Study On The Synergetic Removal Of Heavy Metal Pollutants By Zero Valent Iron/Pyrite

The remediation of heavy metal pollutants by zero-valent iron?ZVI?is regarded as a promising remediation technology for the contaminated environment.However,the reaction between ZVI and heavy metal ions increases pH of medium obviously,which can produce more iron oxides as an passive shell on the surface of ZVI,inhibit the electron transfer between ZVI core and pollutants,and reduce the reactivity and repeated usability of ZVI.Pyrite?FeS₂?is the most abundant iron sulfide mineral in the earth-crust,and has the reactivity to reduce and remove heavy metal ions.However,different from ZVI,the reaction between pyrite and pollutants can decrease pH of medium.Therefore,the mixture of ZVI and pyrite with suitable dose can effectively buffer the changes in pH of medium,and improve their reactivity for removing pollutants.In this study a method simulated PRB column was adopted,and the mixture of ZVI and pyrite was firstly used as the active packing in the column to remove Cr?VI?from artificial wastewater.We investigated the effect of mixed manner and proportion of ZVI and pyrite,pH of medium on the removal efficiency of Cr?VI?,and compared the removal efficiency of the mixed system with that of the single system packed with either pyrite or ZVI alone.In combination of batch experiments,the changes in pH values and component concentrations of various reaction systems were monitored.In addition,SEM-EDS,XPS and XRD were used to analyze the composition and bonds on the surface of ZVI and pyrite,in order to elucidate the synergetic mechanism of ZVI and pyrite on the removal of Cr?VI?.In addition,a negatively charged sodium bentonite bearing high adsorption capacity for Ni?II?was used as the carrier to support NZVI to fabricate sodium bentonite supported NZVI?NZVI/Na-bent?,and the removal and reduction efficiencies of Ni?II?by NZVI and NZVI/Na-bent were also compared.In combination of the study on changes in pH of medium during reaction and element composition on the surface of NZVI and NZVI/Na-bent before and after reaction,and the repeated usability of both NZVI samples,the synergetic mechanism on the removal of Ni?II?by NZVI and NZVI/Na-bent were investigated.The results were listed as follows:?1?In the simulated PRB column experiment,the removal efficiency of Cr?VI?by homogeneous mixture of ZVI and FeS₂ was obviously higher than the sum of that by either ZVI or FeS₂ at the same dose,showing the synergetic effect between ZVI and FeS₂ on the removal of Cr?VI?,and this synergetic effect was better in the medium at lower pH.?2?In the removal of Cr?VI?by mixed system of ZVI and FeS₂,pH of reaction medium in the mixed system was lower than that in ZVI system alone,and the concentration of SO₄^2-and Fe²⁺in the eluent of mixed system were higher than that of ZVI or FeS₂ alone;the produced FeS₂ in the mixed system could more effectively transform the Fe?III?generated from Cr?VI?reduction into active Fe?II?in cycle than ZVI with passive surface,showing the improved reduction capacity per mass of both ZVI and FeS₂ in the reduction of Cr?VI?by mixed system of ZVI and FeS₂.?3?The results of batch experiment showed ZVI,FeS₂,aqueous and surface bound Fe?II?and S₂^2-were the active reducing components in the mixed system.Therein,aqueous and surface bound Fe?II?contributing the reduction of Cr?VI?with the rate of 55.9%were the two most important reductants.?4?The removal efficiency of Ni?II?by NZVI/Na-bent was notably higher than the sum of that either by NZVI or by Na-bent alone,creating the good synergetic effect in the removal of Ni?II?between NZVI and Na-bent.Na-bent as a carrier can improve the dispersion of NZVI particles,enrich Ni?II?on the surface of supported NZVI,effectively buffer the changes in pH of reaction medium by silicon and aluminum hydroxyl groups,decrease the production of iron oxides on the surface of NZVI,and transfer partial corrosion products away from NZVI surface to Na-bent surface.These superior functions of Na-bent promoted NZVI/Na-bent to effectively transform the mobile and toxic Ni?II?into immobile Ni⁰ with lower toxicity,with the highest efficiency in the reported literature so far.