Synthesis Of Nanoscale Zero-Valent Iron By Self-Assembly Technology And Its High-Performance Removal Of Environmental Pollutants

Nanoscale zero-valent iron?NZVI?particles have attracted much attention in the field of environmental pollutants removal,such as nitrite,selenate,organic dyes,aromatic halides and heavy metals etc.,due to their large specific surface area,high reactivity and reducibility.However,NZVI particles can come close together to agglomerate into larger particles because of the strong van der Waals force as well as magnetic force between NZVI particles.Moreover,NZVI particles can be oxidized easily after the removal of contaminants even in anoxic environments.All these factors will lead to a significant decrease in activity,stability,removal efficiency and regeneration of NZVI particles,which limit their widely application.Therefore,it is still a challenge to maintain the high activity and stability of NZVI particles via modification and control the particle size.In addition,the preparation and storage of NZVI particles should be in inert gas because of the high reactivity,which result in the increase of the complexity of preparation process and cost.Therefore,it is an important research subject to improve the activity and stability of NZVI particles via developing new technology to precisely control the size and morphology of NZVI particles.In this work,three novel NZVI particles with good dispersibility,high activity and high stability were firstly prepared via vesicle-assisted method,micelle-assisted method and NH₂-SiO₂ support method at ambient temperature and pressure.The crystal phase,morphology,microstructure,particle size distribution and element distribution of the prepared NZVI particles were fully characterized by X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?,transmission electron microscopy?TEM?,scanning electron microscopy?SEM?,large angle ring dark field scanning transmission electron microscopy?HAAD-STEM?,X-ray photoelectron spectroscopy?XPS?,and dynamic light scattering?DLS?.The synthesis mechanism of three novel NZVI particles was systematically studied.Three template pollutants including nitrobenzene,p-nitrophenol and chromium?VI?were chosen to investigate the reactivity and stability of as-prepared NZVI particles.The main research contents and results were summarized as follows:?1?For the first time,poly?1-vinylpyrrolidone-co-vinylacetate??PVV?was selected to form vesicles in aqueous tetrahydrofuran?THF?solution by self-assembly due to its amphipathy,low toxicity and biodegradability.The amphiphilic block copolymer poly?1-vinylpyrrolidone-co-vinylacetate?can self-assemble into vesicles in aqueous-tetrahydrofuran media,and then iron ions?III?enter into the vesicles through coordination.After the reduction by sodium borohydride,the NZVI partic les were formed in the vesicles.NZVI in ves icles were characterized with the aid of XRD,FTIR,SEM-EDX,TEM,XPS and DLS techniques.It was found that NZVI in ves icles showed some unique features,such as uniform nanometer size distribution?from 70 to 100 nm?,vesicle-like morphology,and good dispersion.Furthermore,the NZVI particles were well encapsulated in the ves icles,and they could be stored directly in the air as solids,but the dried NZVI particles could be easily released from the vesicles once they were put in water.Activity and stability of the NZVI in ves icles were examined by using Cr?VI?and nitrobenzene as the model pollutants,and compared with the bare NZVI particles synthesized with the same procedures but without PVV.A dramatic difference in activity and stability was observed for the two different NZVI particles.The NZVI in vesicles showed a good chemical stability in the air,and still maintained its high reactivity in water.For example,even after exposing the dried NZVI in ves icles to the air for 90 days,only4.4%decrease was observed in the removal efficiency of Cr?VI?,while the corresponding decrease was88%for the bare NZVI particles.Finally,the degradation mechanism of pollutants?Cr?VI?and nitrobenzene?by NZVI in vesicles were discussed in detail with the aid of XPS and HPLC techniques.?2?The zero-valent iron nanoclusters?<1 nm?were synthesized with the aid of micelles self-assembled by hexadecyl trimethyl ammonium bromide?CAT B?at ambient temperature and pressure.The micelles of CTAB serve as soft templates to direct the synthesis and assembly of nanoparticles,and the thiourea as a controlling agent to ensure the size control for final nanoclusters.Zero-valent iron nanoclusters were characterized with the aid of XRD,TEM,STEM-EDS and XPS techniques.It is found that a plentiful of zero-valent iron nanoclusters with a diameter of<1 nm have been assembled into quasi-spherical assemblages?with around 5 nm in width?.The assemblages clearly show that most of the nanoclusters exist discretely because of being coated by entangled hydrocarbon chains of CTA⁺.The ZVI nanoclusters exhibit a good chemical stability and remarkable catalytic activity in water.Compared with traditional noble metal catalysts,record-high catalytic performance is observed in the model reaction of the catalytic reduction of p-nitrophenol to p-aminophenol.For example,100%of p-nitrophenol can be decomposed by NZVI nanoclusters only in 60 seconds with an activity factor as high as 68.3 s^-1 g^-1.After 8recycles,the degradation effect did not decrease significantly.?3?A novel Fe⁰@NH₂-SiO₂ nanospheres were prepared a simple and ingenious method under mild conditions,and the NZVI particles were successfully grafted onto the surface of the nanospheres.Further,the synthesis mechanism of Fe⁰@NH₂-SiO₂ composites was investigated.It was found that a large number of spherical zero-valent iron aggregates?50 nm?were distributed on the surface of NH₂-SiO₂ microspheres,which consist of many zero-valent iron particles?5-10 nm?.The good dispersibility of NZVI particles was possibly due to the winding and coating of hydrocarbon chains of silylating reagents.Further,the activity and stability of Fe⁰@NH₂-SiO₂ composites were studied using p-nitrophenol and Cr?VI?as template pollutants,and the influence of concentration of sodium borohydride and reactants on the recycling of the nanocomposites were investigated in detail.It was found that the nanocomposites had good chemical stability,and the removal efficiency of p-nitrophenol by Fe⁰@NH₂-SiO₂ is 10 times that of bare NZVI particles synthesized with the same procedures but without NH₂-SiO₂.These results illustrate that the modification and grafting of NH₂-SiO₂ microspheres can not only inhibit the agglomeration of zero-valent iron nanoparticles,but also control the size of nanoparticles successfully.