Degradation Of Sulfamethoxazole In ZVI/H₂O₂ And ZVI/K₂S₂O₈ Processes Activated By Sulfidation And Weak Magnetic Field

With the development of social economy,frequent detection of antibiotics in water has attracted worldwide attention to the safety of water environment.Sulfamethoxazole?SMX?was a typical sulfonamide with high detection frequencies in vairous aquatic environments.Recently,advanced oxidation processes?AOPs?which quickly removal various organic pollutants have attracted more attention in water treatment.Fe²⁺ions as a low cost,non-toxic and environment-friendly activator applied in AOPs.However,the addition of Fe²⁺introduced other anions which could consume oxidants or free radicals.Zero-valent iron?ZVI?as an reasonable source of iron could replace Fe²⁺in AOPs for its wide source and simple operation.The low reactivity of zero-valent iron has become an important barrier to promote the activity of zero-valent iron in AOPs.Thus new technologies which improved the activity of zero-valent iron has become a research hotspot of AOPs based on ZVI.In this study,SMX was selected to objective contaminants.Efficiency and mechanisms of SMX removal by ZVI-based AOPs activated by sulfidation and weak magnetic field?WMF?were studied.Removal of SMX by ZVI-based AOPs activated by sulfidation and WMF were invetigated in batch experiments.Then various characterization methods were employed to clarify mechanisms of the enhancement of zero-valent iron activity,quenching experiments and electron paramagnetic resonance spectroscopy?EPR?were carried out to identify radicals in reactions,and mass spectrometry experiments were conducted to obtain possible degradation pathways of SMX in AOPs.Sulfidated zero-valent iron(S-ZVI^bm)was applied to activate H2O2 and K2S2O8to degrade SMX.When initial pH>4.0,ZVI^bm/H2O2 and S-ZVI^bm/H2O2 processes were both difficult to degrade SMX.The removal rate of SMX was slightly promoted by S-ZVI^bm/H₂O₂ at initial pH?4.0 compared with ZVI^bm.With the increase of initial pH from 3.0 to 7.0,the removal rate of SMX at the end of the reaction in S-ZVI^bm/K2S2O8 processes were 12.5%,27.8%,30.8%,27.7%,and 26%higher than that in ZVI^bm/K2S2O8 processes,respectively;and rate constants of SMX degradation in S-ZVI^bm/K2S2O8 processes was 1.47-2.66 times that in ZVI^bm/K₂S₂O₈ processes.Rate constants of SMX degradation in S-ZVI^bm/K₂S₂O₈processes were 2.27-3.72,1.55-2.47,and 2.27-3.6 times those in ZVI^bm/K2S2O8processes in the range of initial zero-valent iron concentration from 5.6 mg/L to 112mg/L,initial K2S2O8 concentration from 0.1 mmol/L to 2 mmol/L,and initial SMX concentration from 5?mol/L to 50?mol/L,respectively.Meanwhile,rate constants of SMX degradation in S-ZVI^bm/K2S2O8 processes in the presence of sulfate ion,chloride ion,and nitrate ion at different concentrations were 1.56-2.01,0.97-2.63,and 1.27-3.47 times those in ZVI^bm/K2S2O8 processes,respectively.With the increase of anion concentrations,the enhancement of SMX degradation efficiency in S-ZVI^bm/K₂S₂O₈ system would be decreased.The removal rate of SMX by sulfidated aged ZVI(S-AZVI^bm)/K2S2O8 system was 65.8%higher than that of aged ZVI(AZVI^bm)/K2S2O8 system,and the activation time of passivation layer on the aged ZVI was shortened.WMF enhanced SMX degradation by ZVI activated H2O2 and K2S2O8 were investigated.The degradation rate of SMX was accordant with the release rate of total dissovled iron in activated H2O2 and K2S2O8 by eight ZVI samples with different origins.WMF did not change the available pH range of ZVI/H2O2 and ZVI/K2S2O8 processes.At initial pH 3.0 and 4.0,removal rates of SMX in WMF/ZVI/H2O2 processes were 36.4%and 32.2%higher than ZVI/H2O2 processes,respectively,and the lag phase of SMX degradation was greatly shortened by WMF.In the range of initial pH 3.0-7.0,the degradation rate constant of SMX in WMF/ZVI/K2S2O8 processes was 1.49-9.38 times that in ZVI/K2S2O8 processes.At different zero-valent iron concentrations,oxidant concentrations,and initial SMX concentrations,WMF enhanced the degradation of SMX in ZVI/H2O2 and ZVI/K2S2O8 processes.In the presence of background anions,WMF enhanced the degradation of SMX by ZVI/H2O2 and ZVI/K2S2O8.The removal rate of SMX in K2S2O8 activaed by sulfidated ZVI was enhacned by WMF.Sufidation could shorten the lag phase of SMX removal by ZVI in the presence of K2S2O8 with WMF.The electrochemical analysis showed that ZVI after sufidation possessed a higher corrosion current and a lower polarization resistance,so the electron transfer rate of ZVI was enhacned.Raman spectroscopy indicated that ZVI with a passivation film covered by iron oxide showed higher activity towards oxidants in the presence of WMF.WMF which generated induced magnetic field on the surface and inside of ZVI particles dissolved the original passivation film of zero-valent iron,and active sites of ZVI could exposed to oxidants and accelerate the degradation of pollutants.Chemical quenching experiments and EPR spectra indicated that sulfidation treatment or WMF did not change species of radicals in AOPs.A case study of WMF/ZVI/H2O2 and WMF/ZVI/K2S2O8 processes was employed to investigate possible degradation pathways of SMX.SMX could produce ortho-hydroxyl substituted SMX in WMF/ZVI/H2O2 processes.However,this important pathway was not detected in WMF/ZVI/K2S2O8 system,and·SO₄^-was easier to attack the S-N bond of SMX directly.