochemical degradation of4-CP and the degradation pathways of4-CPwere proposed. Besides, the biodegradability and the toxicity of4-CP degradationsolution were investigated. The results showed that, after120min ofelectro-catalytic oxidation, the biodegradability of degradation solution wasimproved and the toxicity was decreased significantly.Ce and CNT co-doped PbO₂（Ce-LAS-CNT-PbO₂） electrode was prepared byadding Ce（NO3）3into the electrodeposition solution of LAS-CNT-PbO₂electrode.The results of SEM, EDS and XRD revealed that Ce and CNT had been introducedinto Ce-LAS-CNT-PbO₂electrode. Compared with β-PbO₂, Ce-PbO₂, andLAS-CNT-PbO₂electrodes, Ce-LAS-CNT-PbO₂electrode had smaller grain size and higher active surface area. The results of cyclic voltammetry tests in[Fe（CN）6]4–/[Fe（CN）6]3–system, determination of hydroxyl radical generation,electro-catalytic oxidation degradation of4-CP and accelerating lifetime testsexhibited that the co-doping of CNT and Ce further improved the electro-catalyticactivity and lifetime of PbO₂electrode based on LAS-CNT-PbO₂electrode. Thecomparison of Ce-LAS-CNT-PbO₂electrode and Ce-CNT-SnO₂electrode indicatedthat the electro-catalytic activity of former was superior to that of latter, but theservice life of latter was far longer.