The Preparation Of Amorphous Zero-valent Iron And Its Application In Typical Environmental Contaminants Removal

The environmental problems accompanying with the rapid development of the economy around the world are becoming increasingly prominent.It is urgent to seek economical and environmentally friendly remediation technology.The common water remediation technology mainly includes activated sludge process,membrane technology,advanced oxidation technology and zero-valent iron technology,etc.Among the various technologies,zero-valent iron(ZVI)has attracted much attention in recent years at home and abroad because of its cheap and eco-friendly properties.ZVI permeable reactive barrier(PRB)technology has been applied to remove heavy metal ion and decompose halogenated organic in groundwater.However,the wider application of ZVI technology is limited due to its low efficiency in electron transfer and slow dissolution rate of iron ions.Therefore,researchers have developed a series of strategies to regulate the activity of ZVI,such as sulfidation and hybridization of iron oxide shell,or modified iron with polymer.Although these strategies improvemed the performance of ZVI significantly,they failed to provide a fundmental information about the relationship between the structure of inner iron core and activity in contaminants removal.It is well known that the activity of material relys on its structure.For the core of ZVI,the essence of its structure is the arrangement of iron atoms.The long-range ordered arrangement of iron atoms resulted in crystal zero-valent iron(CZVI),and the long-range disordered and short-range ordered arrangement resulted in the amorphous zero-valent iron(AZVI).Comparing the structure and performance of AZVI and CZVI would constribute to understand the structure-activity relationship of ZVI.Therefore,the objectives of this doctoral dissertation are to prepare AZVI and CZVI by liquid phase reduction,compare the differences in structure and performance in pollutant removal of AZVI and CZVI,establish the relationship of structure and activity of ZVI from the inner iron core and external iron oxide shell,and provide foundmental guidelines for the rational design of iron with satisfactory performance.The main contents and results of this doctoral dissertation are summarized as follows:1.AZVI shows excellent performance in the field of environment remediation and catalysis industry.However,the traditional preparation method for AZVI requires special equipment and ultra-fast cooling rate(105?107 K/s),therefore,it is necessary to develop a more moderate method to prepare amorphous ZVI.Although ZVI prepared by borohydride usually shows low crystallinity,it is limited to prepare AZVI via this method.According to the supersaturation theory of crystal growth,the driving force of crystal growth is the difference in chemical potential between solution phase and solid phase,namely supersaturation(??).Considering the chemical potential value of metal atoms,amorphous ZVI and crystalline ZVI,it is possible to prepare metastable AZVI via decreasing the value of ??.Therefore,organic amine ligands were utilized to decrease the reduction rate of iron ions and ?? to prepare AZVI in this chapter.It was found that ethanediamine(EDA)would increase the reduction potential of ferrous ions,and decrease the reduction potential of sodium borohydride.Meanwhile,the decreased H2 generation in the presence of EDA during the reaction further verified the diminished reduction rate of ferrous ions.In this study,SEM,TEM,XRD,synchrotron radiation XRD and PDF technology were used to characterize products prepared by ammonia,diethylamine,ethylenediamine and diethylenetriamine,and these characterization results showed that only ethylenediamine and diethylenetriamine could induce the formation of AZVI,and the order degree of amorphous zero-valent iron were 10 A and 12 A,respectively.Further isothermal calorimetry experiment results revealed that the strong coordination ability of ethylenediamine and ferrous iron,suggesting that stable ligand was essential to the formation of amorphous structure.This study provided a new method for the preparation of amorphous ZVI under atmospheric pressure,and avoided the utilizaiton of special devices or ultra-fast cooling rate.Meanwhile,this research provided materials for the study of the relationship of ZVI's inner iron core structure and its performance in pollutants removal.2.ZVI is easy to be oxidized in the practical application,and the composition of iron oxides is closely related to the adsorption,precipitation and migration of pollutants,therefore,it is vital to study the oxidation process of zero-valent iron.In this research,AZVI and CZVI were used to study the oxidation mechanism of ZVI.XRD,Raman spectra,XPS,SEM and TEM were used to characterized the composition and morphology of products during oxidation process.From the results of XRD and Raman spectra,we found that AZVI transformed into lepidocrocite at 0.5 h,and the crystallinity of lepidocrocite increased with the increase of reaction time.In constrast,CZVI first transformed into magnetite,and then lepidocrocite.Further SEM,TEM and XPS results demonstrated that the oxidation process of AZVI mainly occurred in solution and the inner iron core would dissolved finally.While CZVI transformed into magnetite at the beginning as Fe(?)absorbed on the surface of CZVI resulted in the formation of the crystal nucleus of magnetite.Meanwhile,the released iron ion of CZVI turned into lepidocrocite.Further high resolution XPS spectra of O 1s and contact angle test showed that the different oxidation pathways of AZVI and CZVI were originated from the different concentration of surface hydroxyl.The study in this chapter provided a new perspective understanding about the effect of ZVI's the oxidation rate and surface propeties on the oxidation products of iron.3.The performance of heavy metal removal is strongly related to the types of iron minerals,it is vital to investigate the relationship between oxidation pathway of ZVI and the performance of heavy metal removal.Considering that adsorption and coprecipitation are always acommpany with reduction and oxidation,it is necessary to choose suitable pollutants to study the ability of oxidation,reduction and adsorption or coprecipitation of ZVI.Due to the fact that Ni(?)could not be reduced by Fe(?),and As(?)could be oxidized to As(?)by hydroxyl radical generated through oxygen activation,Ni(?)and As(?)were served as model pollutants to study the ability of reduction and oxidation,respectively.This chapter compared the removal efficiency of Ni(?)and As(?)by AZVI and CZVI in the air and argon conditions.The results showed that the removal rate of Ni(?)and As(?)by AZVI were higher than that by CZVI,and oxygen had little effect on the removal process of Ni(?).In contrast,the removal rate of As(?)under air condition by AZVI and CZVI increased by 1.4 times and 6.2 times than that under Ar condition.In order to reveal the removal mechanism of Ni(II)and As(?),XPS was used to analyze the states and composition of nickel and arsenic before and after the reaction.The results showed that 35.5%and 21.15%of Ni(?)were reduced to Ni0,and 59.8%and 51.8%Ni(?)were adsorbed directly in the AZVI/Air and CZVI/Air systems,respectively.AZVI/Air and CZVI/Air system could oxidize 64.1%and 60.6%of As(?)to As(?),and adsorbed or co-precipitated 34.5%and 37.2%As(?),respectively.Further detection of dissolved iron ions and hydroxyl radical results revealed that the higher removal rate of Ni(?)by AZVI was originated from the fater electron transfer rate of AZVI,and the faster rate in As(?)removal could be attributed to the faster dissolution rate iron ions.In a word,this chapter reported the reduction,oxidation and adsorption ability of AZVI and CZVI by studing the removal mechanism of Ni(?)and As(?).It is helpful to understand structure-activity relationship of ZVI and the application of amorphous ZVI.4.The results in last chapter revealed that the removal of Ni(?)and As(?)were related to oxidation pathway of ZVI,however,the relationship between oxidation pathway and organic pollutants removal is still unknown.In this chapter,thiamphenicol(TAP),a kind of antibiotics used in aquaculture industry,was served as model contaiminant to study the relationship between ZVI oxidation and organic pollutants decomposition.The degradation experiments results showed that 5%,27%and 100%of TAP could be removed by C-nZVI,C-mZVI and A-mZVI within 80 min.Meanwhile,the concentration of Fe(?)released by C-nZVI,C-mZVI and A-mZVI were 0.02 mmol/L,0.04 mmol/L and 0.12 mmol/L,respectively.Obviously,the degradation percentage of TAP was positively related to the the concentration of released Fe(?).While further control experiments and scavenger experiments demonstrated that TAP degradation was not attributed Fe(?)in the system,but the electron released by iron core.To reveal the mechanism of the difference performance of CZVI and AZVI,we compared the PDF data to obtain the local structure information of CZVI and AZVI,and the results demonstrated that the Fe-Fe bond of CZVI and AZVI were 2.5 A and 2.6 A,respectively.The longer Fe-Fe bond resulted in lower energy for the cleavage of Fe-Fe,and faster rate in Fe(?)and electron release.This finding emphasized the effect of amorphous process on the activity of ZVI,and provided a fundamental understanding about the relationship between the inner iron core and its performance in pollutants removal.It would also contribute to the rational design of highly efficient ZVI materials.5.In this chapter,amorphous zero-valent iron(AZVI)and iron-nickel bimetal(AZVIN)were prepared firstly.Subsequently,AZVIN was characterized by spherical aberration transmission microscope,and the characterization results revealed that AZVIN was surrounded with nickel and showed a typical nuclear shell structure.Comparing the TAP degradation efficiency and the results of SEM,TEM,XRD and specific surface area,we put forward two conclusions.First,the TAP degradation was related to the active site on surface of AZVIN.Secondly,the efficiency of TAP degradation by AZVIN was 14 times higher than that of AZVI.Considering the results of electrochemical test and electron spin resonance,we found that surface negative charge of nickel is favorable for proton adsorption and hydrogen radical generation.Meanwhile,metal nickel showed better recycling performance on the surface of the zero-valent iron due to the reductibility of the zero-valent iron.Therefore,iron-nickel bimetal showed a better performance of the removal of thiamphenicol.