Research On The Mechanism Of The Removal Of Azo Dye In Water By Zero Valent Iron Accelerated With Sulfidation And Weak Magnetic Field

Due to the characteristics of high chrominance,high organic content,complex composition,large pH fluctuation and high toxicity(carcinogenic,teratogenic and mutagenic effects)of dye wastewater,there is a more serious impact on the health of human beings and other creatures when dye wastewater is discharged to the environment.Zerovalent iron(ZVI)is rich in content,cheap and easy to obtain,strong reduction ability and environment-friendly,so it has been widely used in environmental remediation and treatment and more and more attention has focused on the treatment of dye wastewater using ZVI.However,in the process of production and utilization,the passive film can form on the surface of ZVI,and this can reduce its reactivity and decrease the removal rate of pollutants.In this study,sulfidation and weak magnetic field(WMF)were used to improve the reactivity of ZVI and accelerate the decolorization efficiency of azo dye(Orange ?),and good results were achieved.Fe/FeS particles with an average particle size of 30 ?m were prepared by commercial m-ZVI and sodium sulfide.The effects of WMF,sodium sulfate,the dosage of Fe/FeS,Fe(II)and isopropanol on the reaction were investigated in neutral and aerobic solution.The results were compared with that of pure ZVI under the same experimental conditions.The experimental data show that the reactivity conforms to the following trend:Fe/FeO-WMF<<Fe/FeS-WMF<Fe/FeS + WMF ? Fe/FeO+ WMF,where Fe/FeO represents common commercially available ZVI and Fe/FeS stands for ZVI after sulfidation.All the experimental results are in accordance with pseudo-first-order kinetics.According to the observed rate constants(kobs),it shows that the addition of sulfidation and WMF can greatly improve the decolorization rate of Orange ?,2.4-71.8 times,meanwhile the combination of sulfidation and WMF also has certain synergistic effect.This paper systematically studied the corrosion process of sulfidated ZVI and ordinary ZVI under aerobic conditions,and constructed a model for oxygen-driven passivation during reaction.The model fit data agrees well with the original data,which indicates that the surface passivation of ZVI is mainly affected by dissolved oxygen,and this model has similarity with the "oxygen vertical curve".According to the results of UV-Vis and LC-MS,the sulfidation and WMF accelerated the reduction of azo-bond(chromophore),but did not change the mechanism of ZVI to remove Orange I.At the same time,after the breakage of azo-bond,1-amino-4-naphthol is formed,and it is unstable under aerobic conditions and easily becomes the corresponding quinone.The strengthening effect of the WMF is mainly due to the magnetic field gradient force accelerates the paramagnetic substances(such as the Fe(II)produced in reaction)in solution to the direction of high magnetic field strength,thus weakening the passivation of the overall surface reaction sites of ZVI and promoting the corrosion of ZVI.However,the reason for the improvement of ZVI reactivity by sulfidation may be mainly due to its catalytic effect and the reduction of adsorbed ferrous iron.Both sulfidation and WMF can improve the reusability of ZVI.In this paper,the respike experiments of continuous application,alternating application and non-application of WMF were carried out for the first time.The results in alternating experiments show that the reaction activity of ZVI does not decrease after WMF is removed but becomes stronger.This indicates that the "remanence" effect of WMF may exceed the effect produced when WMF is always present.Therefore,it is not necessary to apply WMF continuously when WMF and the ZVI are applied to the actual sewage treatment.This also provides a new theoretical basis for future practical application.