Research On The Performance And Mechanisms Of Cr(?) Sequestration By Sulfidated Zerovalent Iron

The heavy metal pollution of water has caused serious threats to the natural environment and human health,becoming a hot and difficult issue in the environmental field.A large amount of Cr-containing waste water is produced,as chromium(Cr)is widely used in tanning,electroplating,and mining industries.Zero-valent iron(ZVI)technology has broad application prospects due to its wide range of raw materials,low cost,non-toxicity,and high reduction.However,the formation of passivation film on the surface of ZVI greatly limits the further contact reaction between contaminants and ZVI,decreased the reactivity of ZVI.In this study,sulfur compounds were used to vulcanize ZVI based on the corrosion characteristics of sulfur(S),which effectively improved the removal efficiency of Cr(VI)by ZVI.S-ZVI particles with an average particle size of 30 p.m were synthesized by sodium sulfide and micron ZVI,and the difference of S/Fe were controlled adding different volumes of sodium sulfide.The synthesized S-ZVI was investigated by scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDS),X-ray photoelectron spectroscopy(XPS)and atomic force microscopy(AFM)to characterize the physical and chemical properties of S-ZVI.The results show that the S-ZVI surface is rougher and present a porous structure,and FeS is the main form of S load to ZVI.At the same time,due to the difference of S/Fe,S-ZVI exhibits different physical properties.When S/Fe = 0.056,S-ZVI reaches the maximum specific surface area and roughness.The experiments mainly studied the removal of Cr(VI)under anaerobic and aerobic conditions for ZVI and S-ZVI(S/Fe = 0.056)materials,respectively.The results show that the coupling of sulfidation and oxygenation significantly improves Cr removal,and the reactivity conforms to the following trend:S-ZVI + aerobic<S-ZVI +anaerobic<ZVI + anaerobic<ZVI + aerobic.To further explore the effect of oxygen and S/Fe on the removal of Cr(VI)by S-ZVI,selecting different oxygen concentrations under S/Fe = 0.056 and different S/Fe under aerobic conditions.The results show that the removal rate of Cr(VI)by S-ZVI increases as the oxygen concentration increases from 0%to 100%.However,the trend of Cr(?)removal is bimodal distribution under different S/Fe.When S/Fe of S-ZVI is 0.056,Cr(?)achieves the best removal efficiency.Based on the optimal S/Fe and aerobic condition,the Cr(?)removal was investigated under various condition,such as initial Cr(?)concentration,dosage,initial pH and mixing rate.The dynamics and mechanism of Cr(?)removal by S-ZVI was investigated by analyzing the change of Cr(?)concentration in liquid phase,the SEM,EDS,XPS of S-ZVI after reacting 120 min,and adding phenanthroline as Fe(?)complexing agent under reaction process.The results show that all the experimental results accord with the pseudo first-order kinetics reaction.The effect of various factors on Cr(?)removal by S-ZVI can be seen according to rate constants.The removal of Cr(?)by S-ZVI is mainly due to the reduction of Cr(VI)to Cr(?)using Fe(?)as a reducing agent,and then precipitated and removed.In addition,a novel time series correlation was developed between aqueous Fe(?)and Cr(VI)based on data collected in the presence and absence of 1,10-phenanthroline,to probe for changes of reductants during the reaction time course.The analysis indicated that Fe(0)was responsible for the initial small amount of Cr(?)removal,which then transitioned to a phase controlled by surface Fe(?).The slopes of the time series correlations during the latter phase of the reaction vary with experimental conditions but are mostly much higher than the theoretical stoichiometric ratio between Cr(?)and Fe(?)(i.e.,0.33),indicating that Fe(?)regeneration contributes significantly to Cr removal.Finally,we experimentally combined Fe(?)with phenanthroline to speculate that the regeneration of Fe(II)leads to the removal of Cr(?),ie,Fe(0)and Fe(?)binding reaction Fe(?)is generated for the removal of Cr(?)in water.