Efficacy And Mechanism Of Nitrogen-sulfur-iron Doped Carbon Cathode In Electro-fenton For Sulfonamides Degradation In Water

Advanced oxidation technology is considered to be one of the ideal method for treating antibiotic wastewater.Among many advanced oxidation metho ds,electro-Fenton（EF）technology has been favored by researchers for its advantages such as no need for H₂O₂ addition and easy operation condition.However,the EF system suffers from low efficiency of cathodic catalytic oxygen reduction reaction（ORR）to generate H₂O₂ and requires severe p H conditions（p H=3）,making it difficult to spread the use of EF technology.Based on this,the present study provided active sites for H₂O₂ production by doping carbon materials with different contents of carbon nitride.In additon,this study enhanced H₂O₂ accumulation and improved current efficiency by co-doping nitrogen and sulphur on carbon materials.Moreover,iron was doped onto nitrogen and sulphur co-doped carbon materials as well to broaden the applicable p H range of the system while enhancing H₂O₂generation..Also,active sites and catalytic mechanisms of the ORR were also investigated.A carbon nitride-modified carbon electrode was prepared to explore the effect of carbon nitride doping amount on the electrode performance,and build it as a cathode for the degradation of sulfathiazole（STZ）in an EF system.The effects of g-C₃N₄ doping on electronic properties were explored by liner scanning voltammetry（LSV）,density functional theory（DFT）calculation and rotating disk electrode（RDE）.LSV showed an improvement of the electrocatalytic activity,while DFT calculation showed that the active center increased after carbon nitride-doping.RDE result indicated that modified cathode favored 2 electron reduction of oxygen.Regarding H₂O₂ accumulation,the electrodes with graphite and g-C₃N₄ doping ratio of 1.5 showed the highest H₂O₂ accumulation.In addition,approximately 100%of STZ degradation could be achieved after 180 min electrolysis under the following conditions:applied current 50 m A,[STZ]₀=500mg L^-1,[Fe²⁺]=100μM and p H=3.In addition,the attack sites of STZ by different active species including·OH,·O₂^-and ¹O₂ were theoretically proposed by frontier orbital theory and Fukui function.Ultimately,it was proposed that STZ was degraded mainly by oxidation of amino groups,hydroxylation and the breaking of S-N and S-C bonds.N and S were co-doped into the carbon-based cathode to further enhance H₂O₂accumulation and improve current efficiency.The optimized H₂O₂ accumulation of the N and S co-doped cathode was 7.95 mg L^-1 h^-1 cm^-2 and the current efficiency reached 50.14%,an increase of 21.74%compared to the current efficiency of the N doped electrode.According to DFT calculation,N and S co-doped structure favored the“end-on”O₂ adsorption and reduced the adsorption energy of O₂ on the active site.In addition,the C-O bonds in the N and S co-doped cathodes were longer than the O-O bonds,which is favorable for*OOH desorption,promoting the ORR to generate H₂O₂.Furthermore,theoretical calculations of the frontline orbitals of sulfadimethoxine（SDM）,STZ and sodium sulfadiazine（SDZ）showed that the absolute hardness（η）of the three contaminants ranged fromη_SDM>η_STZ>η_SDZ,indicating that the SDM molecular system was the most stable,while SDZ was the most susceptible to alteration,which was susceptible to cleavage by free radical attack.The results of the intermediate biotoxicity assessment showed that most of the intermediates were less toxic than the parent.However,the products from C-S/S-N cleavage and amino oxidation tended to be more toxic in the long term than the original contaminants.In order to solve the problems of additional iron catalyst and narrow p H range,a N,S and Fe co-doped cathode was prepared and a heterogeneous EF system was constructed.Results showed that the N,S and Fe co-doped catalysts contain Fe₃N that can promote ORR.Also,it contains Fe₃O₄ and Fe S which can catalyze the Fenton reaction.The electrochemical performance results indicated that the optimized cathode system has low mass transfer resistance(R_ct=2.46Ω),high H₂O₂ accumulation(15.60 mg L^-1 cm^-2)and excellent H₂O₂ selectivity（nearly 90%）.In addition,optimized aeration with a microporous aeration tube can provide microbubbles and change the bubble direction to allow more dissolved and gaseou s oxygen to encounter the electrode active sites.Moreover,the active speci es in the system were explored and proved to contain mainly·OH,¹O₂,·O₂^-and Fe（IV）,and a possible mechanism for their generation was proposed.Furthermore,the system was established to automatically reduce the solution p H by assigning different current densities to the anodes and cathodes.Finally,the BOD₅/COD values before and after the antibiotic wastewater treatment using EF were examined,and it was found that the BOD₅/COD ratios reached 0.350 after treatment from 0.146,which could be biodegraded.In conclusion,the construction of the co-doped carbon electrode can effectively increase the EF reaction efficiency and broden the applicable p H value of the EF system.Also,it is effective for the degradation and toxicity reduction of sulfonamide antibiotics and can be applied as a pretreatment process for antibiotic wastewater.