Study On Mechanism Of Synergistic Removal Of Nitrobenzene By Zero-valent IRON And Pyrite

Zero-valent iron?ZVI?is widely used to remove oxidizing organic pollutants and toxic metal ions in the environment due to its advantages such as low price,non-toxicity,and high activity.However,the reaction of ZVI with contaminants will cause pH increase in reaction media,resulting in the passivation of iron surface and decreased reactivity and longevity of ZVI,which has become the vital limitation for its further application.Pyrite?FeS₂?is ubiquitous in anaerobic environments and is the most widely distributed sulfide mineral in the earth crust.It is capable to reduce some environmental pollutants and also to decrease the pH in reaction media.Therefore,using pyrite together with ZVI for removal of pollutants will do favor to inhibit the surface passivation of ZVI and to enhance its reactivity.In this paper,the mixture of pyrite and ZVI with different ratios was used to remove nitrobenzene in different initial pH.And the results were compared with the reaction systems using pyrite or ZVI alone.The synergistic mechanism of pyrite and ZVI in the reductive removal of nitrobenzene was discussed in depth by investigating the concentrations of reactants and products,pH change during the reactions,and the composition and structure of ZVI and pyrite before and after the reaction,in combination with M?ssbauer spectroscopy,XPS,XANES,SEM-EDS,XRD,and HPLC.The main results are as follows:?1?The removal rate of nitrobenzene by the pyrite/ZVI mixture is much higher than the sum of removal rates by pyrite and ZVI seperately,showing an obvious synergistic effect.The rate constant of nitrobenzene reduction by ZVI at pH₀ 6.0 was enhanced by 8.55-23.1 folds due to the presence of pyrite with the pyrite/ZVI mass ratio ranging from 1.0 to 6.0.?2?Nitrobenzene could be removed completely at pH₀ ranging from5.0 to 10.0 by the pyrite/ZVI mixture,while negligible removal of nitrobenzene by ZVI alone was observed at pH₀?7.0,which implies that the mixed system has wider pH adaptability.?3?The ZVI sample recovered from the reacted pyrite/ZVI mixture was reused for nitrobenzene reduction,and showed much higher efficiency for nitrobenzene removal than the pristine ZVI.?4?The mechanistic study revealed that pyrite presented multiple functions when reducing nitrobenzene with ZVI.Pyrite could remarkably suppress the pH increase in reaction media,decrease the formation of passive?hydr?oxide film,and boost the regeneration of reactive Fe²⁺.Of special interest is that both M?ssbauer spectroscopy and XRD results confirmed the formation of highly reactive FeS@Fe in the reaction of pyrite/ZVI mixture with nitrobenzene,which provides a noval and facile way for in-situ sulfidation and synthesis of highly reactive FeS@Fe during the removal of contaminants.