Enhanced Azo Dye Removal From Wastewater By Coupling Sulfidated Zero-valent Iron With A Chelator And Its Mechanism

Dye wastewater pollution has been a significant environmental problem,which attracted a great deal of attention in the world.Due to its high chrominance,high toxicity,high organic content,complex composition and difficulty-degradation,it causes a great threat to animals,plants and human beings when discharged into the environment.Zero-valent iron(ZVI)technology has been widely used in the remediation/treatment of soil,groundwater and wastewater in recent decades because of its low cost,high efficiency,environmental benignity and wide adaptability.However,the defects of easy passivation,poor selectivity and easy agglomeration greatly limit its application.In this study,the coupling of sulfidation and chelator(EDTA)could accelerate the surface corrosion of ZVI?weaken the oxidation of Fe(?)(adsorbed/dissolved)by oxygen,thus increasing the removal rate and efficiency oft azo dye(acid red 73)by ZVI.Sulfidated ZVI(S-ZVI)used in this study was synthesized via modification of micro-ZVI using sodium sulfide,having an average particle size of 30 ?m.The degree of sulfidation can be changed by controlling the amount of sodium sulfide.The obtained S-ZVI was characterized by Scanning Electron Microscopy(SElM),X-ray Energy Dispersive Spectroscopy(EDS),X-ray Photoelectron Spectroscopy(XPS)and Fourier Transform Infrared Spectroscopy(FTIR)to analyze the surface morphology of ZVI/S-ZVI before and after the reaction.The results showed that sulfidation caused the ZVI surface to be covered with a layer of porous FeSx,which could improve the electron transport of ZVI surface.In addition,the surface roughness and specific surface area were greatly improved and reached the maximum at S/Fe = 0.084.This study systematically investigated the removal of acid red 73(AR 73)by ZVI/S-ZVI(S/Fe = 0.084)coupling EDTA under aerobic and neutral conditions.The results suggested that the coupling of EDTA and sulfidation could significantly enhance the removal of AR 73,and the reactivity followed the following trend:S-ZVI+ EDTA>S-ZVI>EDTA + ZVI>ZVI.To further study kinetics of pollutant removal,the pseudo-first-order dynamics model was used to analyze experimental data.Based on the fitted apparent rate constants(Kobs),it can be seen that the removal rate of AR 73 obtained in the EDTA + S-ZVI system is about 6.5 times faster than that in the ZVI system.In addition,there is no iron sludge produced at the end of the EDTA + S-ZVI system reaction,which indicates that EDTA can inhibit the oxidation of Fe(?)by oxygen into iron precipitate(Fe2O3 or FeOOH)and improve its utilization efficiency to AR 73.In addition,the effects of various experimental factors(S/Fe molar ratio,chelator concentration,AR 73 initial concentration and mixing rate)on the AR 73 removal were also investigated.The results indicated that the Kobs followed a bimodal pattern with increasing S/Fe ratio,chelator concentration,and mixing rate,but a unimodal pattern along with initial AR 73 concentration.Based on the solid/liquid phase analysis and related literatures,we believed that the ZVI reactivity was affected by thickness of the surface FeSx layer,which leaded to a decrease in surface reactive sites at low thickness,and hindered the release of Fe(?)and the transfer of electrons from the ZVI core to the surface at high thickness.Additionally,high stirring rate also affected the coordination bond of Fe(?)-EDTA complex,thus resulting in a decrease in AR 73 when RPM>300.According to the UV-vis and GC-MS analysis of the reaction supernatant,we found that the chromophoric azo groups of AR 73 were broken down through hydrogenation reaction and hydrazo was produced as an intermediate,which was further reduced to 4-amino azobenzene and 4-amino-5-naphthol-1,2-disulfonic acid.The sulfidation and EDTA increased the removal rate of AR 73 by ZVI,but did not change removal process of AR 73.To further investigate the mechanism of azo dye decolorization by S-ZVI coupled with EDTA,this paper performed an in-depth study on the surface morphology and chemical composition of the S-ZVI particle during the reaction.The results showed that EDTA could complex with the Fe(?)(surface bound Fe(?))in the reaction complex to form the Fe?-EDTA complex,which would inhibit the oxidation of Fe(?)by oxygen and improved the reactivity of Fe(?)to AR 73.In addition,an experiment with a hydroxyl radical scavenger indicated that H2O2 generated did not produce Fe(?)through Fenton-like reaction,instead produced Fe(?)and OH-by reacting with Fe(0)due to the neutral to weakly alkaline environment,which all contributed to the enhanced removal of contaminants.