Efficiency And Mechanism Of Intermittent Ultrasound Enhanced ZVI/PS Process On Sulfonamide Antibiotics Degradation

Sulfonamides（SAs）is one of the earliest applied chemical synthetic antibiotics.The widely application and emission of SAs has caused serious pollution to the environment.SAs are detected more frequently in water environment,which has aroused people’s great concern.Advanced oxidation technology（AOPs）based on zero valent iron（ZVI）activated persulfate（PS）process can achieve the effective degradation of SAs.ZVI/PS process has the advantages of low cost and high stability.However,it also faces some practical problems:large usage of ZVI,long reaction time,and incomplete degradation of pollutants.Therefore,an ultrasound（US）enhanced ZVI/PS system was constructed based on the promotion effect of US on chemical reaction.An intermittent operation mode of US was developed.The activation mechanism of PS in different period was revealed by analyzing the promotion effect of US on ZVI performance.Combined with the quantum calculation and the detection results of intermediate products,the degradation pathways of SAs in intermittent US enhanced ZVI/PS system were analyzed.The intermittent US enhanced ZVI/PS system was established based on the development of new operation mode of US.And the optimal operation conditions of intermittent US enhanced ZVI/PS system were explored to achieve the efficient and rapid degradation of SAs.The effect of US operation mode on SDZ degradation was first studied.The removal efficiency of SDZ increased from 89.8%to 97.4%,while the time of US introduced into system was changed from 0 min to 2 min after reaction.Besides,the effect of intermittent US operation mode on SDZ degradation was studied.The results showed that the removal efficiency and reaction rate of SDZ were the highest under the condition of 60 s/60 s（US runs for 60 s,and then stops running for the next 60 s,recycled）,shortening the action time of US by 76.7%.Response surface modeling（RSM）method was used for the optimization of operation parameters including the initial concentration of ZVI and PS,p H and US intensity.The optimal reaction conditions were obtained as follows:the concentration of ZVI and PS were 0.6 and 1.4 mmol/L,initial p H of 6.0 and US intensity of 0.225 W/cm³,respectively.Under the optimized conditions,the degradation efficiency of six typical SAs were all exceeded 95%in 30 min.Moreover,compared with the traditional SAs removal processes such as ZVI/PS,O₃,Fe-C micro electrolysis and UV system,the reaction rate of SAs by intermittent US enhanced ZVI/PS system was increased by0.7-14.0 times,and the energy consumption was reduced by 68.3-98.5%.The intermittent US enhanced ZVI/PS and ZVI/PS systems both conformed to the Langmuir Hinshelwood（L-H）heterogeneous catalytic model by the analysis results of kinetic and thermodynamic study.Moreover,compared with ZVI/PS system,the surface degradation rate constant k_rxn and equilibrium adsorption constant K_ads of intermittent US enhanced ZVI/PS system are increased by 10.9 and 4.2 times,respectively.The apparent rate constants of intermittent US enhanced ZVI/PS system and ZVI/PS system were 0.236 and 0.019 min^-1,respectively,and the surface activation energies were 32.38 and 81.05 k J/mol,indicating that the optimized intermittent US enhanced ZVI/PS system had stronger oxidation ability and lower surface activation energy to accelerate the degradation process of pollutants.The analysis of SEM-EDS,TEM,and XRD confirmed the enhancement of US on ZVI performance.The activation mechanism of PS was proposed as follows:Fe²⁺in the solution was the main activator of PS in active period,while the Fe（II）_（s）/Fe（III）_（s）on ZVI surface was the main activator of PS in silent period.The main types of radicals in intermittent US enhanced ZVI/PS system were identified as hydroxyl radical（·OH）and sulfate radical(SO₄^·-),which accounted for32.2%and 40.1%SDZ,respectively.At the same time,the existence of SO₄^·-at solid-liquid interface was proved by the o-phenanthroline complexation experiment,and the activation mechanism of PS in silent period was proved.The active sites of SAs were predicted to be the number 7,11 N atoms of phenylamino groups and number 9,10 O atoms of and sulfonamide groups by quantum calculation.The degradation intermediates were analyzed by HPLC-MS/MS,which was consistent with the results of quantum calculation.The common degradation pathways of six SAs were S-N bond breaking,C-N bond breaking,benzene ring hydroxylation,aniline oxidation and R substituents.The six membered ring SAs had a special"SO₂extrusion"N-N rearrangement degradation pathway compared with the five membered ring SAs.In summary,intermittent US enhanced ZVI/PS process can improve the activation and surface adsorption performance of ZVI,and the activation efficiency of PS.The intermittent US enhanced ZVI/PS system could achieve the efficient degradation of SAs with different structures and reduce the energy consumption and cost of SAs per unit mass.It is a green and efficient advanced oxidation technology.This study provides a theoretical basis and technical support for the application of intermittent US enhanced ZVI/PS system on the treatment of refractory pollutants in wastewater.