Mechanism Of Anode Passivation In Electrocoagulation Water Treatment

Electrocoagulation（EC）is a water treatment process with excellent treatment performance,easy operation,and no need for external chemicals.However,the anode passivation,which was attributed to the formation of an oxidized layer on the surface of the anode during operation,deteriorates the removal performance and increases the energy consumption.The passivation is becoming one of the biggest limiting factors for the development and application of EC technology.The passivation mitigation methods such as the addition of erosion ions,power supply polarity reversal,and ultrasonic waves all have problems such as lowed effluent quality,complicated operation,and high energy consumption.In order to effectively overcome this problem,the passivation mechanism should be clarified first.Therefore,by investigating the influence of different solution conditions and electrode structure on anode passivation and analyzing the formation process of passivation layer using electrochemical AC impedance and TOF-SIMS,this project initially reveals the formation mechanism of anode passivation and proposes an operation strategy to effectively suppress anode passivation.Firstly,the effects of anode surface roughness,solution p H,and current density on anode passivation have been investigated.The degree of anode passivation was quantified mainly by calculating the charge transfer impedance（Rct）of the anode surface.The results showed that higher anode surface roughness（Ra）inhibited the generation of the passivation layer on the anode surface,and the Rct of the anode decreased from 117.6 ohm·cm² to 67.7 ohm·cm²when Ra increased from 270.0 nm to 2055.5 nm,and the surface of the anode could be appropriately roughened for practical applications.The Rct of the anode at neutral p H increased by 7.3%and 28.7%compared with p H=3 and p H=9,respectively,indicating that neutral conditions are more favorable for the generation of the passivation layer on the anode surface.In addition,a thicker and more resistive passivation layer was generated on the surface of the anode as the current density increased.When the current density increased from 5 A/m² to 50 A/m²,the Rct value increased to almost 4.2 times the original one.The anode area was found to have an important influence on the degree of anode passivation.At the current densities of 5 A/m²,10 A/m²,and 50 A/m²,the Rct values of small anodes decreased by 24.3%,8.7%,and 22.4%,respectively,compared with those of large anodes.At the same current density,the larger the anode is,the more serious the anode passivation is.The simulation of the current density distribution on the surface of the anode revealed that there is inhomogeneity in the spatial distribution of current density,which was affected by the size of the anode.In addition,the passivation of different positions of the anode during the reaction was compared by surface analysis techniques such as SEM,Raman spectroscopy,XPS,TOF-SIMS,EDS Mapping and infrared thermography combined with electrochemical methods.It was found that the passivation layer on the surface of the anode gradually developed from the four sides of the anode to the middle of the anode,and the Rct at the edge was 1.36 times higher than that at the center after 30 min of reaction.Therefore,it is possible to change the size of the anode to regulate the current density distribution of the anode and thus mitigate the anode passivation.Based on this,a anode design strategy was proposed to suppress electrode passivation by using multiple small-size anode instead of large-size anode of equal area,and the results showed that the average voltage of the split anode system was2.6 V lower than that of a large anode system,which effectively alleviated anode passivation.Experiments with simulated water containing humic acid（HA）as a model contaminant were conducted to verify the applicability of the anodic passivation mitigation strategy in real water bodies.The results showed that the presence of HA hardly affect the response pattern of anode passivation to surface roughness and p H.In addition,the effect of initial HA concentration on anode passivation was investigated.When the initial HA concentration was increased from 0 mg/L to 20 mg/L,the Rct of the corresponding anode anode decreased by39.3%,and when the HA concentration was further increased to 200 mg/L,the Rct of the corresponding anode increased again to 1403.2 ohm·cm².The determination of the remaining Al³⁺content in the solution and the surface morphology analysis showed that at low HA concentration,HA may complex with Al³⁺on the surface of the anode,reducing the Al³⁺concentration on the anode surface and thus slowing down the generation of the passivation layer;at high HA concentration,excess HA may form a gel layer on the surface of the anode,reducing the current efficiency and aggravating the passivation.Finally,the anode splitting method was used to remove the HA-containing wastewater,and it was calculated that the energy consumption could be reduced by 21.8%by using the pole anode splitting method,and the removal rate of HA was not affected.This study provides a strong foundation for the inhibition of anode passivation in the process of EC.