Mechanism Of Electrocatalytic Oxidation Of Perfluorooctane Sulfonate On A 3D Graphene-Lead Dioxide （3DG-PbO₂） Composite Anode And Its Toxicity Risk Evaluation

Electrocatalytic oxidation is a kind of water treatment technology which is suitable for treating toxic and refractory organic wastewater.In this study,a three-dimensional graphene-lead dioxide（3DG-PbO₂）composite anode was prepared by electrodeposition technology for electrocatalytic oxidation of perfluorooctane sulfonate（PFOS）.The main research contents and results are as follows:（1）3DG was prepared by Hummers method and hydrothermal reduction,and the3DG-PbO₂ electrode was prepared by electrodeposition.The surface morphology,crystal structure and elemental states of the electrodes were identified by scanning electron microscopy（SEM）,X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）.The hydroxyl radical generation capacity and electrochemical performance of electrode were investigated by fluorescence probe technology and various electrochemical tests.The experiment of electrocatalytic oxidation PFOS was carried out to evaluate the electrocatalytic oxidation capacity of electrodes.Using single variable method to investigated the effect of operating conditions on PFOS removal.The results show that the 3DG-PbO₂ composite electrodes had higher oxygen evolution potential,lower anodic surface charge number and lower charge transfer resistance than pure PbO₂ electrode.The 3DG-PbO₂-0.08 electrode had the strongest·OH radical generation capacity and the best electrocatalytic oxidation of PFOS.Under the conditions of 50 m A/cm² current density,0.05 mol/L Na₂SO₄ concentration and p H=3,10 mg/L PFOS can be completely removed after 120 min of electrolysis.（2）The influence of 3DG on the electrodeposition process of PbO₂ crystal was explored by cyclic voltammetry（CV）,electrochemical impedance spectroscopy（EIS）and chronoamperometry（CA）tests,and the nucleation mode of PbO₂ crystal was investigated by dimensionless processing of CA test data.The results indicate that the electrodeposition of PbO₂ was a continuous nucleation process controlled by diffusion in electroplating solution containing 3DG,and nucleated and crystallized at 1.50-1.70 V potentials;3DG obstructed the deposition of PbO₂ crystals and makes them smaller;in the same electroplating solution,higher potential is more favorable for the electrodeposition of PbO₂ crystal.（3）Linear scan voltammetry（LSV）,CV and EIS were used to test the electrochemical behavior of the electrode in 200 mg/L PFOS+0.05 mol/L Na₂SO₄ solution.The results show that the anodic oxidation of PFOS was controlled by mass transfer and diffusion,the direct oxidation capacity of PbO₂ anode was improved by doping 3DG.Meanwhile,3DG-PbO₂composite anode had higher electrocatalytic oxygen evolution potential and anodic oxidation coefficient（γ）,indicating stronger electrocatalytic oxidation activity poeessed by 3DG-PbO₂composite anode.（4）The degradation mechanism of PFOS was analyzed by liquid chromatography-mass spectrometry（LC-MS）,and the toxicity evolution of degradation solution was examined by acute toxicity assays to zebrafish embryos to evaluate the ecological risk.The results show that the sulfonate group was removed first to form C₈F₁₇·,after that,a variety of short chain polyfluorinated compound ions were generated through the continuous attack of·OH radicals,which were mineralized into CO₂ and HF at the end.With the electrocatalytic reaction processing,PFOS and its toxic intermediate products were continuously dissociated by·OH radicals,and the toxicity of the solution gradually decreased until disappeared.（5）The 3DG and yttrium（Y）commodified PbO₂ anode（3DG/Y-PbO₂）was prepared by coelectrodeposition techonology.The effect of co-modification with Y and 3DG on the physicochemical properties,electrocatalytic activity and stability of PbO₂ electrode was studied by SEM,XRD,XPS,fluorescent probe technology,LV,CV,CA,Tafel and PFOS degradation experiments.The results show that when the Y³⁺concentration in electroplating solution was15μM,the obtained 3DG-Y-PbO₂-15 electrode had a denser film structure,more active sites,high oxygen evolution overpotential and strong hydroxyl radical generation capacity,resulting the higher electrocatalytic oxidation performance and better stability.After120 min of electrocatalytic oxidation,the removal efficiency of 50 mg/L PFOS for on the3DG-Y-PbO₂-15 anode reached 99.56%.