Preparation Of Highly Efficient Metal Doped Lead Dioxide Electrodes For The Degradation Of Pharmaceutical Wastewater

Pharmaceutical wastewater containing toxic and recalcitrant organic compounds leads to severe environmental problems. When such effluents are discharged into the aquatic ecosystem without sufficient treatment, the pharmaceuticals in the wastewater can be transported over long distances due to their high soluble. Therefore, the treatment of pharmaceutical wastewater is of particular concern to scientists and engineers. In recent years, electrochemical oxidation technology has been widely used for the degradation of bio-refractory organic pollutants due to its high efficiency and environment compatibility. For electrochemical oxidation, anode material plays an important role on the organic pollutants degradation. PbO₂ has been regarded as an excellent metal oxide electrode due to its easy preparation, low cost, high chemical stability, as well as high overpotential for the oxygen reaction. However, this anode still needs improvement, such as the lowering the energy consumption and increasing the catalytic activity. Therefore, in the present work, the following investigation have been carried out for the purpose of development of PbO₂ electrodes with excellent electrochemical properties. And we selected aspirin as a representative organic pollutant, and the electrocatalytic oxidation of aspirin was studied in aqueous solutions to characterize activities of the modified electrodes.The electrochemical degradation of aspirin in aqueous solution was studied using a PbO₂ electrode. The effects of electrolyte concentration, current density and initial aspirin concentration on the aspirin degradation were examined in this study. Under an optimized condition of initial aspirin concentration 500 mg/L, electrolyte concentration Na₂SO₄ 0.1 mol/L and current density 50 mA/cm2, the removals of aspirin concentration and COD reached 64.07% and 53.53% after 150 minutes, respectively. IC and GC-MS methods were employed to identify the products resulting from the eletrochemical degradation of aspirin and the degradation pathway of aspirin was proposed.The novel Ni, Al and In doped PbO₂ electrodes were prepared via electrodeposition in nitrate solution. When the molar ratios of Ni2+, Al3+, In3+ to Pb2+ were 1% in the electrodeposition solution, respectively, the prepared 1% Ni-PbO₂ electrode, 1% Al-PbO₂ electrode, 1% In-PbO₂ electrode had the maximum pollutant removal capability. The morphologies and crystal structure of modified PbO₂ electrodes were characterized by SEM and XRD techniques, it could be found that a limited amount of doping metal element such as Ni, Al and In could produce a more compact PbO₂ film and diminish the size of the crystal grains. A compact surface of electrode would contribute to resist towards electrochemical corrosion, and thus help to prolong the service lifetime of the electrode. The polarization curves showed that the Ni-PbO₂ electrode, Al-PbO₂ electrode and In-PbO₂ electrode had the higher oxygen evolution potential than undoped PbO₂ electrode. The higher oxygen evolution potential will benefit to promote the electrochemical oxidation efficiency of organic pollutants. In addition, the Ni-PbO₂ electrode, Al-PbO₂ electrode and In-PbO₂ electrode were used to degradation aspirin in aqueous solution, which gave the direct evidence of the electrocatalytic capabilities of these electrodes. The results showed that, compared to undoped PbO₂ electrode, the modified PbO₂ electrodes obtained the higher kinetic rate constant, COD, TOC removals and the lower energy consumption. On the basis of a single metal element doping, the La, Y co-doped PbO₂ electrode was prepared by electrodeposition technique and the degradation mechanism for aspirin was explored. In contrast with PbO₂ electrode, La-PbO₂ electrode and Y-PbO₂ electrode, La-Y-PbO₂ electrode displayed a more compact surface with a smaller grain size of crystal, accompanying with increased concentration of active oxygen species. The higher voltammetric charge quantity of La-Y-PbO₂ electrode indicated its larger electrochemical active surface area and higher electrochemical activity. The La-Y-PbO₂ electrode was demonstrated to have a superior electrochemical oxidation ability, which had a higher apparent rate constant, mass transport and mineralization current efficiency and lower energy consumption. The evolution of intermediates and the aspirin degradation mechanism were further discussed. Meanwhile, high reusability and safety were achieved by La-Y-PbO₂ electrode when treating aspirin wastewater. Thus, the electrocatalytic activities and electrode stabilities of Ni-PbO₂ electrode, AlPbO₂ electrode, In-PbO₂ electrode and La-Y-PbO₂ electrode were superior to undoped PbO₂ electrode, indicating that the modified electrodes had application prospects and research value.The Al-doped PbO₂ electrode was applied to the electrochemical degradation of chloramphenicol（CAP） in aqueous solution. The Al-doped PbO₂ electrode showed high electrochemical activity, oxygen evolution potential, radical utilization rate, reusability and safety. The influence factors on CAP degradation with the Al-doped PbO₂ electrode were investigated in detail, and under the optimal conditions the removal rates of CAP and TOC reached 87.30% and 52.06% in acid conditions after 2.5 h electrolysis with a 0.2 mol/L Na₂SO₄ at a current density of 30 mA/cm2, respectively. The electrochemical degradation of CAP at Al-doped PbO₂ electrode followed pseudo-first-order kinetics. The degradation mechanism was proposed by cyclic voltammogram tests and it was deduced that hydroxyl radicals generated in the electrochemical process played a key role in oxidizing CAP. Finally, based on the reaction products identified, a possible degradation pathway including radical reaction, ring open and mineralization was proposed.