Efficiency And Mechanism Of DMP Degradation In Hydroxylamine-enhanced Electrode Aeration Electro-fenton Process

In the Electro-Fenton advanced oxidation process,H₂O₂ and Fe²⁺can be produced continuously by the electrochemical reduction of O ₂ and Fe³⁺at the cathode,and then,H₂O₂ reacts with Fe²⁺to produce powerful oxidants(such as·OH)for the degradation of organics.However,its practical application is limited by low H₂O₂ production,slow transformation from Fe³⁺to Fe²⁺,and narrow pH application range.To conquer these defects,a new aeration mode characterized by aeration from inside a carbon fiber electrode was proposed in this dissertation,namely Electrode Aeration(EA)mode.Based on the EA mode,the EA Electro-Fenton process and the hydroxylamine-enhanced EA Electro-Fenton process were constructed.Addit ionally,the application of the EA Electro-Fenton and hydroxylamine-enhanced EA Electro-Fenton processes in degrading refractory organics was evaluated by the degradation of dimethyl phthalate(DMP).Graphite felt was used to construct the EA mode in this dissertation,which was used for the electrocatalytic oxygen reduction to produce H₂O₂.The influences of oxygen flux,current density and solut ion pH on the Electro-generation of H₂O₂were investigated and compared by different aeration modes(Solution Aeration(SA)and EA).In addit ion,cyclic voltammetry and linear sweep voltammetry were used to investigate the oxygen reduction performance and mass transfer effic iency of the different aeration modes.Results showed that when the oxygen flux and current density were 600 m L/min and 17.6 m A/m³,respectively,the H₂O₂ yield in EA mode was 152-169 mg/L in the pH range of 3.0-10.0 and 126 mg/L at pH=11.0.At the same condit ions,the H₂O₂ yield in the SA mode was 113 mg/L at pH=3.0 and30-51 mg/L in the pH range of 4.0-11.0.The H₂O₂ yield in EA mode was much higher than that in SA mode.Besides,the current intensity of oxygen reduction peak and rate of mass transfer in EA mode were 2.2 and 5.4 times higher than those of SA mode,respectively.The EA Electro-Fenton process was constructed,and the degradation of DMP by EA Electro-Fenton process and SA Electro-Fenton process were compared and investigated.Results showed that the EA Electro-Fenton process exhib ited excellent performance for the degradation of DMP.At pH=3.0,removal efficiencies of 200mg/L DMP and solution TOC by EA Electro-Fenton process were 50.0%and 55.4%higher than SA Electro-Fenton process,respectively.At pH=4.0 and 5.5,the EA Electro-Fenton process showed 36.4%-56.1%and 9.4%-35.7%higher removal for20-50 mg/L DMP and TOC than the SA Electro-Fenton process,respectively.High effective degradation performance of the EA Electro-Fenton process was attributed to the enhanced mass transfer of O₂ and the solution,which provided the suffic ient active area for H₂O₂ electro-generation and promoted the Fe³⁺/Fe²⁺cycling,thus increasing the·OH content.Using EA mode,graphite felt could collect iron sludge at low O₂ flux,and then strip it for reuse at large air flux.This process regenerated graphite felt electrode and elided the disposal of iron sludge.Besides,by supplementing one-fifth of the original Fe SO₄ dosage,the iron-sludge-stripping EA Electro-Fenton process was a cost-effective approach.During long-term use,DMP degradation performance of the EA Electro-Fenton process gradually improved due to the increased hydrophilic property and oxygen-containing functional groups,while the decreased DMP removal with the SA Electro-Fenton process was ascribed to the electrode poisoning caused by iron sludge deposition.The hydroxyla mine was added into the(SA and EA)Electro-Fenton processes for the degradation of DMP.The effects of Fe²⁺concentration,hydroxyla mine concentration,solut ion pH,and DMP concentration on DMP degradation by the(SA and EA)Electro-Fenton process and the hydroxylamine-enhanced(SA and EA)Electro-Fenton process were investigated and compared.The results showed that addition of hydroxyla mine could significantly enhance the degradation efficiency of DMP in the Electro-Fenton process,and the degradation effic iency of DMP in the hydroxylamine/EA Electro-Fenton process was much higher than that of the hydroxylamine/SA Electro-Fenton process.Even at low concentration of Fe²⁺,or high pH value,or low concentration of hydroxylamine,the degradation effic iency of DMP by the hydroxyla mine/EA Electro-Fenton process was still higher than that of the hydroxylamine/SA Electro-Fenton process at high concentration of Fe²⁺,or low pH value,or high concentration of hydroxylamine.Moreover,compared with the hydroxylamine/SA Electro-Fenton process,the hydroxylamine/EA Electro-Fenton process could more effectively broaden the application range of pH,and exhibit a higher degradation rate with the increase of DMP concentration(5-50mg/L).Furthermore,the effect of water parameters on DMP degradation were explored and the influences of other common reducing agents(Vitamin C,Na HSO ₃,Na NO₂ and Na₂S₂O₃)on degradation of DMP were also investigated in the(SA and EA)Electro-Fenton processes.Common inorganic cations(N H₄⁺,Mg²⁺,Ca²⁺,and K⁺)and NO₃^-anions in water had no effect on the DMP degradation.The presence of PO₄^3-promoted DMP removal,while Cl^-produced an inhibitory effect to some extent.Common organic compounds and humic acids showed different degrees of inhibit ion.Tap water and river water had no effect on the DMP degradation in the hydroxylamine/EA Electro-Fenton process,and groundwater showed a slight inhibit ion effect(influence<5%),which meant that the hydroxylamine/EA Electro-Fenton process was very effective to resist the interferenc es of co-exist ing species in actual waterbodies.Among other common reducing agents,vitamin C could slightly enhance the degradation performances of the SA/EA Electro-Fenton processes,while other reducing agents showed inhibition effect.Finally,the mechanis m of DMP degradation in the hydroxylamine-enhanced EA Electro-Fenton process was investigated.The experimental results showed that addition of hydroxylamine promoted the Fe³⁺/Fe²⁺cycle in the system,but had no effect on the electro-generation of H₂O₂.The coupling effect of EA mode and hydroxylamine for accelerating Fe³⁺/Fe²⁺cycle could more effective ly catalyze the decomposition of high concentration H₂O₂ into·OH in the hydroxylamine/EA Electro-Fenton process,resulting in the enhancement of DMP degradation.In the hydroxylamine/SA Electro-Fenton process,the main conversion products of hydroxylamine were N₂O and NO₃^-,while the main transformation products of hydroxylamine in the hydroxyla mine/EA Electro-Fenton process was N₂,which was mainly attributed to the accelerated Fe³⁺/Fe²⁺cycle by the coupling effect of EA mode and hydroxyla mine,indicating that EA mode could better control the nitrogen content in water.