Preparation Of Lead Dioxide DSA And Its Electrocatalytic Oxidation Mechanism Towards Organic Pollutants

Electrochemical oxidation?EO?has been proven to be a versatile and powerful technique for removal of a wide range of refractory organic pollutants with promising application prospects.Therefore it became one of the focuses and frontier areas in the field of industrial wastewater treatment.The foremost part of EO is obviously the availability of electrode materials with high performance and low cost.Lead dioxide?PbO₂?dimensionally stable anodes have been studied for many years and continue to attract considerable interests due to their good electrical conductivity and corrosion inhibition,high oxygen evolution over-potential and strong oxidation capacity.In order to further improve the electrochemical activity and stability of PbO₂ electrodes,a novel preparation way for titanium-based PbO₂ anode was developed.Ionic liquids?ILs?were introduced as a modifier to the electrodeposition solution of Pb?NO3?2 in view of their unique electrochemical and adsorption properties.Influences of molecular structure,dosage of ILs,current density and bath temperature on crystalline structure,morphology and electrocatalytic activity of resulting PbO₂ deposits were investigated by X-ray diffraction,scanning electron microscopy and bulk electrolysis of phenol simulated wastewater,respectively.The modification mechanism of ILs was discussed by means of in-situ electrochemical analysis of electrodeposition process in the presence of ILs as well as microstructure characterization of modified PbO₂ coating.Furthermore,the detection method,generation rule and influencing factors of hydroxyl radical?·OH?in the process of electrochemical degradation of organic pollutants by PbO₂ electrodes were systematically studied.The correlation between the amount of ·OH generated and the degradation efficiency of organic matter provided an experimental basis for the revealing of the electrochemical oxidation mechanism.The research work could enrich the contents of environmental electrochemistry and provide the theoretical basis and technical supports for the electrochemical oxidation of refractory wastewater.?1?The structural,compositional and morphological properties of resulting PbO₂ coatings were strongly influenced by ILs modification.PbO₂ coatings with compact and regular morphology and a highly textured structure were obtained on Ti substrate when [Emim]BF4 was present in the deposition electrolyte.PbO₂ coatings with the optimum performance were obtained in the presence of 50 mg/L [Emim]BF4 at 50 ?,10 mA/cm2.The electrocatalytic activity of PbO₂ electrode was strongly dependent on its phase structure.?-PbO₂ showed higher electrocatalytic activity than ?-PbO₂,and higher degree of crystallinity increased the electrocatalytic activity of pure ?-PbO₂.In addition,?-PbO₂ with preferential orientation along?110?plane possessed higher electrocatalytic capacity on phenol degradation than that with random or other preferential orientation.?2?Compared with the PbO₂ modified by F? and SDS,the PbO₂ coatings prepared by ILs modification exhibited higher degree of crystallinity and different crystal orientation.After degradation of phenol simulated wastewater at 10 mA/cm2 for 180 min,the COD removal rate on ILs-modified PbO₂ anodes was 20% higher than that on conventional ones.In the early stage of degradation,current efficiency on ILs-modified PbO₂ anodes reached 38%,which was 15% higher that on normal ones.Besides,the stability of ILs-modified PbO₂ was much higher than normal ones by doubling the accelerating life time.All the results together indicated that the addition of ILs to deposition solution was able to significantly modify the electrosynthesis of PbO₂ and obtain a coating with improved performances.?3?The in-situ electrochemical tests for PbO₂ electrodeposition process showed that the electro-crystallization follows the three-dimensional continuous nucleation model;the addition of ILs in the deposition solution did not significantly change the electro-crystallization mechanism of PbO₂,but can inhibit the nucleation and crystal growth rate of PbO₂ with the latter being dominant.As a result,a density PbO₂ coating with smaller grain size was obtained.For [Emim]BF4-modified PbO₂ films a marked decline of oxygen vacancies was revealed by the Mott-Schottky tests.Meanwhile,the XPS results indicated that there was more lattice oxygen formed in the [Emim]BF4-modified PbO₂ electrode than that of the control.Therefore,both electrochemical and XPS tests supported that the enhanced performance of the [Emim]BF4-modified PbO₂ electrode arose from its lower content of oxygen vacancy,which meant lower probability of oxygen transfer from more effective ·OH into less desirable Olat at the electrode surface.?4?The capture efficiencies of salicylic acid?SA?and 4-hydroxybenzoic acid?4-HBA?for ·OH were compared.It was found that the main hydroxylated product of SA was 2,5-DHBA,the other products such as 2,3-DHBA and catechol was negligible.The hydroxylated products of either SA or 4-HBA were single and stable.The electrochemical oxidation of SA and 4-HBA with ·OH fitted the first-order reaction kinetics with the rate constant k being 2.1833×10-4 s-1 and 1.3500×10-4 s-1 respectively.To achieve the same amount of ·OH trapping,the initial amount and capture time of 4-HBA was 3 times and 2.3 times more than that of SA.Therefore,SA had a higher sensitivity and a stronger trapping ability for ·OH than 4-HBA.Then a stable and reliable ·OH detection method was established using SA as trapping agent by means of high performance liquid chromatography-UV detection?HPLC-UV?.?5?The electrocatalytic activity of the electrode materials was closely related to its ability to produce ·OH.The generation amount of ·OH was barely influenced by the type of supporting electrolyte used.However,the degradation efficiency differed greatly: the highest with NaCl,followed by Na₂SO₄ and NaNO3.Therefore,it can be deduced that the oxidants electrogenerated from supporting electrolyte had a major contribution to pollutants removal.With the rise of p H values of electrolyte solution the COD removal rates of phenol solution increased progressively,which was in agreement with the higher generation rate of ·OH at higher pH value.COD removal rates decreased with the increase in the electrolyte temperature.An increase in DC current density resulted in a higher COD removal rates.The increase of the pulsed frequency and duty ratio leaded to a rise fist and then reduction of the electro-degradation efficiency,with the optimum efficiency being achieved at 500 Hz?50% duty ratio.The influence of various operating conditions on the generation of ·OH followed the same tendency as that on COD removal rate of phenol solution,which indicated that the operating conditions affected the electro-oxidation efficiency through influencing the amount of ·OH generated and entered the electrolysis system.The calculation results of energy consumption for per COD removal indicated that the pulse electrolysis was more energy efficient than the non-pulse one by lowering 44% energy consumption.This can be attributed to the reduction of the concentration polarization in the diffusion layer by liquid phase mass transfer and higher generation capacity of ·OH under pulsed power.?6?The COD removal rate on PbO₂ electrodes followed pseudo-first-order rate kinetics.The apparent reaction rate constants remarkably dropped by 45% with the addition of ·OH scavenger tert-butanol,indicating that the radical type oxidation mechanism played the dominant role in the electrochemical degradation of organic pollutants.According to the analysis results of intermediate products distribution during the phenol oxidation process,it can be deduced that intermediate products such as p-benzoquinone,catechol and maleic acid were formed under the attack of ·OH.Then these species vanished by the electro-oxidation.Ultimately phenol was completely mineralized.A peak for phenol oxidation appeared in the CV curves tested in the aqueous solution containing phenol,indicating there existed direct electro-oxidation by electron transfer.In a word,the electrochemical degradation of phenol on PbO₂ electrodes was a result of both direct oxidation by electron transfer and indirect oxidation by ·OH,wheareas the indirect oxidation by ·OH played an absolute dominating role.?7?Electrocatalytic oxidation technique was applied to the treatment of actual coking wastewater.It was shown that higher temperature,higher current density and larger Na₂SO₄ dosage favored the electrochemical degradation.COD removal rate of coking wastewater with initial COD value of 1009 mg/L can reach more than 90% after electrochemical oxidation for 3 h at 0.25 mol/L Na₂SO₄,30 ?,and 40 mA/cm2.That meant the treated effluent can reach the emission standard of national level when unit COD treatment energy consumption was 178 kWh/kg.Pulse electrolysis can reduce energy consumption by about 35% compared to DC electrolysis.